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Zhang, JA, Gerstoft P, Bromirski PD.  2010.  Pelagic and coastal sources of P-wave microseisms: Generation under tropical cyclones. Geophysical Research Letters. 37   10.1029/2010gl044288   AbstractWebsite

Nonlinear wave-wave interactions generate double-frequency (DF) microseisms, which include both surface waves (mainly Rayleigh-type) and compressional (P) waves. Although it is unclear whether DF surface waves generated in deep oceans are observed on land, we show that beamforming of land-based seismic array data allows detection of DF P waves generated by ocean waves from Super Typhoon Ioke in both pelagic and coastal regions. Two distinct spectral bands associated with different P-wave source locations are observed. The short-period DF band (0.16-0.35 Hz) is dominated by P waves generated in the deep ocean by local wind seas under the storm. In contrast, P waves in the long-period DF band (0.1-0.15 Hz) are weaker and generated closer to the coast of Japan from swell interactions. The accurate identification of DF P-wave microseism source areas is useful to monitor ocean wave-wave interactions due to tropical cyclones and to image Earth structure using ambient seismic noise. Citation: Zhang, J., P. Gerstoft, and P. D. Bromirski (2010), Pelagic and coastal sources of P-wave microseisms: Generation under tropical cyclones, Geophys. Res. Lett., 37, L15301, doi: 10.1029/2010GL044288.

Ying, YZ, Bean CJ, Bromirski PD.  2014.  Propagation of microseisms from the deep ocean to land. Geophysical Research Letters. 41:6374-6379.   10.1002/2014gl060979   AbstractWebsite

Ocean-generated microseisms are faint Earth vibrations that result from pressure fluctuations at the sea floor generated by the interaction between ocean surface gravity waves, and are continuously recorded as low frequency seismic noise. Here we investigate microseism propagation away from deep-ocean source regions using the spectral element method for an oceanic model that contains realistic northeast Atlantic Ocean irregular-layered structure composed of water, sediment, and upper crust. It also includes structural heterogeneities and continental slope and shelf bathymetry. Numerical simulations of coupled acoustic and elastic wave propagation in both simplified models and the full realistic model show that most microseism energy is confined to sediment and water column waveguides. We also show that a significant portion of microseism energy is reflected back to the deep ocean by the continental slope, while only a small fraction of deep-ocean-generated microseism energy reaches land. We conclude that terrestrially observed microseisms are largely generated in shallow water on continental shelves.

White-Gaynor, AL, Nyblade AA, Aster RC, Wiens DA, Bromirski PD, Gerstoft P, Stephen RA, Hansen SE, Wilson T, Dalziel IW, Huerta AD, Winberry JP, Anandakrishnan S.  2019.  Heterogeneous upper mantle structure beneath the Ross Sea Embayment and Marie Byrd Land, West Antarctica, revealed by P-wave tomography. Earth and Planetary Science Letters. 513:40-50.   10.1016/j.epsl.2019.02.013   AbstractWebsite

We present an upper mantle P-wave velocity model for the Ross Sea Embayment (RSE) region of West Antarctica, constructed by inverting relative P-wave travel-times from 1881 teleseismic earthquakes recorded by two temporary broadband seismograph deployments on the Ross Ice Shelf, as well as by regional ice- and rock-sited seismic stations surrounding the RSE. Faster upper mantle P-wave velocities (similar to +1%) characterize the eastern part of the RSE, indicating that the lithosphere in this part of the RSE may not have been reheated by mid-to-late Cenozoic rifting that affected other parts of the Late Cretaceous West Antarctic Rift System. Slower upper mantle velocities (similar to -1%) characterize the western part of the RSE over a similar to 500 km-wide region, extending from the central RSE to the Transantarctic Mountains (TAM). Within this region, the model shows two areas of even slower velocities (similar to -1.5%) centered beneath Mt. Erebus and Mt. Melbourne along the TAM front. We attribute the broader region of slow velocities mainly to reheating of the lithospheric mantle by Paleogene rifting, while the slower velocities beneath the areas of recent volcanism may reflect a Neogene-present phase of rifting and/or plume activity associated with the formation of the Terror Rift. Beneath the Ford Ranges and King Edward VII Peninsula in western Marie Byrd Land, the P-wave model shows lateral variability in upper mantle velocities of +/- 0.5% over distances of a few hundred km. The heterogeneity in upper mantle velocities imaged beneath the RSE and western Marie Byrd Land, assuming no significant variation in mantle composition, indicates variations in upper mantle temperatures of at least 100 degrees C. These temperature variations could lead to differences in surface heat flow of similar to +/- 10 mW/m(2) and mantle viscosity of 10(2) Pa s regionally across the study area, possibly influencing the stability of the West Antarctic Ice Sheet by affecting basal ice conditions and glacial isostatic adjustment. (C) 2019 Elsevier B.V. All rights reserved.

Vose, RS, Applequist S, Bourassa MKA, Pryor SC, Barthelmie RJ, Blanton B, Bromirski PD, Brooks HOE, DeGaetano AT, Dole RM, Easterling DR, Jensen RE, Karl TR, Katz RW, Klink K, Kruk MC, Kunkel KE, MacCracken MC, Peterson TSC, Shein K, Thomas BR, Walsh JE, Wang XLL, Wehner MF, Wuebbles DJ, Young RS.  2014.  Monitoring and understanding changes in extremes: Extratropical storms, winds, and waves. Bulletin of the American Meteorological Society. 95:377-386.   10.1175/bams-d-12-00162.1   AbstractWebsite

This scientific assessment examines changes in three climate extremesextratropical storms, winds, and waveswith an emphasis on U.S. coastal regions during the cold season. There is moderate evidence of an increase in both extratropical storm frequency and intensity during the cold season in the Northern Hemisphere since 1950, with suggestive evidence of geographic shifts resulting in slight upward trends in offshore/coastal regions. There is also suggestive evidence of an increase in extreme winds (at least annually) over parts of the ocean since the early to mid-1980s, but the evidence over the U.S. land surface is inconclusive. Finally, there is moderate evidence of an increase in extreme waves in winter along the Pacific coast since the 1950s, but along other U.S. shorelines any tendencies are of modest magnitude compared with historical variability. The data for extratropical cyclones are considered to be of relatively high quality for trend detection, whereas the data for extreme winds and waves are judged to be of intermediate quality. In terms of physical causes leading to multidecadal changes, the level of understanding for both extratropical storms and extreme winds is considered to be relatively low, while that for extreme waves is judged to be intermediate. Since the ability to measure these changes with some confidence is relatively recent, understanding is expected to improve in the future for a variety of reasons, including increased periods of record and the development of climate reanalysis projects.

Traer, J, Gerstoft P, Bromirski PD, Hodgkiss WS, Brooks LA.  2008.  Shallow-water seismoacoustic noise generated by tropical storms Ernesto and Florence. Journal of the Acoustical Society of America. 124:EL170-EL176.   10.1121/1.2968296   AbstractWebsite

Land-based seismic observations of double frequency (DF) microseisms generated during tropical storms Ernesto and Florence are dominated by signals in the 0.15-0.5 Hz band. In contrast, data from sea floor hydrophones in shallow water (70 m depth, 130 km off the New Jersey coast) show dominant signals in the ocean gravity-wave frequency band, 0.02-0.18 Hz, and low amplitudes from 0.18 to 0.3 Hz, suggesting significant opposing wave components necessary for DF microseism generation were negligible at the site. Florence produced large waves. over deep water while Ernesto only generated waves in coastal regions, yet both storms produced similar spectra. This suggests near-coastal shallow water as the dominant region for observed microseism generation. (C) 2008 Acoustical Society of America.

Traer, J, Gerstoft P, Bromirski PD, Shearer PM.  2012.  Microseisms and hum from ocean surface gravity waves. Journal of Geophysical Research-Solid Earth. 117   10.1029/2012jb009550   AbstractWebsite

Ocean waves incident on coasts generate seismic surface waves in three frequency bands via three pathways: direct pressure on the seafloor (primary microseisms, PM), standing waves from interaction of incident and reflected waves (double-frequency microseisms, DF), and swell-transformed infragravity wave interactions (the Earth's seismic hum). Beamforming of USArray seismic data shows that the source azimuths of the generation regions of hum, PM and DF microseisms vary seasonally, consistent with hemispheric storm patterns. The correlation of beam power with wave height over all azimuths is highest in near-coastal waters. Seismic signals generated by waves from Hurricane Irene and from a storm in the Southern Ocean have good spatial and temporal correlation with nearshore wave height and peak period for all three wave-induced seismic signals, suggesting that ocean waves in shallow water commonly excite hum (via infragravity waves), PM, and DF microseisms concurrently.

