![](media/image1.png){width="5.496527777777778in"
height="0.7298611111111111in"}

Research, methodology, and applications of probabilistic seismic-hazard
mapping of the central and eastern United States -- Minutes of a
workshop on June 13-14, 2000, at Saint Louis University

(on-line edition)

*by* Russell L. Wheeler and David M. Perkins^1^

Open-File Report 00-0390

2000

This report is preliminary and has not been reviewed for conformity with
U.S. Geological Survey editorial standards nor with the North American
Stratigraphic Code. Any use of trade names in this publication is for
descriptive purposes only and does not imply endorsement by the U.S.
Government.

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGIC SURVEY

^1^Golden, Colorado

INTRODUCTION

The U.S. Geological Survey (USGS) is updating and revising its 1996
national seismic-hazard maps for release in 2001. Part of this process
is the convening of four regional workshops with earth scientists and
other users of the maps. The second of these workshops was sponsored by
the USGS and the Mid-America Earthquake Center, and was hosted by Saint
Louis University on June 13-14, 2000.

The workshop concentrated on the central and eastern U.S. (CEUS) east of
the Rocky Mountains. The tasks of the workshop were to (1) evaluate new
research findings that are relevant to seismic hazard mapping, (2)
discuss modifications in the inputs and methodology used in the national
maps, (3) discuss concerns by engineers and other users about the
scientific input to the maps and the use of the hazard maps in building
codes, and (4) identify needed research in the CEUS that can improve the
seismic hazard maps and reduce their uncertainties.

These minutes summarize the workshop discussions. This is not a
transcript; some individual remarks and short discussions of side issues
and logistics were omitted. Named speakers were sent a draft of the
minutes with a request for corrections of any errors in remarks
attributed to them. Nine people returned corrections, amplifications, or
approvals of their remarks as reported. The rest of this document
consists of the meeting agenda, discussion summaries, and a list of the
60 attendees.

AGENDA

Tuesday, June 13

9:00 a.m.: Introduction

Welcome, logistics (Herrmann, Whittington, Frankel, Wheeler)

National maps (Frankel)

Quaternary tectonic faults (Wheeler)

10:00 a.m.: Charleston seismic zone

##### Earthquake chronology (Talwani)

Logic tree (Cramer)

12:00 a.m.: lunch

1:00 p.m.: Sources north and west of the New Madrid seismic zone
(Wheeler)

1:30 p.m.: New Madrid seismic zone

Paleoearthquake chronology (Schweig)

Historic earthquakes (Hough, Johnston)

GPS models (Gomberg)

Logic tree (Cramer)

Stress concentrators (Talwani)\*

#### **Wednesday, June 14**

# 8:30 a.m.: Other sources

Wabash seismic zone, southern Illinois basin (Wheeler, Frankel)

Eastern Tennessee (Chapman)

Northeastern U.S. (Wheeler)

Quaternary faults in Toronto (Mohajer)\*

10:30 a.m.: Engineering concerns

Design maps (Leyendecker, Hunt)

Seismic piezocone method (Mayne)\*

# 12:00 a.m.: lunch

# 

1:00 p.m.: Continuation of "other sources"

U.S. earthquake registry/compendium (Johnston)\*

Humboldt fault zone (Wheeler, Frankel)

2:00 p.m.: Attenuation and ground motion (Campbell\*, Atkinson, Frankel,
Herrmann, Mueller)

3:00 p.m.: General discussion

Summary (Frankel)

CORS GPS (Prescott)\*

Other

5:00 p.m.: Adjourn

\*: unscheduled talks that were volunteered during the workshop

SUMMARIES OF DISCUSSIONS

Tuesday, June 13

INTRODUCTION

National maps

Art Frankel began by noting that the current (1996) USGS national
seismic-hazard maps were produced about 5 years ago, after a series of
regional workshops like this one. The purpose of each workshop is not to
make immediate decisions on how to update or revise the 1996 maps, but
to stimulate discussions on new developments, methodology, and user
concerns. Discussions will feed into production of interim updated maps
in early 2001. The interim maps will be distributed for further formal
and informal review, with completion of the revised maps anticipated
during the fall of 2001.

Frankel then reviewed the production and use of the 1996 maps, which are
the basis for design maps in the 1997 NEHRP provisions. The hazard maps,
documentation, data sets, and numerous derivative products including the
design maps are available at <http://geohazards.cr.usgs.gov/eq/>. The
hazard maps may be thought of as horizontal slices through site-specific
hazard curves, which graph annual exceedance rate, plotted vertically,
vs. ground motion, plotted horizontally. The hazard curves are
calculated at many thousands of closely-spaced points across the
country. The annual exceedance rate is chosen to correspond to a
particular probability of exceedance in 50 years. The corresponding
ground motion values at a given exceedance rate are plotted on the map
and contoured. At each site the exceedance rate is the weighted sum of
the ground motion exceedance rates from all geographically dispersed
sources that can produce shaking at the site. As a weighted sum, the
curves are termed \"mean hazard curves.\" (The process of summing the
exceedances of a given ground motion is not to be confused with summing
ground motions at a given probability level.)

For the central and eastern U.S. (CEUS) east of the Rocky Mountain
Front, the organizing principle of the 1996 maps is to calculate hazard
mainly from smoothed historical seismicity. The methodology assumes that
most, but not all, moderate to large earthquakes will continue to occur
near previous magnitude 3-5 events, as they have been observed to do in
the past. Two large background source zones provide some protection
against rare damaging shocks in areas with little known historical
seismicity. Five other, smaller zones allow the incorporation of local
variations in seismicity rates, b-values, or maximum magnitudes.
(Details of source zones are in Wheeler and Frankel, 2000, Seismological
Research Letters, v. 71, no. 2, p. 273-282; a few reprints were
distributed.) Within two of these smaller zones, the large, recurring
earthquakes at New Madrid and Charleston, South Carolina, are treated as
characteristic earthquakes.

The 1996 maps explicitly included two CEUS faults, the Meers fault in
Oklahoma and the Cheraw fault in Colorado, for which paleoseismological
work had provided estimates of magnitudes and dates of prehistoric
surface ruptures. Additional individual faults or fields of liquefaction
features can be included in the 2001 and future maps as
paleoseismological results become available. The paleoseismological
results are necessary because magnitudes and recurrence intervals of
large earthquakes do not always match extrapolations from historical
seismicity.

The importance of paleoseismology is shown by large characteristic
earthquakes that are not extrapolatable from historical seismicity at
the New Madrid seismic zone and Charleston, South Carolina. For New
Madrid, the 1996 maps used a characteristic earthquake of Mw 8.0, based
on Arch Johnston's isoseismal-based estimates. Paleoseismological data
available at the time indicated a recurrence interval of 1,000 years.
For Charleston, Johnston's estimated Mw is 7.3 and paleoseismological
evidence indicated a recurrence interval of 650 years. Additional
paleoseismological results since the mid-'90's from both areas indicate
recurrence intervals of approximately 500 years.

The attenuation relations used in the 1996 maps are for a geologic site
condition that corresponds to the NEHRP B-C boundary. This corresponds
to a typical rock site at which strong motion data have been recorded in
the western U.S. Relations of Toro et al. (1993) and Frankel et al.
(1996) were given equal weights. The relation of Atkinson and Boore
(1995) will be added for the 2001 maps. In addition, we will produce a
map for a hard-rock site condition.

