Report from TriNet North ShakeMap Workshop of June 28, 1999
By TriNet North Shakemap Working Group (D. Dreger
(UCB), C. Scrivner (CDMG) and J. Boatwright (USGS))
This workshop was held at
the request of the TriNet North Program Management Committee (PMC) to begin the
process of identifying tasks that need to be accomplished to develop shakemap
capability in northern California as new strong motion stations become
available, and existing stations receive telemetry. A region-wide capability will be required that uses methodologies
suitable to a seismic network with an uneven distribution of stations, which in
most areas is likely to be quite sparse. The PMC specifically asked that we
attempt to define how to merge different methods of constructing shakemaps to
meet an OES request for information and maps that are uniform state wide. The current sparse coverage of strong motion
stations and the large region of coverage require the development and integration
of a hybrid methodology that allows for estimation of earthquake strong ground
motions given varied levels of data and source parametric input as well as
knowledge about site variability. The PMC provided several focus items to the
workshop participants:
- We want to meet
public service needs by getting multiple organizations to work together to
produce a hybrid product that is coherent across the state, and will reflect a
broad-based consensus within the seismic community.
- We must process both urban and rural earthquakes, and allow changing
processing over various timescales after an earthquake.
- As a result of the workshop, a task list of assignments should be
developed, and a short report prepared.
The report will have internal, and possibly external, distribution.
The
workshop was structured into a morning informational session and an afternoon
general discussion session. In the
morning three shakemap methodologies were presented. In addition, developments in the modularization of TriNet ShakeMap
software was discussed. In the
following we summarize the method presentations and the afternoon
discussions. Minutes of the meeting
were compiled by C. Scrivner and are available at http:// ftp.gps.caltech.edu/pub/scrivner/minutes.
TriNet ShakeMap: David Wald presented work being done to
improve the southern California TriNet ShakeMap. The methodology is data driven, but to deal with varied station
density an attenuation model is used to aid in interpolation between data
points. The first step to the method is
the determination of a strong motion centroid (a point source description). Once a centroid is determined, ground motion
values are estimated on a coarse grid of points using standard attenuation
curves (Joyner and Boore, 1981) and a fitted magnitude.
The
ground motion estimates are modified based on a set of site corrections using a
regional geologic model developed by Park and Elrick (1998). This model identifies areas in southern
California with Quaternary, Tertiary or Mesozoic surface rocks. This classification scheme is then used to
determine site corrections following Borcherdt (1994). The corrections are
applied to the predicted shakemap values (data values are never modified). Grid points within 30 km of a recording
seismic station are dropped. The coarse
grid of both data and predicted values is then more finely interpolated and the
fine mesh is contoured. Details of this
method may be found at http://www.trinet.org/shake.html.
Deterministic ShakeMap: Douglas Dreger described work at UCB to
develop shakemap capability in areas that lack either dense strong motion
instrumentation or telemetered instruments.
This method focuses on determining the finite source parameters of
earthquakes from continuously recorded and near-real-time telemetered regional
distance broadband stations. The finite
source parameters are then used in a tiered system to generate near-source
strong shaking. The first stage is to
invert the broadband data for the causative fault plane, rupture velocity and
dislocation rise time using the two possible nodal planes obtained from
automated seismic moment tensor analysis.
The results of the finite source inversion are used to derive
directivity parameters as they are needed by the empirical regression equations
of Somerville et al (1997). These
equations are used to determine peak ground acceleration and spectral acceleration
at periods of interest. The third tier
utilizes the finite source parameters to integrate the fault slip both
spatially and temporally to determine near-source seismograms from which strong
ground motion parameters are obtained.
The merit of this approach is that it provides source-specific
directivity sensitive estimates of near-source strong ground motions. It is
noted that this method is not strictly model based, and that it is possible to
integrate direct observations in the same manner as is done by the TriNet
shakemap. The primary difference
between the two approaches in this context is in the details and complexity of
the underlying model used to predict ground motions where direct observations
are not available. A more detailed
description of the method may be found at http://www.seismo.berkeley.edu/~dreger/webrpt.htm.
Estimated Intensity Maps.
Jack Boatwright described the method he developed for estimating Modified
Mercalli Intensity (MMI) using seismic moment tensor input and a detailed
geologic map of the greater SF Bay Area. This method chooses a fault
plane from the seismic moment tensor as the nodal plane closest to the average
strain across the Bay Area. The extent of faulting is determined from the
hypocentral depth and the earthquake magnitude and mechanism-type. Ground
motions and intensities are estimated for the Bay Area from an attenuation
model that incorporates source directivity both along-strike and updip of the
fault. Digital geology, mapped at the 1:125,000 scale and binned into
NEHRP site categories B-E, are used to provide a dense mapping of site
corrections for the ground motions and the estimated MMI's. A more detailed description of the method may
be found at
http://ncweb-menlo.wr.usgs.gov/study/effects/intensity.html.
Data Availability: The
primary obstacle to implementing the southern California TriNet shakemap
algorithm in northern California is the lack of available data. Examination of station density in the
greater San Francisco Bay Area shows that the numbers of stations is likely to
be adequate for a southern California-style implementation, however many of
these stations, although digital, do not presently have adequate near-real-time
telemetry. Outside of the greater San Francisco Bay Area the density of TriNet
North stations is much less. See http://perry.geo.berkeley.edu/seismo/trinet/committees/existing_stat.html
for figures comparing station densities. How well will data driven shakemaps in
regions outside the Bay Area perform?
Considering the extended and distributed nature of the supporting
infrastructure for California the workshop group reached consensus that all
areas of California should be covered by accurate maps. Given the possibility of local station
and/or telemetry failure, as well as the sparse coverage outside the urban area
it was deemed important to understand how shakemaps respond to deteriorating
station coverage, and to develop an appropriate backup capability. In order to
assess performance under varied conditions a testbed experiment was
proposed. The experiment will compare
the different proposed shakemap methodologies in varied station coverage for
two well recorded southern California earthquakes. The testbed experiment is defined later in this report.
Geologic Information: Another
important factor in the production of shakemaps is the geologic map used to
calculate site corrections for the predicted ground motions. In southern
California the QTM site classification and 30m depth shear wave velocity
extrapolation of Park and Elrick (1998) is used in conjuction with Borcherdt
(1994) to obtain site specific corrections.
A continuous map of corrections is needed because they are used where
predicted ground motion estimates are needed in the contouring of the shakemap.
The Park and Elrick (1998) map has a scale of 1:750,000 for greater southern
California and finer scale in the Los Angeles region. To implement a shakemap
in northern California, it is necessary to have a reference site geology model. Maps of poor soils are important, and their
compilation is essential to seismic hazard assessment. As described below, a
digital map with 1:250000 scale is being compiled for the entire state and
should be released by the CDMG in the fall of 1999. This map should be adequate for identifying site type at each
point of ground motion prediction for interpolation and contouring purposes.
The
following is excerpted from Jack Boatwright’s report on the (nationwide)
availability of a geology database:
The state geology is already digitized at
1:750,000. This digitization contains a
relatively crude separation among surficial deposit (unconsolidated sediments,
alluvium… and rock). Similar maps are
available for all the states in the Western Region. The 1:250,000 digital geologic maps that the CDMG is now
compiling are “materials maps.” They
are mostly newly digitized, not newly mapped. The geology is subdivided into
six categories: B, B/C, C, C/D, D, and D/E.
Chris Wills and Walt Silva have been working together to devise these categories. The digital map will be released in early
September, and Jim Davis is committed to having this geologic map used for
making “ShakeMaps” within the state.
Wentworth’s 1:125,000 map covers 10 “Bay Area” counties (Sonoma, Napa,
Solano, Marin, San Francisco, Contra Costa, San Mateo, Alameda, Santa Clara,
and Santa Cruz). This will be improved
to 1:100,000 for the “urban core” of the Bay Area, including 1:24,000 maps for
the 9 Bay Area counties. (All of the above, except Santa Cruz.).
A Critical Question: What is the impact of seismic
hazard and geologic mapping in the rapidly expanding urban fringe of the Bay
Area, that is, the various suburban areas of Hollister, Stockton, Tracy, and
Sacramento? Don Gautier (USGS-Chief of
Western Geologic Mapping) is now “redefining” the Bay Area as extending from
Monterey to Sacramento. Ed Halley,
using oil well and borehole data, made quaternary maps of Sacramento years ago.
Len Gaydos (USGS-Project Chief for Urban Dynamics Project) has compiled a
series of maps showing the History of California Urban Growth that details the
urban/suburban incursion into the Valley since 1954. There are some areas
covered by past mapping that may be used, but in the rapidly developing
‘fringes’ of the greater San Francisco Bay Area there appears to be a lack of
detailed information.
Attenuation Relationships: The southern California group remarked that ground
motion estimates merged from a variety of sources, as has been suggested for
northern California, may result in inconsistent results. The consistency of SoCal ShakeMaps is due
the use of the same attenuation relationship to determine both the centroid
parameters (location and magnitude) and the estimated values used in the
interpolation. Thus if multiple
methodologies are used it would be advantageous to settle on a common
attenuation relationship to estimate ground motions. It is recommended that a more recent attenuation relationship be
used to take advantage of recorded ground motions for recent large
earthquakes. Additionally, the
attenuation relationship should also have the flexibility to incorporate source
directivity information as it becomes available (e.g. Somerville et al.,
1997). The attenuation relationship
must provide estimates of PGA, PGV, PGD, and spectral acceleration at 0.3, 1
and 3 seconds period. These parameters,
except PGD, are presently reported by the southern California ShakeMap project
and PGA and PGV are used in the calculation of instrumental intensity. The attenuation relationship of Somerville
et al (1997) meets these requirements with the exception of regression formulas
for PGV and PGD. It is recommended that
such regressions be performed in a consistent manner to provide all of the
needed strong ground motion parameters.
For uniformity of product throughout California it is advised that the
same earthquake ground motion data sets and regression equations be used in
both northern and southern California.
In
addition, attenuation relationships currently available used in TriNet
ShakeMaps were developed with data from earthquakes of magnitude 5 and greater.
Attenuation relationships extended to lower magnitudes would be useful. Generating maps for smaller earthquakes
increases the number of test events for the system and develops ongoing user
awareness that the maps are available.
In southern California, maps are made for earthquakes down to M 4 or
even 3.5 in urban areas (based on user demand), but the development group has
had to extrapolate the existing attentuation relationships to apply them to the
smaller events.
Update Stages & Versioning: The concept of stage
processing or versioning was discussed.
What our comfort level is in terms of providing information at various
stages is an important question that requires further thought and
analysis. Should there always be a map
generated? Even with no reporting
stations? The response of the workshop group was yes, to both
questions. A system must be developed
that has sufficient redundancy to provide some information even if it is the
most basic ‘bull’s-eye’ map. Subsequent
versioning then depends upon the numbers of reporting stations, and whether
additional more sophisticated modeling approaches are used to calculate ground
motions for interpolation purposes. Can
a consistent versioning system be developed and applied to both southern and
northern California? This question is
much more difficult to answer. In
southern California, ShakeMap has evolved out of the TriNet plan to provide
denser strong motion coverage, and therefore focus has been on a data driven
product minimally modified by ground motion predictions (except for the smaller
events). In northern California with
the larger region of interest and fewer available stations, versioning is
likely to depend upon different methodologies.
The Berkeley model for a shakemap uses a multi-tier approach in which
the seismic moment tensor processing provides the initial estimate of magnitude
and fault orientation. Given this
magnitude it would be possible to produce a ‘bull’s-eye’ map using the
attenuation relationship. Second tier
processing involves the determination of the causative fault plane and source
dimension. A second tier shakemap is
obtained utilizing regression formulas that take source finiteness into
account. Third tier processing involves
the forward prediction of strong motion seismograms from which strong motion
parameters are determined. A composite
map would then be compiled from the larger of the stage two and stage three
values. As noted before these ground
motion values would then be used for interpolation and contouring. As in southern California, if recording
stations are located reasonably close to a predicted value the data would take
precedence and the predicted values would be ignored.
Concerns
about consistency of results between versions should be addressed during
testing. The test-bed should be used to
address these issues and work toward a hybrid methodology, probably “versioned”
and/or “tiered”, to strive for smoothly improved and self-consistent results as
more data and source information becomes available.
Use of Modules: Bruce Worden described the development of
modules that would allow the incorporation of different model-based approaches
into the general shakemap framework.
This effort is deemed to be very important in providing the flexibility
that is required when dealing with large regions of coverage where capabilities
are expected to vary. The northern and
southern California shakemap working groups need to coordinate on the definition
of input formats to provided the flexibility that is required in northern
California. This can be accomplished
during the test bed experiment described later.
The
working group reached consensus that an evaluation of the different
methodologies is needed. The purpose of
this test is several fold. First, it is important to understand how well a
predicted map based on the relatively fewer near-real-time stations performs in
regard to predicting ground motions where observations are not available. This test can be accomplished for the 1992
Landers and Northridge earthquakes using a reduced station set that is
consistent with the numbers and geometry of TriNet stations that would be
available to these earthquakes if they occurred today. The remainder stations may then be used to
compare the shakemap contours to direct observation to assess the level of
uncertainty in the shakemaps. Second,
it is necessary to evaluate the performance of the methodologies under
diminishing station coverage. Reduced coverage is possible following a large
earthquake if there are earthquake induced telemetry problems, and is certainly
a problem in the vast regions of California where dense strong motion coverage
is unlikely to be installed. These
tests can be accomplished using the 1992 Landers and 1994 Northridge
earthquakes as test cases in which random station distributions with
interstation spacings of 20km, 30km and 40 km are tested. Further, in these tests it will be necessary
to evaluate performance in cases without near-fault recordings such as LUC for
Landers or stations in forward directivity region for Northridge. In all tests the combined data/model
shakemaps will be compared to observations from the larger strong motion data
set that has been compiled for each of these earthquakes. To facilitate the
comparisons the various methods should produce maps of PGA, PGV and Sa at
periods of 0.3, 1.0 and 3.0 seconds, and use the contouring algorithm employed
by the TriNet ShakeMap. This experiment
will also serve the purpose of testing the modular programming being developed
for the TriNet shakemap. The results of
the experiment will be posted to a restricted website for use by the southern
and northern California shakemap working groups. The following is a suggested time table:
1.
January 1, 2000 –
Completion of TriNet station test for both the Landers and Northridge
earthquakes including
a.
Comparisons of
data/model shakemaps using different techniques for the estimation of model
input
b.
Comparison of the
data/model shakemaps with the complete strong motion data set for both
earthquakes
2.
March 2000 – Completion
of sparse station tests
a.
20 km random
configuration*
b.
30 km random
configuration*
c.
40 km random
configuration*
d.
40 km without near-fault
or forward directivity observations
* each of
these tests should be performed for a number of randomly generated station
configurations in order to evaluate the sensitivity.
1. Improve the availability of data from
stations that are in the ground.
2. Closer coordination with the TriNet
North Instrumentation Committee to design the future network to provide
adequate coverage of varied site conditions and source regions.
3. Need to conduct an evaluation of
shakemap performance in varied observing station densities.
4. Geology and site response is of
considerable importance. The workshop
group recommends that USGS and CDMG administrators ‘fast track’ efforts to
characterize geology at a scale needed for shakemap.
5. Version 2 of the TriNet ShakeMap
modules need to be flexible enough to allow for incorporation of other model
based approaches (an essential ingredient in the testbed experiment proposed in
3).
6. The southern and northern California
shakemap working groups need to reach a consensus on the underlying attenuation
relationships. HAZUS employs
‘consensus’ relations, however for the northern California shakemap the
relations need to account for directivity affects and the relationships should
include the most complete strong motion data set available. This raises an interesting question; should
we develop guidelines for future updates of attenuation relationships as new
data becomes available?
Borcherdt, R.
D. (1994) Estimates of site-dependent response spectra for design (methodology
and justification), Earthquake Spectra, 10, 617-653.
Joyner, B. and
D. Boore (1981) Peak horizontal accelerations and velocity from strong-motion
records including records from the 1979 Imperial Valley, California, earthquake,
Bull. Seism. Soc. Am., 71, 2011-2038.
Park, S., and
S. Elrick (1998) Predictions of shear-wave velocities in southern California
using surface geology, Bull. Seism. Soc. Am., 88, 677-685.
Wald, D. J.,
V. Quitoriano, T. H. Heaton, H.
Kanamori, C. W. Scrivner, and C.
B. Worden (1999) TriNet ``ShakeMaps'': Rapid Generation of Instrumental Ground
Motion and Intensity Maps for Earthquakes in Southern California, in press
Earthquake Spectra.