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年发生在加州迷信山的两次6级地震。首先，左侧的埃尔默牧场断层破裂。大约12小时后，迷信山右侧的断层在第二次地震中破裂。第一（震源）震中和第二（接收器）震中之间的距离约为10公里。在1784年、1896年和2000年，南冰岛地震带（SISZ）也发生过类似的大地震（至少6级）。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，特别侧重于2000年SISZ地震序列。触发似乎是一个两步过程。在第一步中，地震波在距触发点的距离上传递动应力。来源？事件到（未来）触发的位置？接收器？活动在第二个较慢的步骤中，涉及另一个导致第二个事件的局部过程。在这个启发式模型中，第一步在源和接收器事件之间产生大的空间分离（距离），而第二步产生时间分离（延迟）。这是拟议的研究正在测试的假设之一。其他假说包括：（1）取决于断层的动摩擦力？的滑动速率和物理状态变量;（2）气泡产生的地震波释放气体从挥发性岩浆;（3）aseptic断层滑动;以及更推测，（4）某种应力脉冲传播与观测到的视速度。为了测试这些相互竞争的假设，研究采用了最先进的技术来测量和模拟冰岛的应力转移。测量包括：（1）来自连续记录的全球定位系统接收器的大地测量时间序列，（2）使用卫星雷达图像的干涉测量法（干涉合成孔径雷达），（3）钻孔水位的水文记录，（4）气压、降雨量和温度的气象记录，以及（5）对地震位置和震源机制的精确估计。数值模型采用三维有限元模拟（FEM）来解释断裂带中的孔隙弹性和粘弹性过程。该模型计算三个量：（1）位移场，可与干涉合成孔径雷达和CGPS测量值进行比较;（2）流体压力场，可与钻孔中测量的水位进行比较;（3）应力张量，可将其分解到已知的断层面，以通过库仑准则评估故障。这项活动的知识价值：这项研究试图描绘在超过几个断层长度的距离和超过地震波传播时间的时间尺度上引起触发的过程。它涉及传统上独立的大地测量学、地震学和水文学学科中的测量和模型。这项跨学科的研究似乎准备增加对地震过程的基本理解。活动的更广泛影响：包括冰岛同事在内的研究人员详细了解了观测结果的优点和缺点，这使他们的团队处于进行这些建模研究的理想位置。他们正在培养一名年轻科学家在地震学、大地测量学和水文学三个学科交叉领域的职业生涯。到两年期项目结束时，初级科学家将掌握三种现代地球物理学技术（CGPS、干涉合成孔径雷达和FEM），其应用范围超出大学范围。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。提高对地震触发的认识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.