Geomechanical model of the Marmara Sea region-II. 3-D contemporary background stress field

Abstract

We present the contemporary 3-D background stress field of the Marmara Sea region derived from a geomechanical model. The background stress field (i.e. the component of the absolute stress state that is time-independent over the seismic cycle) primarily depends on the distribution of density and elastic parameters, on the acting far-field stresses from plate boundary forces and on fault geometries. We take these into account to predict the 3-D background stress field including its spatial variations. Technically, our model concept involves the definition of an appropriate initial stress state accounting for the gravitational reference stress state of the crust, which is then changed by plate tectonics until the 3-D background stress field has evolved. The modelled stress field agrees well with observations from earthquake focal mechanism solutions and their formal stress inversion, with orientation of maximum horizontal stress and with the distribution of seismicity in the Marmara Sea. In particular in the vicinity of fault bends the stress field deviates considerably from the regional NW–SE oriented maximum horizontal stress and exhibits variability of the stress regime. Our model results are consistent not only with dynamic observations but also with kinematic ones. Various kinematic observations are understandable from the stress field. We show that a stress regime that indicates normal faulting in the basins is nonetheless reconcilable with almost pure strike-slip motion on the Main Marmara Fault. The distribution of seismicity in the Marmara Sea can be explained in first order by the distribution of critical differential stress, which is closely related to local fault geometries. We refer the wide absence of seismicity between the bend of the Main Marmara Fault near Istanbul and the Central basin to the relatively plane fault geometry of that segment. Normal stress on the Main Marmara Fault is highly variable along strike, which makes segment-wise rupture more likely than a rupture of the seismic gap at once. Comparably low normal stress on the Prince's Islands Segment indicates a shorter interevent time for this fault segment than for the central segment of the Main Marmara Fault.