Toomey, DR, Allen RM, Barclay AH, Bell SW, Bromirski PD, Carlson RL, Chen XW, Collins JA, Dziak RP, Evers B, Forsyth DW, Gerstoft P, Hooft EEE, Livelybrooks D, Lodewyk JA, Luther DS, McGuire JJ, Schwartz SY, Tolstoy M, Trehu AM, Weirathmueller M, Wilcock WSD.  2014.  The Cascadia Initiative: A sea change in seismological studies of subduction zones. Oceanography. 27:138-150. AbstractWebsite

Increasing public awareness that the Cascadia subduction zone in the Pacific Northwest is capable of great earthquakes (magnitude 9 and greater) motivates the Cascadia Initiative, an ambitious onshore/offshore seismic and geodetic experiment that takes advantage of an amphibious array to study questions ranging from megathrust earthquakes, to volcanic arc structure, to the formation, deformation and hydration of the Juan De Fuca and Gorda Plates. Here, we provide an overview of the Cascadia Initiative, including its primary science objectives, its experimental design and implementation, and a preview of how the resulting data are being used by a diverse and growing scientific community. The Cascadia Initiative also exemplifies how new technology and community-based experiments are opening up frontiers for marine science. The new technology shielded ocean bottom seismometers is allowing more routine investigation of the source zone of megathrust earthquakes, which almost exclusively lies offshore and in shallow water. The Cascadia Initiative offers opportunities and accompanying challenges to a rapidly expanding community of those who use ocean bottom seismic data.

Stephen, RA, Duennebier FK, Harris D, Jolly J, Bolmer ST, Bromirski PD.  2006.  Broadband seismic observations at the Hawaii-2 Observatory, ODP Leg 200. Proceedings of the Ocean Drilling Program, Scientific Results. 200   10.2973/   AbstractWebsite

Among the groups of oceanic microfossils, only Radiolaria occur in abundances and preservation states sufficient to provide biostratigraphic control for restricted intervals within sediments recovered in Hole 1223A. The distribution of these microfossils has been divided into four major intervals, A-D. Radiolaria distribution Interval A occupies the depth range 0-3.0 meters below seafloor (mbsf), where the abundance of specimens is very low and preservation is poor. Radiolaria distribution Interval B occupies the depth range 3.02-7.1 mbsf. Radiolaria in Interval B are locally rare to abundant and well preserved, and assemblages range in age from pure early Eocene to early Eocene admixed with late Neogene taxa. Radiolaria distribution Interval C occupies the depth range 7.1-36.99 mbsf and is characterized by sediments either barren of microfossils or containing extremely rare early Eocene specimens. Radiolaria distribution Interval D occupies the depth range 36.99-38.7 mbsf (base of the recovered sedimentary section), where early Eocene Radiolaria are present in rare to common frequencies, but opal-A to opal-CT recrystallization has degraded the preservation state. The late Neogene assemblage of Radiolaria distribution Interval B is dated at 1.55-2.0 Ma, based on occurrences of Eucyrtidium matuyamai, Lamprocyclas heteroporos, and Theocorythium trachelium trachelium. The early Eocene assemblage of Radiolaria distribution Intervals B and D is somewhat problematically assigned to the Buryella clinata Zone.

Shen, WS, Wiens DA, Anandakrishnan S, Aster RC, Gerstoft P, Bromirski PD, Hansen SE, Dalziel IWD, Heeszel DS, Huerta AD, Nyblade AA, Stephen R, Wilson TJ, Winberry JP.  2018.  The crust and upper mantle structure of central and west Antarctica from bayesian inversion of rayleigh wave and receiver functions. Journal of Geophysical Research-Solid Earth. 123:7824-7849.   10.1029/2017jb015346   AbstractWebsite

We construct a new seismic model for central and West Antarctica by jointly inverting Rayleigh wave phase and group velocities along with P wave receiver functions. Ambient noise tomography exploiting data from more than 200 seismic stations deployed over the past 18years is used to construct Rayleigh wave phase and group velocity dispersion maps. Comparison between the ambient noise phase velocity maps with those constructed using teleseismic earthquakes confirms the accuracy of both results. These maps, together with P receiver function waveforms, are used to construct a new 3-D shear velocity (Vs) model for the crust and uppermost mantle using a Bayesian Monte Carlo algorithm. The new 3-D seismic model shows the dichotomy of the tectonically active West Antarctica (WANT) and the stable and ancient East Antarctica (EANT). In WANT, the model exhibits a slow uppermost mantle along the Transantarctic Mountains (TAMs) front, interpreted as the thermal effect from Cenozoic rifting. Beneath the southern TAMs, the slow uppermost mantle extends horizontally beneath the traditionally recognized EANT, hypothesized to be associated with lithospheric delamination. Thin crust and lithosphere observed along the Amundsen Sea coast and extending into the interior suggest involvement of these areas in Cenozoic rifting. EANT, with its relatively thick and cold crust and lithosphere marked by high Vs, displays a slower Vs anomaly beneath the Gamburtsev Subglacial Mountains in the uppermost mantle, which we hypothesize may be the signature of a compositionally anomalous body, perhaps remnant from a continental collision.

