|Observatories and Research Facilities for EUropean Seismology|
|Volume 5, no 2||September 2003||Orfeus Newsletter|
Moment tensor determination for the Ibero-Maghrebian regionDaniel Stich, Jose Morales, Flor de Lis Mancilla and Gerardo Alguacíl
Instituto Andaluz de Geofísica, Universidad de Granada, Campus Universitario de Cartuja s/n, 18071 Granada, Spain
Earthquakes are rarely exceeding magnitude 5 on the Iberian Peninsula, northern Morocco, the westernmost Mediterranean Sea and the Atlantic Ocean south of Portugal. Twenty-four events altogether are included in catalogues of European or global routine moment tensor projects. This small set of moment tensor solutions can not be expected to sample adequately the tectonic deformation over this large area with several geotectonic domains and complex neotectonic evolution. For a more detailed seismotectonic picture in and around Iberia, we apply full-waveform analysis to obtain the source parameters of the more frequent small-to-moderate events. Analysis of smaller events requires the use of seismograms from the combined Iberian broadband networks and implies performing key parts of the data processing manually. Moment tensor inversion is then performed in a routine manner for all regional earthquakes with local magnitude >= 3.5 for the Iberian Peninsula, Northern Morocco and adjacent off-shore areas, and local magnitude >= 4.0 for Northwestern Algeria. To date, our catalogue contains eighty-four moment tensor solutions for the Ibero-maghrebian region (Figure 1), ranging in size from moment magnitudes 3.5 to 5.8 (http://www.ugr.es/~iag/tensor/, Stich et al., 2003a).
Figure 1. Moment tensor mechanisms available from the IAG moment tensor catalogue. Solutions are dense in the southeastern part of the Iberian Peninsula and in the Alboran Sea, and sample several other seismic zones over the Ibero-Maghrebian region as well.
Moment tensor inversion for small events in a geologically heterogeneous environment is not a trivial problem. The basic requirement for inversion is an appropriate set of broadband waveforms, possibly with good azimuthal coverage, and possibly including stations within near-regional distances and tectonically uniform travel-paths. Also, the inversion procedure cannot be fully automated. A crucial manual task is refining the natural distance-dependent amplitude weighting until obtaining a stable inversion result and adecuate waveform matches. This is generally an iterative procedure. Criteria to down-weight or exclude waveforms are high noise level or propagation along complex paths (where the Greens functions for plane layered media do not provide an appropriate correction). Traces may also be weighted to balance the station coverage over the focal sphere. Moment tensor estimates are usually sensitive to focal depth, and we combine the linear moment tensor inversion with a grid search over a range of depths to evaluate this non-linear effect. Based on the L2-misfit-vs.-depth curve and depth dependence of waveform matches, we select a most appropriate combination of depth and mechanism.
Once a moment tensor solution is obtained, it is double-checked by dislocation grid search modeling. This alternative way to invert magnitude and double-couple focal mechanism serves as a resolution test: We compute waveforms for the full ranges of dislocation source orientations and depths, and then compare them with the observed seismograms to identify the range of valuable mechanisms. The fault angle parameters strike, dip and rake are sampled every 10\272. Those double couple sources that produce significantly larger L2-misfits than the global minimum can be excluded (based on waveform matches, we consider 10% difference a conservative estimate). We usually observe equivalence of both, focal mechanism and quality of fit, for inversion and double couple grid-search, supporting the interpretation in terms of double couple force systems. Nevertheless, the non-double couple component (CLVD) included in the general deviatoric moment tensor may be helpful to absorb effects of uncorrected propagation, noise and finite sources.
With our magnitude criteria, we selected hundred-ninety events since autumn 1995, when the permanent broadband networks probably reached a minimum standard for regional time domain inversion for magnitudes Mw >= 4. For seventy-seven events, we obtained moment tensor solutions that adequately fit the main characteristics of the regional waveforms and passed the grid-search resolution test. With seven solutions for events in the 1980s, using data from the temporary NARS experiment, this amounts to a catalogue of eighty-four moment tensors to date. Recent improvements of broadband network coverage contribute to higher success rates. For the 18 months period November 2001 to April 2003, moment tensor solutions were obtained for twenty-four earthquakes, i.e. ~50% of all events with local magnitude >= 3.5. This is noticeable exceeding the amount of eight solutions that provided routine moment tensor projects on the European scale for the same period. Recent moment tensors include multiple event solutions for the seismic sequences in August 2002 in Bulla, Murcia, SE-Spain (MW<=5.0), January 2003 in Zamora, NW-Spain (MW<=4.2) and February 2003 at the Alboran Ridge (MW<=4.8). Currently, sufficient waveform data for the combined Iberian networks become available within weeks after an earthquake. Our inversion results, waveform fits, and grid-search analysis are posted on-line at http://www.ugr.es/~iag/tensor/.
Two small to moderate earthquakes (MW=4.1 and 4.2) occurred within 5 months at a distance of ~75 km from each other in Sevilla (September 15th 2002, 020915) and Cordoba (January 24th 2003, 030124) provinces in western Andalusia. For these epicentral locations we have good azimuthal station coverage, and we used a selection of near-regional broadband waveforms that are available through Geofon (MTE, SFS, CART), IRIS (PAB) and from several IAG stations (SELV, ANER, ARAC, SESP). The inverted moment tensor solutions match the waveforms well in a pass band from ~15 to 35s (Figure 2), except for effects of un-modeled local earth structure at SFS (event 020915) and ANER (event 030124). ANER is a nodal station for the Cordoba focal mechanism, as confirmed by small amplitudes in the vertical component, but waves with transverse polarization are projected onto the radial component because earth structure is not plane layered at this part of the coast. Large amplitudes at the horizontal components of SFS may be attributed to local amplification in the sedimentary environment (this has been observed for several events, Stich et al, 2003a). These traces have been excluded from inversion (weight zero), as well as the radial component of CART (event 030124) with high noise level. For both events, moment tensor solutions are well resolved according to dislocation grid-search modeling.
The mechanisms are fundamentally different from each other: The Sevilla earthquake shows predominately reverse faulting with P-axis azimuth of N17°E, while the Cordoba earthquake is almost pure strike-slip, with P-axis orientation of N283°E, nearly perpendicular to the Sevilla event. Particularly for the Sevilla quake, results are different from the nearest moment tensor mechanisms at distances of about 100km for the Granada Basin, Gulf of Cadiz and Gibraltar regions, and also from the average trend for the Iberian Peninsula. The occurrence of focal mechanisms that are severely rotated relative to the regional reference orientation suggests heterogeneous tectonic stresses on a local scale, and points to fault interaction.