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Feb 19, 2008 1John Anderson - CEE/GE 479/679 Earthquake Engineering GE / CEE - 479/679 Topic 9. Seismometry, Magnitude Scales, and Seismicity John G. Anderson.

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Presentation on theme: "Feb 19, 2008 1John Anderson - CEE/GE 479/679 Earthquake Engineering GE / CEE - 479/679 Topic 9. Seismometry, Magnitude Scales, and Seismicity John G. Anderson."— Presentation transcript:

1 Feb 19, 2008 1John Anderson - CEE/GE 479/679 Earthquake Engineering GE / CEE - 479/679 Topic 9. Seismometry, Magnitude Scales, and Seismicity John G. Anderson Professor of Geophysics

2 Feb 19, 2008 2John Anderson - CEE/GE 479/679 Key Points Seismometers are single-degree-of-freedom oscillators. Different instruments for different applications Different magnitude scales go with different instruments –ML – Wood-Anderson, local –m b – short-period, teleseismic P-waves –M S – long-period, teleseismic surface waves –M coda – local, when calibration is a problem Magnitude scales are calibrated to be similar, but are not identical –M W is now accepted as best –All other scales saturate

3 Feb 19, 2008 3John Anderson - CEE/GE 479/679 Wood-Anderson Seismograph Important because: –Principles of operation are widely used. –Basis for the magnitude scales of earthquakes that are still used today. –Provide data for early southern California earthquake catalog that is still used today.

4 Feb 19, 2008 4John Anderson - CEE/GE 479/679 Optical magnification of the motion of the mass: Record shows d(t)=M x(t) Richter (1958): M=2800 Recent reanalysis: M=2080

5 Feb 19, 2008 5John Anderson - CEE/GE 479/679 Sample seismogram from a WA Original vnta9201

6 Feb 19, 2008 6John Anderson - CEE/GE 479/679 Magnitude M L C. F. Richter was the first person to define the magnitude of an earthquake. The magnitude was defined from measurements taken using a Wood- Anderson seismogram. All subsequent magnitude scales are defined using the same principle.

7 Feb 19, 2008 7John Anderson - CEE/GE 479/679 Magnitude M L The magnitude was defined from measurements taken using a Wood- Anderson seismogram. For these examples, I use the DuHamel Integral to calculate a synthetic Wood- Anderson seismogram from digital strong motion records.

8 Feb 19, 2008 8John Anderson - CEE/GE 479/679 M L = Local Magnitude Defined by Richter in 1940’s Both amplitudes are measured peak amplitudes in mm from a standard Wood-Anderson seismogram. The amplitude of the reference earthquake is taken at the same distance. The reference earthquake: M L =3.0, A= 1.0 mm at R= 100 km.

9 Feb 19, 2008 9John Anderson - CEE/GE 479/679 To find magnitude,need to find distance. This can be done from a single record. vnta9201 Time (seconds)

10 Feb 19, 2008 10John Anderson - CEE/GE 479/679 How to estimate the distance? Use the relative speed of the P- and the S-waves. This shows the simple math behind the process. This is the origin of the rule of thumb used by seismologists for local earthquakes: multiply the s-p time (in sec) by 8 km/s, to get the approximate distance from the station to the epicenter.

11 Feb 19, 2008 11John Anderson - CEE/GE 479/679 Sample distance calculation vnta9201 P-wave - t~1.0 s

12 Feb 19, 2008 12John Anderson - CEE/GE 479/679 Sample magnitude calculation vnta9201 P-wave t~1.0 s S-wave - t~6.0 s

13 Feb 19, 2008 13John Anderson - CEE/GE 479/679 Sample magnitude calculation vnta9201 P-wave tp~1.0 s S-wave - ts~6.0 s ts-tp = (6-1) s = 5 s R~(ts-tp) * 8 km/s ~ 40 km

14 Feb 19, 2008 14John Anderson - CEE/GE 479/679

15 Feb 19, 2008 15John Anderson - CEE/GE 479/679 M L = Local Magnitude Defined by Richter in 1940’s Both amplitudes are measured peak amplitudes in mm from a standard Wood-Anderson seismogram. The amplitude of the reference earthquake is taken at the same distance. The reference earthquake: M L =3.0, A= 1.0 mm at R= 100 km.

16 Feb 19, 2008 16John Anderson - CEE/GE 479/679 This table, from the textbook Elementary Seismology by Richter (1958), gives the distance correction for the local magnitude. This shows that you need the amplitude and the distance to the earthquake to determine the magnitude.

17 Feb 19, 2008 17John Anderson - CEE/GE 479/679

18 Feb 19, 2008 18John Anderson - CEE/GE 479/679 Sample seismogram from a WA Original vnta9201

19 Feb 19, 2008 19John Anderson - CEE/GE 479/679 Sample magnitude calculation vnta9201 R~40 km Peak response = 828 mm ML=log A - log A 0 log A 0 (40 km) = -2.4 ML=log(828)+2.4 ML=2.9+2.4 = 5.3

20 Feb 19, 2008 20John Anderson - CEE/GE 479/679

21 Feb 19, 2008 21John Anderson - CEE/GE 479/679

22 Feb 19, 2008 22John Anderson - CEE/GE 479/679 Magnitude: General Comment Most magnitude scales, like M L, are tied to a certain kind of seismic instrument. Important issue: convenience of determining the magnitude from the seismograms.

