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发帖数: 30882 | 1 美国加州被地震“撕裂”了
据外媒CNET报道，2019年7月5日发生在加利福尼亚州的7.1级地震，给地球造成
了一条巨大的裂缝，加州的地表被撕开了。
晚上8点19分开始，加利福尼亚州开始剧烈的摇晃，许多人争相寻找掩护，这是
该地区在不到48小时内发生的第二次强烈地震。
地震造成的直接损害非常明显，商店里无数打的瓶子被打碎，爆裂的罐子散落
在地板上，很多挂在墙上的东西也倒塌了。
但是让大家始料不及的是，当第二天太阳升起时，这片区域的地形也发生了明
显的变化，人们通过各种媒体消息得知，美国加州被“撕裂”了。
9年前的预测被证实
2012年2月20日我们在《给美国同行的协查通报》中指出，干旱和暖冬是地震前
兆吗？
耿庆国提出了旱震理论：6级以上大地震的震中区，震前1-3年半时间内往往是
旱区。旱区面积随震级大小而增减。在旱后第三年发震时，震级要比旱后第一年内发震
增大半级。
美国的异常干旱和暖冬可以被锁定在旱震理论的范围之内，可检验的异常现象
接踵而来。
我们在2015年4月2日指出，3年过去了，美国加州干旱持续发展，大震不发，干
旱不止。
美国干旱不仅与大震有关，而且与黄石公园超级火山喷发有关。地下的巨大能量
蠢蠢欲动，是干旱、大震和超级火山喷发的动力。
中国地震台网正式测定：2019年07月05日01时33分在美国加利福尼亚州（北纬35
.71度，西经117.51度）发生6.4级地震，震源深度10千米。专家预测，未来几周发生另
一场大于6.4级地震的几率为9% ，一次大于5级的几率为20%。
7月6日11时20分，美国加州又发生6.9级地震 ，大震预测初步得到证实。
事实上，2012年2月20日我们在《给美国同行的协查通报》中指出，干旱和暖冬
是加州地震前兆，大震不发，干旱不止。
本次加州山火是大震发生的前兆。
我们在2019年2月1日指出，美国灾难将进入峰值。
我们在2021年7月5日指出，干旱、山火、高温、火山活动、地震，美国灾难源
于加州地下能量释放，由此引发的点源能量喷发模式即将进入能量释放高潮。
我们在2021年7月5日指出，由同一原因造成的多种灾害，在同一地区，同一时
间，先后依次发生，形成灾害链。
我们在2015年4月2日指出，大震不发，干旱不止。
据加州高温干旱规模加州至少要发生8级以上大震
加州2019年7级地震和2021年6级地震只是预演。2023-2025年为月亮赤纬角最大
值，2024-2025年可能为太阳黑子峰值，据加州高温干旱规模，在此期间，加州至少要
发生8级以上大震。
美国地质调查局宣布，2021年7月8日15时50分左右记录到加州和内华达州边境
地区一次6.0级地震，震源深度约为6英里，位于托帕斯湖以南，距离内华达州史密斯谷
约20英里。据悉，这是1994年该地区发生6.1级地震以来最大的一次地震。这次地震是
1994年该地区发生6.1级地震以来最大的一次地震。
南加州大学南加州地震中心的Jason Ballman表示，人们应该预防这种规模的地
震发生后几天内的余震。
地震学家表示，预计未来一周会有更多的余震发生，甚至有6%的可能性余震的震
级会超过6.0。
加州的大震警钟敲响，美国灾难正进入高潮。
======================================
所谓“旱震理论”是指大震之前必大旱。其学术创立者——旱震理论专家耿庆国。四川
汶川大地震证明了地震科学家耿庆国旱震理论。
中文名旱震理论释 义大震之前必大旱创立者耿庆国提出时间1972年出生日期1941年
1月证明事件四川汶川大地震
目录
1 原理
2 简介
3 学术创立者
4 研究领域
5 地震预测
6 大纪事
原理编辑 播报
耿庆国提出了旱震理论，对唐山地震提前提出了告急，成功预测了1990年代以来的若干
次地震。旱震理论描述：6级以上大地震的震中区，震前1――3年半时间内往往是旱区
。旱区面积随震级大小而增减。在旱后第三年发震时，震级要比旱后第一年内发震增大
半级。其内在的理论机制有两种：
（一）地热成因说
在月亮和太阳对地球产生的万有摩擦力的作用下，地下岩浆是流动的，流动的岩浆有时
会形成旋转上升的岩浆旋泉，岩浆旋泉会在这一地区的下面形成一个高温岩浆洞，使得
这一地区的地温长时间较高，地温升高会将地下水蒸发，并会使这一地区的气温升高，
空气上升，阻碍高空冷暖空气的汇合，造成这一地区无法降雨形成干旱，同时还会使这
一地区的地壳变薄，承受力变小，容易破裂。 在高温岩浆洞的底部，流动的岩浆容易
再次形成岩浆旋泉，这时形成的岩浆旋泉阻力小、威力大，它能将地壳顶破，引发地震
，因此干旱后容易发生地震。
在地震前，由于地下岩浆的旋转带动着地上空气的旋转，旋转的空气中心气压较小，所
以会出现日平均气压最低的现象。由于岩浆旋泉会把温度很高的岩浆从下地幔或外核输
送到上地幔，热量会扩散到大气中，会使气温升高，所以会出现日平均气温、日最高气
温、日最低气温最高的现象。又因旋转的岩浆会产生磁场，当磁场较强时，磁场中的分
子容易放热，这又会使气温降低，所以会出现日最低气温最低的现象。由于临震前气温
的降低，空气会下沉，冷暖空气在此汇合会形成降雨，因此会出现日降水量最大的情况。
（二）云室效应成因说
在地震发生前的地应力的累积会导致地层深处的由多种放射性同位素组成的氡气被挤压
时放到大气中，在宏观大气中形成云室效应，产生地震云，这就是地震云氡气成因论。
在地震云的氡气成因论中，将干旱和地震之间的关系解释为：氡气释放的对应构造有
关，在板块运动中，在构造初期应力的累积时，会缓慢挤压充填断层裂隙，使正常的氡
气释放通道关闭，由于氡气还是形成降雨的雨核，当断层裂隙被关闭时，释放到空气中
的氡气量减少，与之相应的降水减少，造成区域性干旱，按照本人地震云氡气成因论的
观点，干旱是由于震前存在区域性构造应力增加。使得形成降雨雨核的地氡释放减少造
成的，也就是所谓的震前干旱；当地层应力达到一定的程度后，会出现新的细微裂隙，
造成累积的氡气集中释放，形成地震云，也就是临震云；当地震发生后，断层构造完全
开放，这时候累积的氡气完全释放到大气中，形成震后云的同时，造成震后暴雨。这里
需要补充说明两点：（一）氡气是通过断层释放到大气中的，是断层存在的标识性气体
，也是目前监测地震活动的主要参量，我们实际测量的数值也证实了断层口上部的氡含
量是周边背景值的 10 到几百倍，甚至更高；（二）氡气是一种由多种放射性元素组成
的，其测量的标识元素是 Po218，其含量表示组成氡气的放射性元素的多少，也就是其
释放到空中后衰变所产生的放射性带电粒子的量的大小，也就是易于和空气中的游离水
分子结合，凝聚成雾，也就是我们所看到的形成云彩的凝聚核的多少。综合以上两点，
就可以知道，氡气是一种很好的将地震这种固态地理现象与云彩相关联的元素，即由于
板块应力的变化导致氡气释放量的突变增加，与空气中的游离水分子结合成我们看到的
宏观地震云。这也很好的解决了震前干旱和震后暴雨形成的机制问题。
关键词：旱震、岩浆旋泉、高温岩浆洞、气象要素、五项指标、日平均气压、板块运动
、氡异常、云室效应。
简介编辑 播报
旱震理论，打破了地震界主流的共识——破坏性地震（5级以上）的预报与临震预报是
不可能的。耿庆国师承著名地震学家傅承义教授，学习中国传统文化的精华，充分利用
历史文献记载和取象比类的方法等手段，1972年耿庆国提出“旱震关系大地震中期预报
方法”，在中国地震界处于非主流派。争议很多，但实践证明是准确的。耿庆国预报了
1975年的海城地震，特别是1976年的唐山地震。在1980年代出版了专著《中国旱震关系
》（科学出版社）。 | w*********g
发帖数: 30882 | 2 How Droughts Influence Earthquakes
Authors
Affiliations
Citations
Diandong Ren and Rong Fu
Accepted: November 02, 2019 | Published Online: November 04, 2019
Article
Abstract
References
PDF
How Droughts Influence Earthquakes
Abstract
Earthquakes result from strain build-up from without and weakening from
within faults. A generic co-seismic condition is presented that includes
just three angles representing, respectively, fault geometry, fault strength
, and the ratio of fault coupling to lithostatic loading. Correspondingly,
gravity fluctuations, bridging effects, and granular material production/
distribution form an earthquake triad. As a dynamic constituent of the
gravity field, groundwater fluctuation is the nexus between the triad
components. It is pivotal in regulating major seismic irregularity, by
reducing natural (dry, or purely tectonic, stationary seismicity) inter-
seismic periods and by lowering magnitudes. Specifically, to exert stress on
the fault, groundwater does not need to reside deep in proximity to the
locked fault interface, as it can work remotely. It can act mechanically-
direct (MD), by a differential de-loading and superimposing a seismogenetic
lateral stress field, thereby aiding plate-coupling, from without, or
mechanically-indirect (MI) by enhancing fault fatigue, and hence weakening
the fault, from within. To verify this hypothesis, gravity measurements, and
a numerical model, are used. The remote action hypothesis is globally
applicable. Detailed results are presented for the Himalayan and New Zealand
regions. The gravity recovery and Climate experiment (GRACE measurements)
reveals that major earthquakes (Mw 5 and above) always occur in the dry
stage, indicating drought and associated groundwater extraction is an
important trigger for major earthquakes. By exploring 73 historical records
successfully reproduced by the model, it is found that for collisional (e.g.
, the peri-Tibetan Plateau) and strike-slip (e.g., the San Andreas Fault)
systems, the MD mechanism dominates, because the orographically induced