The 1996 maps of 2 percent exceedance probability in 50 years show
similar probabilistic ground motion at New Madrid and on some parts of
the San Andreas fault. This similarity has confused some recent critics
of the maps. The similarity results from lower attenuation in the CEUS
than in the West, and from higher CEUS stress drops, which produce
stronger high-frequency motions. Both geographic differences are based
on observations of CEUS isoseismals and recordings of small to moderate
earthquakes. At this probability level, the largest earthquakes are
being taken into account. Note that at the 10 percent exceedance
probability, the San Andreas hazard is much higher than that of New
Madrid. This difference reflects both the higher ratio of low-magnitude
earthquakes on the San Andreas fault system, and the shorter recurrence
interval of the largest San Andreas earthquakes compared to the largest
New Madrid earthquakes.

Frankel showed estimates of the uncertainties of the values on the 1996
maps. The estimates were derived from Monte Carlo sampling among the
input alternatives, which include re-sampled catalogs, different weights
on the magnitude 3, 4, and 5 smoothing grids, characteristics earthquake
magnitudes and recurrence times for Charleston, S.C.,
attenuation-function median curves, etc. Frankel had been able to
construct suites of alternative hazard curves for a number of important
cities. The uncertainty can be characterized by the ratio of the 85th
percentile to the 15th percentile ground motions. The ratio is typically
approximately 3, being larger where seismicity is sparse and smaller in
more active areas.

In closing, Frankel listed some topics for possible discussion by the
attendees. Logic trees for New Madrid and Charleston require
specification of weights for different candidate magnitudes, recurrence
intervals, and source zone geometries. The Atkinson-Boore attenuation
relation needs to be weighted relative to those of Toro et al. and
Frankel et al. Paleoseismological results published since 1996 show
prehistoric Mw larger than 6.5 in Illinois and Indiana; the new results
require an increase in the maximum magnitude that is assumed for this
part of the craton. Hazard along and near the Humboldt fault zone of
Nebraska and Kansas may be underestimated in the 1996 maps, because the
area has sparse low-magnitude seismicity but has had two historical
earthquakes of magnitude approximately 5.

Quaternary tectonic faults

Rus Wheeler summarized results of a literature compilation and
evaluation that was done with Tony Crone, as part of updating the 1996
national maps. Wheeler and Crone compiled published geologic
(stratigraphic, structural, geomorphic, or paleoseismological) evidence
for Quaternary tectonic faulting at 69 U.S. locales east of the Rockies.
Others had compiled an additional seven locales in the states that
straddle the Rocky Mountain Front, for a total of 76 faults, fault zones
or systems, liquefaction fields, uplifts, and other features.

Of these 76 evaluated features, 15 represent confirmed Quaternary
tectonic faults. Most of the 15 are in or near the central Mississippi
Valley, coastal South Carolina, and the Boston-Washington urban
corridor. Two of the confirmed Quaternary tectonic faults, the Meers and
Cheraw faults, had already been incorporated into the 1996 maps. Some
other confirmed Quaternary tectonic faults do not impact the maps,
either because (1) estimated prehistoric magnitudes do not exceed local
assumed M(max), for example, liquefaction features at Newbury, Mass.,
and in the central Virginia seismic zone; (2) recurrence intervals of
liquefaction are too long, for example, Newbury; (3) the most recent
documented surface rupture is too old, for example, the Goodpasture
fault in Colorado; or (4) magnitudes and dates of individual prehistoric
earthquakes are too poorly constrained, for example, the Thebes Gap --
Benton Hills area, Mo. -- Ill., the Fluorspar district of southern Ill.,
the Western Lowlands of Ark. -- Mo., and the Cape Girardeau -- Saint
Louis area, Mo. -- Ill. Approximately half of the 76 features were
dismissed as either pre-Quaternary or not faults. For example, some are
landslides. The rest of the features need more work before they can be
either accepted or dismissed as Quaternary tectonic faults. Examples
include the Brockton-Froid lineament of eastern Montana, several sites
on large faults in eastern and western Kentucky that await trenching,
and the Lancaster seismic zone west of Philadelphia. Future work on some
of the known features and on others not yet recognized is likely to
impact maps beyond 2001. Results are being assembled into a USGS
Open-File Report and 2-4 journal papers, and will be made available
digitally.

CHARLESTON SEISMIC ZONE

Earthquake chronology

Pradeep Talwani summarized the paleoearthquake chronology that he has
assembled and submitted to the Journal of Geophysical Research. He
assembled all available dates on paleoliquefaction features, calibrated
them to convert them from radiocarbon years to calendar years, and
correlated them to define seven paleoliquefaction episodes. The
geographic distribution of paleoliquefaction sites that record a given
episode allows estimation of the magnitude of the causal earthquake with
respect to the 1886 shock of Mw 7.3. As previously noted by Obermeier in
1996, most of the paleoearthquakes appear to have been approximately
similar in size to the 1886 earthquake. Earlier recurrence intervals are
longer than later ones. An episode at 1600 yr. BP is recorded only
northeast of Charleston, in the Georgetown, S.C. area, and might
represent an earthquake of about magnitude 6 produced by a separate
source. Liquefiable sediments exist between Georgetown and Charleston
but do not appear to have liquefied, so the 1600 yr. BP liquefaction
features are unlikely to represent distant effects of a large earthquake
at Charleston. Another episode at 2000 yr. BP is recorded only southwest
of Charleston, near Bluffton, and might represent either the distant
effects of a large earthquake at Charleston or the near-field effects of
a magnitude 6 produced by a separate Bluffton source. Depending on
choice of paleoearthquakes, recurrence intervals range from
approximately 500 years for the last three earthquakes to more than 600
years if the older episodes are included. The picture that emerges from
this and previous work by several authors is one of repeated large,
characteristic earthquakes with little paleoseismological evidence of
smaller liquefying shocks.

##### 

##### The source or sources of the characteristic earthquakes are unclear. Talwani summarized structures that might provide a basis for defining source zones of the Charleston characteristic earthquakes. (1) Several kinds of seismological, other geophysical, and geological evidence taken together can be interpreted in terms of two intersecting faults. The Woodstock fault strikes northeast and is offset several kilometers northward where it cross the northwest-striking Ashley River fault. (2) Several kinds of geomorphological evidence define a northeast-trending "Zone of River Anomalies" (ZRA) (see Marple and Talwani, Feb. 2000 Geol. Soc. Am. Bull.). The ZRA appears to be a linear zone of recent uplift, and its southwestern end coincides with the modern microseismicity near Charleston. (3) Regionally, the Charleston area lies within the South Georgia rift basin, which is itself within the Atlantic passive margin of crust that was extended during the Mesozoic. (4) Seismicity appears to cluster near possible plutons that have been interpreted from potential-field data.

Lastly, Talwani suggested a logic tree for sources. The tree
distinguishes single faults from areal sources. The fault branch would
be weighted 0.8, and the areal source branch 0.2. On the fault branch,
the Woodstock-Ashley River faults would be weighted 0.7, and the ZRA
0.3. On the areal source branch, the South Georgia rift basin would be
weighted 0.7, and the zone in the 1996 maps would be weighted 0.3. The
resulting weights would be 0.56 for the Woodstock fault, 0.24 for the
ZRA, 0.14 for the areal source of the South Georgia rift, and 0.06 for
the areal source used in the 1996 maps.

# Logic tree

Chris Cramer briefly presented his and Richard Lee's results of a
logic-tree hazard analysis for the Charleston area, for which they
obtained higher hazard than shown on the 1996 maps. The purpose of the
analysis was to determine which aspects of the logic-tree model most
influenced the result. Using Monte Carlo sampling of the alternative
branches in the logic tree, he could produce alternative hazard
estimates. Overall, the coefficient of variation (COV; standard
deviation divided by mean) was 0.6. Component COV's attributed to
elements of the model are 0.9 for fault location, 0.6 for recurrence,
0.4 for characteristic magnitude, and 0.3 for attenuation function.
Uncertainty of the rupture length has little effect. Cramer discussed
fault location and recurrence in more detail. Compared to the 1996 map,
the sources modeled were generally more concentrated geographically. A
small areal source increases the hazard in the center of the study area,
around Charleston; enlarging the areal source to include liquefaction
features up and down the coast increases hazard to the southwest, toward
Georgia, but decreases the value of Charleston hazard.

