Earthquake Report: India!

Today we had a good sized earthquake in eastern India, within the India-Burmese wedge (IBW). The IBW is a part of the convergent plate boundary between the India plate to the west and the Burma (part of Eurasia) plate to the east. This plate boundary has been evolving since India came into the scene about 60 Ma (Curray, 2005). Prior to that, this boundary is thought to have been primarily convergent. Once India came into the region, prior to colliding with Asia (about?), this margin began to accommodate right lateral (dextral) shear. There is a major strike-slip fault, the Saging fault (SF), to the east of the IBW (Wang et al., 2014). The SF accommodates most of this shear, but some continues to be accommodated in the IBW (Maurin and Rangin, 2009).
Below is a map where I plot the epicenter for today’s M 6.7 earthquake, along with the moment tensor. I also include some inset maps. The lower left inset map is from Curray (2005) and shows the regional tectonics. The map on the right (Maurin and Rangin, 2009) shows the details of faulting in this region. In the upper right corner there is a cross section that is the east-west bold black line (at 22 degrees North) in the Maurin and Rangin (2009) map. This cross section is a little south of today’s earthquake, but is still relevant.
There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.
Today’s M 6.7 earthquake (here is the USGS web page for this earthquake) possibly occurred along the Churachandpur-Mao fault (Wang et al., 2014). Based upon our knowledge of the regional tectonics I interpret this earthquake to have a right-lateral oblique sense of motion.


Here is the figure caption for the Maurin and Rangin (2009) map and cross section in the above map.

(a) General structural map of the Indo-Burmese ranges. The arrow shows the motion of the India Plate with respect to the Burma Plate [Socquet et al., 2006]. Figures 3, 5, 7, 8, and 13 are located with black boxes. The black dashed line is the trace of the buried incipient Chittagong Coastal Fault (C. C. Fault) discussed in this paper. The gray dashed line is the approximate position of the deformation front above the de´collement, in the western boundary of the outer wedge (see text for details). The gray area shows the position of the strong negative Bouguer gravity anomaly produced by the Sylhet Trough. (b) E-W synthetic cross section based on field observations and industrial multichannel seismic data discussed in this paper. The cross section is located as a thick black line in the map. Ages and thicknesses are based on unpublished well records and previously published sedimentological studies (see text for details). OIBW, outer Indo-Burmese Wedge; IIBW, inner Indo-Burmese Wedge.

Here is the Curray (2005) plate tectonic map.


Here is a map from Maurin and Rangin (2009) that shows the regional tectonics at a larger scale. They show how the Burma and Sunda plates are configured, along with the major plate boundary faults and tectonic features (ninetyeast ridge). The plate motion vectors for India vs Sunda (I/S) and India vs Burma (I/B) are shown in the middle of the map. Note the Sunda trench is a subduction zone, and the IBW is also a zone of convergence. There is still some debate about the sense of motion of the plate boundary between these two systems. This map shows it as strike slip, though there is evidence that this region slipped as a subduction zone (not strike-slip) during the 2004 Sumatra-Andaman subduction zone earthquake. I include their figure caption as a blockquote below.

Structural fabric of the Bay of Bengal with its present kinematic setting. Shaded background is the gravity map from Sandwell and Smith [1997]. Fractures and magnetic anomalies in black color are from Desa et al.[2006]. Dashed black lines are inferred oceanic fracture zones which directions are deduced from Desa et al. in the Bay of Bengal and from the gravity map east of the 90
E Ridge. We have flagged particularly the 90
E and the 85
E ridges (thick black lines). Gray arrow shows the Indo-Burmese Wedge (indicated as a white and blue hatched area) growth direction discussed in this paper. For kinematics, black arrows show the motion of the India Plate with respect to the Burma Plate and to the Sunda Plate (I/B and I/S, respectively). The Eurasia, Burma, and Sunda plates are represented in green, blue, and red, respectively.

Here is a different cross section that shows how they interpret this plate boundary to have an oblique sense of motion (it is a subduction zone with some strike slip motion). Typically, these different senses of motion would be partitioned into different fault systems (read about forearc sliver faults, like the Sumatra fault. I mention this in my report about the earthquakes in the Andaman Sea from 2015.07.02). This cross section is further to the south than the one on the interpretation map above. I include their figure caption as a blockquote below.

