’Quake caused by explosion?

Press, 10 January 1987, Page 32

’Quake caused by explosion?

NZPA-Reuter San Francisco Geophysicists who studied a 1984 earthquake in Japan say they have confirmed a decades-old theory that quakes can be caused by huge explosions in sediments beneath the sea floor. Earthquakes are normally thought to be caused by faults in the earth’s crust. According to findings being presented to the autumn conference of the American Geophysical Union, an earthquake that occurred near Tori Shima, Japan, on June 13, 1984, was unusual in several respects. The quake caused disproportionately large tidal waves although the tremor was measured at only 5.5 on the Richter scale. The waves were measured at between 130 cm and 150 cm high more than 160 km from the epicentre.

Hiroo Kanamori, a geophysicist at the California Institute of Technology, said he detected other abnormalities which ruled out faulting as the cause of the earthquake. The best explanation, he said, was that a sudden, intrusion of magma into the sea floor, possibly lasting no more than 20 seconds, caused a reaction between the molten rock and water in the seabed’s sediment. “The magma rapidly raised the temperature of this water, causing it to explode sideways within the sediments,” he said. He said that the huge explosion lifted the sea floor over a large area. Dr Kanamori said the theory that injections of magma could cause earthquakes was first proposed by a Japanese scientist nearly 50 years ago but could not be confirmed until recently because of the lack of precise seismic data.