Chiou and Youngs PEER-NGA Empirical Ground Motion Model for ...

Appendix D
Processing and Model Fits to TriNet/ShakeMap Data
(Incomplete)
Dataset
Ground-motion dataset used in this analysis was compiled by USGS (Boatwright,
per. communication) from the TriNet. It includes PGA (in units of %g) and PGV (in units
of cm/sec) from 252 earthquakes (Figure D-1). Since only the larger of the tow horizontal
components is provided, we divide the listed PGA value by 1.13 to adjust the value down
to the expected level of average horizontal component. A subset of 102 events are
selected and used in the analysis; see below for description of selection criteria.
Epicenter location, focal depth, and local magnitude ML of each earthquake are from the
SCSN catalog. Moment magnitude (Mw) is based on the moment tensor solution catalog,
also from SCSN.
Since most earthquakes are small in size and therefore without a finite source
model, rupture distance is imputed using the same procedure as documented in Appedix ?.
For site conditions, we used the Vs30 value in the NGA database or estimated it using the
same correlation between Vs30 and geological unit as was used in NGA. The geological
unit at the station coordinate is provided by Chris Wills using the recently refined CGS
site-conditions map (Wills et al., 2005). For stations without geological unit and NGA
Vs30, we used the site category given in the USGS data file (column ‘C’) to estimate Vs30
using Table ? in Wills et al. (2001?):
C=1, Vs30 = 674 (m/sec)
C=2, Vs30 = 423 (m/sec)
C=3, Vs30 = 281 (m/sec)
C=4, Vs30 = 165 (m/sec).
Selection of Data Subset
When earthquake epicenter is outside or near the boundary of the strong-motion
station network the majority of the recording stations often lie in a limited distance range.
Such earthquakes are removed because they are not very useful to the characterization of
distance attenuation. The removed earthquakes are plotted as blue circles in Figure 1.
Furthermore, the quality of PGA data degrades as distance increases. Because of
the large number of records a full quality control is not feasible. Instead, we removed
PGA from beyond the cutoff distance of 130 * (M-2) (km), slightly more generous than
the cutoff distance of 100 * (M-2) (km) recommended by Boatwright et al. (2003) for
northern California earthquakes. We also removed data that are obviously in errors. The
final subset includes 102 earthquakes, ranging in magnitude from 3.39 to 5.17, and 9060
PGA data, with values ranging from 0.0007 to 24.5 (%g).