Aftershock Activity of the 1999 Izmit, Turkey, Earthquake Revealed from Microearthquake Observations

Abstract

An accurate aftershock distribution of the 1999 İzmit, Turkey, earthquake was obtained by using the data from a local seismic network, IZINET, and 10 temporary seismic stations. More than 2000 aftershocks were relocated for the period of about 2 months following the mainshock. From this aftershock distribution we obtained several pieces of information on the characteristics of the mainshock. First, the mainshock initiated fault rupture from a place adjacent to an active swarm area where many microearthquakes had been occurring for more than 20 yr prior to the mainshock. Second, the aftershock region extended in the east-west direction along the North Anatolian Fault Zone (NAFZ). This confirms that the mainshock was caused by a slip on the NAFZ. Third, the western end of the rupture caused by the mainshock is likely to have reached up to about 29.2Њ E in the İzmit Bay, and hence the total length of the fault rupture caused by the mainshock amounts to about 150 km, as long as the estimate of the fault rupture length is based on the aftershock distribution. This information is important for the discussion on the possibility of future large earthquakes in the west of the source region of the İzmit earthquake. We also found a clear tendency that aftershocks occur in clusters, which implies strong heterogeneity in both the rupture process and the medium along the fault zone.

Spatial variation of the stress field along the fault rupture zone of the 1999 Izmit earthquake

“…The August 17, 1999 Izmit mainshock rupture initiated at the eastern end of the Gölcük segment at a depth of about 17 km (Barış et al, 2002;Ito et al, 2002; and propagated westward along the Gölcük segment and eastward along the Sapanca segment (Yagi and Kikuchi, 2000). An approximately 10-km-long aseismic zone between the Yalova and Gölcük segments is observed in the aftershock distribution map (Fig.…”

“…The aftershocks in Fig. 1 were obtained by Ito et al (2002), and those in Fig. 4 are obtained by the relocations performed in this study.…”

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

“…Hereafter the seismic stations belonging to MARNET and the national seismic network are referred to as KOERI. Another network called IZINET was installed in the eastern part of the Marmara region in August 1992 with six seismic stations, in order to monitor the microearthquake activity and provide the data of high quality in this region (Ito et al, 1994;Bariş et al, 1996a, b;Bariş et al, 2002;Ito et al, 2000Ito et al, , 2002. It was installed in the east of MARNET in view of increasing earthquake risk in the western part of the NAFZ (Toksöz et al, 1979).…”

“…Because of insufficient network coverage, however, the distributions of earthquakes and seismic stations in and around Marmara region have not been good enough for a study of 3-D velocity structure. In the meantime, tomography studies were performed by Kuleli et al (1995) and by Nakamura et al (2002) for some parts of the Marmara region. In particular, Kuleli et al (1995) attempted a tomography of P-wave velocity for the deeper structure of the Aegean Sea region including the western part of the Marmara Sea, whereas Nakamura et al (2002) evaluated P-wave structure of the aftershock area of the 1999 Izmit earthquake.…”

“…In the meantime, tomography studies were performed by Kuleli et al (1995) and by Nakamura et al (2002) for some parts of the Marmara region. In particular, Kuleli et al (1995) attempted a tomography of P-wave velocity for the deeper structure of the Aegean Sea region including the western part of the Marmara Sea, whereas Nakamura et al (2002) evaluated P-wave structure of the aftershock area of the 1999 Izmit earthquake. However, their data sets are limited and both the grid size and the resolution were rather poor.…”