What's the difference in acceleration between area on firm rock and on alluvium?

Question

How does the peak ground acceleration differ when one site in on firm rock and another one on alluvium?

Answer 1

there is no difference in acceleration,
but the difference is in the traction or the grip on the ground.
in dry weather there is almost no difference(assuming that the ground is compacted or firm.)
the difference in wet weather depends on the composition of the soil and the level of water saturation in the soil.
even in firm rock this will also have an effect on your acceleration, maybe not directly ,depending on how you accelerate.
too fast an acceleration at the start may induce aquaplaning and your wheel to spin at the start, and the same thing may cause you to slow down to enable you to control your vehicle.

Answer 2

It can be big, but it is hard to measure. A ground-shaking map by Wong et al. (2002) for Salt Lake valley has as much as a 0.5g difference for bedrock and alluvium within 0.1 km of eachother.

Answer 3

Bedrock is of much higher density (mass per unit volume) than alluvium which has a higher porosity content - which has open air in large cobbels or boulders and/or water in the alluvium.