Distinguishing between stress-induced and structural anisotropy at Mount Ruapehu volcano, New Zealand

Abstract

[1] We have created a benchmark of spatial variations in shear wave anisotropy around Mount Ruapehu, New Zealand, against which to measure future temporal changes. Anisotropy in the crust is often assumed to be caused by stress-aligned microcracks, and the polarization of the fast quasi-shear wave (f) is thus interpreted to indicate the direction of maximum horizontal stress, but can also be due to aligned minerals or macroscopic fractures. Changes in seismic anisotropy have been observed following a major eruption in 1995/96 and were attributed to changes in stress from the depressurization of the magmatic system. Three-component broadband seismometers have been deployed to complement the permanent stations that surround Ruapehu, creating a combined network of 34 threecomponent seismometers. This denser observational network improves the resolution with which spatial variations in seismic anisotropy can be examined. Using an automated shear wave splitting analysis, we examine local earthquakes in 2008. We observe a strong azimuthal dependence of f and so introduce a spatial averaging technique and twodimensional tomography of recorded delay times. The anisotropy can be divided into regions in which f agrees with stress estimations from focal mechanism inversions, suggesting stress-induced anisotropy, and those in which f is aligned with structural features such as faults, suggesting structural anisotropy. The pattern of anisotropy that is inferred to be stress related cannot be modeled adequately using Coulomb modeling with a dike-like inflation source. We suggest that the stress-induced anisotropy is affected by loading of the volcano and a lithospheric discontinuity.

Seismic anisotropy of the upper crust around Mount Fuji, Japan

“…The splitting polarizations at these stations may be affected more by local fracture systems than by the regional stresses. The existence of both stress‐controlled anisotropy and structurally controlled anisotropy in the close proximity to each other is comparable with anisotropic structure obtained in the upper crust of the West Bohemia/Vogtland seismically active area [ Vavryčuk , ; Vavryčuk and Boušková , ] or Mount Ruapehu Volcano [ Johnson et al ., ].…”

Stress Orientations in a Locked Subduction Zone at the Southern Hikurangi Margin, New Zealand

“…Often measurements at a single station may be consistent for the same earthquake location but vary for different earthquake locations (Johnson et al, ; Zhang et al, ). Yet when splitting measurements are averaged regionally, the measurements should align with the maximum horizontal stress (Savage et al, ), although sometimes they are more consistent with regional structures or with faulting (Balfour et al, ; Zinke & Zoback, ).…”

Shear wave automatic picking and splitting measurements at Ruapehu volcano, New Zealand

“…Previous studies using automatic splitting measurements were limited in time and space by the need to provide a shear wave ( S ) arrival time, which was done by manually picking each record. The most comprehensive spatial analysis of anisotropy in the region was carried out by Johnson et al []. They used the MFAST program [ Savage et al , ] to perform shear wave splitting measurements for local events recorded at 34 seismometers deployed around Mount Ruapehu in 2008 to provide background values against which changes could be measured.…”