Earthquakes Chapter 8

8.1 What is an Earthquake? Earthquake: vibration of Earth produced by the rapid release of energy Most (NOT ALL) occur at faults most faults and stress occurs along active plate tectonic boundaries Focus: the point within Earth where the EQ starts; the source of the earthquake Epicenter: location on the surface directly above the focus

Fault Types Normal Strike-Slip

Focus, Epicenter, and Fault

Causes of Earthquakes Elastic Rebound Hypothesis: Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces As rock is stressed, it bends, storing elastic energy. Once the rock is strained beyond its breaking point, it ruptures and releases the stored energy in the form of vibrations (seismic waves) of earthquakes

Elastic Rebound Hypothesis

Other causes of earthquakes: Landslides, rockslides, or slumping of rocks. Movement of magma, gases, or rocks associated with volcanism

Aftershocks and Foreshocks Foreshocks: small earthquakes before a major earthquake Can happen days or years before the major quake Main shock: is the main earthquake disturbance generated at the focus Aftershocks: movements that follow a major earthquake Smaller than the major EQ Can sometimes destroy structures weakened by the major earthquake

Where do earthquakes occur Time for Excel Practice! Log on to computers, open up excel spreadsheet from classroom website Follow directions of excel spreadsheet Do not worry about printing map, just call me over when you are done so I can check over WHERE ARE MOST OF THE EARTHQUAKES OCCURRING?

Earthquake Zones 80% of all occur in circum-Pacific belt Most result from convergent margin activity 15% occur in the Mediterranean –Asiatic belt 5% occur in the interiors of plates and on spreading ridge centers

Measuring EQs Seismographs: instruments that record EQ waves Seismograms: traces of amplified, electronically recorded ground motion made by seismographs

Earthquake Waves Surface Waves (L waves): Travel along Earth’s outer layers Especially damaging to buildings Most destructive of the three types of waves Rolling and side-to-side movement (think of ocean wave movement)

Body Waves-Travel through Earth’s Interior P (primary) waves Push-pull motion Compression waves- material is moved in the same direction as the wave moves Fastest moving wave Travel through solids, liquids, or gases S (secondary) waves Slower than p waves, faster than surface waves Travel through solids only Move material perpendicular (90 deg) to wave movement

Earthquake Waves

How is an Earthquake’s Epicenter Located? Seismic wave behavior P waves arrive first, then S waves, then L Average speeds for all these waves is known After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

Locating an Earthquake Earthquake Distance: Epicenter is located using the difference in the arrival times between P & S wave recordings, which are related to distance We will use a reference table for these measurements Earthquake Direction Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter We will practice this using a compass

Seismograph example 2:33:00 2:35:30 2:35:30 – 2:33:00 = 00:02:30

2:35:10 2:39:20 2:39:20 – 2:35:10 = 00:04:10

We don’t know when the EQ started. But we know how much time there was between the P&S wave arrivals. Let’s say the difference is 0:04:00

We have to find a spot on the graph where P&S (careful We have to find a spot on the graph where P&S (careful!) are separated by 4:00.

Then drop straight down to see the distance to the epicenter. 2,600km

Now that we know the EQ was 2,600km away, when did it start? To travel 2,600km, a P-wave… needs 5:00 minutes

1,400km

The EQ happened somewhere on this line

Measuring Earthquakes Two different types of measurements: intensity and magnitude Intensity: based on observed effects of ground shaking on people, buildings, and natural features. Varies from place to place w/in disturbed region depending on the location of the observer with respect to the EQ epicenter Magnitude: related to the amount of seismic energy released at the hypocenter of the earthquake Based on amplitude of earthquake waves recorded on instruments

Scales to Measure Earthquakes Richter Magnitude Scale Modified Mercalli Intensity Scale Seismic waves are the vibrations from earthquakes that travel through the Earth Recorded on seismographs Richter Scale-1935 Base-10 logarthmic scale Each unit=32 fold energy increase Calculate combined horizontal amplitude Range from 0-10 <2.0 not felt or recorded 6.0-6.9: strong 7.0-7.9: major 8.0-9.9: great 10+: Epic (never recorded) DOES NOT ADEQUATELY ESTIMATE THE SIZE OF VERY LARGE EQS!! Developed in 1931 12 different levels I-not felt except by very few II-felt only by a few at rest III-felt noticeably by persons indoor IV-felt indoors by many, outdoors by a few V-felt by nearly everyone, many awakened VI-felt by all, some heavy furniture moved VII-damage negligible in well-designed buildings VIII-some chimneys broken IX-buildings shifted off foundations X-some well build wooden structures destroyed XI-few structures remain standing XII-total damage

Moment Magnitude Derived from the amount of displacement that occurs along the fault zone Most widely used measurements for EQs because it is the only magnitude scale that estimates energy released by earthquakes Measures very large earthquakes Calculated by different factors including: Avg amt of movement along the fault (a) Area of the surface break (b) Strength of broken rock (c) a x b x c=measure of how much energy rock can store before it slips and releases energy during an earthquake

Moment Magnitude Chart

Notable Earthquakes

Destruction from Earthquakes

Seismic Vibrations The damage to buildings and other structures from earthquake waves depends on several factors. Factors include: intensity and duration of the vibrations nature of the material on which the structure is built design of the structure

Building Design Factors that determine structural damage Intensity of EQ Unreinforced stone or brick buildings Most serious safety threat Nature of material upon which structure rests Design of the structure

Seismic Vibrations Liquefaction Saturated material turns to fluid Underground objects may float to surface Occurs when loosely consolidated soils saturated with water are shaken by EQ waves

Tsunamis Japanese for seismic sea waves Causes: Triggered by an EQ Occurs where slab of the ocean floor is displaced vertically along a fault Can also occur when the vibration of a EQ sets an underwater landslide in motion

Movement of Tsunamis A tsunami is generated by movement of the ocean floor. The speed of a wave moving across the ocean is related to the ocean depth

Tsunami Warning System Large earthquakes are reported to Hawaii from Pacific Seismic Stations Although tsunamis travel quickly, there is sufficient time to evacuate all but the area closest to the epicenter On average, only 1-2 destructive tsunamis worldwide per year On average only 1 tsunami every 10 years causes major damage and loss of life

Predicting Earthquakes Short-Range Predictions Not successful yet  Long-Range Forecasts Data can be important for updating building codes Probability of EQ occurring within 30-100+ years Scientists don’t yet understand enough about how and where earthquakes will occur to make accurate long-term predictions. A seismic gap is an area along a fault where there has not been any EQ activity for a long period of time

Other Dangers Landslides Fire Results from the violent shaking of EQs, causing the soil and rock on slopes to fall The greatest cause of structural damage Fire In the San Francisco EQ of 1906, most of the destruction was caused by fires that started when gas and electrical lines were cut

US Earthquakes 1973-2002