Hawaii-2 Observatory Research Articles

Seismoacoustic observations at deep ocean observatories: ACO, H2O, and OSN1

Deep seafloor (~5000 m) observations of seismoacoustic arrivals are presented from the Aloha Cabled Observatory (ACO), Hawaii-2 Observatory (H20), and Ocean Seismic Network-1 (OSN1). Acoustic observations of local earthquakes from the ACO 24 kHz hydrophone show extraordinarily high frequencies up to 165 Hz at distances to ~200 km—a unique window into earthquake source dynamics. Combined seismic and hydrophone observations show that the traditional T wave propagates as a seismoacoustic polarized interface wave “Ti” coupled to the seafloor. Seismoacoustic Ti waves propagating at the sound speed of water are routinely observed over megameter distances at H2O between Hawaii and California, even though the seafloor site is within the shadow zone for acoustic wave propagation. Internal waves or thermohaline covariation may or may not be sufficient to scatter acoustic energy from the SOFAR channel to the deep seafloor. Ti has also been observed on seismometers SSW of Oahu at the OSN1 site at the seafloor and within an ODP borehole 242 m into the basalt basement. The observation of Ti from an earthquake in Guatemala at OSN1, whose path is blocked by the Island of Hawaii, is consistent with scattering from the vicinity of the Cross Seamount.

ACOustic keyhole

The Aloha Cabled Observatory (ACO) is located on the seafloor about 100 km north of O‘ahu at 4800 m depth. Seismoacoustic T-waves generated by earthquakes in the South Pacific from Tonga-Kermadec to the Solomon Islands travel 40-70°Δ, passing through an acoustic keyhole—the Ka'ie'ie Waho channel—between O‘ahu and Kaua‘i before reaching ACO. In contrast to ACO and the Pacific seafloor at depths greater than 4500 m, the channel shallows to 1 and 3 km in depth, blocking deeper SOFAR ray/mode paths. T-waves from >50 earthquakes (Mw 6.1 to 8) are reviewed, correcting apparent velocities for near-source P-wave path effects. We examine T-waves crossing the Ka'ie'ie Waho channel and descending to ACO, comparing and contrasting with T-waves propagating unhindered across the North Pacific from events in offshore Japan (Tohoku aftershocks), Kuril Islands, Aleutian Islands, and offshore Alaska. Preliminary review indicates that the T-waves exhibit seismoacoustic modal-coupling to the seafloor, exhibiting a structure observed previously* at the Hawai‘i-2 Observatory (H2O) at 5 km depth between Hawai‘i and California. *R. Butler (2006), J. Acoust. Soc. Am. 120(6), 3599-3606. *R. Butler and C. Lomnitz (2002), Geophys. Res. Lett. 29(10), 57 1-4.

Wave–wave interactions and deep ocean acoustics

Deep ocean acoustics, in the absence of shipping and wildlife, is driven by surface processes. Best understood is the signal generated by non-linear surface wave interactions, the Longuet-Higgins mechanism, which dominates from 0.1 to 10 Hz, and may be significant for another octave. For this source, the spectral matrix of pressure and vector velocity is derived for points near the bottom of a deep ocean resting on an elastic half-space. In the absence of a bottom, the ratios of matrix elements are universal constants. Bottom effects vitiate the usual "standing wave approximation," but a weaker form of the approximation is shown to hold, and this is used for numerical calculations. In the weak standing wave approximation, the ratios of matrix elements are independent of the surface wave spectrum, but depend on frequency and the propagation environment. Data from the Hawaii-2 Observatory are in excellent accord with the theory for frequencies between 0.1 and 1 Hz, less so at higher frequencies. Insensitivity of the spectral ratios to wind, and presumably waves, is indeed observed in the data.

