Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

The Mariana subduction zone (MSZ) extends for nearly 2000 km (Figure 1) with a convergent rate of 6.3 cm/yr (Mueller et al., 2012), providing sufficient length to sustain M9-class megathrust events even with only partial ruptures of the MSZ. The four main islands of the Marianas (Guam, Rota, Tinian, and Saipan) represent 500 km of the former volcanic arc, which shut down in the Miocene. The active arc for this portion of the MSZ has now shifted as much as 50-60 km to the west as a result of flattening of the subduction interface due to subduction of oceanic seamounts (Figure 2) (Dickenson, 2020). The shift in the arc indicates a decrease in dip of the subduction interface from an average of about 31 o to 25 o in the upper 100 km, which is similar to much of the arc to the north, as well as other subduction zones that have generated M9 earthquakes. A significant question is whether the uplift observations by Muhs et al. (2020) are a local effect, or whether they are representative of the entire fossil arc. We have collected preliminary observations from Guam that argue that the entire southern half of the arc is rising at a rate similar to or higher than the coupled Chilean subduction zone. Fig. 1: General tectonic setting of the Mariana Islands and subduction complex (modified from Muhs et al., 2020). Note the many seamounts being subducted (Dickenson, 2020). G=Guam, R=Rota, T-Tianin, S=Saipan Fig. 2: Map showing the main Mariana Islands of Guam, Rota, Tinian and Saipan, which represent the fossil Miocene island arc (modified from Dickenson, 2000). The Plio-Quaternary arc lies 45-60 km to the west of the fossil arc. Also note the location of the 1993 Mw7.8 Guam earthquake, which produced zero coastal uplift.