Structural properties of the Southern San Andreas fault zone in northern Coachella Valley from magnetotelluric imaging
Structural properties of the Southern San Andreas fault zone in northern Coachella Valley from magnetotelluric imaging
复制标题
大地电磁成像显示科切拉山谷北部南圣安德烈亚斯断层带的结构特性
DOI：
10.1093/gji/ggac356
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发表时间：
2022
影响因子：
2.8
通讯作者：
Shunguo Wang
中科院分区：
文献类型：
--
作者：
P. Share;J. Peacock;S. Constable;F. Vernon;Shunguo Wang
南圣安德烈亚斯断层（SSAF）是加州最大的地震风险之一。然而，关于科切拉山谷周围更深层次的结构特性存在很多模糊性，这在很大程度上是由于日常地震活动相对较少。在这里，我们使用非地震方法，即大地电磁测深（MT），以帮助通知深度相关的断层带几何形状，流体含量和孔隙度的SSAF的多股部分的图像。所获得的MT数据和由此产生的反演模型突出显示了一个包含SSAF带的导电柱，其中包括一个2-3 km宽的垂直至陡峭的东北倾斜导体，深度为104 km（最大值为101 Ω.m，深度为2 km），以及韧性地壳中的另一个突出导体（101 Ω.m，深度为12 km，略位于表面SSAF西南）。我们估计孔隙率为18- 44%的导电最高的500米，10- 15%的孔隙度在2公里的深度和少量（0.1- 3%）的高度互连的高盐度流体产生更深的导体。位于该导电区域东北部的主要是电阻性地壳，表明干燥的结晶岩，在某些地方向下延伸至20 km。当地大部分的地震活动都与这个电阻区有关。位于更远的东北部仍然是一个导电区，深度>13公里，与西南部的导电区分开。成像的异常允许两种解释。SSAF带在上地壳中垂直于陡峭的东北方向倾斜，并且（1）在更大的深度处接近垂直，为东北方向的流体迁移创造了一个不可渗透的屏障，或者（2）继续向东北方向倾斜，但在高达13 km的深度处相对干燥和具有电阻，在那里它表现为次级深部韧性地壳导体。我们倾向于解释（1），但需要更多的MT调查。
The Southern San Andreas fault (SSAF) poses one of the largest seismic risks in California. Yet, there is much ambiguity regarding its deeper structural properties around Coachella Valley, in large part due to the relative paucity of everyday seismicity. Here, we image a multi-stranded section of the SSAF using a non-seismic method, namely magnetotelluric soundings (MT), to help inform depth-dependent fault zone geometry, fluid content, and porosity. The acquired MT data and resultant inversion models highlight a conductive column encompassing the SSAF zone that includes a 2–3 km wide vertical to steeply northeast dipping conductor down to ∼4 km depth (maximum of ∼1 Ω.m at 2 km depth) and another prominent conductor in the ductile crust (∼1 Ω.m at 12 km depth and slightly southwest of the surface SSAF). We estimate porosities of 18–44 per cent for the conductive uppermost 500 m, a 10–15 per cent porosity at 2 km depth and that small amounts (0.1–3 per cent) of highly interconnected hypersaline fluids produce the deeper conductor. Located northeast of this conductive region is mostly resistive crust indicating dry crystalline rock that extends down to ∼20 km in places. Most of the local seismicity is associated with this resistive region. Located farther northeast still is a conductive region at >13 km depth and separate from the one in the southwest. The imaged anomalies permit two interpretations. The SSAF zone is vertical to steeply northeast dipping in the upper crust and (1) is near vertical at greater depth creating mostly an impermeable barrier for northeast fluid migration or (2) continues to dip northeast but is relatively dry and resistive up to ∼13 km depth where it manifests as a secondary deep ductile crustal conductor. We prefer interpretation (1), but more MT investigations are required.