The problem of past megatsunami reconstructions on the southern Kurils

Tsunamis are reconstructed on the basis of distribution of tsunamigenic sediments in coastal lowland sections. Reflections of anomalous tsunamis are recorded in detail in the lacustrine–boggy sections of the Lesser Kuril Ridge, while only fragments of these sediments have been found on the islands of the Greater Kuril Ridge. The distribution and composition of the sediments left by recent large-scale tsunamis (locally documented 1994 and 1894 Shikotan tsunamis and transoceanic 2011 Tohoku tsunami) are analyzed for the purpose of understanding deposition features during large and megatsunamis. Interregional correlation of the events during the last ~2.5 kyr is carried out with estimation of their scales. It is established that large events took place in the 17th and 18th centuries and approximately at 1.0, 1.4–1.6, 1.7–1.8, and 2.0–2.1 ka ago. New data on large tsunami chronology since the Middle Holocene are presented. A unique natural peatland section with abundant tsunamigenic sand layers is studied on the Pacific side of Zelenyi Island (Rudnya Bay), where deposition continued through the entire Holocene. The largest tsunamis which happened on the South Kuril Islands during the last ~7.5 kyr and can be classed as megatsunamis are revealed.     

Age estimates of coastal terraces in the Andaman and Nicobar Islands and their tectonic implications

The great Indian Ocean earthquake of December 26, 2004 caused significant vertical changes in its rupture zone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arc ridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands, in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates, which vary from 1.33 mm yr^− 1 in the Little Andaman to 2.80 mm yr^− 1 in South Andaman and 2.45 mm yr^− 1 in the North Andaman. Our radiocarbon dates converge on  600 yr and  1000 yr old coastal uplifts, which we attribute to the level changes due to two major previous subduction earthquakes in the region.     

Revealing the coastal event-history of the Andaman Islands (Bay of Bengal) during the Holocene using radiocarbon and OSL dating

Earthquakes that trigger tsunamis are of great geological, ecological and socio-economic importance. The knowledge of the recurrence interval of these events will give information about the hazard for a region. Coastal sediments on the Andaman Islands located in the eastern Bay of Bengal were investigated to find evidence for palaeotsunamis and palaeoearthquakes. Fieldwork was conducted on Red Skin Island and North Cinque Island, south of South Andaman. Sediment material from event-layers was dated by optically stimulated luminescence and radiocarbon dating method. The results show evidence possibly for one earthquake at about 1,000 or 3,000 years before the present together with deposits from possible tsunamis and storms. The complex pattern of co- and post-seismic uplift and subsidence of the Andaman Islands is reflected in the investigated sections and made it possible to reconstruct an event-history for the last 3,000 years.     

Role of crustal heterogeneity beneath Andaman–Nicobar Islands and its implications for coastal hazard

The Andaman–Nicobar (A–N) Islands region has attracted many geo-scientists because of its unique location and complex geotectonic settings. The recent occurrence of tsunamis due to the megathrust tsunamigenic north Sumatra earthquake (Mw 9.3) with a series of aftershocks in the A–N region caused severe damage to the coastal regions of India and Indonesia. Several pieces of evidence suggest that the occurrence of earthquakes in the A–N region is related to its complex geodynamical processes. In this study, it has been inferred that deep-seated structural heterogeneities related to dehydration of the subducting Indian plate beneath the Island could have induced the process of brittle failure through crustal weakening to contribute immensely to the coastal hazard in the region. The present study based on 3-D P-wave tomography of the entire rupture zone of the A–N region using the aftershocks of the 2004 Sumatra–Andaman earthquake (Mw 9.3) clearly demonstrates the role of crustal heterogeneity in seismogenesis and in causing the strong shakings and tsunamis. The nature and extent of the imaged crustal heterogeneity beneath the A–N region may have facilitated the degree of damage and extent of coastal hazards in the region. The 3-D velocity heterogeneities reflect asperities that manifest what type of seismogenic layers exist beneath the region to dictate the size of earthquakes and thereby they help to assess the extent of earthquake vulnerability in the coastal regions. The inference of this study may be used as one of the potential inputs for assessment of seismic vulnerability to the region, which may be considered for evolving earthquake hazard mitigation model for the coastal areas of the Andaman–Nicobar Islands region.     

