Paleoseismological data for the Wasatch and San Andreas fault zones have led to the formulation of the characteristic earthquake model, which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum. Analysis of scarp-derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes. At all of the sites studied, the displacement per event has been consistently large; measured values range from 1.6 to 2.6 m, and the average is about 2 m. On the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry, six major segments are recognized along the Wasatch fault. On the basis of the most likely number of surface faulting events (18) that have occurred on segments of the Wasatch fault zone during the past 8000 years, an average recurrence interval of 400–666 years with a preferred average of 444 years is calculated for the entire zone. Geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south-central segment of the San Andreas fault suggest that the M 8 1857 earthquake is a characteristic earthquake for this segment. Comparisons of earthquake recurrence relationships on both the Wasatch and San Andreas faults based on historical seismicity data and geologic data show that a linear (constant b value) extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes. Only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes. The characteristic earthquake appears to be a fundamental aspect of the behavior of the Wasatch and San Andreas faults and may apply to many other faults as well.

Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas Fault Zones

We investigate the distribution of focal depths for earthquakes that do not appear to be associated with zones of recent subduction, using both new results from analyses of individual events recorded at teleseismic distances and published data for both microearthquakes and larger events. The deepest events in oceanic regions occur in old lithosphere (≥100 Ma), and excluding earthquakes in active mountain belts, the deepest crustal events occur in old cratons (tectonic age ≥800 Ma). Therefore, the temperature at the source region is likely to be an important factor determining whether deformation occurs seismically or not. From estimates of the temperatures at depths of the deepest events, we conclude that those limiting temperatures are about 250°–450°C and 600°–800°C for crustal and mantle materials, respectively. In several regions of recent continental convergence, in addition to shallow crustal seismicity, there is seismic activity in the uppermost mantle. The lower crust, however, is essentially aseismic. We infer that both the upper crustal and the mantle seismic regions correspond to zones of relatively high strength and that they are separated by a zone of lower strength in the lower crust where aseismic, ductile deformation predominates. This simple interpretation is qualitatively in agreement with extrapolated values of brittle and ductile strengths of geologic materials studied under appropriate pressure and temperature conditions in the laboratory. A low-strength zone in the lower crust might allow detachment of crystalline nappes from the underlying mantle (and lower crustal) lithosphere. The apparently greater strength of mantle materials than crustal materials at the same temperature implies that oceanic lithosphere is much stronger than continental lithosphere, and this difference may account for why plate tectonics works well in oceanic regions but not in continents.

Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere

stress provinces indicate that these transitions can be abrupt, occurring over <75 km in places. In the western United States, a region of active tectonism characterized by high levels of seismicity and generally high heat flow, the stress pattern is complex, but numerous stress provinces can be well delineated. Despite relative tectonic quiescence in the eastern and central United States, a major variation in principal stress orientation is apparent between the Atlantic Coast and midcontinent areas. Most of the eastern United States is marked by predominantly compressional tectonism (combined thrust and strike slip faulting), whereas much of the region west of the southern Great Plains is characterized by predominantly extensional tectonism (combined normal and strike slip faulting). Deformation along the San Andreas fault and in parts of the Sierra Nevada is nearly pure strike slip. Exceptions to this general pattern include areas of compressional tectonics in the western United States (the Pacific Northwest, the Colorado Plateau interior, and the Big Bend segment of the San Andreas fault) and the normal growth faulting along the Gulf Coastal Plain. Sources of stress are constrained not only by the orientation and relative magnitude of the stresses within a given province but also by the manner of transition of the stress field from one province to another. Much of the modem pattern of stress in the western United States can be attributed to present transform motion and residual thermal and dynamic effects of Tertiary subduction along the western edge of the North American plate. Abrupt stress transitions around actively extending regions in the western United States probably reflect shallow sources of stress and anomalously thin lithosphere. Large areas characterized by a uniform stress field in the central and eastern United States suggest broad scale plate tectonic forces. In the midcontinent region, both ridge push and asthenospheric viscous drag resistance to lithospheric motion can explain the NE-SW compression in the cold, thick lithosphere of the craton, although drag-induced stress directions (resistance to absolute plate motion) correlate better with the data than do the ridge push directions. Asthenospheric counterflow models do not apply in this region, as predicted stress orientations are about 90 o off. A region of compression oriented approximately perpendicular to the continental margin and Appalachian fold belt is defined by the stress data along the Atlantic Coast. This NW-SE compression is in direct contrast with previous models predicting extension perpendicular to passive continental margins due to lateral density contrasts at the continental-oceanic crust interface. Ridge push forces, while capable of producing a component of compression across the coastal area, do not explain the observed orientation of the stress. Two speculative mechanisms are suggested to explain the observed orientations: (1) rotation of stress (or strain) due to anisotropic Appalachian basement structure and (2) flexural effects associated with erosion and isostatic rebound of the Appalachians.

