A transportable scale for central Europe and implications for low-magnitude discrimination

Abstract

SUMMARY We have extended the mb(Lg) method of Nuttli using root-mean-square (rms) amplitudes corrected for noise and a Δ−1 dependence for geometrical spreading. Lg waves recorded on the German Regional Seismic Network (GRSN) for earthquakes in south-central Europe were used to develop an mb(Lg) formula requiring a new calibration constant Crms to keep rms mb(Lg) on the same baseline as Nuttli's traditional formula based on 3rd-peak amplitudes. GRSN stations had to be calibrated for site terms and for Lg attenuation. Lateral variations in LgQ appear to be significant across the study area, and a regional Q model consisting of constant-Q partitions north, south and in the central Alps was developed using measurements based on interstation and two-event, single-station methods. When plotted against surface wave estimates of Mw, rms mb(Lg) measurements in central Europe are found to be consistent with Mw–mb(Lg) relationships for north America and southern Asia, thus supporting the transportability of our mb(Lg) formula. Frequency–wavenumber processing of Grafenberg Array data enabled us to extract Rayleigh waves for small events, and regional Ms were measured using the Marshall and Basham formula. Our Ms–mb(Lg) relationship extends to Ms 2.5 and agrees well with observations in other regions including the western United States. The discrimination potential of Ms–mb(Lg) observations was examined under realistic monitoring conditions, where path corrections were inferred from earthquake data and applied uniformly to natural sources and explosions. Under these conditions, mb(Pn, P) are greater than mb(Lg) for large NTS explosions; however, Ms–mb scaling slopes are steeper for P waves than they are for Lg, and Ms−mb observations for NTS explosions converge near mb 4. Thus, allowing for measurement errors and additional uncertainty in mb(Pn) due to regional bias, there is little difference in the discrimination potential for Pn and Lg waves at small magnitudes. As such, a regional Ms−mb discriminant based on Lg might be preferred owing to the better detectability of Lg waves for small earthquakes. These results need to be confirmed for explosions at other test sites. Compared to teleseismic experience, regional Ms−mb observations extend the discrimination capability to lower magnitudes by at least one Ms unit.