EARTHQUAKES AND SEISMOLOGY

An earthquake is a naturally produced shock event which causes the Earth to vibrate.

Of interest are:

their effect why they occur where they occur scientific study of the Earth's interior and dynamics:

the structure of the Earth, composition of the Earth kinematics and dynamic of plate tectonics.

How many Earthquakes?EQs (earthquakes) are very common, many thousands per day all over the world., e.g. map for 22-29th Oct 2003:

Most are weak (low energy) and undetected. Here are those with Mag > 2.5 (see later for definition) for the past week:

Update time = Wed Oct 29 12:00:04 UTC 2003

MAG DATE UTC-TIME LAT LON DEPTH region y/m/d h:m:s deg deg km map 2.8 2003/10/29 11:21:33 61.650 -150.210 45.0 SOUTHERN ALASKAmap 2.8 2003/10/29 10:19:08 63.087 -144.253 5.0 CENTRAL ALASKAmap 3.5 2003/10/29 07:25:52 57.460 -143.110 1.0 GULF OF ALASKAmap 3.1 2003/10/29 07:18:29 34.659 -116.295 1.6 SOUTHERN CALIFORNIAmap 2.5 2003/10/29 03:34:52 62.972 -152.859 180.0 CENTRAL ALASKAmap 2.9 2003/10/29 02:54:35 63.585 -147.866 1.0 CENTRAL ALASKA…….…………………………………………………map 3.5 2003/10/22 19:23:16 51.479 -176.185 23.7 ANDREANOF ISLANDS, ALEUTIAN IS., ALASKAmap 3.0 2003/10/22 17:57:27 51.223 -178.156 14.4 ANDREANOF ISLANDS, ALEUTIAN IS., ALASKA

Large Earthquakes

Large energy EQs are rare but cause wide scale social disruption.

July 1976  Tangshan, China  650,000 fatalities May 1970  Lima, Peru    66,000 fatalities May 1927  Nan-Shan, China  200,000 fatalities Sept. 1923  Tokyo, Japan   143,000  fatalities

Largest Recorded Earthquake

Chile 1960 May 22 19:11:14 UTC, Magnitude 9.5

More than 2,000 killed, 3,000 injured, 2,000,000 homeless, and $550 million damage in southern Chile; Tsunami caused 61 deaths, $75 million damage in Hawaii; 138 deaths and $50 million damage in Japan; 32 dead and missing in the Phillippines; and $500,000 damage to the west coast of the United States.

Why Earthquakes?EQs result from sudden displacement of crust, usually along faults.

Often come in a series of quakes - "minor" aftershocks common.

The earth rings like a bell after a large earthquake occurs.

Very long wavelength surface waves circle the globe and interfere constructively to set up harmonic oscillations.

The shape of an oscillation is determined by the wavelength of the waves, and the frequency of the oscillation by the internal velocity structure.

Analyzed by spherical harmonics….

Shocks give rise to landslips, turbidity currents,…..

….and tsunamis:

Discuss:

earthquake cause and mechanism earthquake locations and distribution earthquake size: magnitude and intensity seismic waves and the interior of the earth seismotectonics earthquake prediction and control

MEASUREMENT OF SEISMIC WAVES Earthquake waves recorded on a SEISMOMETER -> SEISMOGRAM.

Seismometer originally consisted of a rigid frame attached to the Earth, a chart recorder attached to frame, a heavy mass suspended from frame by a pivot and a pen attached to mass.

Modern ones digitised, etc.

When frame is shaken by quake, the mass and pen remain stationary.  The scale of the displacement is recorded on chart.

Mass is stationary because of inertia.

Pivot restricts recording to just one plane. 

Need 3 seismometers to measure EQ. 

One for vertical movements and 2 for horizontal movements.

Often have more seismometers with different masses to record shocks with different frequencies - seismic vibrations have spectrum of frequencies.

Typically record waves with short-periods - 12s,

and  long-periods  - 20s.

 EARTHQUAKE CAUSES AND MECHANISMS Major EQs occur at plate boundaries - infer that cause of EQs is relative motion of sections of the lithosphere.

Explanation of EQs is the ELASTIC REBOUND THEORY, put forward by H. F. Reid in 1906 after San Francisco EQ.

