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R. L. Wesson

Publications and source records attributed to R. L. Wesson.

At least 19 recordsLinked to original sources
USGS2013

Challenges in making a seismic hazard map for Alaska and the Aleutians

We present a summary of the data and analyses leading to the revision of the time-independent probabilistic seismic hazard maps of Alaska and the Aleutians. These maps represent a revision of existing maps based on newly obtained data, and reflect best current judgments about methodology and approach. They have been prepared following the procedures and assumptions made in the preparation of the 2002 National Seismic Hazard Maps for the lower 48 States, and will be proposed for adoption in future revisions to the International Building Code. We present example maps for peak ground acceleration, 0.2 s spectral amplitude (SA), and 1.0 s SA at a probability level of 2% in 50 years (annual probability of 0.000404). In this summary, we emphasize issues encountered in preparation of the maps that motivate or require future investigation and research.

Alaska↗
USGS2012

Predicted variation of stress orientation with depth near an active fault: Application to the Cajon Pass Scientific Drillhole, southern California

Preliminary measurements of the stress orientation at a depth of 2 km in the Cajon Pass Scientific Drillhole, about 3.6 km from the San Andreas fault in southern California, indicate that the local direction of maximum compression is nearly normal to the fault zone. The measurements have been interpreted to indicate that the regional orientation of the maximum compression is normal to the fault, and taken as evidence for a very weak fault. The orientation of the regional maximum compression expected from plate tectonic arguments is about 66° NE from the strike of the fault. Geodetic data indicate that the orientation of maximum compressive strain rate is about 43° NE from the strike of the fault, and show nearly pure right-lateral shear acting parallel to the fault. These apparent conflicts in the inferred orientation of the axis of maximum compression may be explained in part by a model in which the fault zone is locked over a depth interval in the range of 2-5 to 15 km, but is very weak above and below that interval. An analytic solution for this model predicts orientations of the principal stresses that are nearly parallel or normal to the fault, for observations made close to the fault and above and below the locked zone, but an orientation consistent with plate tectonic estimates at depths corresponding to that of the locked interval. This solution does require, however, a few mm/yr of creep at the surface on the San Andreas or nearby sub-parallel faults (such as the San Jacinto), which has not yet been observed, or a shallow zone near the faults of distributed deformation.

Geophysical Research Letters↗
USGS2012

Interpretation of changes in water level accompanying fault creep and implications for earthquake prediction

Quantitative calculations for the effect of a fault creep event on observations of changes in water level in wells provide an approach to the tectonic interpretation of these phenomena. For the pore pressure field associated with an idealized creep event having an exponential displacement versus time curve, an analytic expression has been obtained in terms of exponential-integral functions. The pore pressure versus time curves for observation points near the fault are pulselike; a sharp pressure increase (or decrease, depending on the direction of propagation) is followed by more gradual decay to the normal level after the creep event. The time function of the water level change may be obtained by applying the filter—derived by A. G. Johnson and others to determine the influence of atmospheric pressure on water level—to the analytic pore pressure versus time curves. The resulting water level curves show a fairly rapid increase (or decrease) and then a very gradual return to normal. The results of this analytic model do not reproduce the steplike changes in water level observed by Johnson and others. If the procedure used to obtain the water level from the pore pressure is correct, these results suggest that steplike changes in water level are not produced by smoothly propagating creep events but by creep events that propagate discontinously, by changes in the bulk properties of the region around the well, or by some other mechanism. In addition, simplistic calculations show that significant pressure field variations and water level changes near the surface may be expected to accompany a propagating creep event on a buried fault. Water level changes of as much as several meters may be expected at the surface for a creep event having a dislocation amplitude of l m on a semi-infinite dislocation surface extending downward from a depth of 10 km and propagating horizontally at a rate of 10 km/day. The maximum near-surface effect should be observed at a horizontal distance from the fault about equal to the depth of the top of the dislocation surface. These results are consistent with the observations made in China of large water level changes preceding large earthquakes, if some sort of aseismic creep event at depth precedes these earthquakes.

Journal of Geophysical Research Solid Earth↗
USGS2007

Stress before and after the 2002 Denali fault earthquake

Spatially averaged, absolute deviatoric stress tensors along the faults ruptured during the 2002 Denali fault earthquake, both before and after the event, are derived, using a new method, from estimates of the orientations of the principal stresses and the stress change associated with the earthquake. Stresses are estimated in three regions along the Denali fault, one of which also includes the Susitna Glacier fault, and one region along the Totschunda fault. Estimates of the spatially averaged shear stress before the earthquake resolved onto the faults that ruptured during the event range from near 1 MPa to near 4 MPa. Shear stresses estimated along the faults in all these regions after the event are near zero (0 ?? 1 MPa). These results suggest that deviatoric stresses averaged over a few tens of km along strike are low, and that the stress drop during the earthquake was complete or nearly so.

