Slip Rates along the Yunnan Fault Zones Estimated From Repeating Microearthquakes

Fault slip rate has been widely estimated by geologic and geodetic observations. Most of the geologic and geodetic observations are, however, surficial measurements which require an assumed rheology to estimate the slip rate at seismogenic depth. The discovery of repeating microearthquakes has provided a new means for inferring the slip rate at depth along active faults as well as for imaging potential asperities. Repeating earthquakes are a series of seismic events regularly occurring on a patch of a fault plane. These earthquakes usually generate nearly identical waveforms when recorded at the same station. In this study, we examined the waveform data recorded by the Yunnan digital Seismic Network (YSN) between 1999 and 2006 to estimate slip rates along active fault zones in the Yunnan province, south China. A cross- correlation based analysis yielded the identification of 412 doublets and 353 multiplets, which count up to ~45% of the local seismicity. Most of the sequences are aperiodic with recurrence intervals varying from a few minutes to hundreds of days. There are, however, several quasi-periodic sequences which allow us to infer slip rates. For example, using two clusters occurring in the Lijiang and Ninglang areas, we obtained a slip rate of approximately 5 mm/yr at depth of ~23 km. This estimate is consistent with geological and surface GPS measurements. We also found some variations in slip rate in the study region. Different segments along the same fault could have distinct difference in slip rate. For example, the Xiaojiang fault is characterized by a low slip rate in the northern segment, and a high slip rate in the central and southern segments that is consistent with the local seismic activity.