Overthrowing the model

Recently we mentioned how you don’t want to mistake a model’s assumption for a result. A new paper in Science by Inbal et al. makes some claims about deformation in the mantle that are interesting, but it is something totally outside their field of view that makes this of interest here.

Back in the 1980s, after the Coalinga earthquake of 1983 showed that fold could pose a seismic hazard as much as surface faults, some researchers tried to see what kinds of hazardous faults might be hiding at depth.  Tom Davis and Jay Namson, two consulting geologists, were particularly enthused and soon had a model for Southern California. When GG was a postdoc at Caltech, one of the authors came up to show us the model; it looked something like the version published in 1989:


SSW to NNE section across the Los Angeles Basin, Davis et al., JGR, 1989

It is hard to see (you can click here for a bigger version), but the area where the shaded horizon is deepest is under the Los Angeles Basin.  The red highlight is where the trend of the Newport-Inglewood fault passes through, and below that is a detachment fault extending all the way from the San Gabriel Mountains on the right to offshore Palos Verdes on the left. The orange section in particular is of interest here, as it suggests that the Newport Inglewood fault is cut at depth. When this was presented to us at Caltech, GG asked, why is that orange segment required? At the time, this was being presented as a seminal threat to Los Angeles.  The short answer really came to be: the means by which this model is constructed require it, but after some hemming and hawing there was the admission that you could have two detachments, one rooting to the right, one to the left.  Nevertheless, this is what was published.

How does a paper on faulting into the mantle come into this?

The new paper was focused on a part of that same Newport-Inglewood fault, which has very small earthquakes going to great depth.  By using a very dense network of seismometers across the area, the authors could (they argue) estimate the density of earthquakes across this fault.  What they found is this:


Earthquake density across the Newport-Inglewood fault (NIF), where redder/darker colors indicate more earthquakes per square kilometer.

In the section above drawn using fault-bend and fault-propagation fold geometries, this fault should be cut at a depth of 12-15 km.  But it appears that it plows on through that horizon and on into the mantle. While there are still a lot of questions about this technique and its interpretations, it certainly seems to show that the basin-spanning detachment inferred by Davis et al. in the 1989 paper does not exist. This is the kind of evidence that helps us overthrow the tyranny of the model’s assumptions.

To be clear, though, most of the faults that Davis and others infer have a far more solid basis in fact; the large folds at the north edge of the Los Angeles Basin are most probably seismogenic. We’ve seen one or maybe two significant southern California earthquakes on structures like this (Northridge and maybe Whittier Narrows). It is just those big flat faults going all the way across the very bottom of the model that pose a risk of being mere artifacts.


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