Earthquakes and Related Phenomena

Earthquake - vibration in the earth caused by sudden internal movements along faults

- Earthquakes occur at plate boundaries or within plates (intraplate earthquakes)

- fault – a fracture or system of fractures along which rocks have been displaced

- they usually occur along plate margins, but may also occur in the stable interiors of

continents.

- Fault zones and fault segments

- faults rarely occur as a single rupture

- occur as fault zones (groups of subparallel faults) meters to kilometers wide

- long faults (i.e. San Andreas) are segmented, with each segment having its

own history and style of movement

- segments generally rupture as a unit

- segments are important is assessing seismic hazard

- Fault activity

- inactive – generally greater than 1.65 million years without movement

- potentially active – had displacement is last 1.65 million years

- active – had displacement in past 10,000 years

- activity involves:

- slip rate: ratio of slip (displacement) to time interval over which slip has

occurred (example: 1 m over 1000 yr, then rate is 1mm/yr)

- recurrence interval – repeat times – determined by:

- paleoseismic data in geologic record

- slip rate – assume given displacement per event, then divide by

number by slip rate ( i.e. 1 m per event, slip rate 2mm/yr, then

average recurrence interval is 500 years)

- seismicity – use historical earthquake data

- Tectonic Creep

- gradual displacement along fault zone

- damage roads, buildings, etc.

- warehouse at Irvington (Hayward fault) – 7.6 cm (1921-1966)

- RR tracks at Niles (Hayward fault) – 20.3 cm (1910-1966)

- football stadium at Cal Berkley, 3.2 cm in 11 yeas

- winery near Hollister (San Andreas) 8.3 cm in 7.5 years

- may be continuous or discontinuous

- definitions

- focus - the point at which vibrations of an earthquake originate

- epicenter - point on surface of earth immediately above focus

- fault plane - the break in the earth on which movement occurs

- rupture surface - area on a fault plane that experiences movement during an

earthquake event

- during rupture, crack propagates ~5,800 mph or 2.7 km/s

- theory - elastic rebound theory of earthquakes

- slow deformation of the crust (creep) until strength of rock is exceeded.

Then, rupture occurs. Start over.

- in the 50 years before the 1906 san francisco earthquake, surveys

taken in the area recorded an offset by creep over 3 meters

- movement during the 1906 earthquake was only 6m although this

movement took place in 40 seconds as opposed to 50 years

- Earthquake cycle:

- stage one

- long period of seismic inactivity (creep)

- major earthquake

- aftershocks – minutes to months after major quake

- stage two

- increased seismicity as accumulated elastic strain approaches and locally

exceeds strength of rocks, producing small earthquakes

- stage three

- foreshocks (hours to days before next large earthquake)

- does not always occur

- stage four

- major earthquake

- Dilatancy Diffusion Model

- build up of elastic strain from tectonic forces

- rocks dilate by microfracturing, causing ‘low water pressure’, sliding friction

increases (Stage two)

- water then infiltrates fractured rock, causing rise in fluid pressure, increasing

seismic velocity, and weakening rock (Stage three)

- weakening causes earthquake (Stage four)

- Fault-related landforms

- scarp- vertical displacement of the land

- sag pond – body of water filling in low area along fault zone

- offset channel – stream offset horizontally due to fault motion

- linear valley – valley that parallels the fault. It forms because the rocks along the

fault are usually broken (fractured) and are more susceptible to weathering.

- When an earthquake occurs, seismic waves are given off. This is similar to throwing a stone into a

quiet body of water. Waves are created which move out from the point of impact.

- Energy is being propagated along these paths and as it moves some of the energy is lost.

- The farther the wave travels the lower its energy.

- Earthquake waves (or the waves produced when an artificial quake is created) can be

subdivided into two types:

- Body waves

- P-waves, Compression waves, Primary waves

- fastest waves, travel 5 to 15 km/s

- similar to sound waves, like the motion of a spring or slinky, a push-pull

motion

- p-waves may pass through any kind of solids, liquids, or gases

- movement of rock particles is parallel to direction of wave propagation

- S-waves, shear waves, Secondary waves

- s-waves are slower, 4-7 km/s

- movement of rock particles is perpendicular to direction of wave propagation

- like sending a ‘wave’ through a rope

- s-waves may pass through solids only

- Surface waves – generally slower than body waves

- Rayleigh waves

- rock particles move in a vertical rolling (orbital) motion, something like

ocean waves

- Love waves

- rock particles move side to side in a horizontal plane.