Rudman, AJ, Mallick S, Frazer LN, Bromirski P.  1993.  Workstation computation of synthetic seismograms for vertical and horizontal profiles: A full wavefield response for a two-dimensional layered half-space. Computers & Geosciences. 19:447-474.   10.1016/0098-3004(93)90095-m   AbstractWebsite

FORTRAN code for generation of full wavefield synthetic seismograms is presented for two-dimensional horizontally layered models bounded by a free surface and a half space. Model layers are user defined by compressional and shear velocities, Q factors, densities and thicknesses. The algorithm is based on the reflectivity method and uses the propagator matrix approach. Explosion (point) and double couple (fault) sources are generated with a moment tensor representation. As evaluation of the slowness integrals involves time consuming numerical Hankel transforms, these computations are made with a generalized Filon method that saves computational time. The architecture of the program is unusual because the outermost loop is over temporal frequency and the innermost loop is over slowness. This permits the use of frequency-dependent seismic velocities, necessary for causality, while giving a factor of seven speed-up from vectorization. The codes are applicable for both vector computers and workstations. Two test cases demonstrate successful applications of the codes for both horizontal seismic profiles (receivers at one depth at successively larger offsets) and for vertical seismic profiles (receivers arranged in a vertical array at any offset). Receivers and source may be positioned within any layer. The seismograms display direct, refracted, reflected, and head-wave arrivals and their multiples. Mode converted events of compressional and shear propagation are generated and identified. The code generates seismograms for pressure, vertical and horizontal displacement sensors and for models combining acoustic and elastic layers.

Rasmussen, L, Bromirski PD, Miller AJ, Arcas D, Flick RE, Hendershott MC.  2015.  Source location impact on relative tsunami strength along the US West Coast. Journal of Geophysical Research-Oceans. 120:4945-4961.   10.1002/2015jc010718   AbstractWebsite

Tsunami propagation simulations are used to identify which tsunami source locations would produce the highest amplitude waves on approach to key population centers along the U.S. West Coast. The reasons for preferential influence of certain remote excitation sites are explored by examining model time sequences of tsunami wave patterns emanating from the source. Distant bathymetric features in the West and Central Pacific can redirect tsunami energy into narrow paths with anomalously large wave height that have disproportionate impact on small areas of coastline. The source region generating the waves can be as little as 100 km along a subduction zone, resulting in distinct source-target pairs with sharply amplified wave energy at the target. Tsunami spectral ratios examined for transects near the source, after crossing the West Pacific, and on approach to the coast illustrate how prominent bathymetric features alter wave spectral distributions, and relate to both the timing and magnitude of waves approaching shore. To contextualize the potential impact of tsunamis from high-amplitude source-target pairs, the source characteristics of major historical earthquakes and tsunamis in 1960, 1964, and 2011 are used to generate comparable events originating at the highest-amplitude source locations for each coastal target. This creates a type of ``worst-case scenario,'' a replicate of each region's historically largest earthquake positioned at the fault segment that would produce the most incoming tsunami energy at each target port. An amplification factor provides a measure of how the incoming wave height from the worst-case source compares to the historical event.

Moon, JH, Song YT, Bromirski PD, Miller AJ.  2013.  Multidecadal regional sea level shifts in the Pacific over 1958-2008. Journal of Geophysical Research-Oceans. 118:7024-7035.   10.1002/2013jc009297   AbstractWebsite

