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Geology topics

T. Parsons

Publications and source records attributed to T. Parsons.

At least 19 recordsLinked to original sources
USGS2011

Assessing historical rate changes in global tsunami occurrence

The global catalogue of tsunami events is examined to determine if transient variations in tsunami rates are consistent with a Poisson process commonly assumed for tsunami hazard assessments. The primary data analyzed are tsunamis with maximum sizes >1m. The record of these tsunamis appears to be complete since approximately 1890. A secondary data set of tsunamis >0.1m is also analyzed that appears to be complete since approximately 1960. Various kernel density estimates used to determine the rate distribution with time indicate a prominent rate change in global tsunamis during the mid-1990s. Less prominent rate changes occur in the early- and mid-20th century. To determine whether these rate fluctuations are anomalous, the distribution of annual event numbers for the tsunami catalogue is compared to Poisson and negative binomial distributions, the latter of which includes the effects of temporal clustering. Compared to a Poisson distribution, the negative binomial distribution model provides a consistent fit to tsunami event numbers for the >1m data set, but the Poisson null hypothesis cannot be falsified for the shorter duration >0.1m data set. Temporal clustering of tsunami sources is also indicated by the distribution of interevent times for both data sets. Tsunami event clusters consist only of two to four events, in contrast to protracted sequences of earthquakes that make up foreshock-main shock-aftershock sequences. From past studies of seismicity, it is likely that there is a physical triggering mechanism responsible for events within the tsunami source 'mini-clusters'. In conclusion, prominent transient rate increases in the occurrence of global tsunamis appear to be caused by temporal grouping of geographically distinct mini-clusters, in addition to the random preferential location of global M >7 earthquakes along offshore fault zones.

Geophysical Journal International↗
USGS2011

Seismic survey probes urban earthquake hazards in Pacific Northwest

A multidisciplinary seismic survey earlier this year in the Pacific Northwest is expected to reveal much new information about the earthquake threat to U.S. and Canadian urban areas there. A disastrous earthquake is a very real possibility in the region. The survey, known as the Seismic Hazards Investigation in Puget Sound (SHIPS), engendered close cooperation among geologists, biologists, environmental groups, and government agencies. It also succeeded in striking a fine balance between the need to prepare for a great earthquake and the requirement to protect a coveted marine environment while operating a large airgun array.

Washington↗
USGS2011

Absence of remotely triggered large earthquakes beyond the mainshock region

Large earthquakes are known to trigger earthquakes elsewhere. Damaging large aftershocks occur close to the mainshock and microearthquakes are triggered by passing seismic waves at significant distances from the mainshock. It is unclear, however, whether bigger, more damaging earthquakes are routinely triggered at distances far from the mainshock, heightening the global seismic hazard after every large earthquake. Here we assemble a catalogue of all possible earthquakes greater than M 5 that might have been triggered by every M 7 or larger mainshock during the past 30 years. We compare the timing of earthquakes greater than M 5 with the temporal and spatial passage of surface waves generated by large earthquakes using a complete worldwide catalogue. Whereas small earthquakes are triggered immediately during the passage of surface waves at all spatial ranges, we find no significant temporal association between surface-wave arrivals and larger earthquakes. We observe a significant increase in the rate of seismic activity at distances confined to within two to three rupture lengths of the mainshock. Thus, we conclude that the regional hazard of larger earthquakes is increased after a mainshock, but the global hazard is not.

Nature Geoscience↗
USGS2011

Diffuse Pacific-North American plate boundary: 1000 km of dextral shear inferred from modeling geodetic data

Geodetic measurements tell us that the eastern part of the Basin and Range Province expands in an east-west direction relative to stable North America, whereas the western part of the province moves to the northwest. We develop three-dimensional finite element representations of the western United States lithosphere in an effort to understand the global positioning system (GPS) signal. The models are constrained by known bounding-block velocities and topography, and Basin and Range Province deformation is represented by simple plastic (thermal creep) rheology. We show that active Basin and Range spreading by gravity collapse is expected to have a strong southward component that does not match the GPS signal. We can reconcile the gravitational component of displacement with observed velocity vectors if the Pacific plate applies northwest-directed shear stress to the Basin and Range via the Sierra Nevada block. This effect reaches at least 1000 km east of the San Andreas fault in our models.

