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## ISNS 3359 Earthquakes and Volcanoes (aka shake and bake)

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**ISNS 3359 Earthquakes and Volcanoes**(aka shake and bake) Lecture 6: Locating EQ’s, EQ Magnitude and Intensity Fall 2005**Development of Seismology**• Seismology: study of earthquakes • Earliest earthquake device: China, 132 B.C. • Instruments to detect earthquake waves: seismometers • Instruments to record earthquake waves: seismographs • Capture movement of Earth in three components: north-south, east-west and vertical • 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**Waves**• Amplitude: displacement • Wavelength: distance between successive waves • Period: time between waves • Frequency: number of waves in one second (1/period)**Seismic Waves**b Seismic waves come in two families: those that can pass through the entire Earth (body waves) and those that move near the surface only (surface waves) • Body waves: faster than surface waves, have short periods (high frequency – 0.5 to 20 Hz), most energetic near the hypocenter • Two types of body waves: • P waves and S waves**Body Waves**P (primary) waves • Fastest of all waves • Always first to reach a recording station (hence primary) • Move as push-pull – alternating pulses of compression and extension, like wave through Slinky toy • Travel through solid, liquid or gas • Velocity depends on density and compressibility of substance they are traveling through • Velocity of about 4.8 km/sec for P wave through granite • Can travel through air and so may be audible near the epicenter**Body Waves**S (secondary) waves • Second to reach a recording station (after primary) • Exhibit transverse motion – shearing or shaking particles at right angles to the wave’s path (like shaking one end of a rope) • Travel only through solids • S wave is reflected back or converted if reaches liquid • Velocity depends on density and resistance to shearing of substance • Velocity of about 3.0 km/sec for S wave through granite • Up-and-down and side-to-side shaking does severe damage to buildings**Seismic Waves and the Earth’s Interior**• Waves from large earthquakes can pass through the entire Earth and be recorded all around the world • Waves do not follow straight paths through the Earth but change velocity and direction as they encounter different layers • From the Earth’s surface down: • Waves initially speed up then slow at the asthenosphere • Wave speeds increase through mantle until reaching outer core (liquid), where S waves disappear and P waves suddenly slow • P wave speeds increase gradually through outer core until increasing dramatically at inner core (solid)**Surface Waves**• Surface waves • Travel near the Earth’s surface, created bybody waves disturbing the surface • Longer period than body waves (carry energy farther) • Love waves • Similar motion to S waves, but side-to-side in horizontal plane • Travel faster than Rayleigh waves • Do not move through air or water • Rayleigh waves • Backward-rotating, elliptical motion produces horizontal and vertical shaking, which feels like rolling, boat at sea • More energy is released as Rayleigh waves when the hypocenter is close to the surface • Travel great distances**Sound Waves and Seismic Waves**• Seismologists record and analyze waves to determine where an earthquake occurred and how large it was • Waves are fundamental to music and seismology • Similarities: • More high frequency waves if short path is traveled • Trombone is retracted, short fault-rupture length (small earthquake) • More low frequency waves if long path is traveled • Trombone is extended, long fault-rupture length (large earthquake)**Seismic Velocity**• Seismic velocity is a material property (like density). • There are two kinds of waves – Body and Surface waves. • There are two kinds of body wave velocity – P and S wave velocities. • P waves always travel faster than S waves. • Seismic velocities depend on quantities like chemical composition, pressure, temperature, etc. • Faster Velocities • Lower temperatures • Higher pressures • Solid phases • Slower Velocities • Higher temperatures • Lower pressures • Liquid phases**Locating the Source of an Earthquake**• P waves travel about 1.7 times faster than S waves • Farther from hypocenter, greater time lag of S wave behind P wave (S-P) • (S-P) time indicates how far away earthquake was from station – but in what direction?