Geoinformatics report (2014.5~2015.6) En-Jui Lee, Po Chen, and Thomas Jordan
1. Validation of Southern California Community Velocity Models (CVMs) using
earthquakes in greater Los Angeles area.
We tested the waveform predictions of the three CVMs, CVM-S4, CVM-H11.9 and CVMS4.26, against more than 900 three-component broadband seismograms recorded from two
Los Angeles earthquakes, 17 Mar 2014 MW4.4 Encino and 29 Mar 2014 MW5.1 La Habra (fig
1). All synthetic and observed seismograms were band-pass filtered using a Butterworth
filter with corners at 0.02 Hz and 0.2 Hz. We measured the difference between an individual
observed seismogram 𝑢𝑘 (𝑡) and its corresponding synthetic 𝑢̃𝑘 (𝑡) within the time window
[tk, tk'] by the relative waveform misfit (𝑅𝑊𝑀) statistic, defined by the integral
𝑡′
𝑅𝑊𝑀𝑘 =
∫𝑡 𝑘[𝑢𝑘 (𝑡) − 𝑢̃𝑘 (𝑡)]2 𝑑𝑡
𝑘
𝑡′
√∫𝑡 𝑘 𝑢𝑘 (𝑡)2 𝑑𝑡
𝑘
𝑡′
∫𝑡𝑘𝑘 𝑢̃𝑘 (𝑡)2 𝑑𝑡
The 𝑅𝑊𝑀 is the ratio of the energy in the waveform difference normalized by the
geometrical mean of the observed and synthetic waveform energy. In this study, we set the
window [tk, tk'] for the kth record to run from the first arrival to the end of the main surface
wave group, so that 𝑅𝑊𝑀 measures the net waveform difference across all of the main
phases on the seismograms. In general, synthetics computed using CVM-S4.26 provide
substantially better fit to observed seismograms at frequencies below 0.2 Hz than those
computed using the other two CVMs (Figure 1).
2. Full-3D Tomography (F3DT) of Crustal Structure in Central California
The starting model was constructed by combining the most recent Southern California
F3DT model, CVM-S4.26, with a 3D velocity model of Northern California obtained by
embedding the U.S. Geological Survey (USGS) Bay Area Velocity Model Version 08.3.0 into
Lin et al.’s (2010) California statewide seismic velocity model. Our F3DT procedure
iteratively uses a highly optimized staggered-grid finite-difference code to simulate seismic
wave propagation through 3D structures, frequency-dependent phase misfit measurements
to capture the waveform differences between observed and synthetic waveforms, the
scattering integral (SI) method to calculate the Fréchet sensitivity kernels, and a highly
optimized LSQR algorithm to solve the linear system. We first used the available ambient
noise Green’s functions (ANGFs) to invert the central California crustal velocity model and
have done 5 iterations (Figure 2). The iteratively inversions have reduced the variance of
phase delay measurements by over 35% relative to the starting model, and the changes in
velocities have begun to heal the velocity artifacts inherited from starting model (Figure 2).
Many features revealed in the model are consistent with independent geophysical
observations in Central California, including controlled-source tomography, gravity
anomalies, and the locations of active faults.
3. Full-wave centroid moment tensor (CMT) inversion in an updated 3D velocity
model for earthquakes in Central California
We have applied full-wave CMT inversions in an updated 3D velocity model to more than
180 earthquakes in Central California (Figure 3). Our full-wave CMT inversion algorithm
relies on the use of receiver-side Green’s tensors (RGTs), which comprise the spatialtemporal displacements produced by the three orthogonal unit impulsive point forces
acting at the receivers. We have constructed a RGT database for more than 180 broadband
stations in Central California using an improved version of the 3D velocity model (Figure 2).
The updated CMT solutions for earthquakes in Central California could provide a more
robust catalog for earthquake simulations for further F3DT inversions.
Broader Impacts
1. The waveform validation demonstrates the capability of reliable ground motion
predictions based on our high-resolution 3D velocity model, CVM-S4.26, for earthquake
recordings not used in the inversion. Accurate ground motion predictions could improve
seismic hazard estimates and therefore benefit the CyberShake hazard models in Southern
California.
2. The synthetic seismograms based on the updated 3D velocity model for Central California
provide more accurate ground motion predictions and could benefit seismic hazard related
studies in the region.
3.Our full-wave CMT inversion in an updated 3D velocity model provides more robust
earthquake source parameter estimates. An accurate estimate of hypocenter is critical for
both geologic interpretation of active faults and ground-motion predictions for seismic
hazard assessments.
Publication
Lee, E., Chen, P., and Jordan, T. H. (2014) Testing Waveform Predictions of 3D Velocity
Models Against Two Recent Los Angeles Earthquakes, Seismological Research Letters,
85(5), 1274-1284.
Poster & Presentation
Lee, E., Chen, P., Thomas, H. J., Maechling, J. P., Denolle, M., & Beroza, G. (2014) Full-3D
Tomography of the Crustal Structure in Southern California Using Earthquake Seismograms
and Ambient-Noise Correlagrams. Abstract and poster, 2014 AGU Fall Annual Meeting.
Lee, E., Thomas, H. J., Chen, P., Maechling, J. P., Boué, P., Denolle, M., Beroza, G., & Eymold, W.
K. (2015) Full-3D Tomography of Crustal Structure in Central California. Abstract and
presentation, 2015 SSA Annual Meeting.
Invited Presentations
Lee, E., Chen, P., Thomas, H. J., Maechling, J. P., Denolle, M., & Beroza, G., Full-3D Waveform
Tomography for Crustal Structure in Southern California, Seminar, Department of Earth
Sciences, National Cheng Kung University, Dec. 25, 2014
Lee, E., Chen, P., Thomas, H. J., Maechling, J. P., Denolle, M., & Beroza, G., A High-Resolution
Seismic Structure Model of Southern California Crust Imaged Through Full-3D Tomography
and Its Potential Applications, Seminar, Department of Geosciences, National Taiwan
University, Mar. 20, 2015
Lee, E., Chen, P., Thomas, H. J., Maechling, J. P., Denolle, M., & Beroza, G., A High-Resolution
Seismic Structure Model of Southern California Crust Imaged Through Full-3D Tomography
and Its Potential Applications, Seminar, Department of Earth Sciences, National Central
University, Apr. 24, 2015
List of countries visited for research on the project
Taiwan
Figures
Figure 1. Spatial distributions of the average RWM values for the three CVMs interpolated
from the average RWM values calculated at about 150 stations. White solid line: boundaries
of CVM-S4.26; white dash lines: boundaries of CVM-H11.9; black solid lines: coastlines and
active faults in Southern California; black dash lines: LARSE-I and LARSE-II profiles; stars:
epicenters of the earthquakes; white circles: broadband stations whose waveforms were
used for RWM calculations. The corresponding RWM histograms are shown below the maps
and the vertical dash lines on the histograms indicate the mRWM values.
Figure 2. Shear wave (S wave) velocity at (top) 2 km, (middle) 10 km, and (bottom) 20 km
depths in (left) the initial model CCA00, (middle) the 5th iteration model CCA05, and (right)
the perturbations. The color bar on the lower right corner of each plot shows the range of
the color scale with red indicating relatively slow S wave velocities and blue indicating
relatively fast S wave velocities. Black solid lines show major faults in our study area.
Figure 3. The CMT solution for earthquakes analysed in Central California. Yellow triangles
indicate the locations of broadband stations. Red dots indicate the epicenters of those
earthquakes. The box indicates our study area. Major faults in this area are plotted in black
solid lines. The background color shows topography.