CSULA - Geology: Faulting and Earthquakes in Southern California

Faulting and Earthquakes in Southern California

Faulting and Earthquakes in Southern California | Buildings and Earthquakes | Conclusions | Links

Faulting and Earthquakes in Southern California

Faulting
- Normal Faults
  - Produced by tension or stretching of the crust
  - Sierra Nevada fault is the most important example in California
    - March 27, 1872 Owens valley earthquake
    - 8.5 M - California's greatest
    - 29 killed
  - Cities on normal faults - Bishop, Reno, Carson City
- Thrust faults
  - The southern boundary of the transverse range is a series of east trending thrust faults
    - Santa Monica fault system
    - Sierra Madre fault system
  - The 1994 Northridge earthquake was an example
    - These faults may be characterized by very high horizontal acceleration
  - Thrust faults may be buried beneath parts of the Los Angeles basin and not visible on the surface
  - Cities on or very near thrust faults - Pasadena, all foothill communities
- Strike slip faults
  - These faults dominate the geology of California
    - Active for 30 m.y.
    - 10's to 100's of miles of displacement
  - These are the most active faults in California
    - 6 out of 10 earthquakes occur on these faults
  - Right lateral faults are most common
    - They dominate the peninsular range and coast range
    - They trend NW-SE
    - Mostly active
  - San Andreas-San Jacinto faults are most important
    - Right lateral
    - 600 miles in length
    - 300+ miles of displacement
    - Fault makes a major bend through the transverse range
    - Studies by Caltech indicate 9 major San Andreas earthquakes since the 6th century
      - a) Past events ~ 575, 665, 860, 965, 1190, 1245, 1470, 1745, 1857
      - b) Average repeat interval of 160 years
    - 2 major historic earthquakes
      - a) 1857 Fort Tejon - 8 M
      - b) 1906 San Francisco - 8.3 M
  - Offshore geology
    - Important because of proximity to large population centers
    - Geology is mostly right-lateral strike-slip faults
    - Active
  - Cities on strike slip faults - San Bernardino, San Francisco
  - Map from http://www.scecdc.scec.org/clickmap.html showing "epicenters of historic earthquakes (as far back as 1812) of particular note plotted over the background topography. Shown, too, are major highways (in tan) and the surface traces of major faults (in greenish-blue)."
Historic Earthquakes
- The 1971 San Fernando Earthquake
  - Geology of the San Fernando earthquake
    - Time: February 9, 1971, 6:01 am PST
    - Magnitude: Mw 6.6; Intensity: XI
    - Depth: 8.4 km
    - Length of shaking: 60 seconds!
    - Type of faulting: thrust
    - Fault involved: San Fernando Fault Zone;
    - Minor offset reported on the eastern Santa Susana Fault Zone
    - Surface rupture in the Sylmar – San Fernando Valley area 19 km (12 miles) long
    - Maximum slip was up to 2 meters (6 feet)
  - Damage
    - Over $500 million in property damage
    - Several hospitals suffered severe damage
    - Newly constructed freeway overpasses also collapsed
    - 65 deaths
    - Most of the deaths occurred when the Veteran's Administration Hospital collapsed
    - Loss of life would have been greater had the EQ struck later in the day.
  - Results:
    - Building codes were strengthened
    - Alquist Priolo Special Studies Zone Act was passed in 1972.
      - The purpose of this act is to prohibit the location of most structures for human occupancy across the traces of active faults and to mitigate thereby the hazard of fault rupture.
  - Isoseismal map for the San Fernando, California, earthquake of February 9, 1971. Isoseismals are based on intensity estimates from data. (http://neic.usgs.gov/neis/eqlists/USA/1971_02_09_iso.html)

The April 18, 1906 San Francisco earthquake

Geology of the 1906 earthquake
- Magnitude of 8.3 is estimated
- Earthquake produced 50 seconds of extreme shaking
- The epicenter was 27 miles north of San Francisco
- But the damage extended far south and east
  - a) Santa Rosa on deep, soft alluvium had worse damage than San Francisco even though it is 19 miles from the San Andreas
- A surface rupture 270 miles long extended north and south of San Francisco
  - a) Surface offsets of 20 feet
  - b) Right-lateral displacement
The damage to man-made structures
- Unreinforced brick and combination brick-frame buildings collapsed
  - a) The damage was most severe in the areas of bay fill
  - b) Especially in the "south of market" area
- The great fire
  - a) Water mains were broken to complicate an already inadequate system
  - b) 500 blocks burned - 28,000 buildings
  - c) Fire burned for 3 days and 3 nights
The death count in San Francisco

