Interplay Between Megathrust Topography and Tsunamigenic Structures Offshore West Mexico

Abstract

The megathrust topography is key in conditioning the structural integrity of the overriding plate, and thus, the generation of tsunamigenic structures. The Rivera subduction zone, offshore the Mexican Pacific coast, is known for hosting large megathrust tsunamigenic earthquakes (Mw > 7.5), and yet, little is known regarding the distribution of tsunamigenic structures along the margin. Here, we use spatially coincident 2D multichannel seismic (MCS, 5.85 km long-streamer) and wide-angle seismic (WAS) data acquired during the TSUJAL survey in 2014 to assess structural variations of the overriding plate and the megathrust interface. We have jointly inverted refracted and reflected travel-times (TT) from both MCS and WAS data to constrain the P-wave velocity (Vp) structure of the overriding plate and the geometry of the megathrust. To increase the amount of refracted TT we have applied the downward continuation technique to MCS field data. MCS data has a higher spatial sampling than OBS data, which translates into a higher tomographic resolution. Thus, the resulting tomographic model displays small-scale velocity structure variations of the overriding plate and the megathrust relief that would not be resolved with TT from OBS data only. We used further refracted and reflected TT from OBS data to constrain the Vp structure of the subducting oceanic plate and the geometry of the oceanic Moho. Time-migrated MCS sections were also obtained to constrain the tectonic framework of the shallow subduction zone. The results reveal the lack of an extensive accretionary prism, implying that subduction-erosion dominates the structure of the margin in this region. The inverted megathrust interface shows clear topographic features in its shallow portion (<~10 km from the trench). Such topographic variations are smaller than the average size of seamounts of the Rivera plate, but they are similar to the seafloor fabric generated by a relict East Pacific Rise segment identified west of the trench. We find that megathrust highs correlate with low-velocity anomalies, suggesting the presence of fluids, and with the presence of extensional faults in the overriding plate. This correlation demonstrates the control that megathrust topography exerts on the formation of tsunamigenic structures along the Rivera plate boundary