Abstract The Alaska Amphibious Community Seismic Experiment (AACSE) is a shoreline-crossing passive- and active-source seismic experiment that took place from May 2018 through August 2019 along an ∼700 km long section of the Aleutian subduction zone spanning Kodiak Island and the Alaska Peninsula. The experiment featured 105 broadband seismometers; 30 were deployed onshore, and 75 were deployed offshore in Ocean Bottom Seismometer (OBS) packages. Additional strong-motion instruments were also deployed at six onshore seismic sites. Offshore OBS stretched from the outer rise across the trench to the shelf. OBSs in shallow water (<262 m depth) were deployed with a trawl-resistant shield, and deeper OBSs were unshielded. Additionally, a number of OBS-mounted strong-motion instruments, differential and absolute pressure gauges, hydrophones, and temperature and salinity sensors were deployed. OBSs were deployed on two cruises of the R/V Sikuliaq in May and July 2018 and retrieved on two cruises aboard the R/V Sikuliaq and R/V Langseth in August–September 2019. A complementary 398-instrument nodal seismometer array was deployed on Kodiak Island for four weeks in May–June 2019, and an active-source seismic survey on the R/V Langseth was arranged in June 2019 to shoot into the AACSE broadband network and the nodes. Additional underway data from cruises include seafloor bathymetry and sub-bottom profiles, with extra data collected near the rupture zone of the 2018 Mw 7.9 offshore-Kodiak earthquake. The AACSE network was deployed simultaneously with the EarthScope Transportable Array (TA) in Alaska, effectively densifying and extending the TA offshore in the region of the Alaska Peninsula. AACSE is a community experiment, and all data were made available publicly as soon as feasible in appropriate repositories.
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Frequency Limit for the Pressure Compliance Correction of Ocean-Bottom Seismic Data
Abstract Vertical records of ocean-bottom seismographs (OBSs) are usually noisy at low frequencies, and one important noise source is the varying ocean-bottom pressure that results from ocean-surface water waves. The relation between the ocean-bottom pressure and the vertical seafloor motion, called the compliance pressure transfer function (PTF), can be derived using background seismic data. During an earthquake, earthquake signals also generate ocean-bottom pressure fluctuations, and the relation between the ocean-bottom pressure and the vertical seafloor motion is named the seismic PTF in this article. Conventionally, we use the whole pressure records and the compliance PTF to remove the compliance noise; the earthquake-induced pressure and the seismic PTF are ignored, which may distort the original signals. In this article, we analyze the data from 24 OBSs with water depth ranging from 107 to 4462 m. We find that for most stations, the investigated frequency range (0.01–0.2 Hz) can be divided into four bands depending on the water depth. In band (I) of lowest frequencies (<0.11, <0.05, and <0.02 Hz for water depth of 107, 1109, and 2650 m, respectively), the vertical seafloor acceleration is composed mostly of pressure compliance noise, which can be removed using the compliance PTF. The compliance PTF is much smaller than the seismic PTF, so distortion of earthquake signals is negligible. In band (II) of higher frequencies (0.11–0.20, 0.05–0.11, and 0.02–0.05 Hz for water depth of 107, 1109, and 2650 m, respectively), the vertical acceleration and ocean-bottom pressure are largely uncorrelated. In bands (III) and (IV) of even higher frequencies (>0.11 and >0.08 Hz for water depth of 1109 and 2650 m, respectively), the compliance noise is negligible, and the ocean-bottom pressure is mostly caused by the seafloor motion. Thus, the compliance can be safely ignored in frequency band (I).
