Some friends of mine got it into their heads that they wanted to visit an ophiolite. As a reader of John McPhee's geology books, I thought "of course, what could be more distinctively Californian?" But as I researched and visited likely field sites and did some reading, the subject grew more and more complex. So let me share an outline of what my friends and I are contemplating in this new article on ophiolites. They aren't something you can pull over by the road and point to; they must be delineated and inferred.

California has lots of ophiolites. This photo is from the Trinity Ophiolite, but I think I'll take my friends to the Del Puerto Ophiolite.
_{Peridotite, Trinity Ophiolite — Geology Guide photo}

Yesterday I visited the Del Puerto Ophiolite, in preparation for a small field trip I'm planning later this month. Every time I go in the field I return amazed at the energy and insight of geologic mappers. Even though I followed a good field-trip guide, I found it difficult to discern everything on my own (fortunately I'll be seeing it again soon). The work that has been done to advance geology to its present state is immense, and highly skilled, and cumulative over centuries of effort. Things don't reveal themselves in one big "aha" insight.

That's why I can't stand the armchair cranks who blithely push theories like creationism, Earth expansion and abiotic oil, assuring their readers that the scientists have everything wrong. I would challenge any of them to approach a real outcrop.

Related:
Earth mysteries
On roadside "mystery spots"
Warren Carey, father of Earth expansion
Wild and bemusing Earth theories

The classroom treatment of aftershocks is fairly simple: An earthquake hits, and then a series of aftershocks occurs that is well summarized by three mathematical expressions called the Gutenberg-Richter relation, Bath's law and Omori's law. The first two relate the aftershocks to the mainshock in terms of number and size (G-R says their numbers rise by an order of magnitude with each unit fall in seismic magnitude, and Bath says the largest aftershock averages about 1.2 magnitude units smaller). Omori's law describes how aftershocks tail off to a steady background rate as the reciprocal of time (that is, 10 years later aftershocks occur at 1/10 the rate).

The great majority of earthquakes and their aftershocks are easy to deal with because they occur near plate boundaries. There the seismic background rate is high and aftershock sequences therefore are short, around 10 years (because the background noise swamps their long tails). But away from plate boundaries the background noise is very low, and also the physics within plates appears to favor extra-long aftershock sequences. In sum, Seth Stein and Mian Liu argue in today's Nature, in places like the New Madrid earthquake zone in the central United States, the seismicity we see today could be aftershocks from earthquakes hundreds of years ago. They wouldn't represent the buildup to the next big one.

If we don't recognize these as aftershocks, they will mess up our models for determining earthquake hazards. To places like Memphis, overstrengthening buildings based on a poor New Madrid zone model is wasted money. In more complex plate settings like China, Stein and Liu argue, even distinguishing these aftershocks would only remove one veil from the mysterious and subtle patterns of deadly quakes like the 2008 Sichuan event.

More:
About aftershocks
Earthquake basics
U.S. earthquake hazard map
China earthquake hazard map

After a slow drive last month through the eastern Klamath Mountains, with many stops to take pictures and examine rocks, I now consider myself well acquainted with peridotite. That can be hard to do, because peridotite is almost all deep in the mantle and lower oceanic crust. And most of the peridotite that does make it onto the continents is altered into serpentinite. Of the little that is left, California has the nation's largest share, and most of that is in the eastern Klamaths along the upper Trinity River north of Trinity Lake on state route 3 and Trinity County route 17, the Trinity Heritage National Scenic Byway. Most of the pictures in this new gallery come from there.
_{Peridotite hand specimen — Geology Guide photo}