Geos 218: Earthquakes & Seismology PDF

Geos 218: Geological Disasters & Society Unit 4b: Earthquakes Seismology P-waves, S-waves, Surface waves Locating EQs Magnitude vs Intensity (Unit 5a) 1 Deforming Earth’s Crust 3 Types of stress: Extensional Compressional Shear Results in 3 kinds of faults: Nor...

Geos 218: Geological Disasters & Society Unit 4b: Earthquakes Seismology P-waves, S-waves, Surface waves Locating EQs Magnitude vs Intensity (Unit 5a) 1 Deforming Earth’s Crust 3 Types of stress: Extensional Compressional Shear Results in 3 kinds of faults: Normal Thrust or Reverse Strike slip Earthquakes occur on faults 2 1  The Rupture Area Faults are complex zones of breakage with irregular surfaces, many miles wide and long n Stress builds up until rupture occurs at weak point and propagates along fault surface n Point where rupture first occurs is hypocenter or focus n Point directly above hypocenter on surface is epicenter Fig. 3.12 3 Extensional Stress Compressional Stress = Normal Fault = Thrust/Reverse Fault Shear Stress = Strike-Slip Fault (right-lateral shown) 4 2  Identifying Faults: Right or Left? Strike-slip faults: n Dominated by horizontal movement n When looking across a fault, if the far side moved towards the right, it is a right-lateral fault n When looking across a fault, if the far side moved towards the left, it is a left-lateral fault n Convention works in either direction n Which is like the San Andreas? 5 Development of Seismology n Seismology: study of earthquakes n Earliest earthquake device: China, 132 B.C. n Instruments to detect earthquake waves: seismometers n Instruments to record earthquake waves: seismographs n Capture movement of Earth in three components: north-south, east-west and vertical n One part stays as stationary as possible while Earth vibrates: heavy mass fixed by inertia in frame that moves with the Earth, and differences between position of the frame and the mass are recorded digitally Figure 3.16 6 3  Development of Seismology Waves: n Amplitude: displacement n Wavelength: distance between successive waves n Period: time between waves (= 1/frequency) n Frequency: number of waves in one second Figure 3.17 7 Seismic Waves Seismology - the study of earthquakes, earth structure and seismic waves Surface waves travel along Earth’s surface Body waves travel inside Earth 8 4  Types of seismic waves P wave Fast S wave Intermediate Surface Waves Slow 9 http://www.iris.edu/hq/inclass/animation/1component_seismogram_building_responds_to_p_s_surface_waves 10 5  Quick Review Which travel the fastest? A. P-waves B. S-waves C. Surface waves D. All of these are the same 11 A seismograph is the record of ground motion produced by the earthquake seismometer 12 6  Seismometers typically measure vibrations in all 3 directions (N/S, E/W and Up/Down) ß seismometer seismographs 13 Seismic Waves n Waves from large earthquakes can pass through the entire Earth. Waves do not follow straight paths through the Earth but change velocity and direction as they encounter different layers of different density Fig. 3.19 14 7  https://www.iris.edu/hq/inclass/animation/3component_seismogram_records_seismicwave_motion 15 On-Line Lecture n Part 1: Earthquake basics n Part 2: Faults (3 kinds) n Part 3: Seismic Waves n Part 4: Locating Earthquakes n Part 5: Magnitude vs Intensity 16 8  Locating the Source of an Earthquake P waves travel about 1.7 times faster than S waves Farther from hypocenter, greater lag time of S wave behind P wave (S-P) Figure 3.22 17 Travel Time Graph P waves are P wave faster than S wave S waves P waves arrive before S waves Difference S-P interval in the arrival depends on times of P & S waves distance to increases as earthquake the distance increases 18 9  https://www.iris.edu/hq/inclass/video/travel_time_curves_described 19 http://www.iris.edu/hq/inclass/animation/4station_seismograph_network_records_a_single_earthquake 20 10  Locating the Source of an Earthquake (S-P) time indicates how far away earthquake was from station – but in what direction? 5 minutes = ~3500 km 11 minutes = ~9100 km Figure 3.21 21 Locating the epicenter of an earthquake Use the S-P San Francisco time to determine the distance of Denver earthquake from three seismic stations Intersection of circles determines epicenter Tucson 22 11  Locating an EQ Must determine distance of EQ from at least three seismic stations. Intersection of the circles gives the location. Fig. 3.23 23 On-Line Lecture n Part 1: Earthquake basics n Part 2: Faults (3 kinds) n Part 3: Seismic Waves n Part 4: Locating Earthquakes n Part 5: Magnitude vs Intensity (next time) 24 12