Detecting offshore earthquakes with backprojection and matched-filter method

Tian Feng, Lingsen Meng, & Hui Huang

Published August 14, 2019, SCEC Contribution #9540, 2019 SCEC Annual Meeting Poster #064

To better understand the offshore seismicity and slow slip around recent megathrust earthquakes, we improve the capability of detecting offshore events by combining two recently developed techniques: Backprojection imaging (BP) and matched-filter detection (MF). The BP method, back-tracing the seismic waves recorded by dense arrays, allows effectively detecting missing aftershocks immediately after megathrust earthquakes. The MF method searches for similar patterns by cross-correlating waveforms of known template events with continuous seismic records. By stacking the traces of cross-correlation coefficients, the MF method is capable of detecting many times more small events than routine catalogs and thus reduce the catalog’s magnitude of completeness. The capability of the MF method depends on the quality and quantity of templates, therefore we use the missing aftershocks detected by BP as templates to expand the MF detection. We performed the BP and MF in the period between the mainshock of 2011 Mw 9.0 Tohoku earthquake and 600 days later. Our template library is composed of 1080 BP event templates and 2201 JMA event templates (Mw > 4.5). The hypocenter, original time, and magnitude error of BP events after temporal and spatial calibration are around 10 km, 5 seconds, and 0.5 respectively. Through the BPMF method, we double the size of the original JMA catalog from 223,897 events to 417,128 events, update the earthquake density and b value distribution map. The BPMF catalog highlight the seismicity near and outer the trench, which is barely seen in the original JMA catalog. Besides, the BPMF catalog filled the gap in the b value distribution map of JMA catalog, where the seismicity is too low to calculate the b value. The b value of the BPMF catalog is overall larger than JMA catalog and closer to the value 1.0.

Key Words
Backprojection, Matched-filter, offshore earthquakes

Citation
Feng, T., Meng, L., & Huang, H. (2019, 08). Detecting offshore earthquakes with backprojection and matched-filter method. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology