Stress Drops and Directivity of Induced Earthquakes in Western Canada Sedimentary Basin

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

The Western Canada Sedimentary Basin (WCSB) has experienced an increase in seismicity during the last decade due to primarily hydraulic fracturing. Understanding the ground motions of these induced earthquakes is critical to characterize the increase in hazard. Stress drop is an important parameter in this context because it is a measure of the high-frequency content of the energy released. We use the Empirical Green’s Function (EGF) method to investigate corner frequencies and stress drops of 80 events of M2.3-4.4 in the WCSB. The EGF method is an effective technique to isolate the source effects of a target earthquake since it does not require assumptions on path and site. Instead, it extracts the source component by dividing out the path and site components in the frequency domain, using a smaller, collocated earthquake as an EGF. The corner frequency of the source displacement spectrum of the target event is solved for assuming a Brune spectral shape, and the stress drop is computed from the corner frequency.

The average stress drop observed for induced earthquakes in the WCSB is 7.2 MPa, in agreement with similar studies conducted for induced events in Oklahoma. We find that there are clear directivity effects for many events, despite their small-to-moderate size. For about half of the events, corner frequency varies by as much as a factor of 5 with azimuth. Thus directivity is an important consideration when estimating stress drop for events having large azimuthal gaps in the station coverage. We model such effects where evident, using a Haskell source model assuming unilateral rupture. By placing constraints on source parameters, the results of this study are useful to reconcile the source, path and site contributions to observed ground motions in the WCSB.

Presenting Author: Joanna M. Holmgren

Authors