Three-dimensional structure of Vp, Vs and Vp/Vs in the upper crust of the Marmara region, NW Turkey

Abstract

We applied a 3-D seismic tomography inversion algorithm to arrival-time data obtained, during 18 years from 1985 to 2002, from local seismic networks and aftershock studies in the Marmara region, in order to better understand the upper crustal structure of the complex tectonic region. We integrated all the available data set into a common data set and relocated the events, using a 1-D velocity model. We then selected 3,949 earthquakes and obtained 92355 arrival times, in total, consisting of 59,313 P-wave and 33,042 S-wave arrival times. In this paper we present detailed crustal structures for V p, V s and V p/V s ratios from the surface down to 15 km depth with good resolution in terms of the hit count analysis of seismic rays, the checkerboard and restoring resolution tests for the studied region. The results obtained from the inversion suggest that the western part of the North Anatolian Fault Zone shows strong lateral heterogeneity. We concluded that no clear pattern exists between the distribution of microearthquakes and aftershocks and the velocity perturbations presented in this paper. This is probably due to complex tectonic and geological structures. Large coseismic slip associated with the two recent strong earthquakes was found to correspond to higher velocity anomalies, as was often found recently. On the other hand, low velocity values correspond to the sedimentary units or the alluvium regions, as supported by the low resistivity and gravity values. Thus the results presented in this paper are quite consistent with the other data such as gravity, resistivity and magnetic anomalies, indicating that our model is reliable and efficient and it should be useful for further interpretation of tectonic and geological problems in this region. It should be pointed out, however, that the results of S-wave perturbations and V p/V s ratio could not be discussed in detail because of insufficient quality of the S-wave data and the reliability of the results is not very high. Nonetheless, the fact that recent large Izmit earthquake and moderate aftershocks that occurred in or around the high velocity zones near the low velocity region suggests that high velocity regions found in the area close to Istanbul in the Marmara Sea and also at the Iznik-Mekece fault are potential sites for strain energy accumulation and release.

Evidence for a bimaterial interface along the Mudurnu segment of the North Anatolian Fault Zone from polarization analysis of P waves

“…Despite considerable differences in resolved scales, it is informative to compare results from those studies with those derived in the present paper. Tomography images of Nakamura et al (2002) and Baris et al (2005) indicate that the crust in northwestern Turkey has a velocity contrast over the top ~7 km of up to 6%. The Vp and Vs perturbations confirm that the Mudurnu segment represents a boundary between a faster block to the south and a slower block to the north.…”

Shear wave splitting as a proxy for stress forecast of the case of the 2006 Manyas-Kus Golu (<i>M</i><sub>b</sub> = 5.3) earthquake

“…Relocations and the magnitudes are calculated according to Pand S-waves readings. The duration dependent formula is useful for small and local earthquakes (Bariş et al, 2005). V p /V s = 1.73 value is chosen for routine locations of local seismic events.…”

The Local Earthquake Tomography of Erzurum (Turkey) Geothermal Area

“…These active tectonic units give rise to geothermal energy resources (Haklidir 2015). Previous studies such as Baris et al (2005), Ozer and Polat (2017a, b), Ozer et al (2018) indicate that geothermal fluids are born in the crust by thermal conduction, particularly in volcanic and tectonically active areas. Furthermore, Kocyigit and Canoglu (2017) claimed that the Erzurum pull-apart basin is also a geothermal field, but this aspect of the basin has not been studied yet (Fig.…”