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- Award ID(s):
- 1842989
- NSF-PAR ID:
- 10218244
- Date Published:
- Journal Name:
- Seismological Research Letters
- Volume:
- 91
- Issue:
- 2A
- ISSN:
- 0895-0695
- Page Range / eLocation ID:
- 967 to 976
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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P sf) were dominated by Rayleigh waves of periods between 5 and 50 s, with peak amplitudes of 3–4 kPa at most sites. Waveform similarity and the linear scaling betweenP sfand vertical ground acceleration indicate forced acceleration of the water column being dominant in governingP sfduring long‐period surface‐wave arrivals, with an additional component of elastic oscillation occurring at higher frequencies (>0.1 Hz) causing extra pressure signals. Analysis of formation pressure variations due to various types of ocean loading of distinctly different frequencies (e.g., tides, tsunami, and infragravity waves) shows stable one‐dimensional vertical loading efficiencies that depend on lithology at each borehole site, with loading response being strongly influenced by the presence of free gas at shallow depths within the Cascadia accretionary prism. Inter‐site comparisons of seismic and seafloor pressure waveforms demonstrate a key role of sediment thickness in the amplification of surface wave amplitudes. -
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SUMMARY We present a new compilation and analysis of broad-band ocean bottom seismometer noise properties from 15 yr of seismic deployments. We compile a comprehensive data set of representative four-component (seismometer and pressure gauge) noise spectra and cross-spectral properties (coherence, phase and admittance) for 551 unique stations spanning 18 U.S.-led experiments. This is matched with a comprehensive compilation of metadata parameters related to instrumentation and environmental properties for each station. We systematically investigate the similarity of noise spectra by grouping them according to these metadata parameters to determine which factors are the most important in determining noise characteristics. We find evidence for improvements in similarity of noise properties when grouped across parameters, with groupings by seismometer type and deployment water depth yielding the most significant and interpretable results. Instrument design, that is the entire deployed package, also plays an important role, although it strongly covaries with seismometer and water depth. We assess the presence of traditional sources of tilt, compliance, and microseismic noise to characterize their relative role across a variety of commonly used seismic frequency bands. We find that the presence of tilt noise is primarily dependent on the type of seismometer used (covariant with a particular subset of instrument design), that compliance noise follows anticipated relationships with water depth, and that shallow, oceanic shelf environments have systematically different microseism noise properties (which are, in turn, different from instruments deployed in shallow lake environments). These observations have important implications for the viability of commonly used seismic analysis techniques. Finally, we compare spectra and coherences before and after vertical channel tilt and compliance noise removal to evaluate the efficacy and limitations of these now standard processing techniques. These findings may assist in future experiment planning and instrument development, and our newly compiled noise data set serves as a building block for more targeted future investigations by the marine seismology community.
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SUMMARY A fleet of autonomously drifting profiling floats equipped with hydrophones, known by their acronym mermaid, monitors worldwide seismic activity from inside the oceans. The instruments are programmed to detect and transmit acoustic pressure conversions from teleseismic P wave arrivals for use in mantle tomography. Reporting seismograms in near-real time, within hours or days after they were recorded, the instruments are not usually recovered, but if and when they are, their memory buffers can be read out. We present a unique 1-yr-long data set of sound recorded at frequencies between 0.1 and 20 Hz in the South Pacific around French Polynesia by a mermaid float that was, in fact, recovered. Using time-domain, frequency-domain and time-frequency-domain techniques to comb through the time-series, we identified signals from 213 global earthquakes known to published catalogues, with magnitudes 4.6–8.0, and at epicentral distances between 24° and 168°. The observed signals contain seismoacoustic conversions of compressional and shear waves travelling through crust, mantle and core, including P, S, Pdif, Sdif, PKIKP, SKIKS, surface waves and hydroacoustic T phases. Only 10 earthquake records had been automatically reported by the instrument—the others were deemed low-priority by the onboard processing algorithm. After removing all seismic signals from the record, and also those from other transient, dominantly non-seismic, sources, we are left with the infrasonic ambient noise field recorded at 1500 m depth. We relate the temporally varying noise spectral density to a time-resolved ocean-wave model, WAVEWATCH III. The noise record is extremely well explained, both in spectral shape and in temporal variability, by the interaction of oceanic surface gravity waves. These produce secondary microseisms at acoustic frequencies between 0.1 and 1 Hz according to the well-known frequency-doubling mechanism.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1785/0220190259
