Double-frequency Microseism Research Articles

Ambient noise levels in north central Italy

The characteristics of background seismic noise in north central Italy have been investigated by means of velocity power spectral analysis within the frequency range 0.1–15 Hz. The method proposed by McNamara and Buland (2004) has been applied to estimate the probability density function (PDF) of power spectra computed for ten different stations. Since the target region is the most industrialized area of Italy, a large variability among the power spectra for different sites is observed in the frequency range 1–15 Hz, with the noise levels at two stations exceeding the New High Noise Model (NHNM) of Peterson (1993). The 95th percentile of the PDF varies from −165 to −125 dB (relative to (m/s)2/Hz). This variability could significantly affect the detection capabilities of a network installed for recording the small to moderate size seismicity occurring in north central Italy. We also observed that the dispersion of the powers, estimated at each site as the difference between the 95th and the 5th percentiles, shows a positive trend with frequency that can be ascribed to the diurnal variation of the background noise levels. In the frequency range 0.1–1 Hz, the dominant feature is the double frequency (DF) peak of microseisms generated by oceanic storms. At one of the considered stations, the seasonal variability of the maximum amplitude of the DF peak has been observed in the period from April 2004 to December 2005. Considering the barometric maps provided by the UK Meteorological Office, we observed that the strongest powers in the range 0.10–0.25 Hz occur when intense storms are present over the North Atlantic Ocean, whereas the measurements of the height, frequency, and azimuth of the sea waves at two buoys of the Rete Ondametrica Italiana deployed in the Adriatic and Tyrrhenian seas suggest that the DF microseisms in the frequency range 0.25–0.50 Hz are generated by storms over the Mediterranean Sea. Finally, the analyzed region is characterized by two large‐scale geologic features, namely, the Po Plain and the Alps. The impedance contrast between these two units causes a noise reduction of about 10 dB in the frequency range 0.2–0.6 Hz.

Observations of infragravity waves at the Monterey ocean bottom broadband station (MOBB)

Infragravity waves can be observed at the 1000 m deep ocean bottom broadband seismic station MOBB on stormy as well as quiet days. When compared to the energy of the short‐period ocean waves recorded at the local buoys, infragravity waves in the longer than 20 s period band are found to be mainly locally generated from shorter‐period waves. Two types of modulation of the infragravity signal are observed. First, the entire infragravity band signal is modulated in‐phase with tides, possibly as a result of the nonlinear exchange of energy between the short‐period waves and tidal currents. Second, a longer‐period modulation of the infragravity signal is observed and is best correlated with the energy of the 14 s period ocean waves. This correlation indicates that the mechanism of generation of double frequency microseisms and infragravity waves are likely strongly related. Previously recorded data during the Oregon ULF/VLF experiment at 600 m water depth also indicate that infragravity waves are primarily locally generated.

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.

Feedforward reduction of the microseism disturbance in a long-base-line interferometric gravitational-wave detector

Standing ocean waves driven by storms can excite surface waves in the ocean floor at twice the wave frequency. These traverse large distances on land and are called the double-frequency (DF) microseism. The Laser Interferometer Gravitational-wave Observatory (LIGO) detector relies on length servos to maintain optical resonance in its 4 km Fabry–Pérot cavities, which consist of seismically isolated in-vacuum suspended test mass mirrors in three different buildings. Correcting for the DF microseism motion can require tens of micrometers of actuation, a significant fraction of the feedback dynamic range. The LIGO seismic isolation design provides an external fine actuation system (FAS), which allows long-range displacement of the optical tables that support the test mass suspensions. We report on a feedforward control system that uses seismometer signals from each building to produce correction signals, which are applied to the FAS, largely removing the microseism disturbance independently of length control servos. The root-mean-squared displacement from the microseism near 0.15 Hz can be reduced by 10 dB on average.

The near‐coastal microseism spectrum: Spatial and temporal wave climate relationships

Comparison of the ambient noise data recorded at near‐coastal ocean bottom and inland seismic stations at the Oregon coast with both offshore and nearshore buoy data shows that the near‐coastal microseism spectrum results primarily from nearshore gravity wave activity. Low double‐frequency (DF), microseism energy is observed at near‐coastal locations when seas nearby are calm, even when very energetic seas are present at buoys 500 km offshore. At wave periods >8 s, shore reflection is the dominant source of opposing wave components for near‐coastal DF microseism generation, with the variation of DF microseism levels poorly correlated with local wind speed. Near‐coastal ocean bottom DF levels are consistently ∼20 dB higher than nearby DF levels on land, suggesting that Rayleigh/Stoneley waves with much of the mode energy propagating in the water column dominate the near‐coastal ocean bottom microseism spectrum. Monitoring the southward propagation of swell from an extreme storm concentrated at the Oregon coast shows that near‐coastal DF microseism levels are dominated by wave activity at the shoreline closest to the seismic station. Microseism attenuation estimates between on‐land near‐coastal stations and seismic stations ∼150 km inland indicate a zone of higher attenuation along the California coast between San Francisco and the Oregon border.

