Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982. / Microfiche copy available in Archives and Science / Two folded maps in pocket. / Includes bibliographies. / by Kip Vernon Hodges. / Ph.D.

Arrival times from intermediate-depth (110-150 km) earthquakes within the region of flat subduction beneath central Peru provide constraints on the geometry and velocity structure of the subducting Nazca plate. Hypocenters for these events, which are beneath the sub-andean and eastern Peruvian basins, were determined using a best-fitting onedimensional velocity-depth model with a 15-station digitally-recording network deployed in the epicentral region. For that model, P-wave travel times to coastal stations, about 6° trenchward, exhibit negative residuals of up to 4 seconds and have considerably more complexity than arrivals at the network stations. The residuals at coastal stations are conjectured to result from travel paths with long segments in the colder, higher velocity subducting plate. Travel time anomalies were modeled by 3-D raytracing. Computed ray paths show that travel times to coastal stations for the eastern Peru events can be satisfactorilymodeled if velocities relative to the surrounding mantle are 6% lower within the uppermost slab (a 6 km thick layer composed of basaltic oceanic crust) and 8% higher within the cold peridotitic layer (which must be at least 44 km thick). Raytracing runs for this plate model show that "shadow zones" can occur if the source-slab-receiver geometry results in seismic rays passing through regions in which the slab undergoes significant changes in slope. Such geometries exist for seismic waves propagating to some coastal stations from sources located beneath the eastern Peruvian basin. Observed first-arrival times for such cases do in fact have less negative residuals than those for geometries which allow for \direct\ paths. Modeling such arrivals as trapped mode propagation through the high-velocity part of the plate produces arrival times consistent with those observed. / Master of Science

Focal mechanisms of earthquakes, two of them in the Gulf of Antalya and one in the eastern Aegean Sea, were determined by using both P-wave polarities and S-wave polarization angles. Additionally, a focal mechanism solution for a third Gulf of Antalya earthquake was determined from P-wave polarities only. The polarity data were read from long period vertical (LPZ) seismograms of World Wide Standard Seismograph Network (WWSSN) stations. Additional polarity data were obtained from published earthquake listings. S-wave polarization angles were determined from particle motion diagrams for both the linear, and non-linear distance ranges. To determine polarization angles for the non-linear range stations, a correction technique by Nuttli (1964) was applied. A revised version of the computer program written by Guinn (1977) was employed to obtain. the mechanism solutions. The focal mechanisms for the Gulf of Antalya events indicated strike-slip normal (28 May 1979 and 26 April 1981 events) and strike-slip thrust faulting (1 June 1977 event). All of the earthquakes occurred in this region are not far more than 50 kilometers from each other with focal depths ranging from 66 to 98 kilometers. The occurrence of opposite type of faults in the Gulf of Antalya indicate a complex tectonics in that region. Additionally, the focal mechanism of the Aegean Sea earthquake showed east-west striking normal faulting. The solution is in agreement with the focal mechanisms of the previous regional events (McKenzie, 1972 and Alptekin, 1973), indicating the continuation of the east-west oriented graben systems of western Turkey in the Aegean Sea. / M.S.

No description available.

published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

Paleomagnetic data were collected from Miocene volcanic rocks in the Turtle Mountains, Clipper Mountain, Colton Hills, and Piute Range of the southern Basin and Range (SBR) province in southeastern California as well as in the Soledad Mountains of the Mojave block in southern California. The data from these two tectonic provinces yield significantly different paleomagnetic directions, which probably indicates the existence of a major crustal and /or lithospheric discontinuity in the area between the Barstow Basin and the Clipper Mountain. Comparing the mean direction from the SBR data to the Miocene expected direction indicates no statistically significant rotation (R = -0.2° ± 18.2°) or flattening (F = -6.5° ± 9.2°). A similar comparison for the Soledad Mountain data, which were combined with data of Burke et al. (1982) from the Barstow Basin, yields a significant rotation of -43.5° ± 12.9° and flattening of 19.3° ± 10.6° for the Mojave block. These Mojave block values may be exaggerated a few degrees due to inadequate averaging of secular variation and possible improper structural corrections.

