Investigation of fault zone head waves along the 1912 MW7.4 Şarköy–Mürefte (Ganos) earthquake rupture zone in NW Türkiye

Abstract

Fault Zone Head Waves (FZHW) are a key diagnostic tool to identify bimaterial interfaces along fault zones. We detect and analyse FZHW recorded in the waveforms from the local MONGAN (MONitoring of the GANos Fault) seismic network along the Ganos section of the North Anatolian Fault Zone, northwestern Türkiye, between October 2017 and July 2019. MONGAN covers the Ganos fault with different interstation distances ranging from 25 m to ∼4 km. To detect FZHW, an automatic detector is used as a preliminary analysis method followed by manual revision and particle-motion analyses to distinguish between FZHW and direct P waves. FZHWs are predominantly detected at the southern side of the fault. The observed FZHWs have a moveout (∆t) with respect to the direct P arrivals, increasing with distance travelled along the fault and indicating a deep bimaterial interface down to the bottom of the seismogenic crust. The average velocity contrast is estimated to be 5.9 per cent across the fault. Near fault-recordings indicate that the Ganos Fault is offset by ∼250 m with respect to the surface trace obtained from literature. To a lesser extent, FZHW are also observed in the northern stations from the fault, indicating a shallow wedge-shaped low-velocity portion constituted by highly fractured material to either side along the southwestern section of the Ganos Fault between the fast Eocene block to the north and the slow Miocene block to the south. The seismic velocity contrast and geological complexity have important implications for the rupture evolution during future earthquakes on the Ganos fault in that they would progress predominantly westward, away from Istanbul and Tekirdağ. Furthermore, an asymmetric aftershock distribution skewed to the northern block can be expected, with subsequent implications for site-dependent risk there. Our results allow to revise focal mechanism solutions by separating FZHW from direct-P wave for previous Sea of Marmara earthquakes.