Postcard Friendship Friday - California Geology

A simplified Geologic Map of California, from the California Geological Survey, established 1880. Oversized postcard, 5 by 7 inches.

Red areas are intrusive granitic rocks, mostly Mesozoic in age. They hold up the great mountain ranges of CA including the Sierra Nevada and the Yosemite area, the Peninsular Ranges and the San Gabriels north of L.A. In the theme of Christmas, the dark green rocks are older metamorphic rocks, and the swath of yellow is the Great Central Valley, covered with recent sedimentary rocks. The lighter green areas closer to the coast are part of the Franciscan Complex, rocks of rather high pressure metamorphism that went down a subduction zone, came back up, and got stuck here because of the San Andreas Fault.

Here is a state with a lot of geology, and taxpayers who don't mind paying for the Geological Survey. Er, except maybe now, as CA is nearly out of money. As they note: "The CGS staff is dedicated to the fulfillment of our stated mission of providing information and advice to protect life and property from natural hazards and to promote a better understanding of California's diverse geologic environment" BUT "All CGS offices will be closed the first 3 Fridays of every month due to the furlough program mandated by the Governor."

Gold was discovered in the Sierra Nevada Mountains, starting the Gold Rush of 1849 - 1850. California has many other natural resources.

California has the major plate tectonic boundary of the San Andreas fault, and other minor faults. And plenty of earthquakes. How many?

Note that there are 638 earthquakes on this map, and all happened in the last week (!). Most were very small, and detected only by seismometers. The USGS and the CA Geological Survey spend a lot of time and effort on earthquake preparedness. This includes tsunami warning systems.

It's hard to believe it's been 20 years since the 1989 Loma Prieta quake, which stopped game 3 of the World Series.

Other San Andreas links:

Carrizo Plain National Monument

Wallace Creek area guide

Hayward fault: Slow fault creep in Hollister

National Parks and seashores in CA:

Death Valley

Yosemite

Point Reyes

Muir Woods

All of which I want to visit, some for the second or third time.

Viridian

Geology Postcard: Earthquake hazards map

Published by the US Geological Survey.

From the back: "USGS Earthquake Hazard Map showing earthquake ground acceleration having a 10 percent probability of being exceeded in 50 years for a firm rock site condition. This map is based on seismicity and fault-slip rates and takes into account the frequency of occurrence of earthquakes of various magnitudes. Locally, hazard may be greater than that shown, because site geology may amplify ground motions. Map for other parameters and probabilities of exceedance may be found on the Internet."

OK, in everyday language:

Where earthquakes (EQ) have happened in the past, they are likely to happen again. We also know the location of the San Andreas fault and other faults. These areas are likely to have earthquake shaking in the future.

Big earthquakes are rare but the ground shakes a lot. Little earthquakes are more common but the shaking is much less.

Red areas can be interpreted as areas where EQ are likely to happen in the future, and the shaking will likely be pretty strong. This includes the San Andreas fault area in California, but also the area from Death Valley north along the state line. Also Yellowstone National Park. White areas are safest. In terms of statistics, Wisconsin and Minnesota have had the least number of earthquakes of all the states.

Surprising areas of past EQ activity: Charleston, SC. There was a devastating EQ there in the 1880's and nothing much since. Central USA: 3 huge EQ's in 1811 - 12 that knocked down most houses, scared the **** out of everyone, dropped large areas of the land and momentarily made the Mississippi River run backward. Click on that link if you don't believe me. Still, many tiny earthquakes happen in that area weekly.

See a map of current earthquake activity at http://earthquake.usgs.gov/. There are several largish earthquakes in the world EACH DAY.

viridian