Seismotectonics in Utah and California

Paper:

Magistrale et al., 1992, Forward and inverse three-dimensional P-wave velocity models of the southern California crust

CHJ Thesis:

Jones, 1987, Systematic errors and uncertainties of local earthquake parameters: An Example from northern Utah and southeastern Idaho
Jones et al., 1987, Seismicity accompanying extension within the Basin and Range Province in northern Utah and southeastern Idaho

Abstracts:

Jones et al., 1993, Possible expression of low-angle normal faulting in the seismicity of the Hansel Valley-Pocatello Valley region, Utah and Idaho
Qian et al., 1990, Seismotectonics of southern Sierra Nevada, California

Magistrale, H., H. Kanamori , and C. H. Jones, Forward and inverse three-dimensional P-wave velocity models of the southern California crust, J. Geophys. Res., 97, 14,115-14,135, 1992.

(Abstract not yet entered)

Jones, C. H., Systematic Errors and Uncertainties of Local Earthquake Parameters: An Example from northern Utah and southeastern Idaho (Chapter 2), in C. H. Jones, A Geophysical and Geological Investigation of Extensional Structures, Great Basin, Western United States, Ph.D. thesis, Mass. Inst. Tecnol., Cambridge, Mass., pp. 71-120, 1987.

Abstract (from thesis abstract). Results from a microearthquake survey in northern Utah and southeastern Idaho were scrutinized both for systematic errors and for tectonic implications. Arrival times from more than 330 earthquakes were inverted for both earthquake locations and a one-dimensional (1-D) velocity structure; uncertainties of the velocities of the best-fit 1-D structure, termed M8, were about +/- 4%. Earthquakes recorded by more than 11 stations and located using structure M8 were estimated to have epicenters and depths with uncertainties of about 300 m and 1 km, respectively; these uncertainties reflect both noise in the arrival times and uncertainties in the velocity structure, assuming the velocity structure is one-dimensional. Relocating these events using a best-fit three-dimensional (3-D) velocity structure, M3D, produces differences between the 1-D and 3-D locations and velocities greater than those estimated using the 1-D structure and far in excess of the standard errors. The large differences of locations are due to several systematic errors caused by the assumption of a 1-D structure, several of which might be anticipated: For example, epicenters are closer to regions with laterally lower velocities using M3D, and depths of earthquakes differ in proportion to the mean 1-D structure near the earthquake. One error, not easily anticipated, results in errors in the depths of earthquakes increasing with the number of stations recording the events; the depth changes of the 1-D locations compensate for the greater lateral heterogeneity sampled by rays from the better-recorded events. Focal mechanisms are most sensitive to the location of an earthquake relative to a strong velocity gradient with depth. Focal mechanisms of earthquakes with a lower P-wave velocity at the calculated than at the true hypocenter will have too large a component of strike-slip; mechanisms of events with a higher P-wave velocity at the calculated than at the true hypocenter will have too great a component of dip-slip.

C. H. Jones, P. H. Molnar, S. W. Roecker , R. B. Smith, and D. Hatzfeld, Seismicity accompanying extension within the Basin and Range Province in northern Utah and southeastern Idaho (Chapter 3), in C. H. Jones, A Geophysical and Geological Investigation of Extensional Structures, Great Basin, Western United States, Ph.D. thesis, Mass. Inst. Tecnol., Cambridge, Mass., pp. 71-120, 1987

Abstract (from thesis abstract) Several features in the seismicity of the Hansel Valley--Pocatello Valley region were found to be inconsistent with a simple horst-and-graben structure in the region. Seismicity occurred not only under mountains near Hansel Valley but also under Pocatello Valley farther north; a vertical discontinuity existed in the seismicity south of about 41.82°ree;N at a depth of about 4-5 km; and a sparsely distributed group of earthquakes occurred near a depth of 4-5 km with mechanisms consistent with slip on a low-angle normal fault. At about 41.82°ree;N the seismicity is characterized by oblique-normal, right-lateral slip on a west-northwest striking, north-dipping plane; this seismicity lies at the northern edge of the vertically discontinuous seismicity and at the southern edge of a lateral low-velocity region that has P- and S-wave velocities 10-15% lower than the regions to the north or south. The existence of a low-angle normal fault at a depth of 4-5 km beneath the Hansel Mountains and Hansel Valley is compatible with these observations and the regional geology of the region to the west; this fault is considered to lie at a greater depth north of a "step" at about 41.82°ree;N. This step and its associated oblique-normal, right-lateral sense focal mechanisms appear to be the down-plunge continuation of the offset of the trace of the low-angle faults east of the Albion Mountains to those east of the Raft River Mountains. Hence the seismicity observed in the Hansel Valley--Pocatello Valley region is consistent with the presence of an active low-angle normal fault within the seismically active portion of the crust.

