Comparison of orientations of stress and strain tensors based on fault plane solutions in Kaoiki, Hawaii

Abstract

The stress tensor orientation was estimated based on inversion from 238 first motion fault plane solutions of earthquakes with mostly M = 3.5 + 0.6 located in the 10-km radius Kaoiki cmstal volume. Separate inversions for subvolumes containing 20-50 events yielded the same results in several adjacent volumes, suggesting that the stress tensor is homogeneous in those parts of the Kaoiki area and that the inversion results are stable and meaningful. Five spatial subsets of the data were found for which the orientation of at least one of the principal axes was different from that in the other sets by 200-80 ø and at confidence levels exceeding 95%. The volcano summits of Kilauea and Mauna Loa, and their rift systems, are identified as the source of stress in the Kaoiki cmst, because the greatest principal stress points to Kilauea and Mauna Loa. In addition, the strain tensor due to energy released by these 238 earthquakes was computed for the Kaoiki area, and several subvolumes of it, by summing the moment tensors. The moment tensor of each earthquake was constmcted from the individual fault plane solutions and from an estimate of the scalar moment derived from the moment-magnitude relationship. A comparison of the directions of strain and stress tensors showed close agreement for subvolumes with predominantly strike-slip faulting. In these volumes the inversion process for stress directions led to misfits of approximately equal size for the conjugate near vertical nodal planes. These observations are interpreted to show that in the strike-slip regime of the upper part of the cmst, neither of the nodal planes is preferred for faulting. Rupture probably occurs along the NW and along the NE striking nodal planes in separate earthquakes. Subvolumes with more decollement faulting showed significant differences of 300-40 ø between the principal strain and stress directions. In these volumes the near-horizontal nodal planes showed noticeably smaller misfits in the inversion for the stress directions. These facts are interpreted to indicate that the decollement plane is weak, allowing slip on it even if the principal stresses are inclined at a large angle to it. It is proposed that comparison of strain and stress tensor calculations may be able to differentiate between tectonic regimes uniform in strength (no well developed fault plane) and regimes in which a fault with low frictional strength dominates. As a function of time, significant rotations of the strain tensor by approximately 45 ø can be observed, which seem to be related to the occurrences of Kaoiki mainshocks. During three background periods of about 7 years each, the average strain tensor showed an orientation typical for predominant decollement faulting, while two premainshock periods of 2.5 years each showed an orientation closer to strike-slip faulting. It is proposed that this pattern may be repeated before the next Kaoiki mainshock. The strain released seismically is less than the geodetically observed strain by approximately an order of magnitude. (Stres...

Stress Orientations Obtained from Earthquake Focal Mechanisms: What Are Appropriate Uncertainty Estimates?

“…A high average misfit indicates that the stress field is highly heterogeneous and hence that the stress inversion result may not be meaningful. The maximum acceptable misfit has been quantified for LSIB by Michael (1991) and for FMSI by Wyss et al (1992), Gillard et al (1996), and Lu et al (1997).…”

Seismic strain and seismogenic stress regimes in the crust of the southern Tyrrhenian region

“…They may be di¡erent because stress generally produces seismicity on pre-existing surfaces of weakness in the lithosphere (see e.g. [49]). The orientation of the weakness surfaces which were seismogenic under a given stress ¢eld during the study period controls the seismic strain orientation we ¢nd by Wyss et al's algorithm.…”

“…These solutions were utilized to estimate the seismic strain tensor orientations in the study area ( Fig. 7) using Wyss et al's algorithm [49] derived from Kostrov [50]. In this regard, earthquake magnitudes were converted into scalar moments by the relationship log M 0 = 1.5M+16.27 proposed for the Italian region by Kiratzi [51].…”

Influence of volcanic activity at Mauna Loa, Hawaii, on earthquake occurrence in the Kaoiki Seismic Zone

“…We evaluate DCFS on receiver planes oriented subparallel to the Kaoiki strikeslip faults (strike/dip/rake 40°/90°/180°at 7 km depth), and on receiver planes parallel to the subhorizontal décolle-ment plane (45°/0°/À90°at 10 km depth). The fault plane orientation and slip direction is based on focal mechanism solutions by Endo [1985], Wyss et al [1992], and Got and Okubo [2003]. In the next sections we investigate how intrusions at Mauna Loa encourage or discourage earthquakes.…”

“…Two types of earthquakes occur in the KSZ [e.g., Endo, 1985;Wyss et al, 1992;Got and Okubo, 2003]: The first types of earthquakes are right-lateral strike-slip events along SW-NE striking vertical faults at 5-9 km depth. The second types of earthquakes are low angle thrust events along a décollement fault between the volcanic edifice and the paleo-seafloor at 9 -11 km depth ( Figure 1B).…”