Palaeotsunamis in the Sino-Pacific region

© 2020 Elsevier B.V. Palaeotsunami research in the Sino-Pacific region has increased markedly following the 2011 Tōhoku-oki tsunami. Recent studies encompass a variety of potential sources and cover a full range of research activities from detailed studies at individual sites through to region-wide data collation for the purposes of database development. We synthesise palaeotsunami data from around the region drawing on key examples to highlight the progress made since 2011. We focus on a wide range of spatial and temporal scales, from region-wide to local events, from multi-millennial site records to estimates of magnitude and frequency along national coastlines. The review is based on sub-regions but in reviewing the combined records highlights common events and anomalies. In doing so we identify future research opportunities and notable findings arising from our review

Sedimentological analysis of tsunami deposits along the coast of Peru

The Peru-Chile-Trench is one of the most active seismic areas in the world (Kulikov et al., 2005). The subduction of the Nasca Plate under the South American Plate causes earthquakes with magnitudes greater than 8 every 5 to 10 years. Consequently, the risk for destructive tsunami along the coast of Peru is very high. The greatest historical tsunami events in this region are the two Arica tsunami in 1604 and 1868 (Okal et al., 2006) and the Chile tsunami in 1960 (Cisternas et al., 2005). The most recent tsunami are the Chimbote tsunami in 1996 (Bourgeois et al., 1999) and the Camaná tsunami in 2001 (Jaffe et al., 2003). Additionally, in 2007, a magnitude 7.9 earthquake 150 kilometres SSE of Lima generated a tsunami with run up heights of 10 m along the southern Paracas Peninsula (Fritz et al., 2008). Despite a large increase in tsunami studies in the last years, there is still no complete tsunami facies model. Furthermore the hydrodynamical processes leading to deposition of sediment by a tsunami wave are still not well understood. We surveyed various locations along the 2400 km Peruvian coastline to locate deposits of recent and historical tsunami events. Deposits were studied in trenches and boreholes down to depths of 3 m. We separated the foraminifera content for identification and inference of water depths of sediment entrainment by the tsunami. The grain-size distributions of the sampled deposits were optically determined with a PartAn 2001 particle analyser. The grain-size data were used to re-model the flow depths, using the inverse tsunami model of Jaffe & Gelfenbaum (2007)

Seismites and paleotsunamis deposits, assessing for paleoseismicity in Peru

[ENG] Human occupation records in Perú provide historical record of large earthquakes prior to the 20th century. In this study, we extend our knowledge of major events by evaluating the stratigraphy and chronology of sediments exposed in various sectors of the Central Andes. These observations suggests that strong seismic activity occurred during the Quaternary, either along the subduction megathrust or on crustal faults. Indeed in Cusco and Colca regions, ,active faults affect fluvio-glacial and alluvial Holocene to Pliocene deposits. High in the topography, lacustrine deposits as well as Quaternary moraines display multiple geomorphic evidences of displacements ans seismites attesting for regional seismotectonic activity. Similarly along the Peruvian coast, 90 excavations succeeded in identifying for the first time paleo-tsunami deposits in southern Peru. Among them, the most impressive are encountered in Puerto Casma and Boca Rio and sign the historic 1619 subduction event and former unknown events (1641 ± 26 years B.P. ie 1668, as well as 2.26 ± 0.37 ka and 1.98 ± 0.23 ka respectively)

Sedimentological aspects of recent and historical tsunami events along the coast of Peru

The coast of Peru is greatly endangered by tsunami events. The subduction of the Nasca Plate below the South American Plate triggers strong submarine earthquakes that are capable of causing tsunami. High-energy wave events are major coast shaping processes. In some regions, e.g. the Caribbean, a distinction between storm/hurricane and tsunami deposits is difficult. Therefore, the absence of heavy storms makes the Peruvian coast a good target for tsunami research. Other meteorological phenomena, like El Niño events that occur in Peru are not associated with strong storms or surges. Deposits of El Niño-caused flooding can easily be distinguished from tsunami events, since their sedimentary structures imply transport from the land to the sea, the deposited material derives from the mountain ranges and no indicators (e.g., foraminifera, shells) of marine inundations are present. In our study we re-surveyed locations of the three most recent regional tsunami events in order to learn about the sedimentary structures and their preservation potential. We visited the areas affected by the Chimbote-Tsunami of 21st February 1996 (5 m run up; Bourgeois et al., 1999; Kulikov et al., 2005), by the Camana-Tsunami of the 23rd June 2001 (9 m run up; Jaffe et al., 2003) and by the Pisco-Paracas-Tsunami of 15th August 2007 (10 m run up; Fritz et al., 2007). Secondly, we surveyed the coast of Peru in order to find traces of historical or paleotsunami events. All sediments were sampled for grain size analysis, foraminifera determination and optically stimulated luminescence dating. For historical events, the inverse tsunami model of Jaffe & Gelfenbaum (2007) was applied to calculate onshore tsunami flow depths. Both recent and historical tsunami deposits are present as (1) (graded) layers of coarse sand, some including shell fragments or pieces of rock, (2) (imbricated) shell layers, (3) heavy mineral accumulations and (4) mud caps or mud balls. Imbricated shells can give information on flow directions and hence can help to distinguish between run up and backwash sediments. Unfortunately, the preservation potential of onshore tsunami deposits is very low. Erosion by wind, rivers or heavy rain falls (e.g., during El Niño events) and bioturbation (e.g., by crabs) can modify or destroy the sediments. For recent events, human activity (e.g., the use of beach / tsunami sand for rebuilding) is a limiting preservation factor. This study shows that muddy tsunami sediments and backwash sediments have the highest preservation potential. This is due to the cohesion of mud that makes the deposits less sensitive for erosion during backwash and due to fast hardening of mud layers in the dry Peruvian climate

Sedimentological analysis of the Ordovician and Devonian basins in southern Peru and northern Bolivia

We present data from a study of the evolution of the Early Paleozoic Peru-Bolivia Trough, its facies development and the provenance of sediments deposited during Ordovician and Devonian time. We measured and sampled sections in the Ordovician successions in the Cordillera Oriental of southern Peru (Ollantaytambo, Verónica, San José, Sandia and Calapuja Formations) and northern Bolivia (Coroico, Amutara and Cancañiri Formations), and in the Upper Silurian to Devonian Lampa Formation on the Altiplano, and the Devonian Cabanillas Group on the Altiplano and the Peruvian Coastal Cordillera (Arequipa Massif). Our data contribute to a better understanding of the plate tectonic evolution of the Western Gondwana margin during early Paleozoic times

Sedimentología y dataciones por luminiscencia estimulada ópticamente (OSL) de depósitos de paleotsunamis a lo largo de la costa peruana

La costa del Pacífico fue testigo de innumerables tsunamis desde la formación de la margen y seguirán ocurriendo por millones de años más, es así que se reportan olas de hasta 20 m (Kulikov et al., 2005), generando extensa destrucción y pérdidas de vidas. Lockrige (1985), determina según estadísticas que entre Perú y Chile son los países que sufren más terremotos y erupciones volcánicas por kilómetro cuadrado en todo el planeta. Sólo si se considera el siglo XX, uno de cada tres tsunamis del Océano Pacífico se originó en las costas peruanas y/o chilenas. A fines de los ochenta, la aparición de dos publicaciones generó un importante cambio en la forma de evaluar el riesgo de tsunami. Atwater (1987) observó capas de sedimentos arenosos anómalas en la estratigrafía palustre, interpretándolos como depósitos prehistóricos de tsunamis. Posteriormente, Dawson et al. (1988) describió un inusual depósito contenido en la estratigrafía de la costa de Escocia, explicándolo como el resultado de un mega-tsunami producido por la avalancha submarina Storegga ocurrida hace 8 100 años en el norte de Europa. Tanto Atwater (1987) como Dawson et al. (1988) enfrentaron dificultades en sus interpretaciones, debido a que no fue posible, en aquel tiempo demostrar que aquellos estratos fueran similares a los depósitos dejados por tsunamis modernos, pues nunca habían sido estudiados. Así, Atwater (1987) utilizó información sismológica de un gran terremoto (1700 DC), para sustentar que un tsunami, inducido por aquel sismo, había depositado dichos sedimentos. A pesar de la importancia de los sismos y tsunamis en Perú, existen muy pocos estudios científicos relacionados a los registros sedimentarios y geomorfológicos dejados por estos eventos en el litoral de nuestro país. En el presente trabajo presentamos resultados sobre la sedimentología y dataciones de los depósitos de tsunami a lo largo de la margen peruana utilizando OSL como técnica de datación

Coral-rubble ridges as dynamic coastal features – short-term reworking and weathering processes