EQ derives from FAULT, that separate two blocks which are attempting to move relative to each other.

Friction or cementing will resist the movement. 

Under these conditions crustal stresses do not move blocks, but a state of STRAIN builds up in the region of the fault.

If crustal stresses increase (because of movement elsewhere on fault, etc.) will reach a level where they overcome frictional restraint, and the two blocks will slip relative to each other -> EQ.

This process can occur time after time, so called stick-slip movement.

Time between movements can be years. 

The longer the time, the larger the stress and strain, the larger the EQ.

There are three main types of fault movement, each give distinctive EQ patterns (see below):

normal fault

thrust fault

strike-slip fault

EARTHQUAKE LOCATION AND DISTRIBUTION

see http://vcourseware2.calstatela.edu/eec/Earthquake/

FOCUS- The source - a small part of a fault (approx. a few kms3) - can be regarded as a point . EPICENTRE - The point on the surface, vertically above the focus. FOCAL DEPTH  - The distance from the epicenter to the focus.

Shock waves radiate out from focus in all directions. 

They are first felt at the surface at the epicentre.

Location of epicentre-

If we have seismographs from 3 stations relatively close to the epicentre, we can deduce the position of the epicentre by the time intervals between the P and the S waves at each station - given a knowledge of P and S wave velocities.

If local P and S near-surface velocities are Vp and Vs and the station is X km from epicentre, then

 Time for P wave arrival  (tp) =  X / Vp  Time for S wave arrival  (ts) =  X / Vs

Therefore interval between P and S arrival  

Δt = ts-tp  =  X / Vs  -  X / Vp

Thus re-arranging to find X gives

X  =  Vs.Vp.(Ts-Tp) / (Vp - Vs)

Units - Vp,s in kms-1; Δt in s; X in km

To locate epicentre need values of X from THREE stations -> Triangulation. 

Epicentre at intersection of 3 circles of radii X1, X2 and X3 based at stations 1, 2 and 3.

World-wide have approx. 1000 stations.  

Often arranged as SEISMIC ARRAYS in well defined geometrical distribution.

Can use arrays to give near-surface data and information about deeper structure of Earth.

Depth of focus (h) can also be deduced, but needs a more refined approach.

First, need to determine epicentral angle – Δ This is done from travel time curve, as ts-tp is a simple function of Δ:

Then, a plot of ts-tp v. tp for several stations give t0, the time the earthquake occurred. Then, if V is the P-wave velocity, distance of detector (D) to focus is:

D = (tp-t0)V

Now can obtain h, since:

X = R . Δ . (2π/360) and so:

h ~ √(D2 – X2)

Global Distrubition

All earthquakes occur at depths < 700 km and are confined to the rigid lithosphere. Have 3 classifications of EQs -

Shallow focus    0-70 km Intermediate focus  70-300 km Deep focus   300-700 km

With large Earthquakes (magnitude > 7 ):o 75% Shallow,   o 90%  Intermediate , o Approx. 100% Deep occur around the

margins of the Pacific

Those not in circum-Pacific belt occur in Alpine-Himalayan belt.

Deep Earthquakes largely from subducted slabs:

Also high concentration of shallow Earthquakes (mag. 4-6) along ocean ridge system.

Intra-plate Earthquakes are rarer (but do occur, e.g. Lisbon 1755).

Shallow Earthquakes much more common than Intermediate and Deep.

Most surface damage is caused by Shallow Earthquakes.

EARTHQUAKE SIZE : MAGNITUDE AND INTENSITY Earthquake magnitude is an "ABSOLUTE" measure of size, and is related to the energy released.  

Determined from seismic wave amplitude.

C.f. Earthquake intensity refers to amount of damage caused – more subjective.

Magnitude

Magnitude, given by general equation:-  

M  =  log10 A/T + M = magnitude log - base 10 A = max. amplitude of wave in µm (= 10-6m) T = period of wave, (P or S) in secs. (time taken for one wavelength to pass a fixed point) = factor correcting for epicentral distance, focal depth and type of wave.  These can be read from published tables. However, since needs to be obtained from observation the "absolute" measure of Earthquake magnitude is still subject of debate.  Error in magnitude should be < ± 0.3.

