ISNS 3359 Earthquakes and Volcanoes (aka shake and bake)

1. ISNS 3359 Earthquakes and Volcanoes (aka shake and bake) Lecture 6: Locating EQ’s, EQ Magnitude and Intensity Fall 2005

2. 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

3. Waves • Amplitude: displacement • Wavelength: distance between successive waves • Period: time between waves • Frequency: number of waves in one second (1/period)

4. 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

5. 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

6. Body Waves

7. 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

8. Seismic Waves

9. 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)

10. Seismic Waves and the Earth’s Interior

11. 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

12. 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)

14. 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

15. 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?

16. 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

17. Fig. 4.23

31. 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.

37. 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

38. 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

39. Magnitude of Earthquakes

40. Magnitude of Earthquakes

41. Magnitude of Earthquakes 21,688 earthquakes recorded by NEIC in 1998 http://www.iris.iris.edu/volume2000no1/RevFigure2.big.gif

42. Magnitude of Earthquakes 21,688 earthquakes recorded by NEIC in 1998 http://www.iris.iris.edu/volume2000no1/RevFigure2.big.gif

43. 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)

44. 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

45. 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

47. 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

48. 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

49. 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

50. 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)