Seismic structure in central Mexico: Implications for fragmentation of the subducted Cocos plate
The fine-scale seismic structure of the central Mexico subduction zone is studied using moderate-sized (M4-6) intraslab earthquakes. Regional waveforms from the Mapping the Rivera Subduction Zone (MARS) seismic array are complicated and contain detailed information about the subduction zone structure, including evidence of lateral heterogeneity. This waveform information is used to model the structure of the subducted plates, particularly along the transition from flat to normal subduction, where recent studies have shown evidence for possible slab tearing along the eastern projection of the Orozco Fracture Zone (OFZ). The lateral extent of a thin ultra-slow velocity layer (USL) imaged atop the Cocos slab in recent studies along the Meso America Subduction Experiment array is examined here using MARS waveforms. We find an edge to this USL which is coincident with the western boundary of the projected OFZ region. Forward modeling of the 2D structure of the subducted Rivera and Cocos plates using a finite difference algorithm provides constraints on the velocity and geometry of each slab's seismic structure in this region and confirms the location of the USL edge. We propose that the Cocos slab is currently fragmenting into a North Cocos plate and a South Cocos plate along the projection of the OFZ, in agreement with observations of variable Cocos plate motion on either side of the OFZ. This tearing event may be a young analogy to the 10 Ma Rivera-Cocos plate boundary, and may be related to the slab rollback in central Mexico.
Additional Information© 2012 American Geophysical Union. Received 14 June 2012; revised 21 August 2012; accepted 22 August 2012; published 29 September 2012. This study was supported in part by the Gordon and Betty Moore Foundation through the Tectonics Observatory at California Institute of Technology. This is contribution number 207 from the Caltech Tectonics Observatory. We are grateful to the Incorporated Research Institutions for Seismology Data Management Center (IRISDMC) for making the waveform data available. We thank Arturo Iglesias for providing the Servicio Sismológico Nacional event catalog. We also thank Editor Robert Nowack and two anonymous reviewers for helpful comments which greatly improved the manuscript.
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