Earthquake clustering in the tectonic pattern and volcanism of the Andaman Sea region

- Shallow seismicity is characterised by frequent occurrence of earthquake swarms.
- Earthquake swarm occurrence is limited to two narrow zones.
- Southern zone coincides with the submarine portion of the Sunda volcanic arc.
- Northern zone coincides with zigzag spreading rift of the Andaman Basin.
- The swarms are induced by intrusions of calc-alkaline and basaltic magma.

Seismicity pattern in the Andaman Sea region has been analysed to interpret recent dynamics of regional submarine volcanic provinces. Hypocentral determinations and focal mechanisms of available global seismological data have been used. Frequent occurrence of earthquake swarms proved to be an important characteristic of seismicity pattern in the investigated region. Epicenters of the earthquake swarms are situated almost exclusively along a narrow belt forming the easternmost limit of the epicentral area of the Andaman Sea region. The southern zone of the belt between 6° and 10.5° N is trench parallel and coincides with the northward prolongation of the Sumatra volcanic arc. The northern zone of the belt between 10.5° and 13° N is deflected by about 45° northeast and precisely follows the complicated zigzag pattern of the rift system in the middle of the Andaman Basin. Earthquake occurrence in these two zones, the northern and the southern, differs by several aspects - by shape of the epicentral zones of individual swarms, by focal mechanisms and by response to the 2004 Sumatra-Andaman great earthquake. These differences, together with available information on composition of magmas found at the seafloor, lead us to the conclusion that the swarms of the southern zone are induced by intrusions of subduction-generated calc-alkaline magmas whereas the swarms in the northern zone by intrusions of basaltic magmas associated with the seafloor spreading. Earthquake swarm occurrence defines a brittle, seismogenic layer at depths between 9 and 35 km, excludes the existence of large magma reservoirs in respective depth interval and puts their hypothetical location to a greater depth. Episodes of magma ascent from deeper reservoir to shallow magma chamber (depth < 9 km) or up to the surface induce earthquake swarms. The region deserves attention of volcano seismologists through its almost continuous activity and represents an attractive area for on-site geophysical monitoring and direct seafloor observations.