Domenico Giardini

Improving our understanding of induced and natural earthquakes benefits from controlled experiments in insitu laboratories. To investigate the processes during an Mw∼0 earthquake, we performed the “ Mzero ” experiments in a densely instrumented testbed of the Bedretto Underground Laboratory. These multi‐day hydraulic stimulation experiments went the opposite direction to typical induced seismicity research in that they were designed to enhance seismic rupture. In the first experiment, MzeroA, the rock mass was preconditioned by injecting water for 4 days at a pressure just below fracture reactivation pressure followed by a hydraulic stimulation above reactivation pressure. This strategy aimed at increasing the fractures area near critical stress conditions for shear failure, thus facilitating larger ruptures. During MzeroA, an event with Mw–0.54 was induced, followed by a distinct aftershock sequence. This event was one magnitude unit larger than any preceding event. In the second experiment, MzeroB, stimulation was initiated directly, without preconditioning. Compared to MzeroA, MzeroB exhibited higher seismicity rates, a larger seismicity cloud, and a pronounced migrating seismicity front. Combining seismological and hydromechanical observations, we discuss mechanisms that may have influenced the contrasting seismicity responses. In addition to fluid preconditiong, stress transfer from the Mw–0.54 mainshock, and natural seasonal pressure changes, may have contributed to modulating the seismogenic response. However, the relative importance of these processes remains uncertain given experimental limitations. Our results highlight both the potential and challenge of designing hydraulic stimulations to enhance or suppress seismic rupture, with implications for earthquake physics and induced seismicity.

Back‐propagating earthquake rupture (BPR), where slip migrates opposite to the main rupture direction, is observed in several earthquakes but its origin remains poorly understood, particularly the role of fluids. We use dynamic simulations of frictional sliding coupled with fluids to explore the mechanisms of BPR. Our simulations show that BPR occurrence and rupture mode are controlled by the ratio of initial shear stress to effective normal stress : low produces pulse‐like rupture with BPR, whereas high promotes crack‐like rupture without BPR. Fluids modulate this ratio by reducing . Localized patches of differential pore pressure act as asperities or barriers, inducing slip rate and shear stress perturbations that generate a secondary backward‐propagating front. BPR emerges from incomplete stress release, temporal healing and stress perturbations. These findings highlight the role of fluids in rupture dynamics and provide an energy framework for BPR in earthquakes.

Here we illustrate the use of parameterized models to address fundamental questions about Earth's mantle and core structure. For this, we invert a large set of normal‐mode center frequencies and quality (attenuation) factors, along with astronomic‐geodetic data, for the radial anelastic seismic structure of the Earth. We consider two distinct parameterizations that rely on physically and polynomially parameterized models. In the first approach mantle models are constructed using petrologic phase equilibria in combination with a laboratory‐based viscoelastic model connecting dissipation from seismic to tidal periods (100 s–20 years), whereas seismic properties for a homogeneous and adiabatic core are computed using equations‐of‐state. The polynomially parameterized models follow the Preliminary Reference Earth Model (PREM) and rely on a polynomial representation of density, P‐ and S‐wave velocity, attenuation, and anisotropy. To quantify uncertainty estimates on the derived structure—a feature absent from radial seismic reference models—we employ a stochastic sampling‐based approach in solving the inverse problem. To further constrain mantle and outer‐core seismic wave velocity structure, we also inverted globally‐averaged P‐ and S‐wave and multiple core‐mantle boundary underside‐reflected S‐wave travel times, respectively. The results indicate considerable deviations from PREM, including a denser outer core and less dense inner core, and thus a diminished density contrast across the inner‐core boundary. Also, outer‐core stratification, associated with the layer, appears not to be required to match data. Finally, and of general interest to the wider community, uncertainty measures on all inverted properties are provided with the models presented here.