Nevada↗
USGS2011

No correlation between Anderson Reservoir stage level and underlying Calaveras fault seismicity despite calculated differential stress increases

Concerns have been raised that stresses from reservoir impoundment may trigger damaging earthquakes because rate changes have been associated with reservoir impoundment or stage-level changes globally. Here, the idea is tested blindly using Anderson Reservoir, which lies atop the seismically active Calaveras fault. The only knowledge held by the author going into the study was the expectation that reservoir levels change cyclically because of seasonal rainfall. Examination of seismicity rates near the reservoir reveals variability, but no correlation with stage-level changes. Three-dimensional finite-element modeling shows stress changes sufficient for earthquake triggering along the Calaveras fault zone. Since many of the reported cases of induced triggering come from low-strain settings, it is speculated that gradual stressing from stage-level changes in high-strain settings may not be significant. From this study, it can be concluded that reservoirs are not necessarily risky in active tectonic settings.

California↗
USGS2010

A simple algorithm for sequentially incorporating gravity observations in seismic traveltime tomography

The geologic structure of the Earth's upper crust can be revealed by modeling variation in seismic arrival times and in potential field measurements. We demonstrate a simple method for sequentially satisfying seismic traveltime and observed gravity residuals in an iterative 3-D inversion. The algorithm is portable to any seismic analysis method that uses a gridded representation of velocity structure. Our technique calculates the gravity anomaly resulting from a velocity model by converting to density with Gardner's rule. The residual between calculated and observed gravity is minimized by weighted adjustments to the model velocity-depth gradient where the gradient is steepest and where seismic coverage is least. The adjustments are scaled by the sign and magnitude of the gravity residuals, and a smoothing step is performed to minimize vertical streaking. The adjusted model is then used as a starting model in the next seismic traveltime iteration. The process is repeated until one velocity model can simultaneously satisfy both the gravity anomaly and seismic traveltime observations within acceptable misfits. We test our algorithm with data gathered in the Puget Lowland of Washington state, USA (Seismic Hazards Investigation in Puget Sound [SHIPS] experiment). We perform resolution tests with synthetic traveltime and gravity observations calculated with a checkerboard velocity model using the SHIPS experiment geometry, and show that the addition of gravity significantly enhances resolution. We calculate a new velocity model for the region using SHIPS traveltimes and observed gravity, and show examples where correlation between surface geology and modeled subsurface velocity structure is enhanced.

Washington↗
USGS2009

Probabilistic tsunami hazard assessment at Seaside, Oregon, for near-and far-field seismic sources

The first probabilistic tsunami flooding maps have been developed. The methodology, called probabilistic tsunami hazard assessment (PTHA), integrates tsunami inundation modeling with methods of probabilistic seismic hazard assessment (PSHA). Application of the methodology to Seaside, Oregon, has yielded estimates of the spatial distribution of 100- and 500-year maximum tsunami amplitudes, i.e., amplitudes with 1% and 0.2% annual probability of exceedance. The 100-year tsunami is generated most frequently by far-field sources in the Alaska-Aleutian Subduction Zone and is characterized by maximum amplitudes that do not exceed 4 m, with an inland extent of less than 500 m. In contrast, the 500-year tsunami is dominated by local sources in the Cascadia Subduction Zone and is characterized by maximum amplitudes in excess of 10 m and an inland extent of more than 1 km. The primary sources of uncertainty in these results include those associated with interevent time estimates, modeling of background sea level, and accounting for temporal changes in bathymetry and topography. Nonetheless, PTHA represents an important contribution to tsunami hazard assessment techniques; viewed in the broader context of risk analysis, PTHA provides a method for quantifying estimates of the likelihood and severity of the tsunami hazard, which can then be combined with vulnerability and exposure to yield estimates of tsunami risk. Copyright 2009 by the American Geophysical Union.

