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

446 1 2 T H I N T E R N AT I O N A L I N Q U A M E E T I N G O N PA L E O S E I S M O L O G Y , A C T I V E T E C T O N I C S A N D A R C H A E O S E I S M O L O G Y ( PATA ) , O C T O B E R 6 T H - 1 1 T H , 2 0 2 4 , L O S A N D E S , C H I L E PATA Days 2024 K E Y W O R D S Coastal vertical changes, historical earthquakes, diatoms, Metropolitan Chile (1) Programa de Doctorado en Ciencias Geológicas, Facultad de Ciencias Químicas, Universidad de Concepción. (2) Instituto de Geografía, Pontificia Universidad Católica de Valparaíso. *Email: bsaldana@udec.cl / marco.cisternas@pucv.cl, matias. carvajal@pucv.cl, alexandra.carreno@pucv.cl, mario.guerra@pucv.cl Bladimir Saldaña (1) Marco Cisternas (2) Matías Carvajal (2) Alexandra Carreño (2) Mario Guerra (2) ECOLOGICAL EVIDENCE OF SUDDEN COASTAL LEVEL CHANGES ASSOCIATED TO HISTORICAL METROPOLITAN CHILE EARTHQUAKES A B S T R A C T Conspicuous coastal uplift associated to the 1822 and 1906 earthquakes in Metropolitan Chile has been profusely reported in historical accounts. However, no geological evidence of such changes has been reported to date. In the case of the 1730 earthquake, neither historical nor geological reports are known. Here, we report for the first time lithological and ecological evidence of vertical changes triggered by these three historical earthquakes. Lithological changes accompanied by shifts in diatom assemblages suggest subsidence in 1730 and confirm uplift in 1822 and 1906. Different sense of vertical motion in these historical earthquakes support the hypothesis of varying rupture depths. Coasts adjacent to subduction zones experience sudden vertical changes triggered by earthquakes, involving either uplift or subsidence (Plafker, 1969; Plafker & Savage, 1970). Sedimentary environments respond quickly to these changes, producing distinguishable geologic records (Atwater, 1987). Such records are usually marked in the stratigraphy by sharp lithological changes, which may be accompanied by shifts in the microfossil assemblages preserved in the sediments (e.g., diatoms), revealing sudden ecological changes as well (Nelson et al., 1996; Pilarczyk et al., 2014; Shennan et al., 2014; Dura et al., 2016). Although numerous historical accounts have reported conspicuous coastal uplift following the 1822 and 1906 earthquakes in Metropolitan Chile, no studies have shown geologic evidence of such vertical changes. Coseismic uplift of about 1m attributed to each of these historical earthquakes shouldhave left a distinctive signature in the coastal sediments. On the other hand, historical records reporting vertical changes during the giant earthquake of 1730 have not been found, leaving an unsolved mystery surrounding this event. Here, we report pioneering lithological and ecological evidence of vertical changes triggered by these three historical earthquakes. R E F E R E N C E S Atwater, B. F. (1987). Evidence for great holocene earthquakes along the outer coast of Washington state. Science (New York, N.Y.), 236(4804), 942–944. Carvajal, M., Cisternas, M., & Catalán, P. A. (2017). Source of the 1730 Chilean earthquake from historical records: Implications for the future tsunami hazard on the coast of Metropolitan Chile: Size of the Giant 1730 Chile Earthquake. Journal of Geophysical Research. Solid Earth, 122(5), 3648– 3660. Dura, T., Hemphill-Haley, E., Sawai, Y., & Horton, B. P. (2016). The application of diatoms to reconstruct the history of subduction zone earthquakes and tsunamis. Earth-Science Reviews, 152, 181–197.