Section 8.2 MEASURING

EARTHQUAKES

P Waves

Push (squeeze) and pull rocks in the direction the wave is traveling Fastest and first to be detected Can travel through solids, liquids

and gases

S Waves

Shake rocks perpendicular to the direction the wave is traveling Slower than p waves Can only travel through solids

BODY WAVES

Waves that result when energy from the body waves reaches the surface Slower than body waves, but are larger and thus cause more

damage

SURFACE WAVES

Consist of a weight suspended from a support securely attached to bedrock.

Weight will remain stationary due to its own inertia.

Earth’s surface and the bedrock vibrate during an earthquake.

Pen attached to the weight records the movement of earth relative to the suspended weight.

SEISMOGRAPHS

A time record of ground motion during an earthquake. Shows three different types of seismic waves

SEISMOGRAM

Intensity – a measure of the degree of earthquake shaking at a given location based on observable damage

Magnitude – a measure of the size of seismic waves or the amount of energy released at the focus

MEASURING EARTHQUAKES

Method for locating the epicenter relies on the fact that P waves travel faster than S waves. The difference in arrival times of P and S waves at a station is

directly related to the distance its from the epicenter. Triangulation – precise location of the epicenter is determined

using the distance from three of more seismic stations.

LOCATING AN EARTHQUAKE

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Richter Scale

Based on height of the largest seismic wave recorded.

Uses a scale based on tens.

Thus, a 6-magnitude earthquake is 10 times greater than a 5-magnitude earthquake.

Modified Mercalli Scale

Scale divided into 12 levels based on observed effects Same earthquake can receive

different rating at different locations

MEASURING EARTHQUAKES

Estimates the total energy released along the entire fault surface

Calculated using several factors: Seismographic data Average amount of movement along a fault Area of the surface break Strength of the broken rock

MOMENT MAGNITUDE