spatially highly variable precipitation is channeled into greater depth by
through-cut faults. Droughts elsewhere also are seismogenetic, but likely
through MI effects. In a warming future climate, mechanisms identified here
play a greater role in increasing the recurrence frequency of major
earthquakes, but also in slightly reducing their severity.
Keywords
Tectonic earthquakes, GRACE, Groundwater fluctuations, Bridging effects,
Granular material production
Significance Statement
This study formulates co-seismic conditions involving three specific angles
that relate to balancing of the magnitude and direction of the associated
forces that collectively when perturbed can precipitate an earthquake event.
The hypothesis developed in this research (1) Enlarges our knowledge base
by extending the canonical frictional phenomena into fatigue conditions at
the plate interface; 2) Indicates that large scale, extended droughts, by
exerting cyclical unloading, create two mechanisms for enhancing tectonic
instability and the associated triggering earthquakes; and (3) Integrates
the role of (generic) groundwater fluctuations at the nexus thereby linking
climate change caused extremes and perturbations of the tectonic earthquake
cycle. With this hypothesis, mechanics-based process modelling has the
potential to be used to determine the locations and development of the
evolving regional stresses and to answer the questions of why, when and how
extreme will be a specific earthquake event.
Introduction
With enhanced hydrological cycles, severe droughts and extreme floods are
becoming more frequent. In addition to their direct socio-economic
consequences, they also influence seismic activity. There are salient remote
sensing signatures from droughts and floods. Relying on the groundwater
fluctuation signals provided by gravity satellites, this study focused on
revealing the intrinsic link between large scale hydrological cycles and
tectonic earthquakes, especially the role of groundwater, and water residing
in the rhizosphere, in affecting seismic activity. Tectonic earthquakes are
frictional phenomena between plates in contact [1]. At the wedge junction,
the upper plate's weight (or more generally, the compressive stress) aids
friction in resisting the increasing plate-coupling stress (τ) resulting
from differential plate motion. Figure 1 is an idealized configuration of
uniform fault zone geometry, which assumes a constant slope angle, θ,
unlimited width in the transverse direction, and uniform material with a
constant maximal frictional coefficient μ′= tan (θf). In this cases, the
co-seismic condition is:
ϕc= θ + θf (1)
where the repose angle is ϕ = arctan(τ/G), and G is the compressive
stress on the shear (fault) zone. The subscript 'c' means the critical value
, θ is the slope of the fault zone, determined by the geometry of the plate
interface, and θf is the maximum static friction angle corresponding to
fault strength. Values of θf depend on wall rock material properties and
lithology [2,3]. Between seismic events, the repose angle increases steadily
and approaches the sum of the fault slope angle and the maximum static
friction angle. The driving stress increases and the overall resistive
stress decreases (i.e., θf decreases); both are seismogenetic [1]. In
reality, erosion from within and the loading from without likely are
simultaneous. Earthquake cycles are stress adjustment cycles. Closely
related to the three angles in Eq. (1) are the earthquake triads: i.e.,
weight fluctuations of the overriding crust, bridging effects, and fatigue [
4]. With the fault strength set as constant, the fault geometry and plate-
coupling stress determine a natural, limiting, earthquake occurrence
frequency [5]. However, the observed earthquake occurrence seldom obeys this
limit and typically occurs well ahead of schedule, due to various fault-
weakening factors. Among many other first order mechanisms explainable by Eq
. (1), the present study singles out the weight loss (i.e. de-loading) of
the upper crust as seismogenetic. As recently reported, for the central
Californian valley [6] the weight loss of the upper crust can result from
extended droughts that deplete the soil moisture content residing in the
rhizosphere and/or by groundwater loss (extraction). By synthesizing
geodetic dislocations, obtained from radar and GPS measurements of surface
deformation, using an elastic dislocation model, Gonzalez, et al. [7] found
that the areas of fault slip, along the Alhambra de Murcia Fault of
southeast Spain, correlated well with positive Coulomb stress changes
induced by groundwater extraction from nearby aquifers. Water table
decreases of hundreds of meters were detected. Thus, the nucleation and main
rupture of the Mw 5.1 Lorca earthquake (May 11, 2011) were assumed to be
attributable to a loss of groundwater. In the following discussion,
groundwater is used as a generic term for both soil moisture, and for
drainage into a groundwater reservoir. For relatively weak faults (e.g.
subduction megathrusts), the lateral stresses exerted by groundwater
extraction are of the same order of magnitude as the fault strength. The
normal stress component exerted by groundwater transfers readily into
lateral stress in the fault environment. The following discussions use
subduction faults and collisional systems with thrust faults as prototypes.
By substituting compressive stress for lithostatic loading, the same
reasoning also applies to strike-slip faults, as explained in the
Supplementary Material of Ref. [6].
Displacements caused by a major earthquake (Mw of 5 or greater) usually
range between 0.5-3 m, i.e., they are at least 3 orders of magnitude smaller
than the dimension of the seismically locked fault zone, which can be
several hundred kilometers. Consequently, material in the locked fault zone
has experienced numerous such wearing and tearing events. When simulating
near-future earthquakes, the geological backgrounds, including topography,
material properties and fault geometry, can reasonably be assumed to be
constant. Thus, the natural limiting frequency of seismic slip/rupture
occurrence of a fault is highly stable. The irregularity in earthquake
occurrence is primarily due to fault weakening mechanisms and to external
forcing from the upper boundary. This study focuses on more dynamic
fluctuations affecting the earthquake triad. In the Supplementary Material
of this study (SM), a stability index is introduced, based on Eq. (1) to
explain, simply, the interaction of the triads and the role played by
groundwater fluctuations.
As an under-determined problem, multi-solutions (i.e., uncertainties in the
causal mechanisms) in seismogenesis can result from insufficient information
about the lateral interactions among different segments of the overriding
crust (Figure 2). Rather than the self-weight, the groundwater is
influential as it affects the lateral interaction between crust segments. In
Figure 2, the upper (usually continental) crust is represented as four "
carriages" of a train, each carriage signifying different segments of the
upper crust, which is in force balance between the frictional force at the
bottom and the drag in the front and therefore it is stationary. Depending
on how each carriage is footed (analogous to the friction between the upper
and lower crust plates), there are several possible configurations/scenarios
(the four rows in Figure 2). If the elastic plate can maintain rigidity
over the entire shear zone, all segments of the shear zone must reach stress
saturation for coherent sliding to occur. Prior to that critical moment,
all segments 'saturate' at the same rate, as the compressive driving stress
increases (the top train in Figure 2). In reality, earthquakes involving the
'saturation' of the entire shear zone seldom occur, because tectonic plates
have difficulty in maintaining rigidity over extended distances, manifested
as the existence of through-cut faults, heave bumps and many small
magnitude earthquakes within close proximity of each other. Consequently,
many locations do not contribute their shares of resistance and rely on
neighbors to stay in place. Depending on how is distributed on the fault
zone, there are various possible combinations of saturated and partially
saturated segments along the fault zone (trains 2-4 in Figure 2). In Figure
2 it is assumed the distance that plate maintains its rigidity also is the
horizontal dimension of each segment. Thus, once saturated, a corresponding
co-located earthquake can occur. For example, in the second train (second
row of the scenarios) the first and the last segment are saturated. As they
are separated, two small magnitude earthquakes (or one event with two small
ruptures [8]) are generated. The third train has two adjacent segments
saturated. In this case, a larger magnitude earthquake is triggered. The
fourth train is unique in that it involves interlaced compression and
dilation among the segments, corresponding to a saddle-shaped strain
distribution. Consequently, the three adjacent segments are simultaneously
saturated and a super earthquake is formed.
Even in the case of this highly idealized fault, with its uniform maximum
friction coefficients and uniform lithostatic loading, very different
spatial patterns are possible, fostering earthquakes of drastically
different magnitudes and at different spatial locations. These examples
represent only a fraction of the myriad possibilities that can unfold in
reality. In particular, earthquake occurrence can be compounded by any
factors that affect τ, G, and θf (or fault strength μ') in Eq. (A1).
Multi-solutions in seismogenesis result largely from inadequate information
as constraints on, or under- determination of lateral interactions. For
example, historical data should be treated with caution when predicting
future earthquake scenarios because they usually do not provide information
on the rupture length, width and slip; they provide essentially no
indication of the horizontal coherence of the different plate segments.
Earthquakes, because of the relative motion of plates involved, inevitably
have footprints on the mass [9,10], energy [11] and momentum fields (Ref. [
12] and references therein). Hence, earthquake predictions are increasingly
based on observations of these physical parameters and their fields. The
mechanism discussed here, i.e. the seismic consequences of groundwater
fluctuations, fits this situation exactly. It is a hitherto largely
neglected factor that plays a far larger role than anticipated in the
existing approaches, which include only local weight effects as some extra
weight (negligible compared with the ~20 km thick overriding crust plate).
The effect of the groundwater fluctuations is magnified by the bridging
effects and the related granular material (GM) generation, which is a
fatigue for brittle material.
Groundwater fluctuation caused fatigue accumulated temporally because there
is no healing mechanism for the granular fatigue and each leaves an
accumulative memory on the plate interface. Unlike the reported good
correspondence between minor seismic activity on the San Andreas fault and
the central California droughts and groundwater depletion [6], there seems
to be no one-to-one correspondence between extreme precipitation and drought
cycles and the large earthquakes. It is a multiple drought-wet cycles
preceding one major earthquake phenomenon. More critically, the footprint in
the lateral stress field accumulates spatially, especially along the
frontier mountain chains that act as waveguides by maintaining plate
segments' rigidity for extended distances. As it is a grand challenge, it is
not surprising that there is no simple equation available that predicts
earthquakes. All expressions and equations here are intended to convey the
concepts of the earthquake triads. A sophisticated numerical modeling
approach, as elaborated in the next section, is needed for reliable seismic
hazards predictions.
Materials and Methods
The SM outlines the interactions of the earthquake triads in the simplest
possible manner. From Eq. (A1), the weight of the overriding plate always
serves as a stabilizing factor. A weight reduction (e.g., from extended
droughts) contributes to instability as it is seismogenetic. Fluctuations in
the weight of the overlaying plate not only have local stability
consequences, they also affect neighboring regions through the bridging
effect and a unique fatigue process of the fault zone [13,14], by GM
generation and transportation, a common fault-weakening mechanism. Two
objects, when pressed against each other, wear and tear at the interface, as
a manifestation of material fatigue. The interfaces of tectonic plates are
no exception. Production of granular debris is the primary form of fault
interface fatigue. Regions of strong contact, likely as a result of the
bridging effect, enhance active granular material generation. GM has a much
smaller viscosity and acts as a lubricant for the plates involved. Regions
carrying more loading now effectively rest on a 'slippery floor' (i.e., more
weight is loaded on smooth contacts and less is loaded on rough contacts,
along the shear zone). Thus, the bridging effect, by affecting granular
material generation and redistribution, influences the frictional properties
of the fault interface. It therefore achieves a negative spatial covariance
between loading and fault strengths, thereby being seismogenetic according
to Eq. (A1). Sources of bridging effects are numerous. For example, large
scale structures in the overriding plate, such as mountains and valleys,
signify bridging effects at depth on the seismogenetic fault zone [15].
Bridging effects from static geological backgrounds, although they may cause
variations in the frictional properties of the seismogenetic zone, change
only slowly and are not responsible for short-term variations in major
earthquake occurrence. Large scale variations in groundwater, however, are
more dynamic and thus are viable candidates for explaining earthquake
occurrence irregularity on decadal to centennial time scales. Cyclic forcing
is the most effective means of causing fatigue [16]. Cyclic groundwater
fluctuations, especially when acting in concord with the resonance frequency
of the plates, assist GM generation.
Groundwater time series within and outside the gravity satellite
measurements period
Fluctuations in groundwater, which are remotely sensed by gravity satellites
such as the Gravity recovery and Climate experiment (GRACE) [17-19] are
used to highlight the strong correlations with the timing of major
earthquakes. Launched in 2002, GRACE has revolutionized the detection of
large- scale mass changes on Earth.
Water changes, irrespective of whether they result from accelerated ice
sheet melt, extreme precipitation events, or extended droughts, are the
dominant drivers of regional mass fluctuations. In addition to measurement
noise, the global monthly one-degree data used in this study have the
following missing months: June and July of 2002, June 2011, May and October
of 2012, March, August and September of 2013, February and November of 2014,
and June 2015. Missing values are obtained by linearly interpolated from
neighboring months. In total, there are 15 years of GRACE measurements of
time variations in gravity changes over the globe, before the quality of the
data degrades. Before 2002, or for areas with land surface areas that are
too small to be resolved by GRACE data, a land surface scheme within a model
referred to as Scalable Extensible Geofluid Modeling framework for
ENvironmenTal issues (SEGMENT [14,20,21], will be detailed in Subsection 2.2
), driven by atmospheric parameters, is used to simulate groundwater
fluctuations. The earthquake triad is interlinked with groundwater
fluctuations. Groundwater-aided GM generation is the mechanism modulating
earthquake cycles. As an initial attempt at supporting this hypothesis,
global evidence is provided of the earthquake events and gravity
fluctuations in the neighboring regions. The present study uses the global
seismic records from the USGS Significant Earthquakes Archive (https://
earthquakes.usgs.gov).
From Eqs. (1) and (A1), groundwater, by affecting the gravity field,
contributes to fault instability. Its spatial fluctuation, through enhancing
GM generation, also weakens the fault from within (Section 2.2 of the SM).
To obtain the groundwater fluctuations over the past years from 2000-2015, a
continuous time marching of the land surface scheme is performed with a 30-
minute time step. Atmospheric parameters such as near surface air
temperature, humidity, pressure, winds, and radiative components are from
the NCEP/NCAR reanalysis. Precipitation data from the Global historical
climatology network (GHCN, 1900-2015), GPCP, and The tropical rainfall
measuring mission (TRMM, 1998-2015) are taken as inputs during the
observational periods to drive the land surface scheme in SEGMENT to
diagnose how much precipitation is percolated into ground water reservoir.
For the recent decade (2002 onward), GRACE measurements provide a reliable
means for the groundwater recharge/discharge. Model simulations are found to
be satisfactory when verified against GRACE measurements. GRACE has a very
coarse resolution and SEGMENT can provide the spatial distribution of
groundwater at much finer resolution. Obtaining groundwater distribution in
a warmer climate is challenging. A small ensemble of 27 climate models,
listed in Table S1, and an innovative approach in estimating extreme
precipitation [20] is used for the prediction of the future occurrence of
earthquakes. The partitioning into surface runoff, evapotranspiration and
ground water percolation also is performed within SEGMENT.
In addition to the geodetic measurements and atmospheric parameters, the
earthquake model used here also requires some static data, such as fault
geometry, material property and thermodynamic parameters of the entire
simulation domain. In summary, CRUST1.0 [22] is used to set up the geometry
and physical properties (e.g., density values) over the grids within the
crust layer. It refers to the Advanced Solver for Problems in Earth's
ConvecTion (ASPECT) [23] for setting up the thermal and initial flow fields.
Model spin up runs are constrained by present GPS measurements of the
surface elastic deformation.
Numerical earthquake model
Like landslides, tectonic earthquakes are frictional phenomena. However, the
prediction of earthquakes is, however, very challenging and cannot be
summarized in just one or a few simple equations. Formulae are used here to
explain only the physics involved. At present, the only viable approach is a
coordinated solving of a numerical modeling system that includes energy,
momentum, and mass conservation principles, under prescribed rheological
properties. SEGMENT is such a realization. The current rubric on earthquake
prediction and forecasting (e.g., Ref. [24]) comprises four steps: The fault
model provides physical geometry of the faults (especially large and active
ones) considered; the deformation model provides slip-rate and creep
estimates for each fault section, and derives the deformation rate of the
modelled faults; the earthquake rate model provides long-term rate of all
earthquakes throughout the region of interest; and, finally, an earthquake
probability model provides the actual earthquake prediction. SEGMENT
essentially covers the first three models (with the fault physical geometry
and mechanical parameters as inputs to the modeling system). There is no
application of probabilistic methods in earthquake forecasting and the
epistemic uncertainties are assumed to be model uncertainty (e.g., model
parameter setting and input data inaccuracies), which reflect the still
incomplete understanding of earthquake mechanisms. In actual predictions,
SEGMENT modeling is a set of parallel code and requires super computers for
execution.
An understanding of the basic earthquake mechanics presented here is based
on numerical simulations from SEGMENT. SEGMENT is applied as a global
spherical crown model, with the stress decomposed into driving stress and
resistive stress components, following Van der Veen and Whillans [25]. By
solving the equations for conservation of mass, momentum and energy, SEGMENT
provides a 3D distribution of strain, stress, and the three components of
velocity in the simulation domain. Identifying a major earthquake from these
prognostics is described in Section 2.3. In addition to numerical details
such as fault-following grid stencils, Section 2.3 also describes approaches
in obtaining the current global distribution of GM thickness and its future
evolution.
Numerical model setup and parameterization of the earthquake triads
The theoretical concepts/hypothesis discussed above and also in the SM,
namely, GM generation facilitated by groundwater fluctuation, is implemented
in a global spherical crown model SEGMENT. As a dynamic model unlike
kinematic and empirical statistical models, SEGMENT solves the equations of
mass, momentum (under assumed multiple rheology), and energy conservation in
3D spherical geometry. The model prognostic equations include the three
components of velocity; temperature; and six components of the deviatoric
stresses. Their evolution is depicted by forward time marching of the
governing equations. Variables in Eqs. (1) and (A1), such as the degree of
stress saturation, are diagnostic equations of SEGMENT rather than
prognostic equations. Through these diagnostics, the stress build-up during
the inter-seismic stage can be followed. A major earthquake in the model is
identified from the predicted motion velocities. Once an event is identified
, the released energy is estimated within a time window that covers fully
the time period with macroscopic slip. An epicenter is diagnosed as the
geological locations of the saturated grids, weighted by the respective
energy release. The degree of stress saturation S is not directly used to
identify earthquakes, because the grid elements are not isolated; there are
interactions among adjacent grids. If the unlocking/rupture at a grid point
releases large amount of energy, a neighboring grid point, even if distant
from (stress) saturation, can be unlocked and set into motion. In fact, some
very large earthquakes are formed because there are some sections of the
fault (not necessarily adjacent) are simultaneously ruptured and the
released energy triggered neighboring sections, resulting in an upward
spiral that causes a domino effect in propagating away of the rupture region.
