SCEC/USGS Workshop: California 3-D Seismic Velocity Models
Conveners: Cliff Thurber, Egill Hauksson, Peter Shearer, and Felix Waldhauser
Place: SCEC 2006 annual meeting in Palm Springs, Hilton Hotel
Time/Date: 9:00 am to 4 pm; Sunday, 10 September 2006
The goal of the workshop was to bring together interested scientists to develop a plan for the
construction of California-wide 3-D seismic velocity models (Ca3D). These models will be used
for a variety of purposes, such as improving earthquake locations, calculating 3-D finite-fault
wave-propagation effects, modeling source mechanisms, and interpreting tectonic structures.
There are other valuable aspects to consider as well, including providing a 3-D structural
framework for facilitating statewide hazard estimates, and research using data from the USArray
stations deployed in California.
About 65 scientists registered for the workshop; we estimate that on the order of 50 attended.
The format of the workshop was a series of 5-minute, 3-slide presentations followed by open
discussion, on the following general themes:
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The “Vision” for a 3-D statewide model
Model construction, representation, and use
Current state of the art and utilization
Discussion and action items
Summary of the 26 presentations:
C. Thurber briefly reviewed the UW-Madison group's work on a northern California 3-D model
as an example of the elements required to assemble a state-wide model, including a high-quality
dataset of earthquake arrival times and extensive active-source data. The former is readily
available for northern California, but significant effort will be required to assemble a relatively
complete southern California active-source dataset. Use of quarries identified from satellite and
other imagery data (e.g. GoogleEarth®) can be used to provide additional controlled-source data,
albeit without origin time information. Analysis methods for constructing a state-wide model are
generally in place, including double-difference, spherical-earth, and adaptive-mesh tomography
codes (although a code incorporating all three of these does not yet exist) and practical model
resolution and covariance matrix calculation techniques.
R. Clayton discussed a column grid approach for representing a 3-D model - a database of
latitude, longitude, depth, physical properties, and "characterization" (e.g., lithology, sediments,
crust, mantle, etc.). Properties at any point are determined from interpolation among the
columns. Such an approach is flexible (multiple interpolation methods and length scales are
possible) and extensible (columns and properties are easily added), and allows for a simple
calculation of model variance. An example map of column types and distribution for Baja
California was shown.
D. Okaya presented an overview of the Earthworks "On Demand Synthetic Seismograms"
system using the "workflow technology" approach. The user selects a geographic region and
chooses an event (including mechanism) and one of several Earth models, and the system uses
available computer resources and codes to generate synthetic seismograms and any additional
desired derived products (e.g., PGA). A demonstration version is available now.
A. Plesch described the SCEC Community Fault Model (CFM) and its derived product, the
Community Block Model (CBM). The CFM has 171 faults, and the CBM has 39 blocks and
includes topography, basement interface, and the Moho. Substantial effort is going into ensuring
consistency between the CBM and the Community Velocity Model (CVM).
P. Davis argued for inclusion of the mantle in the Ca3D effort. He made the point that the mantle
may provide significant geodynamic driving forces that influence crustal deformation. Receiver
functions to delineate the Moho and upper mantle features, SKS splitting studies to characterize
mantle flow patterns, and surface- and body-wave tomography can all contribute.
P. Seuss presented an overview of the Harvard v. 4.0 Community Velocity Model (CVM). The
model has Vp, Vs, and density for the consolidated sediments, based mainly on industry data,
atop the 3D tomographic model of Hauksson for the basement. Boundaries are defined by
triangulated surfaces. The model can be queried simply by providing tabulated lat-lon-depth
values.
R. Clayton described v 4.0 of the SCEC 3D CVM. From top to bottom, it has a geotechnical
layer (with limited coverage), basins with formula-based properties, a background tomographic
crustal model (Hauksson's model updated with one iteration including the basins), a Moho
(interpolated from Zhu and Kanamori's work, smoothed), and a tomographic upper mantle model
(Kohler's).