Cramer used observed recurrence intervals based on the paleoliquefaction
studies. Two alternatives, which assume lognormal recurrence intervals,
were based either on the most recent recurrence intervals or on all of
them. For the first alternative, Cramer obtained a median recurrence
interval of 453 years and, with a standard deviation of 0.5, a mean
recurrence interval of 513 years. For the second alternative, he
obtained a median recurrence interval of 701 years and, with a standard
deviation of 0.8, a mean recurrence interval of 963 years.

##### Thus, the most important considerations at Charleston are the choice of source configurations and the decision as to whether to emphasize the most recent recurrence intervals. Cramer distributed a handout that summarizes the trial logic tree.

With respect to the different possible sources of the large
characteristic Charleston earthquakes, Arch Johnston presented results
from his and Paul Bodin's analyses of descriptions of railroad track
shortening that was observed immediately after the 1886 earthquake. The
locations and orientations of shortened tracks, the opposed senses of
shortening northwest and southeast of the highest isoseismals, and the
direction of elongation of the innermost isoseismals, together argue for
right-lateral motion on a fault that strikes north-northeast between
shortened tracks of opposed senses. The deduced fault might be expected
to have produced surface rupture and extended railroad tracks along its
trace; neither was reported. Johnston suggested that the absence of such
reports might be a result of swampy ground between the locations of
shortened track, or of a deep rupture zone that did not break to the
surface.

Sue Hough suggested that little would be gained by reevaluating
intensity reports from the Charleston 1886 earthquake. She raised this
point now because her remarks this afternoon, on her reassessment of New
Madrid magnitudes, might lead attendees to wonder whether the magnitude
of the Charleston 1886 earthquake should also be reassessed. She has
reevaluated intensity reports of the 1811-12 New Madrid earthquakes, to
produce estimated magnitudes smaller than the Mw 8.0 used in the 1996
maps. Hough had revised several of the New Madrid intensity reports
downward because of the older tendency to assign a site the largest
intensity value reported from it, because of improved recent
understanding of the role of site effects, and because of a few older
transcription errors. However, the Charleston earthquake of 1886
produced far more reports that were more uniformly spatially distributed
than those of 1811-12, and, in 1977, Bollinger and colleagues had
carefully reevaluated the 1886 reports using three independent
evaluators.

Rus Wheeler noted that the recurrence intervals lengthen backwards in
time, and suggested that this could be attributed to an incomplete
mid-Holocene record, lower water tables before 2,000 years ago, or a
change in fault behavior. In any of these cases, should the older
recurrence intervals be used at all? Cramer suggested that the older,
longer recurrence intervals should stay in the logic tree, albeit
perhaps with a low weight, to include the possibility that the present
recurrence interval will turn out to be another long one.

Don Wells suggested that probably the older part of the
paleoliquefaction record actually is incomplete because of lower water
tables before 2,000 - 4,000 years ago, following earlier suggestions by
other workers in the area. Wells and his colleagues had developed three
models in their attempt to estimate the large-earthquake recurrence at
Charleston: (1) assigning all the liquefying earthquakes to the
Charleston source, (2) having one of the earthquakes at a northern
source, one at a southern source, and the rest at the Charleston source,
and (3) combining, into a single earthquake, two of the groups of
liquefaction features that formed at two similar times with overlapping
uncertainties in their ages. In all models, Wells and colleagues
discarded the older recurrence intervals as too long because of
incompleteness. Cramer agreed, preferring to weight most heavily the 450
year interval obtained from the three most recent earthquakes.

There followed a lengthy but inconclusive discussion about how to weight
the various source models. Questions included how much or how little to
weight the various likely fault sources and how to constrain the
boundaries of an areal source zone. There appeared to be a general
sentiment for sources that are more areally concentrated than the source
zone used in the 1996 maps. Frankel asked for opinions about earthquake
rates and opinions on the logic tree and weights proposed by Talwani,
either during or after the workshop.

# SOURCES NORTH AND WEST OF THE NEW MADRID SEISMIC ZONE

Rus Wheeler set the afternoon's discussion of New Madrid in context by
summarizing paleoseismological results from five other study areas north
and west of the New Madrid seismic zone. (1) Work in the southern
Illinois basin, which includes the Wabash Valley seismic zone and the
region between St. Louis and Indianapolis, has defined epicentral areas
of eight Holocene and latest Pleistocene earthquakes that were large
enough to cause liquefaction. Estimated moment magnitudes range from
approximately Mw 6 to 7.5, and dates are sufficient to constrain
recurrence intervals. Thus, the southern Illinois basin, like the New
Madrid seismic zone, can be incorporated into the national maps. In
contrast, work in four other areas so far has established the occurrence
of prehistoric earthquakes large enough to have produced surface
ruptures and liquefaction, but magnitudes and dates of individual
earthquakes are too poorly constrained to be included in the maps. These
other areas are (2) the Western Lowlands of Arkansas and Missouri, (3)
the St. Louis-Cape Girardeau map area, (4) the Thebes Gap (Illinois) --
Benton Hills (Missouri) area, and (5) the Fluorspar District of
southeastern Illinois.

# NEW MADRID SEISMIC ZONE

# Paleoearthquake chronology

Buddy Schweig summarized the paleoearthquake chronology of the New
Madrid seismic zone. (Note that the zone does not include the
prehistoric earthquakes that have been recognized during the past decade
in southern Illinois, southeastern Missouri, and Indiana.) The 1811-12
liquefaction field is unique globally in its extent and in the size and
number of individual features, with dikes that are often meters wide and
collapsed areas the size of football fields. Results come from study of
sand blows and dikes. Crosscutting relations, radiocarbon dates, and
archeological artifacts provide age constraints. Ages and sizes of
liquefaction features support correlations between individual features
at different sites. Widespread liquefaction occurred in A.D. 1811-12,
1450, 900, and perhaps 500. Dating uncertainties allow recurrence
intervals from 200 to 800 years.

Years ago the late Roger Saucier noticed that 1811-12 liquefaction
features in the central section of the seismic zone were made of three
fining-upward sequences of sand without intervening soils. Since then,
Schweig, Tish Tuttle, and colleagues have observed this property
throughout the seismic zone. They also found that sand blows dated at
A.D. 900 contain three fining-upward sequences, but that sand blows
dated at A.D. 1450 contain only two. The stacked, fining-upward
sequences without intervening soils are the record of two or three
strong shaking events that were separated in time by intervals long
enough for sand to settle out of suspension, but short enough that
soil-forming processes could not significantly modify the recently
erupted sand. Furthermore, the stacked sequences are recognizable over
large distances, so they are not attributable to aftershocks in the
usual sense of the word. The single, less widely distributed,
fining-upward sequences that would be produced by moderate earthquakes
are few. Thus, the earthquakes in 1811-12, 1450, and 900 were pairs or
triplets of large to very large earthquakes, were not clusters of many
moderate earthquakes, and appear to be characteristic.

The Ambraseys curve, which plots distance to the farthest liquefaction
feature versus magnitude, gives estimated magnitudes of 6.9, 6.7, and
7.6 for the earthquake sequences of A.D. 900, 1450, and 1811-12,
respectively. These estimates are likely to be low because the sediments
of the seismic zone are only moderately liquefiable. A lack of
recognized older liquefaction features and a lack of large cumulative
Quaternary tectonic deformation indicate that there is little evidence
that the seismic zone was active at its present level before a couple of
thousand years ago.

