PLANETARY MUSINGS

Sumatra-Andaman Islands Earthquake

Bilham, R, A Flying Start, Then a Slow Slip, Science, 308, 1126-1127, (2005), http://dx.doi.org/10.1126/science.1113363

Lay, T, H Kanamori, CJ Ammon, M Nettles, SN Ward, RC Aster, SL Beck, SL Bilek, MR Brudzinski, R Butler, HR DeShon, G Ekstrom, K Satake, S Sipkin, The Great Sumatra-Andaman Earthquake of 26 December 2004, Science, 308, 1127-1134, (2005), http://dx.doi.org/10.1126/science.1112250

Science published a series of four papers on the Sumatra earthquake in their May 20, 2005 issue. We are reading the articles above in Journal Club this week.... The devastating Dec. 26, 2004 Sumatra earthquake has opened a new box of scientific questions about the earth because of how the earthquake proceeded. These articles indicate that there was no place on Earth that did not move at least a centimeter as a result of the earthquake. Other previous papers indicated that the Earth's rotation even changed, but imperceptively I think. Anyway, there are two main points in these articles that I think are worth noting. First, with the normal plate convergence in this region at ~14 mm/yr, the ~10 meters of slip that occured in seconds is phenomenal at a minimum, and these authors rightfully point out that would normally take ~700 years. So, how much of the total plate movement is due to large slips like this earthquake compared to the smaller, daily-to-yearly earthquakes? Following on from that is the second point: The variation in sliprate along the rupture from south to north. I would guess that variations in stress (and release of it) are the most important controls on this process, but are there others? For example, the total slip in the north according to Lay et al is of similar magnitude as the south (maybe a little smaller), but must have been slow (aseismic) because it was picked up by tiltmeters and GPS, not broadband seismometers. What controls this process? Might it be related to the 30 Myr variation in age of the crust along the rupture too?