- Ground-motion model is:
where Y is in cm∕s2 for horizontal PGA (for whole Taiwan) a = -0.852, b = -0.0071, c = 1.027, d = 1.062,
h = 1.24km and σ = 0.719 and for vertical PGA (for whole Taiwan) a = -1.340, b = -0.0036, c = 1.101,
d = 1.697, h = 1.62km and σ = 0.687. Also report coefficients for equations derived for three different
- Do not differentiate site conditions.
- Focal depths, h, between 2.72 and 29.98km.
- Data from high-quality digital strong-motion networks of Taiwan Strong Motion Instrumentation Program
(TSMIP) and Central Mountain Strong Motion Array (CMSMA).
- Select data from earthquakes with h ≤ 30km and with records from ≥ 6 stations at de ≤ 20km.
- Select events following the 1999 Chi-Chi earthquake (Mw7.7) with ML > 6.
- Do not use data from the Chi-Chi earthquake because: a) earlier analysis of Chi-Chi data showed
short-period ground motion was significantly lower than expected and b) the Chi-Chi rupture triggered
two M6 events on other faults thereby contaminating the ground motions recorded at some stations.
- Data uniformly distributed for Mw ≤ 6.5 and 20 ≤ rhypo ≤ 100km. Significant number of records for
rhypo > 100km.
- Use data from the Chi-Chi earthquake and the 2003 Cheng-Kung earthquake (Mw6.8) for testing
applicability of developed equations.
- For 32 earthquakes (mainly with Mw < 5.3) convert ML to Mw using empirical equation developed for
- Develop regional equations for three regions: CHY in SW Taiwan (16 earthquakes, 1382 records), IWA in
NE Taiwan (14 earthquakes, 2105 records) and NTO in central Taiwan (13 earthquakes, 3671 records) and
for whole Taiwan to compare regional differences of source clustering in ground-motion characteristics.
- Use Mw since corresponds to well-defined physical properties of the source, also it can be related directly
to slip rate on faults and avoids saturation problems of other M-scales.
- Use relocated focal depths and epicentral locations.
- Do not use rjb or rrup because insufficient information on rupture geometries, particularly those of small
earthquakes, even though believe such distance metrics are justified. However, for small earthquakes do
not think using rhypo rather than rrup will introduce significant bias into the equations. Also use rhypo
because it is quickly determined after an earthquake hence early ground-motion maps can be produced.
- From equations derived for different sub-regions and from site residual contour maps that ground motions
in CHY are about four times higher than elsewhere due to thick, recent alluvial deposits.
- Find predictions for Chi-Chi and Cheng-Kung PGAs are close to observations.
- Plot contour maps of residuals for different sites and relate the results to local geology (alluvial plains and
valleys and high-density schist).
- Divide site residuals into three classes: > 0.2σ, -0.2–0.2σ and < -0.2σ for four NEHRP-like site classes.
Find the distribution of residuals is related to the site class particularly for the softest class. Find residuals
for C (very dense soil and soft rock) and D (stiff soil) are similar so suggest combining them. Believe
geomorphology may also play an important role in site classification because a geomorphologic unit is often
closely related to a geologic unit.