Earthquake swarms reveal submarine magma unrest induced by distant mega-earthquakes: Andaman Sea region

• Several earthquake swarms followed Indian Ocean mega-earthquakes of 2004 and 2012.
• The swarms were situated beneath distinct seamounts of the submarine volcanic arc.
• Epicenters of the swarm earthquakes migrate at a rate of 0.25 km per hour.
• The swarms were probably generated by magma unrest induced by mega-earthquakes.

Little is known about earthquake-triggered magma intrusions or eruptions of submarine volcanoes. The analysis of teleseismic earthquake occurrence performed in this study offers a tool to address such enigmatic and inaccessible processes. In the past ten years, the Andaman Sea region repeatedly became a site of shallow earthquake swarms that followed distant mega-earthquakes by days to weeks. The MW 9.1 December 26, 2004 Sumatra–Andaman earthquake was followed by two earthquake swarms about 600 km northward in the Andaman Sea region, delayed by 30 and 35 days, respectively. Earthquakes of one of these seismic episodes, the extensive January 2005 earthquake swarm, migrated laterally at a rate of about 0.25 km per hour during the swarm evolution. The strong Indian Ocean MW 8.6 and 8.2 April 11, 2012 earthquake doublet west of Northern Sumatra was followed by an earthquake swarm approximately 800 km northward in the Andaman Sea region, delayed by 13 days. All the three swarms that followed the 2004 and 2012 mega-earthquakes occurred beneath distinct seamounts and seafloor ridges. Based on the observations of migration of earthquakes during the swarm and swarm occurrence beneath distinct highs at the seafloor, we conclude that these earthquake swarms probably resulted as a consequence of magma unrest induced by static and/or dynamic stress changes following the distant mega-earthquakes. Repeated occurrence of such a phenomenon suggests that the arc magma reservoirs beneath the Andaman Sea have recently reached some form of criticality and are vulnerable to even small stress changes. The Andaman seafloor could thus become a site of submarine volcanic eruptions in near future and deserves close attention of Earth scientists.