University of California at Berkeley
Civil and Environmental Engineering
Instructor: Stephen A. Mahin
Spring Semester 2009
CEE 227 -- Earthquake Resistant Design
Finding Seismic Design Maps, Information on Seismic Hazard
and Ground Motions on the Web
There are several useful sources of information on the web related to the likely intensity of
earthquake ground motions at a particular location in the U.S. These include some
powerful tools being developed at http://www.opensha.org/ . In addition, there are many
practical tools for design and research purposes at:
http://earthquake.usgs.gov/research/hazmaps/
The USGS site includes “Probabilistic Ground Motion Maps,” “Seismic Design Values for
Buildings” (i.e., plots of Ss and S1 as used by various building codes), and some “Custom
Mapping and Analysis Tools,” along with some background information and links related
to seismic hazard analysis. We will illustrate the use of some of these tools, as well as
some features of openSHA.
Getting started: Finding Latitudes and Longitudes
Some of these tools on the USGS and similar sites allow you to look up a site by its zip
code. This is not particularly accurate in the vicinity of major faults. For other tools, you
may need the latitude and longitude for the site of interest. Many Internet mapping utilities
used to give you the latitude and longitude of a site
when you looked up an address, but this feature is no
longer supported by many of these mapping utilities
(though some will locate a site if you give them the
latitude and longitude). Perhaps the best and
certainly most fun way of finding the latitude and
longitude is using Google Earth. You can type in
the street address, or move a cursor to a spot on a
map or satellite view and it will give you the lat/long
values for the site (see the lower left hand corner of
satellite image window). My house is shown in
Google Earth in the figure to the right, along with
the lat. and long. values.
To get the free application Google Earth, see:
http://earth.google.com/ Note that the USGS site
wants the lat/long values in decimal degrees, so by
going to the preference window in Google Earth and
clicking degrees instead of degrees, minutes and
Lat/Long values from Google Earth
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seconds, you will avoid having to make the conversions manually. Note also that the
USGS site only takes numerical input, so latitude in the northern hemisphere is a positive
value (no N for north), and a longitude for the US is a negative value (no W for west).
Note also that there is are plug-ins for Google Earth that will plot all of the earthquakes in
the world over a certain magnitude that have occurred within a specified time window, as
well as show earthquake faults.
If you do not have a computer where you can access or load Google Earth, try one of these
sites:
http://www.census.gov/cgi-bin/gazetteer
http://www.melissadata.com/lookups/addressverify.asp
Fault Locations
If you click on the “U.S. Quaternary Faults” button on the US National Seismic Hazard
Map page, you will find several useful ways of finding faults that may be located near a
construction site. For instance, in the Fault Map –Interactive, section, you can zoom in on a
region and see faults plotted along with city and county lines, major and minor roads, and
so on, as shown below.
Hayward Fault
San Andreas Fault
Image of Bay Area showing Hayward and San Andreas Faults
(http://gldims.cr.usgs.gov/qfault/viewer.htm)
The static map viewer is similar, but it does not allow you to zoom in as closely. You can
see that the faults are irregular and may be discontinuous, bent, or have several adjacent
parallel ruptures.
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On the “U.S. Quaternary Faults” page there is a link to “Google Earth Files” that gives you
access to a variety of KMZ files showing faults on Google earth pages. You can see recent
earthquakes, photos taken near the fault (little cameras), locations of trenches (blue lines),
data logs (squares with numbers) and so on, in addition to fault locations (red lines). One
needs to be careful of using some of this data for design purposes, as the accuracy of
placement of various features may not be high (see warnings on USGS web site). A Google
Earth view of the Berkeley campus showing the Memorial Stadium and Davis Hall is
shown below. There are various tools in Google Earth (and in the USGS tools) that will
allow you to estimate the distance of your building to the near by faults.
Google Earth view of UC Berkeley showing Hayward and trench locations
Seismic Hazard at a Site
Looking at the “Seismic Design Values for Buildings” link on the USGS site, you will be
able to down load a web applet called Java Ground Motion Parameter Calculator, which
downloads as NSHMP_HazardApp.jar. The resulting application looks like the one shown
below, when you choose the option for Probabilistic Hazard Curves, enter the location of
my house and select the hazard curve for PGA. This tabulates the frequency of exceedence
for ground motion PGAs of various amplitudes. The curve is plotted in a popup window,
and is shown below as well. A log-log or semi-log plot is more user friendly than a linearlinear scale.
