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The crustal structure in south‐central Alaska has been influenced by terrane accretion, flat slab subduction, and a modern strike‐slip fault system. Within the active subduction system, the presence of the Denali Volcanic Gap (DVG), a ∼400 km region separating the active volcanism of the Aleutian Arc to the west and the Wrangell volcanoes to the east, remains enigmatic. To better understand the regional tectonics and the nature of the volcanic gap, we deployed a month‐long north‐south linear geophone array of 306 stations with an interstation distance of 1 km across the Alaska Range. By calculating multi‐component noise cross‐correlation and jointly inverting Rayleigh wave phase velocity and ellipticity across the array, we construct a 2‐D shear wave velocity model along the transect down to ∼16 km depth. In the shallow crust, we observe low‐velocity structures associated with sedimentary basins and image the Denali fault as a narrow localized low‐velocity anomaly extending to at least 12 km depth. About 12 km, below the fold and thrust fault system in the northern flank of the Alaska Range, we observe a prominent low‐velocity zone with more than 15% velocity reduction. Our velocity model is consistent with known geological features and reveals a previously unknown low‐velocity zone that we interpret as a magmatic feature. Based on this feature's spatial relationship to the Buzzard Creek and Jumbo Dome volcanoes and the location above the subducting Pacific Plate, we interpret the low‐velocity zone as a previously unknown subduction‐related crustal magma reservoir located beneath the DVG.

 

Abstract We construct a 3D shear velocity model of the Salt Lake Valley using Rayleigh waves excited by the 31 March 2020 Mw 6.5 central Idaho earthquake recorded on a 168-station temporary nodal geophone network and the 49-station permanent regional network. The temporary array—deployed in response to the March 18 Mw 5.7 Magna earthquake—serendipitously recorded clear surface waves between 10 and 20 s period from the Idaho event at ∼500 km epicentral distance, from which we measure both Rayleigh wave phase velocity and ellipticity (H/V ratio). In addition, we employ multicomponent earthquake coda cross correlation to extend the measurements down to 5 s period. Because Rayleigh wave ellipticity features outstanding shallow sensitivity, we invert for a 3D upper crust VS model of the Salt Lake Valley. Our model shows basin structure in general agreement with and complements the current Community Velocity Model, which is mostly constrained by borehole and gravity measurements. Our model thus provides critical information for future earthquake hazard assessment studies, which require detailed shallow velocity structure. 

Abstract We discuss general structural features of the Banning and Mission Creek strands (BF and MCF) of the southern San Andreas fault (SSAF) in the Coachella Valley, based on ambient noise and earthquake wavefields recorded by a seismic array with >300 nodes. Earthquake P arrivals show rapid changes in waveform characteristics over 20–40 m zones that coincide with the surface BF and MCF. These variations indicate that the BF and MCF are high-impedance contrast interfaces—an observation supported by the presence of seismic reflections. Another prominent but more diffuse change in SSAF structure is found ∼1 km northeast of the BF. This feature has average-to-low arrival times (P and S) and ambient noise levels (at <30 Hz), and likely represents a relatively fast velocity block sandwiched between broader MCF and BF zones. The maximal arrival delays (P ∼0.1 s and S ∼0.25 s) and the highest ambient noise levels (>2 times median) are consistently observed southwest of the BF—a combined effect of Coachella Valley sediments and rock damage on that side. Immediately northeast of the MCF, large S minus P delays suggest a broad high VP/VS zone associated with asymmetric rock damage across the SSAF. This general overview shows the BF and MCF as mature but distinctly different fault zones. Future analyses will further clarify these and other SSAF features in greater detail. 

Abstract We image the shallow structure across the East Bench segment of the Wasatch fault system in Salt Lake City using ambient noise recorded by a month-long temporary linear seismic array of 32 stations. We first extract Rayleigh-wave signals between 0.4 and 1.1 s period using noise cross correlation. We then apply double beamforming to enhance coherent cross-correlation signals and at the same time measure frequency-dependent phase velocities across the array. For each location, based on available dispersion measurements, we perform an uncertainty-weighted least-squares inversion to obtain a 1D VS model from the surface to 400 m depth. We put all piece-wise continuous 1D models together to construct the final 2D VS model. The model reveals high velocities to the east of the Pleistocene Lake Bonneville shoreline reflecting thinner sediments and low velocities particularly in the top 200 m to the west corresponding to the Salt Lake basin sediments. In addition, there is an ∼400-m-wide low-velocity zone that narrows with depth adjacent to the surface trace of the East Bench fault, which we interpret as a fault-related damage zone. The damage zone is asymmetric, wider on the hanging wall (western) side and with greater velocity reduction. These results provide important constraints on normal-fault earthquake mechanics, Wasatch fault earthquake behavior, and urban seismic hazard in Salt Lake City. 

