The major asperities of the 1999M_w = 7.4 Izmit earthquake defined by the microseismicity of the two decades before it

Abstract

Summary We compare the rupture location of the Mw 7.4 Izmit earthquake to the local seismic hazard estimated by the technique of mapping local recurrence time, TL, based on the microseismicity. After correcting for a magnitude shift in 1990, the declustered earthquake catalogue, produced by the University of Istanbul for the Marmara Sea region, is homogeneous for Md ≥ 2.9 during 1983–1999. We mapped TL in the area bounded by 40°–41° latitude and 27.6°–30.5° longitude. TL 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. TL varies strongly as a function of space, since a‐ and b‐values also vary strongly. In our interpretation, the 5–20 per cent of locations with the shortest recurrence times map major asperities. In the Marmara region, we mapped four anomalies of short TL, together covering about 12 per cent of the total area. 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 epicentre 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 the years before the event as locations that produced more large microearthquakes than average, and hence showed anomalously short TL. The TL method does not contain information about when earthquakes are expected, and the absolute values of the recurrence time could be inaccurate. The method only specifies the most likely locations of main shocks. Since the method is new, it will have to be tested for many cases and in many areas before its reliability can be assessed.

Seismic behaviour of the North Anatolian Fault beneath the Sea of Marmara (NW Turkey): implications for earthquake recurrence times and future seismic hazard

“…3) verify high seismic activity in the region (Crampin and Ü çer 1975;Dewey 1976;Ayhan et al 1984;Ambraseys and Finkel 1987;Kalafat et al 2000;Ö ncel and Wyss 2000;Ö zalaybey et al 2002).…”

“…In spite of some successes, the concept is indicated to be still in the testing process (Wyss and Matsumura 2002). Spatial mapping of FMD over the surface area bounded by 40°-41°N latitude and 27.6°-30.5°E longitude in the Marmara region has been previously implemented by Ö ncel and Wyss (2000) using the background M D C 2.9 microseismicity data covering the time period post-1983 and prior to 1999 _ Izmit mainshock. They obtained strong spatial variability of b values in the range 0.9-1.6 in the Sea of Marmara.…”

“…Based on FMD, they estimated shortest local recurrence time (T L ) value of 500 year over the anomalously low b value areas in the western Sea of Marmara. In a later study using the post-1981 and pre-1999 _ Izmit mainshock background M D C 2.9 seismicity Utkucu et al (2007) computed spatial variability of b values over the same map area selected by Ö ncel and Wyss (2000). Depending on the varying data volumes selected for computations, Utkucu et al (2007) obtained b values in the range 0.80-0.84 and 1.90-2.04.…”

Physically based probabilistic seismic hazard analysis using broadband ground motion simulation: a case study for the Prince Islands Fault, Marmara Sea

“…Current investigations have definitely confirmed that the majority of the long-term fault slip occurs along the offshore section of the northern fault segment following the northwest striking Prince Island Fault (PIF) combined with the eastwest striking Central Marmara Fault (CMF) immediately south of Istanbul (Le Pichon et al 2001;Armijo et al 2005;Ergintav et al 2014). Together with seismological and seismotectonic studies (Gürbüz et al 2000;Oncel and Wyss 2001;Oncel and Wilson 2006;Kalafat et al 2011), recent GPS and kinematic studies (Meade et al 2002;Flerit et al 2003) have indicated that the northern branch of the NAFZ is more active and also creates much larger motion than the southern branch. This hypothesis has also been verified by historical period (Ambraseys and Jackson 2000;Ambraseys 2001aAmbraseys , b, 2002a, instrumental period (Kalafat et al 2011), and recent detailed microearthquake seismicity studies (Sato et al 2004;Bohnhoof et al 2013).…”

Caspian sea level change impacts regional seismicity

“…It is conventional that the global average of b-value is approximately one (e.g. Oencel and Wyss, 2000) but it has not been extended to some cases of seismicity such as induced seismicity (Nuannin, 2006) and volcanic areas (Schorlemmer et al, 2003). The regional variations of b-value are found to have a correlation with stress level variations.…”