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Plot from USGS Java Ground Motion Parameter Calculation
50% in 50 years = 1.39x10-2
10% in 50 years = 2.1x10-3
2% in 50 years = 4.04x10-4
Seismic hazard curve for my house in terms of PGA
By looking at the annual probability of exceedence (the inverse of the return period), one
can estimate the peak ground acceleration at a site. The application allows you to also plot
the hazard curve for the elastic pseudo spectra acceleration corresponding to structural
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periods ranging from 0.1 to 2.0 seconds (the values for 0.2, 1.0 and 2.0 are particularly
useful in terms of generating smoothed design spectra).
This information can be used to generate a design response spectrum where the median
spectral response is plotted as a function of period for a given hazard level (say 2%
probability of exceedence in 50 years). Such plots are called uniform hazard curves. They
are not representative of actual motions, but the median for many earthquakes. One for my
house is shown below. It is developed using the same applet, but selecting the Probabilistic
Uniform Hazard Response Spectrum option, and for the case shown below, the 2%
probability of exceedence (PE) in 50 year level (the 10% in 50 year hazard level is possible
also).
Probabilistic Uniform Hazard Spectrum (elastic)
This is based on fairly old data (2002) and will be updated soon. However, you can refer to
OpenSHA for a similar capability using more recent data. This spectrum is generated for
5% viscous damping and assumes soil on the boundary of the NEHRP B/C categories.
A spectral plot based on this data (View UHS button) is shown below.
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Uniform hazard response spectrum plot (2% in 50 year exceedence level) at my house.
The same applet conveniently plots the code MCE (2% in 50 year) spectra, along with
modifications for soil conditions and to bring the spectrum to the design level (multiplying
by 2/3). This is done for the NEHRP seismic design provisions, ASCE 7, the IBC and
several other codes. This is done by selecting ASCE 7 in the options, and the various
buttons are selected to get intermediate values (Ss, S1, SM and SD, etc. Pressing “Show
Spectrum” give a plot of the code design spectrum. These are shown below.
MCE soil type B
MCE soil type D
Design spectra (soil type D)
MCE and Design Spectra for my house based on ASCE 7
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You can access the probabilistic maps of likely PGA and Spectral Pseudo Acceleration by
clicking on “US National, Urban, & Foreign Probabilistic Ground-Motion Maps”, then “48
Conterminous States” (or other region of the US and world), and then 2008. By clicking on
“Interactive Maps,” you can see the various probabilistic maps and by zooming in you can
get detailed information on probabilistic estimates of PGA, or S0.2 and S1.
Probabilistic Hazard Maps for the US from USGS (here PGA with 2% in 50 year
exceedence probability)
OpenSHA also has utilities for computing seismic hazard and uniform hazard spectrum for
particular locations. These are more modern in that they include more recent data and have
more capabilities for calculating other hazard levels, and so on. However, they are far more
difficult for novice users to use. However, if you have an actual project, you would likely
use OpenSHA to carry out the ground motion hazard analysis.
Interactive Deaggregation -- What kind of earthquakes contribute to this
hazard?
By going back to the main USGS page, and clicking on the “Custom Mapping and Analysis
Tools” link, and then “Interactive Deaggregation – 2002, ” you can see a small map of the
vicinity surrounding a site when you enter the lat/long location values, the return period of
interest. You can put in a structural period of interest (say a period of 1 second) by entering
its FREQUENCY (i.e., 1 Hz), and what you are interested in (PGA, Sa, etc) for the given
hazard level, and find out what types of earthquakes contribute to the hazard level at the
site. Thus, for the site in the above example, you can get two types of plots. The first of
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these is a “Seismic Hazard Map” which shows a map with the earthquake events considered
in the hazard analysis and their contribution to the overall hazard. For the site being
considered, considering PSa at T = 1sec, and a 2% probability of exceedence, we get the
following map. The site is near the intersection of three faults (Calaveras, Greenville,
Concord), and the hazard is dominated by a magnitude 6.5-7.0 event on the near by
Calaveras fault. For this hazard level and site, the Hayward fault contributes negligibly to
the seismic hazard.
