David A. Okaya

Frequency-time decomposition of seismic data using wavelet-based methods

Spectral analysis is an important signal processing tool for seismic data. The transformation of a seismogram into the frequency domain is the basis for a significant number of processing algorithms and interpretive methods. However, for seismograms whose frequency content vary with time, a simple 1-D (Fourier) frequency transformation is not sufficient. Improved spectral decomposition in frequency-time (FT) space is provided by the sliding window (short time) Fourier transform, although this method suffers from the time-frequency resolution limitation. Recently developed transforms based on the new mathematical field of wavelet analysis bypass this resolution limitation and offer superior spectral decomposition. The continuous wavelet transform with its scale-translation plane is conceptually best understood when contrasted to a short time Fourier transform. The discrete wavelet transform and matching pursuit algorithm are alternative wavelet transforms that map a seismogram into FT space. Decomposition into FT space of synthetic and calibrated explosive-source seismic data suggest that the matching pursuit algorithm provides excellent spectral localization, and reflections, direct and surface waves, and artifact energy are clearly identifiable. Wavelet-based transformations offer new opportunities for improved processing algorithms and spectral interpretation methods.

Anisotropy of schists: Contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations

We have made sets of five independent compressional and shear wave velocity measurements, which with density, allow us to completely characterize the transverse isotropy of samples from five metamorphic belts: the Haast schist terrane (South Island, New Zealand), Poultney slate, Chugach phyllite, Coldfoot schist, and Pelona schist (United States). These velocity measurements include compressional wave velocities for propagation parallel, perpendicular, and at 45° to the symmetry axis, shear wave velocity for propagation and particle motion perpendicular to the symmetry axis, and shear wave velocity for propagation parallel to the symmetry axis. Velocity measurements were made up to pressures of 1 GPa (∼35-km depth) where microcracks are closed and anisotropy is due to preferred mineral orientation. Our samples exhibit compressional wave anisotropy of 9–20% as well as significant shear wave splitting. Metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments. Our data suggest that a significant thickness (10–20 km) of appropriately oriented (steeply dipping foliation) schist in the crust could contribute as much as 45% of observed shear wave splitting. Our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data.

Preliminary results from a geophysical study across a modern continent-continent collisional plate boundary - the Southern Alps, New Zealand

Abstract The Southern Alps of South Island, New Zealand, is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement. A joint US-NZ geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three-dimensional model of the deformation. The measurements undertaken include active source and passive seismology, magnetotelluric and electrical studies, and petrophysics. Preliminary models for the active source seismic measurements across South Island confirm, in general terms, a thickened crust under the Southern Alps, a high-velocity lower crustal layer, and a major crustal discontinuity associated with the Alpine fault. The anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements, shear wave splitting and resistivity. The mid-crust under the Southern Alps coincides with a major electrical conductivity high, which possibly corresponds to fluid in the crust. The top lies at about 15 km, close to the base of shallow seismicity east of the Alpine fault. Offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to South Island. These data are showing complex structure, particularly off western and southeastern South Island. The complexity in structure, high-quality data and consistency in results from several techniques indicates that the South Island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular, and the continental lithosphere in general.

Teleseismic P wave delays and modes of shortening the mantle lithosphere beneath South Island, New Zealand

A high-speed zone in the mantle directly beneath the Southern Alps of New Zealand is required by the recorded pattern of teleseismic P waves. Two parallel lines of 80 seismographs spaced at ∼2 km intervals recorded three earthquakes from the western Pacific with epicentral distances of 52°, 53° and 78°. Azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines. Differences between measured delays and those predicted from the crustal structure reach 0.8 s along one line and 1.0 s along the other, with the rays for the earliest arriving signals passing the depth of ∼120 km beneath the center of the island. Assuming these early arrivals are due to structure within the mantle shallower than 200 km, they imply that the core of the high-speed zone lies beneath the thickest crust, which has been shortened by ∼100 km of convergence during the past 6–7 Myr. Although the shape and position of the high-speed body cannot be fixed uniquely, a roughly symmetric body centered about a depth of 120 km, 80–100 km wide, with a depth extent of 100 km and with a maximum speed advance of ∼7% satisfies the observations. The pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other. Rather, the residuals favor thickening of mantle lithosphere by a more homogenous straining of it, as if mantle lithosphere beneath continental crust behaved as a continuum. An excess mass in the mantle is also required by the observed gravity anomalies, once allowance is made for the seismically determined crustal thickness. This high-density mantle anomaly provides sufficient force (per unit length) to maintain the crustal root, which is approximately twice as thick as that necessary to support the topography.

Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California

A seismic refraction and low-fold reflection survey, known as the Los Angeles Region Seismic Experiment (LARSE), was conducted along a transect (line 1) extending from Seal Beach, California, to the Mojave Desert, crossing the Los Angeles and San Gabriel Valley basins and San Gabriel Mountains. The chief result of this survey is an interpreted cross section that addresses a number of questions regarding the crustal structure and tectonics of southern California that have been debated for decades and have important implications for earthquake hazard assessment. The results (or constraints) are as follows. (1) The maximum depth of the Los Angeles basin along line 1 is 8–9 km. (2) The deep structure of the Sierra Madre fault zone in the northern San Gabriel Valley is as follows. The Duarte branch of the Sierra Madre fault zone forms a buried, 2.5-km-high, moderately north dipping buttress between the sedimentary and volcanic rocks of the San Gabriel Valley and the igneous and metamorphic rocks of the San Gabriel Mountains. (For deeper structure, see following.) (3) There are active crustal decollements in southern California. At middle-crustal depths, the Sierra Madre fault zone appears to sole into a master decollement that terminates northward at the San Andreas fault and projects southward beneath the San Gabriel Valley to the Puente Hills blind thrust fault. (4) The dip and depth extent of the San Andreas fault along line 1 dips steeply (∼83°) northward and extends to at least the Moho. (5) The subsurface lateral extent of the Pelona Schist in southern California is as follows. Along line 1, the Pelona Schist underlies much, if not all of the San Gabriel Mountains south of the San Andreas fault to middle-crustal depths. North of the San Andreas fault, it is apparently not present along the transect.

Low seismic-wave speeds and enhanced fluid pressure beneath the Southern Alps of New Zealand

A region of low seismic-wave speed is detected beneath the central Southern Alps of New Zealand on the basis of traveltime delays for both wide-angle reflections and P-waves from teleseismic events. Respective ray paths for these P-waves are mutually perpendicular, ruling out anisotropy as a cause of the delays. The low-speed region measures about 25 km by 40 km, has a speed reduction of 6%–10%, and is largely above the downward projection of the Alpine fault. The most likely cause of the low-speed zone is high fluid pressure due to excess water being released by prograde and strain-induced metamorphism into the lower crust. Because enhanced fluid pressure reduces the work required for deformation, the existence of the central Southern Alps low-speed zone implies that this part of the Australian-Pacific plate boundary is relatively weak.

Magma addition and flux calculations of incrementally constructed magma chambers in continental margin arcs: Combined field, geochronologic, and thermal modeling studies

Incrementally constructed magma systems have been recognized from studies of the resulting plutons for more than three decades. However, magma addition rates, fluxes, growth durations, sizes of increments, and sizes and durations of the resulting magma chambers have been difficult to ascertain, emphasizing the need for a better understanding of how magmatic systems evolve. Our results from studies of plutons and arc sections in the North American Cordillera indicate that a large range exists in all of these values. Although arc sections and individual plutons clearly have dramatic temporal changes in volumetric magma additions, true volumetric flux calculations are particularly difficult to determine. Thus, although subduction beneath arcs may have active durations of hundreds of millions of years, volumetrically most magmatism is emplaced during magmatic flare-ups of ∼10–30 m.y. duration. Individual plutons and batholiths in these arcs can grow in

Magmatic lobes as "snapshots" of magma chamber growth and evolution in large, composite batholiths: An example from the Tuolumne intrusion, Sierra Nevada, California

