Optically stimulated luminescence (OSL) dating of sand deposited by the 1960 tsunami in south-central Chile Annaliese Eipert Senior Integrative Exercise March 10, 2004 Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree from Carleton College, Northfield, MinnesotaTable of Contents Abstract Introduction………………………………………………………………………….1 Tectonic setting……………………………………………………………...6 Tsunami properties and deposition…………………………………….........8 Optically stimulated luminescence (OSL) dating ………………………......10 OSL dating of tsunami deposits………………………………………..........17 Field Site…………………………………………………………………………….18 Methods……………………………………………………………………………..22 Field methods……………………………………………………………….22 Laboratory methods………………………………………………………...26 Results………………………………………………………………………………27 Discussion…………………………………………………………………………..28 Conclusion………………………………………………………………………….32 Acknowledgements…………………………………………………………………34 References cited…………………………………………………………………….35Optically stimulated luminescence (OSL) dating of sand deposited by the 1960 tsunami in south-central Chile Annaliese Eipert Senior Integrative Exercise March 10, 2004 Sara Gran Mitchell, Advisor Abstract This study checks the feasibility of optically stimulated luminescence (OSL) in the dating of very young (<200 yr) sandy tsunami deposits in south-central Chile. OSL dating determines the most recent deposition of silicate grains within a potential range of decades to one million years. In the case of tsunami sand sheets, accurate OSL ages will not be achieved unless sand grains were exposed to a certain amount of sunlight during their deposition or before burial. In Chuyaquén, a series of buried sand sheets in the recent geologic record provides a potential means for using OSL to help determine earthquake recurrence intervals. Quartz grains extracted from the top centimeters of the 1960 deposit returned a predicted OSL age of 40 +/- 15 yr BP, but modern tidal flat sediments (~12 cm deep) gave an age older than expected, of 130 +/- 40 yr BP. These results indicate two plausible options: only bleached tidal flat sand shallower than 12 cm was scoured and deposited by the 1960 tsunami, or sand received sufficient sunlight during or after deposition by tsunami. OSL can provide accurate and precise dates on a <200 yr scale, and shows potential to supplement current radiocarbon techniques in the dating of young sandy tsunami deposits. Keywords: Chile earthquake 1960, tsunamis, sand sheets, luminescence analysis, deposits, Holocene1Introduction On the afternoon of May 22, 1960 a magnitude 9.5 earthquake occurred off the coast of central and southern Chile in a complex thrust fault along 1000 km of the subduction zone of the Nazca Plate under the South American Plate (Fig. 1) (Kanamori and Cipar, 1974; Plafker and Savage, 1970). The earthquake’s estimated 20 to 40 m of slip radiated more energy than any other in the 20th century, and caused land level changes in 200,000 square km of Chile’s coastline (Fig. 1b) (Barrientos, 1992). The 1960 earthquake also generated a train of giant ocean waves known as a tsunami. In Chile these waves reached heights of 20 m (Dudley and Lee, 1998). Together, the earthquake and tsunami killed 5,000 to 10,000 people in Chile (Bryant, 2001), and over 2,000 people in Hawaii and Japan (Bryant, 2001). Total damage in 1960 currency was estimated at nearly $900 million (Fig. 2) (Bryant, 2001; Dudley and Lee, 1998). The tsunami proved more deadly than the earthquake itself. All residents of Maullín, a small town in south-central Chile (Fig. 1b), survived the earthquake, but 122 died from the subsequent tsunami (Atwater et al., 1999). Many of these deaths probably could have been avoided if residents had known that a tsunami might result from an earthquake and that they should evacuate to high ground in order to escape it. A better understanding of the behavior of earthquakes and tsunamis could help to prevent such massive loss of life in future events. Written and geological records in Chile show a pattern of recurring earthquakes (Lomnitz, 1970). The great length of the South American subduction zone enables it to generate the largest trans-Pacific tsunamis; both the magnitude and frequency of events make the subduction zone a useful area to study (Beck et al., 1998; Bryant, 2001).= Subduction zone(teeth on upper plate)CHILENazca PlateSouth AmericanPlate= Tectonic plateboundary= GeographicalboundaryNFig. 1- a. Map of Chile, illustrating subduction of the Nazca plate under the South American plateb. South-central Chile with amounts of vertical displacement after 1960. Hatched area representssubsided area (after Plafker and Savage, 1970)78°72°38°44°100 kmNPACIFIC OCEANCHILEa.b.Maullín+2-2-2+1+6limit of subsidence(dashed lines areinferred)**PuertoMonttMaullín2Fig. 2- Aftermath of May, 22, 1960 earthquake and tsunami in Maullín, south-centralChile. a. Car buried in sand after tsunami b. and c. Flooded streets of Maullínd. Collapsed house in Maullín e. Maullín waterfront (Photos by Alejandro Gallardo)a.b.c.d.e.34Historical records of other large earthquakes span back to the arrival of the Spanish around 1570 AD (Beck et al., 1998) but events older than this must be identified and dated from closer study of the geological record. Subduction zone earthquakes are of scientific and public interest because approximately half the world’s population lives near active tectonic plate boundaries (Burbank and Anderson, 2001). The recency of the 1960 Chile earthquake and tsunami makes them valuable events to study in the field and lab, as sedimentary evidence can be supplemented and correlated with written records, maps, tide meters, GPS data, and eyewitness accounts from tsunami survivors.