Geological records of western Pacific tsunamis in northern Taiwan: AD 1867 and earlier event deposits

Abstract Located in the western circum-Pacific seismic zone, Taiwan is an ideal area for recording tsunamis that originate in the tectonically and volcanically active systems of the Ryukyu Arc, Okinawa Trough, and Luzon Arc. However, because of Taiwan's short historical record, there is only credible evidence for one disastrous tsunami, which occurred in northern Taiwan in AD 1867. To better understand the western Pacific tsunamis, an Upper Holocene deposit of a coastal plain inundated by the AD 1867 tsunami was investigated to search for geological evidence of tsunamis. Based on facies characteristics and sediment composition, we identified two sharp-based, decimetre-thick marine quartzose sandy layers that fine upward and landward, intercalated with floodplain marsh sand and mud. The fluvium are rusty red, rich in volcanic clasts and Fe, and comparable to modern riverine sediments of local andesitic provenance. Extending 0.8 km inland, the marine quartzose sandy layers are light-coloured and rich in quartz and Si, have high Ca/Ti values, and are comparable to the modern beach–dune sediments of regional siliciclastic provenance. Radiocarbon dating indicates that the marine sandy layers are correlated with the AD 1867 tsunami and an older event at the boundary between the 17th and 18th centuries. Both events were probably associated with reported earthquakes involving normal faulting. In addition, the distribution patterns of the two marine sandy layers are similar, suggesting that the inundation distances, run-ups, and flow heights of the two events are comparable. This indicates that the disastrous tsunamis were frequent in the area between northern Taiwan and the Okinawa Trough of the western circum-Pacific seismic zone.