Fault tectonics and earthquake hazards in parts of southern California

The author has identified the following significant results. Four previously unknown faults were discovered in basement terrane of the Peninsular Ranges. These have been named the San Ysidro Creek fault, Thing Valley fault, Canyon City fault, and Warren Canyon fault. In addition fault gouge and breccia were recognized along the San Diego River fault. Study of features on Skylab imagery and review of geologic and seismic data suggest that the risk of a damaging earthquake is greater along the northwestern portion of the Elsinore fault than along the southeastern portion. Physiographic indicators of active faulting along the Garlock fault identifiable in Skylab imagery include scarps, linear ridges, shutter ridges, faceted ridges, linear valleys, undrained depressions and offset drainage. The following previously unrecognized fault segments are postulated for the Salton Trough Area: (1) An extension of a previously known fault in the San Andreas fault set located southeast of the Salton Sea; (2) An extension of the active San Jacinto fault zone along a tonal change in cultivated fields across Mexicali Valley ( the tonal change may represent different soil conditions along opposite sides of a fault). For the Skylab and LANDSAT images studied, pseudocolor transformations offer no advantages over the original images in the recognition of faults in Skylab and LANDSAT images. Alluvial deposits of different ages, a marble unit and iron oxide gossans of the Mojave Mining District are more readily differentiated on images prepared from ratios of individual bands of the S-192 multispectral scanner data. The San Andreas fault was also made more distinct in the 8/2 and 9/2 band ratios by enhancement of vegetation differences on opposite sides of the fault. Preliminary analysis indicates a significant earth resources potential for the discrimination of soil and rock types, including mineral alteration zones. This application should be actively pursued

Oceanus.

v. 17, winter (1973-1974

Toward Regional Characterizations of the Oceanic Internal Wavefield

Many major oceanographic internal wave observational programs of the last 4 decades are reanalyzed in order to characterize variability of the deep ocean internal wavefield. The observations are discussed in the context of the universal spectral model proposed by Garrett and Munk. The Garrett and Munk model is a good description of wintertime conditions at Site-D on the continental rise north of the Gulf Stream. Elsewhere and at other times, significant deviations in terms of amplitude, separability of the 2-D vertical wavenumber - frequency spectrum, and departure from the model's functional form are noted. Subtle geographic patterns are apparent in deviations from the high frequency and high vertical wavenumber power laws of the Garrett and Munk spectrum. Moreover, such deviations tend to co-vary: whiter frequency spectra are partnered with redder vertical wavenumber spectra. Attempts are made to interpret the variability in terms of the interplay between generation, propagation and nonlinearity using a statistical radiative balance equation. This process frames major questions for future research with the insight that such integrative studies could constrain both observationally and theoretically based interpretations

The generation, propagation, and mixing of oceanic lee waves

Lee waves are generated when oceanic flows interact with rough seafloor topography. They extract momentum and energy from the geostrophic flow, causing drag and enhancing turbulent mixing in the ocean interior when they break. Mixing across density surfaces (diapycnal mixing) driven by lee waves and other topographic interaction processes in the abyssal ocean plays an important role in upwelling the densest waters in the global ocean, thus sustaining the lower cell of the meridional overturning circulation. Lee waves are generated at spatial scales that are unresolved by global models, so their impact on the momentum and buoyancy budgets of the ocean through drag and diapycnal mixing must be parameterised. Linear theory is often used to estimate the generation rate of lee waves and to construct global maps of lee wave generation. However, this calculation and subsequent inferences of lee wave mixing rely on several restrictive assumptions. Furthermore, observations suggest that lee wave mixing in the deep ocean is significantly overestimated by this theory. In this thesis, we remove some common assumptions at each stage of the lee wave lifecycle to investigate the reasons for this discrepancy and to motivate and inform future climate model parameterisations. Firstly, we investigate the way that seafloor topography is represented in lee wave parameterisations, finding that typical spectral methods can lead to an overestimate of wave energy flux. Next, we make the case for considering lee waves as a full water column process by modelling the effect of vertically varying background flows and the ocean surface on lee wave propagation. Finally, we take a holistic view of topographic mixing in the abyssal ocean, finding that deep stratified water mass interfaces may modify the nature of the lee wave field, and themselves contribute to mixing and upwelling in the deep ocean through topographic interaction.Open Acces

