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Lindner, F, Laske G, Walter F, Doran AK.  2019.  Crevasse-induced Rayleigh-wave azimuthal anisotropy on Glacier de la Plaine Morte, Switzerland. Annals of Glaciology. 60:96-111.   10.1017/aog.2018.25   AbstractWebsite

Crevasses and englacial fracture networks route meltwater from a glacier's surface to the subglacial drainage system and thus influence glacial hydraulics. However, rapid fracture growth may also lead to sudden and potentially hazardous structural failure of unstable glaciers and ice dams, rifting of ice shelves, or iceberg calving. Here, we use passive seismic recordings to investigate the englacial fracture network on Glacier de la Plaine Morte, Switzerland. Glacier dynamics and the drainage of an ice-marginal lake give rise to numerous icequakes, the majority of which generate dispersed, high-frequency Rayleigh waves. A wide distribution of events allows us to study azimuthal anisotropy between 10 and 30 Hz in order to extract englacial seismic velocities in regions of preferentially oriented crevasses. Beamforming applied to a 100-m-aperture array reveals azimuthal anisotropy of Rayleigh-wave phase velocities reaching a strength of 8% at high frequencies. In addition, we find that the fast direction of wave propagation coincides with the observed surface strike of the narrow crevasses. Forward modeling and inversion of dispersion curves suggest that the azimuthal anisotropy is induced by a 40-m-thick crevassed layer at the surface of the glacier with 8% anisotropy in shear-wave velocity.

Doran, AK, Laske G.  2019.  Seismic structure of marine sediments and upper oceanic crust surrounding Hawaii. Journal of Geophysical Research-Solid Earth. 124:2038-2056.   10.1029/2018jb016548   AbstractWebsite

We present models of compressional and shear velocity structure of the oceanic sediments and upper crust surrounding the Hawaiian islands. The models were derived from analysis of seafloor compliance data and measurements of Ps converted phases originating at the sediment-bedrock interface. These data were estimated from continuous broadband ocean bottom seismometer acceleration and pressure records collected during the Plume-Lithosphere Undersea Mantle Experiment, an amphibious array of wideband and broadband instruments with an aperture of over 1,000km. Our images result from a joint inversion of compliance and Ps delay data using a nonlinear inversion scheme whereby deviation from a priori constraints is minimized. In our final model, sediment thickness increases from 50m at distal sites to over 1.5km immediately adjacent to the islands. The sedimentary shear velocity profiles exhibit large regional variations. While sedimentary structure accounts for the majority of the compliance signal, we infer variations in shear velocity in the uppermost bedrock on the order of 5%. We also require relatively high values of Poisson's ratio in the uppermost crust. Lower crustal velocities are generally seen to the north and west of the islands but do not appear well correlated with the Hawaiian Swell bathymetry. A region of strong low velocity anomalies to the northeast of Hawaii may be associated with the Molokai fracture zone.

Blackman, DK, Boyce DE, Castelnau O, Dawson PR, Laske G.  2017.  Effects of crystal preferred orientation on upper-mantle flow near plate boundaries: rheologic feedbacks and seismic anisotropy. Geophysical Journal International. 210:1481-1493.   10.1093/gji/ggx251   AbstractWebsite

Insight into upper-mantle processes can be gained by linking flow-induced mineral alignment to regional deformation and seismic anisotropy patterns. Through a series of linked micro-macro scale numerical experiments, we explore the rheologic effects of crystal preferred orientation (CPO) and evaluate the magnitude of possible impacts on the pattern of flow and associated seismic signals for mantle that includes a cooling, thickening young oceanic lithosphere. The CPO and associated anisotropic rheology, computed by a micromechanical polycrystal model, are coupled with a large scale flowmodel (Eulerian Finite Element method) via a local viscosity tensor field, which quantifies the stress: strain rate response of a textured polycrystal. CPO is computed along streamlines throughout the model space and the corresponding viscosity tensor field at each element defines the local properties for the next iteration of the flow field. Stable flow and CPO distributions were obtained after several iterations for the two dislocation glide cases tested: linear and nonlinear stress: strain rate polycrystal behaviour. The textured olivine polycrystals are found to have anisotropic viscosity tensors in a significant portion of the model space. This directional dependence in strength impacts the pattern of upper-mantle flow. For background asthenosphere viscosity of similar to 10(20) Pa s and a rigid lithosphere, the modification of the corner flow pattern is not drastic but the change could have geologic implications. Feedback in the development of CPO occurs, particularly in the region immediately below the base of the lithosphere. Stronger fabric is predicted below the flanks of a spreading centre for fully coupled, power-law polycrystals than was determined using prior linear, intermediate coupling polycrystal models. The predicted SKS splitting is modestly different (similar to 0.5 s) between the intermediate and fully coupled cases for oceanic plates less than 20 Myr old. The magnitude of azimuthal anisotropy for surface waves, on the other hand, is predicted to be twice as large for fully coupled power-law flow/polycrystals than for linear, intermediate coupled flow/polycrystal models.

