Deep slip rates along the Longmen Shan fault zone estimated from repeating microearthquakes

We investigated deep slip rates along the Longmen Shan fault zone where the devastating 2008 M7.9 Wenchuan earthquake occurred using repeating earthquake data recorded by two digital seismic networks near the fault. Using cross‐correlation analysis of waveform data, we identified a total of 231 doublets and 224 multiplets that have highly similar waveforms. Most of these sequences are aperiodic with recurrence intervals varying from a few minutes to hundreds of days. For each multiplet, we first constructed a reference seismogram at each station and used it to measure the delay time of the P and S waves of each event within the cluster. We used the delay times to determine the relative distances between each event and the cluster centroid. The relative distances were used in screening repeating events in each multiplet. We identified a total of 12 repeating earthquake clusters in the source region of the M7.9 Wenchuan earthquake. Most of the clusters are located at the edge of locked areas where large coseismic slips were observed during the 2008 Wenchuan earthquake, suggesting a close spatial relationship between microearthquakes and impending large devastating earthquakes. The measured in situ deep slip rates appeared to increase with depth and varied from 3.5 to 9.6 mm/yr at depth range of 4–18 km. They are approximately twice as large as those inferred from surface GPS and geological data. Our results here have significant implications to the understanding of faulting as well as risk assessment of seismic hazards.