During July 1974, two reversed deep seismic sounding (DSS) profiles extending about 75 km were recorded in the Explorer Ridge region of the northeastern Pacific, one parallel and the other perpendicular to the ridge. A two-ship operation was used to record near-vertical incidence to wide-angle reflected waves and refracted waves with penetration from the ocean bottom to the upper mantle. Signals from six individual hydrophones suspended at 45 m depth from a 600 m cable trailed behind the receiving ship were recorded in digital form. The shooting ship detonated charges ranging from 2.3 kg to 280 kg and recorded the direct arrival plus the WWVB time code. Processing of the data recorded at distances beyond 4 km included demultiplexing, stacking, and filtering. Before the data were presented in record section form, traveltime corrections were made for topography and shot distance, and amplitude corrections were made for amplifier gain, charge size, and spherical spreading. The interpretation procedure consisted of two steps. A homogeneous, layered velocity-depth model was initially constructed from first arrival traveltime data. The p-A curve corresponding to this model was then altered until an amplitude fit was obtained using synthetic seismograms. Weichert-Herglotz integration of the resultant p-A curve produced the final velocity-depth model. This traveltime and amplitude interpretation required the introduction of velocity gradients into the model. The profile run across the ridge showed no anomalous behaviour as the ridge was crossed; the profile on the Juan de Fuca plate, paralleling the ridge, exhibited traveltime branch offsets and delays. These have been interpreted as due to faulting with a. vertical component of offset of about 5 km. The reversed upper mantle velocities are 7.8 and 7.3 km/s in directions perpendicular and parallel to the ridge. Anisotropy is proposed to explain these different velocities. Compared with crustal sections from other ridge areas, the data require a thick "layer 3" (up to 7 km) near the ridge crest. The total depth to the base of the oceanic crust varies between 10 and 12 km except in the faulted region. The results of this study favor the hypothesis that Explorer Ridge is presently an inactive spreading center. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

No description available.

The largest earthquakes and the majority of the seismic energy are released on megathrust faults in subduction zones. The goal of this dissertation is to characterize the seismic behavior, structural and physical properties of the megathrust fault, so that we can better understand the controls on slip behavior and large earthquakes. To address this goal, I analyzed seismicity data collected by a local seismic network deployed in southern Alaska and multi-channel seismic (MCS) data from an active-source survey offshore of the Alaska Peninsula. This dissertation work revealed seismicity patterns associated with a large asperity, downdip transitions in megathrust fault structure, and along-strike variations in the properties of subducting sediment on the shallow part of the subduction zone. All of these observations have important implications for seismic behavior of the megathrust.

A relocation study is made for 50 Virginia earthquakes (1959-1981) using a variation (JED/SE77) of the Joint Hypocenter Determination (JHD) method. Joint Epicenter Determinations (JED) were used in conjunction with a companion single event location program (SE77) to relocate hypocenters from data sets in the Giles County seismic zone (GCSZ) in southwestern Virginia and the central Virginia seismic zone (CVSZ). In the GCSZ six earthquakes, recorded both regionally and teleseismically and previously relocated jointly by Dewey and Gordon (1980), and nine earthquakes, recorded by a local network and individually located using the HYPOELLIPSE program (Lahr, 1979), were relocated. The JED/SE77 relocated hypocenters virtually all fall within the 70% confidence ellipses of the corresponding original hypocenters. In the CVSZ 25 events, originally located individually using HYPOELLIPSE, were relocated. The JED/SE77 relocations produce, for the most part, only small changes in epicenters (<5 km average displacement) and comparably small changes in focal depths. Geometrically, they can be grouped into three planes and one cluster of events. The majority (17/25) of the events are contained in two of the planes, both of which have an ENE trend and a 45° southerly dip. The JED/SE77 results: 1) indicate that the original HYPOELLIPSE hypocenters are quite accurate and should constitute the preferred hypocenters; 2) give added confidence to the instrumental definition of the GCSZ and the CVSZ; and 3) show by the depth distribution of foci that the entire upper portion of the crust is involved in the release of seismic energy. / Master of Science

Transportation facilities and services provide the cornerstones to the rescue and response operations after a big earthquake. This study appraises the transportation actions taken by the authorities in the immediate aftermath of the Loma Prieta Earthquake of October 17, 1989. The failure of several transportation structures had a significant impact on rescue operations, traffic congestion and change in travel patterns in the San Francisco Bay Area. Emphasis is placed on travel demand management strategies adopted to meet the travel needs in the Bay Area and to return traffic to normalcy. The short-term and long-term impacts of closures of certain highways due to a 7.5 magnitude earthquake are also addressed in this research. Recent predictions by the United States Geological Survey show that there is a 67 percent chance of a big earthquake of 7.5 magnitude happening in the Bay Area before the year 2020. Therefore, there is a dire need to look at the transportation problems that the Bay Area might face if the "Big One" really hits. It is also important to note that certain bridges play a major role in the cross-bay transportation. Hence, the failure of such critical links would greatly influence the mobility of the citizens in the region. A macro-level measure referred to as "Weighted Roadway Congestion index" (RCIW) is developed to assess the severity of the closures of these links. To fulfill this objective, scenario analysis is performed for the expected closures in the San Francisco Bay Area. It is important to note that the macro-level measure developed is applicable only to urban areas. This research also aims at identifying the key network parameters, such as number of lane-miles per freeway exit and freeway network connectivity that impact roadway congestion after earthquakes. / Master of Science