Present cross section based on industrial multichannel seismics and field observations. The seismicity from USGS catalog and Engdahl [2002] is represented as black dots. Focal mechanisms from Global CMT (http://www.globalcmt.org/CMTsearch.html) catalog are also represented.

This figure shows the interpretation from Maurin and Rangin (2009) about how the margin has evolved over the past 10 Ma.


Wang et al. (2014) also have a very detailed map showing historic earthquakes along the major fault systems in this region. They also interpret the plate boundary into different sections, with different ratios of convergence:shear. I include their figure caption as a blockquote below.

Simplified neotectonic map of the Myanmar region. Black lines encompass the six neotectonic domains that we have defined. Green and Yellow dots show epicenters of the major twentieth century earthquakes (source: Engdahl and Villasenor [2002]). Green and yellow beach balls are focal mechanisms of significant modern earthquakes (source: GCMT database since 1976). Pink arrows show the relative plate motion between the Indian and Burma plates modified from several plate motion models [Kreemer et al., 2003a; Socquet et al., 2006; DeMets et al., 2010]. The major faults west of the eastern Himalayan syntax are adapted from Leloup et al. [1995] and Tapponnier et al. [2001]. Yellow triangle shows the uncertainty of Indian-Burma plate-motion direction.

Here is a map from Wang et al. (2014) that shows even more details about the faulting in the IBW. Today’s fault occurred nearby the CMf label. I include their figure caption as a blockquote below. Wang et al. (2014) found evidence for active faulting in the form of shutter ridges and an offset alluvial fan. Shutter ridges are mountain ridges that get offset during a strike-slip earthquake and look like window shutters. This geologic evidence is consistent with the moment tensor from today’s earthquake. There is a cross section (C-C’) that is plotted at about 22 degrees North (we can compare this with the Maurin and Rangin (2009) cross section if we like).

Figure 6. (a) Active faults and anticlines of the Dhaka domain superimposed on SRTM topography. Most of the active anticlines lie within 120 km of the deformation front. Red lines are structures that we interpret to be active. Black lines are structures that we consider to be inactive. CT = Comilla Tract. White boxes contain the dates and magnitudes of earthquakes mentioned in the text. CMf = Churachandpur-Mao fault; SM = St. Martin’s island antilcline; Da = Dakshin Nila anticline; M= Maheshkhali anticline; J = Jaldi anticline; P = Patiya anticline; Si = Sitakund anticline; SW= Sandwip anticline; L = Lalmai anticline; H = Habiganj anticline; R = Rashidpur anticline; F = Fenchunganj anticline; Ha = Hararganj anticline; Pa = Patharia anticline. (b) Profile from SRTM topography of Sandwip Island.

Here is the Wang et al. (2014) cross section. I include their figure caption as a blockquote below.

Schematic cross sections through two domains of the northern Sunda megathrust show the geometry of the megathrust and hanging wall structures. Symbols as in Figure 18. (a) The megathrust along the Dhaka domain dips very shallowly and has secondary active thrust faults within 120 km of the deformation front. See Figures 2 and 6 for profile location.

Early reports show that four people have been killed. AP report.
Here is a cross section that shows seismicity for this region. The earthquakes are plotted as focal mechanisms. This comes from Jacha Polet, Professor of Geophysics at Cal Poly Pomona.


Here is a map showing the seismicity and focal mechanisms, also from Jacha Polet.

References:
• Curray, J.R., 2005. Tectonics and history of the Andaman Sea Region in Journal of Asian Earth Sciences, v. 25, p. 187-232.
• Wang, Y., K. Sieh, S. T. Tun, K.-Y. Lai, and T. Myint, 2014. Active tectonics and earthquake potential of the Myanmar region in J. Geophys. Res. Solid Earth, 119, 3767–3822, doi:10.1002/2013JB010762.
• Maurin, T. and Rangin, C., 2009. Structure and kinematics of the Indo-Burmese Wedge: Recent and fast growth of the outer wedge in Tectonics, v. 28, TC2010, doi:10.1029/2008TC002276