A few days ago (11 April 2012) many of us were alerted to a magnitude 8.6 earthquake in the Indian Ocean south of Sumatra. However, even with such a significant force, the earthquake did not produce a tsunami. The USGS Earthquake website is a fantastic resource, and detailed technical earthquake information is available only minutes after the event. In this case, the technical page related to the earthquake showed that the earthquake was largely a strike-slip motion – where the blocks on either side of the fault move past each other horizontally, rather than displacing the water column vertically as would occur in a reverse or normal fault (see Wikipedia for more info).
If you click on the “Technical” tab on the USGS earthquake site, then you will be provided with a table and a seismic focal mechanism (moment tensor), more commonly know as a “beachball”. The moment tensor here is a classic example of strike-slip motion that would generally not produce a tsunami. The motion is either along a fault oriented N-NE, or S-NW. However, we know that the tectonic fabric in the Indian Ocean is largely oriented N-NE, highlighted by the gravity anomalies and fracture zone geometries.
Another interesting feature of this earthquake and its aftershocks is that they are generally distributed within the Indo-Australian plate. This region is undergoing transtensional deformation as India’s motion continues generally north while Australia’s plate velocities are slightly north-east, causing this part of the plate to buckle even with the immense strength of oceanic lithosphere. Many suggest, including the model of Peter Bird, that this region of diffuse deformation delineates a separate tectonic plate called the Capricorn Plate. The analysis of plate deformation and strain rates by Kreemer et al. (2003) also clearly outlines this region as being under significant strain.
You can download and use the gravity anomaly and strain rate grids in GPlates. They are simple JPG files that you import as a raster into GPlates.
If you use these files. make sure you attribute the original source of the data:
Fracture Zones – MATTHEWS, K. J., MÜLLER, R. D., WESSEL, P. & WHITTAKER, J. M. 2011. The tectonic fabric of the ocean basins. Journal of Geophysical Research, 116, B12109.
Gravity Anomalies – SANDWELL, D. & SMITH, W. 1997. Marine gravity anomaly from Geosat and ERS 1 satellite altimetry. Journal of Geophysical Research, 102, 10-10.
Strain Rate – KREEMER, C., HOLT, W. & HAINES, A. 2003. An integrated global model of present-day plate motions and plate boundary deformation. Geophysical Journal International, 154, 8-34.
Sabin Zahirovic
14 April 2012
geodynamix 