The
1854 Ansei-Tōkai earthquake occurred at about
09:00
local time on 23 December, 1854. It
had a
magnitude of 8.4 and
caused a damaging
tsunami. More than 10,000
buildings were completely destroyed and there were at least 2,000
casualties.
Less than two days later, southern Honshu
was hit by
another
earthquake
of similar size.
Background
The
southern coast of Honshu runs parallel to the Nankai Trough, which marks the subduction of the Philippine Sea Plate
beneath the Eurasian
Plate. Movement on this
convergent plate boundary leads to
many earthquakes, some of them of
megathrust type. The Nankai megathrust
has five distinct segments (A-E) that can rupture independently,
the segments have ruptured either singly or together repeatedly
over the last 1300 years.
Megathrust earthquakes on this
structure tend to occur in pairs, with a relatively short time
gap between them. In addition to the two events in 1854, there were
similar earthquakes in 1944 and 1946. In each case the northeastern
segment ruptured before the southwestern segment.
Damage
Much of central Japan experienced seismic intensities of 5 (on the
JMA
scale).
Damage from this earthquake was particularly
severe in the coastal areas of Shizuoka Prefecture
from Numazu to
Tenryu
River
, with many house being damaged or
destroyed.
On the
east side of the Izu
Peninsula
, Shimoda was hit by the tsunami one hour
after the earthquake. A series of nine waves struck the
city, destroying 840 houses and claiming 122 lives.
Diana,
the flagship of a visiting Russian admiral,
Putyatin, was spun round 42 times on its
moorings and was so badly damaged that it sank in a later
storm.
At
Suruga
Bay
, on the west side of the Izu Peninsula, the village
of Iruma was completely destroyed and a 10 m high sand dome
was deposited, on which the village was later reconstructed.
Characteristics
Earthquake
The rupture area, magnitude and epicenter have been estimated from
seismic intensity measurements, information about tsunami arrival
times and evidence of co-seismic uplift/subsidence.
Tsunami
In most of the affected areas, run-up heights were in the range of
4–6 m.At Iruma, run-up heights of 13.2 and 16.5 m have
been measured, much higher than most of the surrounding area. This
and the deposition of the unusual sand dome, with an estimated
volume of 700,000 m
3, is interpreted to have been
caused by the effects of resonance in the V-shaped Iruma bay.
References