The sound of water flowing from the subducting slab beneath Japan

Both the Philippine and the Pacific Plates are subducting beneath Japan, and these subduction zones are responsible for the relatively frequent earthquakes and volcanoes experienced in that country.  In view of those threats, Japan has developed what is arguably the most intensive and sophisticated geophysical monitoring program in the world.  The Japanese have a network of almost 1000 GPS stations to monitor crustal deformation, and in 2000 they completed installation of a network of 600 high sensitivity seismometers to detect microearthquakes (Hi-net).  Each of those stations has a three-component (2 horizontal and one vertical) seismometer situated at 100 to 200 m depth within a borehole.  The output from each component at each station is digitally sampled 100 times per second and then relayed to a central processing facility. Scientists from the National Research Institute for Disaster Prevention have used Hi-net data to detect very low amplitude seismic tremors in southern Japan (Obara, 2002). Seismic tremor (or “harmonic tremor”) is normally associated with the relatively shallow underground movement of magma in volcanic areas (such as along rift zones in Hawaii).  The seismic tremor recently detected in Japan originates at depths of between 30 and 45 km in the general vicinity of the subducting Philippine Plate.

The Philippine Plate is moving towards the Eurasia Plate at about 40 mm/y and is subducting beneath the southern part of Japan (and also beneath China, Taiwan and the Philippines).  The Pacific Plate is moving in roughly the same direction at about 80 mm/y, and is subducting beneath the northern half of Japan (plus Kamchatka and Alaska).  the Pacific Plate is also subducting beneath the Philippine Plate along the Izu-Bonin Volcanic Arc.  In this region the Philippine Plate is between 15 and 30 m.y. old, and hence it is considerably warmer than the 130 m.y. old Pacific Plate (Julian, 2002). (Volcanoes are shown as red triangles.)

Distribution of seismic tremor events during 2001.  The thick grey line is the leading edge of the subducting Philippine Plate.  The thinner grey contours show the interpreted depths of that plate.

Tremor events persist for several days, and in some cases for several weeks.  The tremor signals have been observed to migrate several tens of km along the strike of the subduction zone at rates in the order of 10 km/day.  Several of the tremor events observed during 2000 and 2001 started immediately after moderate to large local earthquakes (M 4.0 to 6.7), but at least two tremor events stopped immediately after local earthquakes. 

Time-sequence of tremor episodes in the Shikoku area.  The arrows represent significant earthquakes which occurred near to the area of active tremors.

Obara (2002) suggests that the seismic tremor is related to the movement of water that is generated within the subducting Philippine Plate due to the heat-induced hydration of minerals within the rock.  The correlation with earthquakes may be related to strain changes in the rock (Julian, 2002), or to stimulation of the fluid-generation process by the shaking (Obara, 2002).  The 30 to 45 km depth corresponds generally with the 45 to 55 km predicted depth of hydration in the Philippine Plate (Julian, 2002).  The cooler and more quickly subducting Pacific Plate is not expected to start dehydrating until about 100 km depth in this region (Julian, 2002).  Water produced from the dehydration of minerals is generally considered to be the trigger for the melting of the mantle rock immediately above the plate and ultimately of subduction-related volcanism.  There is no volcanism above the area of volcanic tremor in southern Japan.  This could be related to the relatively shallow depth at which the process is taking place (compared with other subduction zones). 

References

Julian, B., 2002, Seismological detection of slab metamorphism, Science, V. 296, p. 1625-26 (May 2002)

Obara, K., 2002, Nonvolcanic deep tremor associated with subduction in southwest Japan, Science, V. 296, p. 1679-81 (May 2002)

Steven Earle, 2002. Return to Earth Science News