Paleoseismic Records of Multiple Great Earthquakes from the Subduction Zones of Sumatra, Chile, and Alaska

Thursday, 18 December 2014: 4:30 PM
Tina Dura1, Ben Horton2, Richard W Briggs3, Marco Cisternas4, Lisa L Ely5, Harvey M Kelsey6, Alan R Nelson3 and Charles Martin Rubin7, (1)University of Pennsylvania, Philadelphia, PA, United States, (2)Rutgers University New Brunswick, Marine and Coastal Sciences, New Brunswick, NJ, United States, (3)US Geological Survey, Denver, CO, United States, (4)Pontifical Catholic University of Valparaíso, Escuela se Ciencias del Mar, Valparaiso, Chile, (5)Central Washington University, Department of Geological Sciences, Ellensburg, WA, United States, (6)Humboldt State University, Arcata, CA, United States, (7)Nanyang Technological University, Singapore, Singapore
Instrumental and historical records have proved too short to estimate the potential magnitudes and recurrence intervals of rare events such as the 2004 Indian Ocean and 2011 Tohoku-Oki great earthquakes and tsunamis. Paleoseismology improves our understanding of subduction zone hazards by extending earthquake histories thousands of years into the past. Through paleoseismic investigations at subduction zones, we scrutinize coastal sediments in low-energy depositional environments to reconstruct relative sea-level (RSL) changes related to upper-plate deformation from past earthquakes and tsunami inundation. Microfossils—long recognized as valuable RSL indicators—in coastal sediment provide an independent test of earthquake related RSL change and tsunami deposition inferred from coastal stratigraphy. Here, we illustrate the value and potential of paleoseismic methods at megathrust sites in Sumatra, Chile, and the eastern Aleutian Islands to identify both coseismic subsidence and uplift. In western Sumatra, coastal sediments beneath a lowland near the city of Padang contain stratigraphic evidence for two mid Holocene earthquakes, each resulting in > 1 m of coseismic subsidence. A site along the most populated portion of the central Chilean coast near Valparaíso records stratigraphic and diatom evidence of six instances of early to mid-Holocene coseismic uplift (< 1 m) accompanied by tsunamis. Coastal marsh sediment on the eastern Aleutian Island of Sitkinak contains stratigraphic and diatom evidence for three instances of uplift and two of subsidence during great earthquakes. Such biostratigraphic evidence of past great earthquakes and tsunamis shows that the absence of great megathrust events near our sites in the last 200-300 years is not representative of the potential for great events on the megathrusts. In addition, we discuss how our earthquake and tsunami records fit within the context of past regional RSL histories and emphasize the importance of RSL on the preservation and interpretation of evidence of past events.