Rufus D. Catchings

SEISMIC EVIDENCE FOR ACTIVE MAGMATIC UNDERPLATING BENEATH THE BASIN AND RANGE PROVINCE, WESTERN UNITED STATES

Near-vertical and wide-angle seismic reflection data provide evidence for the presence of a magma body at the base of the crust beneath Buena Vista Valley in northwestern Nevada. The seismic response of this hypothesized magma body is characterized by high-amplitude, near-vertical P wave reflections and a comparably strong P-to-S converted phase. The magma body, referred to here as the Buena Vista Magma Body, is probably a single sill with thickness no greater than 200 m and length no greater than 1.8 km. The melt fraction in the sill is undoubtedly greater than 20–30%, and probably exceeds 50%. Melt composition is unconstrained. Although the age of the Buena Vista Magma Body is difficult to determine precisely, it is probably no older than 500,000 years. This suggests that magmatism in the Basin and Range Province is an ongoing process, despite the relative paucity of volcanic rocks erupted at the surface during the last 6 m.y.

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....

Origins of deep crustal reflections: Implications of coincident seismic refraction and reflection data in Nevada

We compare seismic refraction and reflection results along the PASSCAL/COCORP 40°N transect in the northern Basin and Range of Nevada in order to determine the origin of the prominent reflections from the deep crystalline crust. Reflection data along the transect show a thick zone of discontinuous, subhorizontal reflections, beginning at 4-6 s two-way traveltime (10-20 km depth) and ending at 9-11 s (27-35 km). Two independently derived velocity models, based on refraction data, are largely similar and agree on many important aspects of the reflectivity-velocity relation. Both models show that the top of the reflective zone lies 3-8 km above a prominent mid-crustal velocity discontinuity, which is interpreted to separate bulk silicic from bulk dioritic-gabbroic crust; in most places, the silicic mid-crust is more strongly reflective than the mafic lower crust. This pattern is expected in areas where ductile shearing is the mechanism responsible for the reflectivity. One of the velocity models, however, suggests that, in places, the strongest reflectivity spans both the middle (6.1-6.3 km/s) and lower (6.6 km/s) crust; this pattern suggests that the combined influence of ductile strain fabrics and mafic intrusions gives rise to crustal reflections. Both models show that the lowermost crust and crust/mantle transition are highly reflective, also suggesting the presence of mafic and/or ultramafic intrusions. Thus the observed reflection patterns suggest that ductile shearing and the intrusion of mantle-derived magma—both of which are likely to have accompanied the extreme Cenozoic extension—are important factors in generating deep crustal reflections.

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.

Seismicity, faulting, and structure of the Koyna‐Warna seismic region, Western India from local earthquake tomography and hypocenter locations

Although seismicity near Koyna Reservoir (India) has persisted for ~50 years and includes the largest induced earthquake (M 6.3) reported worldwide, the seismotectonic framework of the area is not well understood. We recorded ~1800 earthquakes from 6 January 2010 to 28 May 2010 and located a subset of 343 of the highest-quality earthquakes using the tomoDD code of Zhang and Thurber (2003) to better understand the framework. We also inverted first arrivals for 3-D Vp, Vs, and Vp/Vs and Poissons ratio tomography models of the upper 12 km of the crust. Epicenters for the recorded earthquakes are located south of the Koyna River, including a high-density cluster that coincides with a shallow depth (<1.5 km) zone of relatively high Vp and low Vs (also high Vp/Vs and Poissons ratios) near Warna Reservoir. This anomalous zone, which extends near vertically to at least 8 km depth and laterally northward at least 15 km, is likely a water-saturated zone of faults under high pore pressures. Because many of the earthquakes occur on the periphery of the fault zone, rather than near its center, the observed seismicity-velocity correlations are consistent with the concept that many of the earthquakes nucleate in fractures adjacent to the main fault zone due to high pore pressure. We interpret our velocity images as showing a series of northwest trending faults locally near the central part of Warna Reservoir and a major northward trending fault zone north of Warna Reservoir.

Relationships among seismic velocity, metamorphism, and seismic and aseismic fault slip in the Salton Sea Geothermal Field region

The Salton Sea Geothermal Field is one of the most geothermally and seismically active areas in California and presents an opportunity to study the effect of high-temperature metamorphism on the properties of seismogenic faults. The area includes numerous active tectonic faults that have recently been imaged with active source seismic reflection and refraction. We utilize the active source surveys, along with the abundant microseismicity data from a dense borehole seismic network, to image the 3-D variations in seismic velocity in the upper 5 km of the crust. There are strong velocity variations, up to ~30%, that correlate spatially with the distribution of shallow heat flow patterns. The combination of hydrothermal circulation and high-temperature contact metamorphism has significantly altered the shallow sandstone sedimentary layers within the geothermal field to denser, more feldspathic, rock with higher P wave velocity, as is seen in the numerous exploration wells within the field. This alteration appears to have a first-order effect on the frictional stability of shallow faults. In 2005, a large earthquake swarm and deformation event occurred. Analysis of interferometric synthetic aperture radar data and earthquake relocations indicates that the shallow aseismic fault creep that occurred in 2005 was localized on the Kalin fault system that lies just outside the region of high-temperature metamorphism. In contrast, the earthquake swarm, which includes all of the M > 4 earthquakes to have occurred within the Salton Sea Geothermal Field in the last 15 years, ruptured the Main Central Fault (MCF) system that is localized in the heart of the geothermal anomaly. The background microseismicity induced by the geothermal operations is also concentrated in the high-temperature regions in the vicinity of operational wells. However, while this microseismicity occurs over a few kilometer scale region, much of it is clustered in earthquake swarms that last from hours to a few days and are localized near the MCF system.

Are large explosive sources applicable to resource exploration

Seismologists from government and academia are finding that large chemical explosions are an excellent seismic source, not only for profiling the deep crust but also for imaging the shallower geologic targets of interest to the oil and gas industry. (For the purposes of this article, we define a “large” explosion as one that utilizes at least 100 kg—about 220 lbs—of ammonium nitrate or equivalent explosive. In general, shot sizes ranging from this minimum up to 2700 kg—about 3 tons—are used in deep crustal seismic profiling programs.)

Structure of the Koyna‐Warna Seismic Zone, Maharashtra, India: A possible model for large induced earthquakes elsewhere

The Koyna-Warna area of India is one of the best worldwide examples of reservoir-induced seismicity, with the distinction of having generated the largest known induced earthquake (M6.3 on 10 December 1967) and persistent moderate-magnitude (>M5) events for nearly 50 years. Yet, the fault structure and tectonic setting that has accommodated the induced seismicity is poorly known, in part because the seismic events occur beneath a thick sequence of basalt layers. On the basis of the alignment of earthquake epicenters over an ~50 year period, lateral variations in focal mechanisms, upper-crustal tomographic velocity images, geophysical data (aeromagnetic, gravity, and magnetotelluric), geomorphic data, and correlation with similar structures elsewhere, we suggest that the Koyna-Warna area lies within a right step between northwest trending, right-lateral faults. The sub-basalt basement may form a local structural depression (pull-apart basin) caused by extension within the step-over zone between the right-lateral faults. Our postulated model accounts for the observed pattern of normal faulting in a region that is dominated by north-south directed compression. The right-lateral faults extend well beyond the immediate Koyna-Warna area, possibly suggesting a more extensive zone of seismic hazards for the central India area. Induced seismic events have been observed many places worldwide, but relatively large-magnitude induced events are less common because critically stressed, preexisting structures are a necessary component. We suggest that releasing bends and fault step-overs like those we postulate for the Koyna-Warna area may serve as an ideal tectonic environment for generating moderate- to large- magnitude induced (reservoir, injection, etc.) earthquakes.

Detailed P- and S-Wave Velocity Models along the LARSE II Transect, Southern California

Abstract Structural details of the crust determined from P -wave velocity models can be improved with S -wave velocity models, and S -wave velocities are needed for model-based predictions of strong ground motion in southern California. We picked P - and S -wave travel times for refracted phases from explosive-source shots of the Los Angeles Region Seismic Experiment, Phase II (LARSE II); we developed refraction velocity models from these picks using two different inversion algorithms. For each inversion technique, we calculated ratios of P - to S -wave velocities ( V P / V S ) where there is coincident P - and S -wave ray coverage. We compare the two V P inverse velocity models to each other and to results from forward modeling, and we compare the V S inverse models. The V S and V P / V S models differ in structural details from the V P models. In particular, dipping, tabular zones of low V S , or high V P / V S , appear to define two fault zones in the central Transverse Ranges that could be parts of a positive flower structure to the San Andreas fault. These two zones are marginally resolved, but their presence in two independent models lends them some credibility. A plot of V S versus V P differs from recently published plots that are based on direct laboratory or down-hole sonic measurements. The difference in plots is most prominent in the range of V P =3 to 5 km/s (or V S ∼1.25 to 2.9 km/s), where our refraction V S is lower by a few tenths of a kilometer per second from V S based on direct measurements. Our new V S - V P curve may be useful for modeling the lower limit of V S from a V P model in calculating strong motions from scenario earthquakes.

Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations

The San Andreas fault (SAF) is one of the most studied strike‐slip faults in the world; yet its subsurface geometry is still uncertain in most locations. The Salton Seismic Imaging Project (SSIP) was undertaken to image the structure surrounding the SAF and also its subsurface geometry. We present SSIP studies at two locations in the Coachella Valley of the northern Salton trough. On our line 4, a fault‐crossing profile just north of the Salton Sea, sedimentary basin depth reaches 4 km southwest of the SAF. On our line 6, a fault‐crossing profile at the north end of the Coachella Valley, sedimentary basin depth is ∼2–3  km and centered on the central, most active trace of the SAF. Subsurface geometry of the SAF and nearby faults along these two lines is determined using a new method of seismic‐reflection imaging, combined with potential‐field studies and earthquakes. Below a 6–9 km depth range, the SAF dips ∼50°–60° NE, and above this depth range it dips more steeply. Nearby faults are also imaged in the upper 10 km, many of which dip steeply and project to mapped surface fault traces. These secondary faults may join the SAF at depths below about 10 km to form a flower‐like structure. In Appendix D, we show that rupture on a northeast‐dipping SAF, using a single plane that approximates the two dips seen in our study, produces shaking that differs from shaking calculated for the Great California ShakeOut, for which the southern SAF was modeled as vertical in most places: shorter‐period (T<1  s) shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall, compared to shaking calculated for a vertical fault.