Earthquakes

Earthquakes are a natural shaking of the crust caused by a release of energy from the movement of rocks, usually along a fault.

Earthquakes are caused by a shifting of plates along a fault. A fault is a crack or break in Earth’s crust that occurs along a plane of weakness. When stress is acting on a rock it will build until the rock reaches its elastic limit and breaks.

Focus and Epicenter The energy of an earthquake is transmitted in waves that spread out from the origin of the earthquake, the focus. The focus is always below Earth’s surface. The place on Earth’s surface directly above the focus is the epicenter.

The focus is the point under the ground where the waves are actually produced, and the epicenter is the point in the crust directly above the focus.

Epicenter  The epicenter is the first place to feel the tremors of an earthquake, and often suffers the most damage from the quake.  Epicenter

The magnitude (strength) of these waves is measured with a seismograph. The report generated by a seismograph is a seismogram.

Seismic Waves When faulting occurs, seismic waves are released and spread in all directions. Waves travel at different speeds through objects of different densities, moving faster through dense objects, and slower through less dense objects. Some types of waves are not able to travel through liquids. Scientists have been able to ascertain that the outer core of the earth is liquid, the inner core is solid, and the mantle is a semi-solid by studying seismic wave data.

There are two different types of waves that spread from the epicenter, P and S-waves.

P Waves Primary waves are the fastest waves generated by an earthquake [6 mi/sec] They are the first wave to arrive, so they are called Primary waves. P waves are also called compressional waves because they cause the material that they are traveling through to vibrate in the direction of the wave and compress. P-Waves can pass through solids and liquids.

More P Waves. . . P-Waves, or longitudinal waves, travel in a series of compressions and expansions, just like a slinky. The compression and expansion occurs in the same direction as the movement of the wave.

S Waves S-Waves or transverse waves are slower waves that resemble ocean waves. As this type of wave travels through a medium, the medium is displaced, or moved, perpendicular to the direction of the wave. These waves cause the most damage to buildings because they move along the surface of the Earth and shake it like a rope. S-waves can not go through liquids (like the outer core)

More S Waves . . . S-Waves are the second wave on scene, so they are called Secondary waves. S-Waves can only travel through solids and are slower than P-Waves [S-waves = 4mi/sec]

Wave Strength Velocity of seismic waves depends on the physical properties of the material the waves are passing through. If material is under pressure, then the velocity of the waves will increase. The greater the density of a material the greater the velocity.

S/P Wave Review . . . P waves are back-and-forth, compressional waves that are fast-moving and can travel through any material. S waves are up and down waves that are slower moving and can only travel through solids.

Measuring Earthquakes The Mercalli scale is used to measure the damage done by these earthquakes. It ranges from 1 - 12, twelve being total devastation. Unlike the Richter scale, the Mercalli number cannot be tabulated immediately; the damage must be surveyed. The Mercalli Scale measures damage. The Richter scale uses a seismograph to measure the vibrations produced by earthquakes. The Richter scale ranges from 1 - 10, with each number 10 times as powerful as the one before it. Chile recorded a 9.5 in 1960 ~ highest on record. The Richter Scale measures force/energy.

Effects of Earthquakes Ground shaking- Bedrock shakes very little, but loose sediments (like in a landfill) undergo liquefaction and behave like a liquid. Buildings are the #1 cause of deaths by an earthquake and buildings built on fill fall most often.

Ground Failure The vibrations from an earthquake cause rockslides and landslides, which can wipe out entire towns.

Surface Faulting In surface faulting, the surface actually cracks and rips apart due to the earth’s movement. In the famous Alaskan earthquake neighbors ended up ¼ mile apart!

Tsunamis Earthquakes can cause large ocean waves called Tsunamis. These waves can travel over 500 mph (that’s as fast as a 747!) and rise to over 100 feet. Oddly if you were out to sea, you would have no sensation of the tsunami passing you; but the same wave can devastate coastal communities.

So . . . . Now that we know all about Earthquakes, let’s talk about how to locate them!

How do I locate an Earthquake’s epicenter? To figure out just where an earthquake happened, you need to look at your seismogram and you need to have at least two other seismographs recorded for the same earthquake. You will also need a map of the world, a ruler, a pencil, and a compass for drawing circles on the map.

One-minute intervals are marked by the small lines printed just above the squiggles made by the seismic waves. The distance between the beginning of the first P wave and the first S wave tells you how many seconds the waves are apart. This number will be used to tell you how far your seismograph is from the epicenter of the earthquake.

Finding the distance to the Epicenter: Measure the time difference between the first P wave and the first S wave. For example, let’s say the first P and S waves are 24 seconds apart.

Find the approximate point for 24 seconds on the left side of the chart and mark that point. Using either a ruler or piece of scrap paper, find a point between the S-Wave and P-Wave lines that is the same distance apart. According to the chart, the epicenter would be 200 km away.

Finding the Epicenter You have just figured out how far your seismograph is from the epicenter, but you still don't know exactly where the earthquake occurred. This is where the compass, the map, and the other seismograph records come in.

Check the scale on your map Check the scale on your map. It should look something like a piece of a ruler. All maps are different. On your map, one centimeter could be equal to 100 kilometers or something like that. Figure out how long the distance to the epicenter (in centimeters) is on your map. For example, say your map has a scale where one centimeter is equal to 100 kilometers. If the epicenter of the earthquake is 215 kilometers away, that equals 2.15 centimeters on the map.

3. Using your compass, draw a circle with a radius equal to the number you came up with in Step #2 (the radius is the distance from the center of a circle to its edge). The center of the circle will be the location of your seismograph. The epicenter of the earthquake is somewhere on the edge of that circle. 4. Do the same thing for the distance to the epicenter that the other seismograms recorded (with the location of those seismographs at the center of their circles).

All of the circles should overlap All of the circles should overlap. The point where all of the circles overlap is the approximate epicenter of the earthquake.

EQ Location Summary: Earthquakes release P and S Waves. P Waves are primary waves that are fast and can pass through any material; S waves are secondary waves that can only pass through solids. You need a minimum of three seismograph stations in order to pinpoint the exact location of an earthquake because each station narrows the area down to a circle around the station; where all three circles intersect is the epicenter.

As the waves get farther away from the epicenter, the distance between them increases because the S wave is slower. Therefore, the greater the distance from the epicenter, the greater the distance between the P and S waves.

So, in conclusion . . . We locate the epicenter of an earthquake by: Retrieve seismographs of the earthquake from three different locations. Calculate the distance to the epicenter for each station using page 11 of the ESRT. Construct a circle of the distance around each station. 4. Where all three circles intersect is the epicenter!