Booms and Busts in the Deep

AbstractDeep sea (5 km) pressure and velocity at the Hawaii-2 Observatory (H2O), midway between Hawaii and California, exhibit a number of remarkable features that are interpreted using the Longuet–Higgins theory of acoustic radiation from oppositely directed surface waves. A change in the slope of the bottom spectra near 5 Hz can be associated with a transition near 2.5 Hz (25-cm wavelength) of the surface wave spectrum from the classical κ−4 saturated (wind independent) Phillips spectrum to a distinct band of ultragravity waves. Bottom spectra are remarkably stable. Occasional 15-dB busts in the gravities and booms in the ultragravities are prominent features in the bottom records and can be associated with calms and storms at the sea surface. For strong winds, two broad lobes in the directional spectrum of the gravity waves are nearly perpendicular to the wind; as the wind drops, the lobes become narrower and more nearly aligned with the wind, leading to busts.

Deep water ambient noise on the seafloor.

Long, continuous time series of ocean bottom seismometer (OBS) and co‐located hydrophone data from the seafloor offer an unprecedented opportunity to study the physical mechanisms of seafloor ambient noise in the frequency band from 1 to 100 Hz. The Ocean Seismic Network Pilot Experiment (OSNPE) in 1998 compared noise and signal levels for a hydrophone at the seafloor with three‐component observations from OBSs on the seafloor, buried 1 m into the soft sediment, and clamped in a borehole 10 m into basaltic basement. The Hawaii‐2 Observatory (H2O) acquired over 3 years of nearly continuous, real‐time data from a shallow‐buried seismic and seafloor hydrophone system. On the North Pacific Acoustic Laboratory experiment in 2004 (NPAL04) three vertical‐component geophone/hydrophones were deployed beneath vertical line arrays in the water column. In addition to ambient noise data, controlled source transmissions were received from ranges out to 3200 km. In the absence of shipping noise, whale calls, and local sea state, what controls the noise floor on the deep seafloor? Interesting observations include banded coherence structure between the vertical geophones and hydrophones, peaks in ambient noise levels due to resonances in the sediment layer, and Stoneley/Scholte wave particle motions. [Work supported by the NSF, ONR and WHOI.]

Observations of polarized seismoacoustic T waves at and beneath the seafloor in the abyssal Pacific ocean

Combined seismic and hydrophone observations show that the traditional T wave propagates as a seismoacoustic polarized interface wave (Ti) coupled to the seafloor. Seismoacoustic Ti waves propagating at the sound speed of water are routinely observed over megameter distances at the deep (4979 m) seafloor Hawaii-2 Observatory (H2O) between Hawaii and California, even though the seafloor site is within a shadow zone for acoustic wave propagation. Ti has also been observed on seismometers 225 km SSW of Oahu at the OSN1 site at the seafloor and within an ODP borehole into the basalt basement. Analyses of timing, apparent velocity, energy, and polarization of these interface waves are presented. At low frequency (< approximately 5 Hz) Ti propagates dominantly in the sediments and is consistent with higher-mode Rayleigh waves. At higher frequencies the observed Ti waves dominantly propagate acoustically with characteristics suggesting local scattering. The observation of Ti from an earthquake in Guatemala at OSN1, whose path is blocked by the Island of Hawaii, is consistent with scattering from the vicinity of the Cross Seamount.

Source areas of midocean microseismic noise

Comparison of microseismic noise data collected at the ocean‐bottom Hawaii‐2 Observatory (H2O), located midway between Hawaii and California, with wind and wave data shows three frequency bands having different source locations. At frequencies from about 0.3 to 0.5 Hz, the noise spectrum shows a strong correlation with overhead wind speed and direction, implying that this energy is generated locally. Correlation of swell height above H2O with microseism energy at frequencies below 0.3 Hz is generally poor, implying that these signals originate at distant locations. The highest noise levels observed occur in the 0.2‐ to 0.3‐Hz band, apparently resulting from the interaction of waves forced by pre‐ and postfrontal winds from relatively large coincident storms in the same vicinity. Correlation of the H2O microseism levels with NOAA buoy data, with hindcast wave height data over the North Pacific, and with seismic data from mainland and island stations, defines likely source areas of the 0.1‐ to 0.2‐Hz signals. Most of the microseism energy at H2O between 0.08 and 0.2 Hz appears to be generated by high‐amplitude storm waves impacting long stretches of coastline nearly simultaneously. [Work supported by the California Department of Boating and Waterways.]