T. Brocher described the USGS 3D velocity models for the SF Bay region and northern
California as a whole. They are based on the "3D geologic map" of Jachens and coworkers.
Empirical velocity-depth relations for the lithologies in the geologic model were used to
construct the velocity model, including both Vp and Vs. In the process, a new regional Vp-Vs
empirical relationship was developed. The model has been put to use for simulations of strong
motion from the 1906 great San Francisco earthquake.
H. Zhang presented double-difference tomography models at several scales for central and
northern California. A relatively fine-scale model (mainly 1 to 5 km horizontal grid spacing) has
been developed for the region around Parkfield, extending from San Simeon to Coalinga. The
2004 Parkfield earthquake rupture zone appears to be associated with a high-velocity body on the
northeast side of the San Andreas fault. An intermediate-scale model (mainly 5 to 10 km
horizontal grid spacing) has been developed for the San Francisco Bay region. The model clearly
images a number of bedrock highs and sedimentary basins, and shows strong velocity contrasts
across portions of several of the major faults. A large-scale model (mainly 10 to 20 km
horizontal grid spacing) has been developed for most of Northern California. In addition to
imaging the down-going Gorda slab beneath the Mendocino region, a distinct high-velocity body
is found beneath the northern part of the Great Valley that is interpreted to be ophiolitic in
nature.
G. Lin reported on the use of "composite events" and quarry blasts to improve the tomography
model for Southern California. Using an on-line waveform dataset for 450,000 earthquakes
covering 1981 to 2005, waveform correlation calculations have been expanded to 76 million
event pairs. The composite event approach makes the inversion of the absolute picks more
efficient, the use of quarries identified in remote-sensing imagery provides absolute calibration
control, and the massive relative arrival time data provides excellent constraints on relative
locations.
J. Hardebeck presented a new preliminary Vp and Vp/Vs model for the central coast region of
California. The model covers the area from about 34.5° to 36° N and 120° to 121.75° W, along
the coast from Lompoc to San Simeon. A key to the development of this model was access to
catalog picks from the PG&E array of 3-component stations to augment the available CISN data.
G. Fuis recommended the extensive use of borehole and refraction data (existing and new) to
provide critical constraints on the structure of the uppermost crust. He used a comparison of
models along LARSE Line 1 (refraction model, earthquake tomography model, SCEC model) to
illustrate the improvement possible with active-source and in-situ data. He also suggested the use
of gravity data in joint inversions to help constrain the shallow crust where refraction data do not
exist.
J. Murphy focused on the critical need for more S-wave data. In addition to recommending
taking advantage of empirical velocity relations, she presented an example of S picks obtained
from vertical-component records of LARSE shots. Clearly "mining" existing refraction data for
S waves could be quite fruitful.
R. Clayton presented new receiver function results for Southern California using the expanded
dataset available from Trinet and a stacking algorithm. The new results show evidence for abrupt
changes in Moho depth, from receiver function differences at nearby stations and from azimuthal
variations at a given station. The typical Southern California station spacing of 40-50 km still
greatly exceeds the 10-20 footprint of the receiver function of one station, so the observations
remain severely aliased in most places.
C. Tape and Q. Liu described the use of spectral element forward modeling and adjoint method
inverse modeling to improve earthquake source mechanisms and identify deficiencies in the
Southern California 3D model. The Yorba Linda earthquake was used as an example showing
the improvement in forward modeling a 3D model provides compared to a 1D model. The fit is
of course not perfect, requiring time shifts for arrivals as well as amplitude and complexity
changes. Back-projection of time-delayed, residual-weighted waveforms "images" deficiencies
in the model. The adjoint approach is an efficient method for a formal inversion. A simple
checkerboard test was used to illustrate the procedure.
P. Chen presented an alternative scattering-integral approach for full waveform inversion for 3D
structure and source properties. They isolate the waveform segment to be modeled by windowing
the complete finite-difference synthetic seismogram and obtaining the so-called isolation filter.
