Nearby faults interact with each other through the exchange of stress. However, the extent of fault interaction is poorly understood. In particular, interactions may lead to slow-slip activity, resulting in episodes of transient surface motion. Our study concentrates on Northwest Sulawesi (Indonesia), which hosts two fault zones with potential for major earthquakes and tsunamis: the strike-slip Palu-Koro fault and the Minahassa subduction zone. Thanks to a 20-year-long effort in geodetic monitoring, we are able to identify multiple periods during which surface velocities deviate from their interseismic trend. We use a Bayesian methodology with forward predictions of slip on the two fault interfaces to match the observations following the 2018 Mw7.5 Palu earthquake, and infer that both deep afterslip on the Palu-Koro fault and slow slip on the Minahassa subduction interface have caused the observed transient surface motion. This finding represents the first recording of a slow slip event on the Minahassa subduction interface. We also infer that the subduction interface and the strike-slip fault are likely interacting on a regular basis.

The Granada Basin (Spain) is a Neogene sedimentary depression with irregular geomorphology and deep depocenters. It is located in the most seismically hazardous part of the Iberian Peninsula with an historically experienced extremely destructive earthquakes, followed by periods of low to moderate seismicity. In 1980s the Chevron Oil Company collected a set of 30 deep seismic reflection sections in this Basin of which only the results on paper are kept. Due to the fact that many of these seismic profiles are currently located in urban areas and the economic cost of carrying out a similar exploration, it was decided to recover these old data and apply a post-stack treatment to improve their quality. The purpose of this study is to show the applied reprocessing flow and, with the new sections, to present a spatial model of the basin. The first stage of recovery and enhacement of seismic sections has consisted in three phases: first, high-resolution scanning of paper copies to TIFF images followed by the transformation of TIFF images to SEG-Y format; second, poststack processing workflow to increasing resolution and lateral coherence of these seismic lines; and third, it has been used a machine learning algorithm, among others, increasing the spatial resolution, signal-to-noise ratio, and coherence of the seismic signals. In addition, basement horizons, as well as three sedimentary sequences, were identified in all seismic sections and interpolated to create a three-dimensional basement model composed by normal faults, horst and grabens related to the seismotectonic behavior of the basin. As an overall assessment, this work is an example of the usefulness of ‘recycling’ legacy seismic data, which nowadays are usually in archived boxes, but at the time required a great economic and acquisition effort.

In this study, we investigate the stress field in the southeastern margin of the Tibetan Plateau. We first determine the focal mechanism solutions of 1537 small and moderate (3.2 ≤ MW ≤ 6.7) regional earthquakes from January 2009 to June 2021, and then use the focal mechanisms to invert for the spatial variation of crustal stress field by a damped linear inversion method. Our result suggests that in the southeastern margin of the Tibetan Plateau the seismogenic zone is in the upper crust above 15-km depth, and the stress field is predominantly strike-slip in the Sichuan-Yunnan Rhombic Block (SYRB). The maximum compressional stress axis is oriented in a fan-shaped pattern, rotating clockwise from nearly east-west in the Songpan-Ganzi Terrain in the north to northwest-southeast in the SYRB to nearly north-south across the Red River Fault in the Indo-China Block (ICB), consistent with the GPS-derived surface strain rate. The stress field around the border of the Tibetan Plateau with high elevation relief appears to be largely caused by gravitational effect with the maximum extensional axis perpendicular to the topography gradient. The stress field in the vicinity of the Longmenshan Fault Zone and in the Yangtze Craton is mainly thrust as a result of the eastward expansion of the Tibetan Plateau and the resistance of the Sichuan Basin. Near the epicenter of the 2008 Wenchuan earthquake and in the northeastern end of the Longmenshan Fault Zone, the thrust stress field shows spatial variations as a result of the perturbation by complex geometry and the post-seismic healing process. Our result provides multi-resolution images of the stress field for better understanding about the mechanisms of seismic activity and crustal deformation in the southeastern margin of Tibetan Plateau.

The Burma Plate is a microplate that extends along the boundary between the Indian and Eurasian plates. It is characterized by an extraordinarily complex lithospheric tectonic setting, resulting from the continental collision in the north, the oceanic crustal subduction in the south, and the large amount of sediment from the Tibetan Plateau. The lithospheric strength is a key to understanding the tectonic evolution of the Burma Plate. In this study, we use topography and gravity disturbance data to estimate the spatial distribution of effective elastic thickness Te, which is a measure of lithospheric strength. The Tevalues range from ∼10 km to 80 km, with higher values in the Indian Plate than those in the other regions. The non-isostatic flexural effects of sediment loading and subducting slab pull can bias the Te estimation, with maximum reductions of ∼50 km and ∼10 km, respectively. The consistent distributions of the Te and the shear wave velocity anomaly ΔVsat 100 km depth suggest that the lithospheric strength is generally controlled by the thermal structure of the upper mantle. Meanwhile, the Te variations are highly related to the geometry of the subducting Indian Plate along the collision and subduction zones, indicating that the plate tectonics play a dominant role in determining the lithospheric strength of the Burma Plate.

The Pacific Plate within the collision zone between the Louisville Ridge and the Tonga-Kermadec Trench was formed at the Osbourn Trough, a paleo spreading center that became inactive during the Cretaceous. In this region, the trench shallows from a depth of 8–11 km to ∼6 km below sea surface, while the outer rise topography is obscured by Louisville seamounts that rise 4–5 km above the adjacent seafloor. We derive 2-D P-wave (Vp) and S-wave (Vs) velocity-depth models along a wide-angle seismic profile oriented sub-parallel to the trench axis, intersecting the 27.6°S seamount. The seismic profile is located in the down-going Pacific Plate eastwards from the trench axis (∼100 km distant at the south end and ∼ 150 km at the north end), where bending-related faulting is limited or absent. Using the derived P- and S-wave velocity-depth models we calculate the corresponding Vp/Vs ratio model which shows values of 1.7–1.85 throughout the oceanic crust either side of the Louisville Ridge where it is unaffected by magmatism associated with its formation. This range of observations lies within those documented by laboratory measurements on basalt, diabase, and gabbro. Conversely, in the vicinity of the summit of 27.6°S seamount, the relatively elevated Vp/Vs (∼1.9) ratio observed can be attributed to water-saturated cracks within the shallow sub-seabed section of the intrusive core. Beneath the seamount the uppermost mantle has a Vp ranging from 8.0 to 8.9 km/s. Comparing our P-wave model with a pre-existing model running sub-perpendicularly along the Louisville Ridge axis, we observe an anisotropy of up to ∼6% at a depth of 3–4 km below the Moho. The predominant orientation of the faster axis follows the direction of paleo spreading flow when the plate was formed at the Osbourn Trough.

Earthquakes are the dynamic rupture of faults governed by fault weakening processes. Critical slip-weakening distance (Dc) is a crucial source parameter of earthquakes, and the determination of Dc is of great concern to semiologists. However, determining Dc for natural earthquakes is challenging due to the trade-off in inversed source models. To solve this problem, Fukuyama and his coworkers proposed a simple method (denoted as the F&M method) to estimate Dc directly from slip-velocity functions. According to the F&M method, the fault slip at the peak slip velocity (Dc') can be used as an approximation of Dc. However, the feasibility of this method has not been completely resolved. Here, we performed laboratory earthquake rupture experiments to examine the validity of the F&M method. Experiments were conducted on faults with different roughness and stress level. We studied the effect of fault roughness on the validity of the F&M method. Our results show that the increase in fault roughness could complicate the fault weakening process, producing repeated weakening and strengthening phase, which leads to a prominent deviation between Dc' and Dc. Furthermore, we also observed a correlation between the critical slip-weakening distance Dc and the final fault slip D. Such correlation implies the scale-dependent nature of Dc, which is consistent with seismic observations in the field.

Crustal fluids play an essential role in the activity of crustal earthquakes. The north Ibaraki area in northeastern Japan has shown intense crustal seismicity after the 2011 Tohoku-oki earthquake. In this area, it is discussed that crustal fluids are supplied from the deep part of the Earth's crust and contribute to the genesis of these crustal earthquakes. To investigate the distribution of crustal fluids in this area, we focused on reflected S-waves, which are highly sensitive to the presence of crustal fluids. We developed an approach based on the Markov chain Monte Carlo method to precisely and quantitatively estimate the location of the crustal reflector and its geometry. The obtained results showed that the crustal reflector was located at depths of 15–25 km and dipped shallowly to the northwest. The crustal reflector was positioned above the region characterized by low seismic wave velocity and electrical resistivity anomalies in the lower crust, suggesting that the crustal reflector is the uppermost boundary of a fluid-rich zone. The distribution of the fluid-rich zone closely corresponded to the cutoff depths of crustal earthquakes. This fluid-rich zone was likely the source of the fluid that enhanced seismicity in the shallow part of the crust in the target area. In contrast, the fluid-rich zone itself may have suppressed the genesis of crustal earthquakes. We hypothesized that hydrothermal fluids might affect the shallowing of these cutoff depths. If the hydrothermal fluid was contained in the fluid-rich zone, it could induce a shallow brittle-ductile transition by increasing the temperature of the surrounding rocks.

