Measuring EQ, Tsunamis

Transcription

1 16 June 16 Summer Session 2016 Horst Rademacher HH Lect 6: Earth s Interior Measuring EQ, Tsunamis

2 Recap from Midterm Questions 4: Question 8: Question 15: Plate tectonics Wastewater Injection 4-10 km/sec

3 HH Any Questions?

4 Exploring Earth s Interior If Earth had no internal structure EQ focus Pressure, temperature increases with depth Seismic waves go straight through Seismic waves are refracted (bent)

5 Exploring Earth s Interior Example I Andrija Mohorovičić ( ) discovered in 1910 the boundary between Crust and Mantle, the Moho Thin under Oceans, much thicker under continents

6 Exploring Earth s Interior Example II Observation of shadow zones reveals liquid outer core P-wave S-wave

7 Exploring Earth s Interior complete Structure of the Earth

8 Exploring Earth s Interior Naming of Seismic Phases

9 Exploring Earth s Interior Show seismic wave animation

10 HH Any Questions?

11 Computerized Axial Tomography CAT- Scan Exploring Earth s Interior.. in our own backyard

12 Exploring Earth s Interior.. Seismic Tomography

13 Seismic Tomography Exploring Earth s Interior.. in our own backyard Examples

14 Seismic Tomography Exploring Earth s Interior.. in our own backyard Richard Allen, BSL

15 Exploring Earth s Interior.. Seismic Tomography Core-Mantle Boundary under Africa and S-Atlantic

16 Exploring Earth s Interior.. Seismic Tomography under Western North America Karin Sigloch, Karlsruhe

17 Exploring Earth s Interior East Asia: Himalayas to Japan Min Cheng, Rice University

18 Exploring Earth s Interior Barbara Romanowicz, BSL

19 HH Any Questions?

20 How big is an earthquake? We have to define what we mean by BIG How strong is it? How much force (acceleration) does it generate? Loma Prieta, 1989 M=6.9 Kobe, Japan, 1995, M=6.8

21 How big is an earthquake? We have to define what we mean by BIG How many inches the needle of a seismograph swings?

22 How big is an earthquake? We have to define what we mean by BIG How many people died? Dollar value of damage? Insured losses Size of rupture area on the fault? Size of area where EQ was felt? None of these measure are very good for comparing and classifying EQ

23 How big is an earthquake? Seismologists have developed two scales: Intensity Detailed observations of effects on ground: Damage to structures and nature Cracks in ground, Fault offsets Magnitude Precise measurements with seismometers, take distance into account Determines energy packed in an EQ

24 Intensity Scale Developed by Giuseppe Mercalli Italy Originally based on observation of damage to buildings in southern Italy 10 subjective units Scale was modified over time

25 Intensity Scale Modified Mercalli Intensity

26 Intensity Scale Now more objective

27 Intensity Scale Intensity visualized by Shake Map minutes after EQ

28 Intensity Scale We can even go back in the past

29 Intensity Scale Did You Feel It? Citizens contribute to earthquake science

30 Intensity Scale Example: Alum Rock EQ 7 Jan 2010 M = 4.1

31 HH Any Questions?

32 Magnitude Scales Developed by Charles Richter ( ) and Beno Gutenberg ( ) in Pasadena

33 Magnitude Scales First some simple mathematics: Logarithms Let s look at five members of the animal kingdom 1 cm = 0.01 m 10 cm = 0.1 m 100 cm = 1 m 1000 cm = 10 m

34 Magnitude Scales Logarithms Ant = 1 cm Mouse = 10 cm Human = 100 cm Giraffe = 1000 cm A M H G

35 Magnitude Scales Definition of the Richter magnitude: magnitude is log 10 of the maximum amplitude (in mm) recorded on a Wood-Anderson seismometer at 100 km from the epicenter

36 Magnitude Scales Torsion seismometer Frame twists around inertial mass (C) Light beam reflected off mirror writes on photographic paper

37 Magnitude Scales Richter magnitude 1. Measure S-P time. Provides distance 2. Measure amplitude in mm on Wood- Anderson seismometer 3. Draw line between the two points to obtain magnitude

38 Magnitude Scales Richter magnitude How much smaller would the amplitude be if the earthquake was 1 mag unit smaller? factor of mm

39 Magnitude Scales Richter magnitude How much closer would you need to be to get an amplitude of 23 mm for a magnitude 4 earthquake? move from 210 km to 50 km 50 km

40 Magnitude Scales Like intensity scale magnitude scales have evolved Coda or Duration magnitude (M C ) reason: on old (analog or digital) records, recordings were often clipped Example: magnitude 3.5 occurred at 11:48 UTC June 13, 2000,

41 Coda or duration magnitude was developed to relate the length of a record to the local magnitude (measured at standard stations equipped with Wood Anderson seismographs)

42 Magnitude Scales Like intensity scale magnitude scales have evolved Body wave magnitude: m b Measured from P-wave amplitude of any waveform around the world T A m b = log A log T Δ +5.9 Surface wave magnitude: M s Measured from surface-wave amplitude of any waveform around the world M s = log A log Δ +2.0

43 Magnitude Scales Like intensity scale magnitude scales have evolved Developed by Hanks and Kanamori (1979) M w = (2/3) log M Moment magnitude: M w where M 0 is the scalar moment of the best double couple in dyne-cm. The factor 10.7 is to match M w to m b and M s at some standard magnitude. Moment Magnitude is not based on instrumental recordings of a quake, but on the area of the fault that ruptured in the quake. This means that the moment magnitude describes something physical about an earthquake. Seismic moment Moment Magnitude is calculated in part by multiplying the area of the fault's rupture surface by the distance the earth moves along the fault.

44 Magnitude Scales Earthquake energy To measure all the energy released in an earthquake we must integrate over space and time difficult! We can approximately relate magnitude to energy: log 10 E = M s A magnitude 5 earthquake releases about 30 times as much energy as a magnitude 4 Even when you add together the energy release from all the small earthquakes, it is small compared to the one big event

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46 Magnitude Scales Magnitude-frequency relation Gutenburg-Richter relation: log N = a bm Global average for b is ~1 There are ten times as many magnitude 4 as there are magnitude 5

47 Magnitude Scales Magnitude and frequency

48 HH Any Questions?

49 The largest Earthquakes and Tsunamis

50 EQ in North America Alaska Largest EQ ever recorded in US M=9.2, 27 Mar 1964 Anchorage, 4 th Avenue

51 The largest Earthquakes and Tsunamis

52 Sumatra, 24 Dec 2004, M w =9.1 The largest Earthquakes and Tsunamis Longest Rupture ever measured 1200 km

53 The largest Earthquakes and Tsunamis Tohoku, 11 Mar 2011, Mw=9.0 Further Reading:

54 The largest Earthquakes and Tsunamis Tohoku, 11 Mar 2011, Mw=9.0 Coseismic slip along the rupture up to 25 m

55 The largest Earthquakes and Tsunamis Tohoku, 11 Mar 2011, Mw=9.0

56 What is the mechanism of the largest EQ Uplift George Plafker, USGS Subsidence

57 EQ in North America Alaska Subsidence and uplift: >30m Suggested reading:

58 What is the mechanism of the largest EQ Similar pattern with other large EQ 1923 Great Kanto EQ, M = 8.0

59 What happens when megathrust EQ happens under water? Megathrust-EQ

60 The largest Earthquakes and Tsunamis Result: Tsunami

61 Tsunamis Sumatra, 26 Dec 2004, Mw=9.1 Speed in open ocean: ~ 500 mph, jet plane Vasily Titov, NOAA

62 Tsunamis Sumatra, 26 Dec 2004, Mw=9.1

63 Tsunami Warning Systems

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65 HH Any Questions?

66 Tsunamis

67 Tsunamis Speed in open water similar to that of jetplane

68 Tsunamis

69 Japan, 11 Mar 2011, Mw=9.0 Tsunami in Sendai Prefecture

70 HH Any questions?

71 Tsunamis in our backyard Japan, 11 Mar 2011, Mw=9.0 Emeryville Santa Cruz Harbor Richardson Bay

72 Tsunamis in our backyard Although San Francisco Bay is very shallow tsunami risk in is low.

73 Tsunamis in our backyard Different Story in Crescent City, Northern California

74 Tsunamis in our backyard Crescent City hit by Tsunamis 32 times between 1933 und 2011 Wave Heights: 2011 Japan: 2.4 m 1964 Alaska: 6.1 m

75 HH Any Questions?

76 Other causes of Tsunamis Volcanoes

77 Other causes of Tsunamis Volcanoes Krakatoa 28 Aug 1883

78 Other causes of Tsunamis Krakatoa 28 Aug 1883 Wave height in Sunda Strait 30 m people killed

79 Other causes of Tsunamis Meteorites

80 Other causes of Tsunamis Landslides

81 Other causes of Tsunamis Tsunami with largest run-up Ever recorded: run-up 524 m 7.8 EQ on Fairweather Fault caused massive landslide Lituya Bay, Alaska Fritz et al., 2001, Science of Tsunami Hazards 9 July 1958 D.J. Miller, USGS

82 Other causes of Tsunamis Lake Tahoe >12,000 years ago Such tsunamis are also possible in fresh water lakes Vajont Dam, Italian Alps, 1963

83 HH Any questions?

84 Tsunami Warning Systems US Tsunami Warning Systems

85 Tsunami Warning Systems Based on DART Buoys Deep-ocean Assessment and Reporting of Tsunamis

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87 Tsunamis Japan, 11 Mar 2011, Mw=9.0 Further Reading:

88 Tsunami Warning Systems

89 Tsunami Warning Systems German-Indonesian Tsunami Early Warning System

90 Tsunami Warning Systems Warning systems are in place But the earthquake is your natural warning! Also: See Tsunami Leaflet in suggested reading on our website

91 HH Any Questions?

92 Next Tuesday: Seismic Hazard Seismic Risk Personal EQ preparedness Guest Lecture by Aaron Litwin Office of Emergency Management UC Berkeley

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