Grid stencil of SEGMENT: For this study, the original 3D spherical model of
the upper ~4 km medium (suffices for ice sheets and landslides applications)
is now applied to the 500 km thick lithospheric material (including elastic
crust and visco-elastic upper mantle). The horizontal resolution is
generally 30 km at the Earth's surface and refined to 2 km at the subduction
zones (Figure S1). The vertical resolution varies using 101 stretched
vertical layers to represent the 500 km depth. A stretching scheme is used
so that the shear zone gets higher resolution and is better represented. In
setting up the model grids in the simulation domain, it is ensured that
there always is a vertical level that has a surface parallel to the oceanic
plate's upper surface. That is, SEGMENT uses a fault zone-oriented
coordinate system.
Static geological parameters and viscosity parameterizations: The SEGMENT
had been used satisfactorily for landslide research in the past. As
frictional phenomena, landslides and tectonic earthquakes share high degree
of similarity. Both involve a shear zone and weakening of the shear zone
leads to instability. Reducing the weight of the overlying portion also is
prone to instability for landslides. For example, around manmade reservoirs
formed by occluding a river branch with a dam, landslides along the banks
always occur during the discharge stage of the reservoir, same de-loading
mechanism as presented in Eq. (1) for earthquakes. Although the similarity
between landslides and tectonic earthquakes makes the conversion of SEGMENT
into an earthquake model feasible, the pressure and thermal environments of
the fault interface and the shear zone of landslides are very different.
Special attention to the viscosity parameterization is required. Also,
direct verification of the schemes become difficult for earthquake
applications as the plate interface is deep-buried that direct measurements
of either stresses or dislocations are rarely available. Inverse methods are
necessary for model verification, spin-up and initialization.
The CRUST1.0 module is used to set up the density values over the grids
within the crust layer (both the oceanic and continental crusts). Underneath
the bottom of the crust layer is assumed to be upper mantle material (
silicates), and is given a uniform reference viscosity of n0= ν = 1019Pa&#
8729;s and a density of 3.3 × 103 kg/m3. In setting up the grids above
present sea level, the Advanced space- borne thermal emission and reflection
radiometer (ASTER) global digital elevation map (asterweb.jpl.nasa.gov/)
and ETOPO1 (downloadable from https://www.ngdc.noaa.gov/mgg/global/global.
html) bathymetry are referenced to. For faults with intensive gravity
measurements, for example the Longmenshan Fault Zone (LFZ, 102-106oE; 30-
33oN) of the eastern margin of the Tibetan Plateau, there was a detailed
gravity survey after the Mw 7.9 Wenchuan earthquake on 12 May 2008. Finer
resolution data on elastic plate thickness, density and rigidity profiles
are available and used in replacement of the CRUST1.0-derived parameters.
Similarly, the Andes survey [26] (and data pointed out by references therein
) also provided information that is merged into the coarse resolution CRUST
1.0.
Based on the fact that the co-seismic ruptures consume only a portion of the
plate interface [27] to the center and the down-dip section creeps, the
upper and lower portions of the plate should be treated respectively as
elastic and visco-elastic material. Grids in the upper crust are considered
elastic and elastic moduli are specified using canonical values. Grids in
the lower crust and upper mantle away from the shear zone are considered to
be viscoelastic [12] but becoming more rigid upwards intending a smooth
transition in the vertical domain. This is achieved by considering the
thermal structure [11] in the parameterization of material viscosity n(T,P)=
n0Dexp[(Q+PV)/R / T]. Here P is confining pressure, T is absolute
temperature, and D depends on the rock composition, grain size and fluid
content. Here Arrhenius' activation energy Q (~75-260 kJ/mol for natural
rocks), specific volume V and universal gas constant R (8.314 J/mol/K) all
are assumed to be constants in the model. In response to different stress
conditions, wall rocks can be compressed or stretched within a narrow range
before onset of brittle fatigue. Arrays are allocated to record the amount
of granular material formed at each grid cube. The shear zone is a granular
zone of variable thickness (Figure S2). In most cases, it is only a small
fraction of the shear zone layer that is granular material (also recorded in
arrays). The parameterization of GM viscosity follows Ref. [13] as
implemented in SEGMENT [14]. The shear-thinning viscosity structure (Eq. 8
in Ref. [14]) implies a shear-localizing effect from GM accumulation. The
present amount of GM (Figure S2c) is deduced from surface velocity using the
method in Ref. [21]. Future variations of GM are diagnosed (relevant
equation is detailed in the next subsection on GM generation that
immediately follows). The granular ensemble size is deduced from its parent
rock type and the associated grain sizes. Re-mapping of the grid stencils is
automatically performed only if the accumulated GM exceeds 20 m or the
earthquake caused displacement is greater than 20 m.
The lower boundary stress conditions are critical for the simulation of
stress build up inside the simulation domain. The GPS measurements of plate
motion speeds V→ are used to fine tune the parameters of upper mantle
layers so that lateral stress (basal drag) exerted on the lowest model layer
at the active zones (mantle driven plate motion) and passive zones (plate
driven mantle motion) nearly balance. From the vertical profile of the
horizontal velocity components it is clear that, for most regions, the
mantle motion is passively forced by the overlain plates and only in the
active thermal hot spots (for Cascadia, it is the Yellowstone area), does
the boundary layer flow structure indicate that it is the mantle flow that
exerts a strong drag to the overlain plate. Reflected in the flow structure,
mantle flow speeds increase away from the lithosphere-asthenosphere
boundary (LAB) rather than being a maximum at the LAB. Therefore, the
driving stress is primarily from lateral stress from neighboring grids
within the lithosphere, rather than from the local basal drag from mantle (
ultimately it is from the convective mantle but local to subduction zones it
is not). The upper boundary condition is assumed to be stress-free (i.e.,
τ.n→= 0 at upper surface). The model spin-up from ad hoc initial
conditions is aided by advanced data assimilation schemes using the remotely
sensed plate motion, (weather signal pre-filtered) gravity signal and
thermal flux measurements available.
