ISNS 4359 Earthquakes and Volcanoes (aka shake and bake)

1. ISNS 4359 Earthquakes and Volcanoes (aka shake and bake) Lecture 6: Locating EQ’s, EQ Magnitude and Intensity Fall 2005

2. 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)

5. 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

6. 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?

7. 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

8. Fig. 4.23

22. 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.

28. 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

29. 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

30. Magnitude of Earthquakes

31. Magnitude of Earthquakes

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

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

34. 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)

35. 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

36. 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

38. 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

39. 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

40. 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

41. 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)

43. Periods of Buildings and Responses of Foundations Just as waves have natural frequencies and periods, so do buildings • Periods of swaying are about 0.1 second per story • 1-story house shakes at about 0.1 second per cycle • 30-story building sways at about 3 seconds per cycle • Building materials affect building periods • Flexible materials (wood, steel)  longer period of shaking • Stiff materials (brick, concrete)  shorter period of shaking

44. Periods of Buildings and Responses of Foundations Velocity of seismic wave depends on material it is moving through • Faster through hard rocks • Slower through soft rocks • When waves pass from harder to softer rocks, they slow down • Must therefore increase their amplitude in order to carry same amount of energy  greater shaking • Shaking tends to be stronger at sites with softer ground foundations (basins, valleys, reclaimed wetlands, etc.)

45. Periods of Buildings and Responses of Foundations • If the period of the wave matches the period of the building, shaking is amplified and resonance results • Common cause of catastrophic failure of buildings

46. Earthquake Intensity – What We Feel During an Earthquake • Mercalli intensity scale was developed to quantify what people feel during an earthquake • Used for earthquakes before instrumentation or current earthquakes in areas without instrumentation • Assesses effects on people and buildings • Maps of Mercalli intensities can be generated quickly after an earthquake using people’s input to the webpagehttp://pasadena.wr.usgs.gov/shake

47. Earthquake Intensity – What We Feel During an Earthquake

48. What To Do Before and During an Earthquake • Before an earthquake: • Inside and outside your home, visualize what might fall during strong shaking, and anchor those objects by nailing, bracing, tying, etc. • Inside and outside your home, locate safe spots with protection – under heavy table, strong desk, bed, etc. • During an earthquake: • Duck, cover and hold • Stay calm • If inside, stay inside • If outside, stay outside

49. Mercalli Scale Variables Mercalli intensity depends on: • Earthquake magnitude • Bigger earthquake, more likely death and damage • Distance from hypocenter • Usually (but not always), closer earthquake  more damage • Type of rock or sediment making up ground surface • Hard rock foundations vibrate from nearby earthquake body waves • Soft sediments amplified by distant earthquake surface waves • Steep slopes can generate landslides when shaken

50. Mercalli Scale Variables