CALIFORNIA GEOLOGIC HAZARDS
VICKI DRAKE
EARTH SCIENCE DEPARTMENT
SANTA MONICA COLLEGE
I.
FLOODS
A. Northern California; along major rivers during extreme rain events
1. Sacramento River, Feather River, Yuba River, San Joaquin River,
Kings River, Merced, etc.
B. Southern California: deserts, local mountains and coastal plains during
extreme rain events
1. Los Angeles River (channelized system), Topanga Creek, Malibu
Creek, Ventura River, Santa Clara River, Santa Paula River
II.
TSUNAMI (Seismic Sea Waves – “Harbor Waves”)
A. Crescent City and north along coast
1. Earthquakes in Pacific Ocean basin, Gulf of Alaska, etc.
2. Historic: Good Friday Earthquake, (1964) Alaska
a. 16 deaths along coasts of California due to Tsunami
b. 5 Tsunami – with crests ranging from 14 feet to 33 feet
(last one)
B. Santa Barbara to Point Sur
III. EROSION AND LANDSLIDES
A. Coastal Ranges: northern California
1. Ranges are steep, folded, and ‘dipping’ west to ocean
2. Materials are sedimentary (mudstone, Claystone and siltstone with
little structure)
B. Santa Monica Mountains, San Gabriel Mountains: southern California
1. Transverse Ranges tectonically active
2. Material continually weathered and eroded out ready for
‘transport’ downslope
3. Steep-slopes
C. Coastal Regions of northern and southern California
1. Steep, folded, dipping west to ocean
2. Sedimentary materials easily loosened and transported downslope
during rainy season
3. Urbanization and wildfires remove native vegetation
IV.
VOLCANIC HAZARDS
A. Cascade Mountains: northeastern California
a. Eruptive style: explosive and destructive
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b. Composite or Stratovolcanoes; Plug Dome volcanoes; Cinder
Cone volcanoes
1. Mt. Shasta – Stratovolcano
a. Formed during four distinct eruptive periods (~600,000
years ago to present)
b. Building process: Explosion – cone-building; explosion –
cone-building, etc.
c. Elevation: 14,162 feet (4319 meters)
d. Shastina: a parasitic cone built on western flank of Shasta
i. 12,000 feet tall
ii. ~9700 years old, built over 300-year eruptive
period
e. Eruptive history: not in this century, last erupted in 1700s
f. Not an extinct volcano: 1988 swarms of small earthquakes
put USGS and CA Division of Mines and Geology on alert
i. Abrupt increases in seismic activity in inactive
volcanoes signals possible reactivation (Mt. St.
Helens, 1980)
2. Mt. Lassen – Plug Dome Volcano
a. Formed from remains of ancient Mt. Tehama
i. Mt. Tehama destroyed over 0.5 million years ago
b. Mt. Lassen formed ~11,000 years ago
c. Elevation: 10,457 feet
d. Eruptive history: last erupted 1914-1921
3. Medicine Lake Highlands – Rhyolite Cones and Basalt Fields
a. Northeast of Mt. Shasta – high elevation region (7,000
feet) of small volcanic cones (last 1500 years)
b. Remnant of large shield volcano whose peaks collapsed
(~10,00 years ago)
c. Most recent volcanic eruptions:
i. Two Volcanic Glass Mountains
1. ~1100 years old
2. Composed of Volcanic Glass: Pumice and
Obsidian
4. Long Valley Caldera – Inyo-Mono Craters – Silicic and Mafic
materials – Cinder Cone, Shield Volcanoes
a. Located eastern slope of Sierra Nevada – Mammoth Lakes
Region (north of Bishop)
b. Dormant Cinder Cone and Shield volcanoes
i. Last eruption ~720,000 years ago
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c. 1980’s swarms of earthquakes, changes in gas emissions,
ground deformation (uplift of caldera floor), and changes in
thermal springs indicate possible reactivation
d. State/city officials authorized building of emergency exit
routes (“Scenic Byways”)
V.
EARTHQUAKE HAZARDS
A. Faults ‘riddle’ California
1. Fault occurs in brittle rocks of Earth’s crust when rocks
break/fracture due to unequal stresses
a. Small slippage to large slippage rates possible (centimeters
to meters)
b. Successive movement timetable still a mystery
3. Longest fault (most documented, too): San Andreas Fault
a. ~1000 kilometers from Gulf of California to Cape
Mendocino (SF)
b. Active plate boundary between Pacific Plate and North
American Plate
c. Transform fault
4. Types of Faults:
a. Normal Faults – extension forces
b. Reverse Faults – compression forces
c. Thrust Faults (reverse) – compression forces
d. Transform Faults – shearing forces
5. Effects of Earthquakes
a. Displacement as large areas of ground shift position along
fault
b. Liquefaction: solid earth flows as if a liquid as
groundwater moves into unconsolidated soils
c. Seiches: Rise and fall of large bodies of water (lakes,
oceans, even swimming pools)
d. Groundwater level fluctuations as Earth compresses and
expands with passage of seismic body waves
e. Tsunami: Seismic sea wave moving across open ocean and
flooding low-lying coastal areas
B. Earthquakes of Note
1. Owens Valley Fault earthquake: 1872 (8.3 magnitude)
2. Fort Tejon earthquake - 1857(~8.0) along San Andreas Fault
4. San Francisco earthquake -1906 (8.3) along San Andreas Fault
a. Displacement: 20 feet
b. Damage: 700 lives lost: $20 billion property damage
(1980’s $$)
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5.
6.
Hollister earthquake – 1961 (5.6) along San Andreas Fault
San Fernando/Sylmar earthquake – 1971 (6.6)
a. Not San Andreas
7. Whitter Narrows earthquake – 1987 (6.1)
a. Along a fault system not previously known for seismic
activity
8. Loma Prieta earthquake – 1989 (7.1)
a. Displacement deep beneath earth: result – 6 feet lateral
movement, 4 feet vertical uplift/downdropping
b. No relief of strain along San Andreas Fault
c. Damage: 62 lives lost, $6 billion property damage
9. Landers earthquake – 1992 (7.5)
a. One of three earthquakes in a row on a strike-slip fault
trending north west in California
b. 50 mile rupture across Mojave Desert
10. Northridge earthquake – 1994 (6.7)
a. Strongest ground motions recorded in North America
b. Greatest financial loss from natural disaster since 1906 SF
earthquake
c. San Fernando Valley – alluvial fill, underlain by many faults
(known and unknown)
d. Both Northridge and San Fernando earthquakes generated
by reverse-thrust fault systems
e. Northridge earthquake generated specifically by: ‘blind’
thrust fault slippage ~ 20 kilometers (11 miles) beneath
valley floor delivering an ‘uppercut’ to valley
f. Damage: 33 lives lost, 7,000 injured, 20,000 homeless,
major sections of southern California freeways collapsed
and made unsafe, $20 billion property damage
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