Precise chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages in combination with detailed field mapping, 40 Ar/ 39 Ar thermochronology, and finite difference thermal modeling in the magmatic lobes of the Tuolumne batholith characterize these 10–60 km 2 bodies as shorter-lived, simpler magmatic systems that represent increments of batholith growth. Lobes provide shorter-term records of internal and external processes that are potentially obliterated in the main body of long-lived, composite batholiths. Zircon ages complemented by thermal modeling indicate that lobe-sized magma chambers were present between ∼0.2 and 1 m.y., representing only a small fraction of the total duration of melt presence in the main body. During these shorter intervals, a concentric pattern of normal compositional zoning formed during inward crystallization and widespread zircon recycling in the lobes. Lobes largely evolved as individual magma bodies that did not interact significantly with the main, more complex magma chamber(s). Antecrystic zircons and the range of autocrysts, used to track the extent of interconnected melt, record only a limited range of ages and have contrasting zircon populations to those found in the same units in the main batholith. We consider lobes to either be single batches formed during continuous magma flow or multiple, quickly coalescing pulses that in either case formed separate magma chambers that failed to amalgamate with other compositionally distinct pulses such as those occurring in the central batholith. Zircon age comparisons between all four lobes and the main body imply that growth of the Tuolumne intrusion was not stationary, but that the locus of magmatism shifted both inward and northwestward.

Crustal anisotropy in the vicinity of the Alpine Fault Zone, South Island, New Zealand

Abstract Petrophysical measurements of rock samples collected within the Haast, Torlesse, and Alpine Fault Zone terranes of the South Island of New Zealand indicate significant seismic P‐wave velocity anisotropy at pressures representing depths of up to 30 km. The percentage of anisotropy increases with increasing metamorphic grade and thus decreases with structural distance from the Alpine Fault. A maximum anisotropy of 17.3% was obtained from a drill‐core sample located within the garnet‐oligoclase zone schist, immediately adjacent to the Alpine Fault. Shear‐wave splitting is another important property of the schists. For propagation parallel to foliation, split shear waves show velocity differences up to 1 km/s. At elevated pressures, the measured seismic velocity anisotropy is caused by preferred mineral orientation and is not due to the presence of cracks. The pronounced velocity anisotropy will significantly affect propagating seismic waves collected during both natural and active source seismic exp...

Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II

We have constructed a composite image of the fault systems of the M 6.7 San Fernando (1971) and Northridge (1994), California, earthquakes, using industry reflection and oil test well data in the upper few kilometers of the crust, relocated aftershocks in the seismogenic crust, and LARSE II (Los Angeles Region Seismic Experiment, Phase II) reflection data in the middle and lower crust. In this image, the San Fernando fault system appears to consist of a decollement that extends 50 km northward at a dip of ∼25° from near the surface at the Northridge Hills fault, in the northern San Fernando Valley, to the San Andreas fault in the middle to lower crust. It follows a prominent aseismic reflective zone below and northward of the main-shock hypocenter. Interpreted upward splays off this decollement include the Mission Hills and San Gabriel faults and the two main rupture planes of the San Fernando earthquake, which appear to divide the hanging wall into shingle- or wedge-like blocks. In contrast, the fault system for the Northridge earthquake appears simple, at least east of the LARSE II transect, consisting of a fault that extends 20 km southward at a dip of ∼33° from ∼7 km depth beneath the Santa Susana Mountains, where it abuts the interpreted San Fernando decollement, to ∼20 km depth beneath the Santa Monica Mountains. It follows a weak aseismic reflective zone below and southward of the main-shock hypocenter. The middle crustal reflective zone along the interpreted San Fernando decollement appears similar to a reflective zone imaged beneath the San Gabriel Mountains along the LARSE I transect, to the east, in that it appears to connect major reverse or thrust faults in the Los Angeles region to the San Andreas fault. However, it differs in having a moderate versus a gentle dip and in containing no mid-crustal bright reflections.