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Ground Motion Prediction (GMP)
Group 1 – GMP | Poster Abstracts
1-049
BROADBAND GROUND MOTION SIMULATION OF THE 07/29/08 MW 5.4 CHINO
HILLS EARTHQUAKE Graves RW
The July 29, 2008 Mw 5.4 Chino Hills earthquake was an oblique-slip event occurring at a depth of
about 14 km. The SCSN moment tensor solution gives the best fitting double couple solution as:
strike=291, dip=59, rake=142 and Mo=1.53e+24 dyne-cm. In the epicentral region, observed PGA
and PGV values were around 0.4 g and 37 cm/s, respectively. I simulate the broadband (0-20 Hz)
waveforms for this event using the methodology of Graves and Pitarka (2004). At low frequencies
(f < 1 Hz) the computation is purely deterministic and uses a 3D anealsitic FD simulation code with
the SCEC CVM4. At high frequencies (f > 1 Hz), the computation uses a semi-stochastic
representation of source and wave propagation effects. For the FD model, I used a grid spacing of
125 m and a minimum shear velocity of 620 m/s. To account for the lower near surface shear
velocities, a frequency dependent amplification function is applied to simulations on a site-by-site
basis. These amplification functions are based on the site-specific values of Vs30 (Wills et al., 2000).
The simulation covers a 125 km by 250 km region of Southern California and broadband threecomponent
waveforms are calculated on a 1 km mesh over this region, resulting in about 32,000
sites.
Using a simple point source model with the 3D SCEC CVM4 velocity structure produces a
reasonable fit to the recorded motions for frequencies less than 1 Hz, and matches the general
characteristics of the broadband motions. At a number of basin sites, the simulation reproduces
later arriving pulses that are seen in the data, and which are presumably due to basin generated
phases. The point source simulation predicts stronger motions to the north and east of the epicenter
than are seen in the recorded data.
The fault area for this Mw 5.4 event is probably on the order of about 20 – 25 km2 (roughly 5 km by
5 km). In the epicentral region, and for frequencies around 1 Hz, the point source approximation is
probably not fully justified. Given a 5 km by 5 km finite-fault with strike=291 and dip=59, I
consider another possible scenario where the rupture begins at the deep eastern end of the fault
and then ruptures updip toward the southwest. The finite-fault simulation produces a slight
westward shift of the peak motions compared to the point source result, which is more consistent
with the observed pattern.
1-050
EARTHQUAKE GROUND MOTIONS RECORDED AT THE FACTOR BUILDING AT
UCLA Hauksson SE, and Kohler MD
We have analyzed ground motions recorded in the steel, moment-frame Factor building at UCLA
from nearby earthquakes. After the 1994 Northridge earthquake, accelerometers were installed on
every floor either at the mid-sections or corners of the building. We have used two methods for
analyzing the recorded building motions. First, we created 2D animations of the building shaking
as observed from four unique points of view. We have uploaded the highest-quality animations for
public view on the website factor.gps.caltech.edu. The animations show time-varying horizontal
displacements as recorded at each sensor in the building. They are played back at true-time speed
and amplified to provide visible building shaking. Second, we have analyzed the rocking of the
building for several of the strongest earthquakes. Two accelerometers each in the basement and
subbasement detect vertical motions. We computed rocking as the difference between vertical
motions on the east and west walls, and between motions on the north and south walls. Rocking
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