Geophysical Journal International

SummaryDetermining earthquake focal mechanisms is a fundamental task in seismology, essential for understanding the fault structures and stress states in faulting regions. We present a new method for determining focal mechanisms of small earthquakes using 360° S-wave polarization alongside traditional P-wave polarity and S/P amplitude ratio. Ideally, measuring accurate 360° S-wave polarizations at near-source stations allows for a full recovery of the double-couple radiation patterns of direct body waves. By employing a process involving P–SV–SH transformation and correction for S-wave splitting, we show that S-wave polarizations for events with magnitudes less than 3 can be measured with average errors smaller than 7°. Our statistical analyses indicate that reliable focal mechanism solutions can be obtained with as few as two to three near-source stations. The method is particularly effective for strike-slip earthquakes, as their highly variable S-wave polarization patterns provide stronger constraints. We applied this method to the ML 2.8 and 2.9 sequences located in the centre of a dense seismic array in southeastern Korea, successfully determining focal mechanisms for events with magnitudes ranging from 2.9 down to −0.4. While the ML 2.8 sequence events display almost identical focal mechanisms along the main fault, those in the ML 2.9 sequence show variable mechanisms associated with off-fault microseismicity. We further validated the approach using the 2023 Mw 4.3 Parkfield and 2011 Mw 5.8 Mineral earthquake sequences, representing different tectonic settings. Despite using only 2–4 S-wave polarization measurements in Parkfield and 1–2 in Mineral, incorporating S-wave polarization significantly improved the accuracy of focal mechanisms in both cases. This research demonstrates that 360° S-wave polarization allows for robust determination of focal mechanisms in small earthquakes and offers a valuable tool for analyzing microseismic activity.

SummaryPore structure is an important parameter controlling the storage capacity and transport properties of porous rocks and determining their pressure dependent elastic properties. However, pore structure is predominantly inverted from velocities with increasing confining pressure and it remains unclear whether the pore structure from velocities with increasing pore pressure differs. We develop an improved pore-structure inversion method that incorporates the linear reduction of stiff porosity with pressure to extract the complete aspect ratio distribution of compliant cracks from pressure dependent velocities. We also measure the compressional and shear wave velocities and porosity of two dry Berea sandstone samples as a function of both increasing confining pressure and increasing pore pressure. The pore-structure inversion method is applied to the two samples to obtain and compare their pore structures from the velocities measured with different pressure paths. The results show systematically higher velocities and lower porosities for the increasing pore pressure path at equivalent differential pressures. The inverted pore structures show a substantially greater cumulative crack porosity and density from velocities with increasing confining pressure, and reveal a markedly smaller population of compliant cracks, albeit distributed over a slightly broader range of lower aspect ratios from velocities with increasing confining pressure. The difference in the pore structures from velocities with different pressure paths is explained in terms of the crack hysteresis mechanism. The results have helped to explain the greater velocities and smaller porosity of the samples measured with increasing pore pressure, and would help for the estimation of capacity and permeability of CO2 and hydrogen stored reservoirs and for the more accurate prediction of pore pressure in hydrocarbon generated overpressure zones.