Seismic evidence for fragmentation of the Cocos slab in central Mexico

The fine-scale seismic structure of the central Mexico subduction zone, particularly the interface between the slab and overriding plate, is studied using shallow (~40-90 km) intraslab earthquakes of moderate magnitude (M4-6). Regional waveforms from the contemporaneous Middle America Subduction Experiment (MASE) and Mapping the Rivera Subduction Zone (MARS) seismic arrays are complicated and contain detailed information about the subduction zone structure. Identification of seismic phases, their arrival times, and any possible complexities in their waveshapes provide evidence of lateral variations in structure. The detailed waveform information obtained 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 low velocity layer imaged atop the subducted Cocos plate in recent studies along the MASE array is examined here using the combined MASE and MARS waveforms. We find an edge to this low velocity layer which is coincident with the western boundary of the projected OFZ region. We use forward modeling of the 2D structure of the subducted Rivera and Cocos plates using a finite-difference algorithm in order to provide constraints on the thickness, velocity, and geometry of each slab's shallow seismic structure in this region. This modeling shows that the best approximation to the observed seismograms is obtained when there is an edge to the low velocity layer coincident with the western boundary of the projected OFZ region. Coupled with the results of recent plate motion studies which show that the Cocos plate north of the OFZ moves differently than that south of the OFZ, we propose that the Cocos slab is currently fragmenting into a North Cocos plate and a South Cocos plate along the eastern projection of the OFZ. This tearing event may be a young analogy to the mature Rivera-Cocos plate boundary.