I said a couple of weeks ago that I would post on folds and faults. This is sort of from my Annual SCEC report (Nov 2003) and Annual USGS report (end Nov 2003). There are a bunch of co-authors-see link. The Northridge 1994 and 1983 (?) Coalinga earthquake, and Whittier (1987?), and others were on faults that do not reach the surface-or, in some cases the faults can reach the surface, but the slip in that particular event does not-these faults are known as blind faults. Sometimes they are imaged on industry seismic reflection data (acoustic data), but usually they are inferred from the shape of the fold. Or, in our case, we see the upper part on USGS and industry data, which projects beneath the fold. In the Gulf of Mexico, and other areas with salt of overpressured shale, folds can form as diapirs-because the salt or overpressured shale is less dense than the compacted sediment around and above-so the low-density material rises. I believe that this process is rare in southern California, and probably does not contribute to the very large anticlinal folds.

So, if an anticline is actively folding (active legally means the last 10 or 11 ka), then there must be an active fault beneath-the anticline is absorbing slip from an underlying fault.

The Palos Verdes Hills are part of a broad anticlinal fold (means convex-up and not upside-down). It is made up of some smaller folds, so is called and anticlinorium. The active folding had been explained by Ward and Velensise as due to slip through a restraining bend on the Palos Verdes fault. But, they only explain the uplift with respect to sea level-they ignore the likelyhood that the offshore is subsiding, so that the increase in structural relief is (much) greater than the uplift. Even without this, the restraining bend provides an element of convergence-which need not be (fully) accommodated on the Palos Verdes fault. So, we think there is a blind fault under the anticlinorium. But, its a bit worse than that. OK, it is twice as bad as that-the anticlinorium contines WNW offshore for another 20 km. It is a difficult problem to determine whether or not that part of the fold is active-we have some evidence that it is, bur are working on this.

With the date we have, we only see the fault down to about 2 or 3 km in this area. If we just project that fault in 3D, it means the blind fault we are mapping is the upper part of the Compton-Los Alamitos fault of Shaw and Suppe (1996). This fault is above most of the earthquakes in Santa Monica Bay (esp. the M5 1979 and 1989 quakes), and thus can not explain them. Alternatively, the fault we mapped steepens downdip and can explain some of these quakes....Our new faults have been provided to the SCEC Community Fault model but have not been released by them yet. Chris

• NEHRP annual report-Santa Monica