The Seismicity of Indonesia

Indonesian seismicity provides important insights into the tectonics and hazards of a region that is characterized by a remarkable diversity in faulting, including subduction, extension, thrusting, and strike-slip faulting. We present a synthesis of Indonesian seismotectonics by documenting the distributions of hypocenters (≥ M 4.6) and focal mechanisms (≥ M 5.0) over ~20 years, quantifying seismicity rates, and comparing observed seismicity trends with proposed tectonic models. Of the 20,622 events ≥ M 4.6 observed in the study region, ~77% of seismicity are shallow (≤ 70 km depth) and of magnitudes ≤ M 5.0 (68%). 61 events ≥ M 7.0 occurred, five of which exceeded M 8.0, including the 2004 Mw 9.1 Sumatra-Andaman earthquake. Regionally, about ~320 ≥ M 5.0 earthquakes occur per year, and rates decrease exponentially between 50-300 km with significantly elevated seismicity in the Mantle Transition Zone (MTZ). Intermediate and deep events (≥ 70 km depth) trace the Wadati-Benioff zones of several subducting slabs exhibiting a geometry consistent with recent tomography models. Seismicity extends to a maximum depth of 678 km. Oblique convergence, lithospheric age, ambient mantle temperatures and viscous resistance at the 410, 520, and 660 km phase boundaries likely contribute to the non-uniform depth distribution of intermediate and deep earthquakes. Shallow seismicity provides insight into how complex oblique convergence is accommodated near the surface, with primary sources including megathrusting, crustal faulting, and shallow intraslab faulting. All sources of shallow seismicity constitute significant seismic hazards.