This isolation filter is then cross-correlated both with the observed and complete synthetic
seismograms. They window the cross-correlograms and then narrow-band filter them at many
frequencies. The phase and amplitude differences between the narrow-band filtered crosscorrelograms give the frequency-dependent phase-delay and amplitude measurements. The
frequency-dependent kernels for the inversion are constructed by convolving the forward
wavefields generated by the earthquake source with the Green tensors for impulsive point
sources located at the receiver.
M. Ritzwoller summarized previous work applying ambient noise tomography to Southern
California, discussed current efforts to refine and extend the method and apply it to new regions,
and highlighted areas of future work. Ambient noise tomography using Rayleigh wave energy is
complementary to traditional surface wave data because of the significantly higher resolution at
shorter period (< 20 seconds), providing constraints on the crust. Efforts to extend the method to
larger and smaller scales and phase velocities and Love waves are ongoing. Joint inversions with
other data types are envisioned.
R. Catchings discussed the importance of refraction data for calibration of and putting constraints
on 3D velocity models. Surface geology, surface structure, and borehole data cannot be extended
to depth and laterally, respectively, without potential problems. Earthquake tomography typically
does not resolve shallow structure well. He showed examples of disagreement between shallow
seismic refraction models and the geology-based 3D Bay Area model. Results from dozens of
shallow reflection/refraction profiles in California are available.
V. Langenheim pointed out that potential-field data (gravity and magnetics) cover the entire
state, including offshore, whereas we don't have active-source profiles uniformly distributed
throughout the state nor can passive source tomography image the entire crust from top to
bottom because earthquakes don't occur everywhere. Thus potential-field data offer an
opportunity to extrapolate geologic structure throughout the state and help guide development of
a statewide 3D seismic velocity model. Isostatic gravity anomalies are particularly good for
mapping location and geometry of faults, detecting dense, high-velocity bodies, and defining the
geometry of Cenozoic basins. Magnetic anomalies are also good for mapping the location and
geometry of faults, and for detecting ophiolite bodies.
C. Thurber reviewed some of the data types and advanced techniques that may prove vital to
constructing a reliable statewide 3D velocity model. Echoing the comments of G. Fuis and R.
Catchings, he emphasized the value of controlled-source and borehole data for constraining
velocities and interface positions, and the need for joint velocity-interface inversions.
Teleseismic body-wave tomography will be important for extending the earthquake tomography
model to greater depth. Joint receiver function and surface wave inversions and ambient noise
tomography inversions can provide critical constraints on shear velocity structure and interface
depths. Joint seismic-gravity inversions show promise for overcoming coverage issues with
seismic data, and constrained inversions using other geophysical observables or an a priori
geology-based 3D model need to be explored.
F. Waldhauser showed how a real-time high-precision relocation system, using differential times
and double-difference location, can be run in parallel with the present NCSN real-time location
system. The effectiveness of the double-difference approach is quite clear, as an example from
Parkfield illustrates. Some practical current and future issues include model parameterization and
implementation, 3D ray tracing or finite difference travel times versus grid search (lookup tables
for travel times and partial derivatives), how to handle dynamic earthquake catalogs or velocity
models, and how to test new velocity models in an efficient (and possibly automated) fashion.
P. Shearer (on behalf of Y. Fialko) presented two examples of the use of InSAR data to
investigate strain accumulation in southern and central California, including the effect of
material heterogeneity. Using 35 interferograms of the southern San Andreas around the Salton
Sea from 1992-2000, Fialko derived line of sight velocities from the stacked InSAR data. In
addition to expected changes across the major faults, non-uniform lateral strain gradients are
visible that may reflect variations in elastic properties. Similarly, Schmalzle and coworkers
analyzed InSAR data for the Carrizo Plain segment of the San Andreas, finding different strain
gradients NE versus SW of the fault that they associate with a likely seismic velocity contrast
between the two sides of the fault.
B. Aagaard summarized the research efforts on broadband simulation of the 1906 great
earthquake and other actual or potential earthquakes. The objective is to generate synthetic
seismograms that accurately capture travel times, reflections, refractions and amplification. The
desirable features in seismic velocity models to be used for the simulations include a unified
structural representation, a standard interface for querying the models, and fast, efficient queries.
He then summarized desirable properties and characteristics of a unified structural representation
and the model query interface. Among them for the former are consistency between the velocity
model and the fault surfaces, the incorporation of topography, and the integration of Vs30
values.
E. Humphreys presented an updated teleseismic tomography model for southern California.
Features in the new model are suggestive of the presence of small-scale convection and
delamination beneath several areas - the Transverse Ranges, Southern Sierras, and possibly NW
Sonora. Short-term improvements include utilizing the finite frequency approach, incorporating
3D ray tracing, and including constraints from receiver functions. Longer-term plans include
iterative, multi-method inversion.
R. Clayton briefly presented some results from the NARS-Baja/RESBAN Array in the Baja
California area. The array spans the Gulf of California. Tomography images show sharp
vertically oriented low-velocity features associated with the rifting in the Gulf of California.
R. Allen presented preliminary results from the OATS teleseismic array along the Newberry
hotspot track. In addition to imaging the high-velocity slab to ~400 km depth and detecting
slightly clockwise-rotating shear wave splitting in the Cascadia back-arc, they find relatively low
mantle velocities in much of the back-arc, indicating that the mantle beneath the Newberry track
is not particularly unusual.
The workshop concluded with a discussion of the needs, potential collaborative efforts, and
possible funding sources for development of a statewide California (and ultimately regionalscale) 3D model. In the area of data needs, the following were identified: longer deployments,
standardized dataset formats, borehole sonic logs, refraction data (1D models, travel times),
potential field data, reflection data (interfaces, stacking velocities), additional earthquakes,
geologic maps and associated information, and industry-donated data. In the area of software
needs, the following were identified: adaptive meshing, spherical earth tomography, expanded
noise tomography analyses, and code benchmarking and validation efforts. In the area of models,
the discussion brought up goals of shared databases for comparisons, beta versions of a statewide model, ways for "bending" rule-based models, and careful consideration of the needs of the
end-users. Finally, concerning opportunities for collaborative efforts, there is a need to
coordinate with the relevant SCEC focus groups (Lithosphere Architecture, Unified Structural
Representation, Seismology) and NEHRP projects (including CISN), target some start-up efforts
for the November SCEC proposal deadline, and take advantage of the March 2007 EarthScope
National Meeting in terms of relevant tools and infrastructure for USArray.
Names of attendees/interested parties:
1) Brad Aagaard, United States Geological Survey, baagaard@usgs.gov
2) Richard Allen, University of California, Berkeley, rallen@berkeley.edu
3) Bob Anderson, California Seismic Safety C., anderson@stateseismic.com
4) Ralph Archuleta, Univ. of California, Santa Barbara, ralph@crustal.ucsb.edu
5) Andy Barth, Indiana University, ibsz100@iupui.edu
6) Glenn Biasi, University of Nevada, Reno, glenn@seismo.unr.edu
7) Jacobo Bielak, Carnegie Mellon University, jbielak@cmu.edu
8) Tom Brocher, United States Geological Survey, brocher@usgs.gov