Geophysical Research Letters↗
USGS2004

Losses to single-family housing from ground motions in the 1994 Northridge, California, earthquake

The distributions of insured losses to single-family housing following the 1994 Northridge, California, earthquake for 234 ZIP codes can be satisfactorily modeled with gamma distributions. Regressions of the parameters in the gamma distribution on estimates of ground motion, derived from ShakeMap estimates or from interpolated observations, provide a basis for developing curves of conditional probability of loss given a ground motion. Comparison of the resulting estimates of aggregate loss with the actual aggregate loss gives satisfactory agreement for several different ground-motion parameters. Estimates of loss based on a deterministic spatial model of the earthquake ground motion, using standard attenuation relationships and NEHRP soil factors, give satisfactory results for some ground-motion parameters if the input ground motions are increased about one and one-half standard deviations above the median, reflecting the fact that the ground motions for the Northridge earthquake tended to be higher than the median ground motion for other earthquakes with similar magnitude. The results give promise for making estimates of insured losses to a similar building stock under future earthquake loading. ?? 2004, Earthquake Engineering Research Institute.

Earthquake Spectra↗
USGS2003

Association of earthquakes and faults in the San Francisco Bay area using Bayesian inference

Bayesian inference provides a method to use seismic intensity data or instrumental locations, together with geologic and seismologic data, to make quantitative estimates of the probabilities that specific past earthquakes are associated with specific faults. Probability density functions are constructed for the location of each earthquake, and these are combined with prior probabilities through Bayes' theorem to estimate the probability that an earthquake is associated with a specific fault. Results using this method are presented here for large, preinstrumental, historical earthquakes and for recent earthquakes with instrumental locations in the San Francisco Bay region. The probabilities for individual earthquakes can be summed to construct a probabilistic frequency–magnitude relationship for a fault segment. Other applications of the technique include the estimation of the probability of background earthquakes, that is, earthquakes not associated with known or considered faults, and the estimation of the fraction of the total seismic moment associated with earthquakes less than the characteristic magnitude. Results for the San Francisco Bay region suggest that potentially damaging earthquakes with magnitudes less than the characteristic magnitudes should be expected. Comparisons of earthquake locations and the surface traces of active faults as determined from geologic data show significant disparities, indicating that a complete understanding of the relationship between earthquakes and faults remains elusive.

California↗
USGS2001

Seismic hazard in Hawaii: High rate of large earthquakes and probabilistics ground-motion maps

The seismic hazard and earthquake occurrence rates in Hawaii are locally as high as that near the most hazardous faults elsewhere in the United States. We have generated maps of peak ground acceleration (PGA) and spectral acceleration (SA) (at 0.2, 0.3 and 1.0 sec, 5% critical damping) at 2% and 10% exceedance probabilities in 50 years. The highest hazard is on the south side of Hawaii Island, as indicated by the MI 7.0, MS 7.2, and MI 7.9 earthquakes, which occurred there since 1868. Probabilistic values of horizontal PGA (2% in 50 years) on Hawaii's south coast exceed 1.75g. Because some large earthquake aftershock zones and the geometry of flank blocks slipping on subhorizontal decollement faults are known, we use a combination of spatially uniform sources in active flank blocks and smoothed seismicity in other areas to model seismicity. Rates of earthquakes are derived from magnitude distributions of the modem (1959-1997) catalog of the Hawaiian Volcano Observatory's seismic network supplemented by the historic (1868-1959) catalog. Modern magnitudes are ML measured on a Wood-Anderson seismograph or MS. Historic magnitudes may add ML measured on a Milne-Shaw or Bosch-Omori seismograph or MI derived from calibrated areas of MM intensities. Active flank areas, which by far account for the highest hazard, are characterized by distributions with b slopes of about 1.0 below M 5.0 and about 0.6 above M 5.0. The kinked distribution means that large earthquake rates would be grossly under-estimated by extrapolating small earthquake rates, and that longer catalogs are essential for estimating or verifying the rates of large earthquakes. Flank earthquakes thus follow a semicharacteristic model, which is a combination of background seismicity and an excess number of large earthquakes. Flank earthquakes are geometrically confined to rupture zones on the volcano flanks by barriers such as rift zones and the seaward edge of the volcano, which may be expressed by a magnitude distribution similar to that including characteristic earthquakes. The island chain northwest of Hawaii Island is seismically and volcanically much less active. We model its seismic hazard with a combination of a linearly decaying ramp fit to the cataloged seismicity and spatially smoothed seismicity with a smoothing half-width of 10 km. We use a combination of up to four attenuation relations for each map because for either PGA or SA, there is no single relation that represents ground motion for all distance and magnitude ranges. Great slumps and landslides visible on the ocean floor correspond to catastrophes with effective energy magnitudes ME above 8.0. A crude estimate of their frequency suggests that the probabilistic earthquake hazard is at least an order of magnitude higher for flank earthquakes than that from submarine slumps.

Bulletin of the Seismological Society of America↗
USGS2001

Spatial correlation of probabilistic earthquake ground motion and loss

Spatial correlation of annual earthquake ground motions and losses can be used to estimate the variance of annual losses to a portfolio of properties exposed to earthquakes A direct method is described for the calculations of the spatial correlation of earthquake ground motions and losses. Calculations for the direct method can be carried out using either numerical quadrature or a discrete, matrix-based approach. Numerical results for this method are compared with those calculated from a simple Monte Carlo simulation. Spatial correlation of ground motion and loss is induced by the systematic attenuation of ground motion with distance from the source, by common site conditions, and by the finite length of fault ruptures. Spatial correlation is also strongly dependent on the partitioning of the variability, given an event, into interevent and intraevent components. Intraevent variability reduces the spatial correlation of losses. Interevent variability increases spatial correlation of losses. The higher the spatial correlation, the larger the variance in losses to a port-folio, and the more likely extreme values become. This result underscores the importance of accurately determining the relative magnitudes of intraevent and interevent variability in ground-motion studies, because of the strong impact in estimating earthquake losses to a portfolio. The direct method offers an alternative to simulation for calculating the variance of losses to a portfolio, which may reduce the amount of calculation required.

Bulletin of the Seismological Society of America↗
USGS2000

USGS National Seismic Hazard Maps

The U.S. Geological Survey (USGS) recently completed new probabilistic seismic hazard maps for the United States, including Alaska and Hawaii. These hazard maps form the basis of the probabilistic component of the design maps used in the 1997 edition of the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures , prepared by the Building Seismic Safety Council and published by FEMA. The hazard maps depict peak horizontal ground acceleration and spectral response at 0.2, 0.3, and 1.0 sec periods, with 10%, 5%, and 2% probabilities of exceedance in 50 years, corresponding to return times of about 500, 1000, and 2500 years, respectively. In this paper we outline the methodology used to construct the hazard maps. There are three basic components to the maps. First, we use spatially smoothed historic seismicity as one portion of the hazard calculation. In this model, we apply the general observation that moderate and large earthquakes tend to occur near areas of previous small or moderate events, with some notable exceptions. Second, we consider large background source zones based on broad geologic criteria to quantify hazard in areas with little or no historic seismicity, but with the potential for generating large events. Third, we include the hazard from specific fault sources. We use about 450 faults in the western United States (WUS) and derive recurrence times from either geologic slip rates or the dating of pre-historic earthquakes from trenching of faults or other paleoseismic methods. Recurrence estimates for large earthquakes in New Madrid and Charleston, South Carolina, were taken from recent paleoliquefaction studies. We used logic trees to incorporate different seismicity models, fault recurrence models, Cascadia great earthquake scenarios, and ground-motion attenuation relations. We present disaggregation plots showing the contribution to hazard at four cities from potential earthquakes with various magnitudes and distances.

Earthquake Spectra↗
USGS2000

Seismic-hazard maps for Hawaii

Probabilistic seismic hazard maps were prepared for Hawaii portraying peak horizontal ground acceleration and horizontal spectral response acceleration for 0.2, 0.3, and 1.0 second periods with probabilities of exceedance of 10% in 50 years and 2% in 50 years. This particular data set is for horizontal spectral response acceleration for 1.0 second period with a 10% probability of exceedance in 50 years. All of the maps were prepared by combining hazard derived from spatially- smoothed historic seismicity with hazard from fault-specific sources. The acceleration values contoured are the random horizontal component. The reference site condition is firm rock, defined as having an average shear-wave velocity of 760 m/sec in the top 30 meters corresponding to the boundary between NEHRP site classes B and C. This data set represents the results of calculations of hazard curves for a grid of points with a spacing of 0.02 degrees in latitude and longitude. The grid of points were contoured to produce the final representation of the seismic-hazard.

Hawaii↗
USGS1992

Triggered earthquakes and deep well activities

Earthquakes can be triggered by any significant perturbation of the hydrologic regime. In areas where potentially active faults are already close to failure, the increased pore pressure resulting from fluid injection, or, alternatively, the massive extraction of fluid or gas, can induce sufficient stress and/or strain changes that, with time, can lead to sudden catastrophic failure in a major earthquake. Injection-induced earthquakes typically result from the reduction in frictional strength along preexisting, nearby faults caused by the increased formation fluid pressure. Earthquakes associated with production appear to respond to more complex mechanisms of subsidence, crustal unloading, and poroelastic changes in response to applied strains induced by the massive withdrawal of subsurface material. As each of these different types of triggered events can occur up to several years after well activities have begun (or even several years after all well activities have stopped), this suggests that the actual triggering process may be a very complex combination of effects, particularly if both fluid extraction and injection have taken place locally. To date, more than thirty cases of earthquakes triggered by well activities can be documented throughout the United States and Canada. Based on these case histories, it is evident that, owing to preexisting stress conditions in the upper crust, certain areas tend to have higher probabilities of exhibiting such induced seismicity. ?? 1992 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1988

Intermediate-term, pre-earthquake phenomena in California, 1975-1986, and preliminary forecast of seismicity for the next decade

Intermediate-term observations preceding earthquakes of magnitude 5.7 or greater in California from 1975 through 1986 suggest that: (1) The sudden appearance of earthquakes in a previously inactive area indicates an increased likelihood of a significant earthquake in that area for a period from days to years; (2) these larger earthquakes tend to occur towards the ends of creeping fault segments; (3) one large earthquake in a region increases the likelihood of a subsequent significant event in the adjacent area; and (4) marginal evidence for the occurrence of a regional deformation event suggests that such events increase the probability of earthquake occurrence throughout the entire area. A common element in many of these observed patterns appears to be the transmission and amplification of tectonic stress changes by the mechanism of fault creep, and suggests that surface fault creep is a sensitive indicator of changes in stress. The preceding critieria are used to construct a preliminary 'forecast' of the likely locations of significant earthquakes over the next decade. ?? 1988 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1988

The northeastern Ohio earthquake of 31 January 1986: Was it induced?

On 31 January 1986, at 11:46 EST, an earthquake of mb = 5.0 occurred about 40 km east of Cleveland, Ohio, and about 17 km south of the Perry Nuclear Power Plant. The earthquake was felt over a broad area, including 11 states, the District of Columbia, and parts of Ontario, Canada, caused intensity VI-VII at distances of 15 km, and generated relatively high accelerations (0.18 g) of short duration at the Perry plant. Thirteen aftershocks were detected as of 15 April, with six occurring within the first 8 days. Two of the aftershocks were felt. Magnitudes for the aftershocks ranged from about 0.5 to 2.5. Focal depths for all of the earthquakes ranged from 2 to 6 km. Except for one small earthquake, all of the aftershocks occurred in a very tight cluster with a north-northeast orientation. Focal mechanisms of the aftershocks exhibit predominantly oblique right-slip motion on nearly vertical nodal planes oriented N15° to 45°E, with a nearly horizontal P axis north of east.

Ohio↗
USGS1987

Modelling aftershock migration and afterslip of the San Juan Bautista, California, earthquake of October 3, 1972

The San Juan Bautista earthquake of October 3, 1972 ( M L = 4.8), located along the San Andreas fault in central California, initiated an aftershock sequence characterized by a subtle, but perceptible, tendency for aftershocks to spread to the northwest and southeast along the fault zone. The apparent dimension of the aftershock zone along strike increased from about 7–10 km within a few days of the earthquake, to about 20 km eight months later. In addition, the mainshock initiated a period of accelerated fault creep, which was observed at 2 creep meters situated astride the trace of the San Andreas fault within about 15 km of the epicenter of the mainshock. The creep rate gradually returned to the preearthquake rate after about 3 yrs. Both the spreading of the aftershocks and the rapid surface creep are interpreted as reflecting a period of rapid creep in the fault zone representing the readjustment of stress and displacement following the failure of a “stuck” patch or asperity during the San Juan Bautista earthquake. Numerical calculations suggest that the behavior of the fault zone is consistent with that of a material characterized by a viscosity of about 3.6×10 14 P, although the real rheology is likely to be more complicated. In this model, the mainshock represents the failure of an asperity that slips only during earthquakes. Aftershocks represent the failure of second-order asperities which are dragged along by the creeping fault zone.

California↗