Agius, MR, Rychert CA, Harmon N, Laske G.  2017.  Mapping the mantle transition zone beneath Hawaii from Ps receiver functions: Evidence for a hot plume and cold mantle downwellings. Earth and Planetary Science Letters. 474:226-236.   10.1016/j.epsl.2017.06.033   AbstractWebsite

Hawaii is the archetypal example of hotspot volcanism. Classic plume theory suggests a vertical plume ascent from the core-mantle boundary to the surface. However, recently it has been suggested that the plume path may be more complex. Determining the exact trajectory of the Hawaiian plume seismic anomaly in the mantle has proven challenging. We determine P-to-S (Ps) receiver functions to illuminate the 410- and 660-km depth mantle discontinuities beneath the Hawaiian Islands using waveforms recorded on land and ocean-bottom seismometers, applying new corrections for tilt and coherence to the ocean bottom data. Our 3-D depth-migrated maps provide enhanced lateral resolution of the mantle transition zone discontinuities. The 410 discontinuity is characterised by a deepened area beneath central Hawaii, surrounded by an elevated shoulder. At the 660 discontinuity, shallow topography is located to the north and far south of the islands, and a deep topographic anomaly is located far west and east. The transition zone thickness varies laterally by 13 km depth: thin beneath north-central Hawaii and thick farther away in a horseshoe-like feature. We infer that at 660-km depth a broad or possibly a double region of upwelling converges into a single plume beneath central Hawaii at 410-km depth. As the plume rises farther, uppermost mantle melting and flow results in the downwelling of cold material, down to at least 410 km surrounding the plume stem. This result in the context of others supports complex plume dynamics including a possible non-vertical plume path and adjacent mantle downwellings. (C) 2017 The Author(s). Published by Elsevier B.V.

Doran, AK, Laske G.  2017.  Ocean-bottom seismometer instrument orientations via automated Rayleigh-wave arrival-angle measurements. Bulletin of the Seismological Society of America. 107:691-708.   10.1785/0120160165   AbstractWebsite

After more than 10 years of the U.S. ocean-bottom seismometer (OBS) Instrument Pool operations, there is still need for a consistent and accurate procedure to determine the orientation of the horizontal seismometer components of passive-source free-fall broadband OBSs with respect to geographic north. We present a new Python-based, automated, and high-accuracy algorithm to obtain this information during postprocessing of the data. As with some previous methods, our new method Doran-Laske-Orientation-Python (DLOPy) is based on measuring intermediate-period surface-wave arrival angles from teleseismic earthquakes. A crucial new aspect of DLOPy is the consultation of modern global dispersion maps when setting up the analysis window. We repeat measurements at several frequencies to lower biases from wave propagation in laterally heterogeneous structure. We include measurements from the first minor and major great-circle arcs to further lower biases caused by uneven geographical data coverage. We demonstrate the high accuracy of our technique through benchmark tests against a well-established "hands-on" but slow technique using data from instruments of the Global Seismographic Network for which orientations are well documented. We present results for all Cascadia Initiative deployments, along with a number of other OBS experiments. Compared to other widely used automated codes, DLOPy requires fewer events to achieve the same or better accuracy. This advantage may be greatly beneficial for OBS deployments that last as short as a few months. Our computer code is available for download. It requiresminimal user input and is optimized to work with data disseminated through the Incorporated Research Institutions for Seismology Data Management Center.

Berger, J, Laske G, Babcock J, Orcutt J.  2016.  An ocean bottom seismic observatory with near real-time telemetry. Earth and Space Science.   10.1002/2015EA000137   Abstract

We describe a new technology that can provide near real-time telemetry of sensor data from the ocean bottom without a moored buoy or a cable to shore. The breakthrough technology that makes this system possible is an autonomous surface vehicle called a Wave Glider developed by Liquid Robotics, Inc. of Sunnyvale, CA., which harvests wave and solar energy for motive and electrical power. We present results from several deployments of a prototype system that demonstrate the feasibility of this concept. We also demonstrated that a wave glider could tow a suitably designed ocean bottom package with acceptable loss of speed. With further development such a system could be deployed autonomously and provide real-time telemetry of data from seafloor sensors. This article is protected by copyright. All rights reserved.

