Seismotectonics of Recent Large Earthquakes at 50 km Depth within Subducted Slabs

The largest earthquakes in most of the world's subduction zones are underthrusting events on the plate interface. The intra-plate seismicity within the subducting oceanic lithosphere is typically less frequent and smaller in size than plate interface seismicity. However, large Wadati-Benioff earthquakes occasionally occur just deeper than the seismogenic plate interface with normal faulting focal mechanisms. Recently, three of these events have occurred in the subduction zones of the eastern Pacific Ocean: the Sept. 30, 1999 Oaxaca event (Mw 7.3); the Jan. 13, 2001 El Salvador event (Mw 7.8); and the Feb. 28, 2001 Puget Sound event (Mw 6.8). While there is considerable variation in the size of these three events, they are all significant events in the context of regional seismicity of their respective subduction zones. Previous global-scale investigations of the space-time occurrence of intermediate-depth slab seismicity have found it difficult to associate the intra-plate events with plate interface seismicity or other tectonic features of subduction zones. In our examination of the seismotectonic setting of these recent events, we note that the three subduction zone segments appear quite different in their characteristics. In particular, Cascadia apparently ruptures in great earthquakes (Mw of about 9) with recurrence intervals of hundreds of years, great earthquakes in Oaxaca (Mw of about 7.8) occur with a relatively short recurrence time, while the El Salvador segment of the Middle America subduction zone is characterized by relatively small plate interface earthquakes. This inverted order of underthrust and intra-slab earthquake sizes does not apply to all subduction zones. Despite the differences in slab geometry and great underthrusting events at Cascadia, Oaxaca, and El Salvador, there are some curious similarities in the occurrence of the slab events. For example, focal depths for the recent three events are in the range from 45 to 55 km, and all three events occurred close to previous large slab events with "recurrence times" that vary from 68 to 19 years. One implication is that while these large slab events are rare in the global sense, their occurrence is not random. It seems that certain "spots" within subducting slabs continue to generate this type of large event.