Principal stress orientations inferred from inversion of focal mechanism data in Hawaii and Iran

Abstract

Fault plane solutions were inverted to estimate stress tensor directions in Hawaii and Iran. These directions were compared to the seismically released strain tensor obtained by summing the moment tensors of the same earthquakes. Attempts were made to determine which of the nodal planes was the fault plane. Regional seismotectonic models were constructed based on these results. The seismotectonic model for west Hawaii explains the seaward motion of the upper crust along a near-horizontal plane under a near-vertical greatest principal stress. The focal mechanism of the 1951 M = 6.9 Kona earthquake in west Hawaii was modeled as a decollement based on a synthesis of teleseismic body waves using a new method designed for sparse data sets. In southeast Hawaii, a single stress tensor orientation is compatible with a complex mixture of decollement, reverse, and normal faults. However, the stress field varies as a function of space and time. The differences between stress and strain orientations are caused by rotations of stress or strain directions, respectively, while the other remains constant. A rotation of the greatest principal stress in 1979 suggests magma movements within the aseismic part of Kilauea's southeast rift zone. Strain directions rotate due to the shifting of seismic activity from one fault to another in a volume of diverse faulting. These results show that the decollement plane at 10 km depth is weak and can slip in response to greatest principal stresses oriented near-perpendicular, sub-parallel or at 45$\sp\circ$ to it. In Iran, stress directions, as estimated from major earthquakes, are homogeneous over areas several hundred kilometers long and mostly coincide with strain directions, suggesting that the strength of the crust is uniform. The quality of the stress inversion results, measured by the size of the average misfit, is similar in Hawaii and Iran, although the dimensions of the study areas vary from tens to several hundreds of kilometers, and the magnitudes of the earthquakes from M = 3.5 $\pm$ 0.5 to M = 6 $\pm$ 0.5, respectively. Average misfits between 2$\sp\circ$ and 6$\sp\circ$ were obtained in both studies and are interpreted as characteristic of crustal volumes with homogeneous stress fields.