Qian, H. X., C. H. Jones, and H. Kanamori , Seismotectonics of southern Sierra Nevada, California, EOS, 71, 1559, 1990.

The southern Sierra Nevada, California, has been one of the most seismically active regions in California over the past decade despite the fact that no throughgoing active faults have been identified. The patterns of epicenters and focal mechanisms of the small earthquakes in the region suggest the probable onset of Basin Range extensionwithin the Sierra (Jones and Dollar, 1986) and the possible formation of an intracontinental transform in the southernmost Sierra (Qian et al, 1989). This seismically active region appears to coincide with low velocities in the upper mantle (C.H. Jones et al., this meeting), suggesting that weakening of the mantle lithosphere has led to a late Neogene onset of seimic deformation in the southern Sierra Nevada.

Seventeen digital seismometers were deployed in the southern Sierra during the summer of 1988, most recording 3 components of ground motion. The improved identification of shear wave arrivals and the greater areal extent of this network (when combined with the permanent Southern California Seismic Network) greatly improves our ability to locate and study earthquakes in this region. More than 300 earthquakes in the region were recorded by both networks during that time. A total of 2871 P and 854 S arrivals were picked from 111 well recorded earthquakes and an explosion in the China Lake Naval Weapons Center.

To improve the accuracy of hypocenters and focal mechanisms in this area, we construct a three dimensional velocity structure of the region by joining together one dimensional structures of the southern Sierra, Coso/Indian Wells Valley, and western Mojave Desert. We invert for these velocities using S. W. Roecker's HYPIT code. Two starting models are used to check the convergence of the inversions. One is constructed from published 1-D models: Jones and Dollar (1986) for the southern Sierra, Walter and Weaver (1980) for the Coso region and Kanamori and Hadley (1975) for the Mojave Desert. The other model modifies the structure for the southern Sierra using velocity information derived from travel times from the blast and well located earthquakes. The new velocity structure and delays from the 111 earthquakes are used to relocate earthquakes from 1983 to the present. Location and focal mechanism from these earthquakes are then used to better constrain seismogenic structures inferred previously in this region.

Jones, C. H., P. H. Molnar, S. W. Roecker , R. B. Smith, and D. Hatzfeld, Possible expression of low-angle normal faulting in the seismicity of the Hansel Valley-Pocatello Valley region (abstract), Utah and Idaho,Geol. Soc. Am. Abstr. Progs., 25 (5), A60, 1993.

From records collected between August and October 1983 of 47 seismometers spaced 3-5 km apart in the Hansel Valley and Pocatello Valley region of Utah and Idaho, we located 334 earthquakes (M<2.5) and determined 102 first-motion focal mechanisms. These events lie within a 10 km wide, N30E trending band and were located with an uncertainty of about 500 m in epicenter and depth using a 3-D velocity structure determined from the earthquake arrival times. Events south of 41.82°ree;N fall into two clusters ~4 km apart, an eastern group above 4 km depth below mean sea level and a western group below 4 km depth. Earthquakes near 4 km depth scatter between and beyond these clusters. North of 41.82°ree;N no lateral offset of seismicity with depth is observed. Focal mechanisms south of 41.82°ree;N are mostly normal-slip on planes dipping between 30 and 60°ree; and striking NNW-NE; several are consistent with slip on low-angle normal faults (LANFs). A knot of seismicity between 41.82°ree;to ~41.9°ree;N in the Hansel Mtns. includes many events that have oblique-slip or strike-slip solutions with a general right-lateral sense on WNW-striking planes. WNW-striking thrust solutions are also present in this area. Farther north, focal mechanisms are again dominantly normal. This pattern of seismicity suggests that a LANF might be present at about 4 km depth south of 41.82°ree;N and that it deepens to the north across a lateral ramp, above which oblique-slip and thrust faulting accommodates local strain incompatibilities. This could represent the down-dip extension of the LANF inferred to be at the east edge of the Raft River and Albion Mtns. Although indicating that a Pliocene or younger LANF has expression in modern seismicity and that slip on it is consistent with patterns in modern seismicity, we cannot determine from this dataset if this LANF generates large earthquakes.

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