Detailed geometry of the subducting Indian Plate beneath the Burma Plate and subcrustal seismicity in the Burma Plate derived from joint hypocenter relocation

Abstract

With the aim of delineating the subducting Indian Plate beneath the Burma Plate, we have relocated earthquakes by employing teleseismic P-wave arrival times. We were able to obtain the detailed geometry of the subducting Indian Plate by constructing iso-depth contours for the subduction earthquakes at depths of 30-140 km. The strikes of the contours are oriented approximately N-S, and show an "S" shape in map view. The strike of the slab is N20 • E at 25 • N, but moving southward, the strike rotates counterclockwise to N20 • W at 20 • N, followed by a clockwise rotation to a strike of N10 • E at 17.5 • N, where slab earthquakes no longer occur. The plate boundary north of 20 • N might exist near, or west, of the coast line of Myanmar. The mechanisms of subduction earthquakes are down-dip extension, and T axes are oriented parallel to the local dip of the slab. Subcrustal seismicity occurs at depths of 20-50 km in the Burma Plate. This activity starts near the 60-km-depth contour of the subduction earthquakes and becomes shallower toward the Sagaing Fault, indicating that this fault is located where the cutoff depth of the seismicity becomes shallower.

Chapter 2 Active tectonics of Myanmar and the Andaman Sea

“…This interpretation most likely arises from the relatively small number of events studied by Stork et al (2008). There is no evidence for this in the larger dataset of Hurukawa et al (2012), who favour a continuous smoothly curving slab. Rangin et al (2013) reviewed the evidence against an active megathrust, and noted that the earthquakes within the downgoing plate have predominantly T-axes pointing down the dip of the slab (indicating downdip extension) and P-axes oriented along the strike of the slab (approximately parallel to Indian Plate motion with respect to the Sunda Plate) (Fig.…”

“…Ophiolite obduction onto the eastern margin of India is sealed by the Late Eocene-Oligocene Jopi Formation, suggesting that oceanic subduction on the northernmost side of the Indo-Burma Ranges may have ceased by this time (Ghose & Chatterjee 2014). Further south, it is clear that subduction of an oceanic plate has occurred in the recent geological past because there is a continuous dipping plane of earthquakes observed in earthquake relocation studies, and because there is Pliocene-Recent calc-alkaline volcanism above the Burma Seismic Zone (Stephenson & Marshall 1984;Stork et al 2008;Hurukawa et al 2012). However, there is an active debate about whether continental material is now being thrust beneath the Tripura Fold Belt, and even the existence of an active megathrust in this region is disputed.…”

“…Earthquake focal mechanisms suggest that the recorded events occurred on steeply dipping planes within the downgoing plate, not on the plate interface (Hurukawa et al 2012;Kundu & Gahalaut 2012). Interestingly, within the western part of the central Myanmar lowlands, both Stork et al (2008) and Hurukawa et al (2012) found evidence for lower crustal or upper mantle earthquakes above the slab. If these depths are correct, it would suggest that this region is relatively strong, and would explain why it has remained relatively low, potentially able to transfer large stresses as a relatively rigid body.…”

“…2.13). Earthquakes within the Indian Plate have been relocated by Stork et al (2008) and Hurukawa et al (2012). Stork et al (2008) focused on a smaller number of events, and attempted to use depth phase identification to give better depth resolution in the absence of local seismometers.…”

“…Seismological evidence has been cited both in support of (Stork et al 2008;Hurukawa et al 2012) and against (Kundu & Gahalaut 2012;Rangin et al 2013) ongoing subduction at the present day. This debate has important implications for our understanding of the seismic hazard and tectonics of the region.…”