Abstract Two devastating earthquakes struck southeastern Türkiye and northwestern Syria on 6 February 2023: an Mw 7.8 mainshock, followed 9 hr later by an Mw 7.6 aftershock. To recover and separate the subsurface geometry and slip distributions along the two earthquake faults, we jointly invert Interferometric Synthetic Aperture Radar, Synthetic Aperture Radar pixel offset tracking, burst overlap interferometry (BOI), Global Navigation Satellite System, and aftershock datasets. We introduce a new Kalman filter-based approach for merging spatially dense azimuth offset (AZO) data with the more precise yet spatially sparse BOI data. This procedure yields improved measurements of the displacements parallel to the near north-south satellite tracks, which are critical for resolving slip along most of the Mw 7.8 fault segments. We optimize the inversion using a new metric for assessing the degree of spatial correlation between the coseismic slip gradients and early aftershocks, resulting in a stable solution honoring the complementarity between the geodetic and aftershock datasets. The analysis suggests that the Mw 7.8 rupture consisted of three large segments and two short fault branches, covering about 300 km along the East Anatolian fault (EAF), whereas the Mw 7.6 rupture consisted of three segments extending for about 160 km along the nearby Sürgü fault (SF). On the basis of moment-to-stress-drop scaling relations, we show that the Mw 7.6 stress drop is four times larger than the Mw 7.8 stress drop, consistent with the larger recurrence intervals for Mw > 7 earthquakes on the SF than on the EAF. The moment released during the 2023 Mw 7.8 earthquake is 2–4 times larger than the sum of the moments released during individual historical Mw > 7 earthquakes along the three segments of the 2023 Mw 7.8 earthquake. Thus, when considering moment release for multisegment earthquakes, one should note that the final moment of fault coalescence is likely larger than the arithmetic sum of individual segment ruptures.
Abstract The July 2019 Ridgecrest earthquake sequence consists of an Mw 6.4 foreshock and an Mw 7.1 mainshock, which ruptured a complex set of orthogonal faults in the eastern California shear zone. We measure the co- and postseismic deformation associated with this sequence using the Burst Overlap Interferometry (BOI) technique in addition to the commonly used Interferometric Synthetic Aperture Radar (InSAR). The BOI technique, which provides displacement in the satellite’s along-track direction, offers important information on the postseismic deformation that cannot be measured by traditional InSAR and is only sparsely measured by the Global Navigation Satellite System networks. The BOI data reveal up to 4 cm displacement in the along-track direction, 10 km north of the northern tip of the seismic rupture, and up to 3 cm displacement along the coseismically active faults. These results rule out the possibility of significant shallow afterslip near the mainshock hypocenter, suggesting that the previously suggested poroelastic rebound is likely to be the cause for the significant postseismic line of sight deformation near the mainshock rupture. Based on the aftershocks’ moment tensor distribution, surface rupture, and simple forward modeling, we propose that the postseismic deformation north of the Ridgecrest rupture is caused by an aseismic slip along a north-trending normal fault of the Ridgecrest rupture that was induced by the Ridgecrest earthquake. Furthermore, we observed that both deformation and seismic activity decay slower over time as the distance from the Coso geothermal area increases. This decay is influenced by the mechanical properties of the crust, which are affected by the increased heat flow at Coso and thus suppress deformation and seismicity, ultimately controlling their temporal evolution.
Abstract We report systematic seismic velocity variations in response to tidal deformation. Measurements are made on correlation functions of the ambient seismic wavefield at 2–8 Hz recorded by a dense array at the site of the Piñon Flat Observatory, Southern California. The key observation is the dependence of the response on the component of wave motion and coda lapse time τ . Measurements on the vertical correlation component indicate reduced wave speeds during periods of volumetric compression, whereas data from horizontal components show the opposite behavior, compatible with previous observations. These effects are amplified by the directional sensitivities of the different surface wave types constituting the early coda of vertical and horizontal correlation components to the anisotropic behavior of the compliant layer. The decrease of the velocity (volumetric) strain sensitivity S θ with τ indicates that this response is constrained to shallow depths. The observed velocity dependence on strain implies nonlinear behavior, but conclusions regarding elasticity are more ambiguous. The anisotropic response is possibly associated with inelastic dilatancy of the unconsolidated, low‐velocity material above the granitic basement. However, equal polarity of vertical component velocity changes and deformation in the vertical direction indicate that a nonlinear Poisson effect is similarly compatible with the observed response pattern. Peak relative velocity changes at small τ are 0.03%, which translates into an absolute velocity strain sensitivity of S θ ≈5 × 10 3 and a stress sensitivity of 0.5 MPa −1 . The potentially evolving velocity strain sensitivity of crustal and fault zone materials can be studied with the method introduced here.