Journal of Geophysical Research C: Oceans↗
USGS2009

On near-source earthquake triggering

[1] When one earthquake triggers others nearby, what connects them? Two processes are observed: static stress change from fault offset and dynamic stress changes from passing seismic waves. In the near-source region ( r ≤ 50 km for M ∼ 5 sources) both processes may be operating, and since both mechanisms are expected to raise earthquake rates, it is difficult to isolate them. We thus compare explosions with earthquakes because only earthquakes cause significant static stress changes. We find that large explosions at the Nevada Test Site do not trigger earthquakes at rates comparable to similar magnitude earthquakes. Surface waves are associated with regional and long-range dynamic triggering, but we note that surface waves with low enough frequency to penetrate to depths where most aftershocks of the 1992 M = 5.7 Little Skull Mountain main shock occurred (∼12 km) would not have developed significant amplitude within a 50-km radius. We therefore focus on the best candidate phases to cause local dynamic triggering, direct waves that pass through observed near-source aftershock clusters. We examine these phases, which arrived at the nearest (200–270 km) broadband station before the surface wave train and could thus be isolated for study. Direct comparison of spectral amplitudes of presurface wave arrivals shows that M ∼ 5 explosions and earthquakes deliver the same peak dynamic stresses into the near-source crust. We conclude that a static stress change model can readily explain observed aftershock patterns, whereas it is difficult to attribute near-source triggering to a dynamic process because of the dearth of aftershocks near large explosions.

Journal of Geophysical Research B: Solid Earth↗
USGS2009

Can footwall unloading explain late Cenozoic uplift of the Sierra Nevada crest?

Globally, normal-fault displacement bends and warps rift flanks upwards, as adjoining basins drop downwards. Perhaps the most evident manifestations are the flanks of the East African Rift, which cuts across the otherwise minimally deformed continent. Flank uplift was explained by Vening Meinesz (1950, Institut Royal Colonial Belge, Bulletin des Seances, v. 21, p. 539-552), who recognized that isostasy should cause uplift of a normal-faulted footwall and subsidence of its hanging wall. Uplift occurs because slip on a dipping normal fault creates a broader root of less-dense material beneath the footwall, and a narrowed one beneath the hanging wall. In this paper, we investigate the potential influence of this process on the latest stages of Sierra Nevada uplift. Through theoretical calculations and 3D finite element modelling, we find that cumulative slip of about 4km on range-front faults would have produced about 1.3km peak isostatic uplift at the ridge crest. Numerical models suggest that the zone of uplift is narrow, with the width controlled by bending resistance of the seismogenic crust. We conclude that footwall unloading cannot account for the entire elevation of the Sierran crest above sea level, but if range-front faulting initiated in an already elevated plateau like the adjacent Basin and Range Province, then a hybrid model of pre-existing regional uplift and localized footwall unloading can account for the older and newer uplift phases suggested by the geologic record.

International Geology Review↗
USGS2009

Uniform California earthquake rupture forecast, version 2 (UCERF 2)

The 2007 Working Group on California Earthquake Probabilities (WGCEP, 2007) presents the Uniform California Earthquake Rupture Forecast, Version 2 (UCERF 2). This model comprises a time-independent (Poisson-process) earthquake rate model, developed jointly with the National Seismic Hazard Mapping Program and a time-dependent earthquake-probability model, based on recent earthquake rates and stress-renewal statistics conditioned on the date of last event. The models were developed from updated statewide earthquake catalogs and fault deformation databases using a uniform methodology across all regions and implemented in the modular, extensible Open Seismic Hazard Analysis framework. The rate model satisfies integrating measures of deformation across the plate-boundary zone and is consistent with historical seismicity data. An overprediction of earthquake rates found at intermediate magnitudes (6.5 ??? M ???7.0) in previous models has been reduced to within the 95% confidence bounds of the historical earthquake catalog. A logic tree with 480 branches represents the epistemic uncertainties of the full time-dependent model. The mean UCERF 2 time-dependent probability of one or more M ???6.7 earthquakes in the California region during the next 30 yr is 99.7%; this probability decreases to 46% for M ???7.5 and to 4.5% for M ???8.0. These probabilities do not include the Cascadia subduction zone, largely north of California, for which the estimated 30 yr, M ???8.0 time-dependent probability is 10%. The M ???6.7 probabilities on major strike-slip faults are consistent with the WGCEP (2003) study in the San Francisco Bay Area and the WGCEP (1995) study in southern California, except for significantly lower estimates along the San Jacinto and Elsinore faults, owing to provisions for larger multisegment ruptures. Important model limitations are discussed.