Doran, AK, Laske G.  2016.  Infragravity waves and horizontal seafloor compliance. Journal of Geophysical Research: Solid Earth. 121:260-278.   doi:10.1002/2015JB012511  
Thomas, C, Laske G.  2015.  D'' observations in the Pacific from PLUME ocean bottom seismometer recordings. Geophysical Journal International. 200:849-860.   10.1093/gji/ggu441   AbstractWebsite

The seismic investigation of the lowermost mantle is in many places hampered by the lack of suitable source-receiver combinations that sample the D '' region and have to meet the requirements of a suitable epicentral distance range. The low velocity regions beneath the central Pacific and Atlantic Oceans in particular have been sampled in fewer places than circum Pacific regions. In this study, we use data from two recent ocean bottom seismometer (OBS) deployments for the Plume-Lithosphere Undersea Mantle Experiment (PLUME) around Hawaii to increase the coverage of the lower mantle with reflected P waves. Through stacking of the data we achieve significant reduction in noise levels. The most favourable epicentral distances to detect D '' reflections are around 70-79 degrees. Most of our source-receiver combinations have distances less than that, thereby limiting the number of candidate observed reflections. Nevertheless, using array methods, we are able to test approximately 70 events for arrivals with slowness values and arrival times that would be consistent with a top-side reflection off a hypothetical D '' structure (PdP wave). Modelling these data with a 1-D reflectivity method, we identify a few places of detectable PdP waves, for which the velocity contrast in P- and S-wave velocity across the D '' reflector have to be relatively large (around 3-5 per cent increase and decrease, respectively) compared to other regions (e.g. beneath the Caribbean or Eurasia where the contrast is closer to 1-2 per cent). For larger distance ranges, smaller velocity contrasts are sufficient to cause observable reflections. This study shows that, despite the possible dominance of microseisms on OBS records, it is possible to use relatively short-period waves, with dominant periods as short as 3-7 s. Our findings suggest that, with future such deployments, OBS deployments will help to extend D '' studies to previously unmapped regions.

Ma, ZT, Masters G, Laske G, Pasyanos M.  2014.  A comprehensive dispersion model of surface wave phase and group velocity for the globe. Geophysical Journal International. 199:113-135.   10.1093/gji/ggu246   AbstractWebsite

A new method is developed to measure Rayleigh- and Love-wave phase velocities globally using a cluster analysis technique. This method clusters similar waveforms recorded at different stations from a single event and allows users to make measurements on hundreds of waveforms, which are filtered at a series of frequency ranges, at the same time. It also requires minimal amount of user interaction and allows easy assessment of the data quality. This method produces a large amount of phase delay measurements in a manageable time frame. Because there is a strong trade-off between the isotropic part of the Rayleigh-wave phase velocity and azimuthal anisotropy, we include the effect of azimuthal anisotropy in our inversions in order to obtain reliable isotropic phase velocity. We use b-splines to combine these isotropic phase velocity maps with our previous group velocity maps to produce an internally consistent global surface wave dispersion model.

Pasyanos, ME, Masters TG, Laske G, Ma ZT.  2014.  LITHO1.0: An updated crust and lithospheric model of the Earth. Journal of Geophysical Research-Solid Earth. 119:2153-2173.   10.1002/2013jb010626   AbstractWebsite

We present the LITHO1.0 model, which is a 1 degrees tessellated model of the crust and uppermost mantle of the Earth, extending into the upper mantle to include the lithospheric lid and underlying asthenosphere. The model is parameterized laterally by tessellated nodes and vertically as a series of geophysically identified layers, such as water, ice, sediments, crystalline crust, lithospheric lid, and asthenosphere. LITHO1.0 is created by constructing an appropriate starting model and perturbing it to fit high-resolution surface wave dispersion maps (Love and Rayleigh, group and phase) over a wide frequency band (5-40mHz). We examine and discuss the model with respect to key lithospheric parameters, such as average crustal velocity, crustal thickness, upper mantle velocity, and lithospheric thickness. We then compare the constructed model to those from a number of select studies at regional and global scales and find general consistency. It appears that LITHO1.0 represents a reasonable starting model of the Earth's shallow structure (crust and uppermost mantle) for the purposes in which these models are used, such as traveltime tomography or in efforts to create a 3-D reference Earth model. The model matches surface wave dispersion over a frequency band wider than the band used in the inversion. There are several avenues for improving the model in the future by including attenuation and anisotropy, as well as making use of surface waves at higher frequency.