Mid‐ocean microseisms

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.

The Hawaii-2 Observatory: Observation of Nanoearthquakes

The Hawaii-2 Observatory (H2O; http://www.iris.edu/GSN/) was established on the seafloor between Hawaii and California (Figure 1) in 1998 using the retired ATT Duennebier et al. , 2002; Harris and Duennebier, 2002; Pettit et al. , 2002). This is the first undersea seismic station in the Global Seismographic Network. The on-site instrumentation includes a broadband Guralp CMG-3 and 4.5 Hz Geospace HS1 geophones, buried 0.5 m into the sea-floor mud, and an OAS E2PD hydrophone 0.5 m above the seafloor. All of the data are natively sampled at 160 sps continuously and sent via the Hawaii-2 cable to Oahu, where they are distributed by the University of Hawaii to IRIS. Data are available in real-time via a Live Internet Seismic Server (LISS) server and are being used by the Pacific Tsunami Warning Center and the National Seismic Network. In early 2002, the Ocean Drilling Program drilled a borehole through 29±1 m of sediment into basalt about 1.5 km northeast of the current H2O instrumentation as a site for a future borehole sensor (Stephen et al. , 2003). Figure 1. (left) The Hawaii-2 Observatory at 27° 52.916 N, 141° 59.504 W is sited 1,750 km east-northeast of Honolulu (Butler et al. , 2000). The site lies between the Murray and Molokai Fracture Zones at 4,979 m depth on predominantly terrigeneous, pelagic clay. The oceanic crust is of Eocene age (45–50 m.y.) and is normal but fast-spreading with a spreading half-rate of 7 cm/y. (top, center) The Hawaii-2 cable terminates in western Oahu and passes between Kauai and Oahu before traversing northeast toward California, (right) The abyssal bathymetry is locally flat in a ∼5,000 km2 region with abyssal hills rising less than 500 m high at about 30 …

The Hawaii-2 Observatory

A permanent deep ocean scientific research facility-the Hawaii-2 Observatory (H2O)-was installed on the retired HAW-2 commercial submarine telephone cable in September 1998. H2O consists of a seafloor submarine cable termination and junction box in 5000 m of water located halfway between Hawaii and California. The H2O infrastructure was installed from a large research vessel using the Jason ROV and standard over-the-side gear. The junction box provides two-way digital communication at variable data rates of up to 80 kbit/s using the RS-422 protocol and a total of 400 W of power for both junction box systems and user equipment. Instruments may be connected by an ROV to the junction box at 8 wet-mateable connectors. The H2O junction box is a smart design, which incorporates redundancy to protect against failure and allows full control of instrument functionality from shore. Initial instrumentation at the H2O site includes broad-band seismometer and hydrophone packages.

Deep Sea Observatories

A survey of some deep-sea observatories that have either been deployed or have been developed is provided. The enabling technologies that facilitate the powering, comrrtartd and control and the transmission of data back to shore is discussed. To facilitate this discussion, examples of a number of observatories are categorized as “Wired to Shore”, “Wired to Surface” or “Wireless”. Among those observatories discussed are Japan’s Hatsushimu Real Time Long-term Deep Sea Floor Observatory, the Hawaii Undersea Geo-Observatory (HUGO), the Hawaii 2 Observatory (H2O), Ocean Net, the Geophysical and Oceanographic Station for Abyssal Research (GEOSTAR) and the NEPTUNE Project. Some relative advantages of each approach are discussed as regards cost, technical and logistical considerations.