Accurate flow fields are critical for accurate estimations of the stress
build-up between plates. Figure 3 shows present plate motion velocities at
two vertical levels: 0-50 km and 100-250 km depth averages. The geodetic
data over land mass in panel (a) were not collected simultaneously (actually
over 1991-2003) but spanned a period of many major earthquakes (e.g., the
Mw 6.7 Northridge earthquake). Because major earthquakes during the time
window of geodetic data collection may leave coseismic and postseismic
effects on the data [28], comparisons between modeled neotectonics and GPS
measurements in the vicinity of active faults are of limited value for model
validation. Compared with GPS measurements on orogens and plates away from
the faults, globally, errors in meridional velocity are < 0.2 mm/yr and in
the longitudinal direction are less than 0.3 mm/yr. For the Andes region,
the error is slightly larger than the global average (0.35 and 0.3 mm/yr,
respectively in the two directions). Over the Cascadia region, however, the
errors are smaller than global average (0.27 and 0.15 mm/yr respectively).
Based on the optimized solution of neotectonics, further experiments are
performed on the sensitivity of groundwater perturbation and the dynamic
evolution of the fault strength and the associated earthquake cycles.
Although the governing thermos-dynamic equations of SEGMENT are similar to
those of the Advanced solver for problems in earth's convection (ASPECT) [23
], it is noted that the ASPECT model, due to its intended research purpose,
has coarser resolution and cannot sufficiently represent surface topography.
In fact, surface topography-induced creeping is a significant component for
GPS sites in mountainous regions and on islands. This results in
discrepancies between model top-level velocity and GPS measurements. By
using careful numerical settings, SEGMENT avoided this issue and
significantly improved the velocity simulation of the near surface (< 100 km
depth) layers, where the stress build-up is critical for earthquakes (
Figure 3a).
The above discussion gives the global setting of SEGMENT. For applications
to specific regions, existing models over that orogen are also referred to
in the parameter setting. For example, in the peri-Himalayan regions, the
static model of Ref. [5] on Persia-Tibet-Burma orogen is referenced. The
stationary creeping fields (as they use a 2D model, only one level of flow
fields is available) are used to constrain SEGMENT's rheological parameters
setting in the fault vicinity, especially near fault junctions. Similarly,
Ref. [29] is referenced in setting the tectonic environment of New Zealand,
especially the numerous fault traces.
Granular material (GM) generation and transportation: In analogous to the
state equation in Ref. [1], the following force-restore relationship is
proposed here for GM generation:
∂a∂t= -c1a+ c2δΔE+ ADV (2)
where α is the thickness of the granularized rock layer, starting from 0, t
is time, c1 is the e-fold time scale, c2 is the Paris coefficient [30], and
c1 signifies the negative feedback as GM accumulates. The unit surficial
energy density of rocks, J (< 300 mJ/m2, Refs. [1,31]), is inversely
proportional to c2. To form new surfaces at high confining pressure (~ 0.1
GPa), the energy requirement far surpasses the surficial energy. Thus c2
also is inversely proportional to the confining pressure. Thermal effects
also are included in c2. The coefficient is 0 when the stress intensity
change falls within the elastic range of the parent rock and is 1 when
exceeding the elasticity range. DE is the energy input rate when stress
perturbation caused imbalance in principal stress exceeds the elastic range.
External (e.g. groundwater fluctuations) tides with resonance frequencies
are most effective in transferring distortion energy into the plates to
cause fatigue and influence the GM production rates. The estimated inherent
resonant frequencies are labeled in Figure S3.
The inter-plate GM chute system, as left behind by seamounts, removes
granular deposits through an advection process (term ADV in Eq. 2) exactly
the same way as surface runoff moves debris [32]. Transport of GM is left to
the Navier-Stokes solver of SEGMENT. As in Figure S2b, the strong contact
regions have the most active GM generation. Its existence in the locking
zone assists the unlocking because the shear resistance GM can provide at
the strain rate of the locking stage is equivalent to a μ′ value of 10-3,
four orders of magnitudes smaller than solid-solid contact, representing a
lubricant between the two elastic plates. The bridging effects by themselves
may not cause fault weakening. However, with the associated GM generation,
they definitely weaken faults through forming a negative spatial covariance
between loading and friction coefficients. During the past two decades, it
is being gradually realized that the maximum apparent frictional coefficient
decreases with time [1]. GM formation is one feasible explanation for slide
weakening. The generation and transport of GM at the plate contact can
reconcile many counter-intuitive characteristics of major earthquakes [33]
and also is critical in organizing small-scale ruptures to generate major
earthquakes. Correct treatment of present GM (Figure S2c) between plates
also may provide a timeframe for the next major earthquake on the same fault
. From Figure S2c, it appears that, globally, the GM thickness distribution
is positively correlated with major earthquakes occurrence frequencies. The
following sections first discuss the correlation of earthquakes and
fluctuations in gravity fields as a result of groundwater extraction, using
two faults of different geometry and strength, and located in very different
climate zones. Using another two subduction faults, the upcoming
earthquakes as a result of hydrological cycle changes with climate warming
are discussed, as actual applications of the numerical earthquake model
discussed in this section. Limited by size, these two subduction faults,
namely the Andes and Cascadian faults, are addressed in detail only in the
SM.
Results
There are three interlaced mechanisms working together to make the
earthquake prediction difficult for present existing rubrics. The mechanisms
identified here are tested positively against 73 historical cases, all
within the NCEP-NCAR reanalyses period, with high quality precipitation data
. The reproduced earthquakes are close to those observed, especially for
cases after 2003 (the start time of GRACE measurements) and before 2015 (the
aging of the current GRACE satellites). By simulating the time scales of
the evolution of strain and stress in the fault zone, using SEGMENT, with
appropriate parameter settings, assisted by remote sensing information, the
timing and location of major earthquakes (Mw > 5 globally) can be estimated.
The uncertainties in timing are reduced to a year, and the location errors
to within 120 km (the root mean squared error for historical records during
1979-2015). The primary limitation on the prediction accuracy is the
resolution of the CRUST- 1.0 model, depending on which SEGMENT resolves
fault geometry and composing material properties. Figure 4 is the hind-casts
of historical earthquakes greater than Mw 8. It is apparent that simulation
of cases within the remote sensing era has significantly smaller spatio-
temporal errors. Over the same fault zone, those cases within the GRACE
observational period have the smallest simulation errors. As with all
prediction systems, the uncertainty grows exponentially with forecasting
lead time. In the following discussion, the uncertainty level will be
presented as necessary.
Although global results have been obtained for earthquake correlations with
fluctuations in gravity fields resulting from groundwater discharge and
extraction, the following discussion focuses on two regions: The Hikurangi
subduction fault-Alpine fault-puysegur mega thrusts of New Zealand (HAP),
and the Tibetan Plateau and its surrounding regions (TP, a continental
collision earthquake zone). Both regions are adequately covered by GPS
measurements and have adequately sizable land areas that can be sensed by
GRACE for the mass fluctuations around the faults. As a result of different
fault geometries, plate opposing motion speeds, and the distribution
patterns of inter-plate GM, earthquakes over the selected regions are of
various morphologies. Major earthquakes over the TP and HAP regions refer
respectively to earthquakes of magnitudes > Mw 5.5 and Mw > 5. In the
Supplementary Material (SM), applications to subduction earthquake zones are
further discussed for two other earthquake-prone regions: the Andes and the
Cascadian subduction zones, along with global maps of time scales of plate
resonance responses to external forcing by groundwater fluctuations.
