Abstract

Seismicity in the intraplate southwest of Western Australia is poorly understood, despite evidence for potentially damaging earthquakes of magnitude > M6. Identifying stress-focusing geological structures near significant earthquake sequences assists in understanding why these earthquakes occur in seemingly random locations across a region of more than 250 000 km². On 16 September 2018, an M_L5.7 earthquake occurred near Lake Muir in the southwest of Western Australia and was followed by an M_L5.4 aftershock. The main earthquake formed a mainly north-trending fault scarp ∼5 km in length and with a maximum vertical displacement of ∼40 cm. The main event was followed by a series of aftershocks, one of which had a magnitude of M_L5.4. Using high-resolution aeromagnetic data, we analyse bedrock geology in a wide area surrounding the new scarp and map a series of major ∼ east–west-trending faults segmenting eight distinct geological domains, as well as a network of less prominent northwest-trending faults, one of which aligns with the southern segment of the scarp. Surface faulting, surface deformation and earthquake focal mechanism studies suggest movements on north- and northeast-trending structures. The main shock, the aftershocks, surface faulting and changes in InSAR-derived surface elevation all occur in a region bounded to the south by a prominent northwest-trending fault and to the north by a west-northwest-trending domain-bounding structure. Thus, we interpret the north-trending thrust fault associated with the main Lake Muir event as due to local stress concentration of the regional east–west stress field at the intersection of these structures. Further, we propose that a particularly large west-northwest-trending structure may be broadly focusing stress in the Lake Muir area. These findings encourage similar studies to be undertaken in other areas of Australia’s southwest to further the current understanding of seismic release in the region.

• KEY POINTS

• A large M_L5.7 earthquake occurred in southwest Western Australia in September 2018, damaging infrastructure and forming a 40 cm-high thrust fault scarp.

• An aeromagnetic interpretation of the Lake Muir region revealed that the new scarp aligned with pre-existing faults that were reactivated.

• Local and regional structures likely acted as regional stress focusers in the Lake Muir area.

• Identifying regional stress-focusing structures is important for the purpose of understanding seismic risk in intraplate areas such as Western Australia.

摘要

尽管有证据表明M6级以上的地震可能造成破坏，但对西澳大利亚州西南部板内地震的了解却很少。识别近显著地震序列助攻应力集中的地质结构了解为什么这些地震发生在跨越超过25万公里，区域看似随机位置²。在2018年9月16日，一个M_大号5.7级地震在西澳大利亚西南部近缪尔湖发生，接着是一个M_大号5.4级余震。这次大地震形成了大约5公里长的北向断裂带，最大垂直位移约为40厘米。主要事件之后是一系列余震，其中一个震级为M _L5.4。利用高分辨率的航空磁学数据，我们分析了新断层带周围广阔区域的基岩地质情况，并绘制了一系列主要的东西向西断裂，将八个不同的地质区域分割开，并绘制了西北向断裂不那么突出的网络，其中之一与陡峭山脉的南部对齐。地表断层，地表变形和地震震源机制研究表明，北向和东北向的构造运动。主震，余震，地表断层和InSAR引起的地表高度变化都发生在南部以一个突出的西北向断裂为界的区域，而北部以西-西北向发展的域界结构为边界的区域。因此，我们将与主要的缪尔湖事件有关的北向逆冲断层解释为由于这些结构相交处区域东西应力场的局部应力集中。此外，我们建议一个特别大的西-西北趋势结构可能将应力集中在缪尔湖地区。这些发现鼓励在澳大利亚西南部其他地区进行类似的研究，以加深当前对该地区地震释放的了解。

• 关键点

• 大中号_大号5.7级地震发生在西南部西澳大利亚州在九月2018年，破坏基础设施，形成40厘米高推力的断层崖。

• 穆尔湖地区的航空磁学解释表明，新的陡坡与已被重新激活的先前断层一致。

• 地方和区域结构很可能充当了Muir湖地区的区域压力焦点。

• 识别区域应力集中结构对于了解板块内区域（例如西澳大利亚）的地震风险非常重要。