Abstract As stresses following rupture are dissipated continuous measurements of postseismic surface deformation provide insight into variations of the frictional strength of faults and the rheology of the lower crust and upper mantle. Due to the difficulty of capturing the earliest phase of afterslip, most analyses have focused on understanding postseismic processes over timescales of weeks to years. Here we investigate the kinematics, moment release, and frictional properties of the earliest phase of afterslip within the first hours following the 2016 M w 7.1 Kumamoto earthquake using a network of 5‐minute sampled continuous Global Positioning System (GPS) stations. Using independent component analysis to filter the GPS data, we find that (1) early afterslip contributes only ~1% of total moment release within the first hour and 8% after 24 hr. This suggests that the lack of a coseismic slip deficit, which we estimate using standard geodetic data sets (e.g., InSAR, GPS, and pixel offsets) and which span the first 4 days of the postseismic period, is largely reflective of the dynamic rupture process and we can rule out contamination of moment release by early afterslip. (2) Early afterslip shows no evidence of a delayed nucleation or acceleration phase, where instead fault patches transition to immediate deceleration following rupture that is consistent with frictional relaxation under steady state conditions with dependence only on the sliding velocity. (3) There is a close correlation between the near‐field aftershocks and afterslip within the first hours following rupture, suggesting afterslip may still be an important possible triggering mechanism during the earliest postseismic period.
Seismic waves excited by human activity frequently mask signals due to tectonic processes, and are therefore discarded as nuisance. Seismic noise-field analysis is, however, a powerful tool for characterizing anthropogenic activities. Here, I apply this analysis to examine seismic precursors to the October 7 Hamas attack on Israel. The precursory activity in Gaza included massive mobilization which took place in the hours leading to the attack, and was documented on multiple media outlets. Favorable conditions, which arise due to a temporary lack of anthropogenic activity in Israel, allow remote seismic stations to record signals due to Gaza vehicle traffic. I use these seismograms to identify anomalous ground-motions, associate them with pre-attack mobilization, and precisely determine their location. By applying array analysis to three seismic stations located tens-of-kilometers from the Gaza Strip, I was able to obtain valuable information on the Hamas attack plans. This suggests that embedding seismic noise-field analysis into decision-making protocols could enhance preparedness, thus providing an opportunity to blunt terrorist attacks and reduce the number of casualties.
Abstract This study analyzes the seismic noise wavefield recorded before the October 7 Hamas attack on Israel. Preattack activity involved large-scale mobilization, as was documented by various media sources. Opportune conditions stemming from a temporary reduction in Israeli anthropogenic activity enabled the identification of signals generated by vehicular movement in Gaza at three regional seismic stations. Seismogram analysis reveals a widespread signal that abruptly emerged above the nighttime noise levels about 20 min before the attack began. Previous Saturday mornings did not exhibit interstation correlations and signal amplitudes as high as the ones observed on the three stations in the minutes before the attack began. Statistical analysis suggests the October 7 preattack signal is highly anomalous and unlikely to emerge by chance. Tripartite array analysis was used to detect surface waves, locate their sources, and demarcate the extent of preattack activity within the Gaza Strip. The signal’s amplitude, frequency, and spatiotemporal distribution appear to be aligned with vehicular traffic emanating from the south-central region of the Gaza Strip and extending toward its peripheries in the half-hour window preceding the invasion. This analysis underscores the potential utility of seismic noise measurements in identifying large-scale terrorist vehicular mobilizations in advance.