Interpretation of the aeromagnetic anomalies of mainland Scotland using pseudogravimetric transformation and other methods

A procedure to upward continue magnetic anomalies observed on an irregular surface onto a horizontal plane has been developed and applied to the aeromagnetic map of Great Britain. Pseudogravimetric transformation was then carried out on this reduced anomaly and both data sets have been used for analysis and interpretation of several prominent anomalies in Scotland along the Great Glen fault and over the Midland Valley. A prominent linear positive magnetic anomaly occurring along the Great Glen fault has been modelled as due to a locally magnetized outward dipping body almost symmetrical about its apex beneath the fault line, together with a magnetized crustal slab to the northwest of the fault. The outward dipping body has its top lying within the upper crust, a magnetization of greater than about 1.0 A/m, a half-width of about 40 km at its base and a thickness of the order of 7-18 km. The origin of the outward dipping magnetized body may possibly be explained by metamorphism produced by frictional heating resulting from the transcurrent fault movement. Alternatively the metamorphism may be associated with some other fault related process such as crustal fluid flow. Thermal modelling has been used to demonstrate this. The magnetization contrast across the fault may be the direct result of blocks of differing magnetization on opposite side, juxtaposed as a result of transcurrent movement. The modelling along a profile over the Clyde Plateau (Midland Valley of Scotland) using a well-constrained lava body reveals the presence of a long wavelength anomaly component due to a deeper crustal source. The basement anomaly is conspicuous on the pseudogravimetric map but not on the aeromagnetic map. A near circular magnetic anomaly near Bathgate in the Midland Valley can be explained by an unexposed intrusive body superimposed on the deep crustal source as above

Analysis of teleseismic P waves with a 5200-station array in Long Beach, California: Evidence for an abrupt boundary to Inner Borderland rifting

We analyze teleseismic P waves from four Mw ≥ 6.5 earthquakes recorded by a petroleum industry survey in Long Beach, California. The survey used a 2-D array with up to 5200 seismometers, 120 m mean spacing, and 7 – 10 km aperture. At frequencies near 1 Hz, P wave travel times and amplitudes exhibit coherent lateral variations over scales as short as ~400 m, including locally delayed travel times and increased amplitudes at the crest of the Long Beach anticline. Deeper heterogeneity is indicated by P wave phase velocities that deviate from reference model predictions for events from southwestern azimuths. We postulate that a sharp northeastward increase in Moho depth from the Inner Borderland (IB) to mainland southern California causes the anomalous phase velocities. Elastic forward modeling finds the travel times are fit well by a Moho that dips 65° to the northeast and flattens ~10 km southwest of the Newport-Inglewood fault zone. Constraining the felsic thickness of mainland crust to 28 km requires an 8 km thick layer with a P-velocity of 7 km/s beneath it, which could result from basal accretion of former Farallon ocean crust or magmatic underplating during Miocene volcanism. Forward models with a 65° Moho dip predict a P-to-s conversion with a phase velocity of ~5 km/s. Deconvolution of the array's mean P wave signal isolates a similar later arriving phase. The steep crust thickness transition supports a locally abrupt boundary to IB rifting. Our results highlight the utility of dense short-period arrays for passive imaging at near surface to uppermost mantle depths

Local site seismic response in an alpine valley: seismic microzonation of the Castel Caldes area (North-western Trentino)

The purpose of this work is to analyse the local-site seismic response in the Castel Caldes area, located in an alpine valley (Val di Sole) of the north-western Trentino region. The final result is the seismic microzonation map of this area. The seismic microzonation aims to characterize at a small scale two contiguous localities in which the seismic motion is amplified (and how), due to the lithostratigraphic and geomorphological specific characteristicsope