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Detecting Offshore Seismicity: Combining Backprojection Imaging and Matched‐Filter Detection
Author(s) -
Feng Tian,
Meng Lingsen,
Huang Hui
Publication year - 2020
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2020jb019599
Subject(s) - seismology , submarine pipeline , geology , aftershock , trench , induced seismicity , slip (aerodynamics) , geodesy , oceanography , chemistry , physics , organic chemistry , layer (electronics) , thermodynamics
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 600 days 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.