Altimeter data have significantly improved our understanding of regional sea level variability and trends, but their relatively short records do not allow either evaluation of the ocean state prior to 1993 or multidecadal low-frequency signals in the ocean. Here we characterize and quantify the multidecadal regional sea level rise (rSLR) and related ocean heat content in the Pacific from a non-Boussinesq ocean circulation model in comparison with data sets from altimeters, two sea level reconstructions, and in situ ocean profiles from 1958 to 2008. We show that the rSLR trends have undergone two shifts, during the mid-1970s and in the early 1990s, with an east-west dipole pattern in the tropical Pacific. In each of these phases, rSLR accelerated on one side of the Pacific, but decelerated on the other side. The multidecadal sea level shifts can be explained by the dynamical (steric) upper-ocean responses to the surface wind forcing associated with the Pacific Decadal Oscillation (PDO), with negligible contributions from internal (depth-integrated) ocean mass changes. Additional model experimentation further confirms that the Pacific wind stress trend over the recent two decades has played an important role in strengthening the rSLR in the western Pacific while suppressing the rSLR in the eastern Pacific. The climate-forced large-scale rSLR variability is likely to impose a long-term and uneven impact on coastal communities.

Moftakhari, HR, Jay DA, Talke SA, Kukulka T, Bromirski PD.  2013.  A novel approach to flow estimation in tidal rivers. Water Resources Research. 49:4817-4832. AbstractWebsite
Graham, NE, Cayan DR, Bromirski PD, Flick RE.  2013.  Multi-model projections of twenty-first century North Pacific winter wave climate under the IPCC A2 scenario. Climate Dynamics. 40:1335-1360.   10.1007/s00382-012-1435-8   AbstractWebsite

A dynamical wave model implemented over the North Pacific Ocean was forced with winds from three coupled global climate models (CGCMs) run under a medium-to-high scenario for greenhouse gas emissions through the twenty-first century. The results are analyzed with respect to changes in upper quantiles of significant wave height (90th and 99th percentile H-S) during boreal winter. The three CGCMs produce surprisingly similar patterns of change in winter wave climate during the century, with waves becoming 10-15 % smaller over the lower mid-latitudes of the North Pacific, particularly in the central and western ocean. These decreases are closely associated with decreasing windspeeds along the southern flank of the main core of the westerlies. At higher latitudes, 99th percentile wave heights generally increase, though the patterns of change are less uniform than at lower latitudes. The increased wave heights at high latitudes appear to be due a variety of wind-related factors including both increased windspeeds and changes in the structure of the wind field, these varying from model to model. For one of the CGCMs, a commonly used statistical approach for estimating seasonal quantiles of H-S on the basis of seasonal mean sea level pressure (SLP) is used to develop a regression model from 60 years of twentieth century data as a training set, and then applied using twenty-first century SLP data. The statistical model reproduces the general pattern of decreasing twenty-first century wave heights south of similar to 40 N, but underestimates the magnitude of the changes by similar to 50-70 %, reflecting relatively weak coupling between sea level pressure and wave heights in the CGCM data and loss of variability in the statistically projected wave heights.

Diez, A, Bromirski PD, Gerstoft P, Stephen RA, Anthony RE, Aster RC, Cai C, Nyblade A, Wiens DA.  2016.  Ice shelf structure derived from dispersion curve analysis of ambient seismic noise, Ross Ice Shelf, Antarctica. Geophysical Journal International. 205:785-795.   10.1093/gji/ggw036   AbstractWebsite

An L-configured, three-component short period seismic array was deployed on the Ross Ice Shelf, Antarctica during November 2014. Polarization analysis of ambient noise data from these stations shows linearly polarized waves for frequency bands between 0.2 and 2 Hz. A spectral peak at about 1.6 Hz is interpreted as the resonance frequency of the water column and is used to estimate the water layer thickness below the ice shelf. The frequency band from 4 to 18 Hz is dominated by Rayleigh and Love waves propagating from the north that, based on daily temporal variations, we conclude were generated by field camp activity. Frequency-slowness plots were calculated using beamforming. Resulting Love and Rayleigh wave dispersion curves were inverted for the shear wave velocity profile within the firn and ice to similar to 150 m depth. The derived density profile allows estimation of the pore close-off depth and the firn-air content thickness. Separate inversions of Rayleigh and Love wave dispersion curves give different shear wave velocity profiles within the firn. We attribute this difference to an effective anisotropy due to fine layering. The layered structure of firn, ice, water and the seafloor results in a characteristic dispersion curve below 7 Hz. Forward modelling the observed Rayleigh wave dispersion curves using representative firn, ice, water and sediment structures indicates that Rayleigh waves are observed when wavelengths are long enough to span the distance from the ice shelf surface to the seafloor. The forward modelling shows that analysis of seismic data from an ice shelf provides the possibility of resolving ice shelf thickness, water column thickness and the physical properties of the ice shelf and underlying seafloor using passive-source seismic data.

