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Coda Envelope Moment Magnitudes and the Re-evaluation of Magnitude Conversion Relations for Seismic Hazard Assessment in Southeastern Canada

Although moment magnitude (M_W) is the preferred magnitude for hazard assessment, M_W for many Canadian earthquakes is derived from conversion relations. Previous research found that a time-dependent difference of about 0.1 magnitude units in the M_N-M_W conversion, where M_N is the Nuttli magnitude, resulted in a 15% difference in hazard for the Western Quebec Seismic Zone. The difference between pre- and post-1995 events was traced to a change in frequencies at which M_N was calculated. For over a decade, M_W has been calculated via regional moment tensor inversion for moderate to large earthquakes in eastern Canada but it is difficult to calculate M_W for events below M_W 3.5. Recently, the coda envelope moment magnitude method has been used to calculate moment magnitude (M_WC) for earthquakes in the active seismic zones of southeastern Canada since the mid-1990s for M_W as low as 1.5, below the threshold used in hazard assessment (~M_W 2.5). As a result, magnitudes for fewer events will require conversion for hazard assessment and the large suite of M_WC measurements allows for the re-evaluation of the M_N-M_W conversion with a much larger data set than was used for the current conversion relation. A comparison of M_WC to converted M_W values shows that the conversion relation still appears valid for the larger events but does not provide a good fit to the smaller ones. A re-evaluation of the relation is underway. There is some suggestion that different conversions should be used for different tectonic provinces (e.g. Shield vs. Appalachians). Seismic hazard in eastern Canada is based largely on past seismicity. The largest earthquakes are the ones of most concern for hazard but the smaller ones control the shape of the magnitude recurrence curve and a change in their population could affect the hazard values for some regions. An increase in the value of M_W for small earthquakes would increase the b-value and thereby decrease the rate of the largest earthquakes.

Session: Rethinking PSHA: Are We Using Appropriate Inputs for the End Goal? I

Type: Oral

Room: Grand A

Date: 4/21/2022

Presentation Time: 08:30 AM Pacific

Presenting Author: Allison L. Bent

Student Presenter: No

Additional Authors