Search for: All records

This study addresses a significant gap in understanding the features of the south‐central Cascadia subduction zone, a region characterized by complex geologic, tectonic, and seismic transitions both offshore and onshore. Unlike other segments along this margin, this area lacks a 3‐D velocity model to delineate its structural and geological features on a fine scale. To address this void, we developed a high‐resolution 3‐D P‐wave velocity model using active source seismic data from ship‐borne seismic shots recorded on temporary and permanent onshore seismic stations and ocean‐bottom seismometers. Our model shows velocity variations across the region with distinct velocity‐depth profiles for the Siletz, Franciscan, and Klamath terranes in the overlying plate. We identified seaward dipping high‐velocity static backstops associated with the Siletz and Klamath terranes, situated near the shoreline and further inland, respectively. Regions of reduced crustal velocity are associated with crustal faults. Moreover, there is significant along‐strike depth variation in the subducting slab, which is about 4 km deeper near the thick, dense Siletz terrane and becomes shallower near the predominantly less‐dense Franciscan terrane. This highlights a sudden tectonic and geologic transition at the southern boundary of the Siletz terrane. Our velocity model also indicates slightly increased hydration, though still minimal, in both the oceanic crust and the upper mantle of the subducting plate compared to other parts of the margin. 

Abstract Sediment thermal history controls the progress of diagenetic reactions that can alter the mechanical behavior of material entering a subduction zone that then: accretes to the margin, hosts the plate boundary interface, or is carried deeper within the Earth. On the Cascadia margin offshore Oregon (USA), hydrothermal circulation in the oceanic crust affects thermally controlled processes, enhancing sediment alteration above the MARGIN seamount, which is buried by the Astoria Fan. Hydrothermal circulation increases temperatures at the summit of the seamount and in the overlying sediment by up to ∼100°C. We use sediment thermal history constrained by heat flux observations to model the expected progress of the smectite‐to‐illite reaction around the MARGIN seamount. Above the seamount, the smectite‐to‐illite reaction is expected to progress to completion ∼250 m below the seafloor; away from the seamount, smectite is likely unaltered to a burial depth of ∼800 m. The altered sediment above the seamount has higher rigidity and p‐wave velocity than the surrounding sediment. Spatial variability in sediment alteration may be present around other buried seamounts. We use vertical gravity gradient anomalies to estimate the locations and heights of additional seamounts. Each of these seamounts may have altered sediment around it, which could affect deformation and seismicity in the margin wedge. Because cemented sediment with greater elastic strength is better able to store elastic strain energy, enhanced sediment alteration and cementation above seamounts entering the subduction zone could facilitate earthquake nucleation for material in the margin wedge that was above a seamount prior to subduction. 

A 3D crustal model for the central Cascadia continental shelf and Coast Range between 44°N and 45°N shows that the crystalline crust of the forearc wedge beneath the coastline is characterized by a NW-trending, vertical slab of high-velocity rock interpreted to represent the dike complex that fed the Yachats Basalt, which was intruded into the forearc approximately 37 million years ago. A spatial correlation is observed between downward deflection of the crust of the subducting Juan de Fuca plate, inferred from inversion of PmP arrivals to image the Moho surface, and the high velocity (and consequently high density) anomaly underlying the Yachats Basalt. Apparent subsequent rebound of the subducting plate at greater depth suggests a primarily elastic response of the subducting plate to this load. Calculations for a range of plausible values for the magnitude of the load and the width and depth of the depression indicate that the effective elastic thickness of the subducted Juan de Fuca plate is < 6 km. Although our simple analytical models do not include partial support of the load of the slab by the adjacent upper plate crust or time dependence to account for the motion of the slab beneath the load, incorporation of those effects should decrease rather than increase the apparent strength of the subducted plate. We conclude that the subducted Juan de Fuca plate beneath the central Oregon margin is elastically thin and has the potential to store elastic strain energy before rupturing. Our model of a well-defined, focused and static upper plate load that locally deforms the subducted plate within the nominally seismogenic or transitional part of the Cascadia plate boundary may be unique in providing a relatively straightforward scenario for estimating the mechanical properties of the subducted Juan de Fuca plate. We extrapolate from these results to speculate that elastic deformation of the subducting plate may contribute to the low level of seismicity throughout much of the Cascadia forearc in the inter-seismic period between great earthquakes but note that our local results do not preclude faulting or elasto-plastic deformation of a thin and weak plate as it subducts. These results also suggest that the subducting plate should deform in response to larger scale variations in upper plate thickness and density. 

Vigorous seepage offshore Oregon provides insight into the relationship between margin permeability and megathrust slip behavior. 

Abstract Establishing an extensive and highly durable, long-term, seafloor network of autonomous broadband seismic stations to complement the land-based Global Seismographic Network has been a goal of seismologists for decades. Seismic signals, chiefly the vibrations from earthquakes but also signals generated by storms and other environmental processes, have been processed from land-based seismic stations to build intriguing but incomplete images of the Earth’s interior. Seismologists have mapped structures such as tectonic plates and other crustal remnants sinking deep into the mantle to obtain information on their chemical composition and physical state; but resolution of these structures from land stations is not globally uniform. Because the global surface is two-thirds ocean, increasing the number of seismic stations located in the oceans is critical for better resolution of the Earth’s interior and tectonic structures. A recommendation for a long-term seafloor seismic station pilot experiment is presented here. The overarching instrumentation goal of a pilot experiment is performance that will lead to the installation of a large number of long-term autonomous ocean-bottom seismic stations. The payoff of a network of stations separated from one another by a few hundred kilometers under the global oceans would be greatly refined resolution of the Earth’s interior at all depths. A second prime result would be enriched understanding of large-earthquake rupture processes in both oceanic and continental plates. The experiment would take advantage of newly available technologies such as robotic wave gliders that put an affordable autonomous prototype within reach. These technologies would allow data to be relayed to satellites from seismometers that are deployed on the seafloor with long-lasting, rechargeable batteries. Two regions are presented as promising arenas for such a prototype seafloor seismic station. One site is the central North Atlantic Ocean, and the other high-interest locale is the central South Pacific Ocean.