Tectonic geomorphology,tsunamis and environmental hazards: reference to Andaman-Nicobar Islands

Geomorphic features associated with earthquakes and tsunamis have received wide attention in estimating uplift and subsidence after the tectonic event. Although various techniques are in vogue in estimating the uplift and subsidence after the 2004 Andaman-Sumatra earthquake and subsequent tsunami, remote sensing techniques have been proved to be quite handy to study the geomorphic changes. In the present study, geomorphic changes associated with the destructive event of December 2004 have been analyzed. The emergent and subsident coasts around the smaller islands in the Andaman region have been identified. The coral reef area that has been subjected to uplift or subsidence in some of the islands of the Andaman and Nicobar region is delineated, and the net areal extents of these coral beds have been computed. Of the six islands studied in Andaman region, coral reef of four islands was subjected to uplift, and around two islands the area was subsided. The uplifted area varied from 0.10 to 11 km², and subsidence was about 0.50 km². In Nicobar region, the subsidence of coral reefs was recorded. This study helps to monitor the coastal environments and the destruction due to natural hazards.     

Sedimentary evidence of prehistoric distant-source tsunamis in the Hawaiian Islands

Over the past 200 years of written records, the Hawaiian Islands have experienced tens of tsunamis generated by earthquakes in the subduction zones of the Pacific ‘Ring of Fire’ (for example, Alaska–Aleutian, Kuril–Kamchatka, Chile and Japan). Mapping and dating anomalous beds of sand and silt deposited by tsunamis in low-lying areas along Pacific coasts, even those distant from subduction zones, is critical for assessing tsunami hazard throughout the Pacific basin. This study searched for evidence of tsunami inundation using stratigraphic and sedimentological analyses of potential tsunami deposits beneath present and former Hawaiian wetlands, coastal lagoons, and river floodplains. Coastal wetland sites on the islands of Hawai΄i, Maui, O΄ahu and Kaua΄i were selected based on historical tsunami runup, numerical inundation modelling, proximity to sandy source sediments, degree of historical wetland disturbance, and breadth of prior geological and archaeological investigations. Sand beds containing marine calcareous sediment within peaty and/or muddy wetland deposits on the north and north-eastern shores of Kaua΄i, O΄ahu and Hawai΄i were interpreted as tsunami deposits. At some sites, deposits of the 1946 and 1957 Aleutian tsunamis are analogues for deeper, older probable tsunami deposits. Radiocarbon-based age models date sand beds from three sites to ca 700 to 500 cal yr bp , which overlaps ages for tsunami deposits in the eastern Aleutian Islands that record a local subduction zone earthquake. The overlapping modelled ages for tsunami deposits at the study sites support a plausible correlation with an eastern Aleutian earthquake source for a large prehistoric tsunami in the Hawaiian Islands.     

Sheltered coastal environments as archives of paleo-tsunami deposits: Observations from the 2004 Indian Ocean tsunami

The 2004 earthquake left several traces of coseismic land deformation and tsunami deposits, both on the islands along the plate boundary and distant shores of the Indian Ocean rim countries. Researchers are now exploring these sites to develop a chronology of past events. Where the coastal regions are also inundated by storm surges, there is an additional challenge to discriminate between the deposits formed by these two processes. Paleo-tsunami research relies largely on finding deposits where preservation potential is high and storm surge origin can be excluded. During the past decade of our work along the Andaman and Nicobar Islands and the east coast of India, we have observed that the 2004 tsunami deposits are best preserved in lagoons, inland streams and also on elevated terraces. Chronological evidence for older events obtained from such sites is better correlated with those from Thailand, Sri Lanka and Indonesia, reiterating their usefulness in tsunami geology studies.     

Great earthquakes of variable magnitude at the Cascadia subduction zone

Comparison of histories of great earthquakes and accompanying tsunamis at eight coastal sites suggests plate-boundary ruptures of varying length, implying great earthquakes of variable magnitude at the Cascadia subduction zone. Inference of rupture length relies on degree of overlap on radiocarbon age ranges for earthquakes and tsunamis, and relative amounts of coseismic subsidence and heights of tsunamis. Written records of a tsunami in Japan provide the most conclusive evidence for rupture of much of the plate boundary during the earthquake of 26 January 1700. Cascadia stratigraphic evidence dating from about 1600 cal yr B.P., similar to that for the 1700 earthquake, implies a similarly long rupture with substantial subsidence and a high tsunami. Correlations are consistent with other long ruptures about 1350 cal yr B.P., 2500 cal yr B.P., 3400 cal yr B.P., 3800 cal yr B.P., 4400 cal yr B.P., and 4900 cal yr B.P. A rupture about 700-1100 cal yr B.P. was limited to the northern and central parts of the subduction zone, and a northern rupture about 2900 cal yr B.P. may have been similarly limited. Times of probable short ruptures in southern Cascadia include about 1100 cal yr B.P., 1700 cal yr B.P., 3200 cal yr B.P., 4200 cal yr B.P., 4600 cal yr B.P., and 4700 cal yr B.P. Rupture patterns suggest that the plate boundary in northern Cascadia usually breaks in long ruptures during the greatest earthquakes. Ruptures in southernmost Cascadia vary in length and recurrence intervals more than ruptures in northern Cascadia.     

The hazard potential of the western segment of the Makran subduction zone, northern Arabian Sea

Evaluating the hazard potential of the Makran subduction zone requires understanding the previous records of the large earthquakes and tsunamis. We address this problem by searching for earthquake and tectonic proxies along the Makran Coast and linking those observations with the available constraints on historical seismicity and the tell-tale characteristics of sea floor morphology. The earthquake of Mw 8.1 of 1945 and the consequent tsunami that originated on the eastern part of the Makran are the only historically known hazardous events in this region. The seismic status of the western part of the subduction zone outside the rupture area of the 1945 earthquake remains an enigma. The near-shore shallow stratigraphy of the central part of Makran near Chabahar shows evidence of seismically induced liquefaction that we attribute to the distant effects of the 1945 earthquake. The coastal sites further westward around Jask are remarkable for the absence of liquefaction features, at least at the shallow level. Although a negative evidence, this possibly implies that the western part of Makran Coast region may not have been impacted by near-field large earthquakes in the recent past??a fact also supported by the analysis of historical data. On the other hand, the elevated marine terraces on the western Makran and their uplift rates are indicative of comparable degree of long-term tectonic activity, at least around Chabahar. The offshore data suggest occurrences of recently active submarine slumps on the eastern part of the Makran, reflective of shaking events, owing to the great 1945 earthquake. The ocean floor morphologic features on the western segment, on the contrary, are much subdued and the prograding delta lobes on the shelf edge also remain intact. The coast on the western Makran, in general, shows indications of progradation and uplift. The various lines of evidence thus suggest that although the western segment is potentially seismogenic, large earthquakes have not occurred there in the recent past, at least during the last 600?years. The recurrence period of earthquakes may range up to 1,000?years or more, an assessment based on the age of the youngest dated coastal ridge. The long elapsed time points to the fact that the western segment may have accumulated sufficient slip to produce a major earthquake.     

L'aléa tsunami en Polynésie française : synthèse des observations et des mesures

Since 1837, 15 tsunamis in French Polynesia have been reported, 11 generated damage. The two last major Pacific-wide tsunamis, 1946 Aleutian and 1960 Chilean, generated damage in most of the archipelagos. Only one in 1999 was generated by a local source, a coastal cliff failure on Fatu-Hiva Island. Since 1965, no earthquake magnitude was greater than 8.4 and, consequently, no ocean-wide tsunami of large amplitude has crossed the Pacific Ocean. Nevertheless, the four tide gauge installed in French Polynesia recorded 33 distinct tsunamis of amplitude from several centimetres to 1.6 m, generated by earthquakes of magnitude between 7.3 and 8.4, two of them damageable in Marquesas bays. To cite this article: F. Schindelé et al., C. R. Geoscience 338 (2006).     

