ISNS 4359 Earthquakes and VolcanoesISNS 4359 Earthquakes and Volcanoes(aka shake and bake)(aka shake and bake)

Lecture 6: Locating EQ’s, EQ Magnitude and Intensity

Fall 2005

Sound Waves and Seismic Waves• Seismologists record and analyze waves to determine

where an earthquake occurred and how large it was• Waves are fundamental to music and seismology• Similarities:

– More high frequency waves if short path is traveled• Trombone is retracted, short fault-rupture length (small

earthquake)

– More low frequency waves if long path is traveled• Trombone is extended, long fault-rupture length (large earthquake)

Seismic Velocity

• Seismic velocity is a material property (like density).

• There are two kinds of waves – Body and Surface waves.

• There are two kinds of body wave velocity – P and S wave velocities.

• P waves always travel faster than S waves.

• Seismic velocities depend on quantities like chemical composition, pressure, temperature, etc.

Faster Velocities

• Lower temperatures

• Higher pressures

• Solid phases

Slower Velocities

• Higher temperatures

• Lower pressures

• Liquid phases

Locating the Source of an Earthquake

• P waves travel about 1.7 times faster than S waves• Farther from hypocenter, greater time lag of S wave

behind P wave (S-P)• (S-P) time indicates how far away earthquake was

from station – but in what direction?

Locating the Source of an Earthquake• Need distance of earthquake from three stations to

pinpoint location of earthquake:– Computer calculation

– Visualize circles drawn around each station for appropriate distance from station, and intersection of circles at earthquake’s location

– Method is most reliable when earthquake is near surface

Fig. 4.23

Solution to epicenter and hyopcenter

• Mathematically, the problem is solved by setting up a system of linear equations, one for each station.

• The equations express the difference between the observed arrival times and those calculated from the previous (or initial) hypocenter, in terms of small steps in the 3 hypocentral coordinates and the origin time.

• We must also have a mathematical model of the crustal velocities (in kilometers per second) under the seismic network to calculate the travel times of waves from an earthquake at a given depth to a station at a given distance.

• The system of linear equations is solved by the method of least squares which minimizes the sum of the squares of the differences between the observed and calculated arrival times.

• The process begins with an initial guessed hypocenter, performs several hypocentral adjustments each found by a least squares solution to the equations, and iterates to a hypocenter that best fits the observed set of wave arrival times at the stations of the seismic network.

Magnitude of Earthquakes• Richter scaleRichter scale

– Devised in 1935 to describe magnitude of shallow, moderately-sized earthquakes located near Caltech seismometers in southern California

– Bigger earthquake greater shaking greater amplitude of lines on seismogram

– Defined magnitude as ‘logarithm of maximum seismic wave amplitude recorded on standard seismogram at 100 km from earthquake’, with corrections made for distance

– For every 10 fold increase in recorded amplitude, Richter magnitude increases one number

Magnitude of Earthquakes• Richter scale

– With every one increase in Richter magnitude, the energy release increases by about 45 times, but energy is also spread out over much larger area and over longer time

– Bigger earthquake means more people will experience shaking and for longer time (increases damage to buildings)

– Many more small earthquakes each year than large ones, but more than 90% of energy release is from few large earthquakes

– Richter scale magnitude is easy and quick to calculate, so popular with media

•

Magnitude of Earthquakes

Magnitude of Earthquakes

Magnitude of Earthquakes

http://www.iris.iris.edu/volume2000no1/RevFigure2.big.gif

21,688 earthquakes recorded by NEIC in 1998

Magnitude of Earthquakes

http://www.iris.iris.edu/volume2000no1/RevFigure2.big.gif

21,688 earthquakes recorded by NEIC in 1998

Other Measures of Earthquake Size• Richter scale is useful for magnitude of shallow,

small-moderate nearby earthquakes• Does not work well for distant or large earthquakes

