Shear wave polarization anisotropy observed in a rift zone in Japan

Abstract We have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in Central Kyushu, Japan. We have recognized two different directions in the polarization of the faster shear waves: about E-W directions at three stations located in the rift zone, and about N-S at two stations possibly located outside and close to the edge of the rift zone. The former case (E-W) is consistent with a hypothesis that shear-wave splitting is caused by vertical cracks or fractures striking in the E-W direction which have formed under the present tectonic stress inside a rift zone extending in the N-S direction, with the maximum horizontal compressive stress striking in the direction about E-W and the minimum compressive stress vertical. In contrast, the latter case (N-S) is inconsistent with the above hypothesis. This inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone. Alternatively, it may result from a complicated stress regime within the crust close to the edge of the rift zone. Arrival-time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0.12 s, while those from the upper mantle events are less than about 0.25 s. The relatively small arrival-time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust.