gy305 lecture4 seismology - southalabama.edu strike-slip example of 1st motion • compressional 1st...
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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
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
1:00:00PM 1:00:10PM 1:00:20PM 1:00:30PM 1:00:40PM
P-wave S-wave
TP=1:00:05PM TS=1:00:12PM
∆t=1:00:12-1:00:05=7 secondsSeismic Station A
Graphical Plot of P- and S-Wave Epicentral Distances
Epicentral Distance (Km)
Time (sec.)
0 10 6020 70
5
10
15
20
∆t=7sec.
7sec.
Seismic Station A
Calculation of the Time of the Seismic Event
Epicentral Distance (Km)
Time (sec.)
0 10 6020 70
5
10
15
20
∆t=7sec.
• 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
P-wave travel time = 5 sec.
Seismic Event time = 1:00:05PM – 5 sec. = 1:00:00PM
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 17 – 7.9 186 – 6.9 1085 - 5.9 8004-4.9 6,2003 – 3.9 49,0002-2.9 300,000
*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 East-West Transform
• Solid circles are compressional 1st Motions
• Open circles are dilational 1st
motions• Circles with crosses are low-
amplitude indeterminate
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