JOURNAL

Vol. 5, No. 4 June 1995

1

UMMU GE

Patricia A. Lockridge

National Geophysical Data Center NOAA/NESDIS

The last three years have seen an unusual apparent increase in the frequency of destructive tsunami events. First, a group of three very destructive tsunamis occurred in the months between September 1992 and July 1993 (Figure 1), resulting in more than 1300 deaths. After an eleven-month period, a second series of five destructive tsunamis began in June 1994. No other 2.3 year period since the turn of century has had eight destructive tsunamis. The following is a brief description of each event between September 1992 and December 1994, and a comparison of the period with previous periods of tsunami activity.

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The September 1992 tsunami in Nicaragua

This event and the Skagway event in November of 1994 were the only two tsunamis of the eight (in this time period) to occur in the eastern Pacific. At 7:16 P.M. local time on September 1, 1992, an earthquake with a surface wave magnitude of 7.2 generated a tsunami with waves between 8 and 15 m high that struck 26 towns along 250 km of Nicaragua's Pacific coast. More than 40,000 people were affected by the loss of their homes or means of income. The waves left 170 dead, and another 500 injured. A tsunami inundation of 1000 m was reported at Masachapa, where at least 15 people were killed. The tsunami caused an estimated U.S. $25 million in damage and losses. Fifty-three percent of this damage occurred to housing. Low income people suffered the most, incurring losses of homes, fishing boats, and income. The tsunami occurred in the evening when the fishing boats were docked, and many of them were lost or

Tsunamis, from page 1

damaged by the waves. Several tourist centers were damaged, and two schools were destroyed; in addition, water supplies, waste disposal, electricity, and port infrastructure were affected. The ecology of the coastal area was injured as the waves and high salinity seawater destroyed plants, fish, turtles, and fish and turtle eggs.

The earthquake epicenter was located about 120 km west-southwest of the city of Managua. The earthquake was caused by the interaction of the Cocos and Caribbean tectonic plates, and occurred within a seismic gap located along the intersection of this event. This may have included a slow earthquake recently modeled by Jordan and Imhle (1995). Several aftershocks of decreasing magnitude followed.

The unusual wave height and destruction from the tsunami resulted from the relatively shallow earthquake depth and a subterranean landslide. The tsunami moved rapidly toward the Nicaragua coast, arriving at some coastal locations just twenty minutes after the earthquake.

The December 1992 tsunami in
Indonesia

Of the eight events described in this article, the December 1992 tsunami in Indonesia was the most devastating. On December 12, 1992, at 05:29 UT a 7.5 magnitude (Ms) earthquake occurred in the Flores region of Indonesia, about 1800 east of Jakarta. The death toll as a result of the combined earthquake and tsunami effects was 2800, including 1,490 at Maumere and 700 on Babi Island. More than 500 were seriously injured, and 90,000 were left homeless. In addition, 28,118 houses, 785 schools, 307 mosques, and 493 commercial buildings were damaged or destroyed.

On Kalaota, 19 people were killed and 130 houses were destroyed. Severe damage occurred at Maumere, with 90 percent of the buildings destroyed by the earthquake and tsunami; damage also occurred on Sumba and Alar. Tsunami inundations of 300 m with wave heights of nearly 30 m were reported on Flores Island, along with landslides and ground cracks at several locations around the island.

The maximum tsunami runup

height of 26.2 m was measured at Riangkroko, where 163 people lost their lives. Severe coastal erosion occurred during the tsunami, exposing eroded coral complexes and lowering coastal island surfaces. Coastline areas were characterized by the deposition of extensive and continuous sediment sheets of up to a meter in thickness. Wave reflection off Flores Island may have been partially responsible for the devastation on Babi Island.

The Hokkaido Nansei-Oki tsunami of
July 12, 1993

On July 12, 1993, at 22:17 local time (1317 GMT), an earthquake with a surface wave magnitude of 7.6 occurred off the west coast of Hokkaido and the small offshore island of Okushiri in the Sea of Japan. In two to five minutes the tsunami engulfed the coastline of Okushiri Island and the central west coast of Hokkaido. More than 330 fatalities were associated with the event, with more than half attributed to the tsunami. The death toll on Okushiri Island alone was at least 165.

This July 12, 1993 earthquake filled a previously identified seismic gap. The resulting 30 m tsunami caused spectacular localized damage, especially on the southwestern shores of Hokkaido and on Okushiri Island. A runup height of nearly one meter was recorded at Aomori, Honshu, where one person was killed on a fishing boat.

The tsunami reached Russian shores in 30 minutes, with runup heights of one to four meters. The tsunami affected much of the southeastern coast of Russia, and also caused damage to a factory at Kamenka, Sakhalin Island. Three people from the southeast coast of Russia were missing after the tsunami. About 50-70 minutes after the earthquake, the first waves arrived in Prymoric, Russia. Three main waves were registered, but at some points up to eighteen minor waves were reported. In enclosed bays, sea level oscillations (seiches) were observed until noon.

Ninety minutes after the earthquake, the tsunami struck the coast of the Republic of Korea, where a maximum tsunami height runup height of two meters was recorded. Approximately 600 fishing boats were damaged

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