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Seismological observations in the lunar mantle, in conjunction with experimental knowledge on the elastic wave velocities and density of lunar mantle minerals, provide important constraints on the composition and mineralogy of the lunar mantle. Here we report elastic wave velocities and density of a lunar orthopyroxene (Mg0.84Fe0.13Ca0.03SiO3) up to 5.5 GPa and 1,273 K. The result shows that the bulk and shear moduli of orthopyroxene decrease with increasing iron content. Based on the mineral elasticity data, we modeled the P‐ and S‐wave velocities and density of petrologically suggested lunar upper mantle rock composition. The petrological lunar upper mantle rock model shows consistent seismic wave velocities with those observed in the lunar upper mantle whereas markedly lower density. Our modeling suggests an iron‐rich (Fe/(Mg + Fe) = 0.20) lunar upper mantle to explain P‐ and S‐wave velocities and density of the lunar upper mantle at 40–740 km depth.

Discontinuities, such as joints and beddings, usually play a significant role in the seismic response and corresponding failure process of slopes, especially for anti-dip rock slide according to field observations. Shaking table tests associated with numerical analyses are carried out in this paper to explore the effect of seismic wave on response of jointed anti-dip rock slopes. Shaking table tests involve anti-dip rock slope models with different rock types and different excitation intensities. Ten accelerometers are installed inside each slope model to monitor the dynamic response and spectrum shifting characteristics. It is found that the area of failure zone in the soft rock anti-dip slope is approximate 1.5 times the size of that in the hard rock anti-dip slope. Meanwhile, the width and ridge number of the fast Fourier-transformation spectrum along the slope surface can reveal the internal damage features within the anti-dip rock slopes, and the multiple failure planes can also be recognized according to the variation of distance between the innermost and outermost ridges in the fast Fourier-transformation spectrum. Moreover, the distinct element method incorporating a damage model is used to interpret the test results and to identify the main influencing factors for seismic instability. It is found that the failure pattern of a jointed anti-dip rock slope is more sensitive to bedding inclination than to joint inclination.

Our study investigates variations of seismic velocity that occur over short time scales of hours. We use coda wave interferometry to inspect 5 months of continuous data from a seismological array in southern Germany. This results in relative seismic velocities (dv/v) that show temporal variations on the order of 10−4. Spectra of the velocity time series contain strong daily and sub‐daily behavior indicating that the daily and sub‐daily changes in the seismic velocity are primarily caused by atmospheric tides. We also note the influence of temperature changes on daily variations, but as a second‐order effect. The explanatory model focuses on depth variations of the groundwater table, linking atmospheric pressure (loading and de‐loading the Earth's surface) to variations in seismic velocity. Our results highlight an important environmental influence on seismic velocity that needs to be considered before seismic velocity variations can be used for inspecting fluid and stress variations in situ. Plain Language Summary Geological and environmental effects manifest themselves in changes of crustal properties, and we record these as variations of seismic wave velocity. This is especially the case for seismic waves that travel within the Earth's crust. Changes in tectonic stress are particularly interesting, since they can cause earthquakes. We would also like to know more about changes in fluid content in the crust at the various depth levels. Both tectonic stress and fluid content affect seismic velocity, but environmental factors also have an effect and these need to be taken into account. This study shows that the interaction of air pressure and groundwater, has considerable influence on measured daily velocity variations. This opens important possibilities in itself. On the other hand, we can remove these pressure effects from the velocity changes, to highlight changes due to tectonic effects, such as earthquakes. Key Points We observe strong daily and sub‐daily variations of seismic velocity using coda wave interferometry The effect is caused predominantly by the coupling of atmospheric pressure and groundwater table variations The coupling allows a better understanding of crustal deformation and suggests novel techniques for characterizing aquifer systems

For the first time, we measured the ellipticity of direct Rayleigh waves at intermediate periods (15–35 s) on Mars using the recordings of three large seismic Martian events, including S1222a, the largest event recorded by the InSight mission. These measurements, together with P‐to‐s receiver functions and P‐wave reflection times, were utilized for performing a joint inversion of the local crustal structure at the InSight landing site. Our inversion results are compatible with previously reported intra‐crustal discontinuities around 10 and 20 km depths, whereas the preferred models show a strong discontinuity at ∼37 km, which is interpreted as the crust‐mantle interface. Additionally, we support the presence of a shallow low‐velocity layer of 2–3 km thickness. Compared to nearby regions, lower seismic wave velocities are derived for the crust, suggesting a higher porosity or alteration of the whole local crust. Plain Language Summary As never before on Mars, we measured the characteristics of seismic waves traveling along the Martian surface that carry information about the crustal structure at the InSight site. We combined these measurements with two other local‐scale independent observations to derive a consolidated model for the crust underneath the InSight lander. Our results suggest a Martian crust with 4 layers and, particularly, one thin layer of about 2 km thickness close to the surface. The crust‐mantle discontinuity was found at ∼37 km depth, where the sharpest change in seismic wave velocity is observed. Overall, the seismic wave velocities of the local Martian crust at the InSight site are lower than those derived in other regions on Mars, which suggests a higher porosity or local alteration. Key Points Rayleigh waves ellipticity was measured between periods 15–35 s at the InSight landing site using large seismic events, including S1222a A 4‐layer crust, including a shallow low‐velocity layer, is required to explain the ellipticity, receiver functions and P‐wave lag times Low crustal velocities are derived for the InSight site, which may be due to high porosity or heavy alteration at local scale

In active volcanic systems, the elevated pressurization of fluids and the movement of molten materials influence the stress state and mechanical behavior of rocks, but the direct measurement of these processes and the related evolution of rocks properties is difficult. By studying seismic velocity variations, we quantify the physical changes in rocks induced by long‐term volcanic deformation and the dynamic changes associated with the 2017 Casamicciola earthquake (Mw 3.9) in the active volcanic complex of Ischia Island, Italy. Our study reveals a significant dynamic velocity reduction (∼0.2%), primarily due to near‐surface damage, with a permanent drop linked to documented landslides and subsidence observed immediately after the earthquake. We also identified a positive long‐term linear trend in velocity variations, indicative of a generalized contraction of the Ischia Caldera, as revealed by geodetic modeling. Our results suggest a depressurization of the shallow hydrothermal system through degassing along faults or sills. Plain Language Summary Volcanic systems are influenced by a variety of complex and rapid processes, including significant temperature variations, intricate stress patterns, and changes in fluid pressure. In our study, we used seismic wave velocity measurements from ambient noise recordings and GPS data to show that seismic velocity changes are highly sensitive to depressurization, ground shaking, and damage levels. These changes reflect the pressure levels of hydrothermal and magmatic fluids in volcanic regions. Our results indicate varied shallow degassing, mainly in the northern part of the island, and lower effective stress in the southern part, where there is higher geothermal activity and highly pressurized fluids. While GPS data provide surface measurements, seismic velocity variations offer insights into the Earth's crust. Together, these measurements help us understand deformation processes at different depths, which is crucial for monitoring volcanic activity. Key Points We characterize short and long‐term seismic wave velocity variations during 8 years at the volcanic Island of Ischia (Italy) We revealed a significant coseismic drop in occurrence of the 2017 Mw 3.9 Casamicciola earthquake tracking the near‐surface damage We observe a remarkable sensitivity of the seismic wave velocity to depressurization processes of the hydrothermal system

Microcracks in geomaterials cause variations in the elastic moduli under applied strain, thereby creating seismic wave velocity variations. These are crucial for understanding the dynamic processes of the crust, such as fault‐zone damage, healing, and volcanic activities. Solid earth tides have been used to detect seismic velocity changes responding to crustal‐scale deformations. However, no prior research has explored the characteristics of the seismic velocity variations caused by large‐scale tidal deformation. To systematically evaluate the tidal response to velocity variations, we developed a new method that utilized the flexibility of a state‐space model. The tidal response was derived from hourly stacked noise autocorrelations using a seismic interferometry method throughout Japan. In particular, large tide‐induced seismic velocity changes were observed in the low S‐wave velocity region of the shallow crust. Overall, the tidal responses of velocity variations can provide new insights into the response mechanisms of the shallow crust to applied strain. Plain Language Summary Rock deformations can open or close microcracks in rocks along with varying their elastic moduli under an applied strain. The temporal variations in the elastic moduli of rocks alter the seismic wave velocity, which can be monitored to provide information on the strain applied to the crust. This is crucial for understanding the geological processes in fault zones and volcanic regions. To utilize the seismic velocity variations for monitoring how much the Earth's structure deforms, the response of the seismic velocity to the deformations must be assessed. The deformation of the Earth's surface caused by lunar and solar gravity, called solid Earth tides, has been used to study seismic velocity variations in response to crustal deformation. However, only a limited number of regions have been studied for the tidal response of the seismic velocity, and the characteristics of its variations caused by tidal deformation were not yet apparent. This study measured the tidal responses to seismic velocity variations throughout Japan with reliable estimations. Notably, the tide‐induced seismic velocity variations tend to increase in the low S‐wave velocity region. Overall, these results provide new insights into the response mechanisms of the shallow crust to deformations. Key Points The spatial distribution of seismic velocity changes caused by tides was determined using a dense network of seismic stations in Japan The tidal response of velocity variations was extracted from ambient noise using an extended Kalman filter with a Maximum Likelihood method Strain‐velocity sensitivities tend to increase at a low S‐wave velocity in the shallow crust

