Probing the Variation in Aseismic Slip Behavior Around an Active Suture Zone: Observations of Repeating Earthquakes in Eastern Taiwan

An examination of repeating earthquakes in eastern Taiwan revealed previously unrecognized quasiperiodic repetition of aseismic creep along two reverse faults in an active suture zone. Using 202 M_L 2.0 to 4.6 repeating earthquake sequences (RES) during the period from 2000 to 2011, we studied where and how certain faults creep. The RES were found to be highly concentrated in the southern segment of the Longitudinal Valley fault (LVF) and in the northern segment of the Central Range fault (CRF). They are mainly located at a depth of 10–25 km and show strong regional differences in creep behavior. Using the moment release rate of RES and geodetically derived long-term slip rate, we re-estimate the empirical relationship between deep creep and seismic moment for creeping sections in eastern Taiwan. For the 30-km-long LVF, the creep rate increased dramatically from 1.5 to 12.3 cm/year under the influence of the M_L 6.4 Chengkung earthquake of 2003. For the 80-km-long CRF, the high creep rate of 4.3 cm/year appears to have been stable over time and is descriptive of a previously unrecognized deep structure underneath the eastern flank of the Central Range. The quasiperiodic pulsing of the deep slip rate has a predominant interval of 1 year for both segments. After the M_L 6.4 event, the predominant interval for the creeping LVF halved in duration. The time-dependent aseismic slip showed a strong correlation with the creepmeter data, suggesting that the positing of a common mechanism is needed to connect the surface and deep creep variation.