Evaluation of ML-MC as a depth discriminant in Yellowstone, USA and Italy

Recent work has shown that the difference between two magnitude scales, ML (local Richter magnitude) and MC (coda/duration magnitude), acts as a depth discriminant in Utah. Shallow seismic sources, such as mining induced earthquakes and explosions, have strongly negative ML-MC values, while deeper tectonic earthquakes have ML-MC values near zero. These observations imply that ML-MC might be effective at discriminating small explosions from deeper natural earthquakes at local distances. In this work, we examine seismicity catalogs for the Yellowstone region and Italy to determine if ML-MCacts as a depth discriminant in these regions as well. We identified 4,780 earthquakes that occurred in the Yellowstone region between Sept. 24, 1994 and March 31, 2017 for which both ML and MC were calculated. The ML-MC distribution is well described by a Gaussian function with a mean of 0.102 and a standard deviation of 0.326. We selected a subset of these events with accurate depths and determined mean ML-MC values in various depth bins. An event depth was considered accurate if the formal depth error was less than 2 km and either (1) the nearest station was within one focal depth or (2) the distance to the nearest station was smaller than the bin size. We find that ML-MC decreases as event depths become shallower than about 10 km. Similar to the results for Utah, the decrease is statistically significant and is robust with respect to small changes in bin size and the criteria used to define accurate depths. We used a similar process to evaluate whether ML-MC was a function of source depth for 63,555 earthquakes that occurred between April 16, 2005 and April 30, 2012 in Italy. The ML-MC values in Italy are also well described by a normal distribution, with a mean of -0.477 and standard deviation of 0.315. We again find a statistically significant decrease in ML-MC for shallow earthquakes. In contrast to the Yellowstone results, for Italy ML-MC decreases at a nearly constant rate as focal depths change from 30 km to 4 km, and then it flattens out for the shallowest events. Our results reinforce the idea that ML-MC acts as a depth discriminant, but because it appears to behave slightly differently in the two regions, more areas need to be evaluated in the future.peer-reviewe

On the performance of ML-MC as a depth discriminant for small seismic events recorded at local distances in Yellowstone, Oklahoma, and Italy

A recent study by Koper et al. (2016) found that the difference between local magnitude (ML) and coda duration magnitude (MC) successfully distinguished shallow seismic events (mining blasts, mining-induced seismicity, and shallow tectonic earthquakes) from deeper seismic events (tectonic earthquakes) in the Utah region and could therefore be helpful for blast discrimination. Here we present tests of the performance of ML-MC as a depth discriminant in three regions and show that it is effective in all of them. Initially, we investigated ML-MC as a function of depth for seismicity in and around Yellowstone National Park recorded by the University of Utah Seismograph Stations. For 2,845 Yellowstone earthquakes with well-constrained depths varying from 0-25 km, we found that ML-MC decreases 0.036 ± 0.014 magnitude units (m.u.) per 1 km in depth over the depth range of 0-8 km. Then, we examined ML-MC values for anthropogenic seismicity recorded by the National Earthquake Information Center in northern Oklahoma and southern Kansas. We found that for 1,692 events with well-constrained depths, the slope of ML-MC for the shallowest 10 km in depth is 0.037 ± 0.016 m.u. per 1 km depth. Finally, we analyzed ML-MC for 28,721 well-located earthquakes in Italy and Sicily recorded by Istituto Nazionale di Geofisica e Vulcanologia. This region showed an increase of 0.017 ± 0.001 m.u. per 1 km depth, up to 30 km in depth. In each case, the quoted error bounds represent 99% confidence regions. We performed several robustness tests in which we varied the depth bin size, the criterion used to define a well-constrained depth, and the depth range used in the linear fit. In nearly all cases we found a positive slope for ML-MC vs. depth at a confidence level above 99%. Our results provide further evidence that ML-MC is useful as a depth discriminant for events recorded at local distances in different physiographic regions.peer-reviewe