Hi Jane,

I can give a little different take on the subject pending Chris's response. His explanation on this thread so far is excellent, but there is a fundamental explanation of magnitudes of large SAF quakes in California that assists in understanding. The magnitude of an earthquake is rather simply due to two factors, and one of them doesn't change from place to place significantly in California. The two factors are: 1) the total area of slippage - i.e the length times the width (with a strike-slip fault, "width" means depth - not how deep the epicenter is, but the vertical or near-vertical dimension of the rupture plane). 2) modulus of rigidity, which can be simply considered the strength of the rock. As either of those factors increase, so does the magnitude.

When considering very large quakes on the SAF, we are rather fortunate. First, #2 can be ignored, because it is pretty much the same throughout the length of the SAF. So, the magnitude simply depends on the area of rupture. Here, we are fortunate again. The SAF, like most strike-slip faults, is relatively, and consistently, short in its vertical dimension. For ease of computation, we'll call it 10 miles depth-wise. So now, things are even simpler. Since a quake of the size we are discussing will, of necessity, rupture the full vertical dimension of the fault, or very nearly so, the sole factor determining magnitude is horizontal length of rupture. And, again, to keep things simple, a very approximate formula is that 100 miles of rupture will produce an M7.5. This, I think, agrees rather closely with Chris's M8.0 resulting from 300 km (185 miles) of rupture. Chris will probably be able to flesh out the explanation regarding New Madrid (which I am not fully up to speed on), but my guess is that two things make the simple distance/magnitude relationship of the SAF inapplicable to New Madrid. First, the rock there, being much less fractured generally in the region, is stronger and thus a greater magnitude will result from the same amount of rupture area, and, second, the fault(s?) involved may well have a vertical dimension significantly greater than that of the SAF. Note, also, that faults that dip at some angle from the vertical will have a larger "vertical" dimension before they reach non-seismogenic depths. In other words, they are then on a diagonal. This is part of the explanation of why the world's largest quakes occur at subduction zones. Our discussion of the SAF, above, is even further simplified by the fact that the SAF is nearly vertical throughout its length, something not necessarily the case with New Madrid.

Michael Williams
Arroyo Grande, CA USA