Modeling a tsunami from the Nicoya,Costa Rica,seismic gap and its potential impact in Puntarenas

Although subduction zones around the world are known to be the source of earthquakes and/or tsunamis, not all segments of these plate boundaries generate destructive earthquakes and catastrophic tsunamis. Costa Rica, in Central America, has subduction zones on both the Pacific and the Caribbean coasts and, even though large earthquakes (Mw = 7.4–7.8) occur in these convergent margins, they do not produce destructive tsunamis. The reason for this is that the seismogenic zones of the segments of the subduction zones that produce large earthquakes in Costa Rica are located beneath land (Nicoya peninsula, Osa peninsula and south of Limón) and not off shore as in most subduction zones around the world. To illustrate this particularity of Costa Rican subduction zones, we show in this work the case for the largest rupture area in Costa Rica (under the Nicoya peninsula), capable of producing Mw ～ 7.8 earthquakes, but the tsunamis it triggers are small and present little potential for damage even to the largest port city in Costa Rica.The Nicoya seismic gap, in NW Costa Rica, has passed its ～50-year interseismic period and therefore a large earthquake will have to occur there in the near future. The last large earthquake, in 1950 generated a tsunami which slightly affected the southwest coast of the Nicoya Peninsula. We present here a simulation to study the possible consequences that a tsunami generated by the next Nicoya earthquake could have for the city of Puntarenas. Puntarenas has a population of approximately eleven thousand people and is located on a 7.5 km long sand bar with a maximum height of 2 m above the mean sea level. This condition makes Puntarenas vulnerable to tsunamis.     

Holocene coastal uplift in the western Pacific Rim in the context of late Quaternary uplift

This paper reviews recent studies of Holocene coastal uplift in tectonically active areas near the plate boundaries of the western Pacific Rim. Emergent Holocene terraces exist along the coast of North Island of New Zealand, the Huon Peninsula of Papua New Guinea, the Japanese Islands, and Taiwan. These terraces have several features in common. All comprise series of subdivided terraces. The highest terrace is a constructional terrace, underlain by estuarine or marine deposits, and the lower terraces are erosional, cutting into transgressive deposits or bedrock. The highest terrace records the culmination of Holocene sea-level rise at ca. 6–6.5 ka BP. Lower terraces were coseismically uplifted. Repeated major earthquakes have usually occurred at ka intervals and meter-scale uplift. The maximum uplift rate and number of terraces are surprisingly similar, about 4 m/ka and seven to four major steps in North Island, Huon Peninsula, and Japan. Taiwan, especially along the east coast of the Coastal Range, is different, reaching a maximum uplift rate of 15 m/ka with 10 subdivided steps. They record a very rapid uplift. Comparison between short-term (Holocene) and long-term since the last interglacial maximum (sub-stage 5e) uplift rates demonstrates that a steady uplift rate (Huon Peninsula) or accelerated uplift toward the present (several areas of Japan and North Island) has continued at least since isotope sub-stage 5e. Rapid uplift in eastern Taiwan probably started only in the early Holocene, judging from the absence of any older marine terraces. Most of the causative faults for the coastal uplift may be offshore reverse faults, branched from the main plate boundary fault, but some of them are onshore faults, which deformed progressively with time.     

