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 were 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
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) and magnitude. 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 the 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
Mercalli Intensity (MMI) using seismic moment tensor input and a detailed geologic map of the greater SF Bay Area. This method chooses a nodal plane from the seismic moment tensor, which is closest to the average strain across the Bay Area, as the fault plane. 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 TriNet-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 in the SF Bay Area to those in Northern California at large. 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 surface-to-30m depth shear wave velocity extrapolation of Park and Elrick (1998) is used in conjunction 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. The point was emphasized in the workshop that 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 TriNet group remarked that ground motion estimates merged from a variety of sources, as has been suggested for northern California, might result in inconsistent results. 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 attenuation regression equations be used in both northern and southern California. This requires consensus of the TriNet and TriNet-North groups on the appropriate strong motion data set to be used to compile attenuation relationships, and agreement on the form of the regression equations. If the same strong motion data set is used to develop standard attenuation relationships and relationships that can account for source directivity then it is likely that there will be greater continuity between subsequent versions of shakemaps using differing degrees of source specific information. This raises an interesting question; should we develop guidelines for future updates of attenuation relationships as new data becomes available?
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 events to test 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 attenuation 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. The workshop group reached consensus that a shakemap should be generated for all significant events even those with no reporting stations. 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 will be addressed during testing..
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 provide the flexibility that is required in northern California.
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 each methodology predicts ground motions based on relatively few near-real-time stations. This test can be accomplished with the 1992 Landers and 1994 Northridge earthquake data sets 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 remaining 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 Landers and Northridge earthquakes as test cases in which random station distributions with inter-station spacing 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 timetable:
1.February 1, 2000 - Completion of TriNet station test for both the Landers and Northridge earthquakes including
a. Comparisons of data/model shakemaps obtained using different techniques for the estimation of model input and a representative TriNet station distribution.
b. Comparison of the data/model shakemaps with the complete strong motion data set for both earthquakes to examine predictive capability of shakemaps obtained with sparse station coverage.
* The completion of this test is contingent upon the development of the modular TriNet shakemap code, which is scheduled to be completed January 2000.
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 with 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.
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.