3D deep electrical structure and seismogenic environment in the western section of the Zhangjiakou-Bohai fault zone

Abstract

The Zhangjiakou-Bohai fault zone (ZBFZ) is an important NW-trending active tectonic zone in North China. The western section of ZBFZ is characterized by frequent moderate and strong earthquakes. This is a typical tectonic area for studying seismic and volcanic activities. The three-dimensional (3D) deep electrical structure of the region was revealed by using 3D electromagnetic inversion of 143 magnetotelluric stations. The results suggest that the deep electrical structure in the western part of the ZBFZ is laterally heterogeneous. To the north of the Shangyi-Chicheng Fault (F4), the upper crust has a high resistivity structure (R), corresponding to the exposed Hanuoba basalts and metamorphic rocks. To the south of F4, high and low resistivity zones alternate, which is attributed to the complex faults and basin-range structures. In the middle crust, the low-resistivity structures (C1 and C2) gradually appear in the northwest and southeast of the study area, showing the characteristics of a high-resistivity structure sandwiched between two low-resistivity structures. In the lower crust, the two low-resistivity structures (C1 and C2) gradually join each other and the whole region becomes characterized by low resistivity. The Shangyi-Chicheng Fault (F4), the northern margin fault of the Huai’an Basin (F7), and the northern margin fault of the Huaizhuo Basin (F11) are definite electrical difference zones, which are connected to a low-resistivity layer in the crust. The results indicate that the focal depth of the 1998 M6.2 Zhangbei earthquake was between 12 km and 15 km and its seismogenic fault was the Dahezhen-Hailiutu Fault (F3), which is concealed beneath the Hanuoba basalt area. It is speculated that owing to the neotectonic activities, the magma originated in the asthenosphere mantle rose and erupted on the surface near Datong, forming the Datong volcanic group. Then the magma continued to migrate and overflow along with the fractures in the NE direction, forming the Hannuoba basalts in the Zhangbei area. The existence of mantle thermal material in this area led to the migration of accompanying fluids to the middle and upper crust, resulting in fault sliding and frequent occurrence of moderate and strong earthquakes in this region.

Deep electrical structure of the hinterland of Yunkai magmatic arc in South China and the seismogenic environment of the 2019 Beiliu earthquake

“…The seismogenic structure and environment of the Beiliu earthquake are similar to those of the 1998 Zhangbei M6.2 earthquake in the Zhangbo seismic belt of North China. The Zhangbei earthquake occurred in the Hannuoba high-resistance basalts (Peng et al, 2022). The surging of deep thermal materials resulted in the activity of conjugate faults, which triggered the earthquake.…”

“…It is the most sensitive method to detect the conductivity of rock masses and is, therefore, commonly used in the detection studies of seismogenic structures (Unsworth and Bedrosian, 2004;Becken et al, 2011;Zhao et al, 2012;Zhang et al, 2016;Zhan et al, 2017;Wang et al, 2018;Ye et al, 2018;Ye et al, 2020). The results of threedimensional MT exploration in recent years suggest that moderate and strong earthquakes and aftershocks are closely related to the resistivity structure of underground media (Zhao et al, 2012;Zhan et al, 2013;Mohan et al, 2015;Aizawa et al, 2017;Arora et al, 2017;Cai et al, 2017;Sun et al, 2019;Ye et al, 2020;Peng et al, 2022). Some moderate and strong earthquakes have occurred near the boundary between high and low resistivity structure of underground media.…”

Analysis of the Stress Field Around Concealed Active Fault From Minor Faults‐Slip Data Collected by Geological Survey: An Example in the 1984 Western Nagano Earthquake Region

“…Active faults have conventionally been determined using ground surface approaches through aerial photo‐decipherment, reading detailed topographic maps, and topographic identification of fault displacement based on field surveys (Imaizumi et al., 2018; Research Group for Active Faults of Japan, 1991). However, magnitude 6–7 earthquakes frequently occur in various tectonic settings where active faults have not been identified (e.g., the 1983 Mw 6.4 Coalinga earthquake (Stein & Ekström, 1992), the 1994 Mw 6.7 Northridge earthquake (Carena & Suppe, 2002), the 1998 M 6.2 Zhangbei‐Shangyi earthquake (Peng et al., 2022), the 2010 Mw 7.1 Darfield earthquake (Barrell et al., 2011) and the 2010 Mw 7.2 El Mayor‐Cucapah earthquake (Gold et al., 2013)). Faults that generate these earthquakes do not appear on the ground surface but exist as concealed active faults.…”