William J. Lutter

Surface seismic and electrical methods to detect fluids related to faulting

In the absence of drilling, surface-based geophysical methods are necessary to observe fault zones and fault zone physical properties at seismogenic depths. These in situ physical properties can then be used to infer the presence and distribution of fluids along faults, although such observations are by nature indirect and become less exact with greater depth. Multiple observations of a range of such geophysical properties as compressional and shear seismic velocity (Vp and Vs), Vp/V5 ratio (related to Poissons ratio), resistivity and attenuation in and adjacent to fault zones offer the greatest hope of making inferences of the fault zone geometry, fluids in the fault zone, and fluid reservoirs in the surrounding crust. For simple geometries, fault zone guided waves can provide information on fault zone width and velocities for faults of the order of 200 m wide. To address the question of whether a narrow fault zone can be imaged well enough at depths of seismic rupture to infer the presence of anomalously high fluid/rock ratios, we present synthetic seismic tomography and magnetotelluric examples for an ideal case of a narrow fault zone with a simple geometry, large changes in material properties, and numerous earthquakes within the fault zone. A synthetic 0.5-km wide fault zone with 20% velocity reduction is well imaged using local earthquake tomography. When sequential velocity inversions are done, the true fault width is found, even to 9 km depth, although the calculated amplitude of the velocity reduction is lower than the actual amplitude. Vp/Vs is as well determined as Vp. Magnetotelluric imaging of a synthetic fault zone shows that a conductive fault zone can be well imaged within the upper 10 km. Further, a narrow (1 km) very low resistivity (3 ohm m) fault core can be imaged within a broad (5 km) low resistivity (10 ohm m) fault zone, illustrating that regions of a fault containing large quantities of interconnected fluids within a broader, conductive fault zone should be detectable. Thus variations in fluid content and fluid pressure can be inferred from electrical and seismic methods but there will always be uncertainty in these inferences due to the trade-off with other factors, such as intrinsic variations in porosity, mineralogy, and pore geometry. The best approach is combined modeling of varied seismic and electrical data.

Two-dimensional seismic image of the San Andreas Fault in the Northern Gabilan Range, central California: Evidence for fluids in the fault zone

A joint inversion for two-dimensional P-wave velocity (Vp), P-to-S velocity ratio (Vp/Vs), and earthquake locations along the San Andreas fault (SAF) in central California reveals a complex relationship among seismicity, fault zone structure, and the surface fault trace. A zone of low Vp and high Vp/Vs lies beneath the SAF surface trace (SAFST), extending to a depth of about 6 km. Most of the seismic activity along the SAF occurs at depths of 3 to 7 km in a southwest-dipping zone that roughly intersects the SAFST, and lies near the southwest edge of the low Vp and high Vp/Vs zones. Tests indicate that models in which this seismic zone is significantly closer to vertical can be confidently rejected. A second high Vp/Vs zone extends to the northeast, apparently dipping beneath the Diablo Range. Another zone of seismicity underlies the northeast portion of this Vp/Vs high. The high Vp/Vs zones cut across areas of very different Vp values, indicating that the high Vp/Vs values are due to the presence of fluids, not just lithology. The close association between the zones of high Vp/Vs and seismicity suggests a direct involvement of fluids in the faulting process.

Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

We investigate the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic under-plating. In the 1980s and early 1990s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted as remnants of the extinct Kula (or Resurrection) plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by large-scale duplex structures that overlie a tectonic wedge of North Slope crust and mantle. There, the Moho has been depressed to nearly 50 km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula (or Resurrection) plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two regions include flat-slab subduction and an orogenic-float model. In the Neogene, the tectonics of the accreting Yakutat terrane have differed across a newly interpreted tear in the subducting Pacific oceanic lithosphere. East of the tear, Pacific oceanic lithosphere subducts steeply and alone beneath the Wrangell volcanoes, because the overlying Yakutat terrane has been left behind as underplated rocks beneath the rising St. Elias Range, in the coastal region. West of the tear, the Yakutat terrane and Pacific oceanic lithosphere subduct together at a gentle angle, and this thickened package inhibits volcanism.

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.

Large-explosive source, wide-recording aperture, seismic profiling on the Columbia Plateau, Washington

Clear subsurface seismic images have been obtained at low cost on the Columbia Plateau, Washington. The Columbia Plateau is perhaps the most notorious of all “bad‐data” areas because large impedance contrasts in surface flood basalts severely degrade the seismic wavefield. This degradation was mitigated in this study via a large‐explosive source, wide‐recording aperture shooting method. The shooting method emphasizes the wide‐angle portion of the wavefield, where Fermat’s principle guarantees reverberation will not interfere with the seismic manifestations of crucial geologic interfaces. The basalt diving wave, normally discarded in standard common midpoint (CMP) seismic profiling, can be used to image basalt velocity structure via traveltime inversion. Maximum depth‐penetration of the diving wave tightly constrains basalt‐sediment interface depth. An arrival observed only at shot‐receiver offsets greater than 15 km can be used to determine the velocity and geometry of basement via simultaneous inversion....