23 Feb 19, 2008 23John Anderson - CEE/GE 479/679 SMA-1 Strong Motion Accelerograph Important because: –Strong motion data is the basis for all quantitative earthquake resistant design. –Most of the early strong motion data is recorded on instruments of this type or with a similar design. –Principles of operation similar to Wood Anderson

24 Feb 19, 2008 24John Anderson - CEE/GE 479/679

25 Feb 19, 2008 25John Anderson - CEE/GE 479/679

26 Feb 19, 2008 26John Anderson - CEE/GE 479/679 Digital Accelerograph

27 Feb 19, 2008 27John Anderson - CEE/GE 479/679

28 Feb 19, 2008 28John Anderson - CEE/GE 479/679 Is there a magnitude scale associated with the strong motion accelerograph? Traditionally, NO. You cannot determine the magnitude of an earthquake by reading the peak acceleration and knowing the distance. YES, in the sense that you can calculate the synthetic Wood-Anderson response easily from a digital accelerogram. ML is thus the scale most conveniently used with the accelerograph. (Above examples are done this way.)

29 Feb 19, 2008 29John Anderson - CEE/GE 479/679 More Sensitive Seismometers Uses –Teleseismic earthquake observations –Global picture of earthquake activity –Basis for Ms and mb magnitude scales –Observe microearthquakes on a regional basis

30 Feb 19, 2008 30John Anderson - CEE/GE 479/679 Some Definitions (not standard) Teleseismic - “distant seismic” - >30 o –Some might use a smaller distance, as little as 15 o or 20 o. Regional - 500 km (5 o ) to 30 o Local - Closer than 500 km. –Some might say closer than 100 km.

31 Feb 19, 2008 31John Anderson - CEE/GE 479/679

32 Feb 19, 2008 32John Anderson - CEE/GE 479/679

33 Feb 19, 2008 33John Anderson - CEE/GE 479/679

34 Feb 19, 2008 34John Anderson - CEE/GE 479/679 Short Period Long Period

35 Feb 19, 2008 35John Anderson - CEE/GE 479/679

36 Feb 19, 2008 36John Anderson - CEE/GE 479/679

37 Feb 19, 2008 37John Anderson - CEE/GE 479/679 Rayleigh Wave

38 Feb 19, 2008 38John Anderson - CEE/GE 479/679 At large distances, seismologists use travel time tables or curves, such as these. The scale goes all the way from zero distance to half way around the world on this chart (20 0 intervals). The time goes from zero to 50 minutes, in 5 minute intervals. Because the Earth is layered, there are more waves than just the P- and S- waves on this chart.

39 Feb 19, 2008 39John Anderson - CEE/GE 479/679

40 Feb 19, 2008 40John Anderson - CEE/GE 479/679 Role of the global networks Large-scale picture of the global seismicity.

41 Feb 19, 2008 41John Anderson - CEE/GE 479/679

42 Feb 19, 2008 42John Anderson - CEE/GE 479/679 Regional Networks

43 Feb 19, 2008 43John Anderson - CEE/GE 479/679

44 Feb 19, 2008 44John Anderson - CEE/GE 479/679 6

45 Feb 19, 2008 45John Anderson - CEE/GE 479/679 Microwave network operated by the Seismological Laboratory to transmit seismic data to Reno.

46 Feb 19, 2008 46John Anderson - CEE/GE 479/679

47 Feb 19, 2008 47John Anderson - CEE/GE 479/679

48 Feb 19, 2008 48John Anderson - CEE/GE 479/679

49 Feb 19, 2008 49John Anderson - CEE/GE 479/679

50 Feb 19, 2008 50John Anderson - CEE/GE 479/679 Coda Duration Magnitude Used by local networks because amplitude is unreliable, and also often clipped. Each network develops its own scale. UNR equations:

51 Feb 19, 2008 51John Anderson - CEE/GE 479/679 Seismic Moment Definition of Seismic Moment M 0 =μAD –μ is the shear modulus of the rock –A is the area of the fault on which slip takes place –D is the average slip on the fault

52 Feb 19, 2008 52John Anderson - CEE/GE 479/679 Moment Magnitude M W =(2/3) (log M 0 -16.05) (exact) M W =(2/3) log M 0 -10.73 (as applied) This is the preferred magnitude scale in the seismological community.

53 Feb 19, 2008 53John Anderson - CEE/GE 479/679 Relate M L with M W Note, moment of zero magnitude earthquake is M 0 (0)=10 16.05 dyne-cm Compare with

54 Feb 19, 2008 54John Anderson - CEE/GE 479/679

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