The official 1906 city death count - only 260 people

A 1987 review by the San Francisco city archivist estimates 1500

Crushed	427
Fire deaths	199
Exposure	116
Heart attacks	80
Suicide	86
Other*	190
Total	1498
*Includes drowning, disease, gunshot, dynamite, dysentery, and amputation

The official San Francisco government policy of 1906 was to deny the importance of the earthquake

Excellent book on the 1906 earthquake - "The San Francisco earthquake" by Thomas Gordon

The Loma Prieta earthquake of October 17, 1989
- Geological setting of the October 17, 1989 earthquake
  - The San Andreas and Hayward faults
    - Right-lateral strike slip faults
      - a) Very active for millions of years
      - b) These faults both go through the bay area and create great seismic concerns
    - The part of the fault that ruptured on October 17th is a southern portion of the 280-mile long 1906 break
    - This area was said to have had a 30 to 60% probability of a m6-7 earthquake in the next 20 years
  - Some basics of the October 17th earthquake
    - Magnitude of 7.1
      - a) Equals 500,000 tons TNT
    - Epicenter was 10 miles north of Santa Cruz
    - Mercalli intensity depended greatly on the firmness of the earth
      - a) Maximum was IX in Oakland and the Marina district
      - b) Just as in 1906 the areas of fill did very poorly
    - Maximum horizontal acceleration was 0.45 - 0.55 g's near the epicenter
    - The San Andreas Fault broke along a 25-mile segment in the Santa Cruz Mountains
      - a) Focus 11 miles deep
      - b) Earth shifted in a right lateral direction
        
        (6.5 feet horizontally
        
        (4.5 feet vertical up on the southwest side
- Types of geological damage during an earthquake
  - Surface faulting
    - The actual fault rupture did not reach the surface
    - This is surprising for a 7.1m earthquake
  - Ground breakage other than faulting
    - Lurch cracks are abundant in an area about 3 miles wide near the epicenter
    - Complex causes
    - Regional stress
      - a) Weak, unconsolidated tertiary sedimentary rocks not being able to respond elastically to severe shaking
    - b) Cracks damaged pavement, sidewalks, buildings et
  - Landsliding
    - Many landslides occurred in the mountains south of S.
    - They are almost entirely on south facing slopes
    - Many are old landslides that were reactivated
    - Some roads were closed
  - Soil liquefaction
    - Liquefaction is caused by increased pore water pressure
    - This is an especially severe problem around the San Francisco bay
      - a) Large parts of the bay have either:
    - (Soft, water-saturated natural sediment deposited by rivers
    - (Soft sandy man-made fills partly from the 1906 earthquake
  - The marina of San Francisco
    - Located 50 miles from the epicenter and still had a IX intensity
    - A lagoon was filled with 1906 debris to form today's Marina district
      - a) Debris from the 1906 earthquake liquefied in 1989
    - Damage included fires, collapsed buildings, broken utilities
  - East bay area
    - Soil liquefaction damaged the Oakland port facility, runway at Oakland international airport, and possibly the Nimitz freeway
    - Comparison with 1906
    - a) Areas that liquefied in 1906 also did so in 1989
    - b) Water mains needed to fight fires that broke in 1906 did so in 1989
    - c) Maps of damage and intensity for 1906 are similar to 1989
- Human damage
  - Injuries
    - 62 deaths
    - 3,757 injuries
    - 12,000 homeless
    - $6 billion in property damage
  - Buildings
    - No building constructed to code collapsed
      - a) But many were severely damaged
      - b) Codes aim to reduce damage & save lives, not eliminate damage
    - Buildings on solid ground in the epicentral area did well if they were built to code
    - Buildings 50 miles away in San Francisco on weak fill did poorly
    - Older buildings sometimes did poorly because
      - a) General deterioration
      - b) Not attached to the foundation
      - c) Unreinforced masonry, brick or stone
      - d) Lack of lateral reinforcement
      - e) Pounding by adjacent buildings
  - Bridges
    - The Nimitz freeway (rte 880) - triple-decker built between 1949-1954
      - a) Southbound lanes fell on the Oakland bound lanes killing 41
      - b) Damage has been attributed to inadequate bracing and poor soil
      - c) Freeway was pre-1971 and funds for retrofitting for earthquake safety had been withdrawn
    - One span of the Oakland bay bridge fell
      - a) Cause uncertain
- Predicted and actual events of October 17, 1989
  - Predicted are based on a model 6.5 to 7.5m earthquake on the Hayward fault
  - The following is a comparison of events predicted by the U.S. geological survey to occur based on a study of the Hayward fault compared to what actually occurred on October 17, 1989.

PREDICTED	ACTUAL
1500 - 4500 deaths	55 deaths
Modern schools are ok	True
Older schools damaged	True
Golden gate bridge open	True
Oakland bay bridge closed	True (opened in 1937)
Major freeway damage	True
SFO closed	True (first ever closure)
Bart closed	True briefly
Electricity off
100% area 24 hrs	True
50% area 48 hrs	True
Water supply severely hurt	True
Communications overloaded	True

The 1994 Northridge Earthquake
- Geology of the Northridge earthquake
  - Time: January 17, 1994, 4:30:55 Am PST
  - Location: 20 Miles West-northwest of Los Angeles, 1 Mile South-southwest of
  - Northridge
  - Magnitude: Mw 6.7
  - Type Of Faulting: Blind Thrust
  - Fault involved: Northridge Thrust
  - Depth: 18.4 Km
  - Duration of shaking: 10-20 seconds
  - Significantly deformed the Earth’s crust over an area of about 4,000 square kilometers
    - Santa Susana Mountains were pushed up by at least 40 centimeters
    - Northridge uplifted at least 20 cm
    - Other parts of the Valley 20-40 cm

Geological damage
- Ground failures of many types at distances up to about 90 kilometers from the epicenter
- Include:
  - Surface Ruptures
  - Landsliding
  - Soil Liquefaction
- Surface Ruptures
  - Most in the Granada Hills-Mission Hills area
  - Was the principal cause of damage to single-family homes and buried utilities
  - Loose or poorly compacted fills primary cause
  - Areas reengineered after 1971 Sylmar EQ performed very well
- Landsliding
  - Thousands triggered over 10,000 square kilometers
  - Mostly in sparsely populated areas
  - Destroyed homes, roads, and utility lines, and blocked streams
  - Created huge dust clouds – Valley Fever
Damage to Buildings:
- Severe damage to buildings, freeways and gas lines due to location
- Thousands of buildings were significantly damaged
  - +1,600 were later “red-tagged”
  - 7,300 buildings were “yellow-tagged”
  - Many thousands of other structures incurred at least minor damage.
- Estimated losses of 20 billion dollars
Human Death and Injury
- Fifty-seven people died
- +9,000 were injured
- +20,000 were displaced from their homes by the effects of the quake
EQ had an immense impact because it was centered directly beneath a heavily populated and built-up urban region.
The early morning timing of the earthquake spared many lives that otherwise might have been lost in collapsed parking buildings and on failed freeway structures.

Buildings and Earthquakes

Types of home construction
- Wood frame buildings
  - By far the most desirable small property investment
    - Light weight - little inertia
    - Flexible
- Why wood frame houses are damaged in earthquakes
  - Soft, unstable ground
  - A weak or inadequately located foundation
  - Old, poorly maintained or a new poorly constructed building
  - Insufficient lateral bracing or inadequate number of bearing walls and columns
  - Inadequate stilts for hillside homes
  - Heavy roofing such as clay tiles
- Lateral bracing adds great strength
  - Generally required in western U. S.
  - 1" x 4" diagonal across studs from top to sole plate
    - a) Generally adequate in following cases
      - (No 6+ earthquakes
      - (Good foundation material
      - (Single story building
      - (No stucco or masonry veneer
  - Shear-wall bracing 3/8" plywood covering over studs
    - a) Recommended for strong tremor areas
    - b) Metal straps for diagonals are a poor substitute
  - Steel framing and anchoring devices strengthen connections of different components
  - a) Slight cost increase
- Wood frame houses with stucco veneers
  - Possible construction methods
    - Stucco is applied to wire mesh that is carefully nailed to plywood sheathing
    - Seldom damaged in large earthquakes
    - Buildings with stucco applied to sheet rock or plasterboard backing or 2 story buildings without plywood shearwalls suffer badly
  - Remedies
    - Carry earthquake insurance to cover the stucco
    - Strengthen the connection between foundation, sill, and studs - much damage occurs here
    - Remove stucco and install plywood sheathing
- Wood frame with masonry veneer
  - Considerably more dangerous
  - Extra weight means extra inertia
  - Poor connection between veneer and building
  - Poor quality mortar
- Unreinforced brick buildings
  - Consistently suffer severe damage
  - Brick is heavy and inflexible & can't withstand lateral force
  - Only a wood frame interior can prevent total collapse
  - Quality of the mortar is particularly important
- Architectural details
  - Foundations
    - Continuous tie wall foundation
      - Reinforced concrete forms a base beneath each bearing wall
      - Allows building to move as a unit
    - Reinforced concrete slab
      - Behave well on inferior ground due to their rigidity
      - Should be reinforced in excess of code
  - Foundation connection to wood frame homes
    - Poor anchorages between the sill and foundation
      - Building slides off the foundation if it is not properly bolted
      - Clamps may be installed on old homes
    - Cripple studs - raise floor from the foundation
      - Often poorly braced - new homes destroyed in S.
      - Install plywood sheathing to prevent damage
    - Foundation damage existing
      - Rotten wood or termites are common
      - Wide cracks in foundation concrete
    - Houses on stilts or pilings
      - Requires exceptionally stable earth
      - Columns and floor joists of welded steel are best
      - Wood columns require a plywood sheath
  - Columns and walls
    - Wood columns may fail because:
      - Rotting due to poor drainage along the top of the sill
      - Termite damage
      - Column not continuous from sill to roof
      - Horizontal members improperly connected to columns
        
        a) Extremely common yet easily corrected
    - Masonry columns
      - Avoid masonry buildings or masonry columns on frame buildings
- Roofs
  - Generally hold up well except:
    - Poor connections with vertical members
    - Too heavy a roof
      - Traditional clay tile is 1200 lbs/100 sq. ft or 9 tons for 1500 sq. ft
- Large windows and doors
  - Damage is often concentrated in these areas
    - Plywood sheathing and interior paneling will help
- Parapets
  - Present a great hazard
- Chimneys
  - Causes of damage
    - Pre-1960 chimneys are commonly not reinforced with vertical steel bars
      - Post 1960 chimneys did well in S. California
    - Pre-1960 chimneys are commonly not strapped to the building
      - Chimney and building bang against each other
    - Chimneys extending >3 feet above roof should be shortened
- Utilities
  - Plumbing
    - Minimize damage by knowing where the main shut valve is
  - Gas
    - Know where the main feeder valve is
  - Water heaters
    - They are very vulnerable and commonly cause fires
    - They should be bolted to the floor and strapped to the wall
- Furnishings
  - Use heavy weight closed hooks on hanging lamps and pictures
  - Free standing bookcases - bolt to the wall studs
  - Use heavy latches on cabinets for food, drink, and glassware
  - Bed - have it clear of heavy objects and glass windows

Conclusions

Similar events will occur repeatedly in southern California
- The 1994 Northridge earthquake is an example
  - Richter magnitude 6.7; approximately 60 killed
  - Severe lurching and cracking occurred
  - CSUN $300 million damage
    - Apartment buildings collapsed
    - Unreinforced buildings collapsed or heavily damaged
- A future San Andreas or Newport Inglewood fault earthquake may produce widespread soil liquefaction
  - The Newport Inglewood could produce a 7.2 m earthquake nearly centered on LA
  - Liquefaction along river channels and much of Orange County is expected
Geological conditions strongly influence damage
- Geology determines:
  - Where fault rupture will occur
  - How hard the ground will shake
  - Where soil liquefaction will occur
  - Where landslides occur
- We know:
  - A large earthquake is expected in s. California within our lifetime
  - Shaking levels and geologic response can be predicted
  - Preparation will greatly reduce the impact
    - "Those who cannot remember the past are condemned to repeat it" - George Santayana - philosopher & poet
    - Make an earthquake kit for your home, your car and your office

LINKS

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