Homogeneous Moment-Magnitude Calibration in Switzerland

Abstract

An earthquake catalog containing a uniform size estimate is important for long-term seismic hazard assessment in regions of low-to-moderate seismicity. During the update of the Earthquake Catalog of Switzerland (ecos), we performed regression analyses to convert all earthquake size information in ecos to physically meaningful moment magnitude M w. For 34 events in and near Switzerland, we determined seismic moment (thus M w) by regional waveform inversion. Independent M w estimates for the same events do not exist; however, M w from European-Mediterranean events, obtained in the same way, agree with M w from Harvard CMT solutions. All other size estimates, M L, M D, m b, M S, and intensities, are calibrated relative to these 34 events. Teleseismic M S and m b from international data centers are directly regressed against M w. Most observations in ecos consist of local magnitudes ( M L, M D) and intensities. For local magnitudes, we first calibrated the Swiss Seismological Service’s M L. Then we calibrated magnitudes from observatories in neighboring countries (France, Germany, Italy) using only events in the border region (e.g., France–Switzerland). Modern instrumental records exist only since the mid-1970s. We calibrated the macroseismic dataset, which represents by far the largest period in the catalog, by determining surface wave magnitude M S for stronger twentieth century Swiss earthquakes from analog seismograms. These M S, which were converted to M w, connect intensities and M w. After calibration, all 20,300 events in ecos have a unified M w, including a class-type uncertainty estimate based on the original magnitude scale. ecos covers the period 250–2001, from 44° N to 51° N and 4° E to 13° E. The largest event in ecos is the 1356 M w 6.9 Basle earthquake.