Ground motion complexity

Here’s a puzzle for the ground motion modelling community. We all know that the average strong ground motion model computes motion as a function of magnitude, distance, fault style (usually) and site conditions (usually). Imponderables that also affect it include rupture direction, slip distribution, and path effects (refraction, reflection and interference as waves are transmitted through a heterogeneous medium).

But who was the first to identify that path heterogeneity due, for instance, to faulting, would add complexity to the transmission of seismic waves? Or if you can’t supply a name, how about a decade?

1980s? 1960s? Astonishingly, the answer is (arguably) the 1760s, and the individual in question is that remarkable pioneer John Michell, who also has to his credit being the first person to posit the existence of black holes.

Here he is:

… there is another very remarkable appearance in the structure of the earth, though a very common one; and this is what is usually called by miners the trapping down of the strata; that is, the whole set of strata on one side a cleft are sunk below the level of the corresponding strata on the other side. If, in some cases, this difference in the level of the strata on the different sides of the cleft should be very considerable, it may have a great effect in producing some of the singularities of particular earthquakes.

While this could be more explicit, Michell knew that earthquake shaking consisted in the transmission of elastic waves, and I don’t think it is too unreasonable to interpret this statement (which is not amplified by the author) as implying complexity in transmission.

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