ALBERT

Description: Sedimentary underplating at the Cascadia mantle-wedge corner revealed by seismic imaging Nature Geoscience 4, 545 (2011). doi:10.1038/ngeo1195 Authors: Andrew J. Calvert, Leiph A. Preston & Amir M. Farahbod Earth’s largest earthquakes occur in subduction zones, along the boundary between the subducting and overriding plates. Non-volcanic tremor generated by slow slip between the plates is thought to originate on, or near, this boundary. Earthquakes also occur in the down-going plate as fluids are released, and zones of anomalously low seismic velocities observed beneath several subduction zones are interpreted to be the subducting oceanic crust. Yet, the exact location of the plate boundary remains uncertain. Here we interpret a three-dimensional seismic tomography model from the northern Cascadia subduction zone in the northwest USA. We find that the low-velocity zone varies considerably along the Cascadia margin. In places, we observe the low-velocity zone to crop out at the surface and separate from the descending plate at depths of 35–40 km. We argue that the low-velocity zone here cannot represent oceanic crust as previously suggested, and instead the zone mostly represents sediments that have been subducted and underplated beneath the North American continent. We also find that tremor signals correlate with the position of the low-velocity zone, implying that slow slip and tremor may be facilitated by trapped fluids and high pore fluid pressures in subducted sedimentary rocks at, or close to the plate boundary. Our results also imply that the plate boundary beneath Cascadia is much deeper than previously thought.

Description: The complex postdetonation geologic structures that form after an underground nuclear explosion are hard to constrain because increased heterogeneity around the damage zone affects seismic waves that propagate through the explosion site. Generally, a vertical rubble-filled structure known as a chimney is formed after an underground nuclear explosion that is composed of debris that falls into the subsurface cavity generated by the explosion. Compared with chimneys that collapse fully, leaving a surface crater, partially collapsed chimneys can have remnant subsurface cavities left in place above collapsed rubble. The 1964 nuclear test HADDOCK, conducted at the Nevada test site (now the Nevada National Security Site), formed a partially collapsed chimney with no surface crater. Understanding the subsurface structure of these features has significant national security applications, such as aiding the study of suspected underground nuclear explosions under a treaty verification. In this study, we investigated the subsurface architecture of the HADDOCK legacy nuclear test using hybrid 2D–3D active source seismic reflection and refraction data. The seismic data were acquired using 275 survey shots from the Seismic Hammer (a 13,000 kg weight drop) and 65 survey shots from a smaller accelerated weight drop, both recorded by ∼1000 three-component 5 Hz geophones. First-arrival, P-wave tomographic modeling shows a low-velocity anomaly at ∼200  m depth, likely an air-filled cavity caused by partial collapse of the rock column into the temporary postdetonation cavity. A high-velocity anomaly between 20 and 60 m depth represents spall-related compaction of the shallow alluvium. Hints of low velocities are also present near the burial depth (∼364  m). The reflection seismic data show a prominent subhorizontal reflector at ∼300  m depth, a short-curved reflector at ∼200  m, and a high-amplitude reflector at ∼50  m depth. Comparisons of the reflection sections to synthetic data and borehole stratigraphy suggest that these features correspond to the alluvium–tuff contact, the partial collapse cavity, and the spalled layer, respectively.

Description: Earth's largest earthquakes occur in subduction zones, along the boundary between the subducting and overriding plates1. Non-volcanic tremor generated by slow slip between the plates is thought to originate on, or near, this boundary2,3. Earthquakes also occur in the down-going plate as fluids are released4, and zones of anomalously low seismic velocities observed beneath several subduction zones are interpreted to be the subducting oceanic crust5-10. Yet, the exact location of the plate boundary remains uncertain5. Here we interpret a three-dimensional seismic tomography model from the northern Cascadia subduction zone in the northwest USA. We find that the low-velocity zone varies considerably along the Cascadia margin. In places, we observe the low-velocity zone to crop out at the surface and separate from the descending plate at depths of 35-40 km. We argue that the low-velocity zone here cannot represent oceanic crust as previously suggested, and instead the zone mostly represents sediments that have been subducted and underplated beneath the North American continent. We also find that tremor signals correlate with the position of the low-velocity zone, implying that slow slip and tremor may be facilitated by trapped fluids and high pore fluid pressures in subducted sedimentary rocks at, or close to the plate boundary. Our results also imply that the plate boundary beneath Cascadia is much deeper than previously thought. © 2011 Macmillan Publishers Limited. All rights reserved.

Description: In preparation for the next phase of the Source Physics Experiments, we acquired an active‐source seismic dataset along two transects totaling more than 30 km in length at Yucca Flat, Nevada, on the Nevada National Security Site. Yucca Flat is a sedimentary basin which has hosted more than 650 underground nuclear tests (UGTs). The survey source was a novel 13,000 kg modified industrial pile driver. This weight drop source proved to be broadband and repeatable, richer in low frequencies (1–3 Hz) than traditional vibrator sources and capable of producing peak particle velocities similar to those produced by a 50 kg explosive charge. In this study, we performed a joint inversion of P‐wave refraction travel times and Rayleigh‐wave phase‐velocity dispersion curves for the P‐ and S‐wave velocity structure of Yucca Flat. Phase‐velocity surface‐wave dispersion measurements were obtained via the refraction microtremor method on 1 km arrays, with 80% overlap. Our P‐wave velocity models verify and expand the current understanding of Yucca Flat’s subsurface geometry and bulk properties such as depth to Paleozoic basement and shallow alluvium velocity. Areas of disagreement between this study and the current geologic model of Yucca Flat (derived from borehole studies) generally correlate with areas of widely spaced borehole control points. This provides an opportunity to update the existing model, which is used for modeling groundwater flow and radionuclide transport. Scattering caused by UGT‐related high‐contrast velocity anomalies substantially reduced the number and frequency bandwidth of usable dispersion picks. The S‐wave velocity models presented in this study agree with existing basin‐wide studies of Yucca Flat, but are compromised by diminished surface‐wave coherence as a product of this scattering. As nuclear nonproliferation monitoring moves from teleseismic to regional or even local distances, such high‐frequency (〉5  Hz) scattering could prove challenging when attempting to discriminate events in areas of previous testing.