A Violent Pulse: Earthquakes. Lecture #2

Transcription

1 A Violent Pulse: Earthquakes Lecture #2

2 Earthquakes Are Always Happening ~ 1,000,000 / yr Most are small o Detected only by instruments Large EQ ~ 20 / yr cause extensive damage ~ 1 catastrophic EQ / year

3 What Is an Earthquake? Def: vibration of the Earth Study of EQs = seismology Occurs along a fault Stressed rocks bend and store up energy Until rocks break, slip, and release energy

4 Offset Along San Andreas Fault Which way does other side move? Toward the right Same either way Rightlateral fault

5 Movement If the San Andreas Fault is moving 6 cm/yr, how many years to make 160 m of offset? 160 m / (6 cm/yr) = 16,000 cm / 6 cm/yr = 16,000 / 6 cm / (cm/yr) = 2667 (cm * yr/cm) = 2667 years D = R * T T = D / R R = D / T

6 EQ Terminology When rocks break, energy radiates outward in all directions Epicenter = surface point above focus Focus = point where rocks first break

7 Body Waves Def: travel through Earth s interior P-wave = compressional = primary o Rocks are squeezed & stretched o Vibrations parallel to travel direction of wave o Fastest waves

8 More Body Waves S-wave = shear = secondary o Rocks rise & fall o Vibrations perpendicular to travel direction o Slower than P-wave

9 P- and S-Waves Illustrated

10 Surface Waves Def: travel along ground surface 2 types described in text (Love & Rayleigh waves) o Various vibrations o Cause the most shaking => most destructive o Slowest

11 Measuring EQs Seismometer: instrument that records ground motion Seismogram: record of an EQ o Shows P-, S-, surface waves o And shows time *

12 A Seismogram Surface P P-wave arrival time S S-wave arrival time Minutes

13 o N-S Motion in 3D A seismometer measures movement in 1 direction Vertical Horizontal o E-W

14 Locating Epicenters Determine S - P Difference in arrival times => Distance from epicenter to seismograph How??

15 Distance, from Travel Times Suppose a car and a bike start at same place & time, travel on same road. Car travels 60 mph, bike travels 10 mph. How far apart after 1 hour? 50 mi After 2 hours? 100 mi If they are 150 miles apart, how long have they traveled? 3 hours

16 Triangulation At least 3 different seismometers Y-axis X-axis S - P time from seismogram Distance from graph Draw a circle around each seismograph Radius = distance Circles intersect in 1 point = epicenter

17 A Triangulation Example Epicenter

18 Measuring the Size of an EQ Richter Magnitude is based on amount of energy released Measure highest wave on seismogram Calculate magnitude o Logarithmic formula o Increase RM by 1 => ~10 x more shaking

19 Increasing Richter Magnitude Seismogram from small quake (RM): Same epicenter, same seismograph, RM + 1:

20 Increasing Richter Magnitude Same epicenter, same seismograph, RM +2:

21 The Richter Scale Magnitude Damage near Epicenter ~ Frequency > 8 Total destruction ~ 1 in 10 years 6 Widespread damage 100 / year 4 Felt, but little damage 35,000 / year < 2.5 Detected only by seismometer Many / year

22 A Graphic Representation of RM Consider a quake with RM = 5 Maximum amplitude on seismogram = 10 5 µm = 10 5 *( m) = 0.1 m

23 Compare RM 5 to Larger Quakes Consider a quake with RM = 6 Max = 10 6 µm = 10 6 * (10-6 m) = 1 m A quake with RM = 7 Max = 10 7 µm = 10 7 * (10-6 m) = 10 m

24 Another Way to Describe the Size of an EQ Mercalli Intensity: twelve levels based on observed destruction Qualitative Interviews & written records Used to estimate size of older EQs

25 The Mercalli Scale Intensity XII IX VI II Description Cataclysmic Violent Strong Weak Damage Observed Total destruction; ground ripples; landscape altered General panic; structural damage; some buildings collapse Felt by all; windows break; furniture moves Felt by some people, esp. on upper floors

26 Mercalli vs. Richter Consider an earthquake One and only one RM o Same at every seismometer Many different MIs, depending on o Distance from epicenter o Type of rock or ground surface o Type and number of structures

27 San Francisco CA 1906: before Richter invented his scale Estimated at 7.8 o How? Extensive records used for Mercalli Intensity map