HR: 14:40h
AN: S43D-05    [Abstracts]
TI: Triggering of Great Earthquakes: Seasonal Variation.
AU: * Sacks, S I
EM: sacks@dtm.ciw.edu
AF: Carnegie Institution/DTM, 5241 Broad Branch Rd., N.W., Washington, DC 20015
AU: Linde, A T
EM: linde@dtm.ciw.edu
AF: Carnegie Institution/DTM, 5241 Broad Branch Rd., N.W., Washington, DC 20015
AB: It is now well established that even great earthquakes are triggered by relatively small stress changes provided that the stress change increases the Coulomb failure. Stress changes of a few tenths to a few bars have both advanced and inhibited earthquakes. Four centuries of earthquake occurrence data in Northeast Japan allowed a ~36 year lag time of subduction events after on-land earthquakes to be determined (Rydelek and Sacks, 1990). The elastic-viscoelastic model of the crust-lithosphere derived from geodetic observations over about 50 years can explain the time lag. The stress diffusion from the 1940's Nankai trough earthquakes, M ~8, slowly unloaded the normal stress clamping the Nojima fault over a 50 year period, resulting in the 1995 Kobe earthquake, m=6.9 (Pollitz and Sacks, 1997). From the earthquake record spanning about 12 centuries, the 1940's Nankai trough earthquakes were themselves advanced in time, the interval since the previous event of 1854 being clearly the shortest on record. Strain diffusion from the on-land great Nobi earthquake of 1891 explains not only the advance, but also the two year delay between the eastern (Tonankai) and western (Nankaido) events. The failure mechanism was modeled by Rydelek and Sacks, 2003. In the above examples, the strain diffusion has created stress changes of a fraction to a few bars at the fault so as to increase the Coulomb failure and modify the occurrence time by a significant amount, i.e. many years on a fault with recurrence interval of more than a century. Here we describe a situation in which strain changes, even though less than a tenth of a bar have governed great earthquake occurrence for more than a thousand years. Continuous GPS observations enable insight into much lower stress triggering of great events. It has long been recognized that most Nankai trough events (since 684), with recurrence intervals of one to two centuries, occur in the winter. A seasonal shortening of the continental plate (Heki, 2004) overlying the subducting Philippine sea plate, causes a reduction of stress on the thrust fault. Even though this stress change is less than 0.1 bar, and the yearly stress loading of the fault may be 0.5 to 1 bar, it seems to be sufficient to influence the failure time.
DE: 7209 Earthquake dynamics (1242)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7260 Theory
DE: 8110 Continental tectonics: general (0905)
DE: 8164 Stresses: crust and lithosphere
SC: Seismology [S]
MN: Fall Meeting 2005