The Study On Earthquake Process Of Some Strong Earthquakes Of Western China In Recent Years

In the vast western region of China,the uplift and lateral extrusion of the Qinghai-Tibet Plateau produces active block motions,with many strong earthquakes at block boundaries,which have historically caused serious casualties.With economic and social development,the risk of strong earthquakes is further increased by the high concentration of urban and new industrial production.In order to reduce the losses from earthquake hazards,a more comprehensive and in-depth understanding of the processes of strong earthquake nuclation,occurrence and termination is needed.Compared with other geophysical parameters,the temporal-spatial evolution characteristics of earthquake sequences can reflect the intrinsic nature of seismogenic structures,inter-fault stress transitions and interactions,which is the key to study the law of earthquake nucleation.Since 2012,several strong earthquakes have occurred in different tectonic units in the western inland region of China,including the 2012 M6.3,2014 M7.3,and 2020 M6.4 earthquakes in the Yutian area at the western boundary of the Bayan Har Block;the May 21,2021 Yangbi M6.4 earthquake at the junction of the Sichuan-Yunnan block and the Southwest Yunnan block;the M6.9 earthquake occurred near the Lenglongling fault in the Haiyuan Fault Zone in early 2022.These five strong earthquake sequences show foreshocks,aftershocks and earthquake triggering phenomena in different tectonic settings,giving us an opportunity to explore the nucleation pattern of inland strong earthquakes.However,the existing earthquake catalogs are not sufficient to show the full picture of the processes of these strong earthquakes,and there is an urgent need to construct a more complete earthquake catalog.According to the distribution of station networks in the earthquake area,different microseismic detection schemes are adopted in this study.Since the stations in the Yutian area are extremely sparse,we use single-station template detection to identify microseismic signals and use polarization analysis to constrain the their locations;in the Yangbi source area with a high-density temporary station network,we first use a deep learning model to build the initial catalog and then combine it with template matching to scan for microseismic;in the Mengyuan area,we directly use the matched filter technique.After scanning and detecting,the number of earthquakes is 3～15 times higher than that in the CENC catalog,which can show a more complete image of seismic activities.In this paper,we analyze the details and generation mechanisms of the above strong earthquake sequences one by one,and compare the characteristics of inland strong earthquake processes in the western region of China horizontally,and gain new insights as follows.(1)The three strong earthquakes in Yutian were all nucleated in the fault at the edge of the basin.The largest foreshock of the M7.3 earthquake in 2014 was located in the South Xiaoerkule fault,and the mainshock was in the Ashikule fault;The M6.4 earthquakes in 2020 are all located in the southern structure of the South Xiaoerkule basin.From the b-value curve from 2012 to 2020,it can be found that the b-value decreased significantly from half a year to one year before the three strong earthquakes.During this period,the foreshock activity showed two stages of spatial activity,scattered around the margin of the Xiaoerkule basin in the early stage,and clustered near the mainshock in the impending stage.In addition,this paper also identified a large number of similar earthquakes before the M7.3 earthquake in 2014.(2)The Yangbi earthquake sequence in 2021 is distributed on a broom structure,which is composed of a strike slip main fault and several secondary oblique faults;The strike of the main fault is 315° with 80° dipping to NE.The earthquakes extend along the strike about 27 km and distribute 2～13 km in depth;The foreshocks are mainly active on the south side of the fault.With the continuous decrease of b-value,the foreshocks in the 8 km long area show an activity mode of segmented migration,stagnation,acceleration to the direction of the main shock,and finally trigger the mainshock;Aftershocks also mainly occurred in the south side of the fault.In the logarithmic time coordinate system,the migration distance of aftershocks increased linearly.(3)In 2022,the Menyuan earthquake mainly occurred on the high angle South dipping strike slip fault on the south side of the Toleshan-north fault,and there was no obvious foreshock activity;But the characteristics of aftershocks are obvious:the early aftershocks expanded along strike to the east and west with a logarithm migration relationship between distance and the elapsed time;The events in the Lenglongling fault occurred before the aftershock expansion front and migrated southeastward along the strike,but soon blocked,until the M5.2 earthquake occurred five days later.(4)By comparing the temporal and spatial characteristics of several foreshock sequences,it can be found that the b-value curve decreased significantly before the earthquake,with a range of more than 30%,reflecting the concentration of regional stress in the process of earthquake nucleation,and proving the effectiveness of the low b-value phenomenon as a precursor index of inland strong earthquakes;In addition,the foreshock activities all have staged spatial migration,which appeared on the marginal structure in the form of scattered earthquake swarms in the early stage,and gathered near the main fault before the impending earthquake.(5)We discussed the possible mechanism of earthquake nucleation:first,the foreshock sequences of Yutian M7.3 earthquake in 2014 and Yangbi M6.4 earthquake in 2021 all have the characteristics of tectonic preslip.For example,the waveform of M5.4 foreshock sequence in 2014 is highly consistent,and the two groups of swarm activities migrated along the fault to the main shock before the Yangbi earthquake.In addition,through the calculation of Coulomb stress,it is proved that the stress disturbance generated by previous strong earthquakes also has an important contribution to seismic nucleation.Therefore,the process of seismic nucleation is the synthesis of two mechanisms:tectonic preslip and cascade triggering.(6)Some commonalities can also be found in the aftershocks extension.In Yangbi and Menyuan earthquake sequences,the extension of aftershocks generally satisfies the linear relationship between epicentral distance and logarithmic time,reflecting the characteristics of fault afterslip.During the extension of aftershocks,the change of Coulomb stress can trigger earthquakes on the structures around the main fault,resulting in a wider range of surface damage.