Graduation date: 1975

No description available.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1984. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 221-226).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1984. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 178-187).

Thesis (M.S.)--University of Wisconsin--Madison, 1985. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 30-36).

University deployed two arrays of three sonobuoys over the Blanco fracture zone and two arrays of four sonobuoys over the Gorda ridge to detect and locate microearthquakes. Microearthquake activity predicted by plate tectonic theory for the Blanco fracture zone and Gorda ridge was observed by these arrays to originate from these features. Microearthquake activity of one event per hour was observed along the southern flank of the ridge associated with the Blanco fracture zone near 128°10'W. Similar seismicity was observed in Cascadia Gap and associated with the fault scarp and basin of the gap. Cascadia Gap, which is located near the center of the Blanco fracture zone, exhibits ridge-like seismic activity based on a b-value of 1.5 determined from microearthquake observations. The seismic activity on the Gorda ridge, including swarm events, averaged 3.5 events per hour. The microearthquakes originated from the median valley floor, valley walls, and on top of the crestal hills. Other events, which could not be located, appeared to originate from the surrounding hills with a predominance of events from west of the Gorda-Blanco intersection. Focal depths at the intersection area are 6.5 to 9 km below a 3.5 km datum, while those farther to the south at 42°41'N range from 2.5 to 3 km below the datum. A composite fault plane solution for the events on the Gorda ridge indicates high angle faulting on the eastern valley wall with the eastern side down. Since microearthquake and earthquake activity are usually closely associated, the data obtained by this study suggests that the 20-30 km eastward offset of large earthquake epicenters from the topographic features is due to an incorrect use of excessively high lithospheric velocities for seismic stations lying predominantly to the east. / Graduation date: 1976

No description available.

A zone of crustal weakness associated with the Zuni Lineament in the southern half of New Mexico has been delineated by lead isotope measurements, a microearthquake study and magnetic depth soundings. The lead results of Slawson and Austin (1962) revealed the possible existence of a northwest trending zone of isotopically similar lead, bounded on either side by isotopically variable leads. This feature passes through Socorro and Alamogordo, New Mexico, and is coincident in this area with the Zuni Lineament. The microearthquake results (Sanford and Long, 1965) show that a shallow crustal zone of weakness is associated with the isotopically similar zone. The magnetic results (Schmucker, 1964; Livingstone, 1966) confirm that the structure is shallow, and also indicate that it may be a region of high electrical conductivity. It is suggested that this zone of weakness centred on the Zuni Lineament has served as a conduit system for hydro-thermal mineralizing solutions which carried the lead to its final sites of deposition. The lead was originally introduced into the Precambrian rocks about 1550 m.y. ago, mixed with radiogenic rock lead of that age about 30 m.y. ago, and then, finally emplaced. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The seismicity at the northern section of the Gorda Ridge has been studied with Ocean Bottom Seismographs. Nearly one hundred earthquakes were located with RMS travel time residuals less than 0.5 seconds. Most of these events lie within or near the array and they are probably associated with the dynamics of crustal formation which creates the observed bench-like features in the Gorda Ridge. An average of 5 microtremors per hour can be detected in the median valley with a lower activity outside of the ridge. Considerable clustering in time can be seen and it is typical of a main-shock sequence with the largest shock at or very near the beginning followed by a progressive decay in number. Clustering was spatial as well as temporal and the records indicate that they have similar mechanisms. A brittle zone of 18 km thickness at 42°N and 13 km at 42.5°N was found implying any possible magma chamber must be at greater depth. These thicknesses are larger than those observed in the Mid Atlantic ridge and in the East Pacific Rise. They may be explained by the depression of isotherms due to the heat lost by the contact of the ridge with the old and cold plates across the Blanco and the Mendocino Fracture Zones. Comparison of earthquakes locations using only OBS with those obtained from land stations indicate a major P-delay for the Gorda Basin. The seismic activity appears to decrease markedly to the south of 42°N. This drop in activity indicates that the southern part of the ridge is more stable than the northern part. This decrease in activity is not consistent with a southward propagating rift if we accept that higher seismicity levels are expected at the tip of the propagating ridge segments. / Graduation date: 1982

In June and July of 1980 a 12 station microseismic detection network was established over a 40 km² area about the Sullivan Mine, Kimberley, B.C. for the purpose of using hypocentre locations of mining induced earthquakes to delineate the position of fault planes in 3-dimensions. During the experiment 1551 microseismic events were recorded by 2 or more digital seismographs, but only 366 events were large enough to be recorded by 4 or more stations. A regional velocity of 5.30 kms⁻¹ was determined from calibration explosions and a 3-dimensional least squares type location program was used. Based on explosions of known location within the mine a location accuracy of 180 m in epicentral position and 500 m in focal depth was attained. Recorded microseismic activity occurred entirely within the mine bounds, centered on the active working areas. An extremely close relationship between level of microseismic activity and mining cycles was observed. The seismic array at the Sullivan Mine was found to be unsuitable for use as a geologic mapping tool, partly because of the lack of resolution. However several interesting features of the induced microseismicity have become apparent: (i) Influence of mining activity at the Sullivan is extremely localized, with microearthquake activity confined to the working areas. (ii) The correlation between mine activity and recorded microseismic activity was extremely high; the majority of microearthquakes, in particular the smaller events, occurred in the first ten minutes after large mine blasts. These appear to be cavity relaxation events. (iii) There appears to be an association between the epicentral locations and the edges of cave areas. The only activity which may be associated with faults occurs when they are quite proximal to the cave edges; that is where tensional stresses in the hanging wall are localized along these pre-existing planes of weakness. (iv) It is believed that regional stress conditions are unsuitable to allow fault reactivation below the mine. Geologic evidence suggests' that the regional stress in the vicinity of the Sullivan is either neutral or extensional, in which case fault reactivation below the mine will be suppressed by the load removal caused by mining. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate