THE
EARTHQUAKE AND TSUNAMI OF JULY 12, 1993 IN THE SEA OF JAPAN/EAST
SEA
(THE
HOKKAIDO NANSEI-OKI EARTHQUAKE AND TSUNAMI OF 1993)
George
Pararas-Carayannis
 On July 12, 1993 a large earthquake
off the west coast of Hokkaido and the small offshore island
of Okushiri in the Sea of Japan/East Sea, generated a destructive
tsunami.
THE HOKKAIDO
NANSEI- OKI EARTHQUAKE OF JULY 12, 1993
Origin Time - The
earthquake occurred at 1317:12 UT, July 12, 1993. Epicenter:
The epicenter of the earthquake was in the Sea of Japan/East
Sea at 42.8 N, 139.2 E, about 15-30 km off the small offshore
island of Okushiri along the west coast of Hokkaido.
Earthquake Magnitude: 7.8
Death Toll and Damages
A total of 239 people
were killed. More than half of the deaths were attributed to
the tsunami. A total of 558 houses were destroyed. There were
about $600 million in property losses caused primarily by the
tsunami.
It was reported that
the western shore of Okushiri Island subsided by at least 80
cm. Subsidence of about 20-50 cm. was reported along the eastern
shore.
Tectonic
Setting, Kinematic Mechanisms and Seismicity of the Region
 Japan is situated in an active
seismic region. The plate tectonics of the Southern Kuril islands-Northern
Hokkaido region are quite complex and very different than those
along the southern portion of the Japanese Trench. The island
of Hokkaido has been created by active arc-arc collision.
The Sea of Japan/East
Sea is a complex basin between Japan and the Korea/Okhotsk Sea
Basin. It represents a sub plate with apparent counterclockwise
rotational movement as it interacts against the Okhotsk plate,
along the inland sea boundary of the Hidaka Collision Zone (HCZ).
A series of significant earthquakes in the Sea of Japan/East
Sea indicates that there may be active collision of the North
American plate with the Eurasian plate off the west coasts of
Honshu and Hokkaido. However this is not confirmed and there
may be a different scenario of tectonic interactions in this
region.
Vertical ground displacements
at Okushiri Island
(Graphic of the Geographical Survey Institute, 1993)
Near Hokkaido, earthquakes occur on the Pacific Ocean side along
the Kuril and the Japan trenches as the Pacific plate moves against
Hokkaido from an east-southeast direction at a rate of about
8 cm per year. The subduction of the Pacific Plate is at a rather
steep angle along the Kuril Trench boundary. North of Hokkaido,
Sakhalin Island which separates the Sea of Japan/East Sea from
the Sea of Okhotsk is probably the result of transpressional
tectonics along the North America-Eurasia boundary.
Shallow earthquakes
occur also on land areas of Hokkaido, along the southern Hidaka
Mountains. These earthquakes may be the result of stresses caused
by the Pacific plate's angular subduction which - according to
some scientists - may even extend to the region along the eastern
margin of the Sea of Japan/East Sea basin. Thus the major tsunamigenic
earthquakes of 1993 and 1983 may be the result of such tectonic
interactions of Pacific plate subduction off the west coasts
of Honshu and Hokkaido, rather than of active collision of the
North American plate with the Eurasian plate - as previously
stated. The severity of the tsunamigenic earthquakes in 1983
and of this 1993 event, and the extensive subsidence that was
observed on Okushiri Island (in 1993) would tend to support a
tectonic mechanism associated with extended oblique subduction
of the Pacific plate beneath a recently postulated Amurian microplate
of the Eurasian tectonic block, along the Sea of Japan/East Sea.
The postulated
Amurian Microplate (After Wei D. and Seno T. 1998)
THE
TSUNAMI OF JULY 12, 1993
The earthquake of
July 12. 1993 generated a destructive tsunami which was particularly
devastating at Okushiri Island and along the central west and
southwestern shores of Hokkaido. Waves ranging from 5 to 10 meters
in height crashed on the nearest coastlines, destroying fishing
villages and killing 239 people. More than half of the fatalities
were caused by the tsunami. The $600 million in property losses
were attributed primarily to the tsunami.
 Tsunami Travel Time
It took between two
to seven minutes after the earthquake for the first of the destructive
tsunami waves to strike the coastlines of Okushiri Island. The
short tsunami travel time was confirmed by numerous battery-operated
clocks that were found scattered with the debris along the shores
of western Okushiri Island.
Tsunami Generating
Area and Tsunami Travel Times in the Sea of Japan and around
the Japanese Islands ( in minutes)
Timely Tsunami
Warning Issued by JMA
Although the Japan
Meteorological Agency (JMA) issued promptly a regional tsunami
warning, because of the proximity of the tsunami source to the
coastline, there was no sufficient time to warn coastal residents
on Okushiri Island. However, since the 1983 tsunami, preparedness
had greatly improved in this region and many coastal residents
fled to higher ground when they felt the strong ground motions
of the earthquake - thus, saving their lives.
Tsunami
Effects in the Sea of Japan/East Sea
The destructive effects
of the July 12. 1993 tsunami occurred mainly on the southwestern
shores of Okushiri and Hokkaido.
Okushiri
Island
The west and southern
coasts of Okushiri Island were struck by waves that exceeded
20 meters in many locations. There was extensive damage to ships,
houses and structures. In a small valley near the harbor town
of Monai, the maximum tsunami wave runup height was documented
to be as much as 31 meters.
Aonae - Waves ranging in height to over
10 meters breached the protective coastal walls of this small
fishing village on Okushiri's southern peninsula, causing extensive
damage to homes, buildings, vehicles and boats. The waves damaged
gas lines and fuel storage containers, thus starting enormous
fires that completed the destruction of all, still standing,
structures. Primarily the earthquake and not the tsunami caused
the destruction of the lighthouse at Cape Aonae.
