Elsevier

Tectonophysics

Volume 356, Issues 1–3, 7 October 2002, Pages 5-22
Tectonophysics

Regional moment tensor determination in the European–Mediterranean area — initial results

https://doi.org/10.1016/S0040-1951(02)00374-8Get rights and content

Abstract

The broadband seismic network in the European–Mediterranean area provides high-quality data. We invert these regional three-component data for the source parameters of moderate-to-strong earthquakes in the entire European–Mediterranean area. Regional seismograms have a good signal-to-noise ratio even for moderate-sized events that are too small for teleseismic analysis. The magnitude threshold for source parameter determination can, thus, be significantly lowered. The threshold depends on the average event–station distances. Within dense broadband networks, we analyze MW≈3.0 earthquakes. In areas far from broadband seismic stations, the lower bound is MW≈4.5–4.8, still considerably lower than the teleseismic analysis threshold (about MW≈5.0–5.3). For larger events, we perform rapid moment tensor analysis using near-real-time data; solutions are posted within hours after event occurrence. In a second step, we merge near-real-time and later available data to obtain a regional moment tensor catalog of moderate-to-large earthquakes for the entire European–Mediterranean area. Within less than 1 year, we have analyzed 67 earthquakes ranging in size from MW=2.9 to 7.5. The solutions cover the seismically active areas of the European–Mediterranean area. Particularly important are solutions for slowly deforming regions where large earthquakes, that could be analyzed with teleseismic data, occur infrequently. The solutions are reliable: for events with independent source parameter estimates, the agreement is generally high. The solutions are robust: variations in epicentral parameters, source depth, or exact choice of stations do not affect source parameter estimates strongly. The moment magnitudes provide a unified estimate of earthquake size for the European–Mediterranean area. We perform regression analyses to link our moment magnitudes with local, body, and surface wave magnitudes.

Introduction

The location (hypocenter) and size (magnitude) of an earthquake are basic parameters in observational seismology. The distribution of earthquakes in terms of epicenter, depth, and size is also an important parameter for seismotectonic analyses and seismic hazard assessment (Giardini, 1999). The epicentral coordinates are usually relatively well determined by standard location procedures, while hypocenter depth and earthquake size are often only insufficiently well known. Moment tensor inversion (Gilbert, 1971) that uses the complete information contained in the seismograms can constrain hypocenter depth and seismic moment. In addition, moment tensor analysis reveals the deformation styles (fault plane solution).

For larger earthquakes globally (moment magnitude; Hanks and Kanamori, 1979, MW≥5.5), source parameters are routinely determined by the Harvard Centroid Moment Tensor (CMT) project Dziewonski et al., 1981, Dziewonski and Woodhouse, 1983 and by the United States Geological Service (USGS) Sipkin, 1982, Sipkin, 1986, Sipkin and Needham, 1993. These two methods use teleseismic waveforms and only stronger events that generate enough signals at distances several thousand kilometers from the source can be analyzed.

Analysis of moderate earthquakes (MW≥3.0) requires the use of local or regional data. At short epicentral distances, modern broadband seismic stations record complete seismograms with a high signal-to-noise ratio over a broad frequency band even for relatively small (MW≈3.0–4.0) earthquakes. At regional distances of up to about 2000-km epicentral distance, broadband stations can record MW≥4.5 earthquakes with a good signal-to-noise ratio particularly for the large-amplitude surface wave part of the seismograms. The growing network of broadband three-component stations (Fig. 1) offers the opportunity to routinely determine the earthquake source parameters (fault plane solution, source depth, and seismic moment) of moderate-to-strong earthquakes in the European–Mediterranean area.

Previous attempts to model European–Mediterranean earthquakes (e.g., Giardini et al., 1993, Braunmiller et al., 1994, Dufumier et al., 1997) using regional waveforms studied stronger events that in most cases could also be analyzed with teleseismic data. Here we determine moment tensor solutions for a large number of earthquakes too small for teleseismic analysis. Routine regional waveform analysis of moderate earthquakes is widely performed in the western United States Romanowicz et al., 1993, Thio and Kanamori, 1995, Braunmiller et al., 1995 where event–station distances commonly are on the order of hundreds of kilometers. In the European–Mediterranean area, average event–station distances are on the order of 1000–2000 km for several earthquake source regions, like northern Africa, Greece, and Turkey, hampering routine analysis. The only previous attempt for routine analysis covering the European area has been presented by Arvidsson and Ekström (1998).

Our goal is twofold. For larger earthquakes, we want to provide rapid moment tensor solutions within a few hours after an earthquake. Data sources are broadband stations accessible in near-real-time. The rapid solutions provide first reliable estimates of earthquake size, depth, and faulting style that could be of importance for disaster relief agencies, the general public, and Earth scientists. Our second goal is to build a moment tensor catalog for the entire European–Mediterranean area. The catalog should contain a large number of moderate events too small for teleseismic analysis. The source parameters have to be of high quality useful for detailed seismotectonic studies, seismic hazard analyses, quantification of earthquake size, and mapping of strain rates.

Section snippets

Data

Regional broadband seismograms are routinely available from several international (ORFEUS, Geofon, Geoscope, USGS, IRIS) and national (Austria, Germany, Israel, Switzerland) data centers and individual stations (TRI). Some data are rapidly available via AutoDRM Kradolfer, 1993, Kradolfer, 1996 (for example, from Austria, Czech Republic, Israel, and Switzerland), others are available within hours-to-days after a larger event (through ORFEUS, IRIS, Geofon, and ReNaSS), while data from most

Rapid analysis

As an example for rapid regional moment tensor analysis, we present our results for the MW=7.2 Dücze earthquake. The earthquake occurred in the early evening hours of November 12, 1999. The epicenter was located on the North Anatolian fault in western Turkey immediately east of the rupture zone of the devastating MW=7.5, August 17, 1999, Izmit earthquake. Several thousand people lost their lives in these two earthquakes and the destruction in the epicentral area was immense Delouis et al., 2000

Fault plane solutions

Moment tensor analysis covering only about 1 year of larger earthquakes already begins to reflect long-term seismicity in the European–Mediterranean area (Fig. 4). Most events occurred in Greece and western Turkey, the seismically most active areas in the study region (Jackson and McKenzie, 1988). Thrust-faulting mechanisms south of Crete and the Peloponnesus are associated with subduction at the Hellenic trench. Normal faulting events in Greece and southwestern Turkey are part of Aegean

Conclusions

The broadband seismic network in the European–Mediterranean area allows routine regional moment tensor analysis of moderate-to-strong earthquakes. Analysis of moderate (MW≈4.5–4.8) earthquakes is possible, because broadband stations can record such events with a good signal-to-noise ratio even at far regional distances of 1200–2000 km. At teleseismic distances signals are more attenuated limiting teleseismic source parameter determination to larger (MW≥5.0–5.3) earthquakes. Regional data thus

Acknowledgements

We thank the data centers and seismic station operators for providing high-quality broadband data that made this study possible. Namely, we thank the BGR (Germany), Geofon (Germany), Geoscope (France), GII (Israel), Gräfenberg (Germany), IRIS (USA), ORFEUS (The Netherlands), ReNaSS (France), USGS (USA), ZAMG (Austria) data centers, and the operators of station TRI (Italy). The people involved in setting up and running our own Swiss and the MIDSEA networks deserve our praise. Special thanks go

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