The Velocity Structure And Seismic Hazard Of Northeastern Bayan Har Block

The Bayan Har block,is a tectonic belt sandwiched between the East Kunlun orogenic belt and the Qiangtang block in northern Qinghai-Tibet Plateau.As an important secondary active block within the Qinghai-Tibet Plateau,the crustal tectonic activity in its interior and margins is intense,and strong earthquakes occur very frequently.Especially since the Mani M7.5 earthquake in 1997,all the strong earthquakes of M≥7 in mainland China have occurred on the faults around the Bayan Har block.In recent years,it suggests that the Bayan Har block is still the main area of the strong earthquakes above M7 after the 2017 Jiuzhaigou Ms7.0 in Sichuan province and the 2021 Maduo Ms7.4 earthquakes in Qinghai province.It is important to study on the seismogenic environment of these two M7 earthquakes around the northeastern Bayan Har block for the research on the dynamics of the Qinghai-Tibet block and the seismic hazard of strong earthquakes.This paper mainly uses the travel time of seismic wave,continuous waveform and earthquake catalog to study the seismogenic structure and stress state of the vicinity of the Jiuzhaigou-Songpan and the Maduo-Yushu regions to obtain the seismogenic environment and the seismic process of the strong earthquakes and discuss the seismic hazard in eastern section of East Kunlun fault.Firstly,the double-difference tomography method was applied to invert the source parameter and the mid-upper crustal velocity structure of Jiuzhaigou-Songpan and its adjacent areas using the travel time data of fixed seismic stations and postearthquake emergency stations from January 2013 to December 2018.The relocation results show that the aftershocks of the Jiuzhaigou earthquake were located on the extension of the Huya fault in the NWW direction.The main shock manifests as a bilateral rupture whose aftershocks spread faster on the southeast side than that on the northwest side and ruptures from the deep to the shallow.The aftershocks zone was characterized by wide in the upper part and narrow in the lower part.The Wenchuan aftershocks spanned the Pingwu-Qingchuan fault and showed a linear distribution in the NE direction with the characteristics of shallow in the east section and deep in the west section,indicating that the aftershocks mainly occurred on the hanging wall of the thrust fault.From the southwest section to the northeast section,the aftershock zone gradually narrowed,indicating that the dip angle of the fault became steeper and reflecting the variation from the thrust mechanism to the strike-slip mechanism.The VP at a depth of 5 km was consistent with the shallow geological structure in Jiuzhaigou-Songpan and its adjacent areas.For instance,the Minshan block,which is the uplifted area confined by the Minjiang fault and Huya fault,manifests as a high VP anomaly.In contrast,the region in the east of the Huya fault and its extension exhibits a low VP zone.At a depth of 10 and 15 km,the VP on the eastern side of the southern Huya fault is significantly higher than that on the western side.The sharp contrast may make it an area susceptible to a strong earthquake swarm.At a depth of 20 km,the Wenxian and Longnan regions present a prominent low-VP anomaly.The electric structure also shows a low-resistivity layer in this region.This low-resistivity and low-VP layer is prone to deformation and absorbs the displacement of the upper crust.The vertical sections of the VP show that a significant low-VP region occurs below a depth of 20 km beneath the Yingxiu-Beichuan fault and Pingwu-Qingchuan fault in the northern Longmenshan fault zone,which may be related to fluids under the Longmenshan fault zone.The aftershocks of the Wenchuan and Jiuzhaigou earthquakes are mainly located in the upper brittle crust above the low-VP zone.Under the tectonic stress field,the upper brittle crust is prone to breaking by sliding along the weak surface with a low friction coefficient or the rheological extension of the material in the weak layer.The Jiuzhaigou earthquake and the Songpan-Pingwu earthquake swarm are both located at the boundary between high-and low-VP anomalies in the eastern margin of the Bayan Har block.The aftershocks of the Jiuzhaigou earthquake are vertically distributed inside the high-VP body and the main shock locates at the boundary between high-and low-VP anomalies.The southeastward flow of low-VP materials in the mid-lower crust makes the high-VP body above it dislocate in a left-lateral strike-slip manner along the SE direction.Similar to the Wenchuan earthquake,the Songpan-Pingwu earthquake swarm may have been caused by the Sichuan basin obstructing the southeastward movement of the Bayan Har block.Secondly,the double-difference tomography method was also applied to invert the mid-upper crustal velocity