Earthquakes Chapter 8
Earthquakes • Approximately 30,000 earthquakes occur every year. • On average only about 75 per year are very large. • Most of these earthquakes occur in remote areas where very little harm is done. • Occasionally, these large earthquakes occur in areas with people and they are one of the most destructive forces on earth.
Earthquake Damage Turkey Japan Taiwan Haiti Chile California
What causes an Earthquake? • The surface of the earth’s crust is made of plates. • Below these plates is hot molten rock called magma. • The heat from this magma causes the plates to move. • There are boundaries between the different plates. • When the plates move it creates pressure and friction between these plate boundaries. • When the pressure that builds up is released, an earthquake occurs.
Where an Earthquake Occurs • When an earthquake occurs, there is a specific place usually along a plate boundary where the pressure is released and a large seismic wave is produced. • The actual specific place where the pressure is released is called the focus. • Most earthquake foci occur at great depths. • The epicenter is a location on the surface of the earth located directly above the focus.
Epicenters and Fault Lines • When geologists are trying to describe to the general public the location of an earthquake, it is not necessary to explain how deep the earthquake was. • Most people generally would like to know the location of the quake in relation to a familiar place. • That is why epicenters are most often used. • A fault line is the fracture or boundary between two plates. It is most commonly where earthquakes occur.
What is the Elastic Rebound Hypothesis? • The elastic rebound hypothesisis an explanation for how seismic waves are produced and generated. • Basically, if you think of the earth’s crust as a stick, when the stick is bent, it stores elastic potential energy. • Eventually the stick (earth’s crust) will build up enough energy to overcome the bonds that hold the stick together. When this occurs the stick will break. • When the stick breaks, it releases all its stored energy, producing a wave (seismic wave).
What are Aftershocks and Foreshocks • If you think of the analogy of the stick from the elastic rebound hypothesis, it is easy to understand where aftershocks and foreshocks come from. • If you were to bend the stick, you might hear cracks occurring before the stick snapped in half. • These cracks are the equivalent to a foreshock in the earth’s crust. Geologists can sometimes detect foreshocks occurring before a large quake.
Foreshocks and Aftershocks • Foreshocks are small earthquakes that often occur before a large earthquake. • Aftershocks are small earthquakes that occur after a large earthquake occurs. • These were recorded from the Japan earthquake that occurred on March 11, 2011.
Seismology and their instruments • Seismology is the study of earthquakes. • Waves produced by earthquakes are called seismic waves. • Instruments that record seismic waves are called seismographs. • Seismographs produce a written or digital record of seismic waves produced from earthquakes. • The recorded seismic waves are called seismograms.
Seismic Waves • Seismologists can tell a lot about earthquakes and the earth’s interior from seismic waves. • There are basically three main types of seismic waves. • Surface Waves– these waves only travel through the earth’s outer layer. These waves move similarly to the way an ocean wave moves. These waves move both up and down, as well as side to side. Surface waves are the most destructive seismic waves.
Seismic Waves • The other two types of waves are called body waves because they can travel through the earth. • The first body waves are called P waves or primary waves. P waves are the fastest seismic waves. Sometimes they are also called push-pull waves or compression waves because they compress and expand the rocks as they travel through the earth. • They move similarly to a spring being compressed.
Seismic Waves • The second body wave is called an S waveor secondary wave. • S waves shake the ground at right angles to the direction the wave is traveling.
Seismograms and Seismic Waves • Seismic waves are recorded on seismograms. • P waves are the fastest waves and are about 1.7 times faster than S waves. • Surface waves are the slowest and are about 90 percent of the speed of the S waves. • Knowing the speed of the waves can tell seismologists the distance to an earthquake’s epicenter and if at least three distances are known, the exact location can be determined by triangulation.
Where do Earthquakes occur? • About 95% of most earthquakes occur along active plate boundaries. • Most of these occur along the circum-Pacific belt, also known as the pacific ring of fire. • Another belt where many earthquakes occur is called the Mediterranean-Asian belt.
How to determine the Magnitude of an Earthquake • The old familiar scale that was used for determining an earthquake’s magnitude is called the Richter Scale. • The Richter Scale is based on the amplitude of the largest seismic wave recorded on a seismogram. • It is a logarithmic scale which means that if the amplitude of a seismic wave increases by a factor of ten, then the Richter Scale magnitude increases by 1. • So the amount of shaking or intensity of a 5.0 earthquake is ten times greater than a 4.0.
Limitations of the Richter scale and the use of the moment magnitude scale • The problem with the Richter Scale is that the intensity of waves decrease with the distance from the epicenter. • Because of this, the Richter Scale is only accurate out to about 500 km or about 300 miles. • Seismologists use another method called the Moment Magnitude Scale, which is believed to be more accurate. • The Moment Magnitude Scale is based on three things; the average displacement along the fault, the surface area of the fault, and the rigidity (strength) of the rock.
Destruction from Earthquakes • At 5:36 PM, March 27 (Good Friday), 1964, an enormous earthquake occurred about 75 miles east of Anchorage. • It was the largest recorded earthquake ever recorded in North America. It was the second largest earthquake ever recorded. (1960 Chile earthquake was the largest) • It had a moment magnitude of 9.2 and lasted about 3 to 4 minutes. (1960 Chile earthquake was a 9.5) • Luckily, very few people lived in Alaska at the time, and it occurred on Good Friday (a holiday), so only 131 people died. Which is a relatively small number considering the enormous magnitude of the earthquake.
What lessons can we learn from the 1964 Alaska Earthquake? • Scientists and engineers have learned a great deal from the 1964 Alaska earthquake. • Approximately $300 million dollars worth of damage. • Many building and homes were destroyed by the intensity and duration of the shaking. • Steel framed and wood framed buildings and home withstood better than unreinforced stone or brick homes and buildings.
What was different between the earthquakes in Alaska (1964) and Haiti (2010)? • The earthquake in Haiti was only a magnitude 7.0 compared with Alaska’s 9.2 (1964). • That is 100 times less intensity. • Alaska only had 131 fatalities. • Haiti had over 300,000 fatalities • The earthquake in Haiti made 1,000,000 people homeless, and 250,000 homes were destroyed. • The primary difference was the building designs. • In addition to building design, population size was also a factor.
What are some other factors that cause damage during and after an earthquake? • Normally, under ordinary conditions soil that has a small amount of moisture in it is very stable. • The cohesive properties of water actually make the soil more stable and help hold the soil together. • But, seismic waves can cause the bonds of the soil and water to break and cause the stable damp soils to become like an unstable soup of mud. • This is known as liquefaction of soils.