Abstract

In the 2017 Jiuzhaigou earthquake sequence, the distribution of aftershocks in the north of the main earthquake was scattered, while the distribution of aftershocks in the south of the main earthquake was linear and concentrated. The objective of this inquiry is to analyze the dynamic causes underlying such divergent patterns, relying on the horizontal strain rosette, areal strain (As), and the coefficient of accommodation (Ca) based on the regional strain rate. The following two conclusions are obtained: (1) approximately one-third of the aftershocks with focal mechanisms in the north of the main shock are thrust-type earthquakes. Because the direction of regional tectonic principal compressional strain is perpendicular to the fault trend north of the main shock, generating thrust-type earthquakes on low dip-angle faults is indeed easy. Simultaneously, the overall thrust-type focal mechanism north of the main shock and the poor consistency between plate tectonic movement and fault movement caused by the seismic sequence lead to substantial scattered aftershocks in the north of the main shock. (2) One of the aftershocks with focal mechanisms in the south of the main shock is a reverse strikeslip type, while the other 30 are strike-slip type earthquakes. Moreover, the angle between the regional tectonic principal compressional strain direction and the fault trend in the south of the main shock is large, which makes it easier for faults in the south of the main shock to produce strike-slip-type earthquakes. Simultaneously, the overall strike-slip focal mechanism in the south of the main shock, the good consistency between fault movements caused by the seismic sequence, and plate tectonic movements lead to more linear and concentrated aftershocks in the south of the main shock. The findings are significant for investigations into the seismogenic properties and activity of the Huya Fault located on the northeastern margin of Bayan Har Block.

中文翻译：

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2017年九寨沟地震序列南北分布不一致的原因

摘要

2017年九寨沟地震序列中，主震北部余震分布较为分散，而主震南部余震分布呈线状、集中。本次调查的目的是根据水平应变花环、面积应变 (As) 和基于区域应变率的调节系数 (Ca) 来分析这种不同模式背后的动态原因。得到以下两个结论：(1)主震以北震源机制约三分之一的余震为逆冲型地震。由于区域构造主压应变方向垂直于主震以北的断层走向，低倾角断层上确实容易产生逆冲型地震。同时，主震以北整体逆冲型震源机制以及地震序列引起的板块构造运动与断层运动一致性较差，导致主震以北出现大量分散的余震。(2)震源机制位于主震以南的余震有1次为逆走滑型地震，其余30次为走滑型地震。而且区域构造主压应变方向与主震以南断层走向夹角较大，使得主震以南断层更容易产生走滑型地震。同时，主震南部整体走滑震源机制、地震序列引起的断层运动与板块构造运动的良好一致性，导致主震南部余震更加线性、集中。该发现对于研究巴颜喀拉地块东北缘虎牙断裂的孕震特性和活动性具有重要意义。