Bulletin of the Seismological Society of America↗
USGS2009

Assessment of source probabilities for potential tsunamis affecting the U.S. Atlantic coast

Estimating the likelihood of tsunamis occurring along the U.S. Atlantic coast critically depends on knowledge of tsunami source probability. We review available information on both earthquake and landslide probabilities from potential sources that could generate local and transoceanic tsunamis. Estimating source probability includes defining both size and recurrence distributions for earthquakes and landslides. For the former distribution, source sizes are often distributed according to a truncated or tapered power-law relationship. For the latter distribution, sources are often assumed to occur in time according to a Poisson process, simplifying the way tsunami probabilities from individual sources can be aggregated. For the U.S. Atlantic coast, earthquake tsunami sources primarily occur at transoceanic distances along plate boundary faults. Probabilities for these sources are constrained from previous statistical studies of global seismicity for similar plate boundary types. In contrast, there is presently little information constraining landslide probabilities that may generate local tsunamis. Though there is significant uncertainty in tsunami source probabilities for the Atlantic, results from this study yield a comparative analysis of tsunami source recurrence rates that can form the basis for future probabilistic analyses.

Marine Geology↗
USGS2008

Tsunami probability in the Caribbean Region

We calculated tsunami runup probability (in excess of 0.5 m) at coastal sites throughout the Caribbean region. We applied a Poissonian probability model because of the variety of uncorrelated tsunami sources in the region. Coastlines were discretized into 20 km by 20 km cells, and the mean tsunami runup rate was determined for each cell. The remarkable ???500-year empirical record compiled by O'Loughlin and Lander (2003) was used to calculate an empirical tsunami probability map, the first of three constructed for this study. However, it is unclear whether the 500-year record is complete, so we conducted a seismic moment-balance exercise using a finite-element model of the Caribbean-North American plate boundaries and the earthquake catalog, and found that moment could be balanced if the seismic coupling coefficient is c = 0.32. Modeled moment release was therefore used to generate synthetic earthquake sequences to calculate 50 tsunami runup scenarios for 500-year periods. We made a second probability map from numerically-calculated runup rates in each cell. Differences between the first two probability maps based on empirical and numerical-modeled rates suggest that each captured different aspects of tsunami generation; the empirical model may be deficient in primary plate-boundary events, whereas numerical model rates lack backarc fault and landslide sources. We thus prepared a third probability map using Bayesian likelihood functions derived from the empirical and numerical rate models and their attendant uncertainty to weight a range of rates at each 20 km by 20 km coastal cell. Our best-estimate map gives a range of 30-year runup probability from 0 - 30% regionally. ?? irkhaueser 2008.

Conference Paper↗
USGS2008

Stress changes from the 2008 Wenchuan earthquake and increased hazard in the Sichuan basin

On 12 May 2008, the devastating magnitude 7.9 (Wenchuan) earthquake struck the eastern edge of the Tibetan plateau, collapsing buildings and killing thousands in major cities aligned along the western Sichuan basin in China. After such a large-magnitude earthquake, rearrangement of stresses in the crust commonly leads to subsequent damaging earthquakes. The mainshock of the 12 May earthquake ruptured with as much as 9 m of slip along the boundary between the Longmen Shan and Sichuan basin, and demonstrated the complex strike-slip and thrust motion that characterizes the region. The Sichuan basin and surroundings are also crossed by other active strike-slip and thrust faults. Here we present calculations of the coseismic stress changes that resulted from the 12 May event using models of those faults, and show that many indicate significant stress increases. Rapid mapping of such stress changes can help to locate fault sections with relatively higher odds of producing large aftershocks. ??2008 Macmillan Publishers Limited. All rights reserved.