The University of Canberra (UC) has been involved in GPS processing since the late 1980s. This processing commenced with the GOTEX 1988 campaign and progressed through a series of project specific regional campaigns to the current daily processing of a distributed set of continuously operating sites for the determination of precise GPS station positions for user applications. Most of these earlier campaigns covered only short periods of time, ranging from a few weeks to multiple occupations of a few days to a time over one to two years. With software developments, these multiple occupations were able to be combined to produce results from which crustal motion velocities could be extracted. This first became feasible with the processing of the Australian National Network (ANN), which yielded realistic tectonic velocities from two occupations (1992 and 1993) of sites 12 months apart. Subsequently, this was successfully extended by a further 12 months, with re-occupation of certain sites for a third time in 1994. Analysis of the results indicated that the accuracy of determining the earth signals improved as the time span from first to last observation was increased. The same was true also for the determination of the position of global references sites. However, by current standards the results achieved were poor. Consequently, the process was extended to combine the results of subsequent campaigns with the original ANN data set. From 1995 to 1999, campaigns were conducted across Australia, covering many State and tide gauge sites included in the original ANN solution. These provided additional multiple occupations to improve the determinations for both position and velocity. UC has maintained a data set of the global IGS sites, commencing with the IGS pilot campaign of 1992. Daily data sets for those global sites, which contained days common to the regional campaigns, were processed to produce our own independent global orbit and reference frame connection. The motivation for doing so was fourfold. �Firstly, to see if historic data could be reprocessed using current modern software and thus be able to be incorporated in this and other analysts research programs. �Secondly, to compare the results of the reprocessing of the original data set using modern software with the original ANN solution and then validate both the solutions. �Thirdly, to extend the timespan of observations processed to include more recent campaigns on as many original sites as possible. This to achieve a stronger solution upon which to base the determination of an Australian tectonic plate velocity model and provide quality assurance on the solution comparisons with re-observed sites. �Fourthly, to develop a set of transformation parameters between current coordinate systems and the GDA94 system so as to be able to incorporate new results into the previous system. The final selection of regional and global sessions, spanning from mid 1992 to late 2002, contained almost 1000 individual daily solutions. From this 10 year data span a well determined rigid plate tectonic motion model was produced for Australia. This site velocity model was needed to develop a transformation between the thesis solution in ITRF00 an the GDA94 solution in ITRF92. The significant advantage of the plate velocity model is that all Australian sites can now have computed a realistic velocity, rather than being given a value which has been interpolated between sites whose velocities had been determined over a one or two year span. This plate velocity model is compared with the current tectonic motion NNR-NUVEL-1A model and other recently published models. To perform the comparison between the thesis solution in ITRF00 and the GDA solution in ITRF92 a transformation was developed between the two reference systems. This set of transformation parameters, in conjunction with the plate velocity model developed, enables site solutions at any epoch in the current ITRF00 to be converted onto the GDA94, and vice versa, with a simple, non-varying seven parameter transformation. The comparisons between the solutions are analysed for both horizontal position and height consistency. There were 77 sites whose differences were compared. The horizontal consistency was within estimated precisions for 75 of the 77 sites. However, the vertical comparisons revealed many of the single epoch sites, especially in 1992, have inconsistent results between the two solutions. The heights from this thesis for some West Australian sites were compared with analysis done by DOLA and the height recoveries are very similar, indicating a weakness in the GDA94 solution for some of the single epoch sites. Some of these differences have been resolved but others are still under investigation. This thesis describes the repocessing of the original ANN data set, the addition of later data sets, the results obtained, and the validation comparisons of the old and new solutions. As well as the plate velocity model, transformation is provided which enables the user to compute between the GDA94 system, and any epoch result in ITRF00. Recommendations are made as to which sites need additional work. This includes sites which only need further analysis or investigation and those which require further observations to achieve a result which will have acceptable accuracy and reliability.

Magnetic and gravity modelling was completed along two E-W transects offshore central Oregon. These models indicate that the backstop-forming western edge of the Siletz terrane has a seaward dip of approximately 40° to 49° at 44°48'N, shallowing to ~28° at 44°1 1 'N. This is a well-determined result, given available a priori information, to a depth of ~7 km. The edge of the Siletz terrane may continue to descend at these dips to the JdF plate, but alternate geometries for the lowermost portion of the backstop are also consistent with the potential field data. The magnetic data also require progressive eastward demagnetization of the subducting JdF crust, which is most likely due to heating of the descending oceanic plate to the Curie temperature. Our southern transect reveals that Heceta Bank is cored by relatively high-density sediments (~2.54 g/cc), consistent with the model proposed by Kulm and Fowler (1974) for submarine bank formation in the Oregon convergent margin. On the basis of magnetic, gravity, and velocity data, we tentatively interpret a deeply buried, linear aseismic ridge to be present beneath the accretionary complex from about 45°N to 42°N. This ridge may collide with the backstop beneath Heceta Bank and may play a role, in concert with imbricate thrust faulting, in the formation of Heceta Bank's high density core. We also speculate that differences in depth to the JdF plate due to juxtaposition of different-aged crust across pseudofaults which intersect the coast at Nehalem and Heceta Banks may be a factor in the construction of these topographic highs. / Graduation date: 1997

The Nicoya Peninsula of Costa Rica is unusually close to the Middle America Trench (MAT), such that interface locking along the megathrust is observable under land. Here, rapid convergence between the downgoing Cocos and the over-riding Caribbean plates at ~85mm/yr allows for observable high strain rates, frequent large earthquakes and ongoing micro-seismicity. By taking advantage of this ideal location, a network of 20 on-land broadband seismometers was established in cooperation between UC Santa Cruz, Georgia Tech, and OVSICORI, with most stations operating since 2008. To evaluate what seismicity tells us about the ongoing state of coupling along the interface, we must consistently evaluate the location and magnitude of ongoing micro- seismicity. Because of large levels of anthropogenic, biologic, and coastal noise, automatic detection of earthquakes remains problematic in this region. Thus, we resorted to detailed manual investigation of earthquake phases. So far, we have detected nearly 7,000 earthquakes below or near Nicoya between February and August 2009. From these events we evaluate the fine-scale frequency-magnitude distribution (FMD) along the subduction megathrust. The results from this b-value mapping‟ are compared with an earlier study of the seismicity 9 years prior. In addition, we evaluate them relative to the latest geodetically derived locking. Preliminary comparisons of spatial and temporal variations of the b-values will be reported here. Because ongoing manual detection of earthquakes is extremely laborious and some events might be easily neglected, we are implementing a match-filter detection algorithm to search for new events from the continuous seismic data. This new approach has been previously successful in identifying aftershocks of the 2004 Parkfield earthquake. To do so, we use the waveforms of 858 analyst-detected events as templates to search for similarly repeating events during the same periods that have been manually detected. Preliminary results on the effectiveness of this technique are reported. The overall goal of this research is to evaluate the evolution of stress along the megathrust that may indicate the location and magnitude of potentially large future earthquakes. To do so, I make the comparison between the FMD and the interface locking. Only positive correlations are observed in the Nicoya region. The result is different from the one derived from the seismic data set that was recorded 9 years before our data. Therefore, to substantiate the causes for the different relationships between the b-value and the coupling degree, we need additional data with more reliable magnitudes.

published_or_final_version / Earth Sciences / Master / Master of Philosophy