New Perspective on the Transition from Flat to Steeper Subduction in Oaxaca, Mexico Based on Seismicity, Nonvolcanic Tremor, and Slow Slip

The Mexico subduction zone has a complex geometry involving along-strike variations in dip of the Cocos plate. What is interesting about the slab dip is that under central Mexico it becomes horizontal for 250 km before it descends steeply beneath the Trans Mexican Volcanic Belt. Numerous studies have examined the geometry of the flat slab, however, few studies have focused on the along-strike transitions from flat to steeper subduction. Some of those studies have suggested the slab tends to tear along the sharp transitions in slab dip as the slab rolls back. In this study, the eastern transition from flat to steeper subduction in Oaxaca, Mexico is examined in greater detail using high resolution seismic data to provide a better understanding of how the slab morphology changes during slab rollback. We determine the geometry based on local seismicity, nonvolcanic tremor (NVT), and slow slip utilizing a deployment of broadband seismometers and continuous GPS receivers distributed in and around Oaxaca. Seismic data are processed with Antelope to detect and locate earthquakes. We construct depth contours of the subducting slab surface down to 100 km to illustrate that the transition from flat to steeper subduction in Oaxaca occurs rapidly via a stronger flexure than what is shown in previous studies. We find no gaps in seismicity to suggest a tear occurs above 100 km depth. A recently determined catalog of NVT in Oaxaca is extended using the same method and additional data that now extends further west. In the east, NVT epicenters are similar to earlier studies and form a continuous trench-parallel band that gradually moves inland. In the west, the distribution of NVT widens, closely following the shape of the newly determined slab contours. There are no gaps in NVT or slow slip across the rapid transition in slab dip, further supporting the notion that the slab is not currently torn in the updip region of the transition from flat to steeper subduction as it rolls back.