Deep seismic structure and tectonics of northern Alaska: Crustal-scale duplexing with deformation extending into the upper mantle

Seismic reflection and refraction and laboratory velocity data collected along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composite picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made: (1) Northern Alaska is underlain by nested tectonic wedges, most with northward vergence (i.e., with their tips pointed north). (2) High reflectivity throughout the crust above a basal decollement, which deepens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these tectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which appears to involve chiefly bending of a relatively rigid plate consisting of the parautochthonous North Slope crust and a 10- to 15-km-thick section of mantle material. (4) This plate is interpreted as a southward verging tectonic wedge, with its tip in the lower crust or at the Moho beneath the southern Brooks Range. In this interpretation the middle and upper crust, or all of the crust, is detached in the southern Brooks Range by the tectonic wedge, or indentor: as a result, crust is uplifted and deformed above the wedge, and mantle is depressed and underthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle of two different origins (and seismic velocities), giving rise to a prominent sub-Moho reflector.

Linearized rays, amplitude and inversion

In this paper, ray theoretical amplitudes and travel times are calculated in slightly perturbed velocity models using perturbation analysis. Also, test inversions using travel time and amplitude are computed. The pertubation method is tested using a 3-D velocity model for NORSAR having velocity variations up to 8.0 percent. The perturbed amplitudes are found to be in excellent agreement with the calculated ray amplitudes. Velocity inversions based on travel time and amplitude are next investigated. Perturbation analysis using linearized ray equations is efficiently used to compute amplitude derivatives with respect to model parameters. The trial linearized inversions use smaller velocity variations of 1.7 percent to avoid possible effects due to ray shift, even though the perturbation analysis is valid for larger variations. The trial 2-D inversion results show that linearized amplitude inversions are complementary and not redundant to travel time inversions, even in smoothly varying models.

Tomographic images of the upper crust from the Los Angeles basin to the Mojave Desert, California: Results from the Los Angeles Region Seismic Experiment

We apply inversion methods to first arriving P waves from explosive source seismic data collected along line 1 of the Los Angeles Region Seismic Experiment (LARSE), extending northeastward from Seal Beach, California, to the Mojave Desert, in order to determine a seismic model of the upper crust along the profile. We use resolution information to quantify the extent of blurring in the LARSE images and to smooth a damped least squares (DLS) image by postinversion filtering (PIF). Most of the original data fit is preserved while minimizing model artifacts. We compare DLS, PIF, and smoothing constraint inversion images using both real and synthetic data. A preferred PIF image includes larger-scale features in the smoothing constraint inversion image and finer-scale features in the DLS inversion image that are consistent with geologic information. We interpret principal model features in terms of geology, including faulting. The maximum depth of low-velocity sedimentary and volcanic rocks in the Los Angeles basin is 8–9 km and in the San Gabriel Valley is 4.5–5 km. A horst-like uplift of basement rocks occurs between these basins. The northeastern boundary of the San Gabriel Valley is imaged as a tabular, moderately north dipping low-velocity zone that projects to the surface at the southernmost trace of the Sierra Madre fault system. In the central and southern San Gabriel Mountains, velocity-depth profiles are consistent with intermediate-velocity mylonites overlying lower-velocity Pelona Schist along a shallowly southwest dipping Vincent thrust fault. Tomography does not provide a definitive dip for the San Andreas fault but, combined with other LARSE results, is consistent with a vertical to steep northeast dip.

Images of crust beneath southern California will aid study of earthquakes and their effects

The Whittier Narrows earthquake of 1987 and the Northridge earthquake of 1991 highlighted the earthquake hazards associated with buried faults in the Los Angeles region. A more thorough knowledge of the subsurface structure of southern California is needed to reveal these and other buried faults and to aid us in understanding how the earthquake-producing machinery works in this region.

Seismic Velocity Models for the Denali Fault Zone along the Richardson Highway, Alaska

Crustal-scale seismic-velocity models across the Denali fault zone along the Richardson Highway show a 50-km-thick crust, a near vertical fault trace, and a 5-km-wide damage zone associated with the fault near Trans-Alaska Pipeline Pump Station 10, which provided the closest strong ground motion recordings of the 2002 Denali fault earthquake. We compare models, derived from seismic reflection and refraction surveys acquired in 1986 and 1987, to laboratory measurements of seismic velocities for typical metamorphic rocks exposed along the profiles. Our model for the 1986 seismic reflection profile indicates a 5-km-wide low-velocity zone in the upper 1 km of the Denali fault zone, which we interpret as fault gouge. Deeper refractions from our 1987 line image a 40-km wide, 5-km-deep low-velocity zone along the Denali fault and nearby associated fault strands, which we attribute to a composite damage zone along several strands of the Denali fault zone and to the obliquity of the seismic line to the fault zone. Our velocity model and other geophysical data indicate a nearly vertical Denali fault zone to a depth of 30 km. Aftershocks of the 2002 Denali fault earthquake and our velocity model provide evidence for a flower structure along the fault zone consisting of faults dipping toward and truncated by the Denali fault. Wide-angle reflections indicate that the crustal thickness beneath the Denali fault is transitional between the 60-km-thick crust beneath the Alaska Range to the south, and the extended, 30-km-thick crust of the Yukon–Tanana terrane to the north. Online Material: Tables of locations for the tact 1986 and tact 1987 lines.