Triggering of earthquakes over long times and distances: models for the June 2000 sequence in Iceland

大量证据表明，一次地震可以通过应力传递触发另一次地震。教科书上的例子是1987年加州迷信山庄发生的两次6级地震。首先，左侧埃尔莫尔牧场断层破裂。大约12小时后，右侧的迷信山断层在第二次地震中破裂。第一(震源)和第二(接收)震中之间的距离约为10公里。类似的大地震序列(至少6级)分别发生在1784年、1896年和2000年的冰岛南部地震带(SISZ)。这项研究试图解释这种触发地震之间在时间和空间上的分离，特别是2000年SISZ地震序列。触发似乎是一个分两步走的过程。在第一步中，地震波在距离触发震源？的距离内传递动态应力。事件发送到被触发接收器的(未来)位置？事件。在第二个较慢的步骤中，涉及另一个导致第二个事件的本地进程。在该启发式模型中，第一步产生源事件和接收方事件之间在空间上的大间隔(距离)，而第二步产生时间上的间隔(延迟)。这是拟议中的研究正在测试的假设之一。其他假设包括：(1)取决于断层的动摩擦力--S滑动速率和物理状态变量；(2)地震波从挥发性岩浆中释放气体时产生的气泡；(3)无地震断层滑动；(4)某种以观测到的视速度传播的应力脉冲。为了验证这些相互矛盾的假设，该研究正在使用最先进的技术来测量和模拟冰岛的应力转移。这些测量包括：(1)来自连续记录的全球定位系统接收器的大地时间序列，(2)使用卫星雷达图像的干涉测量，(3)钻孔水位的水文记录，(4)大气压、降雨量和温度的气象记录，以及(5)地震地点和震源机制的精确估计。数值模型采用三维有限元模型来考虑断裂带中的孔隙弹性和粘弹性过程。该模型计算了三个量：(1)位移场，可与InSAR和CGPS测量进行比较；(2)流体压力场，可与井中测得的水位进行比较；(3)应力张量，可分解到已知断层面上，根据库仑准则进行失效评估。这项活动的学术价值：这项研究试图描绘出引起触发的过程，触发距离超过几个断层长度，时间尺度超过地震波的传播时间。它涉及大地测量学、地震学和水文学等传统上不同学科的测量和模型。这项跨学科的研究似乎将增加对地震过程的基本了解。这项活动的广泛影响：调查人员，包括冰岛的同事，对观测的优势和劣势拥有详细的熟悉，这使他们的团队处于开展这些建模研究的理想位置。他们正在培养三个学科交叉的初级科学家的职业生涯：地震学、大地测量学和水文学。在为期两年的项目结束时，这位初级科学家将掌握现代地球物理的三项技术(CGPS、InSAR和有限元)，并在大学以外的环境中应用。来自这项研究的新信息很可能与理解冰岛依赖时间的地震灾害直接相关。对地震触发的更好理解也有助于实现国家减少地震灾害计划的目标。

Considerable evidence indicates that one earthquake can trigger another by stress transfer. The textbook example is a pair of magnitude 6 earthquakes that shook the Superstition Hills, California in 1987. First, the left-lateral Elmore Ranch fault ruptured. Some 12 hours later, the right-lateral Superstition Hills fault ruptured in a second earthquake. The distance between the first (source) epicenter and second (receiver) epicenter is about 10 km. Similar sequences of large (magnitude 6 at least) earthquakes occurred in 1784, 1896, and 2000 in the South Iceland Seismic Zone (SISZ). The research seeks to explain the separation in both time and in space between such triggered earthquakes, focusing in particular on the 2000 SISZ earthquake sequence. The triggering appears to be a two-step process. In the first step, the seismic waves transmit a dynamic stress over the distance from the triggering ?source? event to the (future) location of the triggered ?receiver? event. In the second, slower step, another local process is involved that leads to the second event. In this heuristic model, the first step produces the large separation in space (distance) between the source and receiver events, while the second step produces the separation in time (delay). This is one of the hypotheses that the proposed research is testing. The other hypotheses include: (1) dynamic friction that depends on the fault?s slip rate and a physical state variable; (2) bubbles produced as the seismic wave releases gas from volatile magma; (3) aseismic fault slip; and much more speculatively, (4) some kind of stress pulse that propagates with the observed apparent velocity.To test these competing hypotheses, the research is employing state-of-the-art techniques for measuring and modeling stress transfer in Iceland. The measurements include: (1) geodetic time series from continuously-recording Global Positioning System receivers (CGPS), (2) interferometry using satellite radar images (INSAR), (3) hydrological recordings of water level in boreholes, (4) meteorological recordings of barometric pressure, rainfall and temperature, and (5) precise estimations of earthquake locations and focal mechanisms. The numerical models employ 3-dimensional finite element modeling (FEM) to account for poro-elastic and visco-elastic processes in a fault zone. The model calculates three quantities: (1) the displacement field, which can be compared to INSAR and CGPS measurements; (2) the fluid pressure field, which can be compared to the water level measured in boreholes; and (3) the stress tensor, which can be resolved onto known fault planes to evaluate failure by the Coulomb criterion. Intellectual merit of the activity: The research seeks to delineate the processes causing the triggering over distances longer than several fault lengths and time scales longer than the travel time of seismic waves. It involves both measurements and models in the traditionally separate disciplines of geodesy, seismology, and hydrology. This interdisciplinary research seems poised to add to fundamental understanding of earthquake processes.Broader impact of the activity: The investigators, including colleagues in Iceland, posses a detailed familiarity with the strengths and weaknesses of the observations that places their team in an ideal position to undertake these modeling studies. They are fostering the career of a junior scientist at the intersection of three disciplines: seismology, geodesy and hydrology. By the end of the two-year project, the junior scientist will have mastered three techniques of modern geophysics (CGPS, INSAR and FEM) with applications beyond the university setting. The new information coming from this study is likely to be directly relevant to understanding time-dependent earthquake hazards in Iceland. Improved understanding of earthquake triggering also contributes to the objectives of the National Earthquake Hazard Reduction Program.