Fault slip source models for the 2014 M_w 6.9 Samothraki‐Gökçeada earthquake (North Aegean Trough) combining geodetic and seismological observations

Abstract

The 24 May 2014, Mw 6.9, Samothraki‐Gökçeada shallow (depth: 11 km) earthquake along the North Aegean Trough (NAT), at the westward extension of the North Anatolian Fault Zone (NAFZ), is investigated using constraints from seismological and geodetic data. A point source solution based on teleseismic long‐period P and SH waveforms suggests an essentially strike‐slip faulting mechanism consisting of two subevents, while from a finite fault inversion of broadband data the rupture area and slip history were estimated. Analysis of data from 11 permanent GPS stations indicated significant coseismic horizontal displacement but no significant vertical or postseismic slip. Okada‐type inversion of horizontal slip vectors, using the new TOPological INVersion algorithm, allowed precise modeling of the fault rupture both as single and preferably as double strike‐slip faulting reaching the surface. Variable slip models were also computed. The independent seismological and geodetic fault rupture models are broadly consistent with each other and with structural and seismological data and indicate reactivation of two adjacent fault segments separated by a bend of the NAT. The 2014 earthquake was associated with remote clusters of low‐magnitude aftershocks, produced low accelerations, and filled a gap in seismicity along the NAT in the last 50 years; faulting in the NAT seems not directly related to the sequence of recent faulting farther east, along the NAFZ and the seismic gap in the Marmara Sea near Istanbul.

Pattern of dynamic displacements in a strike‐slip earthquake

“…P and S wave time arrivals (Figure 1b) were calculated assuming the P wave velocity at 6.5 km/s and S wave velocity at 3.7 km/s [Saltogianni et al, 2015] and were confirmed by GPS-derived velocities. Synchronization of strong motion and GPS time series was based on cross correlation-analysis.…”

“…This is because the high-gradient segment characterizing the first part of the strong motion, as in other near-field stations (018A and 089A in Figure 1b), is followed by additional oscillations. First, forward directivity in the propagation of seismic waves [Wang et al, 2007], perhaps amplified because of the two reactivated faults and of supershear rupture [Evangelidis, 2015], derived from the amplification of seismic intensities in the wider Çanakkale area (Figure 1a) [AFAD, 2014;Saltogianni et al, 2015]. First, forward directivity in the propagation of seismic waves [Wang et al, 2007], perhaps amplified because of the two reactivated faults and of supershear rupture [Evangelidis, 2015], derived from the amplification of seismic intensities in the wider Çanakkale area (Figure 1a) [AFAD, 2014;Saltogianni et al, 2015].…”

“…The 1 Hz data from an array of 15 GPS stations, nearly uniformly distributed around the fault at various azimuths and distances (20 to 200 km), were analyzed in order to model dynamic displacements, focusing on near-field areas with statistically significant (>1 cm) and on far-field areas with no significant (<1 cm) static displacements ( Figure 1a). These two areas are defined by contours which were computed using the double-fault model of Saltogianni et al [2015] and standard elastic dislocation techniques.…”

“…Inversion of coseismic displacements derived from low-rate GPS measurements, in combination with analysis of teleseismic data, was used to model with accuracy the finite fault pattern responsible for the 2014 earthquake [Saltogianni et al, 2015]. Two adjacent strike-slip faults with an average slip of 50 cm, cutting the crust from the depth of~20 km to the sea bottom along a total length of 80 km reactivated during this earthquake Figure 1a).…”

“…Characteristics of the dynamic displacements of the 2014 Samothraki-Gökçeada earthquake. (a) Location map showing GPS stations (red triangles), epicenter (star), faults (solid red lines), and contours of static displacements of 1 and 5 cm (black lines) based on the 2014 faults modeled by Saltogianni et al [2015]. Areas with maximum intensities based on strong motion records (shaded) are identified south and east of the seismic rupture (data from AFAD [2014]) (b) Amplitude of dynamic displacement during the strong motion as a function of time for selected points.…”