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[Geophysical Research Letters]
Supporting Information for
[Rupture history of 2014 Mw 6.0 South Napa earthquake inferred from near fault
strong motion data and its impact to the practice of ground strong motion
prediction ]
[Chen Ji1,2, Ralph Archuleta1,2, and Cedric Twardzik1,2 ]
[
1
2
Earth research institute, University of California, Santa Barbara, CA, 93106
Department of Earth Science, University of California, Santa Barbara, CA, 93106]
Contents of this file
Text S1 to S5
Figures S1 to S7
Tables S1 to S2
Movie S1
Introduction
To support the discussions in the main text, here we provide additional five text blocks,
seven figures, and two tables.
Text S1.
Parameters of fault segments
We approximate the causative fault geometry with two sub-vertical rectangular fault
segments based on the surface fault trace and the relocated USGS hypocenter [Hardebeck
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and Shelly, 2014]. The information for the fault segments is summarized in Table S1.
Text S2.
Searching ranges of inverted parameters
We adopt a simulated annealing method to simultaneously invert for slip amplitude, rake
angle, rupture initiation time, and the shape of an asymmetric function for each subfault.
The inverted parameters are found by finding the best match in the wavelet domain
between the computed synthetics (based on the source parameters) and the strong motion
waveforms [Ji et al., 2002; Ji et al., 2003]. In Table S2 we summarize the ranges of
parameters allowed during inversion. The slip amplitude varies from 0 to 3 m and rake
angle changes from 135o to 225o. We allow the starting time and end time of the
asymmetric slip rate function [Ji et al., 2003] to range from 0.05 s to 1.0 s. The value of
rise time was therefore limited to lie between 0.1 s and 2.0 s. We let rupture initiate at the
relocated USGS hypocenter. The rupture initiation time of each subfault changes within
[𝑣
𝐿
𝑟𝑒𝑓
− τ, 𝑣
𝐿
𝑟𝑒𝑓
+ τ], L is the on-fault distance to the hypocenter and 𝑣𝑟𝑒𝑓 is the average
rupture velocity [Shao et al., 2011]. 𝑣𝑟𝑒𝑓 and maximum perturbation time τ used during
final inversion are 3.0 km/s and 3 s, respectively.
Text S3.
Data alignments
In this study, we initially aligned all data by handpicking the P wave arrival at each
station but quickly determined that this approach failed to match the double pulses.
Fortunately, the rupture of the Napa earthquake initiated so energetically that we are able
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to precisely handpick the S wave first arrivals at many stations. We then shift the
horizontal components to align the synthetic and observed S wave first arrivals. For the
stations for which we cannot properly pick the S wave arrival, we conducted preliminary
inversions in which we assign lower weights to these records. After the inversions, we
adjust the arrivals of these stations by comparing the synthetic seismograms with their
observations.
Text S4.
Static stress distribution
We estimate static right-lateral stress drop during the 2014 South Napa earthquake using
the preferred slip model shown in Figure 3. We first calculate the stress drop using the
Coulomb 3.3 software (http://earthquake.usgs.gov/research/software/coulomb, [Lin and
Stein, 2004]) for a half-space earth model with Poisson’s ratio. Then following Ripperger
and Mai [2004], we simply scale the calculated stress change with the shear modulus of
the GIL7 model (Figure S1). Note that in this approximation, we ignore the impact of
variations in Poisson’s ratio. The values of calculated stress drop change from -25 MPa to
54 MPa. The peak stress drop of NP, HP and P2 slip patch is 54 MPa, 26 MPa, and 16
̅̅̅̅ = ∑ ∆σi Di / ∑ Di
MPa, respectively. The weight average stress drop, defined as ∆σ
(∆σi , Di are stress drop and slip of the i-th subfault, [Shao et al., 2012]), is 10 MPa. The
spatial distribution of stress drop is shown in Figure S5.
Text S5.
References
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Boore, D. M., and G. M. Atkinson (2008), Ground-motion prediction equations for the
average horizontal component of PGA, PGV, and 5%-damped PSA at spectral periods
between 0.01 s and 10.0 s, Earthq Spectra, 24(1), 99-138.
Hardebeck, J. L., and D. R. Shelly (2014), Aftershocks of the 2014 M6 South Napa
Earthquake: Detection, Location, and Focal Mechanisms, Abstract S33F-4927, presented
at 2014 Fall Meeting, AGU, San Francisco, Calif., 15-19 Dec.
Ji, C., D. J. Wald, and D. V. Helmberger (2002), Source description of the 1999 Hector
Mine, California, earthquake, part I: Wavelet domain inversion theory and resolution
analysis, Bull. Seismol. Soc. Amer., 92(4), 1192-1207.
