Homogeneous moment-magnitude calibration in Switzerland

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 Mw) by regional waveform inversion. Independent Mw estimates for the same events do not exist; however, Mw from European-Mediterranean events, obtained in the same way, agree with M, 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 (ML, MD) 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 degrees N to 51 degrees N and 4 degrees E to 13 degrees E. The largest event in ECOS is the 1356 M-w 6.9 Basle earthquake.