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

俯冲带的地震旋回包括俯冲引起的应变积累、地震破裂引起的应变突然释放和震后随时间变化的变形。大俯冲地震后瞬变的GPS观测为研究巨型逆冲构造和岩石圈下地幔的流变学提供了一个自然实验。地震后瞬变的主要机制是：同震破裂的摩擦余震、软流层的粘弹性松弛，还是孔隙弹性反弹，科学家们仍然存在分歧。2006-2007年，在西北太平洋的千岛岛俯冲带中心发生了两次大地震（m8）：俯冲界面发生了一次逆冲事件，随后在千岛岛海沟向海侧翼下方发生了一次伸展事件。这可能是在卫星大地测量和宽带地震学时代观测到的最大的双重态。2006年，千岛岛GPS阵列被安装，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年的数据显示了一个广泛的震后形变带，其初始水平速度高达100 mm/a，以及一个区域性隆起。千岛重偶波后的震后信号大部分是由弱软流层剪切应力的粘弹性松弛引起的。通过水平运动和垂直运动的模式将粘弹性松弛机制与其他候选机制区分开来。我们预测震后变形将在地震偶波后的10年左右消失，因此优选的机制可以通过持续数年的观测来验证。我们的初步结果表明，上地幔粘度是控制应力分布的最重要参数之一，在俯冲带之间存在很大差异。在这1年的建议期内，工作重点将是：(1)保证地震后时间序列的连续性，否则所有未来的工作都将受到阻碍。(2)分析和建立4?期地震后瞬态变形模型。2006年之后的5年？2007年千岛群岛地震，重新审视或测试了迄今为止在工作中做出的几个假设。(3)比较和评估这两个事件的大地测量和地震模型，并在模型中考虑外升事件的影响。(4)绘制摩擦余震的时空分布，特别是研究第二次余震的应力变化对持续余震的影响。(5)模拟Sarychev火山（千岛群岛中部Matua岛）喷发引起的变形。该项目继续了美国和俄罗斯科学家之间的合作。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使人们普遍提高对俯冲带地震潜力的认识，从而可能最终改善其他地方的灾害估计和减灾工作。

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.