Focal properties of the MW=6.5 Skyros, Aegean Sea, earthquake
N.S. Melis1, G.N. Stavrakakis1, J. Zahradnik2|
1National Observatory of Athens, Institute of Geodynamics, Thisio, GR-118 10, Athens, Greece
2Faculty of Mathematics & Physics, Charles University, V Holesovickach 2, CZ-180 00, Prague, Czech Republic |
Introduction -
Seismicity in the Skyros region and the July 26, 2001 event
Focal mechanism from amplitude spectra
Conclusions -
Acknowledgements -
References
Introduction
On July 26, 2001 at 00:21 GMT a strong (moment magnitude MW=6.5) earthquake about 30 km NW of the island of Skyros in the Aegean Sea. The earthquake location is only about 130km NNE of the heavily populated Greek capital Athens. Fortunately, the event caused no fatalities. Approximately 100 houses, mainly old traditional dwellings in the capital of Skyros, were damaged. The nearly 1000 year old monastery of St George the Arab, located inside the main town castle, was heavily damaged. Some rockfalls and landslides were also observed.The earthquake and its aftershock sequence have been recorded by the Greek National Network and were located by NOA (National Observatory of Athens). NOA also determined the earthquake magnitudes; the main event had a surface wave magnitude MS=5.8 and a local magnitude ML=5.3. After the main shock, NOA quickly assembled information on the NOA webpage for the interested public.
The purpose of this study is to understand the basic focal properties of the Skyros main shock using newly available broadband data from the Greek National Network. Another goal of our study is to demonstrate rapid investigation and result dissemination capabilities of an ongoing seismic sequence, utilizing readily available regional network data; these capabilities are useful for the PRESAP project (EU project - EVG1-CT-1999-00001).
Seismicity near Skyros and the July 26, 2001 event
A number of moderate to strong events has been reported for Skyros and adjacent regions during the 20th century. The strongest earthquake occurred about 60km NE of the July 2001 epicentral area close to the island of St Eustratios (MS=6.8 March 4, 1967). Seismicity for the last 3.5 years prior to the July 2001 sequence was low and the area experienced seismic quiescence (Chouliaras and Stavrakakis, 2001). The main event was preceded by three significant foreshocks that occurred on July 21, 2001 (ML=4.1 and ML=4.6) and on July 25, 2001 (ML=4.2). The Skyros main event was followed by a large number of aftershocks. The NOA broadband network recorded over 300 events within two months after the main event with magnitude ML=3.5. The strongest aftershock (ML=4.9) took place on July 30, 2001. The epicentre distribution of the aftershock sequence is elongated in a NNW-SSE direction of about 50km length; we interpret this length as the fault zone area activated by the Skyros main event, but the fault length of the main shock itself was estimated at about one-half of this value.
Figure 1. Distribution of epicentres of the Skyros aftershock sequence. Shown are events with ML=3.5 that occurred less than two months after the main event. Focal mechanisms from first motion data are shown for the main event, two foreshocks and the strongest aftershock.
Focal mechanism from amplitude spectra
The focal mechanism of the main event was determined with the ASPO method, based on amplitude spectra of complete 3-component waveforms and first motion polarities (Zahradnik et al., 2001; Zahradnik, submitted). Waveforms of nine broad-band stations were considered (Figure 2). Eight, VLS, JAN, KZN, RDO, PRK, APE, ATH, ITM are telemetered stations of NOA, equipped with 20s LE-3D/20s Lennartz sensors. One additional station SER (Sergoula), is a stand-alone station equipped with a 100s CMG3-T Guralp sensor. Specific webpages of NOA and Charles University include the exact station co-ordinates and allow a down-load of the most significant SER records for the present sequence.
Figure 2. Broadband stations recorded the Skyros event in Greece. With red triangle NOA stations, green triangle SER (Sergoula) station. Star denotes the main shock epicentre.
All records are instrument corrected, re-sampled to time increment of 0.02 sec, high-pass filtered (frequencies f > 0.05 Hz), and rotated into R (radial), T (transversal), Z (vertical), and integrated to displacement. The first-motion polarities, carefully read from three component seismograms at all stations provide additional constraints on the focal mechanism. Projecting polarities on the focal sphere is a delicate problem (Zahradnik et al., 2001). To avoid unrealistic take-off angles of the first arrivals, formally interpreted as head waves from inter-crustal discontinuities, we use a gradient model GMF. GMF is an approximation of the layered model MF, recently obtained for travel paths from northwestern Turkey to Greece by inversion of Love wave dispersion (Novotny et al., 2001), see Figure 3. The take-off angles in the gradient model were calculated with the ray-method code ANGGRA (Jansky, 2001).
Figure 3. Crustal models used in this study. The gradient model GMF is an approximation of the homogeneous layer model MF. Both models have the same Moho discontinuity at 33km depth.
With the ASPO method, we analyze the displacement amplitude spectra of complete 3-component waveforms (duration of 160 seconds), in the frequency range 0.05-0.08 Hz (below the corner frequency). The observed spectra are compared to synthetic spectra calculated with the discrete-wavenumber method (Bouchon, 1981; Coutant, 1989) using model MF. For a set of trial source depths, we perform a systematic 10 degrees grid search for the strike (0o-360o), dip (0o -90o), and rake (0o-180o), that best fit the synthetic spectra. A grid search 0o-180o degrees for the rake is sufficient, since solutions with rake R and R-180o have the same amplitude. Scalar moment affecting the spectra linearly is not searched, but estimated from the ratio between the observed spectra and the unit-moment synthetic spectra. For details, see Zahradnik (submitted). The best fitted focal mechanism is then used to calculate the synthetic seismograms.