Recall, epicentral distance from the point of measurement to the epicentre is measured by the angle subtended by these 2 points at the centre of the  Earth - the epicentre angle ().

Magnitude scale is Logarithmic, therefore M = 7.0 is 100 times larger than M = 5.0

There are many definitions of magnitude, depending on what is being measured:

Richter was amongst first to develop a scale for earthquakes in 1935 based on P-wave amplitudes. 

An earthquake's Richter magnitude was originally defined to be the amplitude of shaking on a Wood-Anderson seismometer of an earthquake 100km away. Since there are very few working Wood-Anderson seismometers around these days, scientists approximate the magnitude using calibration scales based on the distance from the source and the amplitude of seismic waves.

The Richter local magnitude is given by:

MR = log10(A/T) + g(d,h) + a

o where M is the magnitudeo A is the maximum wave amplitudeo T is the wave periodo g is a function that corrects for the decrease in wave amplitude(A)o d is the angular distance from seismometer to epicentero h is the focal depth of the earthquakeo and (a) is an empirical constant

BODY-WAVE magnitude - At large distances from epicentre use body-wave amplitude (usually P-wave), because body-wave attenuation is less than surface-waves.  

MB  = log10 (A/T) + 0.01 + 5.9  

where T approx. 12 secs. The surface wave magnitude is given by

MS    = log10 (A/T) + 1.66 log10   + 3.3 o where T is between 17 and 23 seconds o d is between 20 degrees and 160 degreeso and h (depth of focus) is less than 50 km

For extremely deep earthquakes the magnitude must be corrected to compensate for the increased distance due to depth. This correction is:

M corr = MS + bMS(h)

b comes from a table.

The seismic moment is given by

MO = G.A.

o where G is the rigidity o A is the surface area of the fault o and is the average fault displacement

and the moment magnitude is given by

MM = 0.666log10MO - 6

The moment magnitude depends on the surface wave magnitude:

log10MO = 1.5MS + 16.1

Largest Earthquakes in the World Since 1900Location Date Mag Coordinates

1. Chile 1960 05 22 9.5 38.26 S 72.15 W

2. Prince William Sound, Alaska 1964 03 28 9.2 61.02 N 147.65 W

3. Andreanof Islands, Aleutian Islands 1957 03 09 9.1 51.57 N 175.34 W

4. Kamchatka 1952 11 04 9.0 52.75 N 159.50 E

5. Off the Coast of Ecuador 1906 01 31 8.8 1.0 N 81.5 W

6. Rat Islands, Aleutian Islands 1965 02 04 8.7 51.23 N 178.52 E

7. India-China Border 1950 08 15 8.6 28.5 N 96.5 E

8. Kamchatka 1923 02 03 8.5 54.0 N 161.0 E

9. Banda Sea, Indonesia 1938 02 01 8.5 5.25 S 130.5 E

10. Kuril Islands 1963 10 13 8.5 44.9 N 149.6 E

Energy and Magnitude

The energy released by an earthquake (E in Joules) is related to magnitude. An empirical relation between ENERGY and MAGNITUDE is given by:  

Log10 E  = 1.5Ms + 4.8

An increase in magnitude by 1.0 units in Ms gives approx. x 30 increase in energy. Estimated annual energy loss via Earthquakes is 1018 J.  

Most of this comes from the few quakes with Ms = 7 or 8.

Energy of Earthquake is turned largely into heat via friction of vibrating rock particles, etc. 

Total contribution of Earthquake to heat loss is small.  

Total Earth heat flow approx. 1021 J per y.  Total U.S.A. energy consumption approx. 1019 Jy-1!  

Intensity

The intensity of an earthquake is based on its effects at the surface.   For a given location intensity is reported as Roman Numerals according to the modified Mercalli

Scale:

