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Abstract During the past few years, distributed acoustic sensing (DAS) has become an invaluable tool for recording high-fidelity seismic wavefields with great spatiotemporal resolutions. However, the considerable amount of data generated during DAS experiments limits their distribution with the broader scientific community. Such a bottleneck inherently slows down the pursuit of new scientific discoveries in geosciences. Here, we introduce PubDAS—the first large-scale open-source repository where several DAS datasets from multiple experiments are publicly shared. PubDAS currently hosts eight datasets covering a variety of geological settings (e.g., urban centers, underground mines, and seafloor), spanning from several days to several years, offering both continuous and triggered active source recordings, and totaling up to ∼90 TB of data. This article describes these datasets, their metadata, and how to access and download them. Some of these datasets have only been shallowly explored, leaving the door open for new discoveries in Earth sciences and beyond. 

Water injection and Enhanced Geothermal System (EGS) technologies have been used to exploit heat resources from geothermal reservoirs. Detecting spatial and temporal changes in reservoir physical properties is important for monitoring reservoir condition changes due to water injection and EGS. Here, we determine high‐resolution models of the temporal changes in the three‐dimensionalPwave velocity and attenuation (Vp and Qp) structures between the years 2005 and 2011 in the northwestern part of The Geysers geothermal field, California, using double‐difference seismic velocity and attenuation tomography. The northwest Geysers has a shallow normal temperature reservoir (NTR) underlain by a high temperature reservoir (HTR) that has substantial underutilized heat resources but may be more fully utilized in the future through EGS. In the southeastern part of the northwest Geysers, however, EGS has been successfully but unintentionally applied for at least 50 years because the waters injected into the NTR have been flowing into the HTR. Our models are well resolved in this area and show that the NTR and HTR have different seismic responses (seismicity, Vp, and Qp) to water injection, which can be explained by the injection‐induced differences in fracturing and saturation that are likely related to their geological properties. Our results indicate that the joint analysis of changes in seismicity, velocity, and attenuation is valuable for characterizing changes in reservoir fracturing and saturation conditions. Our results suggest that high‐permeability zones and/or pre‐existing permeable fault zones are important for the success of EGS at The Geysers and potentially other geothermal systems.

 

SUMMARY Knowledge of attenuation structure is important for understanding subsurface material properties. We have developed a double-difference seismic attenuation (DDQ) tomography method for high-resolution imaging of 3-D attenuation structure. Our method includes two main elements, the inversion of event-pair differential ${t^*}$ ($d{t^*}$) data and 3-D attenuation tomography with the $d{t^*}$ data. We developed a new spectral ratio method that jointly inverts spectral ratio data from pairs of events observed at a common set of stations to determine the $d{t^*}$ data. The spectral ratio method cancels out instrument and site response terms, resulting in more accurate $d{t^*}$ data compared to absolute ${t^*}$ from traditional methods using individual spectra. Synthetic tests show that the inversion of $d{t^*}$ data using our spectral ratio method is robust to the choice of source model and a moderate degree of noise. We modified an existing velocity tomography code so that it can invert $d{t^*}$ data for 3-D attenuation structure. We applied the new method to The Geyser geothermal field, California, which has vapour-dominated reservoirs and a long history of water injection. A new Qp model at The Geysers is determined using P-wave data of earthquakes in 2011, using our updated earthquake locations and Vp model. By taking advantage of more accurate $d{t^*}$ data and the cancellation of model uncertainties along the common paths outside of the source region, the DDQ tomography method achieves higher resolution, especially in the earthquake source regions, compared to the standard tomography method using ${t^*}$ data. This is validated by both the real and synthetic data tests. Our Qp and Vp models show consistent variations in a normal temperature reservoir that can be explained by variations in fracturing, permeability and fluid saturation and/or steam pressure. A prominent low-Qp and Vp zone associated with very active seismicity is imaged within a high temperature reservoir at depths below 2 km. This anomalous zone is likely partially saturated with injected fluids. 

