Earthquake Engineering GE / CE - 479/679

Transcription

1 Earthquake Engineering GE / CE - 479/679 Topic 4. Seismometry John G. Anderson Director February 4-6,

2 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. February 4-6,

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5 Sample seismogram from a WA Original February 4-6, vnta9201

6 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. February 4-6,

7 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. February 4-6,

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9 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. February 4-6,

10 Sample magnitude calculation February 4-6, vnta9201

11 Sample magnitude calculation P-wave - t~1.0 s February 4-6, vnta9201

12 Sample magnitude calculation P-wave t~1.0 s S-wave - t~6.0 s February 4-6, vnta9201

13 Sample magnitude calculation 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 February 4-6, vnta9201

14 Sample magnitude calculation 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= = 5.3 February 4-6, vnta9201

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17 SMA-1 Strong Motion Important because: Accelerograph 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 February 4-6,

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20 20 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.)

21 21 Digital Accelerograph

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25 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 February 4-6,

26 Some Definitions (not standard) Teleseismic - distant seismic - >30 o Some might use a smaller distance, as little as 15 o or 20 o. Regional km (5 o ) to 30 o Local - Closer than 500 km. Some might say closer than 100 km. February 4-6,

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29 MS is determined from the peak of the long period surface waves observed on a long-period seismograph. mb is determined from the peak of the P-wave on a short-period seismograph. These are defined for teleseismic observations. February 4-6,

30 Global Seismic Network Where? Magnitude scales? February 4-6,

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39 39 Isabella

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41 41 Mammoth Lakes

42 Mammoth Lakes (MLAC) 42

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49 I should replace these seismograms with some more recent records February 4-6,

50 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. February 4-6,

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56 Role of the global networks Large-scale picture of the global seismicity. February 4-6,

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61 61 Regional Networks

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64 Microwave network operated by the Seismological Laboratory to transmit seismic data to Reno. February 4-6,

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68 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. February 4-6,

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72 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 February 4-6,

73 Moment Magnitude M W =(2/3) (log M ) (exact) M W =(2/3) log M (as applied) This is the preferred magnitude scale in the seismological community. February 4-6,

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78 Summary of Important Magnitude Scales M L - Local Magnitude From peak deflection of trace recorded on Wood- Anderson seismograph - usually in the S-wave. Defined for km. Developed by Richter (1932) M S - Surface Wave Magnitude From amplitude of 20 s surface waves recorded at over 2000 km (20 o ). m b - Body Wave Magnitude From amplitude of P-wave recorded from 6 o o distance. M W - Moment Magnitude m c - Coda Magnitude m blg - Lg Magnitude February 4-6,

79 ANSS: Advanced National Seismic System February 4-6,

80 ANSS: Seismic Monitoring with a Focus on Urban Hazards Earthquake emergency response Earthquake early warning Warning of volcanic eruptions Warning of tsunamis Seismic hazard assessment Earthquake engineering Seismic research Public information Education February 4-6,

81 ANSS Equipment: Upgrade February 4-6, Current Seismic Networks Weak motion monitoring systems ~1000 modern seismographs to complete the US National Seismic Network and upgrade Regional Centers. Strong motion monitoring systems 6000 new accelerographs 3000 for urban ground motion monitoring 3000 for urban structural monitoring Upgrade Network Operation Centers and Data Management and Distribution Capabilities Sufficient operating costs

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