Collaborative Research: Mechanisms of transient deformation following great 2006-2007 Kuril earthquakes: Frictional afterslip or viscoelastic relaxation?

俯冲带的地震周期包括俯冲作用引起的应变积累、地震破裂引起的应变突然释放和震后的时间相关变形。大俯冲地震后的震后瞬变GPS观测提供了一个自然的实验，研究流变学的巨型逆冲断层和岩石圈下地幔。科学家们仍然不同意什么机制占主导地位的震后瞬变：同震破裂的摩擦后滑，在软流圈粘弹性松弛，或孔隙弹性反弹。在2006-2007年，一对大地震（Mw 8）袭击了西北太平洋千岛俯冲带的中心：俯冲界面的逆冲事件，随后是千岛海沟向海洋侧翼下方的伸展事件。这可能是在卫星大地测量学和宽带地震学时代观测到的最大的双峰。2006年，安装了Kuril GPS阵列，可以测量地震周期的所有组成部分。在2006 - 2007年地震前后收集的2006 -2009年数据概述了一个广泛的震后变形区，初始水平速度高达100 mm/a，以及区域隆起。大Kuril双峰之后的震后信号主要是由弱软流层剪切应力的粘弹性松弛引起的。粘弹性松弛的水平和垂直运动的模式从其他候选机制进行了区分。我们预测，震后变形将在地震双峰之后大约十年内消失，因此可以通过持续几年的观测来检验首选机制。我们的初步研究结果表明，上地幔粘度在各俯冲带之间有很大的变化，这是控制应力分布的最重要的参数之一。在这一年的建议期内，工作的重点将是：（1）保证地震后时间序列的连续性，否则将妨碍今后的所有工作。(2)分析和开发模型的震后瞬态变形为一个时期4？在2006年伟大的五年后？2007年千岛地震，重新审视或测试工作中的几个假设。(3)比较和评估这两个事件的大地测量和地震模型，并在模型中包括外部隆起事件的影响。(4)绘制摩擦后滑的时空分布图，特别是研究第二次事件的应力变化如何影响正在进行的后滑。(5)模拟Sarychev火山（千岛群岛中部的Matua岛）爆发造成的变形。该项目继续与美国和俄罗斯的科学家合作。该项目的成果将对遭受灾难性地震的地区产生影响。这项工作和我们将开发的方法，预计将导致普遍改善的俯冲带的地震潜力的理解，从而最终可能导致改进的危险估计和缓解其他地方。

The earthquake cycle in subduction zones includes strain accumulation caused by subduction, sudden strain release by the earthquake rupture, and postseismic time-dependent deformation. GPS observations of postseismic transients after large subduction earthquakes provide a natural experiment to study rheology of the megathrust and of the sublithospheric mantle. Scientists still disagree on what mechanism prevails in the postseismic transients: frictional afterslip on the coseismic rupture, viscoelastic relaxation in the asthenosphere, or poroelastic rebound. In 2006-2007, a doublet of great earthquakes (Mw 8) struck in the center of the Kuril subduction zone at the northwest Pacific Ocean: a thrust event at the subduction interface followed by an extensional event beneath the oceanward flank of the Kuril trench. It is probably the greatest doublet that has been observed in the era of satellite geodesy and broadband seismology. In 2006, the Kuril GPS Array was installed, which allowed to measure all components of the earthquake cycle. The data collected in 2006-2009 before and after the 2006-2007 earthquakes outlined a broad zone of postseismic deformation with initial horizontal velocities as fast as 100 mm/a, and a regional uplift. Most of the postseismic signal after the great Kuril doublet is caused by the viscoelastic relaxation of shear stresses in the weak asthenosphere. Viscoelastic relaxation was discriminated from other candidate mechanisms by the pattern of horizontal and vertical motions. We predict that the postseismic deformation will die out in about a decade after the earthquake doublet, so the preferred mechanism can be tested with observations continued for several years. Our initial results suggest large variations among subduction zones in the upper mantle viscosity, one of the most important parameters that governs the stress distribution.The focus of work during this 1-year proposal period will be to: (1) Guarantee continuity of the postseismic time series, without which all future work would be handicapped. (2) Analyze and develop models of postseismic transient deformation for a period 4?5 years after the great 2006?2007 Kuril earthquakes, revisiting or testing several assumptions made in the work so far. (3) Compare and assess geodetic and seismologic models for the two events, and include the effect of the outer rise event in models. (4) Map the space-time distribution of frictional afterslip, and in particular study how stress changes from the second event affected ongoing afterslip. (5) Model the deformation caused by the eruption of Sarychev volcano (Matua Island in the central Kurils).The project continues collaboration between the scientists of the US and Russia. The results of the project will have an impact on the region subject to disastrous earthquakes. This work and the methods we will develop are expected to lead to generally improved understanding of the earthquake potential of subduction zones and may thus ultimately lead to improved hazard estimation and mitigation elsewhere.