Focal mechanisms of intermediate depth and deep earthquakes in the North Island, New Zealand

Abstract

The focal mechanisms of 215 intermediate-depth earthquakes between latitudes 38°S and 40°S and 24 deep earthquakes beneath the Bay of Plenty are studied on the basis of P and S wave first motion data. For 155 of the intermediate shocks (group A) a composite focal mechanism solution gives the tension axis parallel to the Benioff zone and inclined in a northward direction at about 30° to the dip of the zone. This orientation is related to the sloping bottom edge of the lithospheric slab, which is sinking through the asthenosphere under its own weight. The compression axis is also parallel to the Benioff zone. For a further 41 intermediate earthquakes (group B) a composite focal mechanism gives the tension axis in the direction of the dip of the Benioff zone and the compression axis normal to the zone. Most of these earthquakes were 150 km to 200 km deep, and it is proposed that at such depths the occurrence of a group A earthquake temporarily removes the effect of the sloping bottom edge of the lithospheric slab on the local stress field. The focal mechanism solution of the deepest well-observed earthquake between latitudes 38°S and 40°S, which was at 274 km, gives a down-dip compression axis, suggesting that at this depth the lithospheric slab has encountered resistance to its downward motion. For 23 of the deep shocks beneath the Bay of Plenty the composite first motion pattern is similar to that for the group B earthquakes, consistent with the bottom edge of the lithospheric slab being horizontal in this region. For the remaining earthquake, which at a depth of 423 km was 52 km below the next deepest, the first motions have opposite polarity to those of the other deep shocks, again suggesting that at the greater depth the lithospheric slab has encountered resistance to sinking.