GY111 Physical Geology Earthquakes and Seismic Waves Earthquake Mechanisms Brittle Mechanical Model: stick-slip Focal point: 3D point inside the lithosphere where the seismic event occurs Epicenter: projection of focal point to the map surface
Seismic Energy Release Radiates from focal point. P-waves: compressional (fastest). S-waves: shear. Surface waves: move only on surface (slowest);
cause damage to structures. Seismic Wave Mechanics P-waves always travel faster than S-waves. Surface waves are slowest. Seismic Wave Summary P(compressional)-waves: move fastest (7-10/sec), vibration direction parallel to wave
path. Transmitted through all materials. S(shear)-waves: intermediate speed (4-6 km/sec), vibration direction perpendicular to wave path. Can only be transmitted through solid material. Surface waves: move along rock/air interface, created by energy transfer from P wave and S waves. These waves cause damage to manmade structures. Seismic Wave Reflection P- and S-waves will reflect off of a surface
that represents a density contrast. An example would be the crust/mantle boundary (2.8 vs. 3.1 g/cm3 respectively) When waves reflect they will do so at the same angle as their angle of incidence. Incidence angle D1 D2 Reflection
angle D2>D1 Seismic Wave Refraction Refraction: the angular bend of a wave that passes density boundary. Why is the path in the adjacent diagram curved? As density increases with depth so http://www.geometrics.com/
does transmission velocity applications/frequency-askedquestions/seismic-refraction/ Incidence angle D1 D2 Refraction angle
D2>D1 Snells Law Snells Law: calculates the refracted angles of incidence across different velocity layers. Note that at some critical angle i that r = 90 so all wave energy is reflected Suppose v1 = 5000m/sec; v2
=10000m/sec and i = 30 degrees therefore: Sin r = sin i (v2/v1) = 0.5(2.0) r = Arcsine(1.0) = 90 degrees Locating the Epicenter Requires readings from 3 seismic stations at 3 different geographic locations
Seismic Moment Magnitude Richter scale is similar Measured from deflection of pen on seismograph Earthquakes & Plate Tectonics Distribution of epicenters outline plate boundaries Focal Depth Deep focal point earthquakes occur only in subduction zones Only shallow focal points are found along divergent ocean ridge systems
Focal Point Depth and Plate Tectonics Divergent Boundaries: only shallow depth seismic events (< 7km). Transform Boundaries: shallow to intermediate (1 35 km). Convergent Boundaries: shallow to deep focal points (1 700 km). Deep Focal Point Seismicity
The lithosphere is normally a maximum of 35 km depth so deep earthquakes should be impossible. The fact that 700 km deep seismicity occurs can only be explained by subduction moving brittle material rapidly into the mantle. Subduction Zone Dip Angle The geographic distribution of focal point depths determines the dip angle of the
subductions zone: If shallow, intermediate, and deep focal point earthquakes occur in a narrow zone the dip of the subduction zone is steep. If shallow, intermediate, and deep focal point earthquakes occur in a wide belt the dip on the subduction zone is shallow. Subduction Zone Dip Examples Tonga Trench (A): shallow, int., and deep focal points occur in a narrow belt.
Andean subduction zone (B): shallow, int. and deep focal points occur in a broad belt. A=Steep B=Shallow Tectonic Significance of Dip Variation on Subduction Zones Steep subduction zones occur when old
(150 Ma) ocean lithosphere cools and become too dense to float on the asthenosphere. Shallow subduction occurs when young buoyant ocean lithosphere is forced to subduct under continental lithosphere. The ocean lithosphere underplates the continent causing unusually high mountains (Andes). Earthquake Damage
Direct destruction via surface waves. Landslides and ground failure. Tsunami. Fires. Tsunamis and Plate Tectonics
Most tsunamis are generated at convergent plate boundaries. The subducted oceanic plate can have sudden vertical displacement of several tens of meters that moves the entire water column (3-10 km) vertically up or down. The generated tsunami radiates from the focal point at approximately 500 mph and may circle the globe several times before dissipating.
Tsunami Video Note that in the tsunami video there are not any giant breaking waves, rather tsunamis are more like a sudden rise in sea level everywhere along the shoreline. A major tsunami is preceded by a sudden drop in sea level exposing the sea floor. Just as destructive as the advance of sea water is the backwash movement as the tsunami recedes. Tsunamis may occur in several cycles the first advance is not necessarily the most intense. https://www.youtube.com/watch?v=wyOPau0gpFw
Tsunami Wave Physics Velocity = 800 kph (500 mph) Wavelength = 200 km in open ocean Amplitude = 1-3 meters in open ocean As the wave begins to feel bottom approaching the shoreline velocity drops to 80 kph (50 mph) but amplitude (wave height) grows to 30 m (100 feet) or more. Funnel shaped embayments may increase wave height even more.
Earthquake Damage Factors Bedrock composition or lack of bedrock Construction material and design.
Proximity to focal point (focal depth). Earthquake magnitude. Structure periodicity. Bedrock Composition Crystalline bedrock (metamorphic or igneous) is most resistant to earthquake damage. Soft-sediments or rocks with an inherent weakness (bedding, cleavage, joint fractures)
are unstable during seismic wave vibration and may undergo liquefaction. Liquefaction: where a solid material transitions to liquid behavior because of some external factor (seismic waves). Construction Material and Design Earthquake resistant structures contain an internal structural frame that can elastically bend to dissipate seismic wave energy (steel frame buildings).
Brick and Cinder Block construction does not fare well under seismic surface waves. Wood frame buildings potentially are resistant if designed properly but this is not generally the case. If seismic surface wavelength matches the periodicity of the structure it will not survive even low magnitude earthquakes. Proximity to Focal Point Focal points range from surface level to
>700 km. Deep earthquakes allow for energy to dissipate before reaching the surface. Rocks become stronger at greater depth therefore it takes massive stress levels to cause fault slip therefore most large magnitude earthquakes (>8.0) originate as deep focal points. Earthquake Magnitude Magnitudes are measured by how violently
a recording pen is deflected on a seismograph. Magnitude scales are exponential each unit increase is 30 times the energy of lower magnitude. The point on the fault surface where the energy release begins is termed the focal point. Structure Periodicity All structures have a vibration periodicity that is
the time period of vibrations or rocking. If a seismic surface wave period matched the period of a building the building is doomed to fail. You can think of periodicity as the period of pendulum the longer the pendulum arm the longer the period, For most seismic waves a 3-4 storie building matches the periodicity of seismic surface waves.
Disaster Management Problems Transportation network destroyed. No emergency services. Power grid down. Many casualties. No communication. Food and water supplies limited. Sanitation difficult.
U.S. Seismic Risk Proximity to active fault zones. Nature of bedrock. Proximity of populations centers and infrastructure. Building codes. World Seismic Risk Exam Summary Know differences between P-, S-, and surface waves.
Be familiar with the stick-slip theory of earthquake propagation. Know how epicenters and focal points are located with seismic data. Be familiar with the association of earthquake types with various plate tectonic boundaries. Be familiar with the differences between wind-generated ocean waves and seismic sea waves (Tsunamis). Be familiar with reflection and refraction and Snells Law.
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