ALBERT

Description: 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 (sub W) . For 34 events in and near Switzerland, we determined seismic moment (thus M (sub W) ) by regional waveform inversion. Independent M (sub W) estimates for the same events do not exist; however, M (sub W) from European-Mediterranean events, obtained in the same way, agree with M (sub W) from Harvard CMT solutions. All other size estimates, M (sub L) , M (sub D) , m (sub b) , M (sub S) , and intensities, are calibrated relative to these 34 events. Teleseismic M (sub S) and m (sub b) from international data centers are directly regressed against M (sub W) . Most observations in ECOS consist of local magnitudes (M (sub L) , M (sub D) ) and intensities. For local magnitudes, we first calibrated the Swiss Seismological Service's M (sub 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 (sub S) for stronger twentieth century Swiss earthquakes from analog seismograms. These M (sub S) , which were converted to M (sub W) , connect intensities and M (sub W) . After calibration, all 20,300 events in ECOS have a unified M (sub 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 (sub W) 6.9 Basle earthquake.

Description: Since 2003, 39 small earthquakes have been detected off the coast of central Oregon in the nominally locked part of the Cascadia subduction zone, where very little seismic activity has been recorded in spite of a paleoseismic record of great subduction events. Although the regional earthquake bulletin reports depths of 29 and 28 km for the two largest events (M (sub w) 4.9 and M (sub w) 4.7, which occurred in 2004), analysis by Trehu et al. (2008) indicates that they were low-angle thrust events that occurred on the plate boundary at depths of 9-11 and 16 km, respectively. Because of sparse onshore station coverage, most of the smaller events have large location uncertainties. Double-difference relative location of 30 of these earthquakes reveals two tight clusters approximately 30 km apart; each cluster is associated with one of the two larger events. Within each cluster, relocation reduces the hypocenter depth spread from 〉15 km to 〈3 km, with uncertainties on the order of 0.1 km. The relocations, combined with independent absolute hypocenter locations for two 2008 events using a deployment of land and ocean-bottom seismometers, suggest that the seismicity occurred at plate boundary depths, possibly on the Cascadia megathrust. This concentrated activity in the seismogenic zone may represent patches on the fault plane with anomalous frictional characteristics, possibly caused by subducted topographic features, which can affect the propagation of a large megathrust rupture.