skip to main content

Title: Teleseismic Attenuation, Temperature, and Melt of the Upper Mantle in the Alaska Subduction Zone

Seismic deployments in the Alaska subduction zone provide dense sampling of the seismic wavefield that constrains thermal structure and subduction geometry. We measurePandSattenuation from pairwise amplitude and phase spectral ratios for teleseismic body waves at 206 stations from regional and short‐term arrays. Parallel teleseismic travel‐time measurements provide information on seismic velocities at the same scale. These data show consistently low attenuation over the forearc of subduction systems and high attenuation over the arc and backarc, similar to local‐earthquake attenuation studies but at 10× lower frequencies. The pattern is seen both across the area of normal Pacific subduction in Cook Inlet, and across the Wrangell Volcanic Field where subduction has been debated. These observations confirm subduction‐dominated thermal regime beneath the latter. Travel times show evidence for subducting lithosphere much deeper than seismicity, while attenuation measurements appear mostly reflective of mantle temperature less than 150 km deep, depths where the mantle is closest to its solidus and where subduction‐related melting may take place. Travel times show strong delays over thick sedimentary basins. Attenuation signals show no evidence of absorption by basins, although some basins show signals anomalously rich in high‐frequency energy, with consequent negative apparent attenuation. Outside of basins, these data are consistent with mantle attenuation in the upper 220 km that is quantitatively similar to observations from surface waves and local‐earthquake body waves. Differences betweenPandSattenuation suggest primarily shear‐modulus relaxation. Overall the attenuation measurements show consistent, coherent subduction‐related structure, complementary to travel times.

more » « less
Award ID(s):
1829440 1460291
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Solid Earth
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    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.

    more » « less
  2. Abstract

    The Central Appalachian Anomaly (CAA) is a region of the upper mantle beneath eastern North America that exhibits pronounced anomalies in its seismic velocity, seismic attenuation, and electrical conductivity structure. The CAA clearly expresses itself in low velocity, high attenuation, and high conductivity values; however, the present‐day composition and state of the asthenospheric upper mantle in the anomalous region remains imperfectly known. The collection of data from densely spaced, co‐located seismic and magnetotelluric arrays during the Mid‐Atlantic Geophysical Integrative Collaboration (MAGIC) experiment affords the opportunity to probe the structure and properties of the upper mantle in the CAA region in detail using multiple types of geophysical observations. Here, we present new observations ofPandSwave travel times from teleseismic earthquakes measured at MAGIC stations, including a determination of how travel times deviate from the predictions of a standard 1‐D reference model. These observations constrain the ratio of thePtoSwave travel time perturbations associated with the CAA, which in turn allows us to estimate the ratio ofPandSwave velocity anomalies. We combine these observations with previously published estimates of seismic attenuation and electrical conductivity in the upper mantle beneath the MAGIC array, and carry out forward modeling to determine reasonable ranges of temperature, partial melt fraction, water content, and composition for the CAA. Our results suggest that 1%–2% partial melt is required to simultaneously explain the velocity, attenuation, and electrical conductivity observations beneath the MAGIC array.

    more » « less
  3. Abstract

    To systematically investigate seismic azimuthal anisotropy in the Sumatra subduction zone and probe mantle dynamics associated with the subduction of the Australian Plate beneath the Sunda Plate, a total of 169 pairs of teleseismic XKS (including PKS, SKKS, SKS) and 115 pairs of localSsplitting parameters are obtained using broadband seismic data recorded at ~70 stations. Additionally, crustal anisotropy in the overriding Sunda Plate is measured by analyzing the moveout ofP‐to‐Sconversions from the Moho using a sinusoidal function. Comparison between the three sets of anisotropy measurements obtained using shear waves with different depths of origin suggests that (1) the crust of the Sunda Plate is anisotropic with mostly trench‐parallel fast orientations and a mean splitting time of 0.28 ± 0.05 s; (2) the mantle wedge is azimuthally anisotropic with dominantly trench‐parallel fast orientations and splitting times ranging from 0.22 to 0.81 s, which generally increase with the focal depth; and (3) subslab anisotropy is mostly trench‐normal beneath the fore‐arc region with an averaged splitting time of 1.48 ± 0.06 s, and becomes trench‐parallel beneath the arc and back‐arc areas with a mean splitting time of 0.33 ± 0.04 s. The resulting lateral and vertical distributions of anisotropy obtained using splitting of three types of shear waves advocate the presence of an entrained subslab flow that is deflected by the mantle transition zone. The flow enters the mantle wedge through a slab window and flows horizontally parallel to the trench.