INTENSITY ----DESCRIPTION I----Not Felt. II----Felt by persons at rest on uppper floors. III----Felt indoors---hanging objects swing. Vibration like passing of light trucks. IV----Vibration like passing of heavy trucks. Standing automobile rock. Windows, dishes, and doors rattle; wooden walls or frame may creak. V----Felt outdoors. Sleepers wakened. Liquids disturbed, some spilled; small objects may be moved or upset; doors swing; shutters and pictures move. VI----Felt by all; many frightened. People walk unsteadily; windows and dishes broken; objects knocked off shelves, pictures off walls. Furniture moved or overturned; weak plaster cracked. Small bells ring. Trees and bushes shaken. VII----Difficult to stand. Furniture broken. Damage to weak materials, such as adobe; some cracking of ordinary masonry. Fall of plaster, loose bricks, and tile. Waves on ponds; water muddy; small slides along sand or gravel banks. Large bells ring. VIII----Steering automobiles affected. Damage to and partial collapse of ordinary masonry. Fall of chimneys, towers. Frame houses moved on foundations if not bolted down. Changes in flow of springs and wells. IX----General panic. Frame structure shiffted off foundations if not bolted down; frames cracked. Serious damage even to partially reinforced masonry. Underground pipes broken; reserviors damaged. Conspicuous cracks in ground. X----Most masonry and frame structure destoryed with their foundations.

Serious damage to dams and dikes; large landslides. Rails bent slightly. XI----Rails bent greatly. Underground pipelines out of service. XII----Damage nearly total. Large rock masses shifted; objects thrown into the air.

Factors that tend to increase intensity: o magnitude of Earthquake o distance to focus o substrate

SEISMOTECTONICS Earthquake Focal Mechanisms

o In any given direction P waves radiate from focus as compression or dilatation. The pattern of initial motion (first motion) characterizes the nature of the fault motion that produced the EQ (normal, reverse, strike-slip)

o Thus of a strike slip fault:

Can define the fault plane from the plane normal to fault (the auxiliary plane)

o The first shock reveals a good deal about the nature of the fault. Depending on whether a particular area is compressed or extended by movement along the fault, the first motion p-wave will be upwards (positive) or downwards (negative), respectively.

o The pattern of first motions obtained from a number of seismic stations can be summarized on a stereogram, depicting quadrants of compression and dilation.

o On a stereonet there are two planes, drawn along great circles, separating the compressional (positive) p-waves from the extensional (negative) p-waves.

o These planes are known as nodal planes. o From this can infer nature of faulting - FOCAL MECHANISM SOLUTIONS (black- compression,

white- dilatation).

o Note: the observed pattern of arrivals does not uniquely determine the orientation of the fault. o From P waves alone cannot tell fault plane from auxiliary plane:o If the source is a single- couple one, can tell from S-waves distribution:

o But if movement on the fault is complex (“double couple”), then the solution is still ambiguous:

o To determine the correct plane, the locations of aftershocks and/or surface geology and regional tectonics may be necessary.

Mixed Faults

It is very unusual to find a pure strike-slip, normal or thrust fault in nature. Most faults consist mainly of one motion with a small component of another. Pure strike-slip faults have first motion diagrams with nodal planes that intersect in the center of the stereonet (as pictured above). Strike-slip faults that have some component of thrust or normal faulting intersect off-center.

This is either a left-lateral or right-lateral strike slip fault with a component of thrust faulting. The nodal planes for pure normal faults and pure thrust faults intersect along the primitive circle (as pictured previously). Strike-slip faults can have components of either normal or thrust faulting, but normal and thrust faults can only have components of strike-slip faulting. Here's the first motion patterns of an oblique normal fault and an oblique thrust fault with a small component of strike slip motion.

Oblique thrust

The first motion pattern for thrust faulting with a small component of left lateral strike-slip motion would look the same (except the compression and extension circles would be reversed). Oblique

normal

Earthquakes and Plate Tectonics

Different earthquakes are characteristic of different tectonic setting.

Revealed by study of focal mechanisms.

Divergent boundaries - shallow focus EQ's,  normal and strike slip faults along mid-ocean ridge:

Convergent bounday- EQ at all depths,  all magnitudes. Extentional or Compressional depending on position:

o Thrust faults in upper part of subducting slab, o normal faults at depth

Continental collision zones show only shallow and intermediate depth EQ.

EARTHQUAKE PREDICTIONSocial impact of EQ in regions of high pop. are obvious.

If we could predict when EQ will occur, could reduce effect of quake on death toll, etc.

U.S.A., Japan and China have large programmes.

Tokyo and Peking are "at risk" cities.

Current results in prediction are poor - still need more data on EQ processes.