Costa-Cabral, M, Rath JS, Mills WB, Roy SB, Bromirski PD, Milesi C.  2016.  Projecting and forecasting winter precipitation extremes and meteorological drought in California using the North Pacific high sea level pressure anomaly. Journal of Climate. 29:5009-5026.   10.1175/jcli-d-15-0525.1   AbstractWebsite

Large-scale climatic indices have been used as predictors of precipitation totals and extremes in many studies and are used operationally in weather forecasts to circumvent the difficulty in obtaining robust dynamical simulations of precipitation. The authors show that the sea level pressure North Pacific high (NPH) wintertime anomaly, a component of the Northern Oscillation index (NOI), provides a superior covariate of interannual precipitation variability in Northern California, including seasonal precipitation totals, drought, and extreme precipitation intensity, compared to traditional ENSO indices such as the Southern Oscillation index (SOI), the multivariate ENSO index (MEI), NiNo-3.4, and others. Furthermore, the authors show that the NPH anomaly more closely reflects the influence of Pacific basin conditions over California in general, over groups of stations used to characterize statewide precipitation in the Sierra Nevada range, and over the southern San Francisco Bay region (NASA Ames Research Center). This paper uses the term prediction to refer to the estimation of precipitation (the predictand) from a climate covariate (the predictor), such as a climate index, or atmospheric moisture. In this sense, predictor and predictand are simultaneous in time. Statistical models employed show the effectiveness of the NPH winter anomaly as a predictor of total winter precipitation and daily precipitation extremes at the Moffett Field station. NPH projected by global climate models is also used in conjunction with atmospheric humidity [atmospheric specific humidity (HUS) at the 850-hPa level] to obtain projections of mean and extreme precipitation. The authors show that future development of accurate forecasts of NPH anomalies issued several months in advance is important for forecasting total winter precipitation and is expected to directly benefit water resource management in California. Therefore, the authors suggest that investigating the lead-time predictability of NPH anomalies is an important direction for future research.

Chen, Z, Bromirski PD, Gerstoft P, Stephen RA, Wiens DA, Aster RC, Nyblade AA.  2018.  Ocean-excited plate waves in the Ross and Pine Island Glacier ice shelves. Journal of Glaciology. 64:730-744.   10.1017/jog.2018.66   AbstractWebsite

Ice shelves play an important role in buttressing land ice from reaching the sea, thus restraining the rate of grounded ice loss. Long-period gravity-wave impacts excite vibrations in ice shelves that can expand pre-existing fractures and trigger iceberg calving. To investigate the spatial amplitude variability and propagation characteristics of these vibrations, a 34-station broadband seismic array was deployed on the Ross Ice Shelf (RIS) from November 2014 to November 2016. Two types of ice-shelf plate waves were identified with beamforming: flexural-gravity waves and extensional Lamb waves. Below 20 mHz, flexural-gravity waves dominate coherent signals across the array and propagate landward from the ice front at close to shallow-water gravity-wave speeds (similar to 70 m s(-1)). In the 20-100 mHz band, extensional Lamb waves dominate and propagate at phase speeds similar to 3 km s(-1). Flexural-gravity and extensional Lamb waves were also observed by a 5-station broadband seismic array deployed on the Pine Island Glacier (PIG) ice shelf from January 2012 to December 2013, with flexural wave energy, also detected at the PIG in the 20-100 mHz band. Considering the ubiquitous presence of storm activity in the Southern Ocean and the similar observations at both the RIS and the PIG ice shelves, it is likely that most, if not all, West Antarctic ice shelves are subjected to similar gravity-wave excitation.

Chen, Z, Gerstoft P, Bromirski PD.  2016.  Microseism source direction from noise cross-correlation. Geophysical Journal International. 205:810-818.   10.1093/gji/ggw055   AbstractWebsite

Inhomogeneous noise sources surrounding stations produce asymmetric amplitudes in cross-correlation functions that yield preferential source directions. Here we show that preprocessing biases the dominant source direction estimate towards the source producing long-duration signals by down-weighting high-amplitude signals. Tests with both synthetic data and observations show that conventional preprocessing, where only earthquakes and local transients (e.g. trawling, fish impacts) are removed, is more sensitive to coherent energy, while one-bit preprocessing and running-absolute-mean preprocessing are more influenced by signal duration. Comparisons between different preprocessing methods are made on data from the Cascadia Initiative ocean bottom seismometer array, where we find that the total energy arriving from pelagic and coastal areas is similar. Moreover, pelagic-generated signals tend to be weaker but have longer duration, in contrast to coastal-generated signals that tend to be stronger but have shorter duration.

