Assessment of Regional Earthquake Hazards and Risk Along the Wasatch Front, Utah

289 Pages·2012·11.68 MB·English

U.S. Department of the Interior
U.S. Geological Survey
Assessment of Regional Earthquake Hazards and
Risk Along the Wasatch Front, Utah
FRONT COVER. View to the southeast of downtown Salt Lake City, Utah, circa
1980. Wasatch Range with prominent Lake Bonneville shorelines in the
background.
Assessment of Regional Earthquake Hazards and
Risk Along the Wasatch Front, Utah
PAULA L. GORI and WALTER W. HAYS, Editors
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1500-K-R
Contributions from
Utah Geological Survey,
University of Utah, and
Utah State University
Chapters K R are issued as a single volume
and are not available separately.
Chapter titles are listed in the volume
table of contents.
U.S. DEPARTMENT OF THE INTERIOR
BRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY
Charles G. Groat, Director
Any use of trade, product, or firm names in this publication is for descriptive purposes only and
does not imply endorsement by the U.S. Government.
Author affiliations given in this volume were correct at the time
the report was approved for publication.
Published in the Eastern Region, Reston, Va.
Manuscript approved for publication October 5,1995.
Library of Congress Cataloging in Publication Data
Assessing regional earthquake hazards and risk along the Wasatch Front, Utah / edited by Paula L.
Gori and Walter W. Hays.
p. cm. (U.S. Geological Survey professional paper : 1500K-R)
Includes bibliographical references.
Supt.ofDocs.no.: 119.16: 1500KZ.
1. Earthquakes Wasatch Range (Utah and Idaho) 2. Earthquakes Utah. I. Gori, Paula. II.
Hays, Walter W. III. Series: U.S. Geological Survey professional paper; 1500.
QE535.2.U6A84 2000
551.2'2'097922 dc20 92-33618
CIP
For sale by U.S. Geological Survey, Information Services
Box 25286, Federal Center, Denver, CO 80225
CONTENTS
[Letters designate the chapters]
Introduction, by Paula L. Gori and Walter W. Hays
(K) Site Amplification in the Salt Lake City-Ogden-Provo Urban Corridor and the Implications for Earthquake-
Resistant Design, by Walter W. Hays
(L) Predicting Strong Ground Motion in Utah, by Kenneth W. Campbell
(M) Probabilistic Analysis of Earthquake Ground-Shaking Hazards Along the Wasatch Front, Utah, by R.R.
Youngs, F.H. Swan, M.S. Power, D.P. Schwartz, and R.K. Green
(N) Relative Ground Response in Salt Lake City and Areas of Springville-Spanish Fork, Utah, by Kenneth W
King, Robert A. Williams, and David L. Carver
(O) In Situ Poisson's Ratio Measurements Near Prove, Utah, by Richard D. Miller, Don W. Steeples, Kenneth
W. King, and Ralph W Knapp
(P) Earthquake Losses in Central Utah, by S.T. Algermissen, E.P. Arnold, K.V. Steinbrugge, Margaret G.
Hopper, and PS. Powers
(Q) Isoseismals of Some Historical Earthquakes Affecting the Wasatch Front Area, Utah, by Margaret G.
Hopper
(R) Seismic-Risk Methods and Estimates for Utility Systems and State-Owned Buildings Along the Wasatch
Front, by Craig E. Taylor and Delbert B. Ward
III

ASSESSMENT OF REGIONAL EARTHQUAKE HAZARDS AND RISK
ALONG THE WASATCH FRONT, UTAH
INTRODUCTION
By Paula L. Gori and Walter W. Hays
THE EARTHQUAKE THREAT of normal faulting that extends approximately 370 km at
the western foot of the Wasatch Range. Earthquakes have
There are many geologists who are very wise, but even they do
been reported since the arrival of the Mormon pioneers in
not understand the forces which produce mountains. And yet it
1847. During the last two centuries, eight earthquakes of
must be admitted, not only that mountains have been made, but
that some mountains are still rising. The mysterious forces magnitude1 greater than or equal to 6 have occurred in
appear to act in different ways in different places, and it is pos Utah. The two largest were the magnitude (ML 7.0) 6.6
sible that their nature is not universally the same. Suffice it to Hansel Valley earthquake of 1934 (fig. 2) and the ML 6.5
say that in the Great Basin the movements they cause are verti
Richfield earthquake of 1901.
cal. It is as though something beneath each mountain was
slowly, steadily, and irresistibly rising, carrying the mountain The historical record of seismicity in Utah has been
with it. broadened by incorporating geologic evidence and moni
toring small earthquakes. Although no large earthquakes
So began an article in 1883 by Grove Karl Gilbert, the first
(M>7.0) have occurred since Utah was settled, clear geo
Chief Geologist of the U.S. Geological Survey (USGS),
logic and geomorphic evidence (Hamblin, 1976) demon
warning the citizens of the Great Basin about the poten
strates that large earthquakes have occurred repeatedly
tial for damaging earthquakes in their region. Describing
throughout the late Pleistocene (2 million years before
the damage and loss of life in a small town in California
present) and Holocene (last 10,000 years) on segments
that he had just visited after an earthquake, Gilbert cau
within the Wasatch fault zone. For this reason and
tioned that a similar earthquake could occur in Salt Lake
because of the continuing low level of seismicity, scien
City. "It is useless to ask when this disaster will occur. Our
tists believe that some parts of segments of the Wasatch
occupation of the country has been too brief for us to
fault zone are overdue for a damaging earthquake
learn how fast the Wasatch grows; and indeed, it is only
(Schwartz and Coppersmith, 1984). Machette and others
by such disasters that we can learn. By the time experi
(this report) have identified 10 segments of the Wasatch
ence has taught us this, Salt Lake City will have been
fault that have produced large earthquakes on the average
shaken down...." (Gilbert, 1883, p. 4).
of every several hundred to 1,000 years.
After more than a century of research, geologists and
other scientists have increased their understanding of the
tectonic processes at work in the Wasatch Front and are
now beginning to answer the questions about the seismic- Magnitude is a "number that characterizes the size of an earth
ity and tectonics of the Wasatch Front that Gilbert and quake, based on measurement of the maximum motions recorded by a
seismograph for earthquake waves of a particular frequency. Scales
others first posed. Scientists, engineers, architects, urban
most commonly used in the Western United States are (1) local magni
planners, and emergency managers are not waiting for a
tude (ML) (commonly referred to as "Richter magnitude"), (2) surface-
major earthquake disaster to learn that measures must be wave magnitude (Ms), and (3) body-wave magnitude (ms). None of these
implemented to mitigate an earthquake's effects. They are scales satisfactorily measures the largest possible earthquakes because
taking actions now to prepare for and to mitigate the each relates to only certain frequencies of seismic waves and because
the spectrum of radiated seismic energy changes with earthquake size.
physical effects of such an earthquake.
The recently devised moment magnitude (M) scale, based on the con
The majority of Utah's population lives adjacent to the cept of seismic moment, is uniformly applicable to all sizes of earth
Wasatch fault zone (fig. 1), an active, north-trending zone quakes" (Ziony, 1985, p. 18).
REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT
113° IDAHO 112
41
h- /'
0 50 100 KILOMETERS
FIGURE 1 Location map of the Wasatch Front study area (shaded), northern Utah. Generalized fault locations are also shown.
INTRODUCTION
The USGS earthquake hazards program in the Wasatch
Front area was conducted under the auspices of the
"Regional Earthquake Hazards Assessment" ele
ment, one of five elements in the USGS' Earthquake
Hazards Reduction Program. The element was estab
lished to provide concentrated and coordinated attention
to geographic regions containing large urban areas at
risk from earthquakes by utilizing past research and
fostering partnerships with universities, the private
sector, local government, and State and Federal agen
cies. The effort provides a technical basis for devising
and implementing mitigation and loss reduction meas
ures.
The five interrelated goals in the Utah comprehensive
research and implementation program were:
Information systems. To produce quality data along
FIGURE 2. Fault scarp formed in Utah's Hansel Valley north of Great Salt with a comprehensive information system, available to
Lake in the 1934 Hansel Valley earthquake. both USGS and non-USGS users, for use in earthquake
hazards evaluations, risk assessment, and implementation
of loss reduction measures.
Utah's largest urban centers, where more than 80
Synthesis of geological and geophysical data. To
percent of the State's 2 million people live, are located
prepare synthesis reports describing the nature, extent,
along the Wasatch Front. A large earthquake centered
frequency of occurrence, and physical effects of the
near Salt Lake City has the potential to cause extensive
earthquake hazards of ground shaking, surface faulting,
damage to buildings, lifelines, and public facilities. The
earthquake-induced ground failure, and tectonic deforma
expected level of peak ground acceleration would be in
tion and recommend future research to increase the
the range of 0.2 to 0.4 g or greater. Surface-fault rupture
knowledge base required for the creation and implemen
and tectonic deformation (fig. 3) as well as landslides and
tation of mitigation and loss-reduction measures.
liquefaction would be expected to occur in many areas
Ground-motion modeling. To produce deterministic
(fig. 4). Death and injury rates could be high, depending
and probabilistic ground-motion models and maps of the
on the time of day that the earthquake struck and the
ground-shaking hazard and commentaries on their use in
nature and extent of pre-event mitigation actions.
building codes and land use regulations.
