2.207 Bommer et al. (2003)

• Ground-motion model is:

where y is in g, C1 = -1.482, C2 = 0.264, C4 = -0.883, h = 2.473, CA = 0.117, CS = 0.101, CN = -0.088, CR = -0.021, σ1 = 0.243 (intra-event) and σ2 = 0.060 (inter-event).

• Use four site conditions but retain three (because only three records from very soft (L) soil which combine with soft (S) soil category):
R
Rock: V s > 750ms, SA = 0,SS = 0, 106 records.
A
Stiff soil: 360 < V s 750ms, SA = 1,SS = 0, 226 records.
S
Soft soil: 180 < V s 360ms, SA = 0,SS = 1, 81 records.
L
Very soft soil: V s 180ms, SA = 0,SS = 1, 3 records.
• Use same data as Ambraseys et al. (1996).
• Use three faulting mechanism categories:
S
Strike-slip: earthquakes with rake angles (λ) -30 λ 30 or λ 150 or λ ≤ -150, FN = 0,FR = 0, 47 records.
N
Normal: earthquakes with -150 < λ < -30, FN = 1,FR = 0, 146 records.
R
Reverse: earthquakes with 30 < λ < 150, FR = 1,FN = 0, 229 records.

Earthquakes classified as either strike-slip or reverse or strike-slip or normal depending on which plane is the main plane were included in the corresponding dip-slip category. Some records (137 records, 51 normal, 10 strike-slip and 76 reverse) from earthquakes with no published focal mechanism (80 earthquakes) were classified using the mechanism of the mainshock or regional stress characteristics.

• Try using criteria of Campbell (1997) and Sadigh et al. (1997) to classify earthquakes w.r.t. faulting mechanism. Also try classifying ambiguously classified earthquakes as strike-slip. Find large differences in the faulting mechanism coefficients with more stricter criteria for the rake angle of strike-slip earthquakes leading to higher CR coefficients.
• Note that distribution of records is reasonably uniform w.r.t. to mechanism although significantly fewer records from strike-slip earthquakes.
• Try to use two-stage maximum-likelihood method as employed by Ambraseys et al. (1996) but find numerical instabilities in regression.
• Also rederive mechanism-independent equation of Ambraseys et al. (1996) using one-stage maximum-likelihood method.