The Southern San Andreas Fault (SSAF) in California is one of the most thoroughly studied faults in the world, but its configuration at seismogenic depths remains enigmatic in the Coachella Valley. We use a combination of space geodetic and seismic observations to demonstrate that the relatively straight southernmost section of the SSAF, between Thousand Palms and Bombay Beach, is dipping to the northeast at 60–80° throughout the upper crust (<10 km), including the shallow aseismic layer. We constrain the fault attitude in the top 2–3 km using inversions of surface displacements associated with shallow creep, and seismic data from a dense nodal array crossing the fault trace near Thousand Palms. The data inversions show that the shallow dipping structure connects with clusters of seismicity at depth, indicating a continuous throughgoing fault surface. The dipping fault geometry has important implications for the long‐term fault slip rate, the intensity of ground shaking during future large earthquakes, and the effective strength of the southern SAF.

 

We analyze seismograms recorded by four arrays (B1–B4) with 100 m station spacing and apertures of 4–8 km that cross the surface rupture of the 2019 Mw 7.1 Ridgecrest earthquake. The arrays extend from B1 in the northwest to B4 in the southeast of the surface rupture. Delay times betweenPwave arrivals associated with ∼1,200 local earthquakes and four teleseismic events are used to estimate local velocity variations beneath the arrays. Both teleseismic and localPwaves travel faster on the northeast than the southwest side of the fault beneath arrays B1 and B4, but the velocity contrast is less reliably resolved at arrays B2 and B3. We identify several 1–2 km wide low‐velocity zones with much slower inner cores that amplifySwaveforms, inferred as damage zones, beneath each array. The damage zones at arrays B2 and B4 also generate fault‐zone head and trapped waves. An automated detector, based on peak ground velocities and durations of high‐amplitude waves, identifies candidate fault‐zone trapped waves (FZTWs) in a localized zone for ∼600 earthquakes at array B4. Synthetic waveform modeling of averaged FZTWs, generated by ∼30 events with high‐quality signals, indicates that the trapping structure at array B4 has a width of ∼300 m, depth of 3–5 km,Swave velocity reduction of ∼20% with respect to the surrounding rock,Q‐value of ∼30, andSwave velocity contrast of ∼4% across the fault (faster on the northeast side). The results show complex fault‐zone internal structures (velocity contrasts and low‐velocity zones) that vary along fault strike.

 

Near‐surface seismic velocity structure plays a critical role in ground motion amplification during large earthquakes. In particular, the local Vp/Vs ratio strongly influences the amplitude of Rayleigh waves. Previous studies have separately imaged 3D seismic velocity and Vp/Vs ratio at seismogenic depth, but lack regional coverage and/or fail to constrain the shallowest structure. Here, we combine three datasets with complementary sensitivity in a Bayesian joint inversion for shallow crustal shear velocity and near‐surface Vp/Vs ratio across Southern California. Receiver functions–including with an apparent delayed initial peak in sedimentary basins, and long considered a nuisance in receiver function imaging studies–highly correlate with short‐period Rayleigh wave ellipticity measurements and require the inclusion of a Vp/Vs parameter. The updated model includes near‐surface low shear velocity more in line with geotechnical layer estimates, and generally lower than expected Vp/Vs outside the basins suggesting widespread shallow fracturing and/or groundwater undersaturation.

 

We present observations and modeling of spatial eigen‐functions of resonating waves within fault zone waveguide, using data recorded on a dense seismic array across the San Jacinto Fault Zone (SJFZ) in southern California. The array consists of 5‐Hz geophones that cross the SJFZ with ~10–30 m spacing at the Blackburn Saddle near the Hemet Stepover. Wavefield snapshots after theSwave arrival are consistent for more than 50 near‐fault events, suggesting that this pattern is controlled by the fault zone structure rather than source properties. Data from example event with high signal to noise ratio show three main frequency peaks at ~1.3, ~2.0, and ~2.8 Hz in the amplitude spectra of resonance waves averaged over stations near the fault. The data are modeled with analytical expressions for eigen‐functions of resonance waves in a low‐velocity layer (fault zone) between two quarter‐spaces. Using a grid search‐based method, we investigate the possible width of the waveguide, location within the array, and shear wave velocities of the media that fit well the resonance signal at ~1.3 Hz. The results indicate a ~300 m wide damaged fault zone layer with ~65%Swave velocity reduction compared to the host rock. The SW edge of the low‐velocity zone is near the mapped fault surface trace, indicating that the damage zone is asymmetrically located at the regionally faster NE crustal block. The imaging resolution of the fault zone structure can be improved by modeling fault zone resonance modes and trapped waves together.