6. Proposed Plans
One caveat associated with the validation simulations of the preliminary statistical models of small-scale
heterogeneities is that AWP currently uses a constant (frequency-independent) coarse-grained Q
implementation following Day (1998) and Day and Bradley (2001). Such approach is appropriate for
frequencies less than about 1Hz. However, data suggest (Jordan study?) that Q decreases as frequency
increases, and that a power law (Q=Q0fn) can be used to properly describe the frequency dependency.
Using exponents of 0.6-0.8 suggested from data Q may be more than tripled from f=1Hz to 5Hz, which
significantly affects the ground motions. We are in the process of incorporating Q(f) into AWP (AWP-Q(f))
assuming a power law model, using a coarse-grained approach with different weights over a specified
frequency band (see Figure 2).
AWP currently uses a constant (frequency-independent) coarse-grained Q implementation following Day
(1998) and Day and Bradley (2001). Such approach is appropriate for frequencies less than about 1Hz.
However, data suggest (Jordan study?) that Q decreases as frequency increases, and that a power law
(Q=Q0fn) can be used to properly describe the frequency dependency. Using exponents of 0.6-0.8
suggested from data Q may be more than tripled from f=1Hz to 5Hz, which significantly affects the ground
motions. We are in the process of incorporating Q(f) into AWP (AWP-Q(f)) assuming a power law model,
using a coarse-grained approach with different weights over a specified frequency band (see Figure 1).
Once the version of AWP-Q(f) with frequency-dependent Q described above has been implemented and
verified, we plan to use the code to compute ground motions up to 5Hz. We plan to use one or several
historical earthquakes, that are sufficiently small to minimize source complexity contributions in the
simulations (such as the 2008 Mw 2008 Chino Hills, CA, earthquake, see Figure 2).
Comparison of simulated and recorded ground motions will allow us to calibrate Q(f) in the CVMs of CA,
including realistic statistical small-scale heterogeneities. Furthermore, we will attempt to estimate the
relative contribution to the high frequency ground motions from the fault complexity and the surrounding
medium, using simulations with sources generated using dynamic rupture propagation along a rough
(non-planar) fault (Shi and Day, 2013), see Figure 3.
Statewide F3DT Targets
1. Assimilation of geological constraints into statewide F3DT
Full-wave seismic data assimilation (FWSDA), which is a variational data assimilation approach to the
seismological inverse problems (Chen 2012), provides an inversion mechanism that allows us to combine
seismological and geological constraints from different sources into a unified structural representation. In
particular, the basin structures and fault geometries can be incorporated into the inversion through explicit
constraints of the model parameters. The development of basin structures throughout California in the
enhanced statewide CVM will provide us new constraints in our statewide F3DT. As our first target, we
will incorporate the Central Valley into our F3DT in a self-consistent way.
2. Collecting and compiling high-quality waveform data throughout California
A collection of statewide high-quality waveform dataset plays a vital role in our statewide F3DT. The
distribution of seismicity, as well as seismic stations, is not uniform throughout California. The amount and
the quality of the waveform data in northern California are not as good as in southern California. Our
second target is to construct a statewide high-quality waveform dataset that provides a relatively uniform
sampling of the crustal structure throughout the state. To reach this target, we will need to utilize both
earthquake recordings from local seismic networks and also ambient-noise Green’s functions.
3. CMT solutions for earthquakes in California
The accuracy of the inverted seismic structure model depends upon how well the source parameters of
the earthquakes, whose waveform recordings are used in the structural inversion, are calibrated. We plan
to carry out CMT inversions for about 400 earthquakes distributed evenly throughout California using the
tomographically updated 3D statewide seismic velocity model CAF2. This work will pave the path for
carrying out further iterations of statewide F3DT.
We will continue to target the shorter length scale features within the California model by inverting
waveforms down to 2 s in southern California, focusing on the newly added basins within CVM-H 11.9
that have not yet been considered within tomographic inversions.
