Cathyrn,
It may have been felt more strongly in Los Angeles than in San Francisco for 2 reasons:
1: The fault may have ruptured from NW to SE-the mainshock is plotted to the NW of most of the
aftershocks. there is a directivity effect that sends more energy in the direction of fault propagation.

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Yes. I've seen this in other instances.

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2: Los Angeles basin is like a bowl of jello. Long-period waves from distant quakes are amplified in the basin.

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As in Mexico City. Much of the land bordering the SF Bay is built on fill, which is also jello and one of the reasons we had minimal damage in 1989 just seven miles from the epicenter, and all h*ell broke loose in the Marina District and the tiered freeways that collapsed.

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But, your comment that the San Andreas focuses its energy to the north is incorrect.

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Really? This is very interesting.

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The damage in 1989
was more to the north because:
1: That is where the cities are

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You should have seen Watsonville and Santa Cruz, both built in basins. Much damage up Hwy. 9 through Ben Lomand and Felton.

2: There are bad ground conditions (fill) in those cities.

Yup.

The 1989 quake ruptured in both directions, I think.

Really? I thought it went mostly north, with shock waves diving and bouncing off the mantle and sort of diving under the peninsula. Not much damage in Cupertino and Redwood City, but plenty here and points north.

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The next San Andreas quake may well direct its
energy the other way, or equally both ways.

This is fascinating. Thanks, Chris. I can understand how a slip/strike fault could rupture both ways, but wouldn't the movement of plate techtonics on the SAF, generally moving north, trump all that in the long run?

Cathryn