Geology of the Hawaiian Islands Class 21 30 March 2004 Any Questions? 100 100 100 96 A B C D F Exam Scores 95 94 94 90 85 83 83 83 Mean 72 67 61 59 59 55 54 41 Mean = 78.5 Median = 83 Chapter 4 Earthquakes and Tsunamis 1
Rock Deformation Large scale deformation of the Earth s crust = Plate Tectonics Smaller scale deformation = structural geology Deformation Changes in volume or shape of a rock body = strain Stress The force that acts on a rock unit to change its shape and/or its volume Causes strain or deformation 2
Compression Action of coincident oppositely directed forces acting towards each other Shear Action of coincident oppositely directed forces acting parallel to each other across a surface in a couple Tension Action of coincident oppositely directed forces acting away from each other Kinds of deformation Elastic vs Plastic Brittle vs Ductile 3
Rocks are: Elastic and brittle near the earth's surface More plastic and ductile deeper in the crust Because of the increasing temperature and pressure Factors that affect deformation Temperature Pressure Folds Most common ductile response to stress on rocks in the earth's crust Strain rate Rock type The variation of these factors determines if a rock will fault or fold. 4
Folding Due to Compressive Stress 5
Fracture Most common brittle response to stress With No displacement = Joint With displacement = Fault Joints commonly form when the surface of a volcanic rock cools and contracts 6
Faults Occur when large stresses build up in the crust, often due to lava movement into shallow magma chambers Classified according to the kind of movement that has occurred along them Normal Fault Hanging wall drops down 7
Reverse Fault Hanging wall moves up Tom Bean Strike-slip Fault 8
Faults Most faults in Hawaii are normal faults Graben are common 9
Faults may move several meters in a second or so This movement within the earth's crust usually generates an earthquake Wildrose Graben,, Southern California 10
Volcanic earthquakes Caused by the movement of lava up toward the surface More common type in Hawaii than tectonic quakes Generally much smaller in size than tectonic quakes Tectonic earthquakes Most common kind Focus = point where an earthquake originates 11
Epicenter = location at the surface directly above the focus Many different kinds of earthquake waves They travel at different speeds and with different kinds of particle motions Earthquake energy moves radially away from the focus Most common types of earthquake waves: P-waves and S-waves Primary waves travel the fastest in the crust and usually are the first waves to arrive Secondary (or ShearS hear) ) waves are slower and therefore take longer to arrive 12
Seismographs record earthquakes P-waves are compressional Waves S-waves are Shear Waves P-waves travel faster than S-waves, so they arrive at the recording station sooner 13
Difference in traveltime for P and S waves tells us how far away the earthquake is from the recording station We need 3 recording stations to determine the location of the epicenter Earthquake Magnitude Describes the strength of an earthquake Magnitude scale is exponential -- magnitude 6 has 10 times greater amplitude of ground shaking than magnitude 5 14
Quake magnitude related to size of P and S wave amplitude and distance from quake Earthquake Magnitude Earthquakes can be very destructive if they are large enough There have been several historic earthquakes in Hawaii that have caused destruction due to ground shaking or collapse Earthquake Magnitude In Hawaii most earthquakes are rather small (M < 3) Associated with the volcanoes Small magnitude quakes not very destructive Homework #7 Due a week from today, April 6th Find it on the web site 15
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Tsunami Series of very long-wavelength waves on the ocean 17
Tsunami = "tidal wave" Tsunami has nothing to do with tides Tsunami waves Very small out in the open ocean Amplitude of only ~ 1 meter Very long wavelengths (up to 100 km) Travel very fast (as much as 500 mph) Video clip: Tsunamis 18
Tsunami waves When waves reach shallow water, they "feel" the shallow bottom, just like ordinary waves, and they slow down (to 20-30 mph) Because of the massive energy, this slowdown causes them to build up very high (up to 50-100 m) Tsunami can be generated by any process that displaces a large amount of water: Sub-sea earthquakes Giant landslides Volcanic eruptions Meteorite impacts 19
Runup Maximum height above sea level reached by a tsunami when it reaches shore Hilo after 1946 tsunami Inundation Horizontal distance from the normal water's edge reached by a tsunami Kaiaka Bay 1952 Tsunami wave passing under bridge at Haleiwa 20
1946 tsunami -- Hilo harbor: 159 fatalities Beach in Laie, O`ahu before tsunami generated by 1957 quake in Aleutians 21
1983 Japan tsunami -- water recedes 1983 tsunami in Japan 104 people drowned 700 boats, 59 houses destroyed $800 Million property damage 22
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Questions? Thursday Web class No formal class meeting S.E. O`ahu Field Trip Saturday, April 3rd 9 am 3 pm Meet Here at 9:00 am We will see Diamond Head, Hanauma Bay, Koko Crater, Castle Junction Bring water, hat, sun screen, walking shoes Next Tuesday Coasts and beaches Read Chapter 10 24