USR:
Task A(2014): Develop an enhanced statewide CVM that includes several new and important
components.
a) Incorporate statewide Community Fault Model (SCFM) in statewide CVM. As in southern California
(CVM-H), the locations and displacements of major faults in northern California will be used to refine
basin models, resulting in a Unified Structural Representation.
b) Incorporate basin structure throughout California, derived from southern California CVMs (CVMS and
CVMH) and Bay Area Model (USGS), in a unified statewide CVM.
c) Implement data constraints using metadata standards in statewide CVM through UCVM framework.
Task B(2014): Develop and implement a stochastic description of small scale velocity structure in the
sedimentary basins throughout the statewide CVM (w/ Olsen, Jordan). A trend of numerical wave
propagation studies to shorter periods (> 2 Hz) has been facilitated by sustained increases in
computational power, and has created a demand for increased resolution velocity models (e.g., Olsen
and Jacobsen, 2011). Building such models is challenging, in part because geologic and seismologic data
indicate that fine-scale elastic inhomogeneities can be strong in sedimentary basins and have spatially
anisotropic statistical distributions (Magistrale et al., 1996; Süss and Shaw, 2003; Brocher, 2005). While
we have local measures of fine-scale velocity structure (down to meter scales) along boreholes with sonic
logs, there is not a sufficient density of such samples to facilitate the development of a deterministic
regional model. Thus, one of the goals of our collaboration is to develop a statistical description of finescale velocity structure, informed by these local observations, to enhance the community models so that
they can support higher-frequency simulations.
To date, we have analyzed wells in the Los Angeles basin and characterized the fine-scale velocity
structure that is not represented in the current southern California CVM-H 11.9. Our analysis shows a
standard variation of 157 ms/m around a mean of 1.3 ms/m for the delta between compressional wave
slowness in the logs and model. Moreover, we have assessed vertical and horizontal correlations of this
fine-scale velocity (Vp) structure using a closely spaced subset of these wells. Variance analysis has
shown horizontal and vertical correlation distances of about 80m and 900m, respectively. Thus, the model
analysis suggests a ratio of horizontal to vertical correlation lengths of about 11:1. Such a ratio would
serve as important constraint on how to parameterize a stochastic representation of fine-scale velocity
structure. Our goal is to use these measure to develop stochastic representation of fine-scale velocity
structure that can be incorporated into new versions of the CVM’s to facilitate higher frequency wave
propagation studies.
Task C(2015): Contribute to the development of a model development lifecycle workflow, that includes
incorporating refinements to the structural model, implementation of the new model in the UCVM
framework, reparameterization of grids and meshes, waveform tomographic inversions, and
benchmarking of these new models using goodness-of-fit methods. To support accurate simulations of
seismic wave propagation and other uses, the CVM’s incorporate a variety of different velocity
constraints, ranging in resolution from 10 cm scale borehole observations in shallow sedimentary sections
to 3D tomographic models that describe the upper mantle at scales of tens of kilometers. These
components must be assembled in a way that ensures their internal consistency. Thus, we developed a
workflow for building the CVM’s that begins with the development of structural representations of the
basins, including fault representations described in companion Community Fault Models (CFM’s). These
sediment volumes are parameterized with their internal velocity structures using direct data constraints,
such as wells and stacking velocities. These basin models are then used as input for the development of
tomographic models of the crust and upper mantle. Integrated basin and tomographic models are used to
develop computational meshes or grids, and then evaluated and improved using 3D, adjoint waveform
tomographic methods. Finally, a precise geotechnical layer based on Vs30 measurements is developed
as an optional overprint to the model in order to facilitate its use in strong ground motion studies and
engineering applications. Together, all of these elements are described as a Unified Structural
Representation (USR).