Rychert, CA, Laske G, Harmon N, Shearer PM.  2013.  Seismic imaging of melt in a displaced Hawaiian plume. Nature Geoscience. 6:657-660.   10.1038/ngeo1878   AbstractWebsite

The Hawaiian Islands are the classic example of hotspot volcanism: the island chain formed progressively as the Pacific plate moved across a fixed mantle plume(1). However, some observations(2) are inconsistent with simple, vertical upwelling beneath a thermally defined plate and the nature of plume-plate interaction is debated. Here we use S-to-P seismic receiver functions, measured using a network of land and seafloor seismometers, to image the base of a melt-rich zone located 110 to 155 km beneath Hawaii. We find that this melt-rich zone is deepest 100 km west of Hawaii, implying that the plume impinges on the plate here and causes melting at greater depths in the mantle, rather than directly beneath the island. We infer that the plume either naturally upwells vertically beneath western Hawaii, or that it is instead deflected westwards by a compositionally depleted root that was generated beneath the island as it formed. The offset of the Hawaiian plume adds complexity to the classical model of a fixed plume that ascends vertically to the surface, and suggests that mantle melts beneath intraplate volcanoes may be guided by pre-existing structures beneath the islands.

Brown, PG, Assink JD, Astiz L, Blaauw R, Boslough MB, Borovicka J, Brachet N, Brown D, Campbell-Brown M, Ceranna L, Cooke W, de Groot-Hedlin C, Drob DP, Edwards W, Evers LG, Garces M, Gill J, Hedlin M, Kingery A, Laske G, Le Pichon A, Mialle P, Moser DE, Saffer A, Silber E, Smets P, Spalding RE, Spurny P, Tagliaferri E, Uren D, Weryk RJ, Whitaker R, Krzeminski Z.  2013.  A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors. Nature. 503:238-241.   10.1038/nature12741   AbstractWebsite

Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the atmosphere) have the potential to do greater damage(1) than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects(2). Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave(3), but owing to lack of observations this is uncertain(4,5). Here we report an analysis of the damage from the airburst of an asteroid about 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (+/- 100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185x10(12) joules). We show that a widely referenced technique(4-6) of estimating airburst damage does not reproduce the observations, and that the mathematical relations(7) based on the effects of nuclear weapons-almost always used with this technique-overestimate blast damage. This suggests that earlier damage estimates(5,6) near the threshold impactor size are too high. We performed a global survey of airbursts of a kiloton or more (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques(8,9). This suggests a non-equilibrium(if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes.

Yang, ZH, Sheehan AF, Collins JA, Laske G.  2012.  The character of seafloor ambient noise recorded offshore New Zealand: Results from the MOANA ocean bottom seismic experiment. Geochemistry Geophysics Geosystems. 13   10.1029/2012GC004201   Abstract

We analyze the characteristics of ambient noise recorded on ocean-bottom seismographs using data from the 2009-2010 MOANA (Marine Observations of Anisotropy Near Aotearoa) seismic experiment deployed west and east of South Island, New Zealand. Microseism and infragravity noise peaks are clear on data recorded on the vertical channel of the seismometer and on the pressure sensor. The noise levels in the infragravity band (<0.03 Hz) on the horizontal seismometer channels are too high to show the infragravity peak. There is a small difference (similar to 0.25 Hz versus similar to 0.2 Hz) in microseism peak frequencies between the two sides of the South Island on all three seismic channels. Our results show clear depth dependence between the peak frequency of infragravity waves and the water depth. We find that the product of water depth and wave number at the peak frequency is a constant, k(o)H = 1.5. This relationship can be used to determine the variation of phase and group velocity of infragravity waves with water depth, and the location of the infragravity peak and corresponding noise notch at any water depth. These estimates of spectral characteristics, particularly low noise bands, are useful for future OBS deployments.

