|Title||Estimating earthquake rupture directivity using surface wave empirical Greens functions: how low can you go?|
|Author||Paul, C; Cassidy, J F|
|Source||2021 Annual Meeting, Seismological Society of America, technical sessions; Seismological Research Letters vol. 92, no. 2B, 2021 p. 1349-1350, https://doi.org/10.1785/0220210025|
|Alt Series||Natural Resources Canada, Contribution Series 20200699|
|Publisher||Seismological Society of America|
|Meeting||Seismological Society of America 2021 Annual Meeting; April 19-23, 2021|
|Media||paper; on-line; digital|
|Province||British Columbia; Western offshore region|
|Area||Vancouver Island; Pacific Ocean|
|Lat/Long WENS||-132.0000 -124.0000 52.0000 48.0000|
|Subjects||geophysics; tectonics; Science and Technology; Nature and Environment; Health and Safety; seismology; earthquakes; aftershocks; plate motions; structural features; faults; seismic data; seismic waves;
seismic arrays; Methodology|
|Program||Public Safety Geoscience Assessing Earthquake Geohazards|
|Released||2021 04 01|
|Abstract||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.
|Summary||(Plain Language Summary, not published)|
Details of earthquake rupture - especially directivity - control earthquake ground shaking (including strength of shaking and frequency content). We use
a recent well-recorded M6.4 earthquake offshore Vancouver Island as a `calibration event` to evaluate the use of distant seismic surface waves to determine details of earthquake rupture. We compare our results with the known earthquake focal
mechanism and aftershock distribution and find excellent agreement. This demonstrates that this technique is useful for earthquakes as small as M6.4 in this region, allowing us to evaluate historic earthquakes here. This in turn will improve our
mapping of offshore earthquakes, and earthquake hazards in southwestern British Columbia.