/pdf/state-of-stress-in-the-conterminous-united-states-12cs1w0fvw.pdf

State of stress in the conterminous United States

Models of fault zones in continental crust, based on the analysis of rock deformation textures, suggest that the depth of seismic activity is controlled by the passage from a pressure-sensitive, dominantly frictional regime to strongly temperature-dependent, quasi-plastic mylonitization at greenschist and higher grades of metamorphism. Sufficient knowledge now exists concerning the frictional and rheological properties of quartz-bearing rocks to construct crude strength-depth curves for different geotherms. In such models, shear resistance peaks sharply at the inferred seismic-aseismic transition. The maximum depth of microseismic activity in various heat flow provinces of the conterminous United States generally correlates well with the frictional to quasi-plastic transition modeled for the different geotherms. Larger earthquakes ( M L > 5.5) also tend to nucleate near the base of the seismogenic zone. This region is postulated to have the highest concentration of distortional strain energy for stress levels at failure, and can be regarded as the prime asperity in crustal fault zones.

Fault zone models, heat flow, and the depth distribution of earthquakes in the continental crust of the United States

▪ Abstract Earthquake triggering is the process by which stress changes associated with an earthquake can induce or retard seismic activity in the surrounding region or trigger other earthquakes at great distances. Calculations of static Coulomb stress changes associated with earthquake slip have proven to be a powerful tool in explaining many seismic observations, including aftershock distributions, earthquake sequences, and the quiescence of broad, normally active regions following large earthquakes. Delayed earthquake triggering, which can range from seconds to decades, can be explained by a variety of time-dependent stress transfer mechanisms, such as viscous relaxation, poroelastic rebound, or afterslip, or by reductions in fault friction, such as predicted by rate and state constitutive relations. Rapid remote triggering of earthquakes at great distances (from several fault lengths to 1000s of km) is best explained by the passage of transient (dynamic) seismic waves, which either immediately induce ...

Earthquake triggering by static, dynamic, and postseismic stress transfer

The Taltal area, which lies within the coastal cordillera of northern Chile, is dominated by a group of major active faults that cut a eugeosynclinal section of predominantly Jurassic andesites overlying Paleozoic metamorphic and plutonic rocks and intruded by Late Mesozoic plutons. The Atacama fault, a suggested regional strike-slip fracture parallel to the coastline, has been obliquely cut and left-laterally offset 10 km by the Taltal fault, which passes through the town of Taltal. Three distinctive features were found to be consistently offset 10 km by the Taltal fault: the easternmost strand of the Atacama fault, an intrusive contact, and a unique volcanic unit. Former continuity of the Atacama fault through the Taltal region is proposed, and subsequent disruption by the Taltal fault appears to have caused major structural readjustments in the still-active Atacama fault zone.

The tentative offshore epicenter and aftershock distribution of the December 28 earthquake are not directly correlative with faults that have been mapped in the nearby on-shore areas; this lack of correlation is not surprising in view of the suggested depths of hypocenters in the lower crust or upper mantle.

/pdf/geologic-structure-of-the-taltal-area-northern-chile-in-1pj1gck4nv.pdf

Geologic structure of the Taltal Area, Northern Chile, in relation to the earthquake of December 28, 1966

We have calibrated new coda-magnitude (MC) equations for local earthquakes digitally recorded since 1981 in the Utah (UT) region and since 1984 in the Yellowstone National Park (YP) region by the University of Utah Seismograph Stations (UUSS) network. The primary motivation for this study was the recognition of systematic time-dependent differences between MC and local magnitude (ML), ranging up to 0.4 and 0.9 units in the UT and YP regions, respectively. The new MC equations, calibrated against MLs determined from paper and synthetic Wood-Anderson records, are: MC = -2.25 + 2.32 log τ + 0.0023Δ in the UT region, MC = -2.60 + 2.44 log τ + 0.0040Δ in the YP region, where Δ is epicentral distance in kilometers and τ is signal duration in seconds on a short-period vertical-component record, measured from the P-wave onset to the time that the average absolute value of the ground velocity drops below 0.01724 microns/sec. To determine the constants in the MC equations, we used an orthogonal regression method rather than linear regression. The latter produces biased results because the errors in log τ and Δ are not negligible compared to the errors in ML. The signal duration definition used in previous UUSS MC calculations was the time from the P-wave onset to when the signal drops below the pre-event noise level. This definition, coupled with changes in noise levels and instrument gains, was largely responsible for the time

Correction of Systematic Time-Dependent Coda Magnitude Errors in the Utah and Yellowstone National Park Region Earthquake Catalogs, 1981-2001

