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The Global Aftershocks of the 2004 Sumatra-Andaman Earthquake
Emily E. Brodsky (UC Santa Cruz), Thorne Lay (UC Santa Cruz), Nicholas van der Elst (UC Santa Cruz)
The 26 December 2004 Sumatra-Andaman Mw 9.15 earthquake was the largest earthquake to occur since the 1964 Alaska
earthquake. In the subsequent 5.5 years, 9 great shallow earthquakes (M>= 8) have struck around the world giving a rate of 1.64
per year, whereas the prior century averaged 0.63 ± 0.34 per year.
Dynamic wave triggering effects combined with highly productive seismicity cascades could explain the rate increase. Seismic
waves can trigger earthquakes to great distances. The resultant seismicity rate increase is observed to be proportional to the peak
amplitude of the waves for a specific region (Fig. 1a). Over a broad swath of the globe, the 2004 Sumatra-Andaman earthquake
generated ground velocities of over 0.01 cm/s (Fig. 1b), which are sufficient to trigger sizable rate changes (greater than a few
percent) in susceptible regions (Fig 1a).
Once propagating seismic waves trigger a region, the remote sequences decay following Omori’s Law [Brodsky, 2006]. Most
of the triggered earthquakes are small, but each earthquake spawns further earthquakes, which occasionally can be larger than
the initial events. These large events can in turn trigger other remote sequences.
References
Brodsky, E.E., Geophys. Res. Lett., 33, L15313, doi:10.1029/2006BL026605, (2006).
Jiang, C. and Z. Wu, H. Ma, L. Zhou, Acta Seismologica Sinica, 3, 189-200, (2009).
Van der Elst, N. J. and E. E. Brodsky, J. Geophys. Res., doi:10.1029/2009JB006681, in press.
(a) Fractional earthquake rate increase as a function of peak dynamic strain from seismic waves [Van der Elst and Brodsky, in press]. The empirical relationship indicates
that seismicity rates should increase significantly if ground shaking exceeds 0.01 cm/s. (b) Global map of recorded peak vertical ground velocities for the 2004
Sumatra-Andaman earthquake (epicenter at magenta star) indicating the widespread occurrence of strong shaking.
2010 IRIS Core Programs Proposal | Volume II | Earthquake Source Studies | II-67