Joan Gomberg asked whether magnitudes estimated from the Ambraseys curve
are extrapolations beyond the data, given that the 1811-12 liquefaction
field is unique. Schweig agreed. Rus Wheeler pointed out that the
Ambraseys curve depends on the distance to the farthest observable
liquefaction feature, which is likely to be small and, for prehistoric
shaking events like those at New Madrid, perhaps poorly preserved. In
addition, the Ambraseys curve is likely to be dominated by small
features that formed in the most highly liquefiable sediments, whereas
the sediments at New Madrid are only moderately liquefiable.
Accordingly, the magnitudes derived from the Ambraseys curve might be
severe underestimates. Ken Campbell noted that this effect would be
countered by lower attenuation in the U.S. midcontinent compared to the
areas Ambraseys studied, many of which are in plate boundaries. Bob
Herrmann suggested that the geographic distribution of liquefaction
features should show a north-south asymmetry, because the thicker soil
columns in the southern part of the seismic zone should damp out the
higher frequencies.

# Historic earthquakes

Sue Hough summarized the basis for her conclusion that the 1811-12
earthquakes had moment magnitudes in the mid-7's, not around 8 as
estimated by Arch Johnston and as used in the 1996 hazard maps. An early
version of her work, which contains estimated magnitudes in the low 7's,
has been submitted to the Journal of Geophysical Research. Hough pointed
out that there were four earthquakes large enough to cause liquefaction,
including the largest aftershock. Liquefaction is influenced by duration
in addition to strong motion, and the ground motion effects are the most
direct evidence we have with which to estimate magnitudes. The reports
that Hough and colleagues examined contain two transcription errors, a
second-hand account, and a report of the largest aftershock instead of
one of the three main earthquakes.

Hough pointed out that the shaken area was sparsely settled at the time
of the earthquakes. The earthquakes happened less than a decade after
the Lewis and Clark expedition. The few settlements were usually in
river valleys, and there were almost no settlements west of the seismic
zone. The town of New Madrid was washing into the Mississippi River even
before the earthquakes. However, some towns were on limestone bluffs and
suffered little or no amplification -- Sainte Genevieve, Mo., sits on
limestone between Cape Girardeau and Saint Louis and still contains
about 50 buildings that predate the earthquakes, including some brick
and masonry houses and many masonry chimneys. Nonetheless, amplification
and site effects were a large component of many reports, and the
reevaluation of individual reports attempted to take these effects into
account.

Hough's most recent results are estimated magnitudes of 7.3, 7.0, and
7.5 for the December 1811, January 1812, and February 1812 earthquakes,
respectively. These estimates are consistent with ordinary stress drops,
likely fault areas, and the Mw 7.3 estimated for Charleston 1886.

Arch Johnston countered with the still-evolving basis for his magnitude
estimates of around 8. Starting with Les Youd's work on liquefaction
severity index (LSI), which relates distance to magnitude, Johnston
showed that the New Madrid and Charleston earthquakes must have had very
different magnitudes -- LSI values of 100 occur only within 100 km of
Charleston, whereas the same values are obtained 200 km from New Madrid.
Johnston accepted that Hough had demonstrated problems with estimating
intensity from isoseismals. He explained that Bill Bakun has developed a
way to estimate epicenter and magnitude from point locations of
intensity reports, to avoid the problems created by drawing isoseismals.
Bakun and Johnston are collaborating to modify Bakun's methods to apply
to the CEUS. They use a CEUS training set of earthquakes that have both
instrumental magnitudes and intensity reports. For a given intensity,
distances are considerably larger in the CEUS than in the West. They use
the median distance for a given intensity level. Distance-median
intensity points define a curve for the CEUS, whereas they fall on a
straight line in the West. The CEUS curve projected inward to zero
distance is three intensity units higher than the Western U.S. line for
magnitude 5, and two units higher for magnitude 7. Johnston suggests
this difference might indicate higher stress drops in the CEUS. The
present form of the method appears to estimate CEUS magnitudes better
than CEUS epicenters. When applied to Street's data for the December
mainshock and aftershock and the January and February mainshocks,
results are Mw 7.8, 7.2, 7.9, and 8.0, respectively. Using Hough's
reassigned intensity values gives 7.8 and 8.1 for the last two
earthquakes in the sequence. Johnston argues that the three New Madrid
mainshocks were probably of Mw 7.8-8.1.

# GPS models

Prior commitments prevented Seth Stein and Andrew Newman from attending
to present their arguments for New Madrid magnitudes of approximately 7.
They were able to prepare a handout, which was distributed. In their
absence, Joan Gomberg presented an overview of GPS models of the CEUS,
drawing on results presented in a 1999 Science paper by Newman, Stein,
and coauthors, and by them and several others at a January, 2000
workshop at the University of Memphis. Some of these results are
described in more detail in the January workshop minutes at
<http://www.ceri.memphis.edu/usgs/hazmap/20jan00minutes.shtml>. The
attendees at the January workshop have prepared a summary for submittal
to EOS, and preprints were distributed.

The observables from GPS are monument velocities relative to some
assumed fixed monument or reference. Midcontinent GPS velocities,
relative to a model of stable North America, are small and uncertainties
are large. Interpretation of these velocities in terms of fault or
localized slip rates requires that some model be invoked. Perhaps the
most commonly invoked model, used both because of its simplicity and
because of its appropriateness in plate-boundary environments, assumes
the driving deformation rate is constant and is relaxed by slippage
along a fault of finite width and infinite length (e.g., as in a
plate-boundary). Moreover, the driving deformation arises from the
relative displacements of the blocks cut by the fault. Use of this model
implicitly produces the lowest possible fault slip rates, because the
measured deformation is assumed to equal the driving displacements that
also must equal the slip across the fault. The appropriateness of this
model in an intraplate setting like New Madrid is questionable for
several reasons. First, the New Madrid area is not a plate boundary,
driven by relative displacement of adjacent plates, but a stress-driven
zone of weakness. The New Madrid fault system also has finite length,
with a length that is probably no more than several hundred km. Finally,
even data from a plate boundary (i.e., the San Andreas) show that the
driving deformation rate is not constant between major earthquakes.
However, a number of alternative models have been developed, that permit
significant earthquake slip on finite-length faults that does not result
in surface displacements of comparable size. These have been applied to
New Madrid by Kenner and Segall, and the models do reproduce the first
order observations.

# Logic tree

Chris Cramer presented a logic tree for New Madrid hazard. He explained
that a logic tree is the best way to incorporate knowledge-based
uncertainty (as opposed to random uncertainty) and to determine the
effect of the uncertainty on the final hazard. The logic tree is
described at <http://www.ceri.memphis.edu/usgs/hazmap/logictree.shtml>.
Cramer explained the various branches of the tree and examined the
sensitivity of hazard to various choices of paths through the tree. The
preferred map given by the logic tree shows the means or expected values
of probabilistic ground motion, and the preferred map resembles the 1996
maps. The hazard is most sensitive to which fault or faults are
considered seismogenic, and about equally sensitive to choice of
attenuation relation, characteristic magnitude, and recurrence interval
of the characteristic earthquake. The COV's for the source and
recurrence interval are two-thirds to half of the respective values
obtained at Charleston, S.C. Uncertainty in the rupture length has
little effect, as with the Charleston model.

Most of the few opinions voiced about choice of seismogenic faults
favored following the alignments of historical seismicity (Gomberg,
Wheeler), and, thereby, reducing the effect in the model of
graben-border faults. Arch Johnston suggested the assumption of faults
that are centered on the liquefaction field.

Pradeep Talwani summarized results from a releveling study of a
seismically active area in Belgium. The overall motion is mostly
incoherent, which would be consistent with numerous small blocks that
are moving up and down. Talwani speculated that New Madrid might behave
similarly, as a strained crustal volume with earthquakes occurring at
stress concentrators such as plutons or intersecting faults.

The magnitude of the 1811-12 earthquakes remains an open question. Art
Frankel asked for opinions on various combinations of recurrence
intervals and magnitudes, hoping to focus discussion on choices of mean
values. Gail Atkinson noted that none of the appropriate attenuation
relations is constrained by data above magnitude 7.5, and higher
magnitudes would require extrapolation. Therefore, the magnitude chosen
should be the one that can reproduce the observed intensities from
whatever attenuation relation is chosen. Most agreed with this need to
close the loop between magnitudes, intensities, and attenuations.

#### **Wednesday, June 14**

# OTHER SOURCES

# Wabash seismic zone, southern Illinois basin

Rus Wheeler summarized how the southern Illinois basin, including the
Wabash Valley seismic zone, had been treated in the 1996 maps, as well
as new findings that require a change in the treatment. The entire
Illinois basin spans most of Illinois, Indiana, and Kentucky, and parts
of Missouri and Tennessee. Within the basin, the Wabash Valley seismic
zone straddles the southern part of the Illinois-Indiana border. The
basin lies within the craton, for which Mw(max) was set at 6.5 for the
1996 maps. At the time of the 1996 maps, eight prehistoric earthquakes
in the southern Illinois basin were known to have been large enough to
cause liquefaction, but magnitude estimates were available only for the
four that clustered in the Wabash Valley seismic zone. Two of these
magnitudes were Mw 7.1 and 7.5, exceeding the local Mw(max) of 6.5.
Accordingly, for the 1996 maps, a source zone was drawn to include the
four clustered epicentral areas, and Mw(max) was set at 7.5 inside the
zone. Since then, the other four prehistoric earthquakes have been
estimated to have equaled or exceeded 6.5. Wheeler proposed expanding
the zone four-fold in size to include all eight epicentral areas, with
Mw(max) 7.5 inside the zone and 6.5 outside. In the future, the zone
could be expanded as needed to include any additional earthquakes that
might be found to have exceeded 6.5.

Art Frankel reported that a Gutenberg-Richter plot shows that these
eight prehistoric earthquakes are consistent with the historical
seismicity in the southern Illinois basin. The plot predicts a mean
recurrence interval of 1,700 years for earthquakes larger than Mw 6.5.
The paleoseismological results indicate six of this size in the last
12,000 years, which about matches the recurrence interval inferred from
the plot, and indicates that an earthquake larger than 6.5 may still be
possible. The enlarged zone of Mw(max) 7.5 that Wheeler proposed would
increase the ground motion at 2 percent exceedance in 50 years by about
10 percent.

The ensuing lengthy and enthusiastic discussion raised many points,
chief among which turned out to be Gail Atkinson's observation that the
occurrence of, say, a 6.8 anywhere in the craton for which Mw(max) was
still as low as 6.5 would undermine the credibility of the 2001 maps.
Frankel recalled that 6.5 was the value chosen for cratonic Mw(max) at a
1994 workshop, and that setting Mw(max) as high as 7.5 for the rifted
rim of North America was controversial then. Wheeler noted that Arch
Johnston's 1994 global compilation had found that cratons have had
historical earthquakes as large as 6.8 +/- 0.3, but rifted rims of
stable continental regions have had larger historical earthquakes.
Therefore, the two parts of the North American stable continental region
do appear to need different Mw(max) values. However, Johnston's finding
supports an Mw(max) larger than 6.5 for the craton. Attendees appeared
to accept that 6.5 is now seen as too low for the craton. Wheeler
offered the straw man of increasing the cratonic Mw(max) to 7.0 and
retaining something like the 1996 source zone with Mw(max) 7.5 to
enclose the 7.1 and 7.5 in the Wabash Valley seismic zone. Alan Kafka
and Frankel suggested that it would be worthwhile to characterize the
uncertainty of the location of the boundary between the rifted rim, with
Mw(max) 7.5, and the craton, with smaller Mw(max).

Ivan Wong asked whether a truncated exponential magnitude model was
suitable for the Wabash area. Art Frankel replied that the recurrence of
the larger prehistoric earthquakes there was consistent with that model,
but that this was clearly not the case at New Madrid and Charleston. At
New Madrid and Charleston, the paleoseismologically-determined rate of
the large, characteristic earthquakes is much higher.

# Eastern Tennessee

Martin Chapman summarized the eastern Tennessee seismic zone, the most
active in the Appalachians but lacking an historical earthquake larger
than m(b) 4.6. The zone is defined by microseismicity. Seismicity occurs
at depths of 5-20 km, beneath the Paleozoic rocks of the Appalachian
thrust sheets. Earthquakes are spatially correlated with potential-field
anomalies. The large spatial extent of low magnitude seismicity suggests
that a large crustal volume may be stressed. However, a statistical test
indicates no clear basis for limiting M(max) on the basis of historical
seismicity. Chapman concluded that no changes are needed from the way
the zone was treated in the 1996 maps.

**Northeastern U.S.**

Rus Wheeler reported that he and John Ebel agree that no new
developments in the Northeast require changes in the way the region was
treated for the 1996 maps. The main new result from the region stems
from work by Ebel that moves the most likely location of the 1727
Newbury, Massachusetts earthquake onshore from its previous offshore
location, and increases its estimated magnitude from m(bLg) 5.0 to m(b)
5.6.

# Quaternary faults in Toronto

Arsalan Mohajer presented results from the Toronto area that he
suggested might also apply to the Northeastern Ohio seismic zone near
Cleveland. Outcrops in the eastern Toronto metropolitan area, near the
Pickering nuclear power plant, contain faults that cut both bedrock and
Quaternary strata. Whether the Quaternary slip is of tectonic,
glaciotectonic, or glacial rebound origin is in dispute, but work
continues. The faulted outcrops lie on the Niagara-Pickering magnetic
lineament. The lineament has been traced southwestward on aeromagnetic
and gravity maps into the Akron (Ohio) magnetic lineament and the
Northeast Ohio seismic zone, near the Perry nuclear power plant. Mohajer
urged further paleoseismological study, both in the Toronto area and in
the Northeast Ohio seismic zone, because of the population and critical
structures that are concentrated in both areas.

Art Frankel and Rus Wheeler responded that we are interested in the
various published reports on this area, including any conclusions of the
Geological Survey of Canada. Frankel pointed out that the computational
methodology used for the U.S. national maps requires estimates of the
magnitudes and recurrence intervals of individual prehistoric
earthquakes before an individual fault, as opposed to an areal source
zone, can be incorporated into the maps. This requirement applies
throughout the map area, including the Northeast Ohio seismic zone.

# ENGINEERING CONCERNS

# Design maps

The 1996 USGS ground motion maps were modified for use in building
codes. The modified maps were prepared by the USGS and are based on the
recommendations of the Building Seismic Safety Council\'s (BSSC) Seismic
Design Procedures Group (SDPG). The SDPG-recommended maps, the Maximum
Considered Earthquake (MCE) Ground Motion Maps, are based on the USGS, 2
percent probability of exceedance in 50 years, probabilistic maps with
additional modifications incorporating deterministic ground motions near
some major faults. The MCE ground motion maps are available at
<http://geohazards.cr.usgs.gov/eq/>, as well as on a CD-ROM. There are
three sets of design maps. (1) The 1997 NEHRP Recommended Provisions map
set consists of 32 large-scale maps. (2) The 2000 International Building
Code (IBC) Maps are derived from the NEHRP maps with changes only in map
scale and the number of maps in the map set. This same set is
incorporated in the national standard ASCE 7, "Minimum Design Loads in
Buildings and other Structures." (3) The 2000 International Residential
Code (IRC) Map is also derived from the NEHRP maps, simplified into site
categories defined by specific contours on the MCE maps.