- very destructive

- travel faster than Rayleigh waves

Detecting and Measuring earthquakes

- the early Chinese devised an ‘earthquake weathercock’ (132 A.D.) to detect earthquakes

and to determine the direction to the epicenter

- eight dragons held in their mouths bonze balls

- an internal mechanism, activated by a slight tremor opened the mouth of one of the

dragons and released a all to sound an alarm as it clanked into the open mouth

of a toad below.

- the direction to the quake was determined from the orientation of the open-mouthed

dragon

- an early seismometer devised in Italy in 1751 used a pendulum with a brass pointer on the

bottom.

- during a tremor, the pointer traced grooves in the sand of a tray kept level in a larger

tray filled with water

- in the 19^th century, a mercury filled bowl (seismoscope) indicated an earthquake, gave its

direction, and some indication of the strength

- Today, we can measure the strength of an earthquake by a seismograph, an instrument

that amplifies and records earthquake displacement caused by shaking.

- The basic principle behind a seismograph is that of inertia, ‘an object at rest, remains at

rest’

- the seismograph is constructed to allow the earth to move the recording chart, but

not the mass that the ‘pen’ is attached to. This allows the instrument to record

only earth motions. (See diagrams in class)

- the most advanced seismographs can measure ground displacements as small as

10^-8cm (atomic scale)

- these cannot be used on earth as there is too much ‘noise’ in the form of

tides, wind, machinery, etc.

- used on moon. Can detect a 1 kg mass (meteorite) striking anywhere on

moon.

- A typical seismogram looks like this: (see drawing on board)

- The dots are spaced at one minute intervals

p = first p-wave arrival

s = first S-wave arrival

L = first surface wave arrival

- from this seismogram, several important pieces of information may be determined.

- the distance to the earthquake epicenter

- determined from the difference in P-wave and S-wave arrival times, as the

wave types travel at different velocities (See example in class)

- a special graph is used.

- The time the earthquake occurred

- determined from the time of first P-wave arrival and the difference in P-wave

and S-wave arrival times

- the Richter magnitude of the earthquake

- determined from the maximum amplitude of the seismic trace

Locating an earthquake

- in order to locate an earthquake, at least three seismograph stations are needed

- if only one station, hen know just distance to epicenter, along a radius from station

- if two stations, then can narrow down to two possible epicenters

- three stations will give a unique point. Usually many more are actually used.

- See diagram in class

Measuring earthquake intensity and magnitude

- Richter magnitude - quantitative

- a measure of the energy released during an earthquake (as calculated above)

- Richter scale is open ended, <1 to infinity

- only 1 to ~9.2 (largest recorded)

- Richter scale is logarithmic (a mag. 2 is 10 times more powerful than a 1, etc.)

example: magnitude 1 circle 0.25 in dia. Area

2 1.5 in

3 5.0 in

6.4 10.0 ft (1971 San Fernando)

8.3 93.0 ft (1906 San Francisco)

- Modified Mercalli Intensity Index - qualitative

- measure of damage and ‘felt’ intensity

- determined by site examination and interviews

- comparison to Richter magnitude

MMI I II III IV V VI VII VIII IX X XI XII

Richter Mag. -----2----- ----4----- ----6----- ---------8---------

------3------- -----5---- -----7-------

- used to make isoseismal maps

- can be used in damage assessment, planning

- Moment magnitude (M_w)

- based on seismic moment of earthquakes

- product of 1) average amount of slip on he fault during the earthquake

2) the area of the fault plane hat actually ruptured

3) the shear modulus (resistance of the rock to distortion

from shear failure)

- information determined from seismographs, determining length of

rupture, and estimating shear modulus of rocks.

- M_w = 2/3logMo - 10.7, where M_o is seismic moment

- better estimate than Richter scale

- Ground Acceleration –

- rate of change of horizontal or vertical velocity during and earthquake

- measured in relation to gravity acceleration (9.8 m/s) = 1g

- 0.5 g = 4.9 m/s

- necessary to know when designing buildings

- some materials/designs can withstand larger accelerations

- average 0.3 to 0.69

- San Fernando quake 1.15 g

- Northridge quake - even higher

Effects of earthquakes:

- Ground Shaking and Rupture:

- immediate

- result of surface accelerations

- shear or collapse buildings, bridges, dams, tunnels, pipelines, and other

structures

- vertical and horizontal displacements

6.5 m horizontal during 1906 SF

- single breaks are rare, usually multiple fissures

- damage a function of local geology, topography

- landslides

- triggered by shaking

- Loma Prieta quake caused considerable landsliding

- Liquifaction- quick condition failure

- transformation of a water saturated granular material from solid to liquid state

- buildings may tilt or sink

- buried tanks and pipelines may rise toward surface

- tsunamis - seismic sea waves -- these are not!!!! "tidal" waves!!!

- damaging ones generally only occur in Pacific

- originate when water is vertically displaced during:

- earthquakes

- undersea landslides (turbidite flows)

- undersea volcanic eruptions (e. g. Krakatoa, 1883)

- in open ocean:

- may travel up to 700-800 km/hr

- wavelength >100-200 km

- wave height <1 m

- not even noticed by ships at sea

- however, as approach coastline,

- wavelength decreases

- height increases to compensate for low velocity ( up to 30 m)

- velocity is reduced to < 60 km/hr

- very predictable, only if trigger is far enough away

- New Guinea tsunami of 1998 is exception as trigger near coast

- hazard dependent on local coastal and seafloor topography

- Tsunami warning system – all countries around Pacific rim

- in 1946, 7.2 earthquake in Aleutians triggered a Tsunami

- first wave hit Hawaii 5h 7m after quake

- 9-17m waves killed 159

- in 1960, earthquake in Chile caused tsunami that reached Hawaii 15 hours later

- seiches

- oscillating waves on surface of bodies of water (lakes, bays, rivers, etc.)

- uncommon

- fire - most destructive

- 1923 Tokyo-Yokahama, Japan, 143,000 died, 40% in firestorm after 8.3 quake

- in 1906 San Francisco, 508 city blocks wee leveled (12 km²)

- surface displacements and shaking:

- break power and gas lines,

- storage tanks rupture

- furnaces/heaters toppled

- firefighting equipment damaged

- roads blocked with debris or destroyed

- water mains broken

- Regional changes in land elevation

- uplift and subsidence

subsidence may be due to compaction of sediments

- may be tens of meters

- 1964 Alaska 10 uplift and 3 m subsidence over 250,000 km²

- 1992 Mendocino quake – 1 m uplift

Man made earthquakes – caused by:

- loading the earth’s crust by building a dam or reservoir

- in years following building of Hoover Dam in AZ and NV, and the formation of

Lake Mead, hundreds of local tremors, some up to 5 magnitude

- caused by:

- load of water

- infiltration of water into pre-existing fractures and faults

- continue today.

- Dam at Lake Kariba, 1958, Zambia, southern Africa

- in first five years, 2000 earthquakes up to 5.8

- Hsingfengkiang Dam, Canton, China, 1959

- in first 12 years, 250,000 earthquakes

- in 1962, 6.2 quake damaged concrete structure

- Koyna, India, 1962

- by 1967, earthquake swarms every rainy season

- 6.5 quake in 1967 killed 177, injured 1500

- disposing of liquid waste into ground through disposal wells

- Rocky Mountain Arsenal example, 1962 to 1965

- liquid waste pumped into 3500 m deep well, 5 in dia.

- by 1966, 165 million gallons of waste injected

- seeped into fractures in metamorphic rock, increased fluid pressure facilitated

slippage on pre-existing faults

- strong correlation between amount of fluid injected and number of

earthquakes

- within one month of initial injection, earthquakes started

- maximum magnitude was 5.0

- 1986 quake in NE Ohio linked to waste injection

- underground nuclear explosions

- trigger release of natural tectonic strain

- may be used to trigger strain release on potentially hazardous faults???

new madrid, MO (1811-1812)

- Winter of 1811-1812

- Dec. 16, 1811 – 7.2

- Jan. 23, 1812 – 7.1

- Feb. 12, 1812 – 7.4

- Mercalli Intensity V occurred over 2.5 million km², compared to 1906 SF of

150,000 km²

- Intensity data derived from eyewitness accounts and newspaper reports

- one lake raised higher than surrounding countryside, water in lake drained through

fissures in bottom and was replaced w/sand covered with dead fish.

- local subsidence up to 12 feet

- Mississippi jumped a meander (changed course)

- peat and sand thrown from swampland, water level rose 25-30 feet

- During the Feb 12^th quake,

- waves rang church bells in Boston (1600 km away)

- cracks developed in brick buildings in Savannah, GA

- a Chimney top thrown down in Richmond, VA

- houses damaged and chimneys thrown down in St. Louis

- town of New Madrid completely destroyed (dropped from 25’ to 12’ elev.)

- near New Madrid, sand blows 12’ to 50’ in diameter covered he land

- later quake, Charleston, MO, 1865, 6.2

- Dyersburg, TN, 1995 3.1

- Suspected structures:

- New Madrid fault system, part of Reelfoot Rift

- started in Precambrian (600 million years ago)

- active periodically since

- present-day compressive stress may cause occasional reactivation

- area has >200 small-micro quakes per year

- 1811-1812 types quakes occur every 550-1200 years

- probability of >7.0 quake (every 254 – 500 years)

- 19-29% by 2050

- probability of >6.0 quake every 70-90 years

- 50% by 2000

- 90% by 2040

Charleston, sc (1886)

- magnitude estimates 6.6 to 7.1

- shocks at 9:51 PM, 9:59 PM, two more before midnight, then 2:00 AM, 4:00 AM,

and 8:30 AM

- sixty seconds of shaking

- 60 people dead

- 90% of Charleston damaged or destroyed

- reported as far as Boston, Milwaukee, Chicago, Cuba, and Bermuda

- maximum intensity of X, 35 by 50 km

- 80 km of severely damaged RR track, making S-shaped curves, vertical

displacement

- numerous sand blows, some reported to have been 4-6m high

- in Summerville, 25 km NW, houses tilted, chimneys crushed at bases

- little ‘shearing’ deformation suggests motion was primarily vertical

- fault movement interpreted to have occurred below 20 km

- rupture zone estimated to be 30 km long by 19 km wide

- at least three similar size earthquakes in past 3000-3600 yeas, for a recurrence rate

of 1000 years.

- microseisms occur today

- last large quake, 1974

- theories as to origin

1) activity on oceanic fracture zone

2) backslippage on ‘Appalachian fault’

Alaska (1964)

- 9.2 magnitude, largest in U.S.

- earthquake plus tsunami took 125 lives, $311 million in damage (intensity X)

- most in Anchorage, 120 km NW of epicenter, 30 blocks of destruction

- shock lasted 3 minutes

- huge landslides occurred as a result of liquifaction

- vertical displacement occurred over 520,000 km²

- 11.5 m uplift to 2.3 meters subsidence

- off Montague Island, 13-15m absolute vertical displacement

- tsunami reached Hawaii, west coast of U.S.

- maximum wave height 67 m at Valdez inlet

- seiche action occurred on rivers, lakes, harbors, bayous in Gulf Coast of U.S.

- recorded on tide gauges in Cuba and Puerto Rico

California Earthquakes

san francisco, CA (1906)

- magnitude 7.7 to 7.9 (8.3?), intensity values to IX

- ruptured 430 km (290 miles) of San Andreas Fault

- shaking lasted 45 to 60 seconds

- 700 known dead, estimates as high as 2800.

- resulting fires did more damage than shaking

- offsets along fault as large as 6-7m (20 feet)

- 4-5 seconds to move gives 4-5 ft/s (3 mph)

- a similar quake today would result in billions of dollars damage and 1000’s to

10,000’s dead

- in the 70 years prior to quake, moderate (magnitude 6-7) quakes were common,

occurring every 10-15 years in the region

- currently in a similar pattern

- USGS reports 67% chance of >7.0 quake in Bay area by 2020

Northridge, CA (1994)

- moment magnitude of 6.7

- 57 killed, 9000 injured, 12,500 structures moderately to severely damaged.

- major freeway damage up to 32 km from epicenter

- estimate of $13-20 billion in damage

- earthquake preparedness resulted in low number of casualties compared

to areas where not prepared

- similar size quakes killed 25,000 in Armenia (1988) and 11,000 in

India (1993)

- quake occurred early in morning (4:31 A.M) on MLK holiday Jan. 17

- in densely populated San Fernando valley

- if occurred during work day, casualties would have been much higher

- triggered 100’s of landslides

- precipitated an epidemic of Valley Fever (dust (contaminated with

soil fungus) related disease)

- second quake in area in 23 years (1971 San Fernando quake)

- moment magnitude of 6.6

- 32 km NE of Northridge epicenter

- 58 killed, 2000 injured.

- epicenter in sparsely populated San Gabriel mountains

- epicenter 32 km NW of LA, San Andreas 100 km east

- quake occurred along a previously unknown thrust fault (blind thrust)

- accelerations of 1.0-1.78g, ground velocity of 40 cm/s

- fault movement occurred at a depth of 8 –19 km

- Santa Susana Mountains pushed up ~40-50 cm

- 5800 aftershocks (M>1.5) occurred in first seven weeks

- ground failure and surface ruptures common

- Tectonic setting:

- in Transverse Ranges, western portion is part of Pacific Plate

- east-west trending mountains with sed. valley fill

- area dominated by effects of north-south compression, attributable

in part to ‘Big Bend’ of San Andreas and NW motion of

Pacific Plate

- an area of active mountain building

- reverse or reverse-oblique displacements on numerous E-W

trending faults

- fold-and-thrust-ramp model deformation

- area contracting at a rate of 7mm/yr

Loma Prieta (1989)

- ruptured 40 km (25 miles) of San Andreas

- magnitude 7.0, shaking lasted 15 seconds, felt over most of central CA and

western NV

- occurred during world Series

- 63 dead, $6 billion in property damage $1.8 billion to transportation alone)

- most severe damage in Oakland and San Francisco (intensity IX assigned to SF

Marina district, and four areas in SF where Nimitz freeway collapsed)

- liquifaction occurred as far as 110 km from epicenter, primary cause of damage

in Marina district, also building, bridges, pipelines, runways, etc.

- engineered building performed well, only minor system or cosmetic damage

- most damage to unreinforced masonry and wood-frame

- 51 aftershocks of >3.0 in first 24h, 87 in three weeks

- over 1000 landslides occurred as a direct result of the quake

California – The Future……….

- Conditional probability of failure of San Andreas fault

- most likely along Parkfield segment, almost 100% by 2018

- based on observation of historic quakes – 1857, 1881, 1902, 1922, 1934,

1966

- every 21-22 years

- Coachella valley, in southern CA, near 40% probability of failure by 2018

- Along Mojave segment of San Andreas, 30% probability based on historical

record and geologic record along Pallett Creek

- excellent record back to ~500 A.D.

- general clustering of quakes in 160 – 360 year time spans.

Pennsylvania -

Pymatuning Earthquake

- September 25, 1998 at 7:52 PM

- magnitude 5.2,

- felt throughout northern Ohio, most of Pennsylvania, and much of southern Ontario. As far

as Chicago, IL and Rochester, NY

- minor damage in Jamestown, PA and NE Ohio

- fallen ceilings, cracked plaster

- greatest problem was the damage to groundwater systems

- many wells gone dry

- within hours to days of earthquake

- created or enhanced fractures, resulted in increased permeability

- groundwater under hills flowed out

- on a ridge that parallels Rt. 58 between Greenville and Jamestown > 100 households

wells went dry.

- on the bottom of the ridge, artesian wells surged and spring-fed ponds grew,

and “ugly stuff, black water and sulfur” emerged from wells

Northeast Ohio

- January 31, 1986

- magnitude 5.0, followed by 13 aftershocks of magnitude 0.3 to 2.4 (within 2 months)

- minor damage in Jamestown, PA and NE Ohio

- fallen ceilings, cracked plaster

- cracked chimneys and foundations

- broken windows and underground pipes

- changes in flow of water from wells (starting, stopping, sediments, etc.)

Earthquake Risk and Prediction

- seismic hazard maps show earthquake risk in a particular area.

- indicate probability of an event, and probability of a certain amount of ground

shaking

- maximum values – 90% probability that values will not be exceeded in 50 years

- Conditional Probability

- estimate of probability of an earthquake of given magnitude occurring along a

given fault segment within a specified time period

- Short term prediction

- same as a forecast

- some success by Japanese and Chinese by using foreshocks and other data

- factors considered useful:

- Deformation of the ground surface (strain)

- broad regional uplifts due to stable fault slip at depth

- 10 years before 1964 Niigata, Japan quake, several cm broad uplift.

- 5 years before 1983 Sea of Japan quake, several cm uplift

- Rock strain detected by:

- tiltmeters- tube connects two water-filled containers.

- tilt of land causes more water to enter one from

the other

- a 30’ tiltmeter can detect changes as small as 1 ten

millionth of a degree

- gravimeters –

- measures changes in gravitational strength brought about

by rising or falling land, of changes in density of the

underlying rock

- creepmeter –

- a hanging weight maintains a tension on a wire that is 30

feet long, and crosses a creeping fault

- when movement occurs on fault, the distance changes

between ends of the apparatus, and the length of the

wire changes

- Proton Precession Magnetometer

- detects changes in the earth’s magnetic field caused by

rock strain

- at regular intervals, a brief magnetic field aligns spinning

protons of hydrogen atoms

- the much weaker earth magnetic field causes the

protons to wobble or precess

- a change in precession from one reading to the next

indicates a change in the local magnetic field

- Laser distance measuring across a fault

- uses lasers to precisely measure the distance across a fault

- subsequent measurements detect any changes

- lasers can also be bounced off of satellites

- Seismic gaps along faults

- areas along active faults that have not recently produced a large

earthquake

- 10 large plate boundary earthquakes have been successfully

predicted since 1965

- Patterns and frequency of earthquakes

- by studying past history of earthquakes on faults, clustering

patterns may be determined

- reductions of small or moderate earthquakes sometimes occur

prior to a larger event

- small earthquakes may ring an area where a larger event is about

to occur

- Anomalous animal behavior

- noticed first in China

- dog barking

- chickens that refuse to lay eggs

- horses and cattle that are skittish or run in circles

- snakes crawling out in winter and freezing

- other: generally based on the process of dilatency,

- when a rock is squeezed, it deforms, and eventually breaks.

- However, just before breaking, it swells due to the opening

and extension of microcracks.

- This is dilatency. It begins when rock stress is at ~ ½ breaking

strength.

- Radon gas emission due to dilatency

- changes in electrical resistivity due to changes in amount of water

present in microcracks

- changes in water level, turbidity, temperature in deep

wells, again due to dilatencey

- Response to Earthquake Hazards

- hazard-reduction programs

- develop an understanding of earthquake source and process

- determine earthquake potential for an area

- predict effects of earthquakes

- transfer knowledge to states, communities, general public

- critical facilities

- facilities that, if damaged or destroyed, might cause significant to

catastrophic loss of life, property damage, or disruption of society.

- schools, medical facilities, emergency response facilities, power plants,

dams, etc.

- retrofit/design for safety

- establish risk and anticipatory response

- Protective measures

- structural protection in terms of retrofit, new construction

- land-use planning- evaluate potential ground response and plan

accordingly

- increase insurance

- increase number of emergency response personnel

- no action???

- Earthquake warning systems

- technically feasible to provide up to a one minute warning in LA

- shut down machinery

- shut down high speed transit

- take cover

- only after an earthquake has occurred, and before the seismic waves reach

the city

- Concern about false alarms