Evidence for non-self-similarity of microearthquakes recorded at a Taiwan borehole seismometer array

Abstract

We investigate the relationship between seismic moment M0 and source duration tw of microearthquakesby using high-quality seismic data recorded with a vertical borehole arrayinstalled in central Taiwan. We apply a waveform cross-correlation method to the threecomponentrecords and identify several event clusters with high waveform similarity, withevent magnitudes ranging from 0.3 to 2.0. Three clusters-Clusters A, B and C-contain 11,8 and 6 events with similar waveforms, respectively. To determine how M0 scales with tw, weremove path effects by using a path-averaged Q. The results indicate a nearly constant tw forevents within each cluster, regardless of M0, with mean values of tw being 0.058, 0.056 and0.034 s for Clusters A, B and C, respectively. Constant tw, independent of M0, violates thecommonly used scaling relation twαM1/30 . This constant duration may arise either becauseall events in a cluster are hosted on the same isolated seismogenic patch, or because the eventsare driven by external factors of constant duration, such as fluid injections into the fault zone.It may also be related to the earthquake nucleation size.