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1Department of Geophysical Engineering, University of İstanbul, Avcılar-Istanbul 34850, Turkey, 90-212-5938237/1037; oncel@hotmail.com.

²Geophysical Institute, University of Alaska, Fairbanks, AK 99775, United States, 907-474-5529; max@giseis.alaska.edu.

SUMMARY

We specify the most likely locations of main shocks in the Marmara Sea and compare their places to the rupture location of the M7.4 Izmit earthquake. We used the technique of mapping local recurrence time (TL) based on the microseismicity that is homogeneous for M_d³2.9 during 1983-1999. We mapped T_L in the area bounded by 40°-41° latitude and 27.6°-30.5° longitude. T_L is the probabilistic estimate of recurrence time, calculated from the a- and b-values of the frequency-magnitude relation of the seismicity within a radius of 20 km from every point on a grid with 5 km spacing. T_L varies strongly as a function of space, since a- and b-values also vary strongly. In our interpretation, the 5% to 20% of locations with the shortest recurrence times map major asperities. They are centred near 40.25°/29.4°, 40.8°/28.3°, 40.75°/28.8° and 40.7°/29.8°. The last two of these coincide with the western end of the rupture and the U.S.G.S. centroid location of the Izmit earthquake, respectively. Thus, we suggest that the major asperity of this rupture and a point, past which it could not propagate, were mapped out by the background seismicity during years before the event as locations that produced more large microearthquakes than average, and hence showed anomalously short T_L. The T_L method does not contain information about when earthquakes are expected, and the absolute values of the recurrence time could be inaccurate.
 
 

INTRODUCTION

Recently, the technique of mapping local recurrence time (T_L(M)) was successfully used to estimate asperities from the earthquakes within r=const of the location in question (5£r£20 km) (Wyss et al., 1999; Zuniga and Wyss, 1999). T_L can be estimated from the parameters of log-linear scaling:
 
 

log N= a - b                                                                                                                                                  (1)
 
 

where N is the number of earthquakes with magnitude M and larger, by
 
 

T_L(M) = dT/10^(a-bM)                                                                                                                                    (2)
 
 

Local changes of T_L(M) are related to the rate of activity (a parameter) and stress (b parameter) and may be used to map asperities. We define asperities as segments of fault planes resisting faulting more than its surrounding, as it is generally used in models for earthquake rupture (e.g. Wyss and Brune, 1967; Lay et al., 1982; Aki, 1984). The bulk a- and b-values of the main shock rupture areas lead often to over- and underestimates of the recurrence time for smaller and larger events respectively, whereas the asperities are found to exhibit the lowest values of b (b»0.5) and the lowest T_L (Wiemer and Wyss, 1997; Öncel and Alptekin, 1999).
 
 

MAPPING LOCAL RECURRENCE TIME

The raw earthquake catalogue was compiled by the Kandilli Observatory and the Earthquake Research Institute (KOERI), augmented by the Turkish national network and by local seismic networks, with good station coverage particularly since 1970 (Öncel et al., 1995). After declustering, using the algorithm of Reasenberg (1985) programmed in ZMAP (Wiemer, 1996), and correcting for a magnitude shift in 1990, the cumulative number of earthquakes reported in the catalogue for mapping T_L we used for the Marmara sea region (http://www.angelfire.com/al/geophysics) (Öncel and Alptekin, 1999).

For mapping T_L we used spatial subdivisions along the Marmara Sea consisting of fixed cylindrical volumes with radius r equal to 20 km and height h equal to 40 km. The centres of the cylindrical volumes are positioned at nodes with 5 km spacing throughout the region. The parameters a and b are then estimated from the events with M³ M_c for each volume by the gridding technique introduced by Wiemer (1996). Mapping of T_L(M) from the resulting matrix of a- and b-values can be computed from equation (2) for a given magnitude of the expected main shock. The map of local recurrence time (Figure 1) calculated by equation (2) must be based on a main shock magnitude on the same scale as the earthquake catalog. This is the M_D scale, on which the Izmit M_s=7.4 earthquake measures M_D=6.7 according to the Kandilli observatory.

We identify four areas of anomalously low T_L (Figure 1). (1) Area I, a segment along the Izmit bay, between 29.5° and 30.0° longitude, which is located adjacent to the moment centroid epicenter. In this area, T_L is estimated as about 1000 to 1500 years. (2) Area II, a segment of strongly anomalous T_L, coincides with the westernmost end of the 1999 Izmit rupture. (3) Area III, located at longitude 28.3° along the northern branch of the north Anatolian fault, is a weaker anomaly with estimates of about 500 to 1500 years for T_L. (4) Area IV is located between longitude 29.0° and 29.5° and has the shortest T_L estimated of about 500 years. It coincides with the largest earthquake in the catalog during the 17 years before the Izmit earthquake of August 1999.
 
 

CONCLUSION

We have shown that mapping the local recurrence time in the Marmara sea region identified as special the segment of the NAFZ near the centroid location and the western end of the Izmit M7.4 earthquake. This identification is based on the roughly two decades of seismicity before the 1999 Izmit event and had no input from the aftershock sequence. Therefore, we suggest that the mapping of T_L provides an important clue for the identification of fault segments which may initiate or stop future large ruptures.

Given the fact that the migration of large earthquakes along the NAFZ from east to west is well established (Ambraseys, 1970; Toksöz et al, 1979, Barka, 1992), and given that stress coupling is observed between the previous instrumental events in the region (Nalbant et al., 1998), we suggest that the areas II and III with anomalously short T_L are likely locations for future main shocks, or one rupture that may connect to two locations. If the latter should happen then the rupture length would be about 100 km, reaching from about longitude 28° to 29° along the NAFZ in the Marmara sea.

Since the method of using local recurrence time to map asperities is not extensively tested yet, our conjectures about the possibility of future earthquakes should be viewed with caution. Also, we have no way of knowing when future events in the asperities II, III and IV would have to be expected.

ACKNOWLEDGMENTS

This work was supported by the Research Fund of the Istanbul University under the project number1038/250897 and AIST in Japan, as well as NSF grant EAR 9902717 and the Wadati foundation at the University of Alaska Fairbanks.
 
 

REFERENCES

Figure 1: Map of local recurrence times, T_L, estimated probabilistically for an M_S=7.4 (equal to M_D=6.7) main shock, using the b- and a-values, respectively. Four areas of anomalously short T_L are labelled.