Ca Geology Chp 14 San Andreas Fault

Front 1

__ marks the boundary between the Pacific & North American Plates. 1,350 km. Probably world's most famous fault. Has relative __ lateral motion between them & most of the faults. __ km in north & ___ km in south.

Back 1

San Andreas fault system;

right-lateral motion;

100 km;

300 km;

Front 2

Along its entire length, movement takes place along one or more active faults. In some areas, movement is confined to a single, narrow fault zone, whereas in others, multiple, complex __ mark the plate boundary.

Back 2

fault strands

Front 3

There are sections of the San Andreas fault where faulting is confined to one or more linear zones of right-lateral movement are seen where the fault is parallel to the plate boundary. These relatively simple ___ include where? Areas not parallel are complex. Where are these at? The left-stepping bends result in ___ while the right-stepping bends has ___ which creates areas called ___ where crust stretches, forming a basin where young sediments can accumulate.

Back 3

fault segements;

Parallel = North section & areas between San Juan Bautista and the Big Bend.

Complex = Big Bend & smaller bend near San Jose.

Transpression; Transtension;

Pull-apart basins;

Hint 3

Front 4

The contrasts in the appearance of the San Andreas system along its length correspond with contrasts in ___ behavior.

Back 4

earthquake

Hint 4

Since 1769, 117 earthquakes with 6.0 magnitude.

Front 5

___ Ranch in Marin County was offset by __ meters in 1906 earthquake. Off set rate since then is __ cm per year. Of course these do not move at an average pace. Offset normally zero, punctuated by meters of movement within seconds.

Back 5

Skinner Ranch; 4.7 meters;

4.8 cm;

Front 6

An understanding of the long-term rate of offset is important for two reasons.

Back 6

1. It's a clue to the history of motion along the plate boundary.

2. If long-term offset rate is known, segments of a fault that lag behind the long-term rate can be identified as overdue for a major earthquake.

Front 7

Geologists have used offset stream deposits to conclude that offset or slip rate has been between 2.1 & 2.7 cm per year during the past 1,800 years. If all of this displacement occurred during 1906-type earthquakes, __ earthquakes would be needed to create the total displacement of the stream deposits. We would expect a great earthquake every 190-260 years along north San Andreas.

Back 7

8-9 earthquakes;

Front 8

The relative motion between the Pacific & North American Plates has been calculated at __ cm. per year during the past 3-4 million years.

Back 8

4.9 cm

Hint 8

Believe that "missing" right lateral plate motion is occurring on faults east of the San Andreas system.

Front 9

__ geologic units are offset more than recent deposits & landforms because they have been around subjected to longer period of displacement. Able to determine rate, recent breaks, new strands, deactivation, & max amount of displacement on San Andreas fault.

Back 9

Older;

Front 10

Pinnacles National Monument high silica volcanic flows & Neenach Volcanic Formation match. All units older that these fields show same amount of offset, __ km. San Andreas fault began its history of right lateral faulting __ mya. Long-term average slip rate of __ cm per year.

Back 10

315 km;

23.5 million years ago;

1.34 cm;

Hint 10

Some think right lateral motion began on the ancestral faults of the modern San Andreas about 28 million years ago.

Front 11

__ & __ schists are distinctive older rocks that have been used to calibrate the offset across the southern San Andreas fault system.

Back 11

Pelona & Orocopia schists;

Front 12

From an airplane, San Andreas fault appears on the ground as a sharp linear scar that cuts across all features of the landscape. Some areas have elongated depressions or ___ line up along the fault.

Back 12

sag ponds

Front 13

North CA coast & Mendocino Triple Junction has lots of faults and earthquakes because it between 3 plates. What are they? Most of it is offshore, but some on land. Low pop in Humboldt County so not much damage. But could be big 8.5 or 9 to rupture entire Cascadia subduction zone from Cape Mendocino to British Columbia. Last ocurred 300 years ago so could happen. Most earthquakes from __ rather than lateral motion in south.

Back 13

Pacific, North American, & Gorda subducting under North along Cascadia subduction zone.

compression;

Front 14

April 25-26th 1992 3 earthquakes 6-7.0 in Humboldt County near Petrolia. $48 mil damage 356 injured. First historic earthquake produced by thrust faulting along the Cascadia subduction zone. 1 meter tsunami generated. 8 months earlier were 4 earthquakes same. __ uplift from earthquake of 3/5 feet.