Vibrations from the “Perfect Storm”

Microseismic vibrations during the famous October 1991 “Perfect Storm” were observed at seismic stations across North America. The extreme wave conditions during this storm, in conjunction with the occurrence of Hurricane Grace to the south, are ideal for studying where such vibrations originate and their inland propagation. High‐amplitude primary and double‐frequency (DF) microseisms were observed at broadband seismic station HRV in eastern Massachusetts. Similar spectral variation observed at seismic station ANMO at Albuquerque, New Mexico, shows transcontinental propagation of vibrations from the Perfect Storm. Cross correlation between wave spectra from widely separated buoy measurements and corresponding DF microseism spectra at HRV give high‐correlation coefficients, R2, from the New England coast to Cape Hatteras. Contours of peak R2 scaled by the magnitude of the lag at the peak, together with similarities between wave and microseism spectral variation, imply that the dominant source area of DF microseisms during the Perfect Storm is near the southern Massachusetts coast, not in the open ocean where the highest waves occurred.

Relationships between ocean bottom noise and the environment

AbstractObservations of ocean bottom low-frequency noise and surface environmental data over a period of 27 days in the northern Atlantic during the SAMSON and SWADE experiments reveal how closely related the noise is to meteorological conditions. Double-frequency microseisms produced by nonlinear interactions of storm-induced surface gravity waves are especially evident in the frequency band 0.16 to 0.3 Hz and show a high variability in both amplitude and peak frequencies. Bifurcated at times, the peak that characterizes the microseism band contains local and distant or “teleseismic” components, which are generated at different locations. Weather and storm fetch appear to be the major contributions to the size and shape of microseism spectra. Storm development on the sea surface is associated with progressively lower microseism frequencies along with a concurrent increase in amplitude. The single-frequency microseism peak is a continuous feature and is observed to portray the same time-dependent spectral characteristics as the portion of the double-frequency peak associated with distant storms. Coherence studies confirm that both peaks (single and teleseismic double) originate at a distant source. These peaks are generated at roughly the same location with some storm component over the coastline.

Concurrent measurements of the directional spectra of microseismic energy and surface gravity waves

This paper presents concurrent measurements of surface wave directional spectra and double‐frequency, long‐wave microseisms. Long‐wave energy rapidly develops during periods of shifting winds which create bidirectional sea states. Theoretically, nonlinear sum interactions of opposing wavenumber vectors of approximately the same frequency create long‐wave energy at twice the frequency, which is only slightly attenuated in shallow water. Bidirectional sea states have been found using a buried ocean bottom seismometer measuring system from which the long wave energy has been measured at double frequencies. This system was incorporated into the Office of Naval Research sponsored Sources of Ambient Microseismic Ocean Noise (SAMSON) experiment for 3 months off the Army Corps of Engineers' Field Research Facility near Duck, North Carolina in the fall of 1990. Four working sensors produced directional spectra results from nearly 22 gigabytes of recorded data, which was collected 2 km offshore of the FRF under 12–13 m of water and approximately 1 m of sediment. Because of the insignificant attenuation the measured energy levels of the double frequency microseisms at the seafloor are of the same order of magnitude as the single‐frequency, surface wave energy induced seafloor motion. Various data sets were analyzed that confirmed Longuet‐Higgins' theory, which proposes that the propagation direction of double‐frequency microseisms occurs in the direction of the vector sum of the opposing single‐frequency seas.

The SAMSON experiment−Initial results.

The ONR sources of ambient microseismic ocean noise (SAMSON) experiment was conducted in October–December 1990 to develop a synoptic view of low-frequency (10 mHz–1 Hz) ocean noise excitation and propagation. In order to understand the effects of ocean-atmosphere coupling, the experiment was conducted in coordination with the ONR surface wave dynamics experiment (SWADE) which provided detailed meteorological and wave dynamics data. The participating universities and laboratories included FlowDrill Industries, the University of Miami, Oregon State University, the Scripps Institution of Oceanography, and Washington State University. The field experiment produced continuous recordings from seismic and hydrophone arrays located on the ocean floor and near the deep ocean surface off the North Carolina coast, at the coastline at Duck, North Carolina, and in the Great Dismal Swamp, inland. During the experiment, a hurricane (Lilly) and a strong Nor’Easter passed through the area causing significant weather, sea surface, and noise pattern changes with frequencies ranging from as low as 5 mHz to more than 2 Hz. The correlations between weather and seismo-acoustic noise will be presented as well as patterns of frequency-dependent evolution of primary noise types including single and double frequency microseisms and pressure fluctuations associated with infragravity waves. Globally distributed earthquakes, which occur at frequent intervals, are responsible for a great deal of noise variability in the frequency band 60–100 mHz.

