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2015
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.

Bromirski, PD, Cayan DR.  2015.  Wave power variability and trends across the North Atlantic influenced by decadal climate patterns. Journal of Geophysical Research-Oceans. 120:3419-3443.   10.1002/2014jc010440   AbstractWebsite

Climate variations influence North Atlantic winter storm intensity and resultant variations in wave energy levels. A 60 year hindcast allows investigation of the influence of decadal climate variability on long-term trends of North Atlantic wave power, P-W, spanning the 1948-2008 epoch. P-W variations over much of the eastern North Atlantic are strongly influenced by the fluctuating North Atlantic Oscillation (NAO) atmospheric circulation pattern, consistent with previous studies of significant wave height, Hs. Wave activity in the western Atlantic also responds to fluctuations in Pacific climate modes, including the Pacific North American (PNA) pattern and the El Nino/Southern Oscillation. The magnitude of upward long-term trends during winter over the northeast Atlantic is strongly influenced by heightened storm activity under the extreme positive phase of winter NAO in the early 1990s. In contrast, P-W along the United States East Coast shows no increasing trend, with wave activity there most closely associated with the PNA. Strong wave power events exhibit significant upward trends along the Atlantic coasts of Iceland and Europe during winter months. Importantly, in opposition to the long-term increase of P-W, a recent general decrease in P-W across the North Atlantic from 2000 to 2008 occurred. The 2000-2008 decrease was associated with a general shift of winter NAO to its negative phase, underscoring the control exerted by fluctuating North Atlantic atmospheric circulation on P-W trends.

2010
Aster, RC, McNamara DE, Bromirski PD.  2010.  Global trends in extremal microseism intensity. Geophysical Research Letters. 37   10.1029/2010gl043472   AbstractWebsite

Globally ubiquitous seismic background noise peaks near 7 and 14 s period are generated via distinct mechanisms that transfer storm-generated gravity wave energy to the seismic wave field. We utilize continuous digital ground motion data recorded by the Global Seismographic Network and precursor instrumentation to chronicle microseism power extreme events for 1972-2009. Because most land-observed microseism surface-wave energy is generated at or near coasts, microseism metrics are particularly relevant to assessing changes in coastal ocean wave energy. Extreme microseism winter storm season event counts reveal the widespread influence of the El Nino Southern Oscillation (ENSO). Individual station and ensemble slopes trend positive for this study period for Northern Hemisphere stations. The double-frequency microseism is particularly volatile, suggesting that the weaker single-frequency microseism directly generated by ocean swell at coasts is likely a more representative seismic proxy for broad-scale ocean wave energy estimation. Citation: Aster, R. C., D. E. McNamara, and P. D. Bromirski (2010), Global trends in extremal microseism intensity, Geophys. Res. Lett., 37, L14303, doi: 10.1029/2010GL043472.

2008
Aster, RC, McNamara DE, Bromirski PD.  2008.  Multidecadal climate-induced variability in microseisms. Seismological Research Letters. 79:194-202.   10.1785/gssrl.79.2.194   AbstractWebsite

Microseisms are the most ubiquitous continuous seismic signals on Earth at periods between approximately 5 and 25 s (Peterson 1993; Kedar and Webb 2005). They arise from atmospheric energy converted to (primarily) Rayleigh waves via the intermediary of wind-driven oceanic swell and occupy a period band that is uninfluenced by common anthropogenic and wind-coupled noise processes on land (Wilson et al. 2002; de la Torre et al. 2005). “Primary” microseisms (near 8-s period) are generated in shallow water by breaking waves near the shore and/or the nonlinear interaction of the ocean wave pressure signal with the sloping sea floor (Hasselmann 1963). Secondary microseisms occur at half of the primary period and are especially strongly radiated in source regions where opposing wave components interfere (Longuett-Higgins 1950; Tanimoto 2007), which principally occurs due to the interaction of incident swell and reflected/scattered wave energy from coasts (Bromirski and Duennebier 2002; Bromirski, Duennebier, and Stephen 2005). Coastal regions having a narrow shelf with irregular and rocky coastlines are known to be especially efficient at radiating secondary microseisms (Bromirski, Duennebier, and Stephen 2005; Shulte-Pelkum et al. 2004). The secondary microseism is globally dominant, and its amplitudes proportional to the square of the standing wave height (Longuett-Higgins 1950), which amplifies its sensitivity to large swell events (Astiz and Creager 1994; Webb 2006).

2006
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/odp.proc.sr.200.007.2006   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.

2005
Bromirski, PD, Duennebier FK, Stephen RA.  2005.  Mid-ocean microseisms. Geochemistry Geophysics Geosystems. 6   10.1029/2004gc000768   AbstractWebsite

The Hawaii-2 Observatory (H2O) is an excellent site for studying the source regions and propagation of microseisms since it is located far from shorelines and shallow water. During Leg 200 of the Ocean Drilling Program, the officers of the JOIDES Resolution took wind and wave measurements for comparison with double-frequency (DF) microseism data collected at nearby H2O. The DF microseism band can be divided into short-period and long-period bands, SPDF and LPDF, respectively. Comparison of the ship's weather log with the seismic data in the SPDF band from about 0.20 to 0.45 Hz shows a strong correlation of seismic amplitude with wind speed and direction, implying that the energy reaching the ocean floor is generated locally by ocean gravity waves. Nearshore land seismic stations see similar SPDF spectra, also generated locally by wind seas. At H2O, SPDF microseism amplitudes lag sustained changes in wind speed and direction by several hours, with the lag increasing with wave period. This lag may be associated with the time necessary for the development of opposing seas for DF microseism generation. Correlation of swell height above H2O with the LPDF band from 0.085 to 0.20 Hz is often poor, implying that a significant portion of this energy originates at distant locations. Correlation of the H2O seismic data with NOAA buoy data, with hindcast wave height data from the North Pacific, and with seismic data from mainland and island stations, defines likely source areas of the LPDF signals. Most of the LPDF energy at H2O appears to be generated by high-amplitude storm waves impacting long stretches of coastline nearly simultaneously, and the Hawaiian Islands appear to be a significant source of LPDF energy in the North Pacific when waves arrive from particular directions. The highest levels observed at mid-ocean site H2O occur in the SPDF band when two coincident nearby storm systems develop. Deep water, mid-ocean-generated DF microseisms are not observed at continental sites, indicating high attenuation of these signals. At near-coastal seismic stations, both SPDF and LPDF microseism levels are generally dominated by local generation at nearby shorelines.