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

Dis G c u u a ss m io a n ndtheMarianashave sustainedmanysignificant earthquakes historically, with perhaps the 1993 Mw7.8 Guam earthquake one of the most significant (Mueller et al., 2012). However, there has not been a great megathrust event recordedhistorically (past 200 years) for theMariana subduction complex, which is one reason that Uyeda and Kanamori (1979) suggested that this subduction zone sustained aseismic subduction. Notably, the 1993 earthquake ruptured at a depth of 60-70 km (Mueller et al., 2012) but produced no uplift of the Guam coastline. Other significant earthquakes in the Marianas occurred in 1825 (Modified Mercalli Intensity VIII), 1834 (VIII), 1849 (IX), 1862 (VII), 1892 (VIII), 1902 (IX), 1903 (VII), 1909 (VIII), 1936 (VIII), 1970 (V), 1975 (VII), and 1978 (VII) (Mueller et al., 2012), although none were considered great megathrust earthquakes. Following that reasoning in their seismic hazard assessment of Guam and theMarianas, Mueller et al. (2012) state: “Inmodeling the Fig. 5: Example of pristine coral of Holocene (left) and Pleistocene (right) age growing over recrystalized Pleistocene coral. Cross-section through probable Holocene coral showing pristine structure and lack of re-crystalization (left). Fig. 6: Uplifted Pleistocene reef tracts on Guam remain to be dated, but the highest reef levels of the Mariana Formation cap Mio-Pliocene limestone of the Barrigada Formation (Riegl et al., 2008). Fig. 7: Fossil coral preserved at the base of the lowest raised notch near Agua Cove, NE Guam. seismic hazard of the region it is important to recognize that no great–– or even very large––thrust earthquake has ever been associated with the Mariana plate boundary. On the basis of its seismic history and tectonic expression, the Mariana megathrust is commonly categorized as the weakly coupled, “aseismic” end-member in global subduction- zone classifications (Uyeda and Kanamori, 1979; Ruff and Kanamori, 1983; Zhang and Lay, 1992).” The observations of coastal uplift in Guam and Saipan argue against this model and suggest that the current understanding of seismic Hazard underestimates the actual seismic potential of Guam and the Mariana Islands. Finally, the occurrence of a megathrust event to the trench may produce a significant tsunami, may cause harbors to become impassable if the amount of uplift exceeds the harbors water depth, as well as other hazards which are currently not accounted for in disaster planning for the region. For all of these reasons, further work is warranted.