Intra-slab stress field and waveform modeling to determine velocity structure of the Indoburman Range

Patcharaporn Maneerat, Doug S. Dreger, & Roland Bürgmann

Published August 12, 2019, SCEC Contribution #9444, 2019 SCEC Annual Meeting Poster #059

The Indoburman Range (IBR), was formed due to the India-Eurasia collision since the Paleogene, is a highly oblique subduction zone. The dominance of observed strike-slip and intra-slab earthquakes brings into question the current activity of subduction in the IBR. In this study, we aim to (1) investigate the intraslab stress field in the IBR to understand plate driving forces and subduction dynamics and (2) improve the velocity structure model for studying mechanisms of events in this region. For the stress inversion, we used the available datasets of moment tensors from GCMT and IRIS. We chose events with M>4.5 and hypocenters below 20 km. We performed stress inversions from the focal mechanisms obtained from both catalogs by applying Stressinverse1.1 (Vavryčuk, 2014). Next, we compared the average principal stress orientations obtained from stress inversion and mean slab dip angles obtained from SLAB2.0 (Hayes et al., 2018). The results show that there is an E-W downdip tension along the Indian slab, consistent with the dip direction of the SLAB 2.0 plate interface, suggesting that slab pull is the dominant force. The observation of a N-S maximum compressive stress orientation is consistent with the characteristics of the oblique subduction zone.
To expand the data base of moment tensor solutions for such stress analysis we are working to develop calibrated velocity structures to enable the estimation of moment tensors for smaller events using regional distance waveform data. We begin with an analysis of a M5.1 event that occurred on April 24, 2018 in the IBR by waveform modeling. The waveforms we used were recorded at seismometers of 5 stations within 350 km of the epicenter and were obtained from IRIS. Our initial velocity structure model was assumed from the receiver function crustal models from Mitra et al. (2005). Through forward waveform modeling we have developed new calibrated velocity models for the regional distance source-receiver paths. The moment tensor inversion for the M5.1 event recovers an oblique reverse faulting type, which is a typical type of the intermediate events in the IBR. Through the modeling process, we have developed velocity models that lead to improve fit at the regional distance stations enabling the analysis of more events in the region to lower magnitude than is in the global catalogs. Greater numbers of events will lead to improved stress tensor analysis better understanding of this complex collision environment.

Maneerat, P., Dreger, D. S., & Bürgmann, R. (2019, 08). Intra-slab stress field and waveform modeling to determine velocity structure of the Indoburman Range. Poster Presentation at 2019 SCEC Annual Meeting.

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