- Ground-motion model is (for shallow earthquakes):
where A is in cm∕s2, a1 = 1.23 (for hard sites), a1 = 1.41 (for soft sites), a2 = 0.385 and a3 = -0.00255 (σ is not
Ground-motion model is (for subduction zone earthquakes):
where A is in cm∕s2, b1 = 2.08 (for hard sites), b1 = 2.32 (for soft sites), b2 = 0.35 and b3 = -0.0025 (σ is
- Use four site categories:
- Stiff soil
- Deep cohesionless soil (≥ 16m)
- Soft to medium stiff clay (≥ 16m)
Categories 1 and 2 are combined into a hard (H) class and 3 and 4 are combined into a soft (S) class. This
boundary established using field evidence at a shear-wave velocity of 400m∕s and at an SPT N count of
- Use data from ground floors and basements of small or low structures (under 3 stories) because believe
that small structures have little effect on recorded ground motions.
- Separate earthquakes into shallow (h ≤ 19km) and subduction (h ≥ 20km) because noted that ground
motions have different characteristics.
- Use epicentral distance for Japanese data because practical means of representing deep subduction
earthquakes with distant and imprecise fault locations.
- Do not use rupture distance or distance to surface projection of rupture because believe unlikely that stress
drop and peak motions will occur with equal strength along the fault length and also because for most
records fault locations are not reliably determinable.
- Note that there is a paucity of data but believe that the few high peak values observed (e.g. Pacoima
Dam and Morgan Hill) cannot be dismissed without the possibility that interpretations will be affected
- For subduction equations, use records from Japanese SMAC instruments that have not been instrument
corrected, even though SMAC instruments show reduced sensitivity above 10Hz, because ground motions
> 10Hz are not significant in subduction earthquakes. Do not use records from SMAC instruments for
shallow earthquakes because high frequency motions may be significant.
- Examine differences between ground motions in extensional (strike-slip and normal faulting) and
compressional (reverse) regimes for shallow earthquakes but do not model. Find that the extensional ground
motions seem to be higher than compressional motions, which suggest is because rupture propagation
comes closer to ground surface in extensional faults than in compressional faults.
- Group records into 1M unit intervals and plot ground motions against distance. When data is numerous
enough the data points are encompassed in boxes (either one, two or three) that have a range equal to
the distribution of data. The positions of the calculated values within the boxes were used as guides for
shaping appropriate curves. Initially curves developed for M = 6.5 were there is most data and then these
were extended to smaller and larger magnitudes.