Tsunami deposit

[citation needed] Sediment entrained in the tsunami wave that is not deposited onshore may either settle out in the shallow water or become involved in debris flows, possibly becoming turbidity currents as velocities increase downslope.

Debris flows and turbidites may be formed by slope failures, which may themselves be directly triggered by the earthquake.

The extent of a particular deposit may help to judge the magnitude of a known historical earthquake or to act as evidence of a prehistoric event.

In the case of the 869 Sanriku earthquake, the identification of tsunami deposits over 4.5 km inland on the Sendai Plain, dated quite closely to a historical tsunami event, enabled the magnitude of this earthquake to be estimated and the likely rupture area offshore located.

[8] Based partly on this information TEPCO revised estimates of likely tsunami heights at the Fukushima Daiichi Nuclear Power Plant to greater than 9 m, but took no immediate action.

[9] The tsunami triggered by the 2011 Tohoku earthquake had a wave height at Fukushima of about 15 m, well above the 5.7 m for which the plant's defences had been designed.