Environmental Geology

Earthquakes

Directivity

The amplitude of seismic waves is greater in the direction of fault rupture

Building Damage and Ground-Structure Interactions

Damage depends on the ground motion and duration of shaking

Ground motion is related to: -the magnitude of the eq and

characteristics of the seismic waves -Proximity of the epicenter to the site Soil conditions at the site

Resonance Amplifying effect

produced when the natural vibration frequency of ground or structure is matched by the frequency of seismic waves.

Building Height / Typical Natural Period

2 story .2 seconds

5 story .5 seconds

10 story 1.0 seconds

20 story 2.0 seconds

30 story 3.0 seconds

buildings suffer the greatest damage from ground motion at a frequency close or equal to their own natural frequency.

Resonance

Building vibration periods roughly = the number of stories

If building and soil have same frequency of vibration, resonation occurs (amplification)

Typically, low-rise buildings (<5 Stories) located on shallow soils (<50 feet) and high-rise buildings (>14 Stories) on deep soils (>150 feet) sustain the most structural damage

Earthquake Damage

A) Direct shaking & ground rupture

B) Secondary effectsLiquefaction

Landslides

Fires

Tsunamis

Flooding due to changes in land elevation

What can we do?

Structural protection Land use planning Earthquake warning system Effective emergency response plan Increased insurance/recovery measures

B) Short-term prediction:

Precursory phenomena:Ground deformation

Seismic gaps

Patterns and frequency of small earthquakesforeshocks

Anomalous animal behavior

Frequency & Period of Earthquake waves (clarification)

Frequency (Hz) = 1/Period (sec) Higher frequency/lower period = more rapid

attenuation (examples?) Body waves (usually .5-20 Hz = .05-2 sec):

• higher frequencies (lower periods) than surface waves Surface waves (usually less than 1 Hz, greater than 1

sec)• Lower frequencies (higher periods)

Effect of waves Buildings have a natural vibrational frequency

Low buildings have higher frequencies (lower periods) than high buildings

• Low buildings shaken by body waves (high freq.)

• High buildings shaken by surface waves (low freq.) High frequency waves attenuate more quickly

• High buildings shaken at longer distance from the epicenter

REALITY: MANY FACTORS GOVERN EARTHQUAKE DAMAGE!!!

Earthquake Damage

A) Direct shaking & ground ruptureB) Secondary effects

LiquefactionLandslidesFiresTsunamisFlooding due to changes in land elevation

Human activity that affects e.q.:

1) Underground nuclear explosions

2) Loading/unloading of the earth’s crustDam or reservoir

3) Deep waste disposalEx: Rocky Mnt. Arsenal (1962-1965)

Liquid waste pumped 3.6 km

What can we do?

Structural protection Land use planning Earthquake warning system Effective emergency response plan Increased insurance/recovery measures

Earthquake prediction

A) Long-term prediction:Estimate relative seismic hazardEstimate conditional probabilities

B) Short-term prediction Precursory phenomena

A) Long-term prediction:

Estimate relative seismic hazardActive faults?

Active: Holocene

potentially active: Quaternary

Inactive: Pre Quaternary

History of fault activity:Paleoseismology (Pallet Creek study & Coyote Creek study;

page)Estimate average recurrence interval

B) Short-term prediction:

Precursory phenomena:Ground deformation

Seismic gaps

Patterns and frequency of small earthquakesforeshocks

Anomalous animal behavior

The San Andreas Fault Zone in Southern California

Faults in San Diego

California Regulations pertaining to earthquakes Alquist-Priolo Fault Zoning Act (1972) (California law)

direct result of the 1971 San Fernando Earthquake requires that zones along active faults with well-defined

surface features be established (http://www.consrv.ca.gov/CGS/rghm/ap/)

Seismic Hazards Mapping Act (1990) addresses non-surface fault rupture earthquake hazards,

including liquefaction and seismically induced landslides

The Natural Hazards Disclosure Act (1998) sellers of real property must provide prospective buyers with a

"Natural Hazard Disclosure Statement" when the property being sold lies within one or more state-mapped hazard areas.

Alquist-Priolo Fault Zoning Act

1972 – Alquist-Priolo Special Studies Zones Act - Passed in 1972 as a direct result of the 1971 San Fernando Earthquake.

Many cities have their own amendments Purpose

• Provides policies and criteria to assist cities, counties, and state agencies in the exercise of their responsibility to prohibit the locations of developments and structures for human occupancy across the trace of active faults

• Fault must have “well-defined” Holocene surface rupture (Blind-thrusts are exempt)

Alquist-Priolo Fault Zoning Act

Summary of Specific Criteria No structure for human occupancy shall be placed within 50 feet

of an active fault• Area within 50 feet of fault trace presumed to be underlain by active

branches of the fault unless proven otherwise Lead agencies (State Geologist, State Mining and Geology Board)

shall provide public disclosure of delineated fault zones to the public

Development permit applications for any project within a delineated zone must be accompanied by a geologic report

The seller’s agent or seller of real property located within a delineated zone shall disclose to the buyer that fact

Alquist – Priolo Fault Zoning Act

Exemptions• Any structure built before May 4, 1975• Single-Family wood-framed dwellings

(2 stories or under)• Conversions or alterations of existing

structures (under 50% the value of the structure).

Seismic Hazards Mapping Act

Passed in 1990, addresses non-surface fault rupture earthquake hazards, including liquefaction and seismically induced landslides.

Seismic Zones

In Seismic Zone 4, you have a one in ten chance that an earthquake with an active peak acceleration level of 0.4g (4/10 the acceleration of gravity) will occur within the next fifty years.

In Zone 1, you have a one in ten chance that an earthquake with an active peak acceleration level of 0.1g (1/10 the acceleration of gravity) will occur within the next fifty years.

Seismic Zones

The Uniform Building Code places San Diego in Seismic Zone 4

Buildings are required to withstand 1/3 more of the lateral force from earthquakes that Seismic Zone 3 mandates

San Diego Seismic Standards

San Diego has been required to enforce the State Earthquake Protection Law (Riley Act) since its enactment in 1933. However, the seismic resistance requirements of the law were minimal for many years and San Diego did not embrace more restrictive seismic design standards until its first adoption of the Uniform Building Code in 1951.

It is estimated that about 1,000 (mainly nonresidential) masonry buildings within the City may constitute structural hazards.

Structural Design and Seismic Performance

Ductile steel and ductile reinforced concrete frame buildings (as defined in Uniform Building Code) - highly resistant to structural damage; may suffer nonstructural damage.

Vertical load-bearing steel and reinforced concrete frame buildings braced against lateral forces - perform well but may suffer some structural as well as nonstructural damage.

Unreinforced masonry buildings of all types - highly vulnerable to damage.

Structural Design and Seismic Performance Reinforced brick and concrete block masonry buildings - perform

well but may suffer some structural as well as nonstructural damage. Pre-engineered and other light steel and sheet metal buildings -

usually perform extremely well. Residential buildings - Traditional wood frames with wood or stucco

siding usually behave well but may suffer damage. Modern design open-type houses with large glass openings, split-level houses, and two-story houses or apartments with large garage openings in the first story are vulnerable to earthquake damage.

probability of being exceeded in 50 years

Tsunami

Seismic sea wave (not a tidal wave) Disturbing forces: vertical movement of seafloor

Offshore faults; subduction zone earthquakes Submarine landslides Volcanic eruptions Meteorite impact

Size: Wavelength: 125 miles (200 km) Wave height:

• Coast: 10’s of meters (largest are ~30-40 m or 100+ ft.)• Open ocean: 1 meter (3 ft)

Velocity: Open ocean: 450 mph Slow as they approach coastlines

Distance of travel: across oceans (1000’s of km)

Waves

Deep Water Waves Those waves traveling with a water depth

greater than ½ the wavelength The speed of deep-water waves depends on the

wavelength of the waves. Waves with a longer wavelength travels at higher speed.

π2/gLc =

Waves

Shallow Water Waves Those waves traveling with a water depth less

than 1/20th of the wavelength. The speed is independent of their wavelength.

It depends, however, on the depth of the water.

gdc =

• Average wavelength for a wind wave is 100 meters (300 ft), tsunami is 100 miles.• Wavelength decreases and wave height increases as waves approach the coast• Usually more than one wave, the first is not always the biggest!

A 10 meter (30 ft) tsunami wave has a lot more energy than a 10 m wind wave

Pacific Ocean has had more tsunami events than any other ocean or sea.

1m = 3ft

4000 deaths from tsunami waves around the Pacificfrom 1990 to 2000.

Chile, South America 1960• 9.5 subduction earthquake • largest earthquake recorded• Tsunami traveled to Hawaii and Japan

• A piece of the Pacific seafloor (Nazca Plate) about the size of California slid fifty feet beneath the continent of South America.

• South American continent offshore snapped upwards as much as 20 ft. while land along the Chile coast dropped about ten feet.

• The tsunami caused tremendous damage along the Chile coast, where about 2,000 people died.

• The waves spread outwards across the Pacific. 15 hours later the waves flooded Hilo, on the island of Hawaii, where they built up to thirty feet and caused 61 deaths along the waterfront.

• Seven hours after that (22 hours after the earthquake) the waves flooded the coastline of Japan where ten-foot waves caused 200 deaths.

• The waves also caused damage in New Zealand. Tide gauges throughout the Pacific measured anomalous oscillations for about three days as the waves bounced from one side of the ocean to the other.

CHILE 1960 SUBDUCTION EARTHQUAKE

• In San Diego: ferry service was interrupted after one passenger-laden ferry smashed into the dock at Coronado knocking out eight pilings.

•A second ferry was forced 1.5 km off course and into a flotilla of anchored destroyers. More than 80 m of dock were destroyed.

•A 100 ton dredge rammed the concrete pilings supporting the Mission Bay bridge tearing out a 21 m section.

•A 45 m bait barge smashed eight slips at the Seaforth Landing before breaking in half and sinking.

•The currents swept away two sections of dockage at the Southwest Yacht Club at Point Loma.

Wave damage in Hilo Hawaii

DART: Deep-ocean Assessment and Reporting of TsunamisEarly detection and real-time reporting of tsunamis in the open ocean

http://www.pmel.noaa.gov/tsunami/Dart/Flash/CODEframe4DART.html

In addition to subduction zone earthquakes, what else could cause a tsunami in Pacific Ocean?

Submarine landslides

Where?

Each slide has resulted in huge land losses to the islands and resulted in large waves that have carried rocks and sediments as high as 1000 ft above sea level. The giant Hawaiian landslides are important to study because, although they occur infrequently, they have potential for enormous loss of life, property, and resources.