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引发另一次地震。教科书上的例子是1987年发生在加利福尼亚州迷信山的两次6级地震。首先，左侧埃尔莫尔牧场断层破裂。大约12小时后，右侧迷信山断层在第二次地震中破裂。第一（震源）震中和第二（震源）震中之间的距离约为10公里。类似的大地震序列（至少6级）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些触发地震在时间和空间上的分离，特别关注2000年四川地震序列。触发似乎是一个两步的过程。在第一步中，地震波在距离触发震源很远的地方传递动应力。事件发送到被触发的接收者的（未来）位置。事件。在第二个较慢的步骤中，涉及到导致第二个事件的另一个本地进程。在该启发式模型中，第一步在源事件和接收事件之间产生较大的空间（距离）分离，而第二步在时间（延迟）上产生分离。这是提议的研究正在测试的假设之一。其他假设包括：(1)动态摩擦取决于断层？S滑移率和物理状态变量；(2)地震波从挥发性岩浆中释放气体时产生的气泡；(3)地震断层滑动；更值得推测的是，(4)某种应力脉冲随着观测到的视速度传播。为了验证这些相互矛盾的假设，该研究采用了最先进的技术来测量和模拟冰岛的应力传递。测量包括：(1)连续记录的全球定位系统接收机（CGPS）的大地测量时间序列，(2)使用卫星雷达图像（INSAR）的干涉测量，(3)钻孔水位的水文记录，(4)气压、降雨和温度的气象记录，以及(5)精确估计地震位置和震源机制。数值模型采用三维有限元模型（FEM）来考虑断裂带的孔隙弹性和粘弹性过程。该模型计算了三个量：(1)位移场，可以与INSAR和CGPS测量结果进行比较；(2)流体压力场，可与钻孔测得的水位进行比较；(3)应力张量，将应力张量分解到已知的断层面上，用库仑准则评估破坏。该活动的智力价值：该研究旨在描述导致触发的过程，其距离超过几个断层长度，时间尺度超过地震波的传播时间。它涉及测量和模型在传统上分开的学科大地测量学，地震学和水文学。这项跨学科的研究似乎有望增加对地震过程的基本理解。研究活动的广泛影响：研究人员，包括冰岛的同事，对观测结果的优缺点非常熟悉，这使他们的团队处于进行这些建模研究的理想位置。他们正在培养地震学、大地测量学和水文学这三个学科交叉领域的年轻科学家。在为期两年的项目结束时，初级科学家将掌握三种现代地球物理技术（CGPS、INSAR和FEM），并在大学以外的环境中应用。来自这项研究的新信息可能与了解冰岛随时间变化的地震灾害直接相关。提高对地震触发的了解也有助于实现国家减少地震危害计划的目标。

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.