Hokkaido faces risk of massive quakes in fault zone, tectonic plate boundary

TOKYO -- A strong earthquake that hit Hokkaido on Sept. 6 was probably caused by the movement of fault planes inside the crust of a tectonic plate, according to the Japan Meteorological Agency (JMA).

A major active fault zone called the Eastern Boundary Fault Zone of Ishikari Lowland runs from north to south about 10 kilometers west of the quake's epicenter, but the focus is 37 kilometers below the surface -- unusually deep if the jolt was triggered by the movement of the active fault. A JMA official therefore said it is not clear if the earthquake and the fault zone are connected.

The Ishikari fault zone has two parts. The main portion is 66 kilometers long and is estimated to be capable of producing quakes of magnitude 7.9, and the second, southern portion running at least 54 kilometers has the potential to trigger a magnitude-7.7 temblor. The government's Headquarters for Earthquake Research Promotion estimates that the probabilities of such quakes occurring in the next 30 years are "nearly zero" for the main portion and "0.2 percent" for the southern stretch.

University of Tokyo professor Naoshi Hirata, who heads the government's Earthquake Research Committee, told the Mainichi Shimbun that the focus was "too deep for an earthquake caused by an active fault," suggesting the link between the Ishikari fault zone and the quake is weak. A magnitude 5.1 jolt hit the area in Hokkaido in July last year with a similar cause. The focus at that time was closer to the fault zone at a depth of 27 kilometers, but the earthquake was judged to have no connection with the fault.

The area where the Sept. 6 quake occurred is prone to seismic activity because the earth's crust is under pressure both from the east and the west, according to Hirata. "Special vigilance must be observed for the next few days as quakes of similar scale may hit," he said.

Meanwhile, University of Tokyo professor Takashi Furumura says it is not possible to deny the possibility that the latest earthquake occurred on the fault plane in the Ishikari fault zone's deeper portion with an eastern tilt. "It could have happened inside the active fault zone or along a hidden fault running along the zone."

Hokkaido faces the possibility of a massive magnitude 9 quake near the tectonic boundary along the Kuril Trench. The latest quake's focus, however, was 50 kilometers above the boundary, and JMA officials say it is unlikely the Sept. 6 temblor has any link with the area.

(Japanese original by Tomohiro Ikeda, Science & Environment News Department)