HR: 1400h
AN: S43B-01    [Abstracts]
TI: Seismicity and State of Stress in the Subduction Zone of Guerrero, Mexico.
AU: * Pacheco, J F
EM: javier@ollin.igeofcu.unam.mx
AF: Universidad Nacional Autónoma de México, Instituto de Geofísica, UNAM, Ciudad Universitaria, Coyoacán, DF 04510, Mexico
AU: Singh, S K
EM: krishna@ollin.igeofcu.unam.mx
AF: Universidad Nacional Autónoma de México, Instituto de Geofísica, UNAM, Ciudad Universitaria, Coyoacán, DF 04510, Mexico
AB: Until recently it was thought that moderate and large inslab earthquakes below Mexico occurred downdip from the strongly-coupled part of the interface. In the last 15 years, however, there have been several moderate/large inslab earthquakes with normal or thrust mechanism with one nodal plane characterized by steep dip. These earthquakes are located below or close to the downdip edge of the coupled plate interface. In order to understand the relationship between these earthquakes, and the state of stress and the geometry of the subducted Cocos plate, we study small to moderate events (M≥4) along the Guerrero seismic zone. Good station coverage in this region permits an accurate determination of depth and focal mechanism. Broadband seismograms and accelerograms (1995-present) were used to relocate earthquakes. Focal mechanisms were obtained from waveform modeling using local or regional data. Apart from the usual shallow thrust events located along the plate interface, we recorded three types of events: (1) shallow normal-faulting earthquakes in the upper continental crust, (2) steeply-dipping normal or reverse-faulting earthquakes, and (3) intermediate-depth normal- faulting earthquakes. Shallow normal events are related to a seismic sequence that started with an Mw 5.8 earthquake in 2001. Steeply-dipping events are located at a depth between 25 and 40 km, landward of the shallow thrust events. Normal-faulting earthquakes cluster around 180 km from the trench and occur at a depth between 50 and 60 km depth. Shallow normal-faulting events indicate a trench-normal tension in the upper plate. This state of stress in the upper plate near a convergent margin may be explained by tectonic erosion and/or backward retreat of the trench. The steeply-dipping mechanisms seem related to the unbending of the Cocos plate. Normal- faulting events, at 50-60 km depth, are located close to where other studies identify the commencement of steep bending of the Cocos plate and its penetration into the continental mantle. There is a complete lack of seismicity (even at a low-magnitude level) in the region between the steeply-dipping and the intermediate-depth normal-faulting events where the slab is subhorizontal (L~100 km). Seismic activity on the subducted Cocos plate is related to bending or unbending of the plate.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8150 Plate boundary: general (3040)
SC: Seismology [S]
MN: 2007 Joint Assembly