Velocity Structure of the Mosha Fault, Northern Iran, from Local Earthquake Tomography

Abstract:

Convergence between the Arabian and Eurasian plates leads to compression between the Alborz mountain belt and the Central Iran plateau. The Central Alborz region, part of this convergent system, is characterized by high seismic hazard due to major active faults, which have generated large historical and instrumentally recorded earthquakes. Tehran, capital of Iran, is located at the southern edge of the Central Alborz and is surrounded by active faults (e.g., Mosha, North Tehran and Parchin faults). Knowledge about the crustal velocities could therefore provide insight about the active fault structures and geometries. We obtained a 3D P-wave velocity model of the Central Alborz using first arrival travel times from more than 2000 micro-earthquakes that were recorded by the Iranian Seismological Center, the International Institute of Earthquake Engineering and Seismology, the Tehran Disaster Mitigation and Management Organization, and temporary networks. After applying strict data selection criteria and precise picking of first arrivals, the nonlinear, iterative, damped least-square program SIMULPS was used to obtain the 3D velocity structure. Perhaps a most remarkable result is the apparent shallowing of the upper-to-mid crustal boundary that coincides with the relatively aseismic part of the eastern Mosha fault, which, others have argued, has the potential to produce an earthquake with the magnitude greater than 6. Independent gravity modeling reveals higher densities in the anomalous region relative to the adjacent area at mid-crustal depths in agreement with the tomographic results.