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Mw From Coda Envelopes for Small Earthquakes in Utah

Session: Network Seismology: Keeping the Network Running While Integrating New Technologies [Poster]

Type: Poster

Date: 4/22/2021

Presentation Time: 11:30 AM Pacific

Description: 

Moment magnitude (Mw) is now routinely assigned to moderate to large earthquakes (M>3.5) by most regional seismic networks. However, the regional waveform modeling techniques used to calculate Mw often can not reach the short periods required for earthquakes M<4, and nearly impossible for M<3. Calculation of Mw for earthquakes M<4 is desirable in order to have a uniform Mw catalog to accurately calculate earthquake catalog statistics for downstream seismic hazard calculations. Here, we calculate Mw utilizing the well-established coda envelope method (Mw,coda) for three regions and magnitude ranges in Utah: 1) the whole of Utah for Mw>3.5, 2) the Magna sequence in Northern Utah for 1.5<=Mw<=5.7, and 3) southern Utah for 1.5<=Mw<=3.5. For all three regions, we start with a calibration based on waveform modeled Mw and apparent stress from the 2008 Wells, NV sequence. The Wells sequence is one of the few in the region that has previously-calculated apparent stress values, which allow for a more accurate calibration. For southern Utah, we also attempt a calibration to a local earthquake sequence. Mw,coda for earthquakes in the whole Utah calibration match well with Mw from moment tensor inversion for the same events. For the Magna sequence, most of the events are too small for moment tensor inversion, but Mw,coda is consistent with another small Mw technique, Mw,direct, which uses the spectra of direct phases and coda. We investigate an apparent bias in the Wells-calibrated Mw,coda between northern and southern Utah. Finally we compare catalog statistics from catalogs composed of Mw,coda, ML, and those converted to a uniform Mw using regional magnitude scaling relationships to see how this direct measurement of Mw,coda affects b-values and other parameters. This work is part of our expansion of the coda envelope method in Utah with the ultimate goal of including Mw,coda in routine University of Utah Seismograph Stations earthquake monitoring and reporting.

Presenting Author: Katherine M. Whidden

Student Presenter: No

Authors