We started with all nine stations, but the synthetic seismograms could not fit the nearest station with a very good signal-to-noise ratio, ATH (133 km). Then we tested several station sub-sets and the best results (unique misfit minimum and good fit to ATH station) were obtained for the amplitude-spectra inversion from three stations: ATH (133 km, azimuth 205o), PRK (167 km, 83o) and APE (244 km, 155o).
The misfit between the observed and synthetic amplitude spectra (sum from all stations, components, and frequencies) for the three NOA stations is plotted against the sequential number of the strike-dip-rake trial (Figure 4). The least misfit values that range from the minimum to 1.05 times the minimum misfit - the range is used to measure solution uncertainty - are marked by blue crosses. Only two minima fall in the error range, one with strike = 150o, dip = 70o, rake = 10o, and its conjugated solution 57o, 81o, 160o. Such a unique solution is quite an exception compared to ASPO applications for other earthquakes; the present solution is very well constrained by the available data. The ASPO method also compares the observed and calculated first-motion polarities. The red diamonds in Figure 4 mark the fault-plane solutions that are consistent with all nine P-polarities and, at the same time, have a spectral amplitude misfit between min and 1.05*min. The amplitude-preferred solution and the first motion polarities are entirely consistent (coincidence of the blue crosses and red diamonds), which is again a very rare case.
Figure 4. Misfit function of the grid-search ASPO modeling for the mainshock. The trial number on the horizontal axis refers to the sequential number of the systematic search (triple loop) over the strike, dip and rake.
The result in Figure 4 is for a focal depth of 8 km, which (together with a depth of 9 km) has the lowest amplitude-spectra misfit of the focal depths tested, see Figure 5.
Figure 5. Variation of the amplitude misfit with the focal depth for the mainshock.
For a focal depth of 8 km, and the above focal mechanism, the ratio between the observed amplitude spectra and the unit-moment synthetic spectra (averaged over the frequency range 0.05-0.08 Hz) yields the scalar seismic moment of M0 = 4.1·1018 Nm (corresponding to moment magnitude MW = 6.5). Our final result, strike = 150o, dip = 70o, rake = 10o, M0 = 4.1·1018 Nm, obtained from regional data, is in good agreement with moment tensor solutions determined with teleseismic (USGS strike = 145o, dip = 85o, rake = 4o, M0 = 5.4·1018 Nm and Harvard strike = 148o, dip = 71o, rake = -1o, M0 = 5.7·1018 Nm) and regional data of relatively distant (mostly > 1000 km) stations (Swiss Seismological Service strike = 148o, dip = 73o, rake = 0o, M0 = 8.7·1018 Nm). Our final result is also close to our fast preliminary determination from eight NOA broad-band stations and 17 NOA polarities, which provided strike = 170o, dip = 70o, rake = 20o, M0 = 4·1018 Nm (NOA webpage). Solving the forward problem and comparing the synthetic and observed displacement waveforms, we find that the final solution provides a better fit than the preliminary one (Figure 6). The main problem of the preliminary solution was its failure to explain the high-quality ATH record. The preliminary solution reverted the 'sign' of ATH's prominent wave group. As seen in Figure 6 the final solution is not ideal, in particular for stations VLS, KZN, RDO, which implies the need for further refinement of the crustal model.
Figure 6. Band-pass filtered displacement transverse components (blue), compared to synthetics of the preliminary (black) and final (red) ASPO fault-plane solution. For some stations the black and red curves coincide with each other. Numbers to the right of the traces indicate their peak values (in m).
Conclusions
The Skyros earthquake was modeled using regional broad-band stations in the distance range 133-341 km. The non-standard, recently developed ASPO method inverts amplitude spectra of complete waveforms (0.05-0.08 Hz) and utilizes also the first-motion polarities to determine the earthquake focal mechanism. The best results were obtained for inverting spectra from the three nearest NOA stations (ATH, PRK, APE), complemented by the polarities from all nine stations. The Skyros main event had a focal mechanism of strike = 150o, dip = 70o, rake = 10o, and scalar moment 4.1·1018 Nm (moment magnitude MW = 6.5). The preferred centroid depth is 8 km. The solution was confirmed by the forward waveform modelling. The epicentre distribution of the aftershock sequence and the focal mechanisms indicate that the rupture plane has a NNW-SSE strike and that motion on the fault was left-lateral strike slip. The NNW-SSE trending plane has also been identified independently as the fault plane by finite-source synthetics (for details see http://seis30.karlov.mff.cuni.cz).Ackowledgements
Thanks are due to Torild Van Eck and an anonymous reviewer for critically reading this paper and for their comments that helped to improve the initial manuscript. This study was supported by the following grants: PRESAP EU project EVG1-CT-1999-00001, NATO Collaborative Linkage grant EST.CLG.976035; and several research projects in the Czech Republic - MSMT J13/98-113200004, ME354, and GACR 205/00/0902. GMT (Wessel and Smith, 1995) was used to produce some of the diagrams.References
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