Nature↗
USGS2008

Distribution of tsunami interevent times

The distribution of tsunami interevent times is analyzed using global and site-specific (Hilo, Hawaii) tsunami catalogs. An empirical probability density distribution is determined by binning the observed interevent times during a period in which the observation rate is approximately constant. The empirical distributions for both catalogs exhibit non-Poissonian behavior in which there is an abundance of short interevent times compared to an exponential distribution. Two types of statistical distributions are used to model this clustering behavior: (1) long-term clustering described by a universal scaling law, and (2) Omori law decay of aftershocks and triggered sources. The empirical and theoretical distributions all imply an increased hazard rate after a tsunami, followed by a gradual decrease with time approaching a constant hazard rate. Examination of tsunami sources suggests that many of the short interevent times are caused by triggered earthquakes, though the triggered events are not necessarily on the same fault.

Geophysical Research Letters↗
USGS2008

Vertical tectonic deformation associated with the San Andreas fault zone offshore of San Francisco, California

A new fault map of the shelf offshore of San Francisco, California shows that faulting occurs as a distributed shear zone that involves many fault strands with the principal displacement taken up by the San Andreas fault and the eastern strand of the San Gregorio fault zone. Structures associated with the offshore faulting show compressive deformation near where the San Andreas fault goes offshore, but deformation becomes extensional several km to the north off of the Golden Gate. Our new fault map serves as the basis for a 3-D finite element model that shows that the block between the San Andreas and San Gregorio fault zone is subsiding at a long-term rate of about 0.2-0.3??mm/yr, with the maximum subsidence occurring northwest of the Golden Gate in the area of a mapped transtensional basin. Although the long-term rates of vertical displacement primarily show subsidence, the model of coseismic deformation associated with the 1906 San Francisco earthquake indicates that uplift on the order of 10-15??cm occurred in the block northeast of the San Andreas fault. Since 1906, 5-6??cm of regional subsidence has occurred in that block. One implication of our model is that the transfer of slip from the San Andreas fault to a fault 5??km to the east, the Golden Gate fault, is not required for the area offshore of San Francisco to be in extension. This has implications for both the deposition of thick Pliocene-Pleistocene sediments (the Merced Formation) observed east of the San Andreas fault, and the age of the Peninsula segment of the San Andreas fault.

Tectonophysics↗
USGS2008

Monte Carlo method for determining earthquake recurrence parameters from short paleoseismic catalogs: Example calculations for California

Paleoearthquake observations often lack enough events at a given site to directly define a probability density function (PDF) for earthquake recurrence. Sites with fewer than 10-15 intervals do not provide enough information to reliably determine the shape of the PDF using standard maximum-likelihood techniques (e.g., Ellsworth et al., 1999). In this paper I present a method that attempts to fit wide ranges of distribution parameters to short paleoseismic series. From repeated Monte Carlo draws, it becomes possible to quantitatively estimate most likely recurrence PDF parameters, and a ranked distribution of parameters is returned that can be used to assess uncertainties in hazard calculations. In tests on short synthetic earthquake series, the method gives results that cluster around the mean of the input distribution, whereas maximum likelihood methods return the sample means (e.g., NIST/SEMATECH, 2006). For short series (fewer than 10 intervals), sample means tend to reflect the median of an asymmetric recurrence distribution, possibly leading to an overestimate of the hazard should they be used in probability calculations. Therefore a Monte Carlo approach may be useful for assessing recurrence from limited paleoearthquake records. Further, the degree of functional dependence among parameters like mean recurrence interval and coefficient of variation can be established. The method is described for use with time-independent and time-dependent PDFs, and results from 19 paleoseismic sequences on strike-slip faults throughout the state of California are given.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Earthquake recurrence on the south Hayward fault is most consistent with a time dependent, renewal process

Elastic rebound and stress renewal are important components of earthquake forecasting because if large earthquakes can be shown to be periodic, then rupture probability is time dependent. While renewal models are used in formal forecasts, it has not been possible to exclude the alternate view that repeated large earthquakes can happen in rapid succession without requiring time for stress regeneration. Here a consistency test between time dependent and time independent recurrence distributions is made using a Monte Carlo method to replicate the paleoseismic series on the south Hayward fault. Time dependent distributions with recurrence interval of 210 years and coefficient of variation of 0.6 reproduce the event series on the south Hayward 5 times more often than any exponential distribution: a highly significant difference as determined using a two-tailed Z-test for relative proportions. Therefore large Hayward fault earthquakes are quasi-periodic and are most consistent with a stress renewal process.

Geophysical Research Letters↗