Earthquakes & Seismic Waves

For a quick laugh…

Geologists use 2 terms when discussing the location of an earthquake: Focus = The area beneath Earth’s surface where the rock that is under stress actually breaks. Epicenter = The point on the surface directly above the focus.

Seismic Waves Given off by an earthquake & carry energy as they travel outward. Travel in all directions. Energy dissipates over time/distance. 3 Types of Seismic Waves: P Waves = Primary waves. Compress & expand the ground. Arrive first. S Waves = Secondary waves. Vibrate side-to-side & up and down. Shake the ground back & forth. Arrive second. Surface Waves = When P & S waves reach the surface. Move more slowly than P & S waves, but can cause severe ground movement & damage.

Three scales have been developed to measure the intensity of earthquakes…

The Mercalli Scale Developed to rate earthquakes according to the level of damage at a given place. The scale has 12 levels represented by roman numerals, I – XII. The same earthquake can have different Mercalli ratings because it causes different amounts of ground motion at different locations. Considered a biased scale when you compare wilderness vs. city damage.

Mercalli Scale

The Richter Scale Magnitude = Number assigned to an earthquake based on the earthquake’s size. Magnitude rating is determined by measuring the seismic waves & fault movement that occur. Seismograph = Instrument that records & measures seismic waves. This scale provides accurate measurements for small, nearby earthquakes – not large, or distant earthquakes. The scale ranges from 0 – 9, and each level is 10 times greater in intensity than the previous level.

The Moment Magnitude Scale This scale takes into consideration both the distance from the epicenter of an earthquake & the amount of damage it causes. It estimates the total energy released by an earthquake. This is the preferred scale of today’s geologists.

Locating the Epicenter Geologists use seismic waves to calculate the epicenter of an earthquake. P waves arrive first at seismographs, followed closely by S waves. Geologists measure the time between the different waves’ arrivals. The greater the difference in time, the greater the seismograph’s distance from the epicenter of the earthquake. Geologists use data from at least three seismographs to determine an epicenter.

Mercalli Scale Activity You will work independently for this activity. Your goal will be to map out various first-hand accounts of people in a town when an earthquake hits in order to find the earthquake’s epicenter. You will be given a Television Script Key to follow, a breakdown of the Mercalli scale, and a map of our fictional town… Wattsville. What you need to do to begin… Once you’re ready, I will hand you your materials. Please take notice that certain pages are “class sets” & you should not write on them!! You need to read & listen carefully during this activity or you will not get accurate results. Listen to callers’ descriptions, find their location on the map, & assign them a value on the Mercalli scale.

KWAT Television Script Key   Jake Wilde: “We interrupt our regularly scheduled programming on KWAT to bring you a special bulletin. This is KWAT news anchor, Jake Wilde. Moments ago the town of Wattsville was shaken by a strong earthquake. Residents in the KWAT broadcast area are invited to call our emergency response number, 555-KWAT, and give us your name, your location, and a brief summary of what you experienced during the quake. Stay tuned for the latest reports of what your neighbors saw and felt. To report your observations, call 555-KWAT. We have caller number 1 on the line.”

Did you find a pattern once you connected all the points that were similar? Explain. Are you able to locate a precise epicenter for the Wattsville earthquake, or just find a general location? Explain. Why is this scale biased and not frequently used? Explain.