PRECISE RELOCATION OF SMALL-TO-MODERATE-SIZED EARTHQUAKES IN THE DATONG VOLCANIC GROUP AND SURROUNDING AREAS
In the present study we collect a large amount of arrival times from 3 218 earthquakes in the Datong volcanic group and surrounding areas from January 2008 to January 2023 through the China Seismic Network Center and relocated these earthquakes using double-difference location algorithm,finally obtain 2 447 relocate earthquakes.Our result shows that most earthquakes occurred above a depth of 16km,and earthquakes in the basin occurred at depths of 5-16km.There are fewer earthquakes occur near the surface at depths of 0-2km,while 6km and 11km are the dominant depths for earthquakes.The overall strike trending of these earthquake sequences is NE-SW,which is consistent with the regional active faults and controlled grabens and semi-graben-type faulting basins.In addition,these earthquakes are more concentrated near the Kouquan fault zone and in the Datong-Yanggao earthquake zone in the eastern part of the volcanic group.The average location errors of these earthquakes in the east-west,north-south,and vertical directions are about 0.21km,0.22km,and 0.30km,respectively,with an average travel time residual of 0.14s.The earthquakes near the Kouquan fault zone changed from deeper and more concentrated in the south to shallower and more scattered in the north.The earthquake sequences in the northern part of the southern section and the southern part of the middle section of the Kouquan fault zone are deeper along the NE-SW direction,roughly vertically distributed on the Kouquan fault.The earthquake sequences in the northern part of the middle section of the Kouquan fault zone did not occur on the Kouquan fault,and the distribution of earthquakes is relatively scattered,and earthquakes with larger magnitude are mostly concentrated at shallow depth,which may be related to the thick sedimentary coal-bearing strata and mining activities in the area.The strike trending of these earthquakes in the northern section of the Kouquan fault zone is,along the NE-SW direction,roughly distributed on the Kouquan fault.However,there are also earthquakes in the northern part of the Kouquan fault zone,which may suggest that the activity of the Kouquan fault zone has extended there.The focal depth in the source areas of the Datong-Yanggao earthquake is mostly concentrated at depths of 3-16km on the hidden fault parallel to the NE-SW trending Dawangcun fault to the east.The hidden fault has a large dip angle and dips towards NW,which intersects with the Tubao fault and the Liulengshan piedmont fault,likely related to the aftershock activity of the Datong-Yanggao earthquake.Earthquakes occur frequently in the middle section of the Huairen fault,followed by the southern section,and there are few earthquakes in the northern section.The seismic activity of the Shuiyu fault,the east fault of the Cailiangshan mountains,and the Yanggao-Tianzhen fault is relatively weaker.There are some seismic activities in the central part of the northern margin fault of the Tianzhen-Yanggao Basin.Earthquakes in volcanic areas occurred at the boundaries of volcanic clusters,while the seismicities inside the volcanic group area were not very strong,which suggests that the boundary of volcanic clusters is more prone to stress accumulation and earthquake generation than the interior of volcanic clusters.Based on the new seismic results of ambient noise tomography in the area,it is found that earthquakes are not only related to faults,but more importantly,most earthquakes occur near the high-to-low-velocity anomaly boundaries.Furthermore,there are obvious low-velocity anomalies visible beneath most earthquake source areas,which may suggest that the occurrence of these earthquakes is closely related to fluids carried by the upwelling of thermal materials rising to the crust from the mantle and reducing the effective normal stress on the fault planes.
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