Temporal and spatial characteristics of small earthquakes in Huoshan area，Anhui Province

The Huoshan area is located in the eastern part of the Chinese mainland, at the intersection of the Qinling-Dabie orogenic belt and the Tanlu seismic belt. The faults in the area are strongly active and earthquakes occur frequently. The main faults developed are the NE-trending Late Pleistocene active fault Luo’erling-Tudiling fault, the nearly EW-trending Mozitan-Xiaotian fault and Meishan-Longhekou fault. The NE-trending Luo’erling-Tudiling fault is the main earthquake-controlling and seismogenic structure. It cuts across the above-mentioned nearly EW-trending faults and passes through the North Dabie block, the North Huaiyang block and the Hefei basin from south to north. This region has always been a key monitoring area for earthquake prediction in the eastern part of China, often referred to as a “window” for studying stress field variations and seismic activity in the East China region. Acquiring a high-precision micro-seismic catalog for the Huoshan region is of significant importance in delineating fault morphology and seismic prediction.

In this study, the seismic processing system RISP （real-time intelligent seismic processing system） based on the artificial intelligence algorithm PhaseNet was used to scan the continuous waveform data of Huoshan area from 2020 to 2022, ranging from （30.2°N—32.4°N, 115°E—117.6°E）. There are 21 seismic stations within 120 km of the Huoshan earthquake swarm. The seismic catalog was screened by parameters such as signal-to-noise ratio and phase （accuracy） probability, and an automatic catalog containing 3839 earthquakes was obtained. The number is three times that of the manual catalog.

The RISP uses deep learning method to detect earthquakes, which includes phase picking, association, location and magnitude measurement. The system currently lacks the capability to identify non-natural earthquakes and locate tele-seismic events, leading to cases of non-natural earthquakes and misidentification of distant earthquakes as local events in the earthquake catalog. In addition to screening the catalogs, this study combined manual and automatic catalogs for relocation based on local monitoring capabilities: the manual catalog was used for earthquakes above the monitoring capacity, while the automatic catalog was used for smaller events. The combined catalog contained 3825 earthquakes, 14 fewer than the automatic catalog. The double-difference earthquake location and imaging method tomoDD is a relative location method that significantly improves the accuracy of earthquake location. After relocation using tomoDD, 3785 earthquakes were obtained, with 40 earthquakes unable to be located due to not meeting the criteria. The RISP system exhibits a 95% earthquake identification matching rate in the Huoshan region, providing earthquake catalogs of each magnitude range, particularly enhancing the seismic monitoring capabilities for micro-earthquakes in the M_L−1.1—0.0, addressing the lack of sub-magnitude 0 earthquakes in the manual catalog.

Two seismic belts appear at the intersection of the Luo’erling-Tudiling fault and the Mozitan-Xiaotian fault. Their dominant distribution direction is consistent with the strike of the fault. The NE-trending belt BB′ is parallel to the Luo’erling-Tudiling fault and is distributed on its west side, whose length is about 16 km. The earthquakes are concentrated about 3—3.5 km wide. The focal depth is between 4—12 km. The belts are distributed in two sections, and their shapes are different. The cross-section of the vertical seismic belt shows a slightly gentler dip in the southwest segment compared to the northeast segment, but both are nearly vertical. The Luo’erling-Tudiling fault strikes northeast, with a northwest dip angle ranging from 64° to 84°. Field fault geomorphological evidence indicates that the most recent activity of the fault occurred from the late Middle Pleistocene to the early Late Pleistocene, predominantly characterized by dextral strike-slip extensional faults. The focal mechanism of the M4.3 earthquake that occurred on this fault in 2014 indicates a dextral strike-slip fault. This seismic belt may be its branch fracture, and the two converge together in the deep. The northwest-trending belt DD′ consists of several small clusters, concentrated within 4 km wide along the Mozitan-Xiaotian fault, extending approximately 20 km. The belt indicates the recent activity of the fault in this part. The cross-section of the DD′ belt shows a dip angle of approximately 65° in the northwest segment and around 78° in the southeast segment, both dip northeast.

Regarding the seismogenic structures in the Huoshan region, Cui et al （2020） revealed through deep electromagnetic surveys that the Luo’erling-Tudiling fault serves as the seismogenic structure, utilizing weak zones formed by early activity along the Mozitan-Xiaotian fault, where fluid from the highly conductive layer below the fault weakens the fault, leading to the occurrence of seismic swarms in these zones of weakness. Xu et al （2022） used precise location results to reveal three nearly parallel seismic bands, suggesting a strike-slip and extensional fault system formed under the combined effects of the right-lateral Tanlu fault zone and the south segment of the Shangcheng-Macheng fault zone. Our results reveal two intersecting seismic belts, indicating significant activity at the intersection of the two faults. During the two-year study period, no significant seismic events were recorded in Huoshan region, thus no clear seismic sequences occurred. However, during a ten-day period from February 1 to 10, 2021, a total of 641 earthquakes were recorded, starting at 11 km on the BB′ belt, then shifting to the northeast at 12 km, and finally developing towards the southwest at 9 km, showing distinct characteristics of seismic activity migration, which may be influenced by fluid.