9) Rufus D. Catchings, USGS, catching@usgs.gov
10) Po Chen, University of Southern California, pochen@usc.edu
11) Rob Clayton, California Institute of Technology, clay@gps.caltech.edu
12) Paul Davis, University of California, Los Angeles, pdavis@ess.ucla.edu
13) Steve Day, San Diego State University, day@moho.sdsu.edu
14) Jeff Eddo, San Diego State University, geo_boy12@hotmail.com
15) Bill Ellsworth, United States Geological Survey, ellsworth@usgs.gov
16) Zijun Fang, University of California, Riverside, zfang@engr.ucr.edu
17) Christina Forbes, Western Illinois University, cullingsong@aol.com
18) Gary Fuis, United States Geological Survey, fuis@usgs.gov
19) Peter Gerstoft, University of California, San Diego, gerstoft@ucsd.edu
20) Rob Graves, URS Corporation, robert_graves@urscorp.com
21) Yonggui Guo, Rice University, yonggui@rice.edu
22) Jeanne Hardebeck, United States Geological Survey, jhardebeck@usgs.gov
23) Egill Hauksson, California Institute of Technology, hauksson@gps.caltech.edu
24) Gene Humphreys, University of Oregon, gene@newberry.uoregon.edu
25) Robert Jachens, USGS, jachens@usgs.gov
26) Tom Jordan, SCEC, tjordan@usc.edu
27) Vicki Langenheim, United States Geological Survey, zulanger@usgs.gov
28) Guoqing Lin, University of California, San Diego, gulin@ucsd.edu
29) Qinya Liu, California Institute of Technology, lqy@gps.caltech.edu
30) Zhen Liu, Stanford University, liuz@pangea.stanford.edu
31) Pengcheng Liu, University of California, Santa Barbara, pcliu@crustal.ucsb.edu
32) Julio Lopez, Carnegie Mellon University, jclopez+scec@andrew.cmu.edu
33) John Louie, University of Nevada, Reno, louie@seismo.unr.edu
34) Shuo Ma, University of California, Santa Barbara, sma@crustal.ucsb.edu
35) Phil Maechling, University of Southern California, maechlin@usc.edu
36) Morgan Moschetti, Univ. of Colorado, Morgan.Moschetti@Colorado.edu
37) Walter Mooney, United States Geological Survey, mooney@usgs.gov
38) Janice Murphy, United States Geological Survey, murphy@usgs.gov
39) Craig Nicholson, Univ. of California, Santa Barbara, nicholson@msi.ucsb.edu
40) David Okaya, University of Southern California, okaya@usc.edu
41) Arben Pitarka, URS Corporation, arben_pitarka@urscorp.com
42) Andreas Plesch, Harvard University, andreas_plesch@harvard.edu
43) Mike Ritzwoller, Univ. of Colorado, ritzwoll@merckx.colorado.edu
44) Karim Sabra, University of California, San Diego, ksabra@mpl.ucsd.edu
45) Sarah Schindler, California State Univ., Bakersfield, sarahschindler@gmail.com
46) Danijel Schorlemmer, danijel@sed.ethz.ch
47) Peter Shearer, University of California, San Diego, pshearer@ucsd.edu
48) Seok Goo Song, Stanford University, seisgoo@pangea.stanford.edu
49) Peter Suess, Harvard University, suess@rupture.harvard.edu
50) Ricardo Taborda, Carnegie Mellon University, rtaborda@andrew.cmu.edu
51) Ying Tan, California Institute of Technology, ytan@gps.caltech.edu
52) Toshiro Tanimoto, Univ. of California, Santa Barbara, toshiro@geol.ucsb.edu
53) Carl Tape, California Institute of Technology, carltape@gps.caltech.edu
54) Leon Teng, University of Southern California, lteng@usc.edu
55) Cliff Thurber, University of Wisconsin, Madison, thurber@geology.wisc.edu
56) Jeroen Tromp, California Institute of Technology, jtromp@gps.caltech.edu
57) Felix Waldhauser, Columbia University, felixw@ldeo.columbia.edu
58) Jochen Woessner, Caltech, jowoe@gps.caltech.edu
59) Yih-Min Wu, National Taiwan Univ., Dpt. of Geosciences, drymwu@ntu.edu.tw
60) Zhimei Yan, California Institute of Technology, yanmei@gps.caltech.edu
61) Wenzheng Yang, University of Southern California, wenzheny@usc.edu
62) Alan Yong, United States Geological Survey, yong@usgs.gov
63) Haijiang Zhang, Univ. of Wisconsin, Madison, hjzhang@ice.geology.wisc.edu
64) Li Zhao, University of Southern California, zhaol@usc.edu
65) Jiancang Zhuang, Univ. of California, Los Angeles, zhuang@moho.ess.ucla.edu