Moment tensor inversion with single-component historical seismograms:: The 1909 Benavente (Portugal) and Lambesc (France) earthquakes

Moment tensor inversion of early XX century mechanical seismograph recordings offers the opportunity to estimate source parameters for a number of important earthquakes. This knowledge is crucial for understanding regional tectonics and estimating seismic hazard; but processing of the historical seismograms is often difficult. Especially, the rotation of horizontal seismograms into radial (P-SV) and transverse (SH) components tends to fail; for example, if one polarity is wrong, if the alignment of the seismograms is not correct, or if one component is lost. To avoid these kind of problems, we use a moment-tensor-inversion scheme that processes the original single-component horizontal traces without rotation, and rotates the theoretical Green's functions instead. We show how this approach simplifies the analysis, and allows a better use of available data. We present moment tensor solutions for two destructive earthquakes in southwestern Europe in 1909. The April 23 earthquake near Benavente in the lower Tagus valley (Portugal) had a moment magnitude M-W= 6.0 and an estimated centroid depth of 10 km. Our preferred moment tensor solution indicates reverse faulting (nodal planes have strike/dip/rake of N51 degrees E/52 degrees/83 degrees and N242 degrees E/38 degrees/99 degrees). We propose a blind thrust beneath the Tagus valley sediment basin as the responsible fault. The June 11 earthquake near Lambesc, Provence (France) was found to be slightly smaller (M-W= 5.5) and related to oblique reverse faulting at 4-km depth. Nodal planes of the preferred solution have strike/dip/rake of N80 degrees E/53 degrees/53 degrees and N311 degrees E/50 degrees/128 degrees, subparallel to the Trevaresse fold.