An intraplate earthquake doublet, with 11-min delay between the events, devastated the city of Varzeghan in northwestern Iran on August 11, 2012. The first Mw 6.5 strike-slip earthquake, which occurred after more than 200 years of low seismicity, was followed by an Mw 6.4 oblique thrust event at an epicentral separation of about 6 km. While the first event can be associated with a distinct surface rupture, the absence of a surface fault trace and no clear aftershock signature makes it challenging to identify the fault plane of the second event. We use teleseismic body wave inversion to deduce the slip distribution in the first event. Using both P and SH waves stabilize the inversion and we further constrain the result with the surface rupture extent and the aftershock distribution. The obtained slip pattern shows two distinct slip patches with dissimilar slip directions where aftershocks avoid high-slip areas. Using the estimated slip for the first event, we calculate the induced Coulomb stress change on the nodal planes of the second event and find a preference for higher Coulomb stress on the N-S nodal plane. Assuming a simple slip model for the second event, we estimate the combined Coulomb stress changes from the two events on the focal planes of the largest aftershocks. We find that 90% of the aftershocks show increased Coulomb stress on one of their nodal planes when the N-S plane of the second event is assumed to be the correct fault plane.

中文翻译：

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Varzeghan双重地震中的断层滑动和第二个断层平面的识别。

两次地震之间的板块内地震成倍增加，两次地震之间有11分钟的延迟，于2012年8月11日摧毁了伊朗西北部的Varzeghan市。随后发生了200多年的低地震，发生了第一次Mw 6.5走滑地震。由Mw6.4距中心约6 km处的倾斜推力事件。尽管第一个事件可能与明显的表面破裂有关，但是由于没有表面断层痕迹，并且没有清晰的余震信号，因此很难确定第二个事件的断层面。我们使用远震体波反演来推导首次事件中的滑动分布。同时使用P波和SH波稳定反演，我们进一步将结果限制在表面破裂程度和余震分布上。所获得的滑移模式显示了两个不同的滑移斑块，它们具有不同的滑移方向，其中余震避免了高滑移区域。使用第一事件的估计滑动，我们计算第二事件的节点平面上的诱发库仑应力变化，并找到对NS节点平面上较高的库仑应力的偏好。假设第二个事件具有简单的滑动模型，我们估计最大余震焦平面上两个事件的组合库仑应力变化。我们发现，当第二次事件的NS平面被认为是正确的断层平面时，余震中90％的余震在其一个节点平面上显示出增加的库仑应力。