Aperiodic recurrence of geologically recorded tsunamis during the past 5500 years in eastern Hokkaido, Japan

Abstract

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.

Possibility of M w 9.0 mainshock triggered by diffusional propagation of after-slip from M w 7.3 foreshock

“…However, because this phenomenon alone cannot alert us to earthquake generation, in order to accomplish such a prediction we need to know beforehand the locations of coupled patches and their tectonic stress levels. Such evaluations could be made possible by improvements in geodetic (e.g., Hashimoto et al, 2009) and seismic (e.g., Uchida et al, 2004) monitoring for plate coupling, combined with paleo-seismological history reconstruction (e.g., Sawai et al, 2009) to quantify elapsed times since previous earthquakes and their magnitudes. It is also essential to develop proper physical fault models to input these data and to translate them into physical fault states.…”

Environment, Trade and Society in Southeast Asia

“…By this means it was calculated that the megatsunamis occurred at an average of 400 years interval, though varying unhelpfully from 100 to 800 years. Most chilling was the conclusion, relevant also for Sumatra, that 'most of these tsunamis were larger than any generated at Hokkaido in the last 200 years' (Yuki Sawai et al 2009). Although very little comparable work was done on Sumatra before the 2004 disaster, the island has now been declared by a Scripps Institution geodesist to be 'the best area in the world to be able to predict a quake' , because of the perceived periodicity of major tears in its subduction zone (Yehuda Bock, as cited in Perkins 2008).…”

Generation Mechanism of Giant Earthquakes in Subduction Zones with Smaller-Size Interplate Earthquakes During Interseismic Period

“…The estimated source area includes the 2003 coseismic slip area, the 1973 Nemuro coseismic slip area (Mw=7.8), and the wide afterslip area. Note that most of the recurrence interval estimated from the tsunami deposits in this area is less than 500 years (Sawai et al, 2009). .…”