利用盐沼泥炭重建长江三角洲北部全新世中期海平面

根据对长江三角洲北部海安地区4个钻孔标志性沉积物(潮上带盐沼泥炭、高潮滩沉积)的年龄测定和高程测量,以及沉积物压实沉降量的分析研究,重建了本研究区全新世中期8.1～7.3 cal kyr BP和5.6～5.4 cal kyr BP的相对海平面位置。结果显示,8.1～7.3 cal kyr BP海平面缓慢上升1.46m,上升速率仅为0.2cm/yr, 与三角洲南部全新世早期海平面的快速上升(2cm/yr)形成鲜明对比,验证了冰盖控制下的全球海平面阶段性波动上升模式。对比长江三角洲地区海平面曲线发现,三角洲北部海平面曲线较南部低5～6m,长江三角洲海平面曲线与世界各地海平面曲线也存在明显差异,分析认为主要是由长江口地区的差异性沉降和中国东部边缘海的水均衡作用两个因素引起的。     

Palaeoenvironment of the Karelian Isthmus, the easternmost part of the Gulf of Finland, during the Litorina Sea stage of the Baltic Sea history

The palaeoenvironment of the Karelian Isthmus area during the Litorina Sea stage of the Baltic Sea history, between 8.0 and 4.5 kyr BP (8.8-5.2 cal. kyr BP), was reconstructed by studying four sites located on the Karelian Isthmus in Russia. Methods included diatom and pollen analyses, sediment lithostratigraphical interpretation and ¹⁴C dating. The brackish-water (Litorina) transgression began c. 7.7 kyr BP (8.45 cal. kyr BP) in the Karelian Isthmus area. The transgression maximum occurred between 6.7 and 5.7 kyr BP (7.6-6.5 cal. kyr BP), depending on the glacio-isostatic land uplift rate. Regarding the vegetation, the maximum occurrence of temperate deciduous trees took place at the same time. The transgression was interrupted by a short-lived sea-level standstill during the middle phase of the main transgression, c. 6.3 kyr BP (7.2 cal. kyr BP), on the eastern part of the isthmus. The highest Litorina shoreline is located between 8 and 13 m above present sea-level and the amplitude of the Litorina transgression has varied between 5 and 7 m. The 8.2-kyr cold event is not evident, but the sea-level standstill around 6.3 kyr BP (7.2 cal. kyr BP) could reflect a cool episode at that time in the Karelian Isthmus area.     

A data-driven approach to assess large fire size generation in Greece

The 2011 Tohoku earthquake and tsunami motivated an analysis of the potential for great tsunamis in Hawai‘i that significantly exceed the historical record. The largest potential tsunamis that may impact the state from distant, Mw 9 earthquakes—as forecast by two independent tsunami models—originate in the Eastern Aleutian Islands. This analysis is the basis for creating an extreme tsunami evacuation zone, updating prior zones based only on historical tsunami inundation. We first validate the methodology by corroborating that the largest historical tsunami in 1946 is consistent with the seismologically determined earthquake source and observed historical tsunami amplitudes in Hawai‘i. Using prior source characteristics of Mw 9 earthquakes (fault area, slip, and distribution), we analyze parametrically the range of Aleutian–Alaska earthquake sources that produce the most extreme tsunami events in Hawai‘i. Key findings include: (1) An Mw 8.6 ± 0.1 1946 Aleutian earthquake source fits Hawai‘i tsunami run-up/inundation observations, (2) for the 40 scenarios considered here, maximal tsunami inundations everywhere in the Hawaiian Islands cannot be generated by a single large earthquake, (3) depending on location, the largest inundations may occur for either earthquakes with the largest slip at the trench, or those with broad faulting over an extended area, (4) these extremes are shown to correlate with the frequency content (wavelength) of the tsunami, (5) highly variable slip along the fault strike has only a minor influence on inundation at these tele-tsunami distances, and (6) for a given maximum average fault slip, increasing the fault area does not generally produce greater run-up, as the additional wave energy enhances longer wavelengths, with a modest effect on inundation.     

We have come a long way, yet we still have far to go

Disaster experts around the world have continually warned governments and the public about the possibility of “worst-case” natural hazard scenarios and their overwhelming impacts. Yet, planning for the occurrence of these events has fallen far short of need. The large earthquake that occurred off the coast of Sumatra in 2004, which resulted in one of the deadliest tsunamis ever recorded, was a painful reminder that living in some of the most desirable areas of the world does have its risks. We all have enjoyed the fun of restful visits to coastal resort communities all around the world, and we rarely think about earthquakes or tsunamis interfering with this enjoyment. Yet, they take us by surprise. Before these events do occur, there should be adequate education for everyone on what actions are appropriate as well as an effective warning system to trigger the right actions.     

