Fault motion in the larger earthquakes of the Kurile-Kamchatka Arc and of the Kurile-Hokkaido corner

Abstract

The focal mechanisms of 120 larger earthquakes (mb ≥ 5.7) of the Kurile‐Kamchatka arc for the period 1962–1973 illustrate particular features of the subduction of the Pacific plate beneath this island arc system. The faulting characteristic of normal‐depth earthquakes is consistent with convergence of the oceanic plate in the direction N60°W with respect to the continental plate. Shallower foci under the trench are tensional; those seaward of the trench and related to the Hokkaido rise are compressional. Intermediate‐depth foci separate into those which represent either axial compression with respect to the plate or axial tension. The occurrence of the two types in close spatial proximity may possibly correspond (cf. Veith, 1974b) to differences in the orientation of principal stresses due to volume change across the olivine‐spinel phase transition boundary. In the northern Kuriles, deep‐focus earthquakes are compressional with respect to the axis of the descending plate. In the vicinity of the Hokkaido corner, east‐west striking normal faults and northward dipping tension axes in intermediate‐ and deep‐focus earthquakes imply hinge faulting and contortion of the Pacific plate between the more moderately dipping Japanese segment of the plate and the steeper dipping Kurile Islands segment. Recurrent Tokachi‐Oki earthquakes seaward of the Hokkaido corner indicate that the direction of convergence of the Pacific plate is unaffected by the presence of the junction of the two island arc systems. Equal‐area projections are available with entire article on rrticrofiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J76‐001; $1.00. Payment must accompany order.

Mechanism of mantle earthquakes near the junction of the Kurile and the northern Honshu arcs.

“…36 in Table 1), a good agreement between the solutions determined by these authors and those determined in the present study was obtained. The solutions for earthquake 36 determined by STAUDER and MUALCHIN (1976) and in the present study are shown in Fig. 6(c).…”

“…These solutions have also been determined by ISACKS andMOLNAR (1971) andMUALCHIN (1976). Except Table 1.…”

“…The above evidence, however, clearly shows the effect of the slab contortion on the mechanism solutions, especially those of earthquakes deeper than about 200km. STAUDER and MUALCHIN (1976) have indicated, on the basis of the east-west oriented nodal planes and the north dipping tension axes, that the east-west trend of normal and intermediate depth earthquakes coincides with the main junction which separates the Kurile zone from the northern Honshu zone. This, however, is not considered to be reliable: the distribution of earthquakes (Figs.…”

“…The source processes of earthquakes 10 and 17 were studied in a previous paper (SASATANI, 1974a), which indicated that a steeply dipping nodal plane which is nearly parallel to the strike of the Kurile trench can be taken as a fault plane, and the north side block on the fault plane moves downward Table 1. In (c), the solid lines and the dashed lines are the nodal planes for the first and the second shocks, respectively, and the dashed and dotted lines are the nodal planes determined by STAUDER and MUALCHIN (1976). relative to the south side block.…”

“…They have suggested that these mechanism solutions seem to be related to stresses inside one of the segments rather than to stresses attributable to the presumed contortion or disruption. Most recently, STAUDER and MUALCHIN (1976) have studied focal mechanism solutions of mantle earthquakes near the same junction and have suggested the effects of the slab contortion upon mechanism solutions. Although reliable solutions were determined from long-period data in these two studies, only few of the solutions were used to derive a relation between the slab contortion and the focal mechanism solutions.…”