Moment and duration of shallow earthquakes from Love-wave modelling for regional distances

Love waves from five earthquakes in southwest Germany, Italy, Yugoslavia, Greece and Algeria, as recorded at the broadband Gräfenberg array in eastern Bavaria, are modelled in detail using an extended reflectivity method. The focal mechanisms are taken from the literature or determined from long-period WWNSS data. The theoretical double-couple moment function used for modelling increases smoothly during the risetime T from zero to the seismic moment M₀. T and M₀ are determined by matching the observed and theoretical Love-wave seismograms in the long-period WWNSS frequency band. T is approximately equal to the rupture duration; values from 1.4 to 20 s are found. Seismic moments generally agree within a factor of about two with those found by other authors from larger data sets. The earthquakes in southwest Germany and Greece were simple events and can be modelled very well by one double couple. In the case of the other earthquakes, the theoretical Love-wave seismograms are too short. For these earthquakes there is some independent evidence for a multiple-event nature. This evidence is condensed into source models consisting of two or three double couples representing separate and complete sub-earthquakes, which then explain the observations quite well. In the case of the Algerian earthquake, the long Love-wave duration may also be due to complications in structure along the wavepath across the Mediterranean and the Alps. The present results show that accurate Love-wave modelling for a single station at regional distance from an epicentre is a reliable method for estimation of seismic moment and rupture duration. Use of a digitally recording station with large dynamic range allows events over a large magnitude interval to be studied. The Gräfenberg array is well suited for such an investigation of European and North African earthquakes.