Quantitative Study Of The Deep Deformation Along The Northern Segment Of The Red River Fault Zone Using Repeating Microearthquakes

The Red River fault zone with long development history and complicated evolution process is one of the longest strike-slip faults in China.After the Wenchuan M7.9 earthquake,the North-South seismic belt enters a new active period of strong earthquakes,the seismic risks of the Red River fault zone have also caused considerable concerns.Quantitative study of the deep deformation along the Red River fault is of great significance to the analysis of fault active behavior and seismic potential.Surficial observations such as geological and geodetic data provide important constrains on the surface deformation,however,have very limited ability in detecting deep deformation.Repeating earthquake sequences which can be used as subsurface creepmeters,provide a useful way to estimate slip rates at seismogenic depths for the quantitative description of the deep deformation.In this study,we used the bulletins and waveform data collected by the Yunnan Digital Seismic Network(YSN)from 1999 to 2015 to investigate seismicity and deep deformation along the northern segment of the Red River fault zone.We computed fault slip rates at depth from identified repeating earthquakes occurring almost at the same place.The main research contents and results obtained as follows:1.We relocated earthquakes occurred along the northern segment of the Red River fault zone using the double-difference location method based on the combined set of catalog and cross-correlation data.Relocated result reveals a focused picture of seismicity,with most of events occurred in the depth range of 5~15 km.Location errors estimates show that the lengths of the 95% confidence ellipsoids axes are less than 2.6 km.The estimated relative location uncertainty is approximately 1.6 km,both horizontally and vertically,which shows good stability.2.Using waveform cross-correlation analysis,we identified a total of 788 similar event sequences with a cc>0.8,which include 390 doublets consisting of two events and 398 multiplets consisting of at least three events.The total number of earthquakes in the 788 sequences is 4636.We used an empirical method to constrain the relative distance between event pairs based on S-P differential times measured at subsample accuracy.Using this method,we finally identified a total of 23 repeating earthquake sequences included 145 earthquakes along the northern segment of the Red River fault zone.We noticed that in each sequence the occurrences of the earthquakes are rather aperiodic.The changing of the stress field caused by large earthquake occurred near the repeating sequences may cause the reduction of recurrence intervals.Some repeating sequences are located closely,the interactions between these sequences are tied to the irregularity of earthquake recurrence.3.The fault slip rates estimated from seismic moments and recurrence intervals of the 12 repeating earthquake sequences along the northern segment of the Red River fault zone vary from 2.3 to 10.0 mm/a at the depth range of 6.0~13.4 km.The spatial distribution of slip rate shows changes at different seismogenic depths.The high slip rate area is located at depth close to the Midu Basin in the southern part of the study area,which may reveal the active deformation in the seismogenic zone around the Midu Basin.