E.V. Leyendecker noted that the goal of engineering design is to achieve
public safety at an economical cost. Buildings will continue to be
constructed using the most recent published ground motion maps and other
criteria. Accordingly, engineers are best served if hazard mappers make
the best seismological and geological estimates that we can now, while
incorporating existing uncertainties. A complete review and revision
cycle for the design maps takes approximately three years, and probably
the next code revision will occur in about six years at the earliest.

A uniform hazard response spectrum can be approximated from the 0.2 s
and 1.0 s spectral ordinates. Leyendecker illustrated the construction
of an approximate spectrum using the two ordinates. This approximation
works well for San Francisco, for example, but less well for the CEUS
because the CEUS spectrum continues to rise with decreasing period to
0.1 s.

Leyendecker described the value of deaggregating the hazard to identify
the magnitudes and distances of the earthquakes that dominate the
estimated hazard for a given site. With knowledge of the dominant
magnitudes and distances, one can more readily estimate durations and
construct time histories. He showed sample deaggregations for San
Francisco and Saint Louis. Deaggregations for numerous cities, as well
as a custom-deaggregation utility, are available at
<http://geohazards.cr.usgs.gov/eq/>.

Leyendecker also explained the choice of 760 m/s for a site condition
nationwide in the hazard maps. In the western U.S., the available ground
motion attenuations are based on empirical data (variation of ground
motion with distance) and can be used for a site condition of 760 m/s in
the top 30 m without too much difficulty. In the CEUS, the existing
ground motion attenuation relations are in terms of hard rock, which has
a much higher shear-wave velocity. It was believed these latter
attenuations could not be confidently extrapolated for materials with
shear-wave velocities less than 760 m/s without encountering nonlinear
site effects. Accordingly, the common site condition of 760 m/s in the
top 30 m was selected for the entire U.S. The map values must be
adjusted for local site-condition variations. It is anticipated that the
2001 maps will include additional maps for a hard-rock site condition in
the CEUS.

Joe Hunt discussed production of the 1997 MCE maps from the perspective
of the BSSC. The 1997 design maps represent at least three changes from
previous design maps. (1) They use 2 percent exceedance probability in
50 years instead of the previous 10 percent. These are considered
collapse-level ground motions. (2) They use spectral accelerations at
0.2 s and 1.0 s instead of the previous peak acceleration. (3) They use
the 1994 NEHRP site condition classes instead of the previous 1991 NEHRP
classes. The performance goal nationwide is life safety and a low
likelihood of building collapse for ground motions larger than the
design ground motion. Engineering experience with earthquake damage
indicates that properly designed structures, using current code design
criteria, can withstand motions larger than their design motion without
collapse. The ratio of estimated collapse motion to the design motion is
termed the seismic margin. By using an estimate of a factor of 1.5
(considered a lower-bound value) for this margin, the collapse level was
chosen to be the previously indicated 2-percent /50-year ground motions,
and the result was design ground motions that are two-thirds of these
values (collapse ground motions divided by 1.5, the seismic margin).
Hazard curves have different slopes for CEUS and western U.S. cities,
with the result that as probabilities of exceedance decrease, ground
motions for cities in the western U.S. show little increase while ground
motions for cities in the CEUS show considerable increase. In the CEUS,
design for 10-percent/50-year ground motion would provide little
protection if 2-percent/50-year ground motions occurred. Use of the
larger 2-percent/50-year ground motion as a collapse prevention load,
divided by the margin of safety, is intended to provide a uniform
reliability against structural failure.

The new design ground motions are clearly larger at New Madrid and
Charleston than the previous design values. Elsewhere, new values may be
larger or smaller than the old values, but in areas with low hazard, the
change has little impact. In some areas, largely in the western U.S.,
the design ground motions are constrained by deterministic ground
motions based on prior earthquake experience and engineering judgement.

Hunt listed problems that need more work. The geologic site condition
classes need better site coefficients. Sensitivity studies could lead to
better understanding of the uncertainties in the maps and their causes.
The correlation between ground motion and damage in different types of
structures is poorly understood. Deterministic seismic sources, CEUS
attenuation, representative earthquake histories for use in design, and
the performance levels for structural design all need better definition.
The seismic margins that are inherent in the design process require more
study, as do ground motions with periods longer than 1 s.

**Seismic piezocone method**

Paul Mayne summarized the use of seismic piezocone measurements to
estimate liquefaction susceptibility of sediments in situ, and to
estimate the magnitude of the earthquake that would be necessary to
cause liquefaction of the sediments. By applying the method to existing
paleoliquefaction features, it may be feasible to estimate the
magnitudes of preinstrumental earthquakes from geotechnical properties
of the liquefied sands. Mayne and colleagues are gathering data at test
sites in Charleston, S.C., Memphis, northeastern Arkansas, and
southeastern Missouri. Calibrating sites in Japan and California are of
uncertain applicability to the CEUS. Recorded strong motion data would
aid calibration.

# CONTINUATION OF "OTHER SOURCES"

# U.S. earthquake registry/compendium

Arch Johnston described his vision of a uniform moment-magnitude
collection of all earthquakes at or above the felt threshold for the
entire U.S. He prefers the terms "registry" or "compendium" because each
earthquake would be represented by considerably more information,
including original data, than is typically contained in current
catalogs. At least half of the felt earthquakes are non-instrumental,
even up to the first half of the twentieth century. There has never been
a combination of instrumental and pre-instrumental earthquakes into a
uniform moment-magnitude catalog. Currently he is funded to make such a
registry or compendium for the central 20 states. He outlined a process
involving collaborators who could add similar material for the western
and eastern states, with all working to common standards to insure
nationwide compatibility.

# Humboldt fault zone, Kansas-Nebraska

Rus Wheeler summarized the geologic setting. The youngest rocks known to
be deformed by slip on the fault zone are about 290 m.y. old. Art
Frankel explained that the hazard presented by the fault zone might be
underestimated in the 1996 maps. Two poorly located historical
earthquakes of magnitudes 5.0-6.0 occurred near the northern part of the
fault zone, but seismicity is sparse at lower magnitudes. Frankel
proposed drawing an areal source zone around the magnitude 5's and
nearby seismicity, to prevent diluting the local hazard during the
seismicity smoothing computation that is used in the maps. Frankel
showed the results of using such a zone. However, there was little
enthusiasm for changing the way the region was treated in the 1996 maps.

# ATTENUATION AND GROUND MOTION

Ken Campbell proposed a method for developing a hybrid empirical
attenuation relation in regions with few strong motion recordings, such
as the CEUS. This would be an alternative to attenuations based on
stochastic models. The strategy would be to rely on relative differences
between a host region with many recordings, such as California, and a
target region with few recordings. One would select an empirical
attenuation model for soft rock from the host region, develop
seismological models for the host and target regions, and use the
seismological models to adjust the empirical attenuation model to the
target region. In this way, the resulting attenuation model for the
target region would have near-source characteristics built in.
Weaknesses of the suggested strategy include a lack of reliable
seismological models, the assumed similarity of near-source
characteristics in host and target regions, and an assumed similarity of
the scaling relations in both regions. Campbell showed some encouraging
examples and comparisons. The hybrid model shows more near-field
saturation than other models. It compares well with the
Lawrence-Livermore National Laboratory composite model for PGA, but
would be more appropriate for extrapolation to Mw 8.

