Sierra Madre Fault Zone

Seismic fault in Southern California

The Sierra Madre Fault Zone highlighted in red

Situated at the boundary to the San Gabriel Valley and San Fernando Valley, the Sierra Madre Fault Zone (also known as the Sierra Madre-Cucamonga Fault) runs along the southern edge of the San Gabriel Mountains for a total of 95 kilometers (59 mi), where the northwesternmost 19 km (12 mi) comprises the San Fernando Fault (the section responsible for the 1971 San Fernando earthquake). A 1980s paleoseismic study that included a trench investigation and mapping revealed that a major earthquake had most likely not occurred to the east of the San Fernando rupture area for at least the last several thousand, and possibly the last 11,000 years.[1]

The 1971 event was the first in a series (1987 Whittier Narrows, 1991 Sierra Madre, 1994 Northridge) of damaging earthquakes which have occurred on reverse faults in the Los Angeles area. The events triggered discussions concerning the largest magnitude earthquake that could be generated by one of the faults, especially in the Transverse Ranges, but the focal point of earthquake hazard assessments in California are often the San Andreas Fault and other associated dextral faults. Although there is a lack of paleoseismic data on reverse faults in the Los Angeles area, a trench excavation at a site on the Sierra Madre-Cucamonga Fault revealed that two large historic earthquakes occurred in the last 15,000 years.[1]

The central Transverse Ranges

The fault was studied again in the late 1990s in the Loma Alta Park near Millard Canyon where a fault scarp larger than 2 m (6 ft 7 in) was accessible along a (late quaternary) elevated stream terrace. The clearly defined fault was exposed in the trench and emerged as a .5 m (1 ft 8 in) band of coarse gravels lining the hanging wall. By studying the truncated rocks and a wedge-shaped accumulation of gravel and soil, it was possible to visually reconstruct the original geometry of the rock prior to the thrust and eventual and partial collapse of the hanging wall back onto the footwall. An estimate for the maximum slip of the event was given as 3.8–4 meters (12–13 ft).[1]

The evidence found at the Loma Alta trench investigation site brought new information into the deliberation regarding the maximum size of earthquakes near Los Angeles. The large amount of slip observed there did not correspond with a short 15–20 km (9.3–12.4 mi) rupture length of the Sierra Madre Fault Zone, and instead suggested that the historical thrust earthquakes were much larger in magnitude than what was seen with the 1971 event, given its smaller 2 meters (6 ft 7 in) of maximum observed displacement. Two methods were employed to infer the scope of the events at the site (one regression-based and the other based on the seismic moment) and produced a maximum magnitude of 7.5 or 7.6 for the most recent movement of the fault. The results supported an earlier hypothesis that seismic energy release on the Sierra Madre Fault Zone is characterized by infrequent but large earthquakes. A duplicate event in modern times would rupture to the south towards populated areas and would produce strong ground motion capable of damaging modern buildings and other critical infrastructure.[1]

## References

[edit]

1. ^ a b c d Rubin, C. M.; Lindvall, S. C.; Rockwell, T. K. (1998). "Evidence for Large Earthquakes in Metropolitan Los Angeles". Science. 281 (5375). American Association for the Advancement of Science: 398–402. Bibcode:1998Sci...281..398R. doi:10.1126/science.281.5375.398. PMID 9665880.

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Seismically active fault zones and tectonic blocks of California

Statewide Faults|

* San Andreas Fault
* Salinian Block

Southern California|

* Brawley Seismic Zone
* Chino Fault
* Elsinore Fault Zone
* Elysian Park Fault
* Garlock Fault
* Hollywood fault
* Hosgri Fault
* Imperial Fault Zone
* Laguna Salada Fault
* Newport–Inglewood Fault
* Northridge Blind Thrust Fault
* Peninsular Ranges
* Puente Hills Fault
* Raymond Fault
* Rose Canyon Fault
* Salton Trough
* San Cayetano Fault
* San Diego Trough Fault Zone
* San Felipe Fault Zone
* San Gabriel Fault
* San Jacinto Fault Zone
* Santa Maria River Fault
* Santa Ynez Fault
* Shoreline Fault
* Sierra Madre Fault Zone
* Ventura Fault
* White Wolf Fault
* Whittier Fault
* Yorba Linda Fault

Northern California|

* Bartlett Springs Fault
* Calaveras Fault
* Clayton-Marsh Creek-Greenville Fault
* Concord Fault
* Hayward Fault Zone
* Healdsburg Fault
* Maacama Fault
* Mendocino Fracture Zone
* Mendocino Triple Junction
* Mount Diablo Thrust Fault
* Monta Vista Fault
* Nacimiento Fault
* Pleasanton Fault
* Rinconada Fault
* San Gregorio Fault
* San Pablo Fault
* Seal Cove Fault
* Serra Fault
* Silver Creek Fault
* Tesla Fault
* West Napa Fault

Sierra Nevada|

* Death Valley Fault Zone
* Furnace Creek Fault Zone
* Honey Lake Fault Zone
* Kern Canyon Fault
* Maria fold and thrust belt
* Owens Valley Fault
* Rescue Lineament-Bear Mountains fault zone
* Sierra Nevada Batholith
* Sierra Nevada Fault
* Sierra Nevada–Great Valley Block
* Smartville Block
* Walker Lane

* Category
* California Geological Survey
* Seismic Hazards Mapping Act
* Mexico (list)

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Major seismically active faults of North America

North America
(crosses national borders)|

* Aleutian Trench (US–RU)
* Basin and Range Province (US–MX)
* Canadian Arctic Rift System (CA–GL)
* Cascadia subduction zone (US–CA)
* Clarendon-Linden fault system (US–CA)
* Denali Fault (US–CA)
* Northern Cordilleran Volcanic Province (CA–US)
* Puget Sound faults (US–CA)
* Rio Grande rift (US–MX)
* San Andreas Fault (US–MX)
* Southern Great Lakes Seismic Zone (US–CA)
* Tintina Fault (US–CA)

Canada|

* Charlevoix seismic zone
* Laurentian Slope seismic zone
* Leech River Fault
* Queen Charlotte Fault
* Saguenay Graben
* Saint Lawrence rift system
* Timiskaming Graben
* Western Quebec seismic zone

United States|

| Washington and Oregon|

* Brothers Fault Zone
* Olympic–Wallowa lineament
* Puget Sound (Seattle Fault, Tacoma Fault)
* Walker Lane

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California|

* Calaveras Fault
* Garlock Fault
* Hayward Fault Zone
* Mendocino Fracture Zone
* Newport–Inglewood Fault
* Puente Hills Fault
* Rose Canyon Fault
* San Jacinto Fault Zone (Elsinore Fault Zone)

Great Basin|

* Independence Valley fault system
* Moab Fault
* Teton Fault
* Wasatch Fault

Great Plains|

* Humboldt Fault
* Long Point–Eureka Heights fault system
* Meers Fault
* Nemaha Fault Zone

Midwest|

* Marianna Fault
* New Madrid seismic zone
* Sandwich Fault Zone
* Wabash Valley seismic zone

Appalachian Mountains
and Atlantic Coast|

* Eastern Tennessee seismic zone
* Northern Appalachians seismic zone
* Ramapo Fault
* Virginia seismic zones

Caribbean and Mexico|

* Cayman Trough
* Chixoy-Polochic Fault
* Enriquillo–Plantain Garden fault zone (Cuba)
* Gulf of California Rift Zone
* Lesser Antilles subduction zone
* Middle America Trench (Central America)
* Motagua Fault (Central America)
* Pedro Miguel Fault
* Puerto Rico Trench

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