Subducting oceanic crusts of the Philippine Sea and Pacific plates and weak-zone-normal compression in the Kanto district, Japan

Abstract

S U M M A R Y Oceanic crusts subducting into the mantle were detected by analyses of mantle earthquakes in the Kanto district, central Japan. Earthquakes in a depth range of 30-80 km in the southwestern Ibaraki prefecture were relocated by a modified method of joint hypocentre determination. We obtained a good distribution map of hypocentres and focal mechanisms. A boundary layer between the Eurasian and Philippine Sea plates was found, where thrust earthquakes with P axes in the NW-SE direction and of which low-angle nodal planes dip northward occurred in a layer with a thickness of about 4 km. The P-wave velocity in this layer is estimated to be less than about 7.0kms-l. Another boundary layer between the Philippine Sea and Pacific plates exists about 15 km beneath the above boundary layer. Thrust earthquakes with P axes in the EW direction and of which low-angle nodal planes dip westward occurred in this layer that also has a thickness of about 4km. Furthermore, normal-faulting earthquakes of which the P axes are almost perpendicular to the boundary layers were found immediately outside the boundary layers. Combining these results, we can conclude that the boundary layers are oceanic crusts that exist at the top of the subducting Philippine Sea and Pacific plates. 'Weak-zone-normal compression' was proposed to result from the existence of low-velocity oceanic crusts at plate boundaries. This hypothesis can be explained, because only stress caused by relative motion of the two plates is consumed inside the weak oceanic crust. The remnant stress after subtracting the above stress from the tectonic one applied at the plate boundary is the stress of which the P axis is normal to the plate boundary.

A Non Double-Couple Earthquake in a Subducting Oceanic Crust of the Philippine Sea Plate.

“…The focal depth of the non double-couple earthquake determined by Hurukawa and Imoto (1992) is 51 km and is about 10 km shallower than those determined routinely by the NIED or Japan Meteorological Agency (JMA). This difference is caused by differences of assumed velocity structures and stations used.…”

“…Since we must compare our results with previous works, we relocate earthquakes changing a velocity model and a number of stations of the NIED to make a focal depth of the event about 60 Ian. Using a method of modified joint hypocenter determination (Hurukawa and Imoto, 1992), we analyzed 63 Kinugawa-cluster earthquakes that were relocated by Hurukawa and Imoto (1992) with magnitude 3.0 or greater in the southwestern Ibaraki Prefecture, the Kanto district. Assuming the velocity structure shown in Table 1, we use 39 stations of the NIED shown in Fig.…”

“…They found a low-velocity layer with a thickness of about 4 km between the EUR and PHS plates by analyses of focal mechanisms of microearthquakes and a velocity in this layer. Then Hurukawa and Imoto (1992) concluded that the low-velocity layer is an oceanic crust that exists at the top of the subducting PHS plate. Hurukawa and Imoto (1992) also found a non double-couple earthquake of Feb. 10, 1987, of which magnitude and focal depth are 4.6 and 51 km, respectively.…”

“…Then Hurukawa and Imoto (1992) concluded that the low-velocity layer is an oceanic crust that exists at the top of the subducting PHS plate. Hurukawa and Imoto (1992) also found a non double-couple earthquake of Feb. 10, 1987, of which magnitude and focal depth are 4.6 and 51 km, respectively. Since it occurred at the boundary between low-and high-velocity subducting oceanic crusts, they suggested that it is probably a gabbro-eclogite transformation earthquake.…”

“…Almost all of them occur in oceanic crusts of the subducting PHS and PAC plates Imoto, 1990, 1992;Hori, 1990). Using data observed by a network of the National Research Institute for Earth Science and Disaster Prevention (NIED) (Hamada et al, 1985), Hurukawa and Imoto (1992) studied southwestern Ibaraki earthquakes at depths of 30 to 80 km in the Kanto district to obtain fine structure of the plate boundary between the PHS and overriding Eurasian (EUR) plates. They found a low-velocity layer with a thickness of about 4 km between the EUR and PHS plates by analyses of focal mechanisms of microearthquakes and a velocity in this layer.…”