    more » « less

    Global variations in the propagation of fundamental-mode and overtone surface waves provide unique constraints on the low-frequency source properties and structure of the Earth’s upper mantle, transition zone and mid mantle. We construct a reference data set of multimode dispersion measurements by reconciling large and diverse catalogues of Love-wave (49.65 million) and Rayleigh-wave dispersion (177.66 million) from eight groups worldwide. The reference data set summarizes measurements of dispersion of fundamental-mode surface waves and up to six overtone branches from 44 871 earthquakes recorded on 12 222 globally distributed seismographic stations. Dispersion curves are specified at a set of reference periods between 25 and 250 s to determine propagation-phase anomalies with respect to a reference Earth model. Our procedures for reconciling data sets include: (1) controlling quality and salvaging missing metadata; (2) identifying discrepant measurements and reasons for discrepancies; (3) equalizing geographic coverage by constructing summary rays for travel-time observations and (4) constructing phase velocity maps at various wavelengths with combination of data types to evaluate inter-dataset consistency. We retrieved missing station and earthquake metadata in several legacy compilations and codified scalable formats to facilitate reproducibility, easy storage and fast input/output on high-performance-computing systems. Outliers can be attributed to cycle skipping, station polarity issues or overtone interference at specific epicentral distances. By assessing inter-dataset consistency across similar paths, we empirically quantified uncertainties in traveltime measurements. More than 95 per cent measurements of fundamental-mode dispersion are internally consistent, but agreement deteriorates for overtones especially branches 5 and 6. Systematic discrepancies between raw phase anomalies from various techniques can be attributed to discrepant theoretical approximations, reference Earth models and processing schemes. Phase-velocity variations yielded by the inversion of the summary data set are highly correlated (R ≥ 0.8) with those from the quality-controlled contributing data sets. Long-wavelength variations in fundamental-mode dispersion (50–100 s) are largely independent of the measurement technique with high correlations extending up to degree ∼25. Agreement degrades with increasing branch number and period; highly correlated structure is found only up to degree ∼10 at longer periods (T > 150 s) and up to degree ∼8 for overtones. Only 2ζ azimuthal variations in phase velocity of fundamental-mode Rayleigh waves were required by the reference data set; maps of 2ζ azimuthal variations are highly consistent between catalogues ( R = 0.6–0.8). Reference data with uncertainties are useful for improving existing measurement techniques, validating models of interior structure, calculating teleseismic data corrections in local or multiscale investigations and developing a 3-D reference Earth model.

    more » « less
  5. Abstract

    We image seismic attenuation near the Hikurangi trench offshore New Zealand, using ocean bottom and land‐based seismometers, revealing high attenuation above a recurring shallow slow‐slip event and within the subducting Hikurangi Plateau. The Hikurangi subduction margin east of the North Island, New Zealand is the site of frequent shallow slow slip events. Overpressured fluids are hypothesized to lead to slow slip at shallow depths close to the oceanic trench. Seismic attenuation, energy loss of seismic waves, can be used to detect high temperatures, melt, the presence of fluids, and fractures. We use local earthquake P‐ and S‐waves from 180 earthquakes to invert fort*, and subsequently invert for Qp and Qs, offshore the North Island directly above the area of slow slip. We image Qp and Qs to ∼25 km depth, increasing resolution of previously identified coastal lowQ(100–300), and finding a new region of even higher attenuation (Qp and Qs < 50–100) directly above the shallow slow slip event of 2014–2015, beneath the offshore seismic array. This highest attenuation is downdip of a subducting seamount, and is spatially correlated with a high seismic reflectivity zone and Vp/Vs > 1.85, all of which provide evidence for the presence of fluids. The Qp and Qs is low at the trench (<50–100) and in the subducting plate (100–200), suggesting that seismic wave scattering due to faults, fractures, and the inherent heterogeneous composition of the Hikurangi Plateau, a large igneous province, plays a role in seismic attenuation.

    more » « less