How Do Mega-Earthquakes Become So Big?
Miaki Ishii, Harvard University
The recent occurrence of giant earthquakes, such as the magnitude 9.3 Sumatra-Andaman earthquake in 2004 and magnitude 9.0 earthquake in Japan last year, combined with the development of a dense network of seismic instruments, is providing an unprecedented opportunity to study the properties and behavior of giant, potentially devastating, earthquakes. Detailed imaging of the rupture processes of these earthquakes shows unexpected features that distinguish large earthquakes from smaller, more frequent events. For example, instead of the simple sliding on a planar fault that we typically associate with earthquakes, rupture during great earthquakes jump over obstacles, trigger slips on adjacent or distant faults, and propagate in multiple directions. The ability to image rupture area precisely also allows us to monitor the evolution of a slip on a plate interface. For many large earthquakes, especially the 2011 Japan earthquake, cascading failure is observed. This suggests that an earthquake larger than magnitude 9.0 can occur along the northeastern coast of Japan. The complexities we are starting to observe are baffling. Every time a large earthquake happens, it seems that we discover something new. Furthermore, this new technique has identified "hidden" earthquakes that are not catalogued using conventional earthquake detection algorithms. Many of the "hidden" earthquakes have high tsunami-generating potential, and as such, they need to be more closely monitored.