- Ground-motion model for shallow crustal earthquakes is:
where A is in cm∕s2, c1 = 2.8096, c2 = 0.8993, c3 = 0.4381, c4 = 1.0954, c5 = 0.0079 and σ = 0.60.
Ground-motion model for subduction earthquakes is:
where A is in cm∕s2, c′1 = 4.7141, c′2 = 0.8468, c′3 = 0.17451, c′4 = 1.2972 and σ = 0.56.
- Note that there is limited site information available for strong-motion stations in Taiwan so do not consider
local site effects.
- Use strong-motion data from Central Weather Bureau from 1994 to 1998 because it is more numerous and
of better quality than older data.
- Separate earthquakes into shallow crustal and subduction earthquakes because of different seismic
attenuation and seismogenic situation for the two types of earthquake.
- Shallow crustal earthquakes are mostly due to continental deformation, shallow collision or back-arc
opening or are the uppermost interface earthquakes. Focal depths depth between 1.1 and 43.7km with
most shallower than 20km. Most records from earthquakes with 4.5 ≤ Mw ≤ 6.0.
- Subduction earthquakes are located in the Wadati-Benioff zone or the deep lateral collision zone and are
principally intraslab. Focal depth between 39.9 and 146.4km.
- Do not use records from earthquakes associated with coseismic rupture because they have complex
near-field source effects.
- To avoid irregularly large amplitudes at great distances reject distant data predicted to be less than trigger
level plus 1 standard deviation using this threshold formula: aMw-blnD+c ≥ lnV , where V is geometric
mean of PGA equal to threshold plus 1 standard deviation. For shallow crustal earthquakes: a = 0.64,
b = 0.83, c = 2.56 and V = 6.93 and for subduction earthquakes: a = 0.76, b = 1.07, c = 3.13 and
V = 6.79.
- For shallow crustal earthquakes examine effect of focal depth on seismic attenuation by finding geometric
attenuation rate using epicentral distance, De, for earthquakes with 5km depth intervals. Find that deeper
earthquakes have slower attenuation than shallow earthquakes. Therefore assume ground motion, A, is
product of fsource (source effects) and fgeometrical-spreading (geometrical spreading effects) where
fsource = C1 exp(c2M)∕Dp-c3 and fgeometrical-spreading = De-(c4-c5Dp) where Dp is focal depth.
- For subduction earthquakes examine effect of focal depth in the same way as done for shallow crustal
earthquakes but find no effect of focal depth on attenuation rate. Therefore use fgeometrical-spreading =
- Plot residuals of both equations against distance and find no trend.
- Note that it is important to separate subduction and shallow crustal earthquakes because of the different
role of focal depth and attenuation characteristics.
- Plot residual maps of ground motion for Taiwan and find significant features showing the important effect
of local structures on ground motion.
- Cite various published models for Taiwan.