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

Abstract

S U M M A R YMoment 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 • E/52 • /83 • and N242 • E/38 • /99 • ). 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 • E/53 • /53 • and N311 • E/50 • /128 • , subparallel to the Trévaresse fold.

Intensity-distance attenuation laws for the Portugal mainland using intensity data points

“…The epicentre location was estimated at 38.9 • N, 8.8 • W, using the available macroseismic information, by Kárník (1969). From different studies, based on the seismic moment estimation, the moment magnitude, Mw, was estimated between 6.0 and 6.2 and Ms close to 6.3 (Teves-Costa et al 1999Dineva et al 2002;Stich et al 2005). Stich et al (2005) determined a focal mechanism of a reverse faulting.…”

“…From different studies, based on the seismic moment estimation, the moment magnitude, Mw, was estimated between 6.0 and 6.2 and Ms close to 6.3 (Teves-Costa et al 1999Dineva et al 2002;Stich et al 2005). Stich et al (2005) determined a focal mechanism of a reverse faulting. This is in agreement with other focal mechanisms calculated for this region (Borges et al 2001).…”

Los terremotos del Bajo Valle del Tajo (Portugal): Lisboa 26 de Enero de 1531 y Benavente 23 de Abril de 1909

“…A throughout search for original records of the event showed that many of them were lost or were destroyed. Nowadays, combined efforts of Teves-Costa et al (1999, Dineva et al (2002) and Stich et al (2005) to locate records of the Benavente earthquake allowed to recover 31 potentially useful seismograms of this event (Table 1). Data sources include original smoked paper recordings and photographic contacts as well as photocopies or reproductions of seismograms in journals.…”

“…Four centuries later occurred another strong earthquake in the LTV on 23 April 1909 (Mw = 6.0, estimated by Stich et al 2005). It produced large damage in several small villages located in the eastern bank of the Tagus River, as Samora Correia, Benavente and Santo Estevão (Figure 1).…”

“…It is for this reason that even today the 1909 earthquake remains very significant, evidencing the seismic activity in the LTV and contributing in a very important way for the regional seismic hazard and risk assessment (Fonseca and Vilanova 2010). Recently, several studies have been performed to better understand its source mechanism, as well as the seismic attenuation processes in the western part of the Iberian Peninsula (Stich et al 2005, Cabral et al 2011, Teves-Costa and Batlló 2011.…”