An airgun refraction line along the length of the caldera of Axial Seamount as
recorded by three Ocean Bottom Seismometers has been analyzed using the tau-zeta
inversion technique (Dorman and Jacobson, 1981). Five resulting velocity profiles
show that the seismic velocities of the upper 1.4 km of the crust are low and similar to Mid-Atlantic Ridge upper crustal velocities. The low velocities and failure to observe shear waves are thought due to a combination of a thick section of pillow basalts and sheet flows, and a high porosity. The porosity is believed mostly due to a large fracture density. Hammond (pers. comm., 1986) has proposed the caldera is the site of overlapping spreading centers. A high fracture density in the upper crust of the caldera could be due to stresses generated in the area of overlap (Macdonald et al., 1984). No systematic variation in velocity structure along the length of the caldera
could be resolved.
Shallow water (about 1560 m), smooth bathymetry and the absence of
sediments allowed direct measurement of the surface velocities of the caldera floor.
An average surface velocity of 3.05 km/s is observed. Assuming a highly simplified
crustal model, this velocity translates into a minimum porosity of 30% near the surface of the caldera floor. This minimum porosity is predicted to gradually decrease to about 0% at 1.4 km depth. Upper limits on the porosity can not be found with the available information.
No magma chamber has been observed to a depth of 1.4 km. A
compressional wave attenuation source in the northwest corner of the caldera below a
depth of 1.4 km, however, might be a small magma body or alternatively an
anomalously highly fractured area. / Graduation date: 1987
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28997 |
| Date | 25 June 1986 |
| Creators | van Heeswijk, Marijke |
| Contributors | Bibee, L. Dale |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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