Abstract We present two new seismic velocity models for Alaska from joint inversions of body-wave and ambient-noise-derived surface-wave data, using two different methods. Our work takes advantage of data from many recent temporary seismic networks, including the Incorporated Research Institutions for Seismology Alaska Transportable Array, Southern Alaska Lithosphere and Mantle Observation Network, and onshore stations of the Alaska Amphibious Community Seismic Experiment. The first model primarily covers south-central Alaska and uses body-wave arrival times with Rayleigh-wave group-velocity maps accounting for their period-dependent lateral sensitivity. The second model results from direct inversion of body-wave arrival times and surface-wave phase travel times, and covers the entire state of Alaska. The two models provide 3D compressional- (VP) and shear-wave velocity (VS) information at depths ∼0–100  km. There are many similarities as well as differences between the two models. The first model provides a clear image of the high-velocity subducting plate and the low-velocity mantle wedge, in terms of the seismic velocities and the VP/VS ratio. The statewide model provides clearer images of many features such as sedimentary basins, a high-velocity anomaly in the mantle wedge under the Denali volcanic gap, low VP in the lower crust under Brooks Range, and low velocities at the eastern edge of Yakutat terrane under the Wrangell volcanic field. From simultaneously relocated earthquakes, we also find that the depth to the subducting Pacific plate beneath southern Alaska appears to be deeper than previous models. 

Imaging silicic systems using geophysics is challenging because many interrelated factors (e.g., temperature, melt fraction, melt composition, geometry) can contribute to the measured geophysical anomaly. Joint interpretation of models from multiple geophysical methods can better constrain interpretations of the subsurface structure. Previously published resistivity and shear wave velocity (Vs) models, derived separately from magnetotelluric (MT) and surface wave seismic data, respectively, have been used to model the restless Laguna del Maule Volcanic Field, central Chile. The Vs model contains a 450 km³low‐velocity zone (LVZ) interpreted as a region with an average melt fraction of 5–6%. The resistivity model contains a conductor (C3) interpreted as a region with a melt fraction >35%. The spatial extents of the LVZ and C3 overlap, but the geometries and interpretations of these features are different. To resolve these discrepancies, this study investigates the resolution of the MT data using hypothesis testing and constrained MT inversions. It is shown that the MT data are best fit with discrete conductors embedded within the larger LVZ. The differences between the MT and seismic models reflect resolution differences between the two data sets as well as varying sensitivities to physical properties. The MT data are sensitive to smaller volumes of extractable mush that contain well‐connected crystal‐poor melt (C3). The seismic data have lower spatial resolution but image the full extent of the poorly connected crystal‐rich magma storage system. The combined images suggest that the LdMVF magma plumbing system is thermally heterogeneous with coexisting zones of warm and cold storage.

 

With substantial postglacial rhyolite eruptions and ongoing rapid uplift, the Laguna del Maule volcanic field in the southern Andes provides an exceptional opportunity to study the dynamics of an active silicic magmatic system. Using 4,093Parrivals from 137 distant earthquakes recorded by 44 local stations over∼2.25 years, we conduct teleseismic tomography to image the crustal structure down to 40 km below the volcanic field. A prominent low‐velocity body at depths between∼0 and 12 km below sea level (b.s.l.), characterized by a volume of∼500 km³and a peak anomaly of−400 m/s (∼9%), overlaps the location of the upper‐crustal magma reservoir detected in recent gravity and surface wave tomography studies. Its estimated averagePwave velocity of∼4.6 km/s corresponds to an average melt fraction of about 14% and a melt volume of∼70 km³. Petrologic observations are also consistent with generation and storage of rhyolitic melts at depths corresponding to the anomalous zone. Moreover, the tomographic results support a lower crust zone of MASH (melting, assimilation, storage, and homogenization) from a depth of∼25 km to the base of the model, which likely reflects a deep crustal source of magma that contributes to and incubates the shallow silicic reservoir.

 

Conceptual models of magma storage and transport under calderas favor a connected system of sills and dikes. These features are individually below the resolution of standard seismic tomography, but radial seismic anisotropy can reveal where they exist in aggregate. We model radial anisotropy at Okmok caldera, Alaska, to demonstrate the presence of a caldera‐centered stacked sill complex and surrounding dike system. We show that ascending magma, inferred from seismicity, either intersects the sill complex, resulting in a larger volume eruption of evolved magma, or bypasses the overlying sill complex via dikes, resulting in a low‐volume mafic eruption. Our results exemplify how the locations of magma storage and paths of transport impact eruption size and composition. As this type of crustal storage is likely common to many calderas, this analysis offers a potential new framework for volcano observatories to forecast the size of impending eruptions.