Collins, JA, Wolfe CJ, Laske G.  2012.  Shear wave splitting at the Hawaiian hot spot from the PLUME land and ocean bottom seismometer deployments. Geochemistry Geophysics Geosystems. 13   10.1029/2011gc003881   AbstractWebsite

We examine upper mantle anisotropy across the Hawaiian Swell by analyzing shear wave splitting of teleseismic SKS waves recorded by the PLUME broadband land and ocean bottom seismometer deployments. Mantle anisotropy beneath the oceans is often attributed to flow-induced lattice-preferred orientation of olivine. Splitting observations may reflect a combination of both fossil lithospheric anisotropy and anisotropy due to present-day asthenospheric flow, and here we address the question whether splitting provides diagnostic information on possible asthenospheric plume flow at Hawaii. We find that the splitting fast directions are coherent and predominantly parallel to the fossil spreading direction, suggesting that shear wave splitting dominantly reflects fossil lithospheric anisotropy. The signature of anisotropy from asthenospheric flow is more subtle, although it could add some perturbation to lithospheric splitting. The measured delay times are typically 1 s or less, although a few stations display larger splitting delays of 1-2 s. The variability in the delay times across the different stations indicates differences in the degree of anisotropy or in the thickness of the anisotropic layer or in the effect of multilayer anisotropy. Regions with smaller splitting times may have experienced processes that modified the lithosphere and partially erased the fossil anisotropy; alternatively, asthenospheric splitting may either constructively add to or destructively subtract from lithospheric splitting to produce the observed variability in delay times.

Wolfe, CJ, Solomon SC, Laske G, Collins JA, Detrick RS, Orcutt JA, Bercovici D, Hauri EH.  2011.  Mantle P-wave velocity structure beneath the Hawaiian hotspot. Earth and Planetary Science Letters. 303:267-280.   10.1016/j.epsl.2011.01.004   AbstractWebsite

Three-dimensional images of P-wave velocity structure beneath the Hawaiian Islands, obtained from a network of seafloor and land seismometers, show an upper-mantle low-velocity anomaly that is elongated in the direction of the island chain and surrounded by a high-velocity anomaly in the shallow upper mantle that is parabolic in map view. Low velocities continue downward to the mantle tansition zone between 410 and 660 km depth and extend into the topmost lower mantle, although the resolution of lower mantle structure from this data set is limited. Comparisons of inversions with separate data sets at different frequencies suggest that contamination by water reverberations is not markedly biasing the P-wave imaging of mantle structure. Many aspects of the P-wave images are consistent with independent tomographic images of S-wave velocity in the region, but there are some differences in upper mantle structure between P-wave and S-wave velocities. Inversions without station terms show a southwestward shift in the location cif lowest P-wave velocities in the uppermost mantle relative to the pattern for shear waves, and inversions with station terms show differences between P-wave and S-wave velocity heterogeneity in the shallow upper mantle beneath and immediately east of the island of Hawaii. Nonetheless, the combined data sets are in general agreement with the hypothesis that the Hawaiian hotspot is the result of an upwelling, high-temperature plume. The broad upper-mantle low-velocity region beneath the Hawaiian Islands may reflect the diverging "pancake" at the top of the upwelling zone; the surrounding region of high velocities could represent a downwelling curtain; and the low-velocity anomalies southeast of Hawaii in the transition zone and topmost lower mantle are consistent with predictions of plume tilt. (C) 2011 Elsevier B.V. All rights reserved.

Tian, Y, Zhou Y, Sigloch K, Nolet G, Laske G.  2011.  Structure of North American mantle constrained by simultaneous inversion of multiple-frequency SH, SS, and Love waves. Journal of Geophysical Research-Solid Earth. 116   10.1029/2010jb007704   AbstractWebsite

We simultaneously invert for the velocity and attenuation structure of the North American mantle from a mixed data set: SH wave traveltime and amplitude anomalies, SS wave differential traveltime anomalies, and Love wave fundamental mode phase delays. All data are measured for multiple frequency bands, and finite frequency sensitivity kernels are used to explain the observations. In the resulting SH velocity model, a lower mantle plume is observed to originate at about 1500 km depth beneath the Yellowstone area, tilting about 40 degrees from vertical. The plume rises up through a gap in the subducting Farallon slab. The SH velocity model confirms high-level segmentation of the Farallon slab, which was observed in the recent P velocity model. Attenuation structure is resolvable in the upper mantle and transition zone; in estimating it, we correct for focusing. High-correlation coefficients between delta lnV(S) and delta lnQ(S) under the central and eastern United States suggest one main physical source, most likely temperature. The smaller correlation coefficients and larger slopes of the delta lnQ(S) - delta lnV(S) relationship under the western United States suggest an influence of nonthermal factors such as the existence of water and partial melt. Finally, we analyze the influence of the different components of our data set. The addition of Love wave phase delays helps to improve the resolution of both velocity and attenuation, and the effect is noticeable even in the lower mantle.