structure and source parameter in Maduo-Yushu and its adjacent areas by using the travel time data of the fixed seismic network from October 2008 to November 2021.The relocation results show that the distribution of aftershocks of the Maduo earthquake is consistent with the surface rupture and rupture process in depth,indicating that the double-difference tomography method have a good constraint on the focal depth.There is obvious lateral heterogeneity in the velocity structure of the upper crustal in the Maduo-Yushu region,which is related to the geological structure of the surface in shallow.The velocity structures are significantly different for the upper crust of the Qiangtang block,the Bayan Har block,and the Qaidam block while the velocity changes are relatively intense at the boundary of the blocks.The Maduo Ms7.4 earthquake is located at the high-VP side of the boundary between high-and low-VP anomalies while the Yushu Ms7.0 earthquake is located at the high-VP body.The fluid in the middle crust of the Qiangtang block may have a certain control effect on the rupture of the earthquake.The occurrence of these two strong earthquakes may be related to the asperity body represented by the high-VP body.In addition,the continuous waveform data of some stations during January and March 2014 were selected from phase Ⅱ of ChinArray project to detect and locate the microseism by Match and Locate method.The results show that the b value of the Jiuzhaigou earthquake swarm in 2014 is significantly lower than the b value of the Jiuzhaigou earthquake sequence in 2017 and the background b value on the northern Huya fault.It suggests that the regional stress level in the Jiuzhaigou region rose and the seismic hazard increased after the Lushan earthquake in 2013.The detected microseisms reveal obvious clustering characteristics in time,which mainly concentrated in 5 days after the ML3.5 and the ML4.2 earthquakes.The Jiuzhaigou ML4.2 earthquake swarm has the characteristics of conjugate intersection along the NW and NE directions,which reveals that there may be secondary ruptures in the NE direction near the epicenter of the ML4.2 earthquake besides the main rupture in the NW direction.The process of microseismic expansion shows that the ML4.2 and the ML3.5 earthquake swarms extended along the northern Huya fault in the NW direction.This synergistic process may provide the conditions for the instability of the NW hidden fault.Finally,the earthquake catalog above ML1.5 from January 2006 to December 2021 on the eastern section of the East Kunlun fault was used to calculate the seismicity parameters at each node by gridding the study area.On this basis,the spatial scanning of b values was carried out on the eastern section of the East Kunlun fault before and after the Jiuzhaigou earthquake and Maduo earthquake.The b value scanning result of the whole period shows that there is significant difference for the b value in different region on the eastern section of the East Kunlun fault.It is significantly lower for the b value on the western side of 101° E than that on the eastern side,indicating that the western side is in a high stress state.This may be related to the decomposition of the eastward movement of the East Kunlun fault to both sides,resulting in a weakening of the slip rate.The high stress on the east side of Maduo may be related to the blocking of the high-velocity body during the southeastward movement of the Bayan Har block.The high b value anomaly on the the A’nymaqen suture belt in constract to both sides may be related to its low velocity,high poisson’s ratio and broken structure.The TL value of the recurrence period is shorter in the Maduo-Maqin and Maqin-Maqu segments than that in other areas,which is consistent with the low b value area.In addition,the spatial distribution of b values in the eastern section of the East Kunlun fault before and after the strong earthquake shows that the changes of b values with time are closely related to the pregnant process of the strong earthquake and the velocity structure of the crust.Before the Jiuzhaigou earthquake,the b value was low integrally,indicading the stress level was relatively high in the eastern section of the East Kunlun fault.After the Jiuzhaigou earthquake,the b value increased significantly in the east of Maqin area,suggesting the stress was released in this area,while the b value did not increase significantly in the west of Maqin area,indicading the regional stress was not released effectively.After the Maduo earthquake,the b value increased notably and the stress was rapidly released in the west of the Maduo earthquake,while the b value reduced and the stress was still relatively high in the Maduo-Maqin area,which indicated the stress loading may take place on the east of the epicenter after the Maduo earthquake.