Deep Slip Rates Along The Active Faults In Gulang Region Estimated From Repeating Microearthquakes

This paper has discussed the repeating earthquakes and estimated the fault velocity from the repeating sequences in 1927 Gulang M8 earthquake region which is after the 1920.12.16 Haiyuan earthquake.On 1927.5.23,the epicenter intensity of the region has reached?degrees,resulting in tens of thousands of casualties and a large number of buildings destroyed.The people's lives and property caused great losses and damage.Since the records of earthquakes in 1980,the region and its adjacent areas has frequent micro-earthquakes.The small earthquakes are relatively concentrated in space which time is continuous.The small earthquakes in this area are also found that it is concentratedly in the northeastern margin of the Qinghai-Tibet block,at the same time,its seismicity may induce that the Qilian Mountains seismic zone has a certain significance to the medium or even strong earthquakes seismicity,defined as "Gulang window." From the tectonic structure,Gulang window is located in the confluence area of the Qilian mountain fault-fold belt and the Haiyuan fault zone in the northeastern part of the Qinghai-Tibet Plateau.Lots of work has been carried out for the seismogenic and models in this area.Since 2000,a large number of digital observations have been accumulated,but the research work on the application of digital observation data in this region is relatively limited.The relationship between the modern small earthquakes in Gulang and the rupture fault of the Gulang M8 earthquake in 1927,The differences between the sliding rate and the surface observation,and the current activity habits of the active faults need to be solved urgently.It is important to understand these problems for the study of the healing process after the earthquake,to research the prediction of the strong earthquakes and the recurrence of earthquakes.The repetition of earthquakes is one of the important mechanisms of spatiotemporal distribution of active faults.Repeating earthquakes are a group of earthquakes that occur at the same fault location and have highly similar waveforms.The discovery and application of repeating earthquakes provide new ideas and methods for studying the deep sliding rate of active faults and finding potential asperities.In this paper,the study have applied the repeating earthquakes to calculate the slip rates along the active fault in Gulang area with the seismic waveform data between 2001 and 2015 from Gansu Digital Seismic Network data.The study have applied the repeating earthquakes to calculate the slip rates along the active faults in Gulang region with the seismic waveform data between 2001.1.16 and 2015.4.30.Through cross correlation technique of available waveforms,it identified a total of(1250 events)200 doublets and 690 multiplets that correlation coefficient >0.8 and show high waveform similarity.Most of these sequences are aperiodic with recurrence intervals varying from a few minutes to hundreds of days.Repeating earthquakes,as special similar earthquakes,have nearly identical focal mechanisms and overlapped rupture area.Thus,based on hierarchical clustering algorithm,we divided the correlation coefficient>0.9 multiplets which are from HJT?SGT?MEY stations into 33 clusters included no less than 4 events(156 events),meanwhile,and measured the delay time of the P and S waves of each event within the clusters to identify 5 repeating microearthquake clusters.Thinking about the limited quantity and quality of seismic data existed in Gulang region,it has chosen HypoDD to relocate the seismicity and based on 5 repeating sequences calculated from a circular fault model with a stress drop of 2Mpa,we find that the deep slip rate on the western part of Huangcheng-Shuangta fault is about 5.1mm/a at depth 8.0km and western part of Tianqiaogou-Huangyangchuan fault,at differential seismogenic depths about 6~10km,the slip rate varied from 3.8~12.9mm/a.The center part of Wuwei-Tianzhu fault is approximately 3.5mm/a at depth 8.6km.As a whole,the slip rate of middle Gulang region is largest,the west is medium,the eastern is smallest.