Why some large earthquakes do not genereate tsunamis

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.

Moment tensor for Sumatran earthquake (USGS)

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.

Gravity anomalies and fracture zones (left) highlight the north-south trends in the seafloor fabric, and strain rates (right) highlight the diffuse deformation within the Indo-Australian plate.

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.

Gravity Anomalies

Strain Rate

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

Introductory post… India plate motion in GPlates

The idea to start up a blog has been floating around for a while now, and I have finally committed to posting an entry. The first entry is an animation (and snapshots) of plate reconstructions in GPlates (v 1.2) depicting the evolution of the Indo-Australian plates in the context of Gondwana breakup from Cretaceous times.

The animation begins at 200 Ma and is based on a recently-published model by Seton et al. (2012) that you can read about here, and you can download the model for use in GPlates from the EarthByte resources page. The plate reconstructions are global and can be interactive manipulated in GPlates, and  can be exported as a series of snapshots (JPG, PNG, BMP) or as other files (Scalable Vector Graphics, ESRI Shapefiles, GMT xy/OGR formats, etc.). The reconstructions show plate boundaries which are dynamic – resulting from polygon topologies resolved at each timestep dynamically from the intersection of individual plate boundaries, a process that is described by Gurnis et al. (2012). Present-day coastlines and topography are reconstructed to give a reference and largely because paleobathymetry/paleotopography models are very uncertain. Plate velocities are also plotted, showing India’s northward advance was very fast compared to the velocities of surrounding plates. Recent research (i.e. van Hinsbern et al. 2012) has suggested that the acceleration of India northward resulted from the combined northward slab pull from Tethyan subduction, but also the northward “plume push” forces derived from the arrival of the Reunion Plume head to the base of Indian/African lithosphere.

The animation itself is more precisely based on the preferred scenario from my first published paper in G-Cubed (Zahirovic et al. 2012) that tests alternative India-Eurasia convergence scenarios using geodynamic models (implemented using GPlates and CitcomS). This preferred scenario has a large back-arc along southern Eurasia, much like the present-day west Pacific, with India first colliding with the associated island arc at ~60 Ma, followed by continent-continent collision and suturing at ~40 Ma.

I am publishing this stuff in a blog largely because I have been meaning to start uploading educational material and other things I find interesting in (geo-) science news for a long time now. It’s also a great opportunity to showcase some of the stuff we do, and the cool software/data we work with.

Sabin Zahirovic

13 April 2012