Why are there so many earthquakes and faults in the Western United States?
This region of the United States has been tectonically active since the supercontinent Pangea broke up roughly 200 million years ago, and in large part because it is close to the western boundary of the North American plate. Since the formation of the San Andreas Fault system 25-30 million years ago, the juxtaposition of the Pacific and North American plates has formed many faults in California that accommodate lateral motion between the plates. North and east of California, the Basin and Range province between the Wasatch Mountains in Utah and the Sierra Nevada Mountains in eastern California is actively spreading and stretching westward.
In New Mexico and west Texas, similar spreading has opened a north-south rift that starts in central Colorado and extends into northern Mexico. The geologic conditions and plate tectonic setting in much of the Western U.S. has resulted in the region being underlain by relatively thin crust and having high heat flow, both of which can favor relatively high deformation rates and active faulting.
In contrast, in the Central and Eastern U.S. (CEUS) the crust is thicker, colder, older, and more stable. Furthermore, the CEUS is thousands of miles from active plate boundaries, so the rates of deformation are low in this region. Nevertheless, the CEUS has had some rather large earthquakes in historical times, including a series of major earthquakes near New Madrid, Missouri in 1811-1812, a large earthquake near Charleston, S.C. in 1886, and the Cape Ann earthquake northeast of Boston in 1755.
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East vs West Coast Earthquakes
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New Audiences, New Products for the National Seismic Hazard Maps
New Audiences, New Products for the National Seismic Hazard Maps
The 1964 Great Alaska Earthquake and Tsunami
On March 27th, 1964, the second largest instrumentally recorded earthquake worldwide rocked southern Alaska for 4 to 5 minutes. In addition to the earthquake, the event triggered a major tsunami that caused casualties and damage from the Kodiak Islands to northern California.
A Virtual Tour of the Hayward Fault
The U.S. Geological Survey has a new website that offers a virtual tour of the Hayward fault.
Denali Fault: Gillette Pass
View northward of mountain near Gillette Pass showing sackung features. Here the mountaintop moved downward like a keystone, producing an uphill-facing scarp. The main Denali fault trace is on the far side of the mountain and a small splay fault is out of view below the photo.
Denali Fault
At pass west of Delta River. Here there was roughly 5 m of offset. Note the push up in the background. There is permafrost at the bottom of the cracks.
San-Andreas Fault
Aerial photo of the San Andreas Fault in the Carrizo Plain. By Ikluft - Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=3106006
Map of known active geologic faults in the San Francisco Bay region
Map of known active geologic faults in the San Francisco Bay region, California, including the Hayward Fault. The 72 percent probability of a magnitude (M) 6.7 or greater earthquake in the region includes well-known major plate-boundary faults, lesser-known faults, and unknown faults. The percentage shown within each colored circle is the probability that a M 6.7 or
...Interactive Quarternary Fault Database
This database contains information on faults and associated folds in the United States that demonstrate geological evidence of coseismic surface deformation in large earthquakes during the Quaternary (the past 1.6 million years).
Traces of the Hayward Fault, California
The purpose of this map is to show the location of and evidence for recent movement on active fault traces within the Hayward Fault Zone, California. The mapped traces represent the integration of the following three different types of data: (1) geomorphic expression, (2) creep (aseismic fault slip),and (3) trench exposures.