Absolute Stress Fields in the Source Region of the 1992 Landers Earthquake

Earthquake focal mechanisms are often inverted to obtain the deviatoric stress field. Because shear stress is equal to the frictional strength of the fault at the time of an earthquake, six components of the absolute stress tensor at the hypocenter can be obtained from a focal mechanism by combining deviatoric stress fields with the Coulomb failure criterion. For a data set of focal mechanisms determined for southern California earthquakes, including the 1992 Landers earthquake sequence, we calculated the absolute stress tensors at their hypocenters using a standard intrinsic friction coefficient under three pore pressure conditions, parameterized by the reference pore pressure at the optimally oriented faults to the stress field. Three absolute stress fields were obtained for southern California immediately before the Landers main shock by applying each data set of the stress tensors to an inversion scheme based on Bayesian statistical inference and Akaike's Bayesian Information Criterion. The coseismic stress field was calculated to obtain the absolute stress fields immediately after the main shock. The variations of the coseismic stress rotation were related to the reference pore pressure. Comparing this relation with that obtained through stress inversion, we determined the absolute stress field and the most plausible reference pore pressure to be hydrostatic. On average, the maximum shear stresses immediately before the main shock were 44 ± 15 and 79 ± 24 MPa at depths of 5 and 10 km, respectively. Earthquakes on the off‐plate boundary faults in southern California occur on faults that are loaded by Anderson‐Byerlee stress conditions

Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3)

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three‐dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre‐condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low‐pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations

Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3)

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three‐dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre‐condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low‐pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations

MOESM2 of Overpressurized fluids drive microseismic swarm activity around Mt. Ontake volcano, Japan

Additional file 2. Roles of pore-fluid pressure and tectonic stress in earthquake generation. (a) Relationship between event magnitudes and pore-fluid pressures activating the events. The vertical axis denotes the overpressure coefficient C (see text). (b) Relationship between event magnitudes and shear stress acting on fault planes. The vertical axis shows shear stress normalized by the maximum shear stress at the hypocentral depth. Open diamonds in (b) denote seismic events triggered by pore-fluid pressures with C < 0.2 (near-hydrostatic pressures). Note that we did not use events with M < 1 for estimating the 3-D pore-fluid pressure field

MOESM1 of Overpressurized fluids drive microseismic swarm activity around Mt. Ontake volcano, Japan

Additional file 1. The 3-D pore-fluid pressure field around Mt. Ontake estimated in Terakawa et al. (2013b). (a) Pore-fluid pressures at a depth of 5 km. (b) Estimation errors of pore-fluid pressures at a death of 5 km. (c) Pore-fluid pressures at a depth of 7.5 km. (d) Estimation errors of pore-fluid pressures at a death of 7.5 km. The pore-fluid pressure fields are superposed on the topographic map in (a) and (c). The white circles in (b) and (d) denote the data used in the FMT analysis within 1.25 km from the horizontal plane at 5 and 7.5 km, respectively. The red triangle indicates Mt. Ontake. The white star denotes the epicenter of the 1984 Western Nagano prefecture earthquake

MOESM4 of Overpressurized fluids drive microseismic swarm activity around Mt. Ontake volcano, Japan

Additional file 4. Difference between the regional and tectonic stress fields. The red lines show the closeness of the two stress tensors measured by the inner tensor product (Michael 1987). The light blue and blue lines denote the estimation errors of the regional and tectonic stress patterns, respectively. The estimation errors of the stress fields are represented with the average value of the inner tensor products of 100 possible stress patterns (AVITP, see text). The black and light gray bars show the standard deviation of the inner tensor products (SDVITP, see text)