/pdf/anss-advanced-national-seismic-system-31i0zewp6p.pdf

ANSS-Advanced National Seismic System

The State of Utah’s rules for dam safety require that an “Operating Basis Earthquake” (OBE) be determined for all dams within seismic zones 2 and 3 of the Uniform Building Code. The OBE is described as the earthquake with a return interval of at least 200 yrs that has the greatest potential to cause damage at the site, considering all active earthquake sources which could affect it. Previous studies have shown that at most sites in the Wasatch Front region, the dominant sources of seismic hazard for 200-yr return periods are moderate-sized earthquakes of ML ≤ 6.5. This result suggests that an earthquake in this size range would be an appropriate OBE for a Wasatch Front region dam. Because such earthquakes are below the usual threshold of surface faulting, their locations and magnitudes cannot easily be predicted. Hence, in seismic hazard evaluations, these moderate-sized earthquakes are usually assumed to occur randomly throughout the region. This randomness complicates the task of selecting a specific OBE for use in engineering design purposes. We evaluate an approach to the problem of selecting an OBE which employs the concept of the “probabilistic epicentral distance” (r0) introduced in the 1980’s by C.K. Wood and D.A. Ostenaa of the U.S. Bureau of Reclamation. In the application considered here, r0 is the radius of a circle within which the annual probability of an earthquake of 5.5 ≤ ML ≤ 6.5 is equal to some specified small number. Using an average rate of occurrence of earthquakes in this size range estimated from historical and instrumental seismicity data for the Wasatch Front region, it is found that r0 = 85, 58, 38, and 17 km for annual probabilities (inverse return periods) of 1/95, 1/200, 1/475, and 1/2373, respectively. Based on these distances, the calculated median magnitude of 5.8, and (for example) the Boore and others (1993) attenuation relation for peak horizontal acceleration (PHA), we calculate that the expected PHA’s are, respectively, 0.04, 0.05, 0.07, and 0.13 g. These PHA’s are similar to those obtained for the same annual probabilities from a simple probabilistic seismic hazard analysis in which only random earthquakes of 5.5 ≤ ML ≤ 6.5 (at distances of up to 100 km) are considered and the uncertainties in the peak accelerations predicted by the attenuation relation are ignored. However, they are factors of 1.5 to 5.7 lower than values obtained from more complete probabilistic hazard analyses for the Wasatch Front region, which take into account smaller and larger potential earthquakes and also the uncertainties in the predicted PHA’s. Thus, at least in this region, the probabilistic distance method yields an OBE which significantly under-represents the ground shaking hazard. We suggest an alternative and more conservative approach to the problem of selecting an OBE which involves the direct use of results from a regional or site-specific probabilistic seismic hazard analysis.

The Problem of the Random Earthquake in Seismic Hazard Analysis: Wasatch Front Region, Utah

Recent developments in digital communication and seismometry
are allowing seismologists to propose revolutionary
new ways to reduce vulnerability from earthquakes, volcanoes,
and tsunamis, and to better understand these
phenomena as well as the basic structure and dynamics of the
Earth. This document provides a brief description of some of
the critical new problems that can be addressed using modem
digital seismic networks. It also provides an overview of existing
seismic networks and suggests ways to integrate these
together into a National Seismic System.
A National Seismic System will consist of a number of
interconnected regional networks (such as southern California,
central and northern California, northeastern United
States, northwestern United States, and so on) that are jointly
operated by Federal, State, and private seismological research
institutions. Regional networks will provide vital information
concerning the hazards of specific regions. Parts of these networks
will be linked to provide uniform rapid response on a
national level (the National Seismic Network).
A National Seismic System promises to significantly
reduce societal risk to earthquake losses and to open new areas
of fundamental basic research. The following is a list of some
of the uses of a National Seismic System.

https://pubs.usgs.gov/circ/1989/1031/report.pdf

National Seismic System science plan

Handle everyday research tasks with reliable, citation-backed results

Your personal Research Agent to handle research tasks with citation-backed results

Popular Tasks used by Researchers

How can I help with your research?

Meet SciSpace

Get more enhanced response by uploading the PDFs you want me to reference.

No relevant PDFs in your library

SciSpace is the AI research assistant for academics. Run systematic literature reviews on 280M+ papers, and write papers with cited sources. Trusted by 1M+ students, PhDs & researchers.

SciSpace | AI for Research

Analyze PDFs

Code & Manuscripts

Funding & Grants

Literature & Patents

Medical & Clinical Data

Systematic Review

Visualize & Present

Web & Data

Build a Google Scholar-like website for your research.

Build a website

Create charts and images for your research

Create a Chart

Write a paper for submission to a journal

Draft a manuscript

Patent Search

Design eye-catching scientific posters in minutes.

Scientific Poster Generation

Systematic Literature Review

One task is running at the moment. Your messages will be shown right after.

Drag and drop or click here to browse

Loved by <highlight>1 million+</highlight> researchers

Extract a list of specific topics and their sources from unstructured text

Topics

Compare and analyze relevant papers that matches with your search

Papers

Get insights from PDFs and bookmarked papers from your library

My library

Recent searches

Try searching for:

Catch AI-generated content in scholarly and non-scholarly content

{ai} Detector

Ai Writer

Get PDF Summaries, highlighted text explanations 

Chat with PDF

Effortlessly create in-text citations and bibliographies in APA and 2,500 other formats

Citation generator

Get explanations, summaries, and answers on academic papers

Ease up your research workflow with {scispace}'s cohort of exciting AI tools

Elevate your academic writing skills and convey your ideas the way you want

Paraphraser

Explore our range of reading and writing tools

Your file is being prepared and should be ready in a few minutes. If it's a large file, it might take a bit longer. You can close this window, and we'll email you the file when it's done.

You have reached a maximum limit of <strong>{limit}</strong> columns in the table. Remove at least <strong>1</strong> column to add or create another one.

Walter J. Arabasz

Author Tools

Chat about Author