A major challenge in the development of CVM’s and USR’s is incorporating new constraints on basin and
near surface structure. For example, new wells or seismic reflection experiments, or the occurrence of a
major earthquake, may help to better resolve some local aspect of the regional structure. To incorporate
this information into a model, however, necessitates completing the entire model workflow –
parameterization, meshing/gridding, and tomographic inversions – in order to ensure internal consistency.
Moreover, new model versions must be benchmarked through comparisons of observed and synthetic
waveforms using goodness-of-fit measures. Our experience in SCEC suggests that it takes about two
years to complete this process. Thus, one of the goals of our Geoinformatics collaboration is to reduce
this development cycle by …. [Phil, Tom, and others …]
Simulation:
We plan to port the GPU code to enable tomography calculations. This is to take the advantage of the
GPU code efficiency to accelerate tomography research. The primary challenge of this porting effort is to
enable scalable IO and efficient transfer of 3D tomographic data from GPUs to CPUs through PCI
Express bus.
Statewide F3DT Targets
1. Assimilation of geological constraints into statewide F3DT
Full-wave seismic data assimilation (FWSDA), which is a variational data assimilation approach to the
seismological inverse problems (Chen 2012), provides an inversion mechanism that allows us to combine
seismological and geological constraints from different sources into a unified structural representation. In
particular, the basin structures and fault geometries can be incorporated into the inversion through explicit
constraints of the model parameters. The development of basin structures throughout California in the
enhanced statewide CVM will provide us new constraints in our statewide F3DT. As our first target, we
will incorporate the Central Valley into our F3DT in a self-consistent way.
2. Collecting and compiling high-quality waveform data throughout California
A collection of statewide high-quality waveform dataset plays a vital role in our statewide F3DT. The
distribution of seismicity, as well as seismic stations, is not uniform throughout California. The amount and
the quality of the waveform data in northern California are not as good as in southern California. Our
second target is to construct a statewide high-quality waveform dataset that provides a relatively uniform
sampling of the crustal structure throughout the state. To reach this target, we will need to utilize both
earthquake recordings from local seismic networks and also ambient-noise Green’s functions.
3. CMT solutions for earthquakes in California
The accuracy of the inverted seismic structure model depends upon how well the source parameters of
the earthquakes, whose waveform recordings are used in the structural inversion, are calibrated. We plan
to carry out CMT inversions for about 400 earthquakes distributed evenly throughout California using the
tomographically updated 3D statewide seismic velocity model CAF2. This work will pave the path for
carrying out further iterations of statewide F3DT.
Evaluation:
KBO. Task B-1. Toward characterizing the variability of shallow sediment amplification further, we have
collected available near-surface velocity data, including several datasets of direct and indirect Vs30
values. These data, along with borehole measurements, were then used to quantify statistical parameters
characterizing the existing shear-wave velocities in Los Angeles Basin through semi-variograms. Our
preliminary analysis suggests that the data are conforming to a fractal distribution with fractal dimensions
of 1.5-1.8 and related Hurst exponents of 0.2-0.5. We have generated statistical models of seismic
velocities and densities in agreement with the results of the data analysis and integrated these models
into the most recent SCEC CVM-S, as a basis for validation against strong motion data.
Visualization: Refer to Tim Scheitlin’s movies presented at SC12?
The effects of the near-surface heterogeneities on ground motion and scattering are being tested in two
different ways. First, we are using simulations of the 2008 Mw 5.4 Chino Hills, CA earthquake in a subset
of the SCEC CVM 4.0, with frequencies up to 2.5-5 Hz. We assume a minimum Vs of 200 m/s using the
highly scalable finite-difference code (AWP-ODC, see Figure 1). Preliminary results indicate a trade-off
between the statistical model (in terms of scattering Q) with anelastic attenuation, and that the currently
employed sediment Q models may need to be tuned (toward higher Q values) for models including the
near-surface heterogeneities. The ratios of peak ground velocities between models with and without the
near-surface heterogeneities (both including the Q model) vary between ~0.5 and 2.
1. References