Attenuation exists in seismic wave propagation in subsurface layers, and relatively high attenuation occurs in oil-bearing reservoirs. Inversion of frequency components of observed seismic data generates values of attenuation factor 1/Q, which produces potential results for determining oil-bearing reservoirs. Beginning with expressions of seismic wave velocity in attenuating media, we involve P-wave maximum attenuation factor to rewrite P-wave velocity driven by an attenuating rock physics model, and we also employ an empirical relationship between P-wave attenuation factor and S-wave attenuation factor to express S-wave velocity in terms of P-wave maximum attenuation factor. Using the derived P- and S-wave velocities, we extend Zoeppritz equations to compute reflection coefficient for an interface separating two attenuating media. Under the assumption that contrasts in elastic properties of two media across the interface are small and the background attenuation is weak, we propose a linearized reflection coefficient of PP-wave as a function of contrasts in elastic parameters (i.e., P-wave velocity, S-wave velocity and density) and attenuation factor, and expression of elastic impedance (EI) is also presented. Based on the EI, we demonstrate an approach of estimating elastic parameters and attenuation factor from frequency components of partially incidence-stacked seismic data, which is implemented as a two-step inversion involving the prediction of EI datasets using a model-based damping least-squares algorithm and nonlinear inversion for elastic parameters and attenuation factor. Noisy synthetic seismic data generated using the extended Zoeppritz equations are employed to verify the robustness and stability of the proposed inversion approach. Applying the proposed approach to a real dataset acquired over an oil-bearing reservoir, we obtain convincing results of P-wave velocity, S-wave velocity, density and attenuation factor, which can reasonably match corresponding well log data.

Seismic tomography has shown that the shear wave velocities (Vs) under continents, especially under cratons, are extremely fast at 100–200 km depth, which is difficult to explain by low temperatures or high Mg#. Alternatively, delaminated eclogitic lower continental crust has been proposed to account for these fast seismic anomalies. However, the thermoelastic properties of jadeite which constitutes up to 60–80 mol% of clinopyroxene in the potentially delaminated lower continental crust are not well constrained. In this study, we measured the single‐crystal elasticity of jadeite by Brillouin spectroscopy under simultaneous high pressure and temperature conditions for the first time. We found that the temperature dependence of Vs of jadeite is extremely small if not negligible. The seismic velocities of the potentially delaminated lower continental crusts were subsequently modeled and found to match the widely observed fast seismic anomalies under cratons between 100 and 200 km depth. Plain Language Summary The seismic wave velocity variation images show the potential composition and temperature heterogeneities inside the Earth. Fast shear wave velocities (∼7% higher than the global average) have been observed under continents at 100–200 km depths. A candidate explanation of this fast shear wave velocity anomaly is the existence of delaminated eclogitic lower continental crust. However, due to the lack of knowledge of the thermoelastic properties of clinopyroxene, which is the dominant mineral phase (up to 60 vol%) in delaminated eclogitic lower continental crust, evaluation of this hypothesis is difficult. Clinopyroxene in the potentially delaminated lower continental crust is jadeite‐rich (up to 60–80 mol%) due to its high Na content (2.5–3.5 wt%). In this study, we report single‐crystal elasticity of jadeite at high pressure‐temperature conditions. We found the Vs of jadeite is much higher than all the other major upper mantle minerals under upper mantle conditions. The calculated seismic velocities of the potentially delaminated lower continental crusts could easily account for the fast shear wave anomalies observed under cratons. Key Points High pressure‐temperature single‐crystal elasticity measurements of jadeite are conducted by Brillouin spectroscopy Jadeite is among the seismically fastest phases in the Earth's upper mantle Delaminated lower crust can help explain the fast seismic anomalies under cratons