**Locating the Source of an Earthquake**• Need distance of earthquake from three stations to pinpoint location of earthquake: • Computer calculation • Visualize circles drawn around each station for appropriate distance from station, and intersection of circles at earthquake’s location • Method is most reliable when earthquake is near surface**Solution to epicenter and hyopcenter**• Mathematically, the problem is solved by setting up a system of linear equations, one for each station. • The equations express the difference between the observed arrival times and those calculated from the previous (or initial) hypocenter, in terms of small steps in the 3 hypocentral coordinates and the origin time. • We must also have a mathematical model of the crustal velocities (in kilometers per second) under the seismic network to calculate the travel times of waves from an earthquake at a given depth to a station at a given distance. • The system of linear equations is solved by the method of least squares which minimizes the sum of the squares of the differences between the observed and calculated arrival times. • The process begins with an initial guessed hypocenter, performs several hypocentral adjustments each found by a least squares solution to the equations, and iterates to a hypocenter that best fits the observed set of wave arrival times at the stations of the seismic network.**Magnitude of Earthquakes**• Richter scale • Devised in 1935 to describe magnitude of shallow, moderately-sized earthquakes located near Caltech seismometers in southern California • Bigger earthquake greater shaking greateramplitude of lines on seismogram • Defined magnitude as ‘logarithm of maximum seismic wave amplitude recorded on standard seismogram at 100 km from earthquake’, with corrections made for distance • For every 10 fold increase in recorded amplitude, Richter magnitude increases one number**Magnitude of Earthquakes**• Richter scale • With every one increase in Richter magnitude, the energy release increases by about 45 times, but energy is also spread out over much larger area and over longer time • Bigger earthquake means more people will experience shaking and for longer time (increases damage to buildings) • Many more small earthquakes each year than large ones, but more than 90% of energy release is from few large earthquakes • Richter scale magnitude is easy and quick to calculate, so popular with media**Magnitude of Earthquakes**21,688 earthquakes recorded by NEIC in 1998 http://www.iris.iris.edu/volume2000no1/RevFigure2.big.gif**Magnitude of Earthquakes**21,688 earthquakes recorded by NEIC in 1998 http://www.iris.iris.edu/volume2000no1/RevFigure2.big.gif**Other Measures of Earthquake Size**• Richter scale is useful for magnitude of shallow, small-moderate nearby earthquakes • Does not work well for distant or large earthquakes • Short-period waves do not increase amplitude for bigger earthquakes • Richter scale: • 1906 San Francisco earthquake was magnitude 8.3 • 1964 Alaska earthquake was magnitude 8.3 • Other magnitude scale: • 1906 San Francisco earthquake was magnitude 7.8 • 1964 Alaska earthquake was magnitude 9.2 (100 times more energy)**Other Measures of Earthquake Size**Two other magnitude scales: • Body wave scale (mb): • Uses amplitudes of P waves with 1 to 10-second periods • Surface wave scale (ms): • Uses Rayleigh waves with 18 to 22-second periods • All magnitude scales are not equivalent • Larger earthquakes radiate more energy at longer periods not measured by Richter scale or body wave scale • Richter scale and body wave scale significantly underestimate magnitudes of earthquakes far away or large**Moment Magnitude Scale**• Seismic moment (Mo) • Measures amount of strain energy released by movement along whole rupture surface; more accurate for big earthquakes • Calculated using rocks’ shear strength times rupture area of fault times displacement (slip) on the fault • Moment magnitude scale uses seismic moment: • Mw = 2/3 log10 (Mo) – 6 • Scale developed by Hiroo Kanamori**Foreshocks, Main Shock and Aftershocks**• Large earthquakes are not just single events but part of series of earthquakes over years • Largest event in series is mainshock • Smaller events preceding mainshock are foreshocks • Smaller events following mainshock are aftershocks • Large event may be considered mainshock, then followed byeven larger earthquake, so then re-classified as foreshock**Magnitude, Fault-Rupture Length and Seismic-Wave Frequencies**• Fault-rupture length greatly influences magnitude: • 100 m long fault rupture magnitude 4 earthquake • 1 km long fault rupture magnitude 5 earthquake • 10 km long fault rupture magnitude 6 earthquake • 100 km long fault rupture magnitude 7 earthquake**Magnitude, Fault-Rupture Length and Seismic-Wave Frequencies**• Fault-rupture length and duration influence seismic wave frequency: • Short rupture, duration high frequency seismic waves • Long rupture, duration low frequency seismic waves • Seismic wave frequency influences damage: • High frequency waves cause much damage at epicenter but die out quickly with distance from epicenter • Low frequency waves travel great distance from epicenter so do most damage farther away**Ground Motion During Earthquakes**• Buildings are designed to handle vertical forces (weight of building and contents) so that vertical shaking in earthquakes is typically safe • Horizontal shaking during earthquakes can do massive damage to buildings • Acceleration • Measure in terms of acceleration due to gravity (g = 9.8 m/s2) • Weak buildings suffer damage from horizontal accelerations of more than 0.1 g • In some locations, horizontal acceleration can be as much as 1.8 g (Tarzana Hills in 1994 Northridge, California earthquake)