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

We investigate the relationship be_tween 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 _twα^M1/30 . 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.