Earthquake activity in the New zealand region
The weight of groundwater alone is unimportant as a normal pressure
component in a fault. However, the lateral stress it exerts on the
surrounding region is critical for fault instability. The second critical
point is that, through fatigue mechanisms, groundwater extraction leaves a
memory on the fault interface and forms a mechanism for temporal
accumulation. Figure 5 shows the occurrence of major earthquakes over New
Zealand since 1995. All major earthquakes occurred during the relatively dry
periods (low groundwater levels as indicated by the low spikes in the blue
curve of gravity anomalies). This high correlation is not by chance. Using
SEGMENT, the transient stress field exerted by the groundwater deficit (
relative to a long-term mean) is simulated for the Kaikoura Mw7.8 earthquake
on November 16, 2016 (Figure 5b). Kaikoura resides in a maximum region of
the Rxz stress field exerted by groundwater deficit, which reaches 5 kPa.
This additional stress, even if was not the effective trigger for the major
earthquake, is sufficient for triggering the following slow slip events (
SSEs, personal communication with L Wallace [9]). For the events shown in
Figure 5, it appears that most of the high correlations result from the
groundwater deficit being an effective trigger rather than being a direct
cause, even though the shear stress exerted by gravity fluctuation (
groundwater extraction through net evaporation) can be orders of magnitude
greater than the weight of the groundwater in the fault conditions (e.g.,
horizontal pushing as in the train carriage illustration in Figure 2). Also
notable is that, through bridging effects and GM generation, each cycle of
groundwater fluctuation leaves a 'mark' on the plate interface. This is a
mechanism for the temporal accumulation of groundwater effects. From the two
major mechanisms described above, many seemingly distinct earthquake events
become explicable. A further examination of the 73 historical records
worldwide suggests the above groundwater hypothesis likely is universally
applicable.
Earthquake activity in the Tibetan plateau region
Large scale (e.g., 1000 km) groundwater fluctuations exert additional stress
on a fault interface. From Figure 6, almost all major (> Mw 5.5)
earthquakes during 2003-2016 over the Tibetan Plateau also occurred during
low groundwater phases (Figure 6b), similar to the New Zealand cases of Sec
3.1. Although the fluctuation of groundwater is secondary to plate motion in
causing stress build-up, its addition can trigger an earthquake. Analyzing
the stress field clearly indicates that once two plates are coupled together
, the underlying plate drags the lighter overriding plate downward and they
move in tandem deeper into the Earth, pushing aside the higher density upper
mantle material. This causes mass loss around the fault region. This
process, although only causing mass loss at a rate of two orders of
magnitude smaller than the groundwater discharge, is however, aided when the
region is experiencing mass loss due to groundwater reduction (e. g., when
evaporation steadily exceeds precipitation during extended droughts). The
recent Nepal earthquakes of April and early May, 2015, and another, third,
earthquake in Burma a year later all reside within the mass loss region. The
temporal sequence of these three major earthquakes and the ruptures'
spatial pattern (starting from northwest and extending to southeast) can be
explained by the GM production and its transportation, as proposed here.
They represent key stages of strain energy release in the lateral direction,
along the transverse direction of the fault, by unlocking the granular '
sticky' spots scattered over the plate interface. The order of unlocking
strictly follows the GM accumulation in the shear zone.
The three epicenter-consisting patches (comprising the three respective
epicenters) all were approaching saturation by April 2015, after ~72 years
of stress accumulation [34-36]. The western-most epicenter ruptured first
because it was aided by groundwater reduction. Based on SEGMENT simulations,
the groundwater recharge over the past 80 years has a decreasing trend over
the Bay of Bengal region (74-104oE; 15-35oN), in agreement with the
monsoonal precipitation trends identified in Ref. [37]. On average, the
groundwater reduction rate was ~31 Gt/yr. This super-imposed stress field
produces a saturated area (as defined in Eq. (A1)) of ~85 km2 annually. The
matured (saturated) area is not spatially contiguous on the fault interface;
it scattered across the inter-seismically locked interface in a complicated
-connected manner. Connection of these sporadically scattered saturated
patches is critical for generating earthquakes. For example, the occurrence
of the first earthquake (April 25, 2015; Mw 7.8; epicenter at 28.147oN, 84.
708oE) redistributed the granular material due underneath and established a
stronger rock-rock contact, lessening the degree of saturation of the second
major earthquake (May 12, 2015 of M7.3 magnitude and epicenter located 200
km to the southeast at 27.973oN, 85.963oE) region, so another month had
passed before its occurrence. The earthquakes over Burma (Mw 6.9 on April 12
, 2016 at 23.13oN, 94.9oE and Mw 6.8 on August 25, 2016 at 20.9oN, 94.6oE),
further to the southeast, were not directly related to the first two
earthquakes. They were a result of the lightening of the Bay of Bengal
region, to be further addressed, below. Unlike some recent studies 24
predicting that the future earthquake after the Nepal earthquake of 2015
would be in the sector to the northwest, the SEGMENT simulation indicates
that the sector to the south-east also is becoming unstable. In actuality,
the maturation pattern jumps between the eastern and western sectors, with
cluster centers at (30oN, 84.4oE) and (27.5oN; 86.2oE). Consequently, major
earthquakes also occur alternatively between these two regions, in a bi-
polar mode.
The LFZ is a classic example of the 'non-unique solution' configuration
presented in Figure 2. The LFZ defines the eastern margin of the Tibetan
Plateau. A very recent synthetic approach, using a gravity survey, seismic
reflection profiles and earthquake focal mechanism data, and sediment
analysis indicated that the LFZ essentially is composed of two segments of
different formations. The central-northern section, which was formed ~ 40
Myr ago [38,39], is a lithospheric through-cut fault zone that is of low
elastic strength but has strong dextral strike-slip motions. In contrast,
the southern sector is a crustal thrust zone dominated by shallow-angle
thrust motion of the TP over the moderately stiff South China Plate. This
Mobius-twisted fault plane and contrasting behaviors along the LFZ, in
addition to providing kinematically favourable conditions for instability,
also has a profound influence on the groundwater reservoir. The Mw 7.8
Wenchuan earthquake offers further evidence of the importance of gravity
field changes in fostering major earthquakes.
The mass change fields averaged over the period 2003-2008 indicate clearly
that Wenchuan resides at the center of a saddle region (Figure 3 in Ref. [10
]). As illustrated in the 4th scenario train in Figure 2, the superimposed
stresses form a strong compressive stress in the horizontal direction,
aiding the existing geological compression. For earthquakes occurring along
the same fault (but temporally consecutive), the compression caused by
tectonic plate motion usually produces very similar saturation patterns. The
primary reason for the randomness of epicenters is the increase in
transient perturbations, such as groundwater fluctuations. The superimposed
field causes a specific point to reach saturation first and then propagates
away to cause a large rupture area. The rupture can grow because, to halt
the motion of the unstable plates, neighboring unsaturated regions may be
activated. If the propagation encounters too many unsaturated regions, the
momentum is quickly lost, strongly limiting the magnitude of the earthquake.
