EARTHQUAKES AND SEISMOLOGY
An earthquake is a naturally produced shock event which causes the Earth to vibrate.Have seen how can
use seismic waves to study Earth's interior. Here look more at processes associated with quakes
themselves.
Of interest are:
their effect
why they occur
where they occur
what the resulting vibrations can tell us about the structure of the Earth.
EQs (earthquakes) are very common, many thousands per day all over the world.
Most are weak (low energy) and undetected.
Large energy EQs are rare but cause wide scale social disruption.
e.g.
July 1976
May 1970
May 1927
Sept. 1923
Tangshan, China
Lima, Peru
Nan-Shan, Chian
Tokyo, Japan
650,000 fatalities
66,000 fatalities
200,000 fatalities
143,000
fatalities
EQs result from sudden displacement of crust, usually along faults.
Often come in a series of quakes - "minor" aftershocks common.
Whole Earth vibrates (c.f. bell)
Shocks give rise to landslips, turbidity currents, tsunamis, etc.
Discuss -
measurement and waves
locations and distribution
size: magnitude and intensity
cause, control and prediction
Measurement and nature 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 (pivots at ┴).
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.
Seismic waves are acoustic waves produced by the rapid release of strain energy (c.f. being produced by
bursting balloon).
As we have seen, in a planetary size body can find four types of seismic waves, two body waves and two
surface waves.
P-waves are longitudinal waves, particles move in the direction of travel of wave. P = primary (travel
fastest) or Push-Pull.
S-waves are transverse waves, that shear the rock.
Vp =  [K + 4/3µ / ρ]
Particles move at ┴ to motion of wave.
Vs =  [µ / ρ]
Since K > 0, Vp > Vs, P = primary. S = secondary or shear
Love waves (L-waves) are shear waves, with displacement within plane of surface.
Rayleigh waves (R-waves) like ocean waves, particles in medium through which wave travels describes
an elliptical path.
Typical P wave velocity near surface is approx. 5-6 kms-1, S waves travel at approx. 3-4 kms-1. R and L
waves are slowest of all.
Earthquake location and distribution
Can quite easily determine latitude and longitude of EQ.
The source - a small part of a fault (approx. a few kms3) - can be regarded as a point - the FOCUS. The
point on the surface, vertically above the focus is the EPICENTRE.
Shock waves radiate out from focus in all directions. They are first felt at the surface at the 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 = X / Vp
Time for S wave arrival = X / Vs
Therefore interval between P and S arrival (_ t)
_ t = X / Vs - X / Vp
Thus re-arranging to find X gives
X = Vs.Vp._t / (Vp - Vs)
N.B. Units - Vp,s in kms-1; _t 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 can also be deduced, but needs a more refined approach.
All earthquakes occur at depths < 700 km.
Have 3 classifications of EQs.
Shallow focus
Intermediate focus
Deep focus
0-70 km
70-300 km
300-700 km
With large EQs (magnitude > 7 - see later)
75%
Shallow
90%
Int.
Approx. 100% Deep
)
) occur around margins of Pacific
)
Those not in circum-Pacific belt occur in Alpine-Himalayan belt. Also high concentration of shallow EQs
(mag. 4-6) along ocean ridge system. Intra-plate EQs are rare (but do occur, e.g. Lisbon 1755). Shallow
EQs much more common than Int. and Deep. Most surface damage is caused by Shallow EQs.
EARTHQUAKE SIZE : MAGNITUDE AND INTENSITY
EQ magnitude is an "ABSOLUTE" measure of size, and is related to the energy released. Determined
from seismic wave amplitude. EQ intensity refers to amount of damage caused - somewhat arbitrary.
Magnitude, given by general equation:M = log A/T + af(_,h) + b
M = magnitude
log - base 10
A = max. amplitude of wave in µm (= 10-6m)
T = period of wave, in secs. (time taken for one wavelength to pass a fixed point)
f ( ) = a function of ...
_ = distance from point of measurement to epicentre, as measured by the angle subtended by these 2
points at the centre of the Earth - the epicentre angle:The function f takes account of the reduction in amplitude with distance due to spreading and absorption
of wave, generally called ATTENUATION. f is found by observation and theory.
N.B. Magnitude scale is Logarithmic, therefore M = 7.0 is 100 times larger than M = 5.0
Richter was amongst first to develop a scale for earthquakes in 1935. Others have been developed
since. Now usually use an equation of Bath (1967) for SURFACE-WAVES, with periods of approx. 20 s
for SHALLOW-FOCUS EQs.
Ms
surface-wave
magnitude
=
log (A/T) + 1.66 log _ + 3.3
Surface wave is usually Rayleigh-wave. This relation is used for observations near epicentre, where
surface-wave amplitude is usually larger than body-wave.
At large distances from epicentre use body-wave amplitude (usually P-wave), because body-wave
attenuation is less than surface-waves.
Mb
body-wave
magnitude
=
log (A/T) + 0.01_ + 5.9
where T approx. 12 secs.
For shallow-focus EQs can related Ms and Mb ->
Mb
=
0.56 Ms + 2.9
N.B. Since "a", "b" and f ( ) in master equation need to be refined from observation the "absolute"
measure of EQ magnitude is still subject of debate. Error in magnitude should be < ± 0.3.
An empirical relation between ENERGY and MAGNITUDE is given by:
log E = 4.8 + 1.5Ms
where E is in Joules (Man-made explosions - calibration).
An increase in magnitude by 1.0 units gives approx. x 30 increase in energy. Estimated annual energy
loss via EQs is 1018J. Most of this comes from the few quakes with Ms = 7 or 8.
Energy of EQ is turned largely into heat via friction of vibrating rock particles, etc. Total contribution of
EQ to heat loss is small. Total Earth heat flow approx. 1021J per y. Total U.S.A. energy consumption
approx. 1019J y-1!
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.
EARTHQUAKE PREDICTION
Social 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. imp. But since people will not move, how and when will these large, plate boundary
quakes occur?
(2)
Predict that there will be a large EQ in San Francisco. 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.
Need to understand EARTHQUAKE PRECURSORS.
Geodetic Measurements
Prior to an EQ, does the earth bulge due to build up of strain, etc.?
These could be detected by geodetic measurements - study of land shape, tilt and position:- periodic
survey.
Studies in Japan, U.S.S.R. and elsewhere, show that there are changes of height, etc. in regions which
subsequently suffer from EQs.
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.
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.
N.B.: No EQ as yet PREDICTED by this method.
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. But when?
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 effects, noble gas escape, animals go made, etc.
State of Art
Still hit and miss. Could we handle to social consequences of accurate prediction??
MAN-MADE EQS - CONTROL AND MODIFICATION
Man 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!)
N.B.: Study of shock effects of explosions 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. Nuclear explosions not
transformed so much into surface waves, therefore Ms smaller than Mb.
Problems:- 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)
(2)
(3)
Causes:(1)
(2)
Epicentres of shocks always close to reservoir.
Seismic activity maximum at early stages of filling. Diminishes with time.
Most EQs are <Ms = 4.
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.
Reservoir -> increase in pore-water pressure in local rocks. 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.