Imaging the Plate Interface in the Cascadia Seismogenic Zone: New Constraints from Offshore Receiver Functions

The Cascadia subduction zone, where the Juan de Fuca (JdF) plate subducts beneath North America, has paleoseismic evidence of Mw∼9.0 megathrust earthquakes (Nelson et al., 1995; Goldfinger et al., 2003). However, there are virtually no instrumentally recorded thrust‐zone earthquakes, hence the location and behavior of the seismogenic zone is known only indirectly. Temperature has been proposed to control seismogenesis with depth, assuming that the locked zone extends from the trench or the 150°C isotherm down‐dip to the 350°C isotherm, with a transition zone extending to 450°C (Hyndman and Wang, 1993; Oleskevich et al., 1999; Cozzens and Spinelli, 2012). These models generally place the down‐dip edge of the locked zone near the coastline. Inversions of onshore Global Positioning System data also can be used to determine the locking behavior and place the locked zone offshore (e.g., McCaffrey et al., 2013). Onshore receiver function (RF) studies have imaged an eastward‐dipping low‐velocity zone (LVZ) with high VP/VS between the coastline and depths of 45 km (Rondenay et al., 2001; Nicholson et al., 2005; Abers et al., 2009; Audet et al., 2009). This structure has been interpreted as overpressured pore fluids, metamorphosed sediments, or a combination thereof at or just above the top of the subducting oceanic crust (Abers et al., 2009; Hansen et al., 2012). Because of this uncertainty, it is unclear if an LVZ should continue up‐dip through the locked zone, since fluid pressure and metamorphism should vary differently with depth (e.g., Hacker et al., 2003; Liu and Rice, 2007; Saffer and Tobin, 2011). However, existing RF images only sample the plate boundary deeper than the locked zone because past broadband arrays are on land. Brillon et al. (2013) analyze RFs from two ocean‐bottom seismometers (OBSs) offshore of Vancouver Island, but poor data quality at these stations contributes to large uncertainties. Receiver funtions are difficult to calculate from OBS instruments, because water column multiples interfere with other arrivals and noise is high particularly on horizontal components (Leahy et al., 2010; Bostock and Trehu, 2012; Ball et al., 2014). The Cascadia Initiative (CI) is a prime opportunity to revisit this challenge (Toomey et al., 2014). In particular, the new trawl‐resistant‐mount (TRM) OBS design not only allows the instruments to be deployed in shallow water, but also greatly reduces horizontal‐component noise (Webb et al., 2013). In this article, we evaluate the ability of all sites of the CI array to calculate RFs, and we focus on results from the 19 OBSs deployed off the coast of Grays Harbor, Washington. These extend the onshore Cascadia Arrays for Earthscope (CAFE) broadband array (Abers et al., 2009) offshore, allowing direct comparisons

High seismic attenuation at a mid-ocean ridge reveals the distribution of deep melt

At most mid-ocean ridges, a wide region of decompression melting must be reconciled with a narrow neovolcanic zone and the establishment of full oceanic crustal thickness close to the rift axis. Two competing paradigms have been proposed to explain melt focusing: narrow mantle upwelling due to dynamic effects related to in situ melt or wide mantle upwelling with lateral melt transport in inclined channels. Measurements of seismic attenuation provide a tool for identifying and characterizing the presence of melt and thermal heterogeneity in the upper mantle. We use a unique data set of teleseismic body waves recorded on the Cascadia Initiative’s Amphibious Array to simultaneously measure seismic attenuation and velocity across an entire oceanic microplate. We observe maximal differential attenuation and the largest delays (Embedded Image s and δTS ~ 2 s) in a narrow zone <50 km from the Juan de Fuca and Gorda ridge axes, with values that are not consistent with laboratory estimates of temperature or water effects. The implied seismic quality factor (Qs ≤ 25) is among the lowest observed worldwide. Models harnessing experimentally derived anelastic scaling relationships require a 150-km-deep subridge region containing up to 2% in situ melt. The low viscosity and low density associated with this deep, narrow melt column provide the conditions for dynamic mantle upwelling, explaining a suite of geophysical observations at ridges, including electrical conductivity and shear velocity anomalies

Crustal structure along the Aleutian island arc: New insights from receiver functions constrained by active-source data

Moho depth and Vp/Vs estimates from stacking phases of receiver functions along the Aleutian island arc give new constraints on its composition and structure. They expand on the current understanding of island arcs and their relationship to continental crust production. We also present an approach for including constraints from active-source data in receiver function analysis in a region with sparse data coverage to complement this analysis. Moho depth averages 37.5 km with an average uncertainty of 2.5 km along the entire arc. Excluding the westernmost island of Attu yields an average crustal thickness of 38.5 ± 2.9 km. The Vp/Vs ratio decreases moving eastward along the arc with an average value of 1.80 in the western and central portion of the arc built on oceanic crust, but 1.63 in the eastern section built on continental crust. This may reflect tectonic and compositional changes along the arc. However, overall the arc appears more mafic than continental crust. Near-constant crustal thickness, despite significant compositional changes, may indicate that nonmagmatic processes such as erosion and isostasy act to regulate arc thickness. Additionally, strong conversions from an upper crustal magma chamber are observed beneath Akutan Island, confirming and clarifying the geometry of the magma body inferred from other techniques. They indicate a volcanic body much larger than the eruptive edifice, a feature that must persist between eruptive cycles

Elastic anisotropy of lizardite at subduction zone conditions

Subduction zones transport water into Earth's deep interior through slab subduction. Serpentine minerals, the primary hydration product of ultramafic peridotite, are abundant in most subduction zones. Characterization of their high-temperature elasticity, particularly their anisotropy, will help us better estimate the extent of mantle serpentinization and the Earth's deep water cycle. Lizardite, the low-temperature polymorph of serpentine, is stable under the P-T conditions of cold subduction slabs (< 260{\deg}C at 2 GPa), and its high-temperature elasticity remains unknown. Here we report ab initio elasticity and acoustic wave velocities of lizardite at P-T conditions of subduction zones. Our static results agree with previous studies. Its high-temperature velocities are much higher than previous experimental-based lizardite estimates with chrysotile but closer to antigorite velocities. The elastic anisotropy of lizardite is much larger than that of antigorite and could better account for the observed large shear-wave splitting in some cold slabs such as Tonga

Reconciling mantle attenuation-temperature relationships from seismology, petrology, and laboratory measurements

Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt-free rocks, but require ∼10²−10³ extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation (Q_S⁻¹), affording an opportunity for calibration. We reanalyze seismic data sets that sample arc and back-arc mantle in Central America, the Marianas, and the Lau Basin, confirming very high attenuation (Q_S ∼ 25–80) at 1 Hz and depths of 50–100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300–1450°C. The Q_S measurements correlate inversely with the petrologically inferred temperatures, as expected. However, dry attenuation models predict Q_S too high by a factor of 1.5–5. Modifying models to include effects of H₂O and rheology-dependent grain size shows that the effects of water-enhanced dissipation and water-enhanced grain growth nearly cancel, so H₂O effects are modest. Therefore, high H₂O in the arc source region cannot explain the low Q_S, nor in the back arc where lavas show modest water content. Most likely, the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body wave Q_S can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs

AACSE earthquake catalog: May-December, 2018

The Alaska Amphibious Community Seismic Experiment (AACSE) comprised 75 ocean bottom seismometers and 30 land stations and covered about 650 km along the segment of the subduction zone that includes Kodiak Island, the Alaska Peninsula and the Shumagin Islands between May, 2018 and September, 2019 (Barcheck et al., 2020). This unprecedented dataset has the potential to support a greatly enhanced earthquake catalog by both increasing the number of detected earthquakes and improving the accuracy of their source parameters. We use all available regional and AACSE campaign seismic data to compile an enhanced earthquake catalog for the region between Kodiak and Shumagin Islands including Alaska Peninsula (51-59N, 148-163W). We apply the same processing and reporting standards to additional picks and seismic events as the Alaska Earthquake Center currently use for compilation of the authoritative regional earthquake catalog. This release includes earthquake catalogs for the time period between May 12 and December 31, 2018 (3829 events total 1132 of which are newly detected). We include monthly CSS database tables and quakeml files. This material is based upon work supported by the U.S. Geological Survey under Grant No. G20AP00026. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the opinions or policies of the U.S. Geological Survey. Mention of trade names or commercial products does not constitute their endorsement by the U.S. Geological Survey.This material is based upon work supported by the U.S. Geological Survey under Grant No. G20AP00026. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the opinions or policies of the U.S. Geological Survey. Mention of trade names or commercial products does not constitute their endorsement by the U.S. Geological Survey