- Ground-motion model is: where a is in g, for horizontal PGA α = -1.09, β = 0.238, b = -0.00050, h = 6.0 and σ = 0.28 and for
vertical PGA α = -1.34, β = 0.230, b = 0, h = 6.0 and σ = 0.27. When use focal depth explicitly: for
horizontal PGA α = -0.87, β = 0.217, b = -0.00117 and σ = 0.26 and for vertical PGA α = -1.10,
β = 0.200, b = -0.00015 and σ = 0.26.
- Consider two site classifications (without regard to depths of deposits) but do not model:
- Select records which have: Ms ≥ 4.0 and standard deviation of Ms known and reliable estimates of
source-site distance and focal depth, h ≤ 25km, regardless of local soil conditions from free-field and bases
of small buildings. No reliable data or outliers excluded. Records from instruments at further distances
from the source than the closest non-triggered instrument were non-excluded because of non-homogeneous
and irregularly spaced networks and different and unknown trigger levels.
- Most data, about 70%, with distances less than 40km. Note strong bias towards smaller values of magnitude
- PGA read from analogue and digitised data, with different levels of processing. Differences due to different
processing usually below 5%, but some may be larger.
- Errors in distances for small shocks may be large.
- Prefer one-stage technique because second step of two-stage method would ignore records from
singly-recorded earthquakes which compose over half the events, also find more realistic, b, and h0 using
one-stage method. Do not use weighting because involves assumptions which are difficult to verify.
- Find inadmissable and positive b for vertical PGA so remove and repeat.
- Remove records from distances less than or equal to half their focal depth and also less than or equal to
their focal depth, find that h0 is governed by near-field data.
- Use focal depth explicitly, by replacing r = (d2+h02)1∕2 by r = (d2+h2)1∕2. Find lower standard deviation
and that it is very significant.
- Repeat analysis on subsets of records grouped by focal depth. Find no correlation between h0 and focal
depth of subset. Use h0 equal to mean focal depth in each subset and find similar results to when focal
depth used explicitly.
- Repeat analysis with geometric attenuation coefficient equal to -0.83, corresponding to the Airy phase,
as opposed to -1.0.
- Find small dependence of horizontal PGA on site classification, note due to level of information available.