Geology topics
Robert W. Simpson
Publications and source records attributed to Robert W. Simpson.
A physically-based earthquake recurrence model for estimation of long-term earthquake probabilities
A physically-motivated model for earthquake recurrence based on the Brownian relaxation oscillator is introduced. The renewal process defining this point process model can be described by the steady rise of a state variable from the ground state to failure threshold as modulated by Brownian motion. Failure times in this model follow the Brownian passage time (BPT) distribution, which is specified by the mean time to failure, μ, and the aperiodicity of the mean, α (equivalent to the familiar coefficient of variation). Analysis of 37 series of recurrent earthquakes, M -0.7 to 9.2, suggests a provisional generic value of α = 0.5. For this value of α, the hazard function (instantaneous failure rate of survivors) exceeds the mean rate for times > μ⁄2, and is ~ ~ 2 ⁄ μ for all times > μ. Application of this model to the next M 6 earthquake on the San Andreas fault at Parkfield, California suggests that the annual probability of the earthquake is between 1:10 and 1:13.
Suppression of large earthquakes by stress shadows: A comparison of Coulomb and rate-and-state failure
Stress shadows generated by California's two most recent great earthquakes (1857 Fort Tejon and 1906 San Francisco) substantially modified 19th and 20th century earthquake history in the Los Angeles basin and in the San Francisco Bay area. Simple Coulomb failure calculations, which assume that earthquakes can be modeled as static dislocations in an elastic half-space, have done quite well at approximating how long the stress shadows, or relaxing effects, should last and at predicting where subsequent large earthquakes will not occur. There has, however, been at least one apparent exception to the predictions of such simple models. The 1911 M >6.0 earthquake near Morgan Hill, California, occurred at a relaxed site on the Calaveras fault. We examine how the more complex rate-and-state friction formalism based on laboratory experiments might have allowed the 1911 earthquake. Rate-and-state time-to-failure calculations are consistent with the occurrence of the 1911 event just 5 years after 1906 if the Calaveras fault was already close to failure before the effects of 1906. We also examine the likelihood that the entire 78 years of relative quiet (only four M ≥6 earthquakes) in the bay area after 1906 is consistent with rate-and-state assumptions, given that the previous 7 decades produced 18 M ≥6 earthquakes. Combinations of rate-and-state variables can be found that are consistent with this pattern of large bay area earthquakes, assuming that the rate of earthquakes in the 7 decades before 1906 would have continued had 1906 not occurred. These results demonstrate that rate-and-state offers a consistent explanation for the 78-year quiescence and the 1911 anomaly, although they do not rule out several alternate explanations.
Surface strain accumulation and the seismic moment tensor
Although the scalar moment accumulation rate within the seismogenic zone beneath a given area is sometimes deduced from the observed average surface strain accumulation rate over that same area (e.g., Working Group on California Earthquake Probabilities, 1995), the correspondence between the two is very uncertain. The equivalence between surface strain accumulation and scalar moment accumulation is based on Kostrov's (1974) relation between the average strain rate over a volume and the moment-rate tensor for that volume. The average strain rate over the volume is replaced by the average strain rate measured at the free surface to deduce an approximate moment-rate tensor. Only in exceptional circumstances will that moment-rate tensor correspond to a double-couple mechanism, a mechanism that can be represented by a scalar moment accumulation rate. More generally, the moment tensor must be resolved into the superposition of two or more double-couple mechanisms, and that resolution can be done in many ways, each with its own scalar moment rate. Thus the resolution is not unique. This is demonstrated by deducing scalar moment accumulation rates for a GPS network that covers most of California south of San Francisco. It is shown that resolutions into different double-couple mechanisms lead to scalar moment accumulation rates differing by factors of ∼2. We suggest that the minimum scalar moment rate equivalent to principal surface strain rates ɛ 1 and ɛ 2 acting over the area A is M 0 (min) = 2 μHA Max (¦ ɛ 1 ¦, ¦ ɛ 2 ¦, ¦ ɛ 1 + ɛ 2 ¦), where μ is the rigidity and H the depth of seismogenic zone, and the function Max is equal to the largest of its arguments. Within the uncertainites of measurement, the scalar moment accumulation rate in southern California based on that approximation is in balance with the average historic seismic moment release rate so that no current earthquake deficit need be accumulating.
The Loma Prieta, California, earthquake of October 17, 1989: Aftershocks and postseismic effects
While the damaging effects of the earthquake represent a significant social setback and economic loss, the geophysical effects have produced a wealth of data that have provided important insights into the structure and mechanics of the San Andreas Fault system. Generally, the period after a large earthquake is vitally important to monitor. During this part of the seismic cycle, the primary fault and the surrounding faults, rock bodies, and crustal fluids rapidly readjust in response to the earthquake's sudden movement. Geophysical measurements made at this time can provide unique information about fundamental properties of the fault zone, including its state of stress and the geometry and frictional/rheological properties of the faults within it. Because postseismic readjustments are rapid compared with corresponding changes occurring in the preseismic period, the amount and rate of information that is available during the postseismic period is relatively high. From a geophysical viewpoint, the occurrence of the Loma Prieta earthquake in a section of the San Andreas fault zone that is surrounded by multiple and extensive geophysical monitoring networks has produced nothing less than a scientific bonanza. The reports assembled in this chapter collectively examine available geophysical observations made before and after the earthquake and model the earthquake's principal postseismic effects. The chapter covers four broad categories of postseismic effect: (1) aftershocks; (2) postseismic fault movements; (3) postseismic surface deformation; and (4) changes in electrical conductivity and crustal fluids.
Digital grid of isostatic residual gravity of North America; a digital database
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Isostatic residual gravity and crustal geology of the United States
A new isostatic residual gravity map of the conterminous United States presents continent-wide gravity data in a form that can be readily used, with geologic information and other geophysical data, in studies of the composition and structure of the continental crust. This map was produced from the gridded gravity data on which the recently released Gravity Anomaly Map of the United States is based. About 1 million onland and 0.8 million offshore gravity observations interpolated to a 4- by 4-km grid serve as the basis for both maps. The Airy-Heiskanen model of isostatic compensation of topography applied to topographic and bathymetric data averaged over 5- by 5-min compartments was used to remove, to first order, the large, long-wavelength Bouguer gravity anomalies caused by deep density distributions that support topographic loads. The parameters used in the Airy-Heiskanen model were topographic density, 2.67 g/cm 3 ; sea-level crustal thickness, 30 km; and density contrast across the base of the model crust, 0.35 g/cm 3 . Many of the conspicuous short-wavelength anomalies (widths less than several hundred kilometers) on the isostatic residual gravity map correlate with mapped or near-surface geologic features, and primarily reflect shallow-density distributions rather than any departures from isostatic equilibrium. In general, gravity highs occur over (1) mafic igneous bodies emplaced in rift or magmatic arc settings or as isolated intrusions controlled by structures; (2) accreted slices of mafic oceanic, island-arc, or transitional crust; and (3) uplifted crystalline basement. Gravity lows are found over (1) thick bodies of felsic intrusive or extrusive rocks; (2) sedimentary deposits in extensional, convergent, or transform settings; and (3) depressed crystalline basement. Anomalies with widths as much as 1,000 km or more also appear to reflect crustal properties in many cases—several broad gravity highs are associated with crust having a high average seismic wave velocity, and comparable broad gravity lows occur over areas of low average seismic velocity. Alternative ways of viewing the isostatic residual gravity data provide additional information about density distributions in the crust. The first-vertical derivative map accentuates gravity anomalies over shallow, abrupt density changes at the expense of those resulting from deeper or more gradual density transitions. The maximum horizontal gradient map contains information about the locations of pronounced density boundaries. Two-dimensional spectral analysis of the gravity data provides a quantitative means for identifying dominant fabrics in the gravity field and for distinguishing various terranes from each other. Neither Bouguer nor isostatic residual gravity anomalies are particularly well suited for practical modeling of deep structure in conjunction with deep seismic information. However, a scheme in which the entire Earth outside the area of interest is approximated by laterally homogeneous layers and isostatically compensated topography, and in which the area of interest is modeled using the seismic constraints applied in a two-and-one-half-dimensional geometry, holds promise for exploiting useful features of both the Bouguer and isostatic residual gravity anomalies.
Average topography, isostatic residual gravity, and aeromagnetic maps of the Parkfield region, California
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Mineral resources of the Whipple Mountains and Whipple Mountains Addition Wilderness Study Areas, San Bernardino County, California
At the request of the U.S. Bureau of Land Management, approximately 85,100 acres of the Whipple Mountains Wilderness Study Area (CDCA-312) and 1,380 acres of the Whipple Mountains Addition Wilderness Study Area (AZ-050-010) were evaluated for identified mineral resources (known) and mineral resource potential (undiscovered). In this report, the Whipple Mountains and Whipple Mountains Addition Wilderness Study Areas are referred to as simply "the study area." Most of the mines and prospects with identified resources in the Whipple Mountains Wilderness Study Area are within areas designated as having mineral resource potential. The area in and around the Turk Silver mine and the Lucky Green group and the area near the northwest boundary of the study area have high mineral resource potential for copper, lead, zinc, gold, and silver. An area along the west boundary of the study area has moderate resource potential for copper lead, zinc, gold, and silver. An area in the east adjacent to the Whipple Mountains Addition Wilderness Study Area has moderate resource potential for copper, gold, and silver resources. One area on the north boundary and one on the southeast boundary of the study area have low mineral resource potential for copper, lead, zinc, gold, and silver. Two areas, one on the north boundary and one inside the east boundary of the study area, have moderate resource potential for manganese. A small area inside the south boundary of the study area has high resource potential for decorative building stone, and the entire study area has low resource potential for sand and gravel and other rock products suitable for construction. Two areas in the eastern part of the study area have low resource potential for uranium. There is no resource potential for oil and gas or geothermal resources in the Whipple Mountains Wilderness Study Area. Sites within the Whipple Mountains Wilderness Study Area with identified resources of copper, gold, silver, manganese and (or) decorative building stone are located at the Stewart mine, New American Eagle mine, Turk Silver mine, Twin Lode mine, decorative stone property, Lucky Green group, Blue Cloud mine, Nickel Plate mine, Crescent mine, Quadrangle Copper group, and the Copper Basin mine. The Whipple Mountains Addition Wilderness Study Area has moderate resource potential for copper, gold, and silver resources and low resource potential for sand and gravel and other rock products. There is no resource potential for oil and gas or for geothermal energy in the Whipple Mountains Addition Wilderness Study Area. Although there are no identified resources in the Whipple Mountains Addition Wilderness Study Area, sites within and immediately adjacent warrant further study because of gold assays from widespread, numerous samples.
A new isostatic residual gravity map of the conterminous United States
In order to display more clearly the gravity anomalies caused by bodies of geologic interest in the crust, a new colored isostatic residual gravity map of the conterminous United States has been prepared using the gravity data set compiled for the Gravity Anomaly Map of the United States (SEG, 1982). The isostatic residual map is based on an Airy-Heiskanen model of local compensation, with the surface load requiring compensation being defined by 5-minute topographic and bathymetric data sets. Because topography-related anomalies have been removed to first approximation, the resulting isostatic residual map highlights density contrasts associated with geologic bodies in the crust. For example, the 2000 km-long gravity high that follows the Appalachian orogenic belt is delineated along its entire length without the topographic overprint characteristic of Bouguer maps. For purposes of displaying gravity anomalies caused by shallow bodies of geologic significance, the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort. Most isostatic models will result in residual maps which appear very similar in their main patterns and features. Anomalies on isostatic residual gravity maps should not be casually interpreted in terms of "undercompensation" or "overcompensation" because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium.
Principal facts for gravity stations in the Big Maria, Riverside, and Whipple Mountains, California
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Digitized aeromagnetic map of part of the Boston 1° x 2° quadrangle, Massachusetts, New Hampshire, Connecticut, Rhode Island, and Maine
No abstract available.
Preliminary Bouguer gravity map of eastern Maine and adjacent Canada
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Bouguer gravity map of the Presque Isle 1° x 2° quadrangle and parts of adjacent quadrangles, Maine, New Brunswick, and Quebec
No abstract available.
Bouguer gravity map of the New York 1° x 2° quadrangle, New York, New Jersey, and Connecticut
No abstract available.
Bouguer gravity map of the Hartford 1° x 2° quadrangle, Connecticut, New York, New Jersey, Massachusetts
No abstract available.