Back 14

coseismic uplift

Front 15

On surface of Cascadia subduction zone is an area of compression marked by numerous thrust & reverse faults & folds known as a ___. Humboldt County has Freshwater syncline near mouth of Mad River. Has salt mash had subsidence because of earthquake. marsh plants & trees drowned & mud accumulated. Similar aged ones in Oregon, etc. Think that 9.0 ruptured entire zone.

Back 15

fold-and-thrust belt.

Front 16

From the Mendocino Triple Junction south to Point Arena, the San Andreas fault lies __. __ Bay is one of the San Andrea's fault's most striking landforms, separating the Point Reyes Peninsula from the "mainland" of Marin County along a narrow slash that extends south of Tomales Bay it to Bolinas Bay. From Bolinas Bay the fault lies __ until intersects the San Francisco Peninsula & remains on land for the rest of its length.

Back 16

offshore;

Tomales;

offshore;

Front 17

From Point Arena south to San Juan Bautista & Hollister, the faults of the San Andreas system cross through the SF bay area. In the same segment, movement on the San Andreas system is shared among 4 major right-lateral fault zones. Several reverse faults produced by compressional forces active. Parallel to SAF. Believe that the complexity of the SAF in the Bay area is at least partly caused by the ___ in the orientation of the fault in the Santa Cruz Mountains.

Back 17

slight bend;

Front 18

1989 ___ earthquake San Francisco Giants & Oakland Athletics. 5 p.m. Named for a peak near its epicenter. Most damaging earthquake since 1906. 62 dead, 3,757 injured, 12,000 homeless. $6-10 billion damage. 6.9 moment magnitude. Didn't rupture the ground surface along the fault, though cracks. Ironically some worse areas because of 1906 earthquake. Ex. SF had exhibition in 1913 filling lagoon with artificial fill that was rubble cleaned up after 1906 earthquake. Failed and liquefaction brought up old debris.

Back 18

Loma Prieta Earthquake

Front 19

1906 __ earthquake 5 a.m. 7.8 magnitude. Fires. 2,000 died there, 189 others. One of the world's most famous disasters , partly because of the lessons learned by geologists, seismologists, & engineers who studied its effects. One important finding was correlation between geologic materials & intensity of ground shaking in different parts. Measured offset along rupture zone found that entire 430 km of San Andreas fault had ruptured at once.

Back 19

San Francisco earthquake

Front 20

Between San Juan Bautista & Parkfield, San Andreas system acts different. This central segment experiences numerous small earthquakes usually less than 4.0. Believe that strain energy is being released in continuous, small amounts, so that the likelihood of a great earthquake on these seismically active segments is __. Calaveras fault & Hayward fault earthquake are proof that moderate earthquakes can happen though.

Back 20

small;

Front 21

Relatively constant, slow displacement causes ___ along the central segment of the San Andreas. Displaces sidewalks, curbs, etc. 1 cm per year. Following Loma Prieta earthquake moved a lot.

Back 21

fault creep;

Front 22

At south end of creeping segment near Parkfield, a 30 km San Andreas fault had 6.0 earthquakes at __ year average intervals between 1881 & 1966. This regular pattern led seismologists to predict earthquake for Parkfield in the late 1980s, but the event has not occurred as of 2003.

Back 22

22

Front 23

Unpopulated grasslands of the Carrizo Plain good to see effects of San Andreas on landscape. Straight, relatively simple fault. 1857 __ earthquake huge. Between Parkfield and Big Bend. Hit military post. 1-3 mins shook. Larger than 1906 earthquake. 8.0. This segment of the San Andreas has remained lock since then.

Back 23

Fort Tejon earthquake

Front 24

The Big Bend San Andreas segment starts after the Garlock fault. Complexity of the fault maximized here. Extreme ___ here so big mountain ranges of San Gabriel, San Bernardino, & San Jacinto 3,000 meters along fault. Have fans beneath steep slopes. Many range-front faults are reverse faults.

Back 24

compression

Front 25

The San Andreas fault itself defines the __ end of the system, and it is presently the most active fault strand.

Back 25

eastern

Front 26

The San Gabriel fault can be traced through the eastern Transverse Ranges & through the western San Gabriel Mountains near Pasadena. The Mint Canyon Formation 12-13 millions year old sedimentary unit contains cobbles of unusual volcanic rocks. Well to the southeast in the Chocolate Mountains, the source for these volcanic clasts can be found.

Back 26

ok

Front 27

The San Gabriel fault is __ today. Geologists believe the San Andreas fault to the east is presently accommodating the slip formerly taken up by the San Gabriel fault.

Back 27

inactive;

Front 28

___ northeast of LA has a marsh that has been cut by a strand of the San Andreas fault. Deposits here record amazing evidence of past earthquakes on the San Andreas fault in south CA. 12 earthquakes have broken the sediments here in past 1,700 years giving geologists the best info about earthquake history along the San Andreas. Clusters of earthquakes 2-3 within 100-200 are followed by 200-300 years without large earthquakes. Average recurrence rate is 132 years.

Back 28

Pallet Creek;

Front 29

Long Beach, San Fernando, Northridge earthquakes not part of this section of SAF system, but indirect result of the same transform motion & damaged LA area. Since 1857, the segment of SAF along the Big Bend north of Cajon Pass has been seismically ___. South is the opposite and passing San Gorgonio Pass at south end of Big Bend shows southern CA's __ level of background seismicity.

Back 29

quiet; highest;

Front 30

___ earthquake 1987 2 within 4 days. Near LA Civic Center. 1st damaging earthquakes in LA since San Fernando 1971. In a blind thrust fault.

Back 30

Whittier Narrows earthquake

Front 31

___, a long desert valley that contains both the Imperial Valley & Salton Sea, is the most striking feature of the southern SAF system. Borders SAF in NE & San Jacinto fault in SW.

Back 31

Salton Trough

Front 32

The transform plate boundary in the region of the Salton Trough shows some features characteristic of the ___ plate boundary in Baja CA. It is a pull-apart basin formed in 4 mya as Peninsular Ranges pulled away from North America. Spreading hasn't formed midocean ridge yet. Salton Buttes domes show evidence of basalt from it though. High heat flow in Salton Trough proves that rifiting is active in the area. High geothermal gradient.

Back 32

divergent;

Front 33

Continued right-lateral motion on the faults bordering the Salton Trough causes the basin to __ in length. At the same time, vertical motion on the Brawley fault, N. Imperial fault, is causing the basin to ___. On flanks of Salton Trough are low mountains one with Imperial Formation, sequence of sandstone, shale, limestone. Marine microfossils indicate that an ocean embayment existed in the region.

Back 33

increase; deepen;

Front 34

Right lateral motion for 3/4 major faults in area during century. SAF proper aseismic in historic time. __ fault doing most of the work. 1940 6.9 __ earthquake orange trees 1st event to be recorded by strong-motion sensors located near the fault. 1979 6.4. At least 10 earthquakes 6.0-6.6 along San ___ system since 1890, more moderate events than on any other southern CA fault zone.

Back 34

Imperial fault;

Imperial earthquake;

San Jacinto;

Front 35

NE edge of Salton Trough no earthquakes in historic time. Only 4 since 1000 a.d. with last being 300 years ago.

Back 35

OK

Front 36

__ fault trends east-northeast from Tejonpass forming northern border for Mojave Desert. Unusual because left-lateral motion. No significant earthquakes in historic time. 7 mm slip rate & 200-3,000 years between earthquakes. 20 mya extension.

Back 36

Garlock fault

Front 37

A zone of active faults ___ runs parallel to SAF through the central Mojave desert. Many young, normal faults, some right-lateral. Could be boundary between Pacific & North American Plates.

Back 37

Eastern California Shear Zone;

Front 38

Two earthquakes on separate faults shook south CA desert on June 28th, 1992. ___ earthquake ruptured parts of Eastern CA Shear Zone 5 a.m. 7.5. Desert so killed only 1. Thought to have triggered __ earthquake 3 hours later which took place on a left lateral fault & had no surface rupture. Concerned that these earthquakes have brought the nearby segment of the San Andreas fault closer to failure.

Back 38

Landers earthquake & Big Bear earthquake;

Front 39

Most recent significant earthquake ___ Oct. 16th, 1999 north of Joshua Tree. Not much damage because little population. Proof that not all transform motion comes from SAF & confirm existence of Eastern CA Shear Zone.

Back 39

Hector Mine earthquake;