– Short-period waves do not increase amplitude for bigger earthquakes

– Richter scale:• 1906 San Francisco earthquake was magnitude 8.3

• 1964 Alaska earthquake was magnitude 8.3

– Other magnitude scale:• 1906 San Francisco earthquake was magnitude 7.8

• 1964 Alaska earthquake was magnitude 9.2 (100 times more energy)

Other Measures of Earthquake SizeTwo other magnitude scales:

• Body wave scale (mb): – Uses amplitudes of P waves with 1 to 10-second periods

• Surface wave scale (ms):– Uses Rayleigh waves with 18 to 22-second periods

• All magnitude scales are not equivalent– Larger earthquakes radiate more energy at longer periods

not measured by Richter scale or body wave scale

– Richter scale and body wave scale significantly underestimate magnitudes of earthquakes far away or large

Moment Magnitude Scale• Seismic momentSeismic moment (Mo)

– Measures amount of strain energy released by movement along whole rupture surface; more accurate for big earthquakes

– Calculated using rocks’ shear strength times rupture area of fault times displacement (slip) on the fault

• Moment magnitude scaleMoment magnitude scale uses seismic moment:– Mw = 2/3 log10 (Mo) – 6

– Scale developed by Hiroo Kanamori

Foreshocks, Main Shock and Aftershocks• Large earthquakes are not just single events but part

of series of earthquakes over years– Largest event in series is mainshockmainshock

– Smaller events preceding mainshock are foreshocksforeshocks

– Smaller events following mainshock are aftershocksaftershocks

• Large event may be considered mainshock, then followed by even larger earthquake, so then re-classified as foreshock

Magnitude, Fault-Rupture Length and Seismic-Wave Frequencies

• Fault-rupture length greatly influences magnitude:– 100 m long fault rupture magnitude 4 earthquake

– 1 km long fault rupture magnitude 5 earthquake

– 10 km long fault rupture magnitude 6 earthquake

– 100 km long fault rupture magnitude 7 earthquake

Magnitude, Fault-Rupture Length and Seismic-Wave Frequencies

• Fault-rupture length and duration influence seismic wave frequency:– Short rupture, duration high frequency seismic waves

– Long rupture, duration low frequency seismic waves

• Seismic wave frequency influences damage:– High frequency waves cause much damage at epicenter

but die out quickly with distance from epicenter

– Low frequency waves travel great distance from epicenter so do most damage farther away

Ground Motion During Earthquakes• Buildings are designed to handle vertical forces

(weight of building and contents) so that vertical shaking in earthquakes is typically safe

• Horizontal shaking during earthquakes can do massive damage to buildings

• Acceleration– Measure in terms of acceleration due to gravity (g = 9.8

m/s2)

– Weak buildings suffer damage from horizontal accelerations of more than 0.1 g

– In some locations, horizontal acceleration can be as much as 1.8 g (Tarzana Hills in 1994 Northridge, California earthquake)

Periods of Buildings and Responses of Foundations

Just as waves have natural frequencies and periods, so do buildings

• Periods of swaying are about 0.1 second per story– 1-story house shakes at about 0.1 second per cycle

– 30-story building sways at about 3 seconds per cycle

• Building materials affect building periods– Flexible materials (wood, steel) longer period of

shaking

– Stiff materials (brick, concrete) shorter period of shaking

Periods of Buildings and Responses of Foundations

Velocity of seismic wave depends on material it is moving through

• Faster through hard rocks• Slower through soft rocks• When waves pass from harder to softer rocks, they

slow down• Must therefore increase their amplitude in order to

carry same amount of energy greater shaking• Shaking tends to be stronger at sites with softer

ground foundations (basins, valleys, reclaimed wetlands, etc.)

