On the portability of ML-MC as a depth discriminant for small seismic events recorded at local distances

In this paper we show that M_L-M_C is a viable and regionally portable depth discriminant and therefore may contribute in nuclear test ban treaty verification. A recent study found that the difference between local magnitude (M_L) and coda duration magnitude (M_C) discriminates shallow seismic events (mining blasts, mining-induced earthquakes, and shallow tectonic earthquakes) from deeper tectonic earthquakes in the Utah region. The shallow seismic events had anomalously high M_C values, with increasingly negative M_L-M_C values as depth decreased. Here we evaluate the performance of M_L-M_C as a depth discriminant in three new regions, finding that M_L-M_C increases between 0–9 km depth in all cases. Initially, we investigated M_L-M_C as a function of depth for naturally occurring earthquakes in the region around Yellowstone National Park, as recorded by the University of Utah Seismograph Stations. For 3,358 Yellowstone earthquakes with well-constrained depths, we found M_L-M_C increased 0.030 ± 0.007 magnitude units (m.u.) for each 1 km increase in depth up to 10 km depth. Next, we examined M_L-M_C values for anthropogenic seismicity in northern Oklahoma and southern Kansas, as recorded by the National Earthquake Information Center. For 1,628 events with well-constrained depths, we computed a slope for M_L-M_C of 0.022 ± 0.010 m.u./km. Finally, we analyzed M_L-M_C for 28,722 well-located earthquakes in Italy, as recorded by the National Institute of Geophysics and Volcanology, and found an M_L-M_C slope of 0.018 ± 0.001 m.u./km. In each case, the quoted error bounds represent 95% confidence regions which exclude zero, implying that the depth-dependence of M_L-M_C is statistically significant. We performed several robustness tests in which we varied 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 M_L-M_C vs. depth at a confidence level above 95%.