Monai - The village of Monai on the
west coast of Okushiri Island was completely destroyed. The waves
reached an elevation of nearly 20 m above sea level, flooding
300 m inland. Ten persons were killed there and 12 houses were
destroyed.
Tsunami Destruction
at Aionae (photo by Y. Tsuji)
Assessment
of Tsunami Generation Potential in the Sea of Japan/East Sea
The Sea of Japan/East
Sea along the west coast of Japan has a greater potential to
generate earthquakes and tsunamis than previously believed. Current
plate tectonic theory indicates a mechanism of extended subduction
of the Pacific Plate underneath Japan and along the eastern boundary
of the postulated Amurian microplate of the Eurasian tectonic
block. However, tsunamis are not generated as frequently along
the west coasts of Japan as they do along the eastern coasts.
Nonetheless, the shallow ocean bathymetry and the morphology
of the western coastlines of Hokkaido and Honshu Islands can
contribute significantly to the destructiveness by tsunamis.
Historic
Tsunamis in the Sea of Japan/East Sea
 The eastern Sea of Japan /East
Sea, along the apparent collision (subduction?)boundary described
above, is an active seismic zone. Earthquakes in this region
have been responsible for the generation of local tsunamis which
have caused considerable destruction along the Japanese, Russian
and Korean coasts. There have been at least 6-8 major historic
tsunamis in the last three centuries in the Sea of Japan/East
Sea. A review of historic records in Japan and Korea will undoubtedly
reveal the occurrence of many more tsunamis. The following are
summaries of some of the better-known events.
Tsunami Damage at
the village of Aonae, Okushiri Island
The Earthquake and
Tsunami of 26 May 1983 -
Almost ten years prior to the 1993 event, a similar earthquake
(the Nihonkai Chubu Earthquake, M 7.7),) off the west coast of
Aomori and Akita Prefectures in the Sea of Japan/East Sea generated
a destructive tsunami which had maximum impact at the Akita Prefecture
in the north-east part of Honshu Island, in northern Japan. The
tsunami source region was close by. The first of the destructive
waves struck the coast of Akita about seven minutes after the
earthquake, completely destroying Oga Peninsula. The maximum
tsunami height reached 14 meters. One hundred four persons perished.
There was extensive destruction of houses, ships and port facilities.
A total of 700 boats and 59 houses were destroyed and the total
property loss was estimated at $800 million (1983 U.S. dollars).
 At Minehama, Honshu, the estimated
tsunami heights were also up to 14 meters. Along southern Hokkaido
and northern Honshu the waves ranged from 2-6 meters. There was
tsunami damage as far away as the Yamaguchi Prefecture in southwestern
Honshu. Destructive tsunami waves ranging in height between 2-7.5
meters struck the eastern and southern coasts of South Korea,
where three people were killed. Also, waves ranging up to 8 meters
struck the coast of Russia (USSR at the time).
Landslide
at Okushiri port triggered by the Earthquake of July 12, 1993
(Photo by Hokkaido Shinbun Co, Ltd.)
The Kamuizaki-Oki
Earthquake and Tsunami of 1949 - This was a major earthquake (M = 7.5) which
generated a destructive tsunami. At Uljin, Kyongsang-Pukdo, and
the island of Ullung, on the east coast of Korea, the tsunami
runup ranged up to 2.0m. There is no much information of its
effects elsewhere.
The Kampo Earthquake
and Tsunami of 1741 -
Records of the Choson Dynasty indicate that an earthquake in
1741 - with an estimated magnitude of M = 7.5 - occurred off
the southwestern coast of Hokkaido, in the Sea of Japan/East
Sea, and that it generated a destructive tsunami that struck
the coast of Kangwondo, destroying houses, boats and killing
about 1,500 people.
Lessons
Learned
The July 12, 1993
event was the second destructive tsunami in the Sea 0f Japan/East
Sea within a ten-year interval. There were many lessons learned
from this event about the potential of tsunami generation along
this very active tectonic boundary. The closeness of the tsunami
generating sources to populated coastal areas emphasized the
need for better preparedness and for automated instrumentation
and systems that can provide immediate warnings following major
earthquakes. The 1993 Okushiri event was extensively documented
and studied by scientists and Civil Defense authorities and much
was learned about how to mitigate the effects of future tsunamis.
Fortunately in Japan there is a great deal of public awareness
of earthquake and tsunami risks.
FOR MORE PHOTOS CONTACT
THE AUTHOR
References
Hokkaido Tsunami Survey
Group, Tsunami devastates Japanese coastal region,
Eos Trans. AGU, 74, 417, 1993.
Pararas-Carayannis,
G., 1983. The Earthquake and Tsunami of 26 May 1983 in the Sea
of Japan
http://drgeorgepc.com/Tsunami1983Japan.html
Pararas-Carayannis,
G., 2006. The Earthquake and Tsunami of 15 November 2006 in the
Kuril Islands
http://drgeorgepc.com/Tsunami2006Kurils.html
Pararas-Carayannis,
G., 2000. Major Earthquakes in Japan in the 20th Century
http://drgeorgepc.com/EarthquakesJapan.html
Seno, T., and Y. Yamanaka.
1996. Double seismic zones, compressional deep trench - outer
rise events and superplumes in Subduction Top to Bottom, edited
by G. E. Bebout, D. W. Scholl, S. H. Kirby, and J. P. Platt Geophys.
Monogr. 96 347-355 1996
Wei D. and Seno T.
1998. Determination of the Amurian Plate Motion. In Mantle Dynamics
and Plate Interactions in East Asia, edited by Martin Flower
, GeoDynamics Series., AGU, 1998
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