Seismic Hazard Map from Interactive Deaggregation Feature
More quantitative information is obtained from the “Hazard Graph” link on the Interactive
Deaggregation report. This is shown below for the site being examined here. We can see
that the hazard is dominated by earthquakes at a distance about 3 km away, with a much
smaller contribution from another fault about 4 km away (in the other direction as can be
seen from the Seismic Hazard Map shown above). There are relatively minor contributions
from events on the Hayward fault (at 17 km) with magnitudes between 6.5 and 7.2 and a
magnitude 8 event on the San Andreas fault (46 km). Thus, if we wanted to pick some
ground motions for this building site, and had a period around 1 second, we would want
them to have a spectral acceleration at 1 second with a median about 107%g and comprised
of a set of motions where half would be magnitude 6.5 events recorded at 2 km, a quarter
would be magnitude 6.25 events at 3 km, and an eighth would be magnitude 6.75 events at
2 km, and the remaining eighth would be a smattering of smaller events at about 3 km and a
magnitude 6.75 event on the Hayward fault just for fun. In reality, there are not enough
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recorded ground motions to do this at this fine level of gradation, and one would end up
with a bin of strike slip ground motions recorded from 0-10 or 0-15 km, having magnitudes
between 6-6.5 and 6.5-7 (plus a bin of 6.5-7.0 events at 15-30 km or so to get the Hayward
event).
Hazard Chart from Interactive Deaggregation Feature (2% in 50-year Hazard Level)
Note that the above graphs will change for the same location, if a different period is
specified for the structure or the hazard level of interest is changed. For example, for a
50% in 75 year hazard level, we have the graph shown below, which shows that the more
probable events on the Hayward and San Andreas faults contribute more to the hazard
associated with serviceability issues. As such, to check the continued occupancy criterion
for a structure, you will want not only ground motions having different intensity levels, but
also different distribution of magnitude and distance characteristics. Nonetheless, a smaller
earthquake on a nearby fault may give higher PGA and Sa values than a more probable
event on a distant fault.
Seismic Maps in Codes and Guidelines
Most building codes or guidelines do not use a strictly probabilistic approach to defining
hazard for design purposes. One reason for this is that values we get from a probabilistic
analysis is sensitive to uncertainty in the data available for some locations and to the type of
statistical distribution assumed in the analysis. Similarly, the intent of the code is not really
to protect for very rare event, which might on average occur only every 2500 years. This
hazard level results from a combination of various factors. For example, we might want the
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site investigated above to be able to resist shaking caused by large earthquakes generated by
dominant faults with high probabilities of rupture in the area.
Thus, for the above example, we would be concerned with the Hayward, San Andreas and
other faults (well, actually, the Calavaras fault is pretty probable, but I am trying to make a
conceptual argument here). However, a moderate earthquake on the nearby faults might
very likely produce much larger ground shaking at the site, if they ruptured. Thus, we want
to protect our structure from these events as well. In this case, however, we would tend to
consider the shaking that would exist if the earthquake occurred on these smaller faults (say
median plus one standard deviation if we are looking at collapse prevention). However,
since a rupture on some of these faults is in itself rare, and if we go to plus one standard
deviation above the median, we may again end up with a highly improbable event in a
statistical sense. In some regions of the US, this is not the case, and in others, the big faults
will control the design spectrum in one period range, and the smaller faults may control in
another.
Hazard Chart from Interactive Deaggregation Feature (50% in 75-year Hazard Level)
Guidelines such as FEMA 356 (ASCE 41) stipulate that the designer can independently
develop a site specific design earthquake by picking other hazard levels, considering a
scenario event on a nearby fault, and so on. This may be very useful in many situations.
Site-specific ground motions are required for certain types of sites (very soft soil, etc.) and
may be required for certain types of structures and analysis methods. In doing this,
structural engineers need to work with knowledgeable geotechnical engineers who
understand the basis for seismic hazard analysis, code guidelines, and structural behavior.
Such individuals are not necessarily easy to find.
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In current codes, where the same approach needs to be applied over a large geographical
area, it is more common to consider the 2% in 50-year hazard level for all faults, but not to
exceed an approximation for a median plus one standard deviation deterministic estimate of
the shaking for the fault that would cause the worst shaking at a site (we did this in an early
homework assignment). Commonly, this deterministic upper bound for the shaking at a site
is taken as 150% of the deterministic value computed for the controlling fault for a 10% in
50-year hazard level.
For political, policy and other reasons, most codes do not reduce the design values less than
considered in their 1994 provisions. Similarly, there are adjustments based on geological,
geographical, jurisdictional and other features. Thus, different codes and guidelines can
result in far different design values compared to one another or to strictly probabilistic
values. This can be easily seen in a comparison of the probabilistic uniform hazard
spectrum and ASCE response spectrum plotted previously.
Maps of S0.2 and S1 for various codes (they are NOT all the same) may be found from a
link at the bottom of the “Seismic Design Values for Buildings” page, which takes you to
http://earthquake.usgs.gov/research/hazmaps/pdfs/.
