Detecting Offshore Seismicity: Combining Backprojection Imaging and Matched-Filter Detection

An important manifestation of the background or coseismic deformation surrounding megathrust earthquakes is the offshore microseismicity, which are difficult to be directly detected by land-based seismic instruments. Here, we improve the capability of detecting offshore events by combining two popular techniques: backprojection (BP) imaging and matched-filter (MF) detection. The BP method is effective in retrieving offshore seismicity (M-w > 4.5) buried in the coda wave of large earthquakes. The capability of the MF method depends on the availability of the template pool; therefore, the BP-inferred events can be used as additional templates to expand the MF detections (BP-MF). We performed the BP-MF approach in the period within 600days after the 2011 M-w 9.0 Tohoku earthquake. We find overall 44.2% more offshore events than those listed in the Japan Meteorological Agency (JMA) catalog. In the near-trench area, we detect 213% more events. Among the newly detected M-w > 4 events, the BP template contributions are twice more than those matched by the JMA templates. Based on the spatial consistency between aftershock-depleted zones and large coseismic slip, we identify a possible large coseismic slip zone in the near-trench region offshore Fukushima. Large b values (>= 1.2) are found close to large aftershocks, possibly indicating localized pockets of small differential stresses. At several locations close to the trench, p values (0.93-1.11) are higher than those in the inland area (0.64-0.85). This may be due to the larger coseismic slip and hence larger stress drop of the outer-rise normal-faulting events compared to the deeper thrust-faulting events.