|Observatories and Research Facilities for EUropean Seismology|
|Volume 2, no 1||March 2000||Orfeus Newsletter|
Application of Regional Moment Tensor InversionRami Hofstetter 1, Gonca Örgülü2, Mustafa Aktar2,3
Although localization of earthquakes are done for many years, since the beginning of the 20th century, only recently the source parameters of strong earthquakes are calculated on a semi-routine basis. Analysis of seismic moment tensors at regional distance is more challenging than at teleseismic scale, due to the strong effect of crustal and upper mantle heterogeneity on seismograms. Nevertheless, this kind of analysis is important for studying smaller events, in a lower magnitude range (4<M<5.5) and which occur frequently. The recent availability of broadband stations covering seismically active regions is the basis for dedicated research in this field. Below we briefly describe applications of the RMT inversion that are independently implemented in Turkey and in Israel.
Synthetic seismograms are computed with a frequency-wavenumber integration code (Saikia, 1994) which is a hybrid one consisted of two algorithms: Filon and Bouchon Integration algorithms. They are calculated for each path between source and station using simple 1-D velocity models for different source depths. The synthetic seismograms are compared with the observed long-period data to find the best fitting double-couple solution. Comparison of the inversion solutions based on several velocity models shows that the inversion results are not strongly model dependent for short propagation paths to near-regional stations.
Figure 1. Broadband stations (squares) located in western Turkey and medium-band stations (triangles) around the Sea of Marmara. The epicenter of the aftershock (black star) and its source mechanism is shown on the map as well as the location of the Izmit and Duzce earthquakes (red star).
An inversion example is given for an aftershock (30 August 1999, M = 5.2) of the Izmit earthquake (Fig. 2) located within close distance of the epicenter of the mainshock. The well fitting between long-period data and synthetic seismograms provides right-lateral strike-slip mechanism having a strike direction of N76E slightly different than the general character of the North Anatolian Fault Zone. A similar solution is provided using a different methodology by Braunmiller et al., 2000 within this issue.
Figure 2. Comparison of three-component long-period (20-50s) displacement data (solid) and synthetic seismograms (dashed) for the aftershock (M = 5.2) that occurred on 31 August 1999.
Additional inversion solutions, among others, the 9/20/1999 earthquake, located at the north of the Marmara Island in the Sea of Marmara, are provided on the Kocaeli web page of KOERI.
Figure 3. Broadband stations (squares) that were used in the waveform inversion for the November 22, 1995 event (red star).
Figure 4. Comparison of three-component long-period (20-50s) displacement data (solid) and synthetic seismograms (dashed) for the 22 November 1995, M = 4.4, event in the Gulf of Aqaba.
In conclusion, we showed that the above-described method can be used, either manually or semi-automatically, to provide useful results for better understanding the seismotectonic setting of a given region. In the future, as the the mean distance between permanent broadband stations is decreased, the RMT solutions will be increased quantitatively as well as qualitatively.
Dreger D., and Helmberger, D., 1993. Determination of Source Parameters at Regional Distances with Single or Sparse Network Data, J. Geophys. Res., 98, 8107-8125.
McKenzie, D., 1970. Plate tectonics of the Mediterranean region, Nature, 226, 239-243.
Montessus de Ballore, F., 1906. Comte de 1851, les treblements de terre, Paris, Colin.
Quennell, A. M., 1958. The structural and geomorphic evolution of the Dead Sea rift, Quart. J. Geol. Soc, 114, 1-24.
Saika, C., 1994. Modified Frequency-Wavenumber Algorithm for Regional Seismograms using Filon~ Rs Quadrature: Modeling of Lg waves in Eastern North America, Geophys. J. Int., 118, 142-158.