Deep fluid circulation likely triggered the large extensional events of the 2016–2017 Central Italy seismic sequence. Nevertheless, the connection between fault mechanisms, main crustal‐scale thrusts, and the circulation and interaction of fluids with tectonic structures controlling the sequence is still debated. Here, we show that the 3D temporal and spatial mapping of peak delays, proxy of scattering attenuation, detects thrusts and sedimentary structures and their control on fluid overpressure and release. After the mainshocks, scattering attenuation drastically increases across the hanging wall of the Monti Sibillini and Acquasanta thrusts, revealing fracturing and fluid migration. Before the sequence, low‐scattering volumes within Triassic formations highlight regions of fluid overpressure, which enhances rock compaction. Our results highlight the control of thrusts and paleogeography on the sequence and hint at the monitoring potential of the technique for the seismic hazard assessment of the Central Apennines and other tectonic regions. Plain Language Summary There is widespread evidence that the Amatrice‐Visso‐Norcia seismic sequence (2016–2017, Central Italy) was triggered by fluid circulation across the Apennine Chain. However, how, and why fluids migrated across the fault network is still under debate. Seismic attenuation describes how seismic waves lose energy during their propagation. When used as an imaging attribute, it has demonstrated the potential to recover the spatial extension and mechanisms of fracturing and fluid movement across volcanoes and faults. Here, we map scattering attenuation through the peak delay measurements in 3D before (2013–2016) and during the 2016–2017 sequence. Scattering attenuation separated fractured zones from regions of compaction, controlled, before and during the sequence by thrusts and lithological differences. High scattering (strong fracturing) increases through time due to intense fracturing, while low scattering (higher compaction of the rocks) marks areas where earthquakes will occur. Our results highlight the importance of the main thrusts, as they separate compartments of the shallow crust characterized by different scattering attenuation anomalies, the Triassic deposits in fluid accumulation, and subsequent triggering of normal faults. Key Points Scattering attenuation detects the control of thrusts and lithology on post‐seismic fracturing and fluid migration during the AVN sequence Overpressurized fluids compact low‐scattering rocks at thrusts' roots before earthquakes Detecting fluid overpressure and fracturing suggests an unexploited monitoring potential of scattering attenuation

Strong waveform complexities, including multipathing of the S diffracted phase and rapid changes in differential ScS‐S times, are observed for multiple deep Fiji earthquakes recorded at the USArray. The complexities occur at the northeastern edge of the Pacific Large Low Shear Velocity Province (LLSVP), about 12 degrees southeast of present‐day Hawaiʻi. Waveform modeling of the multipathing provides good constraints on an ultra‐low velocity zone (ULVZ) with a width of 5 degree located near the inner edge of the LLSVP. Based on the mineralogical‐modeling of the ULVZ as a solid iron‐rich magnesiowüstite‐bearing assemblage with compatible morphology predicted from geodynamical simulations, a ULVZ model with a thickness of 30 km and a shear wave velocity reduction of 18% is preferred. The rapid change in differential ScS‐S travel time is best explained by having both the aforementioned ULVZ and an adjacent high velocity structure near the LLSVP. Furthermore, a low‐velocity plume‐like structure could potentially explain the observed S travel time delay independent of ScS. These seismic features are proposed to be a ULVZ driven toward the edge of the LLSVP while potentially pushed by a subducted slab. This configuration may trigger plume generation due to strong thermal instabilities and is in the same vicinity where mantle flow models place the present‐day Hawaiian plume source. Multiple ScS can potentially be used to verify vertical plume structure in tomographic models but the accuracy of upper mantle structure, which is a key reflection point, needs to be considered. Plain Language Summary Seismic waves from earthquakes in Fiji recorded by seismometers in the United States travel close to the core‐mantle boundary (CMB) and can be used to image fine‐scale structures along the edge of a previously known large province near the CMB with low seismic wave velocity, namely the Pacific LLSVP. The edges of the Pacific LLSVP are of interest because they contain many structural anomalies, thought to be correlated with hotspots on Earth's surface, including Hawaiʻi, and can host the plume sources for these hotspots. In this study, we observed two phenomena: (a) seismic waves with an additional unexpected pulse due to the presence of a very low velocity structure and (b) a rapid change in the travel time behavior of two seismic phases which can be explained by the same low velocity structure adjacent to a high velocity structure. We constrained these two structures to be at the edge of the LLSVP, a configuration favorable for generating long‐lasting plume, that is, the source for the Hawaiian hotspot. The location of these structures is in agreement with the hypothesized source location from recent geodynamical studies. We also showed that this low velocity structure could be composed of a solid iron‐rich material. Key Points Seismic waves sampling the northeastern edge of the Pacific Large Low Shear Velocity Province (LLSVP) show strong waveform complexity and rapid change in differential time Waveform and mineralogical modeling suggest a magnesiowüstite‐bearing ultra‐low velocity zone adjacent to slab at edge of LLSVP, conducive to plume formation The structural anomalies and proposed plume are located ∼12° southeast of present‐day Hawaiʻi, in agreement with recent mantle flow models