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Sunday, April 04, 2010

quick dirty primer on LA Region likelihood of a major quake

We just had a 7.2 mag quake located in northeast corner of Baja pennisula 110 miles east of tijuana and 20-25 miles south of US-Mex border. Time was 3.40 pm PST, Sunday April 4th Easter day. Quake epicenter was 300+ miles from LA Region but was widely felt all over SCal as a long rolling, rocking 45-second motion which did little damage. The longevity of the quake, coupled with it being felt over a wide region of SCal and South Arizona, indicates a major massive plate movement, possibly 3 to 10 meter shifting/fracture of the earths crust but very deep down(6-8 miles)so that SCal was able to absorb the shook. The damage to Baja region of this 7.2 quake close to quake epicenter is unknown as of this time of writing.

The quake is in a very active sesmic region centered in the imperial valley fault zone whch straddles US/Mex border and runs along Imperial Valley south all way to north end of gulf of CA/Sea of Cortez, where it likey contines underneath that sea. This appears to be a spreading rift zone which sometimes occurs along plate boundaries, though we area told that the pacific plate and NA plates are sliding or bumping into each other, not spreading apart. My view is that plate boundaries are complex and characterized by sliding, undercutting, grinding, rifting, uplifting, underthrusting, ect. The NA/PACIFIC plate boundaries here in Scal are mostly characterized by the sliding of the PAC plate northward in relation to the likely westward movement of the NA plate at rates of aveage 6-10 CM per year.

However, in one section of the plate boundary(the north La County section of San Andreas fault), it appears that the PAC plate is undercutting/grinding north/Northeastward against the NA plate, causing numerous secondary offshoot faults in the LA Basin and creating the uplifted youthful masses of the SCal mountains. This also snags and locks this dog-leg bended section of the SA fault which in time wil snap and create one hell of a quake, time uncertain but probablity next 10-50 years.

What is more of immediate concern are the 100's of mid- sized faults(from 70 to 200 Kilometers Long) which crisscross LA Basin. Some of these such as newly discovered LA Puente Hills fault and the famous Newport/inglewood fault, are capable of an LA City-leveling 7.0 or greater mag, though all of these mid-sizes fault lie dormant most of time(like a volcano), snapping once every 100 to a 1000 years. They will accumulate stresses over time however, and a major snappage of one major SCal fault such as SA, san jacinto, elsinore or imperial valley fault, could unleash secondary aftershock faults along any of these minor LA Basin faults.

Before the big one breaks along the SA fault we will likely get a 6-7 mag fault rupture along an undiscovered fault running beneath the LA Basin. Or a break of a established Mid-sized fault within LA county/city such as the la puente hills, newport/imglewood, siera madre, or northridge/ sylmar/ santa susanna faults. The chances of a super-killer 7-8 mag fault rupture is extremely rare for these secondary faults but not 100% failsafe.






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