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A magnitude 8.1 earthquake occurred in the southern Kurils and on northern Hokkaido
on Tuesday, October 5, 1994, (October 4 at 13:23 GMT). It was a sudden event, without
any short-term precursors or foreshocks. The earthquake epicenter was located 80 km
east southeast of Shikotan Island. The slides in this set show views of a newly-created
landslide formation, ground cracks, structural damage, and effects of tsunami runup
on Shikotan Island. The documentation is a scientific overview of this event, written
by a member of the international team that studied it.Viacheslav K. Gusiakov Computing
Center, Novosibirsk, 630090, RussiaA magnitude 8.1 earthquake hit the southern Kurils
and northern Hokkaido on Tuesday, October 5, 1994, at 00:23 local time (October 4
at 13:23 GMT). It was a sudden event, without any short-term precursors or foreshocks.
The earthquake epicenter was located 80 km east southeast of Shikotan Island, the
biggest island in the Small Kuril (Habomai) group of islands. The earthquake was the
largest shallow event in the Pacific since the 1989 Macquarie Ridge earthquake. It
was felt over a large area from Severo-Kurilsk, Paramushir Island (about 1200 km north
of the epicenter) to Tokyo, Honshu Island (about 800 km to the south). The source
parameters of the main shock as determined by different seismological agencies and
tsunami warning centers are shown in Table 1.The main shock was followed by an extensive
series of aftershocks. The network of the Research Center for Earthquake Prediction
(RCEP), Hokkaido University, recorded 440 events during the first day, 2,100 events
during the first week and more than 4,000 events during the first month. The largest
aftershock, Ms=7.3 (RCEP data), occurred on October 9, 1994, at 07:56 GMT.On Shikotan
Island, located closest to the earthquake source region, ground shaking was extremely
intense. The intensity was reported to be between IX and X on the abridged Modified
Mercalli Intensity (MMI) Scale (12 intensity scale). Two major fissures occurred on
Shikotan near Malokurilskoe and Krabozavodskoe villages (see photos 1-9). Numerous
ground cracks were reported from other islands. Communications with the mainland were
interrupted by the shock and were restored only by the next morning. As a result,
a tsunami warning from the regional Tsunami Warning System in Yuzhno- Sakhalinsk was
not delivered to the island in a timely fashion. Heavy destruction was reported on
Kunashir and Iturup Islands. Telephone and electric power, as well as the water supply,
were cut in many villages in the Southern Kurils. Fortunately, there was no fire caused
by this quake. At that time of year heating systems were not inuse and cutting the
electric supply helped prevent igniting fires in domestic locations. At Hokkaido,
highways buckled and were shattered by deep fissures. Some bridges collapsed. The
strong quake shook goods off the shelves of shops in the coastal city of Kushiro,
Japan. Office workers had to hold their computers down to keep them from bouncing
off their desks. NHK public television said that the quake blacked out more than 10,000
homes on Hokkaido and paralyzed rail traffic in Aomori Prefecture (the northern tipof
Honshu). In Kushiro, telephones and electrical power were cut in someareas and road
and rail traffic was halted after the quake.CasualtiesThe total casualty toll was
relatively small for an 8.1 magnitude earthquake. In the Southern Kuril Islands, 11
people were killed and 242 were injured. Of these casualties, seven were buried under
the ruins of a two- story military hospital in Goryachie Klyuchi Village on Iturup
island. On Shikotan Island, two people were killed by falling building debris as they
tried to leave their apartments. Most of the people in the affected area experienced
a heavy shock after almost three minutes of continuous shaking accompanying the low-frequency
sound and spent the night outside buildings even though some of the buildings were
not seriously damaged. None of the casualties in the Kurils were due to the tsunami
itself, mainly because the wave heights in all populated areas did not exceed 1.5
to 2 meters.The Kyodo News Agency reported that more than 140 people were injured.
One 73-year old man was reported dead, apparently from a heart attack, and one woman
was seriously hurt when her car plunged off a collapsing bridge.TsunamiA regional
tsunami warning was issued by Japan Meteorological Agency (JMA) and broadcast on TV
stations with impressive speed. At 13:28 UTC, just five minutes after the quake, the
NHK transmitted a map of Japan with the eastern coast of Hokkaido flashing in red
to indicate a "tsunami warning" area, and the Pacific coast of Honshu flashing in
yellow to indicate a "tsunami caution" area. However, at that time it was not accompanied
by any information about the earthquake, its position and size, leaving TV viewers
ignorant of where the earthquake occurred and how large it was (Abbot, 1994). This
information was released by JMA several minutes later.In Russia, a tsunami warning
for the Southern Kurils was issued by the Yuzhno-Sakhalinsk Seismological Observatory
at 13:32 UTS (00:32 local time). However, it was not transmitted to the threatened
area until the next morning because communications with Shikotan and Kunashir were
broken by the earthquake.At 14:33 UTC, a Pacific-wide tsunami warning was issued by
the Pacific Tsunami Warning Center (PTWC) based on the seismological criterion (magnitude
greater than 7.5) and reported 1.73 meter run-up in Nemuro, Japan. Although no damaging
Pacific-wide tsunamis have ever been generated in the Southern Kuril region, there
was lingering concern of the potential threat for Hawaii, when the Midway tide gauge
reported 0.5 meter oscillations of water level (Tsunami Newsletter, 1995). The highest
wave in Hawaii was only 0.8 meter (peak to trough) observed in Kahuliu, Maui. The
highest recording on the U.S. west coast was 1.1 meter (peak to trough) at Crescent
City, California. The list of observed tsunami wave heights is shown in Table 2. The
majority of the regional data are results of measurements made during the field survey
while most of the distant data are taken from the post-event analysis of mareograph
records.The largest tsunami runup was measured on the eastern coast of Shikotan Island.
The runup varies from 6.0 to 10.4 m, with very smooth variation along the coast line,
indicating the long period of the maximum wave. The highest wave in Japan (nearly
3.5 m, peak to trough) was measured by the tide gauge in Hanasaki (northern Hokkaido).
All distant tsunami heights were less than 1 meter, so that no damaging Pacific-wide
tsunami was generated by this quake. However, this tsunami was recorded clearly by
many tide gauges throughout the Pacific.The earliest and probably the largest cataloged
event (magnitude Ms=8.2) occurred here on April 25, 1843. The exact position of the
source region for this event is unknown. However, severe seismic shaking and damage
was reported in the large area between Urup Island in the north and Kushiro, Japan,
in the south. The data on tsunami heights in the Southern Kurils are not available,
but Iida (1984) reported 4.5 m waves near Kushiro. The next large (Ms=7.9) event in
the area occurred on March 22, 1894, with the reported maximum tsunami runup of 4
m at Miyako, Japan.After this event, there were no large (with Ms>7.9) earthquakes
for a number of years. Then, 64 years later, on November 6, 1958, an earthquake of
Ms=8.2 occurred near Iturup Island. This earthquake generated tsunamis, and the maximum
runup height (about 5 m) was reported on the east coast of Iturup Island. Only 11
years later (on August 11, 1969) another large earthquake (Ms=8.2) struck the same
region, about 150 km south-west of the 1958 event. The maximum runup height (up to
5 m) was measured on the east coast of the Shikotan Island. The tsunami source areas
of the 1958 and 1969 events overlap each other for about 50 km along the trench. The
present Shikotan earthquake of October 4, 1994, occurred near the same location as
the 1969 event. The aftershock areas of both events almost coincide with each other;
the deviation in the area locations was only on the order of 20 30 km. There were
other recent and substantial earthquakes at nearly the same location. An earthquake
of Ms=7.0 occurred on June 10, 1975, at the northeastern edge of the 1994 Shikotan
earthquake source region. Despite its smaller magnitude, tsunamis with 2.5 m height
were measured at the nearest coast. The 1975 event is considered to be an example
of a so-called tsunami earthquake (Kanamori, 1972). Furthermore, the southwestern
edge of the 1994 Shikotan earthquake source is also overlapped by the source area
of the June 17, 1973, magnitude 7.4 earthquake. The latest events in this area were
a series of five tsunamigenic earthquakes of March 1978 which occurred at the northeastern
edge of the 1994 event source area.SurveyThe tsunami reconnaissance survey was conducted
from October 16 through November 1, 1994, by an international team consisting of sixteen
Russian scientists and two members from the United States. Tsunami runup height measurements
were made in Shikotan, Iturup, Kunashir, and small islands between Shikotan and Hokkaido
(Ivaschenko et al., 1996; Yeh et al., 1995). Many measurements along the Hokkaido
coast were carried out soon after the event by the Tohoku University group (Sendai,
Japan).
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