Ji, C., D. V. Helmberger, D. J. Wald, and K. F. Ma (2003), Slip history and dynamic
implications of the 1999 Chi-Chi, Taiwan, earthquake, J. Geophys. Res.-Solid Earth,
108(B9), art. no.-2412.
Lin, J., and R. S. Stein (2004), Stress triggering in thrust and subduction earthquakes and
stress interaction between the southern San Andreas and nearby thrust and strike-slip
faults, J. Geophys. Res.-Solid Earth, 109(B2).
Ripperger, J., and P. M. Mai (2004), Fast computation of static stress changes on 2D
faults from final slip distributions, Geophys Res Lett, 31(18), L18610.
Shao, G. F., C. Ji, and E. Hauksson (2012), Rupture process and energy budget of the 29
July 2008 M-w 5.4 Chino Hills, California, earthquake, J. Geophys. Res.-Solid Earth,
117.
Shao, G. F., X. Y. Li, C. Ji, and T. Maeda (2011), Focal mechanism and slip history of
the 2011 M-w 9.1 off the Pacific coast of Tohoku Earthquake, constrained with
teleseismic body and surface waves, Earth Planets Space, 63(7), 559-564.
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Figure S1. Plot of observed PGA (top) and PGV (bottom) vs. epicentral distance
compared with the NGA ground motion prediction equation of Boore and Atkinson
(2008) Red and dashed lines denote the prediction and its standard deviation.
(http://strongmotioncenter.org/NCESMD/data/southnapa_24aug2014_72282711/hig
hlights_southnapa_24aug2014_72282711.pdf).
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Figure S2. GIL7 velocity model: S wave (thick line) and P wave (thin line).
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Figure S3. Comparison of observed velocity waveforms for the transverse components
(black lines) and synthetic seismograms (red lines) predicted using the preferred slip
model. The waveforms have been bandpass filtered from 0.05 Hz to 4 Hz before the
comparison. For each comparison, the value above the beginning of the trace is station
azimuth relative to the epicenter and the value below is epicentral distance. The value
above the end of trace is the observed peak amplitude in cm/s, which is used to normalize
both the synthetic seismogram and the observation.
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Figure S4. Comparison of observed velocity waveforms (black lines) and synthetic
seismograms (red lines) predicted using the preferred slip model. The waveforms have
been bandpass filtered from 0.05 Hz to 1.25 Hz before the comparison. (See Figure S3
caption for explanation of labels.) (a) Transverse components (T) (b) Radial components
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(R) (c) Vertical components (Z). Note that with the current fault geometry, stations with
an azimuth between 149o and 181o are located near the P and SV nodal plane.
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Figure S5. Comparison of observed velocity waveforms in transverse component (black
lines) and synthetic seismograms (red lines) predicted using the rupture on slip patch P2.
The waveforms have been bandpass filtered from 0.05 Hz to 4 Hz before the comparison.
(See Figure S3 caption for explanation of labels.)
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Figure S6. Distribution of static right-lateral shear stress drop estimated using the
preferred slip model shown in Figure 3. The color denotes the stress value in MPa and
contours with an interval of 0.5 m show the co-seismic slip. Three slip patches, NP, HP,
and P2, are indicated. The circles show relocated aftershocks projected onto the fault.
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Figure S7. Snapshots of cumulative fault slip during the first 2.25 s of the rupture. Time
in seconds is given in the upper right corner of each snapshot. Color shows the fault slip;
white contours show the rupture initiation time at intervals of 0.5 s. The red star denotes
the hypocenter, and the circles show relocated aftershocks projected onto the fault. The
white and red arrows indicate the slip patches that produced the first and second velocity
pulses, respectively.
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Table S1. Fault plane information.
Strike/dip
Length
(along strike)
Width
(down-dip)
Depth of Top
edge
Subfault size
(km2)
Number of
subfaults
Segment I
159o/87o
16 km
11.2 km
0.52 km
0.4 x 0.4
1120
Segment II
179o/87o
3.6 km
11.2 km
0.52 km
0.4 x 0.4
252
Table S2. Ranges of the five source parameters for each subfault
Slip
(m)
Rake (o)
Starting
time (s)
Ending
time (s)
Reference
rupture velocity
(km/s)
Maximum
Perturbation time
(𝑠)
[0, 3]
[135, 225]
[0.05, 1.0]
[0.05, 1.0]
3.0
3.0
Movie S1. Spatiotemporal evolution of 2014 Mw 6.0 South Napa earthquake
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