We present an extension of the nodal discontinuous Galerkin method for elastic wave propagation to high interpolation orders and arbitrary heterogeneous media. The high-order lagrangian interpolation is based on a set of nodes with excellent interpolation properties in the standard triangular element. In order to take into account highly variable geological media, another set of suitable quadrature points is used where the physical and mechanical properties of the medium are defined. We implement the methodology in a 2-D discontinuous Galerkin solver. First, a convergence study confirms the hp-convergence of the method in a smoothly varying elastic medium. Then, we show the advantages of the present methodology, compared to the classical one with constant properties within the elements, in terms of the complexity of the mesh generation process by analysing the seismic amplification of a soft layer over an elastic half-space. Finally, to verify the proposed methodology in a more complex and realistic configuration, we compare the simulation results with the ones obtained by the spectral element method for a sedimentary basin with a realistic gradient velocity profile. Satisfactory results are obtained even for the case where the computational mesh does not honour the strong impedance contrast between the basin bottom and the bedrock.

Distributed acoustic sensing (DAS) is an ideal tool for ambient noise tomography owing to the dense spatial measurements and the ability to continuously record in harsh environments, such as underwater. Although the fine spatial sampling of DAS facilitates the imaging of small-scale lateral velocity heterogeneities, efforts relying on dispersion-based ambient noise tomography are hampered by the underlying premise of negligible lateral variations across the segment used for dispersion curve extraction. To image small-scale structures, this method should be augmented with objective and independent approaches that are not scale limited. Here, we show that power spectral densities (PSDs) and autocorrelations (ACs) of DAS data reveal extremely detailed frequency-dependent resonance and wave propagation characteristics. These observations contain crucial information on lateral and vertical wave propagation. We use these methods to demonstrate the ability to image a complex underwater basin using ambient noise recorded on a fiber deployed offshore Greece. A 2D shear-wave velocity model was derived by analyzing Scholte-wave dispersion. The PSD and AC reveal significant lateral variations across the short 2.5 km long fiber segment, including basin edge effects and scattered waves. These were used to further constrain and modify the velocity model. The modified model is supported by waveform simulations that qualitatively reproduce the PSD and AC observations. Our results demonstrate the advantages of incorporating PSD and AC observations into ambient noise-based imaging. The spatially continuous observation of resonance modes across the basin highlights the benefit of DAS acquisitions for ground-motion estimation.

Interactions between melting ice and a warming ocean drive the present-day retreat of tidewater glaciers of Greenland^1–3, with consequences for both sea level rise⁴ and the global climate system⁵. Controlling glacier frontal ablation, these ice–ocean interactions involve chains of small-scale processes that link glacier calving—the detachment of icebergs⁶—and submarine melt to the broader fjord dynamics^7,8. However, understanding these processes remains limited, in large part due to the challenge of making targeted observations in hazardous environments near calving fronts with sufficient temporal and spatial resolution⁹. Here we show that iceberg calving can act as a submarine melt amplifier through excitation of transient internal waves. Our observations are based on front-proximal submarine fibre sensing of the iceberg calving process chain. In this chain, calving initiates with persistent ice fracturing that coalesces into iceberg detachment, which in turn excites local tsunamis, internal gravity waves and transient currents at the ice front before the icebergs eventually decay into fragments. Our observations show previously unknown pathways in which tidewater glaciers interact with a warming ocean and help close the ice front ablation budget, which current models struggle to do¹⁰. These insights provide new process-scale understanding pertinent to retreating tidewater glaciers around the globe.

The April 2016 Pedernales earthquake ruptured a 100 km by 40 km segment of the subduction zone along the coast of Ecuador in an $Mw$ 7.8 megathrust event east of the intersection of the Carnegie ridge with the trench. This portion of the subduction zone has ruptured on decadal time scales in similar size and larger earthquakes, and exhibits a range of slip behaviors, variations in segmentation, and degree of plate coupling along strike. Immediately after the earthquake, an international rapid response effort coordinated by the Instituto Geofísico at the Escuela Politécnica Nacional in Quito deployed 55 seismometers and 10 ocean‐bottom seismometers above the rupture zone and adjacent areas to record aftershocks. In this article, we describe the details of the U.S. portion of the rapid response and present an earthquake catalog from May 2016 to May 2017 produced using data recorded by these stations. Aftershocks focus in distinct clusters within and around the rupture area and match spatial patterns observed in long‐term seismicity. For the first two and a half months, aftershocks exhibit a relatively sharp cutoff to the north of the mainshock rupture. In early July, an earthquake swarm occurred $∼100km$ to the northeast of the mainshock in the epicentral region of an $Mw$ 7.8 earthquake in 1958. In December, an increase in seismicity occurred $∼70km$ to the northeast of the mainshock in the epicentral region of the 1906 earthquake. Data from the Pedernales earthquake and aftershock sequence recorded by permanent seismic and geodetic networks in Ecuador and the dense aftershock deployment provide an opportunity to examine the persistence of asperities for large to great earthquakes over multiple seismic cycles, the role of asperities and slow slip in subduction‐zone megathrust rupture, and the relationship between locked and creeping parts of the subduction interface.

SUMMARY: We report here an interesting case study of dynamic characterization of a limestone sea cliff area, named the Sutta-Rocca overhang, located near the medieval town of Bonifacio (southern Corsica, France). The site belongs to an UNESCO protected area with an important number of visitors per year. Therefore, it is of particular interest to evaluate the potential rockfall hazard along the cliff. With the objective of evaluating the feasibility of an operational technique to monitor the cliff using ambient vibrations, two seismic stations were installed during six months (from 2016 November to 2017 April) in order to identify and monitor the resonant frequencies of the a priori unstable rock mass. Several techniques of classical seismic monitoring are tested on the site: single-station spectral analysis for identification of frequency peaks related to the unstable mass, relative spectral amplifications between stations and the operational modal analysis of the cliff. In particular, data analysis from a temporary campaign of two days of continuous recordings by a linear array of seismometers perpendicular to the sea cliff highlights the main characteristics of the ground motion: the overall motion of the promontory mainly in the north–south direction (perpendicular to the coast) and bending vertical and parallel to the cliff (east–west direction) at higher frequencies. The strong vertical and east–west seismic amplifications are also confirmed by the analysis of five local seismic events recorded at both semi-permanent stations. Even if ambient vibrations and seismic data analysis allows to decipher the dynamics of the Sutta-Rocca overhang, we conclude that classical seismic monitoring of just one single frequency peak with a limited number of sensors is not recommended for this particular site.

Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 (V_S10), 30 (V_S30), 100 (V_S100), and 300 (V_S300) meters and show its usefulness in quantitative comparisons of V_S profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects.