San Francisco April 18 1906 San Francisco City

San Francisco, April 18, 1906 San Francisco City Hall after the 1906 EQ

Objectives for this Lab: • Understand how to interpret seismograms and record data • Use seismic data and determine magnitude an earthquake • Use seismic data to determine the location of earthquake epicenters

Consider a rock that is dropped in calm water. What happens? Earthquake – vibrations within the earth that produce rapid release of energy in the form of seismic waves

“Good Vibrations” – released seismic waves Three major seismic waves are released: • The P-wave • The S-wave • The L-wave (surface wave) What measures the incoming seismic waves? • The Seismograph The inertia of the suspended mass keeps the seismograph motionless while the seismic waves vibrate the recording drum anchored to the bedrock – produces a seismogram

P-wave (primary wave) • the fastest • compressional wave • moves through solids and liquids S-wave (secondary wave) • 2/3 speed of P-wave • shearing wave • moves through solids only Love wave (surface wave) • slowest waves • side-to-side (surface) • “We” feel these waves Rayleigh wave (surface wave) • slowest wave • “rolling” wave • like floating on a boat type motion

Did you feel the earthquake? – Measuring an EQ What’s the difference between an 8 magnitude and a 6. 5 magnitude? Two ways to measure EQ’s: Mercalli Intensity Scale Richter Scale

Modified Mercalli Intensity Scale • measures the intensity of the EQ – how much damage occurs • closer to the epicenter-- more damage Less intense • farther from the epicenter -- less damage • people report the damage • destruction is assigned a Roman Numeral (I – XII) • I – least intensity • XII – greatest intensity More intense 23

Richter Scale (Charles Richter – 1935) • measures the intensity of an EQ using a seismogram • maximum intensity = largest amplitude recorded • EQ intensity is based on the logarithmic scale. • each step (M 1…M 2…M 3. . ) largestincreases amplitudethe amplitude X 10 • M 5 has an amplitude 10 times larger than an M 4 • M 5 has an amplitude 100 times larger than an M 3 • What about the energy released? • Each increase in Richter amplitude (by 10) increases energy 32 times so, • An M 6 releases 32 more times energy than an M 5. 1, 024 more times energy than an M 4. • An M 6 releases _____ • An M 6 releases 32, 768 _____ more times energy than an M 3. • An M 6 releases 1, 048, 576 ____ more times energy than an M 2. What observations can you make about the increasing energy release from one magnitude to the next?

Interpretation of a seismogram S-P time interval S-P interval P-wave (S-wave subtract P-wave) = in seconds The height of the wave (mm) S-wave Amplitude P-wave S-wave

Calculate the S-P interval: P-wave arrival S-P interval S-wave arrival 21 hr 32 m 40 sec 30 sec 21 hr 33 m 10 sec 15 hr 23 m 19 sec 46 sec 15 hr 24 m 5 sec 3 hr 16 m 32 sec 41 sec 3 hr 17 m 13 sec 19 hr 43 m 12 sec 40 sec 19 hr 43 m 52 sec 20 hr 58 m 42 sec 34 sec 20 hr 59 m 16 sec

Calculating the Richter Magnitude P S 20 5 5 magnitude 220 km distance

Seismograph Stations S-P A S-P B D S-P 8 M C What is the magnitude at each station? SAF How would the Mercalli scale relate to each station?

Interpreting magnitude at various seismic stations More damage A 24 closest 40 2 Although, there are different size triangles, the ratio of (2) remains the same. 32 B 12 20 2 16 C 6 2 10 Analogy Although, there are different S-P interval seismograms, the magnitude remains the same 8 D Less damage 3 2 4 5 How does magnitude relate to the Mercalli scale? ?

Triangulation – locating the epicenter of an EQ SS = seismic station D = distance to epicenter SS-A D D SS-C Epicenter SS-B D

Finding the epicenter using triangulation • intersection of three circles = epicenter location • the radius of circle = distance to epicenter from a single earthquake • use the S-P interval vs. Distance chart to find the distance to the epicenter from a single EQ S-P interval (min, sec) Distance (km, mi)