Prediction at several levels

(1) Most large EQs will occur at plate boundaries. If no one lived in these regions, EQs would not be socially v. important. But since people will not move, how and when will these large, plate boundary quakes occur?

(2) Earthquakes often occur in cycles.Predict that there will be a large EQ in San Francisco again. But when - within 200 years - 100% prob.; 50 years - 50% prob.; 5 years ?

(3) To be socially useful need to be PRECISE as to time and place.

To make progress need to understand:

EARTHQUAKE PRECURSORS.

Geodetic MeasurementsPrior to an EQ, does the Earth bulges due to build up of strain, etc.?

These could be detected by geodetic measurements - study of land shape, tilt and position -> periodic survey :

and GPS monitoring (but need high precision):

Studies in Japan, U.S.S.R. and elsewhere, show that there are changes of height, etc. in regions which subsequently suffer from EQs. Also water levels in wells change:

Studies show upwelling and downwelling before EQ. However, the time scale can be in 10's of years. Cannot easily predict when it will happen.

Tide-gauges can also be useful. Large retreats of sea can occur before large submarine EQ. In 1872 shock in Japan sea went out over 200 m, 20 mins. before shock.

Tide gauge data:

Continuous observation of land level - Tiltmeters (c.f. surveys) e.g. water filled tube 30-40 m. in length.

Find that increase in rate of ground tilting occurs during immediate pre-earthquake period.

Tilting produced by change in volume of rocks during deformation.

As shear rock, open up cracks - dilation - which increases volume.Big EQ occurs when cracks join up to give large fracture.

Now can use SAR to see land deformation after Earthquake (and bulging before):

7.1 magnitude Hector Mine earthquake of October 16, 1999 40 km northwest of Barstow, California (edge of Mojave Desert).

Seismic Activity In seismically active regions, the distribution of seismic activity can give clues as to where large EQs may occur.

Along San Andreas have areas with lots of small earthquakes (microearthquakes Ms approx. 1 to 3). These are continuously releasing the strain in rocks and it is unlikely that a large EQ will happen in these sections.

Some areas are "LOCKED" and show no microseismic behaviour. Expect large build-up of strain in these Locked Sections.

This will be where the big EQs occur – e.g. Loma Prieta Earthquake of 17th Oct 1989:

Earthquake filled the gap:

Major damage where the shockwaves where in soft mud – high amplitude:

Still a gap in San Fransico Area – when will it happen?

Changes in SEISMIC WAVE VELOCITY may give the answer.

When rocks deformed get changes in Vp and Vs.

Find Vp/Vs during stress build up.

Have normal Vp/Vs (often approx. 1.7), which drops prior to EQ, then shows rapid increase to original value of even greater prior to EQ.

Reason:- As stress builds up, get cracks formed. Air-filled cracks show seismic waves (P affected more than S, since effective bulk modulus reduced).

When cracks nearly all joined up, water can flow in. Water replaces air (or vacuum) and increases bulk modulus (water more incompressible than air), but not shear modulus, therefore Vp/Vs increases.

Water in cracks weakens rock so it now fails, and EQ occurs (stress corrosion, lubrication, etc.).

Theory supported by experiments on rocks. Looks promising a way of predicting EQ.

But ....

The "Dilatancy Cycle" takes longer in areas where rocks are strong - i.e. in areas where EQs are likely to be of high energy.

The time interval between Vp/Vs drop and the occurrence of EQ increases with increasing magnitude of quake.

The larger the quake, the longer the warning time - good!

The less precise the estimate of occurrence of quake - bad!

If Ms > 6, Vp/Vs anomaly occurs approx. 3 years before quake!

Other Techniques

Magnetic and electrical effects:

Low frequency magnetic signals recorded for 31 days around the 1989

Due to enhanced water connectivity and so electrical conduction.Also have strain on quartz in rock that may set up piezoelectric field.Friction might give heating and enhanced IR immision:

Noble gas escape due to cracking:Location and radon variations of 6 springs in the vicinity of

a M=2.4 earthquake a few km west of the Abant Lake end of 1990. Shown are changes relative to 1988.

animals go mad, etc.

State of Art Still hit and miss, but this is the general sequence:

Stage I

Elastic strain builds up along a fault due to plate movement: All parameters are at their normal state. No uplift, radon increase, etc.