Maximum Considered Sa1.0 values for 2003 ASCE-7 provisions for New Building Design
for Soil Type B (Site of interest designated by star)
To find such “design” value maps for various editions of different guidelines and codes,
click on “Seismic Design Values for Buildings” on the main USGS page
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(http://earthquake.usgs.gov/research/hazmaps/) and click on “Ground motion maps are now
available in PDF format.,” or go directly to:
http://earthquake.usgs.gov/research/hazmaps/pdfs/
For the overall US, the 2003 NEHRP hazard curves are given at:
SA0.2: http://earthquake.usgs.gov/research/hazmaps/pdfs/NEHRP-2003-Figure3.3-01.pdf
SA1.0: http://earthquake.usgs.gov/research/hazmaps/pdfs/NEHRP-2003-Figure3.3-02.pdf
For California, there are expanded views for
SA0.2: http://earthquake.usgs.gov/research/hazmaps/pdfs/NEHRP-2003-Figure3.3-03.pdf
SA1.0: http://earthquake.usgs.gov/research/hazmaps/pdfs/NEHRP-2003-Figure3.3-04.pdf
For example, considering the 2005 ASCE 7 maps of Maximum Considered Earthquake
(MCE) shown above, one gets for the 150% of the deterministic event on the two faults
near site Sa1 values of 81%g and 76%g. The grey area on the map corresponds to a
uniform value of 60%g for Sa1. One is permitted to interpolate on the maps. Thus, one
might say the site is on the boundary, and would use the 60%g value, or we could take it
being half way between the 81%g and 76%g values and use 78%g. Recall, that the USGS
probabilistic estimate of Sa1 for the 2% in 50-year event was 107%g, which suggests we are
not looking for such a rare event on the nearby faults in setting the design forces using
ASCE7. Nonetheless, selecting appropriate design values from the maps requires some
judgment, and in general approval of the local building official. Thus, using the web
applets discussed previously is much more convenient than using the maps directly.
The reference for the definition of the MCE event is: Leyendecker, E.V., Hunt, R. J.,
Frankel, A.D., and Rukstales, K. S., 2000, "Development of Maximum Considered
Earthquake Ground Motion Maps", Spectra, Earthquake Engineering Research Institute,
Oakland, CA.
Note from the above plot that the MCE event for the Hayward fault near Berkeley is 93%g,
and it falls off as one goes west. The first contour line is roughly at the edge of the Bay, and
corresponds to 75%g. Thus, it is hard (or ambiguous) to determine the appropriate design
value for the case study archetype building project site.
Ground Motion Time Histories
If you want ground motion time histories for a dynamic analysis (say you know the
deagretization discussed above, and you want a ground motion representative of a given
fault, distance to rupture, soil condition and so on), there are several useful databases. For
example,
http://peer.berkeley.edu/nga/
http://db.cosmos-eq.org/scripts/search.plx
From the Search function on the PEER NGA database, by saying we want a 6.5 to 7
magnitude event on a strike slip fault, recorded on a firm soil site at a distance from 0-10
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km, we are returned records from 5 sites from 4 earthquakes (only three of which are
useful). This can be seen below.
Results from PEER Ground Motion Database for Recordings at 0-10 km from Strike Slip
Fault Ruptures having 6.5<M<7 on Soil Type B
The PEER feature that provides response spectra, and permits searches to be made based on
spectral characteristics is currently not available and is being updated.
The COSMOS site lists distances as hypocentral distances, and includes many uncorrected
motions. For this reason, it may return more motions in general than the PEER site, but
they may not be as useful.
Ensembles of earthquake records representing the seismic hazard at a site
An example of a geotechnical report describing the development ground motion time
histories for the new Biological Science Lab building on the western part of the UC
Berkeley campus can be found at:
http://www.peertestbeds.net/UCS/Somerville%20Dec%202001%20UCS%20ground%2
0motions.pdf
And the ground motions can be download from:
http://www.peertestbeds.net/library/Ucs/GM/
This testbed building has a period of 0.45 seconds, and the ground motions are developed
and scaled to emphasize this characteristic period.
A similar, but slightly different approach was used in the SAC Steel Project for a building
in the Los Angeles area (plus Seattle and Boston). In this project, the motions were scaled
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to met the hazard spectrum over a broad period range, ground motion scaling was more
severely limited, and for the 2% in 50 year events ground motions were included based on a
simulation of the earthquake rupture process. See:
http://nisee.berkeley.edu/data/strong_motion/sacsteel/ground_motions.html
For a very good example on how these ground motions are developed and used from a
structural engineering perspective, see:
http://www.ce.berkeley.edu/~mahin/FEMA355F.pdf
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