One year of ambient ocean noise data, 0.4 to 30 Hz, from the Wake Island hydrophone array in the northwestern Pacific are compared to surface wind speeds, 0–14 m/s (0–28 kn). Between 0.4 and 6 Hz, noise levels increase with wind speed at rates of up to 2 dB per m/s until a saturation is reached having a slope of about −23 dB/octave and a level of 75 dB relative to 1 μPa/√Hz at 4 Hz. This noise saturation, called the ‘‘Holu Spectrum,’’ likely corresponds to saturation of short-wavelength ocean wind waves. It is probably a worldwide constant. Between 4 and 30 Hz, noise also increases with wind speed at rates of up to 2 dB per m/s, but no saturation level is observed and the slope increases to about 4 dB/octave. This may be acoustic noise from whitecaps. On a hydrophone less than 3 km from Wake, noise between 0.5 and 10 Hz increases with wind speed at a rate up to 2 dB per m/s, but absolute noise levels are significantly higher than levels on the other hydrophones more distant from Wake, and no saturation is apparent. Surf breaking against the shore of the island is the probable source of this noise.

No abstract

Both P(n), and S(n), phases recorded at distances greater than 3000 kilometers in the western Pacific have substantial amounts of energy at high frequencies, in sorne instances as high as 12 hertz for P(n) and 15 hertz for S(n), A comparison of P(n) and S(n) spectra reveals generally higher energy levels and higher proportions of high-frequency to low-frequency energy for S(n) than for P(n). Estimates of the effective quality factor, Q, indicate that the efficiency of S(n) propagation may be two or three times that of P(n). First arrivals of Pn and Sn have apparent velocities in agreement with values for the uppermost mantle, whereas maximum-energy arrivals have apparent velocities in agreement with values for the lower crust.

Analyses of oceanic P and S data from an earthquake in the Kuril Islands which occurred along the trend of a 1500‐km‐long ocean bottom hydrophone array deployment near Wake Island have yielded constraints on high‐frequency seismic attenuation in the western Pacific. Spatial attenuation and Q were determined for group velocities between 8.2 and 3.0 km/s and for frequencies between 2.5 and 22.5 Hz. There is greater attenuation of oceanic P than oceanic S at all frequencies suggesting either the presence of significant bulk attenuation in the lithosphere or that oceanic P propagates as a leaky mode. The apparent attenuation of oceanic P and S is distance‐dependent, with lower attenuation beyond 30° Δ. The apparent Q of oceanic P increases with frequency as Q ∼ ƒ0.7, from about 300 at 2.5 Hz to 1500 at 17.5 Hz. The apparent Q of oceanic S increases with frequency as Q ∼ ƒ1.1, from about 400 at 2.5 Hz to 3000 at 22.5 Hz.

The April 11, 2012, magnitude 8.6 earthquake off the northern coast of Sumatra generated a tsunami that was recorded at sea‐level stations as far as 4800 km from the epicenter and at four ocean bottom pressure sensors (DARTs) in the Indian Ocean. The governments of India, Indonesia, Sri Lanka, Thailand, and Maldives issued tsunami warnings for their coastlines. The United States' Pacific Tsunami Warning Center (PTWC) issued an Indian Ocean‐wide Tsunami Watch Bulletin in its role as an Interim Service Provider for the region. Using an experimental real‐time tsunami forecast model (RIFT), PTWC produced a series of tsunami forecasts during the event that were based on rapidly derived earthquake parameters, including initial location and Mwp magnitude estimates and the W‐phase centroid moment tensor solutions (W‐phase CMTs) obtained at PTWC and at the U. S. Geological Survey (USGS). We discuss the real‐time forecast methodology and how successive, real‐time tsunami forecasts using the latest W‐phase CMT solutions improved the accuracy of the forecast.