| Term | Definition |
|---|
| focus | an earthquake's underground point of origin |
| epicenter | the point on Earth's surface vertically above the focus of an earthquake (you need at least three seismograms to find it) |
| primary waves | a type of seismic wave that compresses and expands and is the first wave to reach the surface |
| secondary waves | a type of seismic wave that goes up and down or side to side and is the second wave to reach the surface |
| surface waves | a type of seismic wave that forms when P waves and S waves reach the Earth's surface |
| seismograph | a device that records ground movements caused by seismic waves |
| magnitude | the measurement of an earthquake's strength based on seismic waves |
| Richter scale | a scale that rates an earthquake's magnitude based on the size of its seismic waves |
| MMS | a scale that rates an earthquake by estimating the total energy released |
| intensity | what the Mercalli scale measures |
| Mercalli scale | a scale that rates earthquakes according to their intensity and how much damage is caused in a particular place |
| plate boundary | a place where two plates meet |
| fault | a break or crack in the Earth's lithosphere along which the rocks move |
| normal fault | a type of fault that is caused by tension |
| reverse fault | a type of fault that is caused by compression |
| strike-slip fault | a type of fault that is caused by shearing |
| tension | stress that stretches rock so that it becomes thinner in the middle (divergent boundary) |
| compression | stress that squeezes rock, which sometimes causes folding (convergent boundary) |
| shearing | stress that pushes masses of rock in opposite directions, in a sideways movement (transform boundary) |
| liquefaction | the process which turns loose sediment into a liquid mud; for it to happen there has to be loose sediment, the loose sediment has to be saturated, and there has to be an earthquake |
| tsunami | a giant wave caused by compression in the ocean floor; it happens when a non-subducting plate flings up or a reverse fault pushes the water up |
| aftershock | an earthquake that occurs after another earthquake in the same area |
| P-S lag | the time between the beginning of the primary wave and the beginning of the secondary wave; it is used to determine the distance the seismograph is from the epicenter |
| base-isolated building | a building that is mounted on rubber and steel pads; the pads are underground and base-isolation is only used on very tall buildings like skyscrapers |
| How many seismograms at least are needed to locate the epicenter of an earthquake? | three |
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