Chaput, J, Aster RC, McGrath D, Baker M, Anthony RE, Gerstoft P, Bromirski P, Nyblade A, Stephen RA, Wiens DA, Das SB, Stevens LA.  2018.  Near-surface environmentally forced changes in the Ross Ice Shelf observed with ambient seismic noise. Geophysical Research Letters. 45:11187-11196.   10.1029/2018gl079665   AbstractWebsite

Continuous seismic observations across the Ross Ice Shelf reveal ubiquitous ambient resonances at frequencies >5 Hz. These firn-trapped surface wave signals arise through wind and snow bedform interactions coupled with very low velocity structures. Progressive and long-term spectral changes are associated with surface snow redistribution by wind and with a January 2016 regional melt event. Modeling demonstrates high spectral sensitivity to near-surface (top several meters) elastic parameters. We propose that spectral peak changes arise from surface snow redistribution in wind events and to velocity drops reflecting snow lattice weakening near 0 degrees C for the melt event. Percolation-related refrozen layers and layer thinning may also contribute to long-term spectral changes after the melt event. Single-station observations are inverted for elastic structure for multiple stations across the ice shelf. High-frequency ambient noise seismology presents opportunities for continuous assessment of near-surface ice shelf or other firn environments. Plain Language Summary Ice shelves are the floating buttresses of large glaciers that extend over the oceans and play a key role in restraining inland glaciers as they flow to the sea. Deploying sensitive seismographs across Earth's largest ice shelf (the Ross Ice Shelf) for 2 years, we discovered that the shelf nearly continuously sings at frequencies of five or more cycles per second, excited by local and regional winds blowing across its snow dune-like topography. We find that the frequencies and other features of this singing change, both as storms alter the snow dunes and during a (January 2016) warming event that resulted in melting in the ice shelf's near surface. These observations demonstrate that seismological monitoring can be used to continually monitor the near-surface conditions of an ice shelf and other icy bodies to depths of several meters.

Cayan, DR, Bromirski PD, Hayhoe K, Tyree M, Dettinger MD, Flick RE.  2008.  Climate change projections of sea level extremes along the California coast. Climatic Change. 87:S57-S73.   10.1007/s10584-007-9376-7   AbstractWebsite

California's coastal observations and global model projections indicate that California's open coast and estuaries will experience rising sea levels over the next century. During the last several decades, the upward historical trends, quantified from a small set of California tide gages, have been approximately 20 cm/century, quite similar to that estimated for global mean sea level. In the next several decades, warming produced by climate model simulations indicates that sea level rise (SLR) could substantially exceed the rate experienced during modem human development along the California coast and estuaries. A range of future SLR is estimated from a set of climate simulations governed by lower (B1), middle-upper (A2), and higher (A1fi) GHG emission scenarios. Projecting SLR from the ocean warming in GCMs, observational evidence of SLR, and separate calculations using a simple climate model yields a range of potential sea level increases, from 11 to 72 cm, by the 2070-2099 period. The combination of predicted astronomical tides with projected weather forcing, El Nino related variability, and secular SLR, gives a series of hourly sea level projections for 2005-2100. Gradual sea level rise progressively worsens the impacts of high tides, surge and waves resulting from storms, and also freshwater floods from Sierra and coastal mountain catchments. The occurrence of extreme sea levels is pronounced when these factors coincide. The frequency and magnitude of extreme events, relative to current levels, follows a sharply escalating pattern as the magnitude of future sea level rise increases.

Bromirski, PD, Frazer LN, Duennebier FK.  1995.  The Q-gram method: Q from instantaneous phase. Geophysical Journal International. 120:73-86.   10.1111/j.1365-246X.1995.tb05911.x   AbstractWebsite

