Abstract

The 2017 Malard and 2020 Damavand moderate crustal earthquakes (M_w 4.8) occurred about 40 km west and about 55 km northeast of Tehran, the capital and economic heart of Iran, with a metropolitan population of over 15 million. Seismic hazard assessment in the region has been affected by few historically documented destructive earthquakes with magnitudes around 7.0 (e.g., 312–280 B.C, 958, 1177, and 1830 A.D.); however, in the absence of large contemporary earthquakes, a detailed analysis of moderate earthquakes is essential. In this study, seismic sources of the two earthquakes are characterized in terms of focal mechanism, fault geometry, and rupture directivity through waveform inversion, hypocenter relocation, and empirical Green’s function methods. The eastern segment of the well-known Mosha fault is responsible for the 2020 Damavand earthquake, with a left-lateral strike-slip mechanism ruptured unilaterally westward where Tehran is situated. The 2017 Malard earthquake is a peculiar case in a poorly studied region. For this event, we propose a left-lateral strike-slip mechanism corresponding to E-W trending Mahdasht fault. This event was preceded by a swarm of events, 12 km northward, that started a few months earlier and terminated right before the mainshock. The energy released due to this precursory activity was higher than the Malard mainshock and its aftershocks. The events seem to align along an N-S transverse basement fault that, further southward, may intersect with the Mahdasht fault system.

中文翻译：

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德黑兰都市区发生中度地震：以 2017 年马拉德和 2020 年达马万德地震序列为例

摘要

2017 年马拉德地震和 2020 年达马万德中度地壳地震（震级_4.8）发生在伊朗首都和经济中心德黑兰以西约 40 公里、东北约 55 公里处，大都市人口超过 1500 万。该地区的地震危险性评估受到历史记录的7.0级左右破坏性地震的影响（例如，公元前312-280年、公元958年、1177年和1830年）；然而，在当代没有发生大地震的情况下，对中震进行详细分析是必要的。在这项研究中，通过波形反演、震源重定位和经验格林函数方法，对两次地震的震源进行了震源机制、断层几何形状和破裂方向性的表征。著名的莫沙断层东段是 2020 年达马万德地震的罪魁祸首，左旋走滑机制在德黑兰所在的地方向西单侧破裂。2017 年马拉德地震是一个研究不足的地区的特例。对于这一事件，我们提出了与东西向 Mahdasht 断层相对应的左旋走滑机制。在此事件之前，向北 12 公里处发生了一系列事件，这些事件在几个月前开始，并在主震发生前结束。此次前兆活动释放的能量高于马拉德主震及其余震。这些事件似乎沿着南北横向基底断层排列，再向南，可能与马赫达什特断层系统相交。