NASA's InSight mission has provided an unprecedented snapshot of Mars' seismicity, despite data analysis challenges arising from low signal‐to‐noise ratios (SNR) and single‐station constraints. High frequency (HF) events—the most common type—were initially assumed to propagate through shallow crustal layers. However, several impacts that occurred late in the mission provided independent distance constraints, indicating that HF event energy must have propagated through the mantle. We analyzed the full HF data set using an extended catalog and denoised waveforms derived with deep learning (DL) techniques. Using a DL ensemble, we picked phase arrivals on denoised envelopes and estimated SNR‐dependent pick timing uncertainties based on the removed noise. We computed distances consistent with mantle paths using the latest Mars interior models, while the back azimuth remained inconclusive due to local resonance dominating the HF bandwidth. We compared and grouped HF recordings by their similarity and investigated how attenuation properties shape their envelopes. Additionally, we re‐calibrated and assigned magnitudes for the extended catalog. Overall, we (re‐)located 1,430 HF events clustered between epicentral distances of around 1,600–3,600 km, but without constraints on the back azimuth, their source region remains speculative. Based on spatiotemporal similarities, we attributed a subset of 1,357 events to a common source region and labeled them as swarm events. The analysis of envelope shape confirms and extends previous results of stratified attenuation properties. Swarm events, with magnitudes between 1.5 and 2.5, are cumulatively equivalent to a single magnitude 4 event and show a high ‐value and clear seasonal trends in seismicity.

Understanding the seismicity and hydromechanical processes during hydraulic stimulation in hectometer-scale in situ experiments is crucial for bridging the gap between laboratory studies, decameter-scale in situ experiments, and large-scale geothermal projects. These insights can help to optimize the use of geothermal energy. A key example of this is the Bedretto Underground Laboratory in Switzerland, where multiple hydraulic stimulation experiments were conducted between 2021 and 2023 at a hectometer scale. In this study, we present modifications of standard seismic event processing techniques for mesoscale experiments, enabling the accurate detection and location of picoseismic events at these scales. We differentiate between the needs of seismic catalogs for rapid hazard assessment, which require real-time detection of large-magnitude events, and postprocessed catalogs, which are essential for detailed analysis of activated fracture networks, for which a complete seismic catalog is essential. In addition, we evaluate the impact of picking accuracy, background 1D velocity model, and network coverage on the final event locations within a mesoscale underground 3D seismic network, a novelty in experimental seismology.

The structure and nature of Earth’s transition zone, which is delineated by the transformation of olivine to its higher-pressure polymorphs, exerts a strong influence on material transfer between upper and lower mantle. Mars, however, because of its relatively large core, is only expected to exhibit the equivalent of Earth’s uppermost transition zone seismic discontinuity. We searched the InSight seismic data for marsquakes and impacts located in an epicentral distance range favorable for detection of seismic phases that have interacted with Mars’s olivine transition (midmantle) discontinuity. Through application of careful data selection criteria and processing schemes, we found 13 events in the distance range in which body waves are expected to refract through the midmantle of Mars. Although triplicated body waves are potentially present in seven events, the distance distribution is insufficient to allow for unambiguous detection of the triplicated waveform pattern associated with the midmantle discontinuity. Comparison of travel times of the observed waveforms with predictions from recent Mars models indicates the possible presence of a midmantle discontinuity located between 987 and 1052 km or 1075 and 1122 km depth, in which the uncertainty comes from our inability to reliably distinguish first from secondary arrivals.

The InSight lander represents a unique opportunity to correlate seismic data with impact events identified in orbital images, enabling the characterization of the physical properties of the martian crust and mantle. Here, we present the first comprehensive catalog of impacts that occurred during the InSight mission within a 50° radius around the lander. We use a machine learning‐enabled approach to identify 123 date‐constrained impacts with diameters between ∼1 and 22.5 m. We estimate an impact rate of 2.7 × 10 ⁻⁶ /km ² /year for >3.9 m effective diameter, which is ∼1.6–∼2.5 times higher than previously derived for Mars. We identify 49 seismic events with one or several potential impact match(es) including a 21.5 m crater located near Cerberus Fossae. Our catalog will enable a more accurate characterization of the propagation of seismic body waves at intermediate distances to InSight (5–50°), with major implications for estimates of other seismic event distances.