GY305 GEOPHYSICS
Seismology
Seismology & Seismic Waves
• Seismology is the study of the transmission of seismic wave energy
through the Earth
• 3 fundamental seismic waves
• P-wave: compressional wave
• S-wave: shear wave
• Surface wave: wave that travels along the surface of the earth
• Seismic wave transmission can me used to remotely measure
physical properties of the internal layers of the Earth:
• Transmission speed is proportional to density
• Density contrasts cause reflection and refraction according to
Snell’s law
• S-waves cannot be transmitted through a liquid
Physics of Seismic Waves
• P-wave: particle motion vibrates in the direction of wave-front
travel
• S-wave: particle motion vibrates perpendicular to the
direction of wave travel
• Surface Wave: composed of Rayleigh and Love waves:
• Rayleigh: particle motion perpendicular to ground
surface
• Love: particle motion parallel to ground surface
• P-waves and S-waves are considered “Body” waves because
they travel through the Earth’s interior
• P-waves have higher velocities and therefore arrive at
seismograph stations 1st
• S-waves have an intermediate velocity and arrive 2nd
• Surface waves are slower than P- or S- waves and therefore
arrive last
P- versus S-wave
Particle Motion
P-wave
S-wave
Rayleigh versus Love Components
of Surface Waves
Relationship between Density and
Seismic Velocity
• Density versus Seismic wave velocity at (a) 0.2 GPa, (b) 0.6 GPa, and (c) 1.0
GPa confining pressure (depths = 6, 18, and 30 km)
• Solid circles = Igneous & Metamorphic
• Open circles = Sedimentary
Earthquake Seismology Terms
• Seismograph: instrument that records the arrival of seismic
waves at the instrument location over time
• Seismic station network: global array of seismic stations
built to detect the location and magnitude of seismic
events, natural and man-made
• Epicenter: 2D location of seismic event on a map- requires
latitude & longitude
• Focal Point: 3D location- latitude, longitude, and depth
• Magnitude: measure of the release of energy from the
seismic event
Earthquake Epicentral Distance
• Because P-waves travel faster than S-waves the epicentral distance from
the seismic station may be calculated
• The time differential (∆t) is proportional to the epicentral distance
Seismic Station A
P-wave
TP=1:00:05PM
1:00:00PM
1:00:10PM
S-wave
∆t=1:00:12-1:00:05=7 seconds
TS=1:00:12PM
1:00:20PM
1:00:30PM
1:00:40PM
Graphical Plot of P- and S-Wave Epicentral
Distances
Seismic Station A
20
15
Time
(sec.)
10
∆t=7sec.
5
7sec.
0
10
20
Epicentral Distance (Km)
60
70
Plotting Epicenter Location
Seismic
Station
Epicentral
Distance
A
23 km
B
57 km
C
30 km
B
C
A
Calculation of the Time of the
Seismic Event
Seismic Event time = 1:00:05PM – 5 sec. = 1:00:00PM
•
20
Once the epicentral distance is calculated
the time of arrival of the P- or S-wave at
any of the seismic stations can be used to
calculate the time of the seismic event
15
Time
(sec.)
10
∆t=7sec.
5
P-wave travel time = 5 sec.
0
10
20
Epicentral Distance (Km)
60
70
Earthquake Magnitude
• All earthquake magnitude calculations (i,.e. Richter scale) are
derived from the below equation:
• M = Log(A/T) + q(,h) + a
• A = Amplitude of wave in 10-6 meters
• T = period of wave in seconds
• q = function correcting for () angular distance from
seismometer to epicenter, and for (h) the focal depth
• a = an empirical constant that takes into account variations
specific to the seismic station and seismic instrument
• Note the log scale – a magnitude 8 event releases
thousands of times the energy compared to a magnitude 5
event
Earthquake Magnitude Frequency
Magnitude
Number per Year
> 8.0
1
7 – 7.9
18
6 – 6.9
108
5 - 5.9
800
4-4.9
6,200
3 – 3.9
49,000
2-2.9
300,000
*Mean annual frequency of earthquakes recorded 1918-1945
(Gutenberg and Richter, 1954)
Seismic Wave Paths in the Earth
• P- and S-waves travel in curved
paths because of refraction
• Rapid density changes across
contacts may also cause
reflections
• S-waves will not transmit through
the liquid outer core
Reflection, Refraction, and Snell’s Law
• Reflected ray paths match the
incident angle indicated by the
normal to the boundary
• Example:
• Velocity medium 1 = 8.8 km/sec
• Velocity medium 2 = 6.3 km/sec
• Layer 1 incident angle = 40
• V2 * sin (1) = V1 * sin(2)
• 6.3 * sin 40 = 8.8 * sin 2
• sin 2 = 6.3/8.8 * sin(40)
• sin 2 = 0.726
• 2 = 27.4
V1=8.8km/sec
V2=6.3km/sec
1st Motion Studies and Fault Motion Solutions
• P-wave 1st arrivals at seismic stations will be either
compressional or dilational
• This will indicate the relative fault block motion along a
fracture and therefore the type of fault (normal, reverse,
dextral, sinistral)
Sinistral strike-slip
Normal Dip-slip
Reverse Dip-Slip
Dextral Strike-Slip Example of 1st Motion
• Compressional 1st motion displays as a positive “up-tick” on strip chart
• Dilational 1st motion displays as a negative “down-tick” on strip chart
• Note that 1st motion gives 2 possible fault plane solutions- you need some
knowledge of the regional geology to determine the correct fault plane
• Note that the intensity of the P-wave amplitude decreases to 0 at the nodal
plane
Example of Dextral Strike-Slip Motion on an EastWest Transform
• Solid circles are compressional
1st Motions
• Open circles are dilational 1st
motions
• Circles with crosses are lowamplitude indeterminate
Example 1st Motion Data From Dip-Slip Faults
Normal
Reverse
Example 1st Motions from Mid-Atlantic Ridge
Relationship of Seismic Wave Velocity to Earth’s Internal
Layers
• Phase changes
create rapid
density changes
• Physical state
(solid vs. liquid)
generate velocity
gradients
Potential Ray Paths due to Reflection and Refraction
• The ray path
that moves
along the layer
interface is
termed the
“Head Wave”
Seismic Reflection
• Known quantities: shot point offset and geophone
spacing
• Depth = Sqrt(((ray path dist)/2)^2-(ground dist)/2)^2)
• Ray path dist = 2-way travel time * velocity
Seismic Reflection cont.
• 2-way travel times on a horizontal surface follow a hyperbolic trend
Seismic Reflection: Fault Offset
• Fault offset produces an offset in hyperbolic curve
Consolidated Reflection Data
• Multiple Shot points are collected by computers and processed into
a reflection profile
• Below is a profile through the Rio Grande Rift displaying the top of
the rift magma chamber