A Violent Pulse: Earthquakes Lecture #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
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
Offset Along San Andreas Fault Which way does other side move? Toward the right Same either way Rightlateral fault
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
EQ Terminology When rocks break, energy radiates outward in all directions Epicenter = surface point above focus Focus = point where rocks first break
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
More Body Waves S-wave = shear = secondary o Rocks rise & fall o Vibrations perpendicular to travel direction o Slower than P-wave
P- and S-Waves Illustrated
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
Measuring EQs Seismometer: instrument that records ground motion Seismogram: record of an EQ o Shows P-, S-, surface waves o And shows time *
A Seismogram Surface P P-wave arrival time S S-wave arrival time Minutes
o N-S Motion in 3D A seismometer measures movement in 1 direction Vertical Horizontal o E-W
Locating Epicenters Determine S - P Difference in arrival times => Distance from epicenter to seismograph How??
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
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
A Triangulation Example Epicenter
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
Increasing Richter Magnitude Seismogram from small quake (RM): Same epicenter, same seismograph, RM + 1:
Increasing Richter Magnitude Same epicenter, same seismograph, RM +2:
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
A Graphic Representation of RM Consider a quake with RM = 5 Maximum amplitude on seismogram = 10 5 µm = 10 5 *(0.000001 m) = 0.1 m
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
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
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
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
San Francisco CA 1906: before Richter invented his scale Estimated at 7.8 o How? Extensive records used for Mercalli Intensity map