Along Hokkaido's Pacific coast near the town of Kiritappu, sandy deposits in a muddy lagoon and on a nearby beach‐ridge plain provide evidence for 15 tsunamis between 200 and 6000 years ago. Additional sand beds at the lagoon probably represent the historical tsunamis of A.D. 1843 and 1894. We observed the sequences of sandy deposits in continuous slices 2 to 4 m deep. Some of the deposits consist of just a single sand bed, whereas others contain multiple units of sand, muddy sand (or sandy mud), and mud caps including plant detritus. We also found at the lagoon a 17th century tsunami deposit that thickens and thins regardless of elevation or distance inland. We bracketed the ages of most of the inferred tsunamis by radiocarbon dating of detritus, mainly seeds and leaves at the lagoon and charcoal at the beach‐ridge plain, from pretsunami and posttsunami beds. Tsunami dates computed from the bracketing ages commonly have uncertainties spanning 2 to 4 centuries. Within these uncertainties, the inferred sequence of 15 prehistoric tsunamis at the lagoon, beginning almost 6000 years ago, can be matched tsunami by tsunami with the inferred history at the beach‐ridge plain, 15 km away. The sand sheet extents suggest that most of these tsunamis were larger than any generated at Hokkaido in the last 200 years. The intervals between these inferred outsized tsunamis average nearly 400 years but range widely from about 100 to about 800 years.
Four sand units deposited by tsunamis and one sand unit deposited by storm surge(s) were identified in a muddy marsh succession in a narrow coastal lowland along the Pacific coast of central Japan. Tsunamis in ad 1498, 1605, 1707 and 1854 that were related to large subduction‐zone earthquakes along the Nankai Trough, and storm surges in 1680 and/or 1699 were responsible for the deposition of these sand units. These sand units are distinguished by lithofacies, sedimentary structures, grain‐size and mineral composition, and radiocarbon ages; their ages are supported by events in local historical records. The tsunami deposits in the study area are massive or parallel‐laminated sands, with associated intraclasts, gravels, draping mud layers and, rarely, a return‐flow subunit. The storm surge deposits are devoid of these characteristics, and are composed of groups of thin, current ripple‐laminated sand layers. The differences in sedimentary structures between the tsunami and storm surge deposits are attributed to the different characteristics of tsunami and storm waves.
The tsunami heights from the 2004 Sumatra-Andaman earthquake were between 0.4 and 2.9 m along the Myanmar coast, according to our post tsunami survey at 22 sites in Ayeyarwaddy Delta and the Taninthayi coast. Interviews to coastal residents indicate that the tsunami heights were lower than high tide level in rainy season, probably by storm surge. They also testified that the arrival times were between 2 and 5.5 hours after the earthquake but the reliability may be low because nobody felt ground shaking. Much smaller tsunami than the neighboring Thai coast, where the tsunami heights were 5 to 20 m, explains relatively slighter tsunami damage in Myanmar; the casualties were reported as 71, compared to about 8300 in Thailand. The smaller tsunami was probably due to the fact that the main tsunami source did not extend to Andaman Islands. The tsunami travel times and maximum heights computed from a 700 km long source are basically consistent with the observations. For a nearby tsunami source, the tsunami hazard would be more significant in Myanmar, because coastal houses are unprotected for tsunamis and no infrastructure exists to disseminate tsunami warning information.
Tectonic environments, recent stress and crustal strain observations, and historical descriptions of geomorphological changes and eyewitness accounts of the 1762 Bengal earthquake suggest that great earthquakes (M 8.0 or larger) can occur along the northward continuation of the 2004 Sumatra-Andaman earthquake. We describe marine terraces along the Rakhine coast of Myanmar as evidence for three great earthquakes in the past 3400 years. Radiocarbon dating of coral remains suggests that the oldest terrace emerged three times, during 1395–740 BC, AD 805–1220 and AD 1585–1810. We assign the youngest age to the 1762 earthquake, which reportedly raised parts of the Burmese coast by 3–7 m. These indicate that the great subduction-zone earthquakes have repeatedly occurred west off Myanmar with an average recurrence interval of about 1000–2000 years. The time since the last earthquake, ~ 250 years, is much shorter than the average interval, hence the chance of next earthquakes in the near future may be considered as low. However, the variability in both uplift amounts and recurrence intervals suggests the next great earthquake could happen sooner or later than would be expected from the average interval.
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