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 ∆t=1:00:12-1:00:05=7 seconds P-wave S-wave TP=1:00:05PM TS=1:00:12PM 1:00:00PM 1:00:10PM 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 Epicentral Station Distance A 23 km B 57 km B C 30 km C A Calculation of the Time of the Seismic Event Seismic Event time = 1:00:05PM – 5 sec. = 1:00:00PM • Once the epicentral distance is calculated 20 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 V1=8.8km/sec •sin 2 = 6.3/8.8 * sin(40) •sin 2 = 0.726 • 2 = 27.4 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 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.