April 5, 1990 Tsunami: Mariana Trench, N. Mariana Islands, US Territories

April 5, 1990, 21:12 UT. A MW 7.5 earthquake in the Marianas Trench near Saipan produced an observable tsunami at the Tinian dock but no damage. The Civil Defense survey did not find evidence of a tsunami at Saipan, and considered news reports of three to four meter waves at Saipan and Tinian to be greatly exaggerated. However, crewmen of the Jungle Boat on the Talofofo River, Ted,

Andy and Lauren Fairfield told Paul Hattori that they were at the loading area at about 8:00 A.M. when they saw a

rush of foamy sea water coming under the bridge to the loading raft going up the river. The raft rose 5 to 6 feet. The water receded to the bay after 15 minutes or longer and took the form of a strong rip current. As in 1993, debris was carried up the river for a distance of two miles. Talofofo bay is shaped to focus the water to the river mouth at the head of the bay giving a higher water level there. However, there must have been observable waves elsewhere on the northern and western coasts but this area is lightly developed and no known surveys were taken. The tsunami was not reported as having been recorded in Guam and the marigrams have not yet been located. It was recorded in Japan with heights of 26 cm at Hachi, 24 cm at Murotomisaki, 23 cm at Chichijima, 22 cm at Tosashimizu, 19 cm at Yaene, 16 cm at Mera, 15 cm at Kushimoto, 10 cm at Aburatsu, 7 cm at Owase, 6 cm at Choshi, and 4 cm at

Uchiura and Naha, in Hawaii with a height of 24 cm at Kailua-Kona, at Midway Island with a height of 6 cm, at

Wake Island with a height of 4cm, and at Truk Island with a height of 3 cm. (reference #482)

The Japan Meteorological Agency (JMA) has been routinely operating two Permanent Ocean Bottom Seismograph Systems (OBS). One was developed and installed off Cape Omaezaki in 1978 by the Meteorological Research Institute. The other was deployed by the Headquarters of JMA in 1986 off the Boso Peninsula, Kanto District. The OBS off Cape Omaezaki has four seismographs and one pressure gauge for detecting tsunamis, and another system has four seismographs and three pressure gauges. The OBS system of JMA has a long cable for transmitting signals to the station on land. On April 5,1990, another small tsunami was generated by an earthquake of Ms 7.5 near the Mariana Islands, and it was observed at many stations in Japan. We can find in the records of the OBS tsunami-meter not only sea level changes, such as tide and tsunami, but also thermally induced fluctuations and pressure variations caused by ground motion. Traces at TK1 and BSl show the ocean tide fairly well, however the ones at BS2 and Bs3 are surely deformed by thermally induced effects. Another type of thermal noise appears around eight o'clock and continued for about two hours on the records of TK1. Ground motion noise begins at the same time as the earthquake at

6:13 on April 6, JST (9 hours ahead to Greenwich Mean Time) and continues for about two hours. This noise is larger, as the water is deeper. Tsunami waves can be seen for several hours from about ten o'clock, however the amplitudes in the deep sea are much smaller than sea level fluctuations at

the tide stations. (reference #312)