Gail Atkinson reviewed competing CEUS attenuation relations and
presented some new results. She noted that the competing relations have
similar attenuation rates with distance, and differ mainly in absolute
level and in how they model the source. The main effect of this
difference is in long-period ground motion, where the two-corner
Atkinson-Boore 1995 relation estimates lower hazard than the one-corner
relation of Frankel et al. 1996. The difference is small at all
frequencies for magnitude 4.5, but large at frequencies of 2 Hz and
less, for magnitude 7.5. For 5 Hz, hazard at St. Louis is dominated by a
hypothetical magnitude 5.25 earthquake at 7 km distance, and the two
models predict about the same hazard; at 1 Hz, hazard is dominated by a
hypothetical magnitude 7.75 at 100 km distance, for which the two models
differ by a factor of 2. Stochastic simulations of magnitude 6 and
smaller earthquakes at large distances show good agreement with eastern
North American (ENA) ground motion data at 0.2 sec, for both models; at
1.0 sec the Atkinson and Boore model matches the data, while the Frankel
et al model overpredicts the data. Atkinson examined California data and
found a two-corner effect there, as well. She also found that the
California source model nearly matches the ENA source model. She
concluded that empirical California ground motion relations can be
modified for use in ENA, by using simple factors to reflect regional
differences in crustal amplificaton and attenuation. She recommended
that the 2001 maps use 3 sets of ground motion relations: the Atkinson
and Boore 1995 relations (2-corner stochastic model), a 1-corner
stochastic relation (either Frankel et al., 1996 or Toro et al., 1993),
and a modified California empirical relation.

Art Frankel agreed with Atkinson about the need to add the
Atkinson-Boore 1995 model for the 2001 maps, with a weight of at least
1/3. However, he showed some data that do not appear to be consistent
with the two-corner model. The Nahanni, Ungava, Loma Prieta, and other
California data do not appear to have corners where the model would
predict them. However, Atkinson pointed out that the corners could not
be properly determined based on the teleseismic data shown, which are
reliable only for frequencies of 1 Hz and less, whereas the predicted
corners are at higher frequencies. Joan Gomberg noted that it is
dangerous to generalize from individual earthquakes because of slip
variability and azimuthal effects. Frankel's opinion is that it is
fundamentally risky to take regional recordings of older earthquakes and
extrapolate back to characterize the source. He asked whether Campbell's
hybrid method is already in a form that we can use for the 2001 maps.

Bob Herrmann summarized some of his results on high-frequency scaling,
site effects, intensity, and high-frequency ground motion in the central
U.S. (CUS). A goal is to generalize a model to allow extrapolation to
higher magnitudes than have been recorded instrumentally in the CUS.
Computing the effect of the thick filling of the Mississippi embayment
depends critically on the depth dependence of Q. The embayment filling
is asymmetric north-south, thickening southward. Accordingly, adding the
filling to the national maps would produce resonances in the northern,
shallow part of the embayment, attenuation in the southern, deep part,
and corresponding north-south asymmetry in liquefaction and other
effects. If the 1999 results of Dave Wald are correct, then older
intensity maps overestimate peak motions because the isoseismals tend to
follow the most distant reports of a given intensity instead of
something like a median distance. This overestimation would impact loss
estimation. Regional CUS ground motion indicates motions about 1.5 times
larger than predicted by Atkinson-Boore 1995 at distances of 200-500 km.
This effect would only slightly decrease magnitude estimates of the
1811-12 New Madrid earthquakes, but it also favors slightly different
distance scaling for the CUS than would be used for the northeastern
U.S. and southeastern Canada. If the attenuation relation of Toro and
McGuire is used in the 2001 maps, we should collaborate with them to
develop a stochastic forward model. Although the standard FEMA 273
procedure can be used in most of the U.S., it should not be used in
areas of deep soils, such as the Mississippi embayment.

Ken Campbell pointed out that site factors that are based on California
data reflect a velocity gradient that continues to great depths. A site
underlain by high velocity rock at shallow depth would behave very
differently. Herrmann agreed, noting that the FEMA 273 procedure should
not be used in much of the CUS. Sue Hough noted that Wald's peak
accelerations were based on intensities from Tri Net, and that Trifunac
and Brady systematically give lower peak accelerations for a given
intensity than Wald.

Chuck Mueller reported on his characterization of the effects on ground
motion of the seaward thickening wedge of Coastal Plain sediments and
rocks on the Atlantic and Gulf of Mexico seaboards. The strata get
younger seaward and the relative age differences mapped at the surface
persist at depth. Thus, we might expect relative velocities and Q
differences that are seen between sites at the surface to persist at
depth, but to diminish downward. This expectation indicates the
feasibility of mapping site effects at a regional scale, based on
surface geology, its extension down-dip, and depth to basement. This
would be important in assessing hazard throughout the Coastal Plain.
Mueller used surface geology and depth-to-basement information obtained
from the literature. He developed power-law Vs- and Qs-versus-depth
functions for each of six surface geology units. Then, looping over a
grid of sites, he calculated site response for various ground motion
parameters, magnitudes, and distances, using quarter-wavelength
approximations. Haskell-Thompson calculations demonstrate resonance, but
Vs and depth are too uncertain to lend confidence in where
amplifications and deamplifications would occur geographically.
Deamplification is large atop the extremely thick sediment pile in the
lower Mississippi River valley.

There was some discussion about characterizing the Mesozoic basins that
underlie the Coastal Plain, about vertically incident waves, and about
the desirability of including surface waves. Bob Herrmann reported
complexities around 10 Hz that should not present problems at 1-10 Hz.
Art Frankel showed a recording from the Fort Pillow station that shows
trapping in sediments, with a significant surface-wave effect at 2 Hz.

# GENERAL DISCUSSION

**Summary**

Art Frankel summarized some main points of the workshop, emphasizing
that no decisions have been made yet. (1) The Charleston logic tree
identifies many variables, and we heard a proposal for a greater
geographic concentration of sources than was used in the 1996 maps. (2)
There is disagreement about intensity-based estimates of magnitudes of
the New Madrid earthquakes. Recurrence is uncertain because the A.D.
1450 sequence may have been of lower total moment than the 1811-12 or
A.D. 900 sequences. (3) There is a need to increase Mw(max) for the
craton, and to estimate the uncertainty in the location of the boundary
between the craton and the extended rim of North America. (4) It remains
uncertain whether the Humboldt fault zone requires special treatment.
(5) There is a need for a CEUS catalog that uses a uniform magnitude.
(6) It would be desirable to map uncertainty in the hazard, or at least
to estimate uncertainty for individual sites. (7) How should the 2001
maps incorporate the Atkinson-Boore 1995 attenuation relation? (8) The
hybrid attenuation relation proposed by Campbell is distinct from
existing relations. Should it be included in the 2001 maps, and if so,
how? (9) The 2001 maps will include a hard rock map. (10) Hazard maps
that include amplification are in the future. (11) It is important to
close the loop that links intensities, magnitudes, and attenuation. (12)
Modeling of finite CEUS faults should be calibrated against data.

# CORS GPS

Will Prescott observed that in the CEUS, the maps are driven by
historical earthquakes, which have long recurrence intervals.
Accordingly, we don't know whether we have identified all possible
sources of large earthquakes. Searching the entire nation with present
paleoseismological methods would take a long time. Prescott proposed
seeking additional sources by utilizing the network of Continuously
Operating Reference Stations, a nationwide GPS network that is being
established for navigation. Currently CORS includes 140 stations, with
three being added per month. Although these are not highly stable
stations, they are many and the data are free except for an analyst's
time. Prescott suggested determining velocities for many sites,
selecting the few with anomalously large velocities, and attempting to
verify their velocities by comparison to nearby sites. Already five or
six stations have velocities well above noise levels. Prescott presented
a probabilistic argument by which the expected increase in the number of
CORS stations will raise to attractive levels the likelihood of
observing a useful anomaly.

A questioner asked what we would do if we found a high velocity that
could not be discounted. How would we estimate the seismic potential?

**Other**

Gail Atkinson and others approved of Art Frankel's summary. Sue Hough
observed that closing the loop between intensity, magnitude, and
attenuation will require reassessing the original felt reports. Arsalan
Mohajer asked whether the cratonic source(s) in Toronto and northeastern
Ohio were not worth further examination. Frankel agreed, noting that a
first step should be thorough geologic study to determine whether the
exposed faults are tectonic. Even if they are, they will hard to deal
with without a recurrence interval. Norm Hester asked how we are
reaching out to the users of the maps. Frankel replied that we have not
addressed the issue of scenario earthquakes, but there is interest in
it. We are always ready to discuss with users how we make the maps. Some
of us will be doing this with the Washington state Department of
Transportation, and the AASHTO codes may incorporate seismic hazard
provisions.

## APPENDIX

After the meeting Art Frankel provided an outline of his summary
statement. We append it here, with the addition of numbers in
parentheses that key to the numbered sentences in the summary paragraph
on p. 15-16.

# SOURCE ISSUES

1\. Use logic tree to describe Charleston, SC source

a\. magnitude range (1)

b\. recurrence times (1)

c\. geometry- sentiment of workshop participants is to use a smaller
areal source zone plus alternative models with Charleston-type
earthquakes confined to Woodstock fault and Zone of River Anomalies (1)

2\. Use logic tree to describe New Madrid Source

a\. magnitude (Mw7.25 to just above 8.0; preferred value around
7.6-7.7?) (2)

b\. recurrence time (latest paleoseismological evidence indicates about
500 years mean rate) (2)

c\. geometry: don't use fictitious faults from 1996 maps, multiple
models: consider putting faults along current seismicity

3\. Adjust Mmax zones

a\. make Mw(max)=7.5 zone larger for Wabash valley zone, based on
paleoearthquakes

b\. make Mw(max)=7.0 for craton (3)

c\. Consider uncertainty in craton-extended rim M(max) zones by logic
tree with somewhat different boundaries (3)

4\. Examine effect of using areal source zone for Nemaha Ridge rather
than smoothed seismicity. Workshop participants not enthusiastic about
changing to areal zone there (4)

5\. Develop maps of uncertainties derived from Monte Carlo simulations
(6)

**GROUND-MOTION ISSUES**

1\. Include Atkinson and Boore 1995 attenuation relation (7)

2\. Consider including hybrid attenuation relation of Campbell if it is
put in a form we can use (8)

3\. Produce hazard map of CEUS for hard-rock sites, in addition to B-C
boundary sites (9)

4\. Need to close loop between observed intensities and predicted ground
motions from attenuation relations (11)

5\. Need to incorporate finite-fault effects in attenuation relations
for CEUS (12)

## LIST OF ATTENDEES

  ----------------------- --------------------- -------------------------------
  Al-Shukri, Haydar       Univ. Ark./Little     alshukri@seismo.ualr.edu
                          Rock                  

  Ammon, Chuck            Saint Louis Univ.     ammon@eas.slu.edu

  Atkinson, Gail          Carleton Univ.        gma@ccs.carleton.ca

  Bauer, Bob              Ill. State Geol.      bauer@isgs.uiuc.edu
                          Surv.                 

  Bowlby, John R.         Neotectonics Assoc.   bowlby@idirect.com

  Bramel, Brian K.        NAHB Research Ctr.    bramelbk@nahbrc.org

  Caldwell, Philip J.     Square D Co.          caldwelp@squared.com

  Campbell, Ken           EQE                   kwceqe@aol.com

  Chapman, Martin         Virginia Tech.        mcc@vt.edu

  Cooling, Tom            URS Greiner-W'Clyde   tom_cooling@urscorp.com

  Cramer, Chris           USGS                  cramer@ceri.memphis.edu

  Frankel, Art            USGS                  afrankel@usgs.gov

  Guccione, Peggy         Univ.                 guccione@comp.uark.edu
                          Ark./Fayetteville     

  Halchuk, Stephen        Geol. Surv. Canada    halchuk@seismo.nrcan.gc.co

  Herrmann, Bob           Saint Louis Univ.     rbh@eas.slu.edu

  Hester, Norm            Indiana Univ.         hester@indiana.edu

  Hoffman, David          Mo. Div. Geol.        nrhoffd@mail.dnr.state.mo.us

  Hough, Sue              USGS                  hough@gps.caltech.edu

  Howe, Richard W.        EQE                   rwh@eqe.com

  Hunt, Joe               Oak Ridge National    huo@y12.doe.gov
                          Lab.                  

  Jemberie, Alemayehu     Saint Louis Univ.     jemberie@eas.slu.edu
  Lakew                                         

  Johnston, Arch          Univ. Memphis         johnston@ceri.memphis.edu

  Julia, Jordi            Saint Louis Univ.     julia@eas.slu.edu

  Kafka, Alan             Boston College        kafka@bc.edu

  Kiefer, John            Ky. Geol. Surv.       kiefer@kgs.mm.uky.edu

  Kim, Won-Young          Lamont, Columbia      wykim@ldeo.columbia.edu
                          Univ.                 

  Kimball, Jeff           DOE                   jeffrey.kimball@ns.doe.gov

  Kirschner, David        Saint Louis Univ.     dkirschn@eas.slu.edu

  Kosarian, Minoo         Saint Louis Univ.     kosarian@eus.slu.edu

  Lee, Richard            Westinghouse-SRC      richard02.lee@srs.gov

  Leyendecker, E.V.       USGS                  leyendecker@usgs.gov

  Luebbering-Evers, Sue   FEMA                  sue.luebbering-evers@fema.gov

  Mahdi, Hanan            Univ. Ark./Little     mahdi@seismo.ualr.edu
                          Rock                  

  Mancilla, Flor de Lis   Saint Louis Univ.     mancilla@eas.slu.edu

  Mayne, Paul             Georgia Tech          pmayne@ce.gatech.edu

  McCarthy, Jill          USGS                  jmccarthy@usgs.gov

  Mendez, Andres          Impact Forecasting    andres_mendez@aon.com

  Mohajer, Arsalan        U.                    mohajer@scar.utoronto.ca
                          Toronto/Scarborough   

  Mueller, Charles        USGS                  cmueller@usgs.gov

  Ortega, Roberto         Saint Louis Univ.     ortega@eas.slu.edu

  Patterson, Gary         Univ. Memphis         patterson@ceri.memphis.edu

  Perkins, Dave           USGS                  perkins@usgs.gov

  Petersen, Mark          USGS                  mpetersen@usgs.gov

  Pezeshk, Shahram        Univ. Memphis         spezeshk@memphis.edu

  Prescott, Will          USGS                  wprescott@usgs.gov

  Pruess, Ted             EQE                   tpruess@eqe.com

  Raoof, Mehdi            Univ. Mo./St. Louis   raoof@eas.slu.edu

  Saikia, Chandan         URS Greiner-W'Clyde   chandan_saikia@urscorp.com

  Salomone, Lawrence      Westinghouse-SRC      lawrence.salomone@srs.gov

  Schweig, Buddy          USGS                  schweig@ceri.memphis.edu

  Steckel, Phyllis        EQE                   steckel@usmo.com

  Talwani, Pradeep        Univ. South Carolina  pradeep@sc.edu

  Updike, Randall         USGS                  updike@usgs.gov

  Wells, Donald           Geomatrix Consultants dwells@geomatrix.com

  Wheeler, Rus            USGS                  wheeler@usgs.gov

  Whittington, Melanie    Saint Louis Univ.     melanie@eas.slu.edu

  Windeler, Don           Risk Mgmt. Solutions  donw@riskinc.com

  Wong, Ivan              URS Greiner-W'Clyde   ivan_wong@urscorp.com

  Xie, Jiakang Jack       Lamont, Columbia      xie@ldeo.columbia.edu
                          Univ.                 

  Young-Soo, Jean         Saint Louis Univ.     sooy@eas.slu.edu
  ----------------------- --------------------- -------------------------------