Stress interactions of three moderate size earthquakes in Afyon, southwestern Turkey

Abstract

We analyze three moderate size earthquakes occurred in the Afyon region of southwestern Turkey on December 15, 2000 ( M w = 6.0) and February 3, 2002 ( M w = 6.5 and M w = 5.8). This earthquake sequence took place on the Sultandağ Fault having dominantly normal faulting mechanisms with different orientations in an extensional tectonic regime. We relocate these mainshocks and their large aftershocks and use the waveform data to determine the rupture dimensions and directivities. We carry out both static and dynamic stress analysis to investigate stress interactions and possible earthquake triggering mechanisms in this sequence. We show that the 2002, M w = 6.5 rupture started at the termination of the 2000, M w = 6.0 earthquake, where the calculated Coulomb stress changes increased more than 0.5 bar. Thus, we conclude that the coseismic stress increase by the 2000 event favored the nucleation and promoted the timing of the 2002 event. An intense seismic activity, identified as the Kızıldağ aftershock zone, developed following both of the large mainshocks where the third largest earthquake ( M w = 5.8) of the sequence occurred 2 h after the 2002, M w = 6.5 mainshock. The Coulomb stress calculations of the 2000 earthquake shows no influence in this zone while the static Coulomb stress changes due to the 2002, M w = 6.5 mainshock raised the Coulomb stresses an additional 0.3–0.7 bar in the Kızıldağ aftershock zone. Thus, we infer that such changes in the Coulomb stresses positively influence the occurrence of the aftershocks in this zone. Since the Kızıldağ aftershock zone takes place in a geothermal area, we compute dynamic stress changes imposed by the 2000 and 2002 mainshocks. The computed peak dynamic stress levels for both mainshocks are found much higher than the static stress changes calculated within the Kızıldağ aftershock zone. Thus, we propose that transient stress triggering, most likely involving elevated fluid pressures in the hydrothermal systems present in this region, may have played a major role in the initiation of increased seismicity followed by local, multiple elastic stress triggering leading to the further development of aftershocks and the occurrence of the M w = 5.8 mainshock.