Underground storage in geologic formations will play a key role in the energy transition by providing low-cost storage of renewable fuels like hydrogen. The sealing qualities of caverns leached in salt and availability of domal salt bodies make them ideal for energy storage. However, unstable boundary shear zones of anomalous friable salt can enhance internal shearing and pose a structural hazard to storage operations. Considering the indistinct nature of internal salt heterogeneities when imaged with conventional techniques like reflection seismic surveys, we develop a method to map shear zones using seismicity patterns in the US Gulf Coast, the region with the world’s largest underground crude oil emergency supply. We developed and finetuned a machine learning algorithm using tectonic and local microearthquakes. The finetuned model was applied to detect microearthquakes in a 12-month nodal seismic dataset from the Sorrento salt dome. Clustered microearthquake locations reveal the three-dimensional geometry of two anomalous salt shear zones and their orientations were determined using probabilistic hypocenter imaging. The seismicity pattern, combined with borehole pressure measurements, and sonar surveys show the spatio-temporal evolution of cavern shapes within the salt dome. We describe how shear zone seismicity contributed to a cavern well failure and gas release incident that occurred during monitoring. Our findings show that caverns placed close to shear zones are more susceptible to structural damage. We propose a non-invasive technique for mapping hazards related to internal salt dome deformation that can be employed in high-noise industrial settings to characterize salt domes used for storage.

We construct a new shear velocity model for the San Gabriel, Chino and San Bernardino basins located in the northern Los Angeles area using ambient noise correlation between dense linear nodal arrays, broadband stations, and accelerometers. We observe Rayleigh wave and Love wave in the correlation of vertical (Z) and transverse (T) components, respectively. By combining Hilbert and Wavelet transforms, we obtain the separated fundamental and first higher mode of the Rayleigh wave dispersion curves based on their distinct particle motion polarization. Receiver functions, gravity, and borehole data are incorporated into the prior model to constrain the basin depth. Our 3D shear wave velocity model covers the upper 3 to 5 km of the basin structure in the San Gabriel and San Bernardino basin area. The Vs model is in agreement with the geological and geophysical cross-sections from other studies, but discrepancies exist between our model and a Southern California Earthquake Center (SCEC) community velocity model. Our shear wave velocity model shows good consistency with the CVMS 4.26 in the San Gabriel basin, but predicts a deeper and slower sedimentary basin in the San Bernardino and Chino basins than the community model.

The San Gabriel, Chino, and San Bernardino sedimentary basins in Southern California amplify earthquake ground motions and prolong the duration of shaking due to the basins’ shape and low seismic velocities. In the event of a major earthquake rupture along the southern segment of the San Andreas fault, their connection and physical proximity to Los Angeles can produce a waveguide effect and amplify strong ground motions. Improved estimates of the shape and depth of the sediment-basement interface are needed for more accurate ground-shaking models. We obtain a three-dimensional basement map of the basins by integrating gravity and seismic measurements. The travel time of the sediment-basement P-to-s conversion, and the Bouguer gravity along 10 seismic lines, are combined to produce a linear relationship that is used to extend the 2D models to a 3D basin map. Basement depth is calculated using the predicted travel time constrained by gravity with an S-wave velocity model of the area. The model is further constrained by the basement depths from 17 boreholes. The basement map shows the south-central part of the San Gabriel basin is the deepest part and a significant gravity signature is associated with our interpretation of the Raymond fault. The Chino basin deepens towards the south and shallows northeastward. The San Bernardino basin, bounded by the San Jacinto fault (SJF) and San Andreas fault zone, deepens along the edge of the SJF. In addition, we demonstrate the benefit of using gravity data to aid in the interpretation of the sediment-basement interface in receiver functions.