However, if the propagation starts from a region surrounded by a nearly
saturated neighborhood, a positive feedback forms and is supportive of large
magnitude earthquake formation. By initiating the rupture near Wenchuan, a
more efficient energy release is achieved. From the SEGMENT simulations with
future precipitation scenarios under the IPCC SRES A1B scenario provided by
a small-volume climate model ensemble (as listed in Table 1), the next
major earthquake is expected to occur in the Ganshu province (epicenter
located along a 104.8oE longitudinal arc between 33oN and 35oN) to the north
east, ~ 120 ± 45 years later of magnitude ~ Mw7. For the far-larger TP
region, the India Eurasian collisional system, while thickening the elastic
lithosphere, has created numerous vertically extended crevasses. These
crevasses are very effective in channeling groundwater to greater depth.
Combined with the TP's orographic effects, precipitation variation transfers
readily into extra lateral stress on the faults. Therefore, groundwater
fluctuations become a dynamic factor affecting the epicenter location and
the magnitude of major earthquakes. Because of the persistent gravity
reduction in the Bay of Bengal area and Southwest China, coupled with
insignificant changes over the plateau and the northern Tarim and very arid
Qaidam basins, a saddle-shape super-imposed stress field has formed and has
persisted over the past half century. The natural, geological, recurrence
frequency (background-deduced "dry" frequency) is greatly reduced over the
region, especially over the Southeast Asia peninsula. The fault zone over
Myanmar likely will become unstable in approximately two years. Of great
interest is that the epicenters of the upcoming earthquakes, affected by the
weakening of monsoonal precipitation, are located along the 96 ± 1oE
parallel between 17-28oN (most frequently in the 17-23oN sector), roughly
parallel to the Ayeyarwady River basin.
In addition to the pre-Himalayan and the New Zealand regions, the authors
have shown that there are strong, statistically significant correlations
between tectonic plate earthquake irregularities and groundwater
fluctuations over many other global faults, agreeing with several recent
studies (e.g., Refs. [6,7]). For example, the central Italy earthquake (
August 24, 2016) and the Lorca earthquake (May 11, 2011) both occurred at
low groundwater level stages. The main value of SEGMENT is its predictive
ability. Previous models only involve the groundwater induced crustal
unloading, or the direct mechanical effect of groundwater as in Eq. (1).
Without considering the fault weakening effects from groundwater, these
models explain reasonably well the observed fault slip patterns resulting
from groundwater depletion. However, predicting earthquakes is beyond the
capability of such models. The SEGMENT, by placing groundwater fluctuations
at the nexus of all prongs of the earthquake triads, especially emphasizing
the effective fault weakening from the cyclic variations of the induced
crustal loading, possesses the ability to simulate the role of groundwater
in future earthquakes. Using the SEGMENT modeling system, driven by
precipitation scenarios from climate model projections, the future
occurrence of earthquakes over the Cascadian and Andes subduction zones is
investigated (in the SM). It was found that fatigue caused by groundwater
fluctuations, associated with increased precipitation volatility in 21st
Century California, will likely be decisive in shortening the time scale of
the natural earthquake cycle of the Cascadian subduction fault (SM). However
, as a consequence, their magnitudes will be reduced. For the Andes region,
in addition to the regular precipitation pattern from ENSO, the granular
chute system left by seamounts on the Nazca and Jun Fernandez Ridges foster
major earthquakes.
Discussion
Attempts to understand earthquakes have a very long history and are globally
widespread. The novel earthquake theory presented here follows from a
confluence of several factors: Advances in remote sensing technology; recent
, tectonically active stages motivated many researchers to re- evaluate
previous assumptions [33]. Most critically, a well-documented landslide
numerical model is used as a prototype of an advanced numerical modeling
system to represent the mechanics of inter- plate earthquakes, and to verify
the original hypotheses proposed here. This study shows, through data
analysis and modeling, that variations in groundwater loading and occurrence
of drought conditions together influence the occurrence and properties of
earthquakes. Otherwise, the stationary seismicity of neotectonics is very
stable for the majority of major faults mapped on existing neotectonic
models. The temporal scale separation, i.e., the geological conditions vary
at a much longer time scale than the occurrence of major earthquakes, which
underscores why groundwater fluctuations constitute a viable mechanism for
affecting earthquakes.
Earthquakes occur when the repose angle determined by plate-coupling stress
reaches the sum of two angles representing, respectively, the fault geometry
and fault strength. Accurate representation of the evolution of fault
strength and repose angle therefore is critical for earthquake simulation
and prediction. The earthquake triad, as defined in this study, is a generic
framework that has universal applicability. Groundwater fluctuations,
although relatively small, nevertheless play a pivotal role in linking the
three triad components together and thereby determining earthquake
irregularity. Groundwater influences fault instability directly by super-
imposing a seismogenetic lateral stress field working in synergy with plate
coupling stress, and also indirectly, by weakening the fault from within by
enhancing fault fatigue from GM generation. The direct effect alone, the
groundwater fluctuation superimposed stress field is of the same order as
the residual of gravity- aided frictional stress and plate coupling stress;
hence it affects the timing of major earthquakes. These findings agree with
recent studies by Amos, et al. [6] and Gonzalez, et al. [7], and present the
results in a coherent theoretical framework. The present study addresses
the concerns of Avouac [40] and is a significant advance in understanding
earthquake dynamics and thermodynamics for reliable earthquake prediction.
By exploring existing records of earthquakes using SEGMENT, it is found that
for collisional systems, direct effects from groundwater control the
irregularity of major earthquakes. Essentially, the orographically highly
variable precipitation is effectively channeled into deeper depth by the
prevalence of through-cut faults. Droughts elsewhere also are seismogenetic
but likely their effects are indirect. For example, the Elnino-southern
oscillation (ENSO), as the dominant climate driver of regional precipitation
, has a strong influence on the gravity field over the Andes region of South
America. The occurrence of major earthquakes, although bearing the same 4-7
year occurrence frequency of ENSO, has a significant hiatus when compared
with the fluctuation in the gravity field. Similarly, the stability of the
Cascadia fault is found to be remotely affected by Californian droughts. The
input of strain energy from the periodic recharge and discharge of
groundwater in a large regional area increases the prospect of un-locking of
seismically coupled plates. Thus, climate fluctuations can and do determine
the groundwater loading pattern through extended droughts and extreme
precipitation storms, which in turn affect the earthquake cycles. The actual
impacts of a warming climate are fault specific, but for the San-Andreas
Fault, located in a Mediterranean climate zone, an enhanced hydrological
cycle will further reduce the magnitude of earthquakes but enhance the
occurrence frequency.
Existing geological data limited SEGMENT only represent major faults on
orogens. Large number of minor faults [41,42] are not included in the input
data, and their seismic activity in those locations are beyond the
capability of our modeling system. New higher resolution geodetic
information on plate interface and boundaries, sea floor spreading, fault
systems, and geodesy all are necessary for further improvements of the
predicting score, by improved description of continuum deformation and
subsequent rupture/yielding.
Supplementary Materials
Due to size limitation, important details are provided in the Supplementary
Material (SM). The author also will provide assistance if readers wish to re
-produce the results.
Conflicts of Interest
The authors declare no competing financial interests.
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