Large earthquakes and the abandonment of prehistoric coastal settlements in 15th century New Zealand

This paper reports on the effects of large earthquakes and related events, such as tsunamis, on prehistoric coastal settlements in New Zealand. It is based on field observations at several well‐established archaeological sites around the Cook Strait region and on literature reviews. We identify three broad periods of seismic activity in New Zealand since human occupation of the islands: 13th century, 15th century, and the 1750s to 1850s. The most significant, from a prehistoric human perspective, is the 15th century. Using examples from the Cook Strait region, we suggest that the abandonment of coastal settlements, the movement of people from the coast to inland areas, and a shift in settlement location from sheltered coastal bays to exposed headlands, was due to seismic activity, including tsunamis. We expect similar patterns to have occurred in other parts of New Zealand, and other coastal areas of the world with longer occupation histories. © 2003 Wiley Periodicals, Inc.     

Tsunamigenic slope failures: the Pacific Islands ‘blind spot’?

We discuss issues related to a recognised shortcoming in existing tsunami hazard assessments for Pacific Island Countries and Territories (PICTs), that of tsunamigenic slope failures (TSFs). Currently, TSFs are most likely underrepresented as sources in existing tsunami databases for two key reasons. First, relatively low magnitude earthquakes associated with subduction zones are generally assigned as the tsunamigenic source, as opposed to the TSFs they generate. A reassessment of such ‘anomalous tsunamis’ may yield clues that serve to reassign their tsunamigenic source. Second, there are thousands of oceanic islands and seamounts scattered across the Pacific and flank collapse of volcanic edifices such as these is a largely unquantified tsunamigenic threat. However, while it is now possible to model such TSFs, this is unlikely to happen in the near future because of the lack of detailed bathymetry and landslide mass data. Recent developments in the identification of past tsunamis in the Pacific Islands have developed a unique range of indicators that can be used for identifying such events. These are geological, oral tradition and archaeological components that include, but are not limited to, a modified Darwinian model of atoll formation, coastal megaclasts, oral traditions of vanished islands and giant waves, and the abandonment of prehistoric coastal sites. As such, the most logical way forward is to use the multiple indicators available to us to identify evidence of past tsunamis.     

The hydrodynamics of landslide tsunamis: current analytical models and future research directions

Landslide-generated tsunamis are lesser-known yet equally destructive than earthquake tsunamis. Indeed, the highest tsunami wave recorded in recent history was generated by a landslide in Lituya Bay (Alaska, July 9, 1958) and produced run-up in excess of 400 m. In this paper, we review the state of the art of landslide tsunami analytical modelling. Within the framework of a linearised shallow-water theory, we illustrate the dynamics of landslide tsunami generation and propagation along beaches and around islands. Finally, we highlight some intriguing new directions in the analytical modelling of landslide tsunamis to support early warning systems.     

The Earthquake Threat in Southwestern British Columbia: A Geologic Perspective

Ten moderate to large (magnitude 6–7) earthquakes have occurred in southwestern British Columbia and northwestern Washington in the last 130 years. A future large earthquake close to Vancouver, Victoria, or Seattle would cause tens of billions of dollars damage and would seriously impact the economies of Canada and the United States. An improved understanding of seismic hazards and risk in the region has been gained in recent years by using geologic data to extend the short period of instrumented seismicity. Geologic studies have demonstrated that historically unprecedented, magnitude 8 to 9 earthquakes have struck the coastal Pacific Northwest on average once every 500 years over the last several thousand years; another earthquake of this size can be expected in the future. Geologic data also provide insights into the likely damaging effects of future large earthquakes in the region. Much of the earthquake damage will result directly from ground shaking, but damage can also be expected from secondary phenomena, including liquefaction, landslides, and tsunamis. Vancouver is at great risk from earthquakes because important infrastructure, including energy and transportation lifelines, probably would be damaged or destroyed by landslides and liquefaction-induced ground failure.