University of Miami shallow buried OBS experiments at the Oregon Margin.

A new method of OBS burial has been successfully tested in 3000-m-deep waters of the Oregon Margin during the cruise on board R/V Wecoma in September 1991. Guralp CME-3 broadband seismometers were buried approximately 1 m below seafloor. Approximately 50 h each of continuous data were successfully collected at OM-1 (44 42.5’ N, 125 32.3’ W) and OM-8 (45 04.83’ N, 125 24.96’ W) sites. A local earthquake event at 01:36 a.m., 12 September (Oregon time) was clearly recorded in the seismic channels and pressure channel at OM-1 station. The time-averaged frequency spectra of vertical acceleration and pressure show a quiet notch at 0.04 to 0.10 Hz as well as very high coherency between channels both at the infragravity wave band (0.003 to 0.03 Hz) and at the double frequency microseism band (0.10 to 0.50 Hz). The crustal structures of compressional wave velocity, shear wave velocity, and porosity down to 10 km below seafloor are determined at a resolution of a few hundred meters using BSMP inversion of the infragravity data. [Work supported by NSF.]

A comparative study of island, seafloor, and subseafloor ambient noise levels

AbstractA study of seafloor and island stations shows that for the frequency band 0.1 to 10 Hz the seismic noise levels on islands are comparable to the levels on the seafloor. The microseism peak at the seafloor appears to be comparable to the highest levels observed on small islands. For this band, seafloor stations are realistic alternatives when island sites are not available.Seven year averages of the ambient noise levels recorded by Seismic Research Observatory (SRO) stations on three islands (Guam [GUMO], Taiwan [TATO], and New Zealand's north island [SNZO]) are compared with those recorded by the International Deployment of Accelerometers (IDA) station on Easter Island and on and beneath the ocean floor by Ocean Bottom Seismometers (OBSs) and the Marine Seismic System (MSS) deployed in a south Pacific DSDP drill hole at 23.8°S., 165.5°W (Adair et al., 1986). From 0.3 to 2 Hz the SRO displacement power levels fall in the range historically observed by the Scripps' OBSs (decreasing at 70 dB/decade from 1 by 106 nm2/Hz at 0.3 Hz to 1 nm2/Hz at 2 Hz) and are 10 to 15 dB above MSS levels. Above 2 Hz it appears that the same ratios hold (the SRO power levels decrease at 70 dB/decade to 1 by 10−3 nm2/Hz at a frequency of 10 Hz), although this correlation is based on very limited, high gain, short-period data. At frequencies below 0.3 Hz the SRO noise levels peak and decrease to approximately 2 by 103 nm2/Hz at 40 mHz. The noise levels recorded at Easter Island are somewhat higher (decreasing at 70 dB/decade from 1 by 107 nm2/Hz at 0.2 Hz to 1 nm2/Hz at 10 Hz and to 1 by 105 nm2/Hz at 50 mHz). At the microseism peak near 0.2 Hz the MSS levels are from 15 to 20 dB higher than observed by the SRO stations and equivalent to those recorded at Easter Island. There appears to be little dependence of the variance in noise level estimates on frequency. The upper 95 per cent confidence limit generally lies 10 dB above the average noise levels for all island stations.All island noise level curves are dominated by the broad double-frequency microseism peak centered between 0.15 to 0.2 Hz. The single-frequency peak ranges from absent (Easter Island) to discernable (Guam and New Zealand) to obvious (at Taiwan). The center frequency of this peak ranges from 0.07 Hz at Guam and New Zealand to 0.1 Hz at Taiwan. We speculate that the increased amplitude and frequency of the single-frequency microseism peak is due to the interaction between the shallow continental shelf and surface gravity waves and/or the presence of Taiwan in a region of limited fetch.

Earth noise, 5 to 500 millicycles per second: 2. Reaction of the Earth to oceans and atmosphere

Sources of earth noise have been identified and located for different parts of the frequency band from 5 to 500 mc/s. In the typical microseism bands around 75 and 150 mc/s, sources are associated with storm waves arriving at both distant and local sea coasts. During the winter months the primary-frequency microseisms measured at La Jolla come mostly from the coast north of Cape Mendocino and are due to major storms in the North Pacific. Double-frequency microseism sources are generally within a few hundred kilometers of La Jolla, but more distant sources have also been observed. In addition, both ocean and atmosphere produce local loading which becomes significant at frequencies below the storm microseism band.