The width of a seismic pulse increases monotonically with distance and with Q-1. Estimates of Q from pulse width measurements are often not robust for oscillatory arrivals or for impulsive arrivals in the presence of noise. We present a method to estimate Q from two arrivals using measurements of any signal attribute, xiBaR, that is sensitive to propagation loss. The propagation loss is defined as the change in xiBAR divided by the difference in traveltime between the arrivals. The first data arrival is used as the reference wavelet. The Q-gram method is based on propagating the reference wavelet with a plane-wave Q-propagator for various values of Q-1. The Q-propagator includes a dispersion relation and the measured difference in traveltime between the data arrivals. The plot of synthetic propagation loss between the reference and propagated wavelets, versus Q-1, is called a Q-gram. The Q-gram, together with the measured propagation loss of the data, gives the Q of the data. The averaged instantaneous frequency fBAR and the averaged instantaneous pulse width tauBAR make good signal attributes. Tests on synthetic seismograms show that the Q-gram method, using either fBAR or tauBAR for xiBAR, is applicable to both impulsive and oscillatory arrivals and is relatively robust with regard to noise, phase changes and signal clipping. We apply the Q-gram method to horizontal-component airgun ocean-bottom seismometer (OBS) data using the basement-converted shear-wave reflection, PS, as the first arrival and PSSS as the second arrival. We estimate Q(beta), the effective sediment shear-wave Q, with an fBAR-type Q-gram and a tauBAR-type Q-gram for the PS and PSSS sediment shear-wave reflections. The data indicate that Q(beta) almost-equal-to 75 +/- 15, in agreement with results from the application of the spectral-ratio method using windows that exclude interfering arrivals identified by means of the instantaneous frequency.

Bromirski, PD, Frazer LN, Duennebier FK.  1991.  Sediment Q from spectral ratios of converted shear reflections. Shear waves in marine sediments. ( Hovem JM, Richardson MD, Stoll RD, Eds.).:361-368., Dordrecht ; Boston: Kluwer Abstract
Bromirski, PD, Gerstoft P.  2009.  Dominant source regions of the Earth's "hum'' are coastal. Geophysical Research Letters. 36   10.1029/2009gl038903   AbstractWebsite

Hum beam power observations using the USArray EarthScope transportable array, combined with infragravity wave observations, show that the dominant source area of the Earth's hum over the 120-400 s period band during winter months is the Pacific coast of North America, with the western coast of Europe a secondary source region. Correlation of hum with model ocean wave heights indicates that the Pacific coast of Central America is an important hum source region when impacted by austral storm waves. Hum is excited by relatively local infragravity wave forcing as ocean swell propagates along coasts, with no indication of significant deep-ocean hum generation. Citation: Bromirski, P. D., and P. Gerstoft (2009), Dominant source regions of the Earth's "hum'' are coastal, Geophys. Res. Lett., 36, L13303, doi: 10.1029/2009GL038903.

Bromirski, PD, Flick RE, Cayan DR.  2003.  Storminess variability along the California coast: 1858-2000. Journal of Climate. 16:982-993.   10.1175/1520-0442(2003)016<0982:svatcc>;2   AbstractWebsite

The longest available hourly tide gauge record along the West Coast (U. S.) at San Francisco yields meteorologically forced nontide residuals (NTR), providing an estimate of the variation in "storminess'' from 1858 to 2000. Mean monthly positive NTR (associated with low sea level pressure) show no substantial change along the central California coast since 1858 or over the last 50 years. However, in contrast, the highest 2% of extreme winter NTR levels exhibit a significant increasing trend since about 1950. Extreme winter NTR also show pronounced quasi-periodic decadal-scale variability that is relatively consistent over the last 140 years. Atmospheric sea level pressure anomalies (associated with years having high winter NTR) take the form of a distinct, large-scale atmospheric circulation pattern, with intense storminess associated with a broad, southeasterly displaced, deep Aleutian low that directs storm tracks toward the California coast.

Bromirski, PD, Diez A, Gerstoft P, Stephen RA, Bolmer T, Wiens DA, Aster RC, Nyblade A.  2015.  Ross Ice Shelf vibrations. Geophysical Research Letters. 42:7589-7597.   10.1002/2015gl065284   AbstractWebsite

Broadband seismic stations were deployed across the Ross Ice Shelf (RIS) in November 2014 to study ocean gravity wave-induced vibrations. Initial data from three stations 100km from the RIS front and within 10km of each other show both dispersed infragravity (IG) wave and ocean swell-generated signals resulting from waves that originate in the North Pacific. Spectral levels from 0.001 to 10Hz have the highest accelerations in the IG band (0.0025-0.03Hz). Polarization analyses indicate complex frequency-dependent particle motions, with energy in several frequency bands having distinctly different propagation characteristics. The dominant IG band signals exhibit predominantly horizontal propagation from the north. Particle motion analyses indicate retrograde elliptical particle motions in the IG band, consistent with these signals propagating as Rayleigh-Lamb (flexural) waves in the ice shelf/water cavity system that are excited by ocean wave interactions nearer the shelf front.