Comparison of U.S. Geological Survey and Montana Bureau of Mines and Geology Epicenters and Magnitudes for Recent Western Montana Earthquakes

Session: Regional Earthquake Centers: Highlights and Challenges
Type: Oral
Date: 4/28/2020
Time: 05:00 PM
Room: 215 + 220
Description: 

The MBMG Montana Regional Seismic Network and the USGS NEIC both provide solutions for western Montana earthquakes. Here, I compare epicenters and magnitudes for 86 recent Montana earthquakes M >= 3 computed by both MBMG and USGS, with solutions reported in the ComCat. This comparison reveals that 83% of the epicenters differ from each other, including reported location uncertainties, and 12% differ by 5 or more km. Sixty-four percent of USGS epicenters lie SE of MBMG epicenters, implying a systematic bias. Each of the other three compass quadrants contain 16% or fewer of the epicentral difference directions. Several factors imply greater precision of MBMG epicenters. The reported uncertainty for USGS epicenters averages 3.1 km, while MBMG uncertainties average 0.5 km. The root mean square of travel time residuals for MBMG events is two to five times smaller than for USGS events. For areas with multiple events (2017 Lincoln earthquake aftershocks and the ongoing Manhattan swarm), MBMG epicenters cluster more tightly than USGS epicenters. I attribute the epicenter differences to MBMG’s analysis process of: using only readings from stations within 280 km of an event; using a velocity model specific to western Montana; and a single analyst picking all events. A systematic difference also exists between USGS and MBMG magnitudes. A least-squares fit of 66 USGS and MBMG M_Ls indicates that MBMG reports M_Ls consistently 0.2 units larger than USGS M_Ls. Comparing 20 events with USGS M_Ws and MBMG M_Ls also indicates that MBMG M_Ls are consistently about 0.2 units larger than USGS M_Ws. Although the cause of this systematic difference is unknown, it warrants investigation.

This comparison of USGS and MBMG events shows a systematic difference in both locations and magnitudes. These preliminary results indicate the need for systematic reviews of earthquake parameters generated from a national perspective. Identifying the analytical reasons for these differences will be a first step.

Presenting Author: Michael C. Stickney

Authors