Stage II

Cracks begin to develop in crustal rocks in the pre-quake area. The buildup begins to be visible as an uplift of the area. The cracked rocks do not propogate P-waves as easily and their velocity slows in the area. Radon gas can escape through the newly formed cracks, and electrical resitivity decreases. The newly forming cracks and increasing stress may also result in a tiny increase in local seismicity.

Stage III

Groundwater from surrounding areas can now flow into the new cracks. Because the cracks are now filled again, the P-wave velocity can increase back to normal. The ground's uplifing also ceases and radon gas emission decreases. Electrical resistivity is still decreasing.

Stage IV THE EARTHQUAKE

But….If we could predict an major earthquake, could we handle to social consequences of accurate prediction??

MAN-MADE EQS - CONTROL AND MODIFICATIONMan produces EQs in several ways.

The study of these "man-made" EQs sheds light on how it may be possible to control natural EQs and so prevent major, damaging shallow EQs.

Nuclear Explosions

During explosion naturally produce a shock

1 megaton approx. equal Mb approx. equal 6.5

Also find that explosion gives rise to a series of "after shocks". Usually small Mb < 4,but sometimes Mb > 4.

Cause: shock of explosion unsticks locked up faults, and so allows the release of stored strain energy in form of after shocks.

Conclude: Could release strain in a large fault by explosion.

Problem:- Unpredictable Not respectable (Superman stuff!)

Study of shock effects of explosions was not motivated by EQ control, but for nuclear test ban studies, etc.

How to identify a nuclear explosion?

(1) First motion studies - an explosion only generates compressive first waves, no dilational first arrivals. Fault plane solution:

(2) Usual Mb, Ms relations do not hold for nuclear explosions:

Mb = 0.56 Ms 2.9 for natural EQs not for explosions. Mb is larger than predicted for a bomb. Nuclear explosions not transformed so much into surface waves, therefore Ms smaller than Mb.

Also rock in which explosion occurs affects Mb, Ms. Alluvium adsorbs more energy than granite, therefore Mb is less in Alluvium explosion than Mb in granite.

State of Art:- approx. 90% of nuclear tests can be distinguished from natural, larger explosions easier to distinguish than those approx. <Mb = 4.

Reservoirs Now widely known that filling a dammed reservoir initiates a series of EQs, even in previously aseismic areas.

Make a lake, cause a quake!

Largest occurred in December 1967 when after filling local reservoir had Ms = 6.4 quake in Koyna, India. Killed 180 people. This occurred in area of Indian PreCambrian shield, with no previous seismic history.

Also note:-(1) Epicentres of shocks always close to reservoir.(2) Seismic activity maximum at early stages of filling. Diminishes with time.(3) Most EQs are <Ms = 4.

Causes:-(1) Increased load of water may change stress loads, therefore -> EQ?

Unlikely because stress due to loading of even 1 or 2 km. of water is small in comparison with crustal stresses.

(2) Reservoir -> increase in pore-water pressure in local rocks?More likely - water enters cracks and lubricates faults to allow strain release (c.f. Dilatancy Cycle of natural EQs).

Pumping Studies Similar EQ generation found in areas where chemical waste pumped into ground.

EQs started in previously aseismic area.

Find that often increased pumping pressure increased EQ activity. Conclude:-

(1) Increase in pore-pressure reduces shear strength of rock, therefore allows release of stored up strain energy.

(2) Water acts as lubricant (stress corrosion) and hence allows slip.

Earthquake Control Aim: To release strain in locked fault zone in a controlled manner. Hence no more damage!

Approach: Use controlled changes in pore pressure to allow limited slip:

Drill holes A -> D along fault.

-----------A---------------------B----------------------C-------------D------------

Pump out H2O at A and C to lock fault still further - dry fault zone.

Pump in H2O at B to allow limited slip (By locking at A and C will prevent catastrophic failure along whole fault)!

Now repeat, but with centre on C, etc.

Has worked in small scale experiments.

Problems: (1) don't know what pressure is needed to allow slip; (2) can't be certain that movement will be stopped at dry wells; (3) don't know what Ms of EQ will be when it is triggered, (4) relation of Energy and magnitude means that to release same amount of energy as and M=6, need to generate ~ 1000 M=4 shocks.