A coral-rubble ridge built by storm waves at Anegada (British Virgin Islands) underwent remarkable changes in shape and weathering in a 23-month period. The ridge is located along the island's north shore, in the lee of a fringing reef and a reef flat. This coarse-clast ridge showed two major changes between March 2013, when first examined, and February 2015, when revisited. First, a trench dug in 2013, and intentionally left open for further examination, was found almost completely infilled in 2015, and the ridge morphology was modified by slumping of clasts down the slope and by reworking attributable to minor storm waves. In size, composition and overall condition, most of the clasts that filled the trench resemble reworked clasts from the ridge itself; only a small portion had been newly brought ashore. Second, a dark gray patina formed on the whitish exteriors of the carbonate clasts that had been excavated in 2013. These biologically weathered, darkened clasts had become indistinguishable from clasts that had been at the ridge surface for a much longer time. The findings have two broader implications. First, coastal coarse-clast ridges respond not solely to major storms, but also to tropical storms or minor hurricanes. The modification and reworking of the ridge on Anegada most probably resulted from hurricane Gonzalo which was at category 1–2 as it passed about 60 km north of the island in October 2014. Second, staining of calcareous clasts by cyanobacteria in the supralittoral zone occurs within a few months. In this setting, the degree of darkening quickly saturates as a measure of exposure age

Styles of early diagenesis and the preservation potential of onshore tsunami deposits – a resurvey of Isla Mocha, Central Chile, 2 years after the February 27, 2010 Maule tsunami

The style of early diagenesis and preservation of onshore tsunami deposits are poorly constrained. Only tsunami surveys and subsequent re-surveys can fill this information gap. Here we present the results of a first re-survey in 2012 on Isla Mocha following the original survey in 2010 in the wake of the February 27, 2010, Maule earthquake and tsunami in central Chile. As a result of this tsunami, a large number of boulders consisting of clay-rich fine sandstones representing the Miocene age bedrock of the island had been transferred from the tidal to shallow subtidal zone onto the coastal plain. Coarse elastic sediment mixtures of pebbles, granules, and sand entrained at coastal plain terraces and transported up to the maximum runup position c. 600 m from the coast by the inflow had been left behind as extensive backflow blankets on the lower coastal plain. In 2012, vegetation had covered the 2010 tsunami deposits. Sediment beyond 200 m from the coast had been removed by a combination of surface processes and grazing cattle. Grain-size distributions of the preserved sediment show an increase of the sand fraction at the expense of the coarser grain sizes. Boulders show patterns resembling mud cracks on the surface and evidence of disintegration into smaller fragments and sand. Veneers of dried algae documenting the derivation of the boulders from the tidal zones had flaked off partly or completely from most rock surfaces. At the northern, wind-facing coast of the island, a c. 130 m long and 1.2 m high beach ridge had accumulated, most likely from reworked tsunami sediment. Boulders deposited by tsunamis are commonly assigned a high preservation potential. We demonstrate for the first time that such boulders may in fact disintegrate rapidly and disappear from the record over short geological time scales, given a lithology susceptible to weathering. The degree of modification to the Isla Mocha tsunami boulders and deposits strongly questions the applicability of inverse models using paleotsunami deposit thickness and grain-size distributions to infer parameters and magnitudes of the causative tsunamis. Inverse models would consequently underestimate tsunami flow parameters and may result in the inference of erroneous transport modes and event magnitudes. Our study gives strong evidence in support of re-survey studies following the deposition of tsunami sediments by tracing and quantifying the changes affecting them

A quasi-experimental setting of coarse clast transport by the 2010 Chile tsunami (Bucalemu, Central Chile)

On 27th February, 2010 a M(W) 8.8 earthquake occurred in central Chile, causing a tsunami that severely affected the central Chilean coast. In this study we present sedimentological data of the 2010 Chile tsunami from the coastal town of Bucalemu in Central Chile. The tsunami deposited both fine grained sediments as well as cobbles and boulders. The location offers a quasi-experimental setting for coarse clast transport because an artificial pile of cobbles and boulders was piled there for construction purposes some days before the tsunami. Therefore, the pre-tsunami transport setting and transport distances are well defined. Additionally, a local flow depth of ca. 2 m and flow speed of ca. 5.6 m/s are deduced from eyewitness reports and video recordings. The cobbles and boulders have been transported up to 155 m from the pile in landward direction. The size of the quarried clasts was restricted to max. 0.3 m(3) and 720 kg. Bigger boulders were not available at the location, even though calculations show that the tsunami would have been capable to transport them. Calculations of flow speeds required for the initialization of transport in different modes, such as sliding, rolling or saltation, indicate that all clasts that were moved by the tsunami in landward direction, were entrained by the flow given a minimum observed speed of 5.6 m/s, with 65% being transported by rolling and 35% by saltation. The transported clasts are arranged in three groups positioned in morphological depressions on the coastal plain. A landward fining trend is not present. The arrangement of clasts originating from a pile and being deposited in distinctive groups, entails that the landward transport of cobbles and boulders is a function of repeated entrainment, transport and deposition by several waves of the tsunami wave train. Consequently, we interpret the clast distribution as a result of a combination of the number of individual waves of the tsunami wave train, their respective energies, the local topography, and the pre-transport setting of a clast pile which releases wedged clasts with each successive wave