Laske, G, Markee A, Orcutt JA, Wolfe CJ, Collins JA, Solomon SC, Detrick RS, Bercovici D, Hauri EH.  2011.  Asymmetric shallow mantle structure beneath the Hawaiian Swell-evidence from Rayleigh waves recorded by the PLUME network. Geophysical Journal International. 187:1725-1742.   10.1111/j.1365-246X.2011.05238.x   AbstractWebsite

We present models of the 3-D shear velocity structure of the lithosphere and asthenosphere beneath the Hawaiian hotspot and surrounding region. The models are derived from long-period Rayleigh-wave phase velocities that were obtained from the analysis of seismic recordings collected during two year-long deployments for the Hawaiian Plume-Lithosphere Undersea Mantle Experiment. For this experiment, broad-band seismic sensors were deployed at nearly 70 seafloor sites as well as 10 sites on the Hawaiian Islands. Our seismic images result from a two-step inversion of path-averaged dispersion curves using the two-station method. The images reveal an asymmetry in shear velocity structure with respect to the island chain, most notably in the lower lithosphere at depths of 60 km and greater, and in the asthenosphere. An elongated, 100-km-wide and 300-km-long low-velocity anomaly reaches to depths of at least 140 km. At depths of 60 km and shallower, the lowest velocities are found near the northern end of the island of Hawaii. No major velocity anomalies are found to the south or southeast of Hawaii, at any depth. The low-velocity anomaly in the asthenosphere is consistent with an excess temperature of 200-250 degrees C and partial melt at the level of a few percent by volume, if we assume that compositional variations as a result of melt extraction play a minor role. We also image small-scale low-velocity anomalies within the lithosphere that may be associated with the volcanic fields surrounding the Hawaiian Islands.

Anchieta, MC, Wolfe CJ, Pavlis GL, Vernon FL, Eakins JA, Solomon SC, Laske G, Collins JA.  2011.  Seismicity around the Hawaiian Islands Recorded by the PLUME Seismometer Networks: Insight into Faulting near Maui, Molokai, and Oahu. Bulletin of the Seismological Society of America. 101:1742-1758.   10.1785/0120100271   AbstractWebsite

Instrumental limitations have long prevented the detailed characterization of offshore earthquakes around the Hawaiian Islands, and little is known about the spatial distribution of earthquakes in regions outside the vicinity of the well-monitored island of Hawaii. Here, we analyze data from the deployment of two successive networks of ocean-bottom seismometers (OBSs) as part of the Plume-Lithosphere Undersea Melt Experiment (PLUME) to better determine seismicity patterns along the Hawaiian Islands and their offshore regions. We find that earthquake detection rates are improved when seismograms are high-pass filtered above similar to 5 Hz to reduce the background seismic noise. Hypocentral solutions have been determined for 1147 previously undetected microearthquakes, and an additional 2880 events correspond to earthquakes already in the catalog of the United States Geological Survey (USGS) Hawaiian Volcano Observatory (HVO). The spatial patterns of earthquakes identified solely on the PLUME network provide complementary information to patterns identified by the HVO network. A diffuse pattern of seismicity is found to the southeast of the island of Hawaii, and clusters of earthquakes are located west of the island. Many microearthquakes are observed in the vicinity of Maui and Molokai, including some located at mantle depths. A small number of microearthquakes are found to occur near Oahu. There is no evidence from our analyses that the Molokai fracture zone (MFZ) is seismically active at this time, and no evidence was found of a previously hypothesized Diamond Head fault (DHF) near Oahu. However, on the basis of both the PLUME and HVO locations, there is a northeast-southwest-trending swath of epicenters extending northeastward of Oahu that may indicate the locus of moderate-sized historic earthquakes attributed to the Oahu region.