The October 28, 1983, Borah Peak, Idaho, magnitude
Loss estimation models. To devise economical meth
(Ms) 7.3, earthquake reminded scientists and policymak-
ods for acquiring inventories of structures and lifeline sys
ers that large earthquakes will recur in the Intermountain
tems in urban areas, to create a standard model for loss
Seismic Belt (fig. 5). Scientists believe that the Borah
estimation, to produce loss and casualty estimates for
Peak earthquake is a model of what might happen on the
urban areas, and to prepare commentaries giving guide
Wasatch Front (Smith and Richins, 1984 ). A magnitude
lines for use by agencies of State and local governments.
7.0 to 7.5 earthquake is now being considered as a sce
Implementation. To foster the creation and imple
nario earthquake for emergency response planning in
mentation of measures to mitigate the earthquake hazards
Utah.
of ground shaking, surface-fault rupture, earthquake-
induced ground failure, and tectonic deformation in
PURPOSE AND SCOPE OF THIS VOLUME urban areas and to provide high-quality scientific informa
tion that can be used by local government decisionmakers
In 1983, the USGS proposed the Wasatch Front of Utah as a basis for implementing and enforcing loss-reduction
as the target of a 5-year program of focused research on measures.
earthquake hazards and hazards reduction. The goals This report provides a comprehensive treatment of the
were to (1) accelerate the development of the knowledge knowledge gained by the research and implementation
base on seismic sources, size, frequency of occurrence, program in the Wasatch Front area. The two volumes are
and physical effects of earthquakes in a 10-county area divided into sections dealing with individual components
along the Wasatch Front, including Salt Lake, Davis, Juab, of the program that collectively define the nature of the
Weber, Wasatch, Summit, Morgan, Cache, Utah, and Box earthquake hazards in the Wasatch Front area. The first
Elder Counties, and (2) foster implementation of earth volume deals with tectonic framework and earthquake
quake hazards mitigation measures. potential. The second volume covers ground-shaking
REGIONAL EARTHQUAKE HAZARDS AND RISK ALONG THE WASATCH FRONT
FIGURE 3. Interested bystanders studying fault rupture formed in the 1934 Hansel Valley, Utah, earthquake. From the Thomas Adams Collection; courtesy of
the Department of Geology and Geophysics, University of Utah, Provo.
hazards and aspects of loss estimation. The research fact that eight of these segments have demonstrable
results came from studies conducted by scientists and Holocene movement increases the possible number of
engineers of the USGS, the Utah Geological and Mineral separate localities where earthquakes may occur.
Survey, universities, and private consulting firms. Funding Chapter B: Persistent and Nonpersistent Segmentation
was provided primarily by the USGS in the form of grants of the Wasatch Fault Zone, Utah: Statistical Analysis
to non-USGS scientists and engineers. for Evaluation of Seismic Hazard, by Russell L.
Wheeler and Katherine B. Krystinik. The Wasatch
fault zone has been segmented at four salients
TECTONIC FRAMEWORK-AND EARTHQUAKE POTENTIAL
Pleasant View, Salt Lake, Traverse Mountains, and
OF THE WASATCH FRONT AREA
Payson throughout much or all of its 10-m.y. history
The first volume of Professional Paper 1500 contains and will likely continue to be segmented there
chapters devoted to geologic and geophysical studies throughout the next several millenia, which is the
aimed at improving the fundamental understanding of the time span of interest for hazard evaluation.
potential for the occurrence of large, damaging earth Chapter C: Subsurface Geology Along the Wasatch
quakes in the Wasatch Front area. In some cases, Front, by Don R. Mabey. Magnetic data suggest
researchers have drawn on studies of other parts of Utah segment boundaries of the Wasatch fault zone that are
and the Intermountain Seismic Belt to gain an under generally consistent with segment boundaries inferred
standing of the large-magnitude earthquakes that may from surface mapping of the fault zone.
occur in the Wasatch Front. The geologic and seismologi- Chapter D: Observational Seismology and the Evaluation
cal/geophysical studies composing this section are of Earthquake Hazards and Risk in the Wasatch Front
described in Chapters A through J: Area, Utah, by W.J. Arabasz, J.C. Pechmann, and E.D.
Chapter A: Paleoseismology of the Wasatch Fault Zone: Brown. Background seismicity predominates on
A Summary of Recent Investigations, Interpretations, second-order faults in the Wasatch Front area. Small
and Conclusions, by Michael N. Machette, Stephen F. to moderate earthquakes are the largest contributor to
Personius, and Alan R. Nelson. Ten discrete segments the probabilistic ground-shaking hazard for exposure
have been identified on the Wasatch fault zone. The periods of 50 years or less. The earthquake data imply