Seismic attenuation in upper ocean crust at Hole 504B

Online Access: Get full text
doi: 10.1029/98JB02124
Author(s): Swift, Stephen A.; Lizarralde, D.; Stephen, Ralph A.; Hoskins, Hartley
Author Affiliation(s): Primary:
Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA, United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 103(B11), p.27,193-27,206. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 70 refs.; illus., incl. 2 tables
Summary: Seismic attenuation and its relationship to borehole stratigraphy in the upper 1.8 km of ocean basement at Hole 504B are determined from analysis of vertical seismic profile (VSP) data. VSP data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust, and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading, scattering, and intrinsic loss mechanisms to this decay. About 60% of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts. The remaining amplitude decrease is concentrated in two layers, at 500-650 and 800-900 meters below seafloor (mbsf) (225-375 and 525-625 m below basement), across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified. Attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by Q of 10 and 8, respectively. It is likely that the Q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes. The shallow interval coincides with a change in alteration mineralogy deposited by late-stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off-axis flow through large lava tubes. Intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting. There is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf (∼725 m subbasement). Copyright 1998 by the American Geophysical Union.
Year of Publication: 1998
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; 20 Geophysics, Applied; Attenuation; Boreholes; Crust; DSDP Site 504; Deep Sea Drilling Project; Frequency; Geophysical methods; Geophysical profiles; IPOD; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Nazca Plate; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Plate tectonics; Propagation; Reflection methods; Sea-floor spreading; Seismic methods; Seismic profiles; Spreading centers; Upper crust; Vertical seismic profiles; Wave dispersion
Coordinates: N011335 N011338 W0834348 W0834357
N011203 N011400 W0834000 W0834500
N011338 N011338 W0834349 W0834349
N011338 N011338 W0834349 W0834349
N011300 N011340 W0834320 W0834400
N011000 N020000 W0834000 W0835000
N003000 N014000 W0834000 W0863000
N011337 N011338 W0834348 W0834349
S192706 N011400 W0834349 W1294614
Record ID: 1999033422
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute.