Earthquakes

Focus The point beneath Earth’s surface where rock breaks under stress and causes an earthquake

Epicenter The point on Earth’s surface directly above an earthquake’s focus

Seismic Waves

Seismic wave generation Seismic waves are released when rocks that are under pressure bend and then break, releasing energy The point at which the rock breaks is called shear strengh Elastic rebound - When the rocks break and go back to their original shape releasing energy

Seismic Waves A vibration that travels through Earth carrying the energy released during an earthquake

What determines how much the ground shakes during an earthquake? How close a location is to the epicenter The types of rock and soil surrounding the epicenter determines how much the ground shakes

What are the different kinds of seismic waves? There are three types of seismic waves: Primary Waves (P waves) Secondary Waves (S waves) Surface Waves

An earthquake sends out two types of waves P waves and S waves When the waves reach Earth’s surface at the epicenter, surface waves develop.

Primary Waves

Primary Waves (P Waves) A type of seismic wave that compresses and expands the ground The first wave to arrive at a seismomoter

P wave movement P waves compress and expand like an accordion When P waves arrive they vibrate the particles of the crust forward and back along the path of the wave

Secondary Waves

Secondary Waves (S Waves) A type of seismic wave that moves the ground side to side

How are P waves different from S Waves? P waves compress and expand the ground These waves cause buildings to contract and expand P waves travel through solids and liquids S waves vibrate from side to side These waves vibrate from side to side and thrust the ground up and down, or back and forth S waves can not move through liquids

Surface Waves

Surface Waves A type of seismic wave that forms when P waves and S waves reach Earth’s surface Surface (L-waves) cause the most damage

Detecting Seismic Waves

Seismograph A device that records ground movements caused by seismic waves as they move through Earth

Seismometers Old School vs. New School

Measuring Earthquakes

The Mercalli Scale

Mercalli Scale A scale that rates earthquakes according to their intensity and how much damage they cause

Magnitude The measurement of an earthquake’s strength based on seismic waves and movement along faults

The Richter Scale

Richter Scale A scale that rates seismic waves as measured by a particular type of mechanical seismograph

How are the Mercalli scale and the Richter scale similar How are the Mercalli scale and the Richter scale similar? How are they different? Both measure the strength of an earthquake The Mercalli scale measures the strength in terms of extent people notice the earthquake and the amount of damage caused The Richter scale measures the size of seismic waves

Locating the Epicenter

P-S interval

How do scientists calculate how far a location is from the epicenter of an earthquake? Scientists calculate the difference between arrival times of the P waves and S waves The further away an earthquake is, the greater the time between the arrival of the P waves and the S waves

Steps to finding an Earthquakes epicenter Step 1: Calculate the S-P interval This is how much time is between the 1st P-wave and the 1st S-wave

Step 2- determine distance to epicenter using a time-travel graph

Step 3- map circumference showing where EQ could To do this, find the distance to the epicenter and draw a circle at that distance around the seismograph location using the scale on your map.

The point where all three circles intersect is the epicenter Houston 800 Km Savannah 900 km

Direct vs. Indirect Damage Direct Hazards- damage caused by the earth shaking Collapse of buildings, broken power lines, pipelines, roads and bridges Indirect Hazards- a side-effect of the ground shaking Fire, landslides, tsunami

Tsunami Travel at great speeds which make warnings difficult (1000km/hr) Not very high waves on the ocean, only a m, but can be more than 30 m high when crashing on shore

What effects the amount of damage caused during an earthquake? Duration of Shaking Direction of Motion Underlying Earth Materials Loose, wet soil will sustain more damage than bedrock

Designing Earthquake Resistant Structures