The Gulf of Corinth, Central Greece, is a highly active half-graben, characterized by seismicity which is more intense in its western part, while destructive earthquakes have also occurred towards its eastern end. We herein present an analysis of the seismicity in the Central Gulf of Corinth, for the period from June 2018 to December 2022. We applied the EQTransformer machine-learning model to enhance the initially available data, adding missing P- and S-wave arrival-times or improving existing ones. The events were initially located using a new local velocity model and then relocated using the double-difference method, including waveform cross-correlation data from local stations. The hypocenters, generally distributed at depths between 5 and 15 km, along with the focal mechanisms of significant earthquakes (1965 through 2022) and the geometry of mapped faults on the surface were co-examined to better understand their possible connection. It is shown that major outcropping north-dipping structures, such as the East Helike fault and its eastward offshore extension, match only with the southern bounds of seismicity. The Mw = 5.9, 1970 Antikira and Mw = 5.7, 1992 Galaxidi earthquakes cannot be associated with known mapped faults on the surface and likely occurred on low-angle, north-dipping planes. The variability in slip behavior of the low-angle detachment in the Gulf of Corinth, ranging from seismic slip to aseismic creep, probably accounts for the most part of the N-S extensional deformation. The spatial pattern of the 2018–2022 microseismicity delineates the edges of the rupture planes of major events that occurred during the instrumental era, including the Mw = 6.31995 Aigion earthquake. The lack of aftershocks for significant earthquakes, including the Mw = 5.0 8 October 2022 event, south of Desfina, is interpreted in terms of different pore pressure conditions, variations in fault-rock strength, and the preferred accumulation of high stress inside the upper crust.

It is well-established that the thermal state of the lithosphere strongly influences various regional and local geological processes, including crustal deformation, hydrocarbon maturation, hydrogen generation, and geothermal phenomena. Moreover, the thermal structure exhibits high sensitivity to tectonic features, a property of particular significance in Colombia, where three main tectonic plates converge, and lithospheric tearing has been documented. In this contribution, we focus on elucidating the impact of plate architecture on the thermal field in central-eastern Colombia at both shallow and deep levels. To accomplish this, we constructed a series of two-dimensional profiles and derived a numerical solution of the heat equation using the conservative finite difference method. As constraints, we incorporate an inferred distribution of rocks in the deep crust and upper mantle based on global and local lithospheric thickness models. Material parameters for the various rocks, both exposed and inferred, were obtained from the literature. Additionally, we used superficial heat flow estimates and apparent geothermal gradients compiled by the Colombian Geological Survey.Our results suggest a significant influence of the lithospheric Caldas tear on the thermal state of Colombia, with the breaking off occurring in the Nazca plate under the Eastern Cordillera range around 5°N. The modeled asthenospheric heat flow remains approximately 25 mWm−2, except in the northern Eastern Cordillera range, where the background heat flow increases rapidly to 40 mWm−2. Consequently, our model predicts partial melting in the lower crust to the north and a thermally unstable lower crust to the south of the Caldas tear. The material parameters that best fit the surface data suggest the presence of a basement moderately enriched in radioactive elements in the Eastern Llanos basin. After accounting for compaction, we also confirm a strong influence of the tectonic setting on the thermal state of sedimentary basins.

We investigated the active tectonics and earthquake potential of the eastern Siena Basin, a slowly deforming portion of southern Tuscany in the inner Northern Apennines. This region hosts several historical settlements and valuable cultural heritage, but also frequent background seismicity and rare damaging earthquakes in the Mw range 5.0–6.2. We describe in detail an active, capable, and seismogenic fault system that we identified in the eastern Siena Basin, a few kilometers south-east of the city of Siena, thanks to the presence of an active quarry (Cava Capanni) that exploits travertines of Middle Pleistocene-Holocene age. Travertines are unique rock masses that may preserve living evidence of active and seismogenic faulting, thus providing remarkable seismotectonic insight. The active fault system consists of at least two segments rupturing travertines younger than 45 ka, with a cumulative vertical displacement of 111 cm, and an estimated minimum slip rate of 0.02–0.03 mm/y. We maintain that this displacement is the result of at least three coseismic movements accompanied by clastic dykes injected within the fault damage zone due to liquefaction phenomena. The fault system is seen to extend east of the quarry, affecting Pliocene and Mesozoic deposits.The Cava Capanni fault system is evidence of a poorly understood but potentially seismogenic tectonic mechanism of regional extent. Its orientation and kinematics are compatible with the activity of faults that are oriented obliquely or orthogonally to the main chain axis, in contrast with the setting of the axial and outer zone of the Northern Apennines, where extension and compression are accommodated by Apennines-parallel faults.