Geologic, Hydrologic, and Urban Hazards for Design in Desert Environments - Design with the Desert: Conservation and Sustainable Development - page 101

5.5 Earthquakes
Most of the arid and semiarid regions of the southwestern United States are in the Basin and
Range physiographic province (see Chapter 2), which formed as a result of extensional tec-
tonic activity beginning about 20-30 million years ago. 40 Crustal extension caused numer-
ous faults in this region, and many of them are still active and are the source of considerable
seismic hazard to urban areas. Although ground motion is the largest hazard, surface rup-
ture and movement can also cause considerable damage. The southern extension of the most
famous fault in western United States, the San Andreas, crosses the western Sonoran Desert,
extending from Palm Springs, California, through the Salton Sea and beneath the Gulf of
California. Other well-known faults that pose high seismic hazards include the Owens
Valley fault, which is parallel to and east of the Sierra Nevada Mountains in California; the
complex system of faults along the Wasatch Front in Utah, which extends southward via the
Hurricane fault into Arizona; and the central Nevada seismic zone. These faults are promi-
nently displayed in maps of recent seismic activity in the western United States.*
Numerous historical earthquakes have occurred in the desert Southwest. In Arizona, the
most famous of these was a magnitude 7.2 event that occurred on May 3, 1887, with an epi-
center just south of the U.S.-Mexico border in the San Bernardino Valley of Sonora. 41 This
earthquake essentially leveled Tucson, Arizona, then a town composed of mud brick build-
ings, killed 51 people in Sonora, and caused numerous hydrologic changes in the region. The
March 26, 1872, earthquake on the Owens Valley fault at Lone Pine, California, (magnitude
7.4) killed 27 people and leveled most of the houses in town. † Other large earthquakes in
the region include the February 24, 1892, event in the Imperial Valley (magnitude 7.9), the
December 21, 1932, earthquake near Cedar Mountain, Nevada (magnitude 7.2), and the March
12, 1934, earthquake near Kosmo, Utah (northwest of Salt Lake City), the largest in that state's
history (magnitude 6.6). The recent magnitude 7.2 earthquake centered south of Mexicali,
Baja California, ‡ caused substantial damages in that city and in Calexico, California. §
Earthquake prediction remains a goal of seismologists, and although promising
approaches have been identified in recent years, these scientists do not have a reliable
method for forecasting earthquake occurrence. Instead, seismologists rely on earthquake
hazard assessments developed using geophysical models that incorporate the best sci-
entific data and models to predict hazards. 42 This type of approach is used to predict the
probability of occurrence of ground motion that occurs during earthquakes and are useful
for design purposes. Seismic hazard maps ¶ depict earthquake probabilistic data at a vari-
ety of geographic scales of use to urban designers.**
Several metrics of ground motion are used to describe earthquake hazards. †† Peak horizon-
tal acceleration is ground motion expressed as percent of gravity; vertical acceleration is gen-
erally lower than horizontal acceleration and is not used. For moderate to large earthquakes,
the pattern of peak horizontal acceleration is complicated and extremely variable over short
distances because small-scale geologic differences significantly change accelerations. Peak
horizontal velocity reflects the pattern of the earthquake faulting geometry, with largest

* http://earthquake.usgs.gov/regional/states/seismicity/us_west_seismicity.php (accessed June 26, 2009).
† http://earthquake.usgs.gov/regional/states/events/1872_03_26.php (accessed June 27, 2009).
‡ http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/ci14607652.php (accessed June 23, 2010).
§ http://en.wikipedia.org/wiki/2010_Baja_California_earthquake (accessed June 23, 2010).
¶ http://pubs.usgs.gov/fs/1996/fs183-96/fs183-96.pdf (accessed June 26, 2009). 43
** http://earthquake.usgs.gov/research/hazmaps (accessed June 27, 2009).
††

http://earthquake.usgs.gov/eqcenter/shakemap/background.php (accessed June 27, 2009).