Accession Number : ADA304874

Title :   Earthquake Activity near Ascension Island as seen by a Combined Seismic/Hydrophone Array.

Descriptive Note : Final rept. 1 Feb 93-28 Feb 96,

Corporate Author : SCRIPPS INSTITUTION OF OCEANOGRAPHY LA JOLLA CA INST OF GEOPHYSICS AND PLANET ARY PHYSICS

Personal Author(s) : Given, Holly K. ; Hanson, Jeff A. ; Berger, Jonathan

PDF Url : ADA304874

Report Date : FEB 1996

Pagination or Media Count : 25

Abstract : Data from a recently installed Global Seismographic Network station on Ascension Island in the central Atlantic Ocean (IRIS/IDA ASCN) are used in conjunction with data from a network of ocean hydrophones (MThS network) deployed around the island to detect and locate background seismicity associated with the mid-Atlantic Ridge and nearby transform faults. Locations calculated from the combined data set have errors less than 10 km, and frequently less than 3 km. A local magnitude scale based on S-wave amplitudes at the seismic station is devised. We estimate a detection threshold of about magnitude 1.7 for earthquakes occurring on the nearby part of the ridge, 120 km away; this is almost three orders of magnitude smaller than the smallest events from this region in the PDE or REB catalogs. No natural seismicity on Ascension Island itself was seen during a 200-day observation period. The seismic station regularly records large T-phases from moderate-sized events around the Atlantic. T- phases are more pronounced at IRIS/IDA station SHEL on St. Helena Island, 1290 km southeast of Ascension Island, probably due to its steeper topography. Taken together, the seismic stations ASCN, SHEL, and the MILS network represent a unique facility to monitor seismic events in the south Atlantic. (MM)

Descriptors :   *ACOUSTIC ARRAYS, *HYDROPHONES, *EARTHQUAKES, *UNDERWATER ACOUSTICS, *OCEAN BOTTOM, *SEISMIC ARRAYS, DATA BASES, FAULTS(GEOLOGY), SEISMIC WAVES, ATLANTIC OCEAN, OCEAN BOTTOM TOPOGRAPHY, OCEAN RIDGES, SEAFLOOR SPREADING, SEISMOLOGICAL STATIONS, SEISMOGRAPHS, ASCENSION ISLAND, SOUTH ATLANTIC OCEAN, MAGMA, MIDATLANTIC RIDGE.

Subject Categories : Physical and Dynamic Oceanography
      Seismology
      Acoustic Detection and Detectors
      Seismic Detection and Detectors

Distribution Statement : APPROVED FOR PUBLIC RELEASE