Following the Mw 8.8 Maule megathrust earthquake that occurred on February 27, 2010, in central Chile, a sequence of normal faulting crustal earthquakes occurred close to the city of Pichilemu. This activity lasted several months and included two large events (Mw = 6.9 and Mw = 7.0) on March 11, 2010. An initial investigation of this activity analyzed a data set of about 630 earthquakes recorded locally by 8 short-period seismic stations and attributed this seismicity to the activation of the “Pichilemu fault system.” While there is no visible surface faulting associated with this system, it is inferred from discontinuities in lithofacies in the metamorphic complex of Pichilemu and from morphological breaks attributed to post-Pliocene neotectonic activity. There are no recorded shallow crustal earthquakes in the Pichilemu area prior to the 2010 events. In this study, we combine locally recorded earthquake data from the 2010 seismic deployment and data from 20 seismic stations (short period, three components, continuous recording) deployed in 2017 around Pichilemu, Chile, to create a more detailed characterization of the Pichilemu fault system through local earthquake tomography (LET). The combined data set composed of P- and S-wave arrival times from 3691 events was inverted to generate a 3D elastic wave speed model from the surface to about 50-km depth. One hundred twenty-two focal mechanisms for relocated earthquakes with M ≥ 1 were also generated. Relocated hypocenters show that most of the recorded seismicity is associated with the Pichilemu fault system; its main structure is oriented N145°E, and it is seismically active along about 50 km long. Normal faulting mechanisms predominate for events with M≥2, being similar to the mechanisms of the Mw 7.0 March 11, 2010, Pichilemu earthquakes. Low-velocity anomalies correlate with fracture zones associated with the Pichilemu fault, and a high contrast in Vp/Vs coincides with known structures of Paleozoic to Mesozoic age. A high Vp/Vs ratio is observed where a projection of the fault reaches the interplate contact, suggesting that this zone of the forearc crust is likely weakened by the presence of fluids from the slab. A high Vp anomaly is contiguous with the fault system and appears to be related to the presence of granitic rocks which belong to the Coastal Batholith within the Cordillera de la Costa. We suggest that the location of the Pichilemu fault system is governed by rheological contrasts inherited from the evolution of the subduction complex represented by the current Cordillera de la Costa, and we infer that the orientations of those structures, the possible hydration of them from the interplate contact and associated crustal blocks, play a key role in fault activation following the stressing by a great subduction earthquake.

继2010年2月27日在智利中部发生的8.8兆瓦Mw特大推力地震之后，在比奇莱默市附近发生了一系列正常的断层地壳地震。这项活动持续了几个月，并于2010年3月11日发生了两次大事件（Mw = 6.9和Mw = 7.0）。对该活动的初步调查分析了由8个短周期地震台站本地记录的约630次地震的数据集，并将其归因于这种地震活动导致了“ Pichilemu断层系统”的激活。虽然没有可见的与该系统有关的表面断层，但它是由毕赤勒木变质复合物中岩相的不连续性和上新世后新构造活动引起的形态学断裂推断的。在2010年地震之前，Pichilemu地区没有记录到浅层地壳地震。在这项研究中，我们将2010年地震部署中的本地记录地震数据与2017年在智利比奇勒姆附近部署的20个地震台站（短时间，三个分量，连续记录）的数据相结合，以对比奇勒姆断裂系统进行更详细的描述通过当地地震层析成像（LET）。将来自3691个事件的P波和S波到达时间组成的组合数据集进行反演，以生成一个从表面到大约50公里深度的3D弹性波速度模型。还产生了M≥1的一百二十二个震源机制。重新定位的震源表明，大部分记录的地震活动与Pichilemu断层系统有关；其主要结构为N145°E，它在大约50公里长的地方具有地震活动性。对于M≥2的事件，正常的断层机制占主导地位，与2010年3月11日的Pichilemu地震Mw 7.0的机制类似。低速异常与Pichilemu断层相关的断裂带有关，Vp / Vs的高对比度与古生代到中生代的构造相吻合。在断层投影到达板间接触处观察到较高的Vp / Vs比率，这表明前面板地壳的这一区域很可能会由于平板流体的存在而减弱。高Vp异常与断层系统相邻，似乎与花岗岩的存在有关，而花岗岩属于Cordillera de la Costa内的沿海基岩。