Leahy, GM, Collins JA, Wolfe CJ, Laske G, Solomon SC.  2010.  Underplating of the Hawaiian Swell: evidence from teleseismic receiver functions. Geophysical Journal International. 183:313-329.   10.1111/j.1365-246X.2010.04720.x   AbstractWebsite

P>The Hawaiian Islands are the canonical example of an age-progressive island chain, formed by volcanism long thought to be fed from a hotspot source that is more or less fixed in the mantle. Geophysical data, however, have so far yielded contradictory evidence on subsurface structure. The substantial bathymetric swell is supportive of an anomalously hot upper mantle, yet seafloor heat flow in the region does not appear to be enhanced. The accumulation of magma beneath pre-existing crust (magmatic underplating) has been suggested to add chemical buoyancy to the swell, but to date the presence of underplating has been constrained only by local active-source experiments. In this study, teleseismic receiver functions derived from seismic events recorded during the PLUME project were analysed to obtain a regional map of crustal structure for the Hawaiian Swell. This method yields results that compare favourably with those from previous studies, but permits a much broader view than possible with active-source seismic experiments. Our results indicate that the crustal structure of the Hawaiian Islands is quite complicated and does not conform to the standard model of sills fed from a central source. We find that a shallow P-to-s conversion, previously hypothesized to result from the volcano-sediment interface, corresponds more closely to the boundary between subaerial and subaqueous extrusive material. Correlation between uplifted bathymetry at ocean-bottom-seismometer locations and presence of underplating suggests that much of the Hawaiian Swell is underplated, whereas a lack of underplating beneath the moat surrounding the island of Hawaii suggests that underplated crust outward of the moat has been fed from below by dykes through the lithosphere rather than by sills spreading from the island centre. Local differences in underplating may reflect focusing of magma-filled dykes in response to stress from volcanic loading. Finally, widespread underplating adds chemical buoyancy to the swell, reducing the amplitude of a mantle thermal anomaly needed to match bathymetry and supporting observations of normal heat flow.

Wolfe, CJ, Solomon SC, Laske G, Collins JA, Detrick RS, Orcutt JA, Bercovici D, Hauri EH.  2009.  Mantle Shear-Wave Velocity Structure Beneath the Hawaiian Hot Spot. Science. 326:1388-1390.   10.1126/science.1180165   AbstractWebsite

Defining the mantle structure that lies beneath hot spots is important for revealing their depth of origin. Three-dimensional images of shear-wave velocity beneath the Hawaiian Islands, obtained from a network of sea-floor and land seismometers, show an upper-mantle low-velocity anomaly that is elongated in the direction of the island chain and surrounded by a parabola-shaped high-velocity anomaly. Low velocities continue downward to the mantle transition zone between 410 and 660 kilometers depth, a result that is in agreement with prior observations of transition-zone thinning. The inclusion of SKS observations extends the resolution downward to a depth of 1500 kilometers and reveals a several-hundred-kilometer-wide region of low velocities beneath and southeast of Hawaii. These images suggest that the Hawaiian hot spot is the result of an upwelling high-temperature plume from the lower mantle.

Grad, M, Tiira T, Group WESC.  2009.  The Moho depth map of the European Plate. Geophysical Journal International. 176:279-292.: Blackwell Publishing Ltd   10.1111/j.1365-246X.2008.03919.x   AbstractWebsite

The European Plate has a 4.5 Gy long and complex tectonic history. This is reflected in the present-day large-scale crustal structures. A new digital Moho depth map is compiled from more than 250 data sets of individual seismic profiles, 3-D models obtained by body and surface waves, receiver function results and maps of seismic and/or gravity data compilations. We have compiled the first digital, high-resolution map of the Moho depth for the whole European Plate, extending from the mid-Atlantic ridge in the west to the Ural Mountains in the east, and from the Mediterranean Sea in the south to the Barents Sea and Spitsbergen in the Arctic in the north. In general, three large domains within the European Plate crust are visible. The oldest Archean and Proterozoic crust has a thickness of 40–60 km, the continental Variscan and Alpine crust has a thickness of 20–40 km, and the youngest oceanic Atlantic crust has a thickness of 10–20 km.

