Shallow structural deformation associated with the 1956 Amorgos Earthquake, Aegean Sea - an investigation from 3D seismic reflection data

The Santorini–Amorgos Tectonic Zone in the South Aegean Sea is a major hotspot for marine geohazards, where strong earthquakes, pronounced deformation, and tsunamis interact within an actively extending back-arc setting. The 1956 tsunamigenic Mw 7.5 Amorgos earthquake stands out as the largest instrumented earthquake in the region during the 20th century. While previous focal mechanism analyses have provided a good characterization of the seismogenic source as a NE-striking extensional rupture, little is known about the shallow deformation occurring within the upper kilometer below the seafloor. This shallow deformation associated with this large normal-fault earthquake is of fundamental importance for investigating tsunami triggers.

Previous interpretations of 2D seismic, bathymetric, and ROV data provided first-order insight into the regional tectonic framework, but the geometry and segmentation of the fault system could not be fully characterised due to the sparsely spaced profiles. Here, we present newly acquired high-resolution 3D seismic data, integrated with detailed seafloor mapping to unravel the shallow structural configuration and deformation of the southwestern part of the Amorgos Fault Zone, close to the epicentral area of the 1956 earthquake.

Detailed seismic interpretation and seismic attribute analysis reveal distinct segmentation of the shallow part of the fault system and spatially heterogeneous shallow deformation. Our analyses are aimed at shedding light on the specific shallow rupture patterns that triggered the tsunami and, in particular, determining why there was strong regional variability in tsunami run-up heights reported along the surrounding coasts. Our work will help to improve the understanding of how large normal fault ruptures can generate hazardous tsunamis.