|Observatories and Research Facilities for EUropean Seismology|
|Volume 2, no 1||March 2000||Orfeus Newsletter|
Surface-wave Centroid Moment Tensors in the Mediterranean region: the MEDNET-Harvard projectA. Morelli1, G. Ekström2, S. Mazza1, S. Pondrelli1, E. Boschi1, A. M. Dziewonski2
In the Mediterranean area, however, moderate-energy (4.5 < Mw < 5.5) seismicity is particularly important because it is widely spread, and more common than the relatively infrequent larger-magnitude events. Small or moderate earthquakes are impossible to model at teleseismic distance with the classical CMT method, owing to the low signal to noise ratio of the long period body waves used. We resort, then, to modelling surface waves, which exhibit much higher amplitudes, in a modified CMT algorithm. Besides showing prominently on seismograms, surface waves can also be modelled at closer distance, thereby further decreasing the magnitude threshold of the analysis when seismographs are appropriately available at local and regional distance. The method uses detailed surface wave phase velocity maps (Ekström et al., 1997) and is described in Arvidsson and Ekström (1998). Here we briefly review some aspects of the implementation, show some recent applications, and discuss our plans for future activity.
Figure 1. Waveform fit for the surface wave Centroid Moment Tensors. Left panel shows local and regional seismograms (epicenter to station distance ranging from 0.8 to 14.4 degrees) for the smallest of the events analyzed (Mw=4.2). Right panel shows seismograms at larger distances (13.4 to 16.4 degrees) where dispersion of surface waves allows to discriminate between the dominant fundamental mode and overtones (from Morelli et al., 1999).
We use data from available stations at local, regional, and teleseismic distance. Earthquakes with magnitude 5.5 and above are most conveniently recorded and modelled at a global scale, with the standard CMT technique, and are routinely analyzed at Harvard. We are instead normally interested in events smaller than 5.5. Our magnitude threshold may reach 4.2 in the best instrumented areas, but varies depending on data availability.
The surface wave regional centroid moment tensor calculation is very fast. Its speed allows rapid calculation of source mechanisms, a feature of great importance for scientific and relief operations following an earthquake. For this reason, we also analyze strong earthquakes in a rapid manner, depending on the quasi-real time availability of data from a number of seismographic stations. For the determination of rapid RCMTs we rely on data recorded at MedNet (Mediterranean Network) stations accessible by telephone dial-up or the Internet. Long period seismograms are automatically extracted in nearly-real time by the MUSCLES system (Mednet Unmanned Stations CalLer for Extraction of Seismograms), by calling MedNet stations in the occurrence of a seismic event (Mazza et al., 1998). MUSCLES is launched by an e-mail reporting an earthquake, and it is based on five unix shell scripts running independently from each other every minute. When available, we also use data from other seismographic stations, reachable through the ORFEUS or IRIS Spyder® systems.; We estimate the availability of local and regional data generally sufficient to grant approximate completeness for events with magnitude equal or greater than 4.5.
Figure 2: Regional moment tensor solutions for years 1997 and 1998 (Pondrelli et al., 1998). The map also shows, in blue, RCMTs for some older events. The map shows 73 solutions, in red, in the 2-year period, with magnitude between 4.5 and 5.5, which had not been previously analyzed on a global scale.
Figure 3: Regional moment tensor solutions for events of the 1997-98 Central Italy earthquake sequence (Ekstrom et al, 1998; Morelli et al, 1999). Labels indicate month and day of occurrence of events. The radii of the focal spheres are proportional to the moment magnitude. Compressional quandrants are shaded in different colors to distinguish between 1997 and 1998 events.
Being based on a modification of the CMT scheme, our RCMT solutions have similar characters and, for earthquakes whose size allows both to be computed, the two procedures yield compatible and very similar results, as shown in Figure 4. Note that, for each event, the two solutions shown have been computed with different sets of stations, at different distance ranges, and modelling different parts of the seismogram. The level of agreement is also significant as an empirical estimate of stability to possible bias in the data or due to simplifying assumptions in the theory.
Figure 4: Comparison of regional CMTs (top) and standard Harvard CMTs (bottom) for the largest events of the 1997-98 Central Italy earthquake sequence. Full moment tensors are plotted by the red areas, thin lines show best fitting double couple mechanisms. Labels identify event dates (from Morelli et al, 1999).
Figure 5: Centroid moment tensor solutions for earthquakes of the 1976-1977 Northern Italy seismic sequence (after Pondrelli et al., 1999). PO is the Periadriatic overthrust, SF the Sequals fault, and TF the Tricesimo thrust fault. Event locations from Piromallo and Morelli (1998). The size of focal mechanisms is proportional to moment magnitude.
All RCMT determinations, once revised and improved by modelling of off-line data retrieved through ORFEUS or the IRIS DMC, are collected in a regional catalog of moment tensors. The catalog contains considerably more solutions, as nearly-real time data are only available for larger magnitude events and for selected seismographic stations. The catalog is being organized for publication, and will be regularly updated. The Mediterranean regional catalog of seismic moment tensors will shortly be also hosted on a web site, now under construction.
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Pondrelli, S., E. Boschi, A. M. Dziewonski, G. Ekström, S. Mazza, A. Morelli, and C. Piromallo, 1998, Regional Centroid Moment Tensors of the Mediterranean area and their tectonic implications, IUGG99 Abtract Book, A, 169.
Pondrelli, S., Ekström, G., and Morelli, A., 1999, Seismotectonic re-evaluation of the 1976 Friuli, Italy, seismic sequence, Journal of Seismology, in press.