Laske, G, Weber M, Desert Working G.  2008.  Lithosphere structure across the Dead Sea Transform as constrained by Rayleigh waves observed during the DESERT experiment. Geophysical Journal International. 173:593-610.   10.1111/j.1365-246X.2008.03749.x   AbstractWebsite

The interdisciplinary Dead Sea Rift Transect (DESERT) project that was conducted in Israel, the Palestine Territories and Jordan has provided a rich palette of data sets to examine the crust and uppermost mantle beneath one of Earth's most prominent fault systems, the Dead Sea Transform (DST). As part of the passive seismic component, thirty broad-band sensors were deployed in 2000 across the DST for roughly one year. During this deployment, we recorded 115 teleseismic earthquakes that are suitable for a fundamental mode Rayleigh wave analysis at intermediate periods (35-150 s). Our initial analysis reveals overall shear velocities that are reduced by up to 4 per cent with respect to reference Earth model PREM. To the west of the DST, we find a seismically relatively fast but thin lid that is about 80 km thick. Towards the east, shallow seismic velocities are low while a deeper low velocity zone is not detected. This contradicts the currently favoured thermomechanical model for the DST that predicts lithospheric thinning through mechanical erosion by an intruding plume from the Red Sea. On the other hand, our current results are somewhat inconclusive regarding asthenosphere velocities east of the DST due to the band limitation of the recording equipment in Jordan.

Houser, C, Masters G, Shearer P, Laske G.  2008.  Shear and compressional velocity models of the mantle from cluster analysis of long-period waveforms. Geophysical Journal International. 174:195-212.   10.1111/j.1365-246X.2008.03763.x   AbstractWebsite

We present a new technique for the efficient measurement of the traveltimes of long period body wave phases. The technique is based on the fact that all arrivals of a particular seismic phase are remarkably similar in shape for a single event. This allows the application of cross-correlation techniques that are usually used in a regional context to measure precise global differential times. The analysis is enhanced by the inclusion of a clustering algorithm that automatically clusters waveforms by their degree of similarity. This allows the algorithm to discriminate against unusual or distorted waveforms and makes for an extremely efficient measurement technique. This technique can be applied to any seismic phase that is observed over a reasonably large distance range. Here, we present the results of applying the algorithm to the long-period channels of all data archived at the IRIS DMC from 1976 to 2005 for the seismic phases S and P (from 23 degrees to 100 degrees) and SS and PP (from 50 degrees to 170 degrees). The resulting large data sets are inverted along with existing surface wave and updated differential traveltime measurements for new mantle models of S and P velocity. The resolution of the new model is enhanced, particularly, in the mid-mantle where SS and PP turn. We find that slow anomalies in the central Pacific and Africa extend from the core-mantle boundary to the upper mantle, but their direct connection to surface hotspots is beyond our resolution. Furthermore, we find that fast anomalies that are likely associated with subducting slabs disappear between 1700 and 2500 km, and thus are not continuous features from the upper to lower mantle despite our extensive coverage and high resolution of the mid-mantle.

Laske, G, Phipps Morgan J, Orcutt JA.  2007.  The Hawaiian SWELL pilot experiment; evidence for lithosphere rejuvenation from ocean bottom surface wave data. GSA Special Paper. 430:209-233.   10.1130/2007.2430(11)   Abstract

During the roughly year-long Seismic Wave Exploration in the Lower Lithosphere (SWELL) pilot experiment in 1997/1998, eight ocean bottom instruments deployed to the southwest of the Hawaiian Islands recorded teleseismic Rayleigh waves with periods between 15 and 70 s. Such data are capable of resolving structural variations in the oceanic lithosphere and upper asthenosphere and therefore help understand the mechanism that supports the Hawaiian Swell relief. The pilot experiment was a technical as well as a scientific feasibility study and consisted of a hexagonal array of Scripps Low-Cost Hardware for Earth Applications and Physical Oceanography (L-CHEAPO) instruments using differential pressure sensors. The analysis of eighty-four earthquakes provided numerous high-precision phase velocity curves over an un-precedentedly wide period range. We find a rather uniform (unaltered) lid at the top of the lithosphere that is underlain by a strongly heterogeneous lower lithosphere and upper asthenosphere. Strong slow anomalies appear within ∼300 km of the island chain and indicate that the lithosphere has most likely been altered by the same process that causes the Hawaiian volcanism. The anomalies increase with depth and reach well into the asthenosphere, suggesting a sublithospheric dynamic source for the swell relief. The imaged velocity variations are consistent with thermal rejuvenation, but our array does not appear to have covered the melt-generating region of the Hawaiian hotspot.

Laske, G, Widmer-Schnirig R.  2007.  Normal Mode & Surface Wave Observations. Treatise on geophysics. 1, Seismology and structure of the earth. ( Herring T, Romanowicz B, Schubert G, Eds.).:67-125., Amsterdam u.a.: Elsevier Abstract