It is well known that direct surface waves of large earthquakes are capable of triggering shallow earthquakes and deep tremor at long-range distances. Recent studies have shown that multiple surface waves circling the earth could also remotely trigger microearthquakes. However, it is still not clear whether multiple surface waves returning back to the main shock epicenters could also trigger/modulate aftershock behavior. Here we conduct a study to search for evidence of such triggering by systematically examining aftershock patterns of earthquakes with magnitude ≥ 8 since 1990 that produce observable surface waves circling the globe repeatedly. We specifically examine the 2011 M9 Tohoku-Oki event using a composite catalog of JMA, HiNet and newly detected events obtained by waveform cross correlation. We compute the magnitude of completeness for each sequence, and stack all the sequences together to compute the seismicity and moment rates by sliding data windows. The sequences are also shuffled randomly and these rates are compared to the actual data as well as synthetic aftershock sequences to estimate the statistical significance of the results. Our results suggest that there is some moderate increase of early aftershock activity after a few hours when the surface waves return to the epicentral region. However, we could not completely rule out the possibility that such an increase is purely due to random fluctuations of aftershocks or caused by missing aftershocks in the first few hours after the mainshock.

M.Sc. (Geology) / There was a possibility that the rockbursts experienced in the faulty Klerksdorp Goldfield area could be related to the numerous faults, fractures and dykes occurring in the area. Until the end of 1979 it was impossible to establish with any degree of certainty which discontinuity was associated with most of the seismic events, as the location network was too inaccurate at that stage to determine such associations. The accuracy of the network was subsequently improved to 30 m in all directions in a specific area of the macro-network. Twenty seven percent of the seismic events which located in this area, occurred less than 30 m from the faults, 21 percent were less than 30 m from the dykes, while 42 percent occurred less than 30 m from dykes and faults. The remainder ( 10 percent) were located more than 30 m from any of the abovementioned discontinuities.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2007. / Includes bibliographical references (p. 194-197). / Oceanic spreading centers are sites of magmatic, tectonic, and hydrothermal processes. In this thesis I present experimental and seismological constraints on the evolution of these complex regions of focused crustal accretion and extension. Experimental results from drained, triaxial deformation experiments on partially molten olivine reveal that melt extraction rates are linearly dependent on effective mean stress when the effective mean stress is low and non-linearly dependent on effective mean stress when it is high. Microearthquakes recorded above an inferred magma reservoir along the TAG segment of the Mid-Atlantic Ridge delineate for the first time the arcuate, subsurface structure of a long-lived, active detachment fault. This fault penetrates the entire oceanic crust and forms the high-permeability pathway necessary to sustain long-lived, high-temperature hydrothermal venting in this region. Long-lived detachment faulting exhumes lower crustal and mantle rocks. Residual stresses generated by thermal expansion anisotropy and mismatch in the uplifting, cooling rock trigger grain boundary microfractures if stress intensities at the tips of naturally occurring flaws exceed a critical stress intensity factor. / (cont.) Experimental results coupled with geomechanical models indicate that pervasive grain boundary cracking occurs in mantle peridotite when it is uplifted to within 4 km of the seafloor. Whereas faults provide the high-permeability pathways necessary to sustain high-temperature fluid circulation, grain boundary cracks form the interconnected network required for pervasive alteration of the oceanic lithosphere. This thesis provides fundamental constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers. / by Brian J. deMartin. / Ph.D.

Thesis (S.M.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and, the Woods Hole Oceanographic Institution), 2004. / Includes bibliographical references (p. 37-40). / Laterally extensive, well-developed clinoforms have been mapped in Early Cretaceous deposits located in the northeastern 27,000 km2 of the Colville Basin, North Slope of Alaska. Using public domain 2-D seismic data, well logs, core photographs, and grain size data, depositional geometries within the Nanushuk and Torok formations were interpreted in order to constrain the transport conditions associated with progradation of the shoreline and construction of the continental margin out of detritus shed from the ancestral Brooks Range. Using STRATA, a synthetic stratigraphic modeling package, constructional clinoform geometries similar to those preserved in the North Slope clinoform volume (32,400 km3) were simulated. Sediment flux, marine and nonmarine diffusivities, and basin subsidence were systematically varied until a match was found for the foreset and topset slopes, as well as progradation rates over a 6 million year period. The ability of STRATA to match the seismically interpreted geometries allows us to constrain measures of possible water and sediment discharges consistent with the observed development of the Early Cretaceous clinoform suite. Simulations indicate that, in order to reproduce observed geometries and trends using constant input parameters, the subsidence rate must be very small, only a fraction of the most likely rate calculated from the seismic data. Constant sediment transport parameters can successfully describe the evolution of the prograding margin only in the absence of tectonic subsidence. However, further work is needed to constrain the absolute magnitude of these values and determine a unique solution for the NPR-A clinoforms. / by Christina Marie Kaba. / S.M.