Periods of Buildings and Responses of Foundations

• If the period of the wave matches the period of the building, shaking is amplified and resonance results– Common cause of catastrophic failure of buildings

Earthquake Intensity – What We Feel During an Earthquake

• Mercalli intensity scaleMercalli intensity scale was developed to quantify what people feel during an earthquake

• Used for earthquakes before instrumentation or current earthquakes in areas without instrumentation

• Assesses effects on people and buildings• Maps of Mercalli intensities can be generated

quickly after an earthquake using people’s input to the webpage http://pasadena.wr.usgs.gov/shake

Earthquake Intensity – What We Feel During an Earthquake

What To Do Before and During an Earthquake

• Before an earthquake:– Inside and outside your home, visualize what might fall

during strong shaking, and anchor those objects by nailing, bracing, tying, etc.

– Inside and outside your home, locate safe spots with protection – under heavy table, strong desk, bed, etc.

• During an earthquake:– Duck, cover and hold

– Stay calm

– If inside, stay inside

– If outside, stay outside

Mercalli Scale VariablesMercalli intensityMercalli intensity depends on:

• Earthquake magnitude

– Bigger earthquake, more likely death and damage

• Distance from hypocenter

– Usually (but not always), closer earthquake more damage

• Type of rock or sediment making up ground surface

– Hard rock foundations vibrate from nearby earthquake body waves

– Soft sediments amplified by distant earthquake surface waves

– Steep slopes can generate landslides when shaken

Mercalli Scale Variables

Mercalli Scale VariablesMercalli intensity depends on:

• Building style

– Body waves near the epicenter will be amplified by rigid or short buildings

– Low-frequency surface waves are amplified by tall buildings, especially if on soft foundations

• Duration of shaking

– Longer shaking lasts, more buildings can be damaged

Design of Buildings in Earthquake-Prone Areas

• Eliminate resonance:– Change height of building

– Move weight to lower floors

– Change shape of building

– Change building materials

– Change attachment of building to foundation

– Hard foundation (high-frequency vibrations) build tall, flexible building

– Soft foundation (low-frequency vibrations) build short, stiff building

Design of Buildings in Earthquake-Prone Areas

• Floors, Roofs and Trusses– Give horizontal resistance by transferring force to vertical

resistance elements

• Shear Walls– Designed to receive horizontal forces from floors, roofs and

trusses and transmit to ground

– Lack of shear walls typically cause structures like parking garages to fail in earthquakes

Design of Buildings in Earthquake-Prone Areas

• Bracing– Bracing with ductile materials offers resistance

• Moment-resisting frames– Devices on ground or within structure to absorb part of

earthquake energy

– Use wheels, ball bearings, shock absorbers, ‘rubber doughnuts’, etc. to isolate building from worst shaking

A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971

• Earthquake magnitude

– M 6.6, with 35 aftershocks of magnitude 4.0 or higher

• Distance from epicenter

– Bull’s-eye damage pattern

• Foundation materials

– Not a major factor

• Building style

– ‘Soft’ first-story buildings were major problem

– Hollow-core bricks at V.A. Hospital caused collapse

– Collapse of freeway bridges

Fig. 4.32

A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971• Duration of shaking

– Lasted 12 seconds (at that magnitude, can last from 10 to 30 seconds), relatively short time

– Lower Van Norman Reservoir • 11,000 acre-feet of water behind earthen dam, above homes of 80,000

• When shaking stopped, only four feet (of original 30) of dam was still standing above water level

• Another few seconds of shaking might have caused catastrophic flood

Fig. 4.29

Fig. 4.30

Fig. 4.33a

Fig. 4.33b

Fig. 4.34

Problem Set 1, Journals, Term Paper

• Problem set 1 and term paper format will be available on WebCT by tomorrow morning.

• Problem set 1 will now be due on Sept 20, rather than Sept 15.• Submit a topic for approval for term paper by Sept 22, if you

have not already done so• Journals - to reiterate - track 1 event (EQ or V) per week for four

weeks and submit a journal on what you find. Length and format is up to you, but all work (other than figures) should be original and all sources cited