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Estimating Earthquake Rupture Directivity Using Surface Wave Empirical Greens Functions: How Low Can You Go?

Session: Earthquake Science, Hazards and Policy in Cascadia [Poster]

Type: Poster

Date: 4/20/2021

Presentation Time: 11:30 AM Pacific

Description: 

Earthquake rupture directivity is a parameter that has important applications for understanding earthquake impacts. There are many techniques used to evaluate directivity and slip distribution, most of which require dense seismic networks and high-quality digital data. One relatively simple method involves the use of empirical Green's functions with regional and teleseismic surface waves. This has been successfully applied to large (typically M7-8+) earthquakes in the past. As a few examples where rupture directivity and slip distribution have successfully been estimated include the 1992 M7.3 Landers, CA earthquake, the M7.8 Haida Gwaii earthquake of 2012, and the 1992 M6.8 earthquake offshore British Columbia.

In this study, we examine the application of this technique to a smaller (M6.4) offshore Vancouver Island earthquake. This 2014 event was well-recorded by a temporary OBS array and has a well-determined aftershock pattern and focal mechanism – making it an ideal `calibration event`.

For the surface wave EGF analysis we used a nearby Mw 5.3 earthquake as the primary EGF source. To improve SNR we applied stacking of relative source time functions. We considered a Mw 4.8 aftershock as a secondary EGF source. We used broadband seismic data from 105 regional and teleseismic stations in our analysis. The relative source time functions we obtained show an overall rupture direction of 143 ± 6° and extent of 28 ± 2 km. This is in good agreement with the double-difference aftershock relocations (using both onshore and offshore data) that indicated a 32 ± 2 km unilateral rupture with strike of 146 ± 2° and the centroid moment tensor with a nodal plane striking 150 ± 6°.

By demonstrating that this surface wave technique works for smaller (M~6.4) earthquakes, it provides confidence that we can examine historic moderate earthquakes (that have well-recorded surface waves, but otherwise limited datasets) to better assess seismic patterns, active faults, and rupture directivity.

Presenting Author: John F. Cassidy

Student Presenter: No

Authors