Science Starter Describe in your own words what an Earthquake is and what causes it. Answer The MSL
WHAT IS AN EARTHQUAKE AND HOW DO WE MEASURE THEM? Chapter 8, Section 8.1 & 8.2
Looking Back Deserts Wind-shaped environments Water in the Desert Glaciers
Let s Review What are the two basic types of glaciers? Valley Glaciers and Ice Sheets What are the 6 types of sand dunes? Barchan, Longitudinal, Transverse, Parabolic, Barchanoid, and Star Dunes. What is desert pavements? When sand and silt is removed by wind and only leaves a layer of coarser pebbles and cobbles that protect remaining sediment.
Looking Ahead We are going to talk about how damaging Earthquakes can be and the Earth s interior tomorrow. Bill Nye Episode: Earthquakes Take Notes!
Today s Plan Finish Sections 8.1 & 8.2 Paper Fault Blocks Activity If time, Bill Nye episode on Earthquakes
Chapter 8 Vocabulary Earthquake Focus Epicenter Fault Elastic Rebound Hypothesis Aftershock Foreshock Seismograph Seismogram Surface Wave P Wave S Wave Moment Magnitude Liquefaction Tsunami Seismic Gap Crust Mantle Lithosphere Asthenosphere Outer Core Inner Core Moho
Introduction Each year, more than 30,000 earthquakes occur worldwide. Most of them are minor and do little damage Generally only 75 major earthquakes happen each year. Most occur in remote regions Occasionally large earthquakes hit heavily populated areas. Earthquake is one of the most destructive natural forces on Earth under these conditions.
Earthquakes An earthquake is the vibration of Earth produced by the rapid release of energy. Earthquakes are often caused by a break in Earth s crust. The point within Earth where the earthquake starts is called the focus. Released energy radiates in all directions from the focus in the form of waves. The epicenter is the location on the surface directly ABOVE the focus! What they talk about on the News!
Earthquake Anatomy
Faults Lots of evidence shows that Earth is constantly changing. We know that Earth s crust has been uplifted at times. Wave-cut features above highest tide levels Offsets in fence lines, roads, and other structures indicated that horizontal movements of Earth s crust are also common. Earthquakes are usually associated with large fractures in Earth s crust and mangle called faults. Faults are fractures in Earth where movement has occurred.
Faults in Real Life
Anatomy of a Fault It is important to realize that although all faults have movement occur on them, they are not all the same! There are 5 parts of a fault: Fault Plane: Where the action is; it is the flat plane that may be vertical or sloping; it is moving! Fault Trace: The line the fault makes on the ground. Hanging Wall: When the fault plane is sloping, it is the block on top. Footwall: When the fault plane is sloping, it is the block on bottom Fault Scarp: Vertical displacement of ground due to fault movement.
Fault Anatomy Fault Plane Fault Trace
Fault Anatomy Fault Scarp Hanging Wall/Footwall
Types of Faults There are 3 main types of faults classified on their movement: Normal Faults: Form when hanging wall moves DOWN. Extensional forces make these: Pulling slabs APART Reverse Faults: Form when hanging wall moves UP. Compressional forces make these: Pushing slabs TOGETHER Strike-Slip Fault: Fault movement is sideways. No hanging wall or footwall because plane is vertical.
Types of Faults
Cause of Earthquakes Before the great 1906 San Francisco Earthquake, the actual causes and effects of earthquakes were not understood. The San Francisco earthquake caused horizontal shifts in Earth s surface of several meters along the northern portion of the San Andreas Fault. The 1300km San Andreas fracture extends north and south through southern California. During the 1906 earthquake (a SINGLE event) the land on the western side of the San Andreas Fault moves as much as 4.7 meters to the north compared to the land on the eastern side of the fault.
Causes of Earthquakes Based on the measurements from the 1906 Quake, a hypothesis was developed to explain what had been observed: 1. Forces within Earth slowly deform the crustal rocks on both sides of the fault (shown by bent features) 2. Rocks bend and store elastic energy 3. Resistance caused by internal friction (which holds rocks together) is overcome 4. Rock slips at weakest point (focus) 5. Movement exerts forces along the fault where additional slippage will occur until ALL the pent up energy is released. Allows deformed rock to snap back into place Vibrations of Earthquake is the rock elastically returning to its original shape!!
Elastic Rebound Hypothesis
Elastic Rebound Hypothesis The springing back of the rock into its original place is called elastic rebound. Rock acts like a stretched rubber band when it is released. When rock is deformed, they first BEND and then break! This releases the stored energy. The explanation for the release of energy stored in deformed rocks is the elastic rebound hypothesis.
Elastic Rebound Hypothesis Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces. When the strength of the rock is exceeded, it suddenly breaks, causing the vibrations of an earthquake.
Aftershocks and Foreshocks The intense shaking of the 1906 San Francisco Earthquake lasted for about 40 seconds. Most movement along the fault occurred in this short time period. Additional movements along the fault continued for several days. Movements that occur before or after an earthquake are called foreshocks and aftershocks!
Aftershocks vs. Foreshocks Aftershocks Follow a major earthquake Can destroy structures already weakened by main earthquake. Foreshocks Come before a major earthquake. Can happen days or even YEARS before the major earthquake!
San Andreas Fault The San Andreas Fault system is the most studied fault system in the world. Studies have shown that displacement has occurred along segments that are 100 to 200 km long. Each fault segment behaves a little differently than the other segments. Some parts of the fault show slow, gradual movement (fault creep). Happens smoothly. Other segments regularly slip and produce small earthquakes. Some segments stay locked and store energy for hundreds of years before they break loose and cause MAJOR EARTHQUAKES.
Measuring Earthquakes
Measuring Earthquakes The study of earthquake waves, or seismology, dates back 2000 years! The first attempts to discover the direction of earthquakes were made by the Chinese. Seismographs are instruments that record earthquake waves.
Measuring Earthquakes Idea behind seismographs: Waves from an earthquake reach the instrument, the inertia of the weight keeps it stationary while Earth and the support vibrate. Weight stays motionless, and provides a reference point to measure the amount of movement that occurs as waves pass through the ground below. Recorded on rotating drum
Modern Seismographs Amplify and electronically record ground motion, producing a trace. This trace is called a SEISMOGRAM! Seismogram Seismos = shake Gramma = what is written
Earthquake Waves Energy from an earthquake spreads outward as waves in all directions from the focus. Seismograms show that two types of seismic waves are produced by an earthquake: Surface Waves Body Waves
Surface Waves Surface waves are seismic waves that travel along Earth s outer layer. Motion of surface waves is complex: Travel along the ground and cause the ground and anything resting on it to move. Like ocean waves that toss a ship Move in an up-and-down motion as well as side-to-side. Side to side movement is especially damaging to the foundations of buildings. Surface waves are the MOST DESTRUCTIVE earthquake waves.
Surface Waves
Body Waves Waves that travel through Earth s interior are called body waves. Two types depending on how they travel through materials in the earth: P Waves S Waves
Body Waves P Waves S Waves Push-Pull Waves Push (compress) and pull (expand) rocks in the direction the waves travel. Known as compression waves. Temporarily change the volume of the materials they pass through by compressing and expanding it. Shake particles at right angles to the direction of travel. Transverse waves. Temporarily change the shape of the material they pass through. Gases and liquids do NOT transmit S waves because they do not rebound elastically to their original shape.
Body Waves A seismogram will show all three types of seismic waves! Order of Arrival: P Wave S Wave Surface Waves Arrive at different times because they travel at different speeds. In a solid material, P-waves travel about 1.7 times faster than S waves. Surface waves travel the slowest (90% slower than S waves)
Seismogram Wave Identification
Locating an Earthquake The difference in velocities of P and S waves provides a way to locate the epicenter. The longer the distance traveled by the waves, the greater the difference in arrival times of the P and S waves will be. So basically, a long gap means a far away earthquake!
Earthquake Distance A system for locating earthquake epicenters was developed by using seismograms from earthquakes whose epicenters could be easily pinpointed (usually from physical evidence). Travel-time graphs constructed from these seismograms. Two steps to do so: 1 st : Find time interval between the arrival of first P wave & first S wave 2 nd : Find the equivalent time spread between the P and S wave curves on time-travel chart.
Earthquake Direction So now we know distance from the seismic station, but in what direction was it? Precise location can be found when you know the distance for 3 or more seismic stations.
Locating Earthquakes Draw a circle around each station representing the distance of the epicenter from each station. The point where the three circles intersect is the epicenter of the quake! Travel-time graphs from 3 or more seismographs must be used to find exact location of earthquake epicenter.
Earthquake Zones About 95% of the major earthquakes occur in a few narrow zones. Most occur around the outer edge of the Pacific Ocean. Known as the circum-pacific belt Active earthquake areas in this region are: Japan, Philippines, Chile, and Alaska s Aleutian Islands. Second Zone: Around Mediterranean Sea. Mediterranean-Asian Belt Third Zone: Continuous belt extending for thousands of kilometers through the world s oceans. Coincides with oceanic ridge system.
Earthquake Zones
Measuring Earthquakes Historically, scientists have used two different types of measurements to describe the size of an earthquake: Intensity Magnitude Intensity: Measure of the amount of earthquake shaking at a given location based on the amount of damage. Not quantitative because it is based on uncertain personal damage estimates. Magnitude: Quantitative measurements that rely on calculations using the size of seismic waves or the amount of energy released at the source of an earthquake.
Richter Scale A familiar but outdated scale for measuring the magnitude of earthquakes is the Richter Scale. Based on the amplitude of the largest seismic wave (P, S, or Surface Wave) recorded on a seismogram. Earthquakes vary in size so the Richter scale is logarithmic. A tenfold increase in wave amplitude = increase of 1 n the magnitude scale. 5.0 quake is 10 times more shaking than a 4.0 Seismic waves weaken as the distance between the earthquake focus and the seismograph increases. Richter scale is only useful for small, shallow earthquakes within about 500km of the epicenter. Scientists no longer use the Richter Scale
Moment Magnitude In recent years, scientist shave been using a more precise means of measuring earthquakes. Called moment magnitude scale The moment magnitude scale is derived from the amount of displacement that occurs along a fault zone. Does NOT measure ground motion at a distant point. Calculated using several factors: Average amount of movement along the fault Area of the surface break Strength of broken rock (Surface area of fault) x (average displacement along fault) x (rigidity of rock)
Moment Magnitude Together these factors provide a measure of how much energy rock can store before it suddenly slips and releases this energy during an earthquake. Moment magnitude is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes.
Moment Magnitude
Deep Quakes vs. Shallow Quakes Shallow earthquakes do MORE damage than deep earthquakes because their focus is closer to the surface and more movement is felt. Shallow Quakes (70km from the surface or less) Intermediate Quakes (70-300km underground) Deep Earthquakes (300+km underground) Deep earthquakes waves have further to travel and so are not as damaging.
Earthquake Depths
Let s Review List the three different seismic waves: Surface Waves, P Waves, and S Waves What is a fault? A fracture in Earth where movement has occurred. What is the source of an earthquake called? The focus What order to the three basic types of seismic waves reach a seismograph in? P Waves, S Waves, Surface Waves