Kinematics of the Hellenic Trench System from Earthquake Seismology and Field Observations

Abstract:

The kinematics of the Hellenic subduction zone have long been enigmatic; while GPS and seismological data suggest that subduction is mostly aseismic, there is reliable historical evidence for several large (M_W~8) earthquakes in the past. The origins of the Hellenic Trench System, a group of bathymetric escarpments within the overriding plate, are also widely debated. Since most of the region is underwater, earthquake seismology provides the best hope of understanding its kinematics, but reliable estimates of source depth are required to distinguish between earthquakes in the overriding and downgoing plates. We critically review previously assigned earthquake mechanisms and depths and (where data are available) use body-waveform modelling and array seismology to constrain them further, providing an updated catalogue of the seismicity of the Hellenic subduction zone since 1957. We also present new radiocarbon dates from uplifted paleoshorelines on the island of Rhodes, which show that its coast was uplifted in a large (M_W > 7.5) reverse-faulting earthquake around 4000 years BP. Seismological and reflection seismic data suggest that this earthquake and the one responsible for uplift of Crete in AD 365 occurred on steep reverse faults within the overriding lithosphere, presenting a resolution for the paradox of occasional large earthquakes in a region of predominantly aseismic subduction and also a plausible mechanism for the formation of the Hellenic Trench System. The presence of flights of Quaternary marine terraces suggests that uplift of these islands is ongoing, but it is important to distinguish between the effects of the reverse faulting and those of the widespread normal faulting throughout the Aegean; although it seems probable that uplift of Crete and Rhodes is related to reverse faulting, seismological and field evidence suggests that Karpathos (the island between them) is being uplifted as a horst block, as part of arc-parallel extension of the overriding plate.