Low-frequency (0.01 to 0.2 Hz) seismic noise, arising from pelagic storms, is commonly observed as microseisms in seismic records from land and ocean bottom detectors. One principal research objective, in the study of microseisms, has been to locate their sources. This article reports on an analysis of primary and secondary microseisms (i.e., near and double the frequency of ocean swell) recorded simultaneously on three land-based long-period arrays (Alaskan Long Period Array, Montana Large Aperture Seismic Array, and Norwegian Seismic Array) during the early 1970s. Reliable microseism source locations are determined by wide-angle triangulation, using the azimuths of approach obtained from frequency-wave number analysis of the records of microseisms propagating across these arrays. Two near-shore sources of both primary and secondary microseisms appear to be persistent in the sense that they are associated with essentially constant near-shore locations. Secondary microseisms are observed to emanate from wide-ranging pelagic locations in addition to the same near-shore locations determined for the primary microseisms.
A seismic array consisting of nine Hawaii Institute of Geophysics (HIG) ocean bottom seismometers (OBSs) was deployed at the eastern intersection of the Oceanographer Fracture Zone (OFZ) and the Mid‐Atlantic Ridge (MAR). The 12‐day experiment was designed to relate low‐magnitude earthquakes to the structure and tectonics of the MAR‐OFZ intersection. An average of 10 locatable events with duration‐based magnitudes between −1.0 and 2.0 were recorded per day. Excellent hypocentral locations of 112 events were obtained. Earthquake locations based on more than eight observations generally show 50% confidence volume constraints within OBS location errors. The earthquake locations cover a broad swath across the corner of the intersection zone. Magnitude‐weighted earthquake location likelihood maps suggest a decline in magnitudes near the intersection bathymetry low. Composite focal plane solutions suggest source mechanisms which indicate that the region is dominated by extensional tectonics. Alternative source solutions indicating translational movement are presented but are inconsistent with apparent bathymetric trends. The transition from the diverging (MAR) to translational (OFZ) plate margin occurs in the context of reduced magma genesis and crustal thinning due to the influence of the adjacent older lithosphere. The region may be described in terms of semirigid plate tectonics accompanying transform valley genesis.
High‐quality seismic data recorded by a three‐component, ocean borehole seismometer are used to determine the horizontal geophone azimuths to within ±1.5°. Analysis of water waves and first arrivals from 22 explosive charges fired to the downhole instrument show the effects of tilted sensors, shot mislocation, and dip of sediment‐basement interface. Analysis also shows that the ocean subbottom seismometer (OSS IV) borehole package is not positioned below the sediment‐basalt interface as previously believed but is actually locked in the sediments at some distance above the basement. The results of this study indicate that carefully located shots can be used to obtain geophone sensor azimuths to within ±1.0°, a precision adequate for most seismic and acoustic purposes.
Persistent low frequency noise between 10 and 200 mHz due to storm systems is commonly observed as microseisms on seismic records from land and ocean bottom detectors. We report on an analysis of 20‐second microseisms recorded simultaneously on two land‐based long‐period arrays (the Alaskan Long Period Array and the Large Aperture Seismic Array) during November 1973. Azimuths of approach are determined by applying frequency wave‐number analysis and beam‐forming techniques to coherent bandpass‐filtered samples of the microseismic noise field recorded by the arrays. Microseismic source azimuths exhibit sufficient stability over periods of one hour to permit determination of reliable source locations by triangulation with the two arrays. Locations for two microseism noise sources associated with two separate Atlantic and Pacific pelagic storms were found simultaneously with these methods. In both cases, the microseismic noise source appears to be associated with nearshore processes. Although the gross spectral character of the microseisms displays the commonly observed primary‐ and double‐frequency microseism peaks, slight spectral differences are apparent for the two noise sources.
A permanent Ocean Sub-bottom Seismometer (OSS IV) designed and built by the Hawaii Institute of Geophysics was successfully installed in DSDP Hole 581C during Leg 88. The system consists of a borehole package, electromechanical cable, recorder package, recovery rope, and buoy. The borehole package contains vertical and horizontal geophones, tilt meters, and a temperature sensor. Signals from these sensors are multiplexed, digitized, and telemetered through the electromechanical cable to the recorder package near the ocean bottom, approximately 4 km from the borehole. Data are recorded continuously for up to two months on magnetic tape cassettes. The digital data are also monitored during installation and during servicing of the recorder package, which may be performed with a midsized oceanographic ship (>50 m) at any time after the installation.Data in digital format from the OSS IV installation were recorded during installation in September, 1982, and the recorder package was first serviced in May 1983. Data continuously recorded for 65 days were recovered from the first recorder package. A second recorder package was installed in May 1983 and is presently awaiting funding for servicing.
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