Chapter 8 Earthquakes I What are Earthquakes Seismology

Chapter 8 Earthquakes

I. What are Earthquakes? • Seismology is the science devoted to studying earthquakes • Most take place near the edges of tectonic plates • Earthquakes are caused by elastic deformation • Rocks stretch to a certain point and then suddenly return to its original shape in elastic rebound • Energy is released and some of this energy travels as seismic waves that cause earthquakes

A. Faults at Tectonic Plate Boundaries • A specific type of motion takes place at different tectonic plate boundaries • Each motion creates a particular kind of fault - Transform strike-slip fault - Convergent reverse fault - Divergent normal fault

1. Earthquake Zones • Places where a large number of faults are located • Some earthquakes however happen along faults in the middle of tectonic plates

B. How do Earthquake Waves Travel? • Seismic waves that travel through the Earth’s interior are called body waves - P waves - S waves • Seismic waves that travel along the Earth’s surface are called surface waves

1. P Waves (primary waves) • Travel through solids, liquids, and gases • Fastest waves; travel ahead of other seismic waves • Move rock back and forth, squeezing and stretching the rock

2. S Waves (sear, or secondary waves) • Cannot travel through parts of the Earth that are completely liquid • Second-fastest seismic waves • Shear rock side to side

3. Surface Waves • Produce motion mostly in the upper few kilometers of Earth’s crust • Travel more slowly and are more destructive • Produces motion up, down, and around or back-and-forth

II. Earthquake Measurement A. Locating Earthquakes • Seismographs are instruments located at or near the surface of the Earth that record seismic waves • When the waves reach a seismograph, the seismograph creates a seismogram

1. Determining Time and Location of Earthquakes • The start time is determined by comparing seismograms and noting the differences in arrival times of P and S waves • Seismograms are also used to find the earthquake’s epicenter - An epicenter is the point on the Earth’s surface directly above an earthquake’s starting point

• A focus is the point inside the Earth where an earthquake begins

2. The S-P Time Method • Seismograms from different locations are compared • Seismograms are placed on a time-distance graph • The horizontal axis tells the distance between a station and the earthquake’s epicenter

• A circle is drawn around three seismograph stations with the radius of the circle coming from the horizontal axis of the time-distance graph • The point where all three circles intersects is the earthquake’s epicenter

B. Measuring Earthquake Strength and Intensity 1. The Richter Magnitude Scale • Charles Richter created the scale in the 1930 s • Compares earthquake’s by measuring ground motion recorded by seismograms

2. Earthquake Ground Motion • Magnitude is a measure of the strength of an earthquake • Magnitude values are from 2 to 7 • Each time the magnitude increases by one unit, the measured ground motion becomes 10 times larger Ex) Magnitude of 5 is 10 x greater than magnitude of 4 and 100 x greater than magnitude of 3

3. Modified Mercalli Intensity Scale • Intensity is a measure of the degree to which an earthquake is felt by people and the amount of damage • Scale of Roman numerals from I to XII - An earthquake not felt by people to total damage of an area • Intensity values are usually higher near an earthquake’s epicenter

III. Earthquakes and Society A. Earthquake Hazard • A measure of how likely an area is to have damaging earthquakes in the future • The West Coast has a high earthquakehazard level because it has a lot of seismic activity

B. Earthquake Forecasting 1. Strength and Frequency • Strength of earthquakes is related to how often they occur

2. The Gap Hypothesis • A hypothesis that states that sections of active faults that have had relatively few earthquakes are likely to be the sites of strong earthquakes in the future • The areas along a fault where relatively few earthquakes have occurred are called seismic gaps

C. Earthquakes and Buildings • The process of making older structures more earthquake resistant is called retrofitting - Securely fastening buildings to their foundation - Steel can be used to strengthen structures made of brick • Architects and engineers use the newest technology to design and construct buildings and bridges to better withstand earthquakes