Earthquakes

ESSC 100 – Intro to Earth Science EARTHQUAKES Ancient cultures offered a variety of explanations for earthquakes activity (seismicity), most of which involved the action or mood of a giant animal or god. Today we know that an earthquake is the motion or trembling of the ground produced by several factors, including: • sudden displacement of rock in the Earth's crust; • volcanic eruptions; • giant landslides; • a meteorite impact, or • underground nuclear-bomb tests. EARTHQUAKES AND EARTH’S INTERIOR STRESS AND STRAIN, PLASTIC AND BRITTLE DEFORMATION •Stress is the push, pull or shear that a material feels when it is subjected to a force. •Strain is the change in shape of a material in response to the application of a stress. •Brittle deformation: permanent deformation in which the rock fractures or crack, instead of flowing or bending. •Plastic (ductile) deformation: permanent deformation in which a rock may change its shape by flowing or bending. ELASTIC REBOUND • Along plate boundaries, rocks are under stress. • Rocks initially deform plastically, but when the stress exceeds the strength of rocks, they break along a fault. • The accumulated strain is suddenly released as seismic waves. • Rocks are somewhat elastic, so they try to snap back after they break. • The fault remains as a weakness in the rock. See textbook figure 7.4 SAN FRANCISCO, 1906 EARTHQUAKES General features • Vibration of Earth produced by a rapid release of energy • Associated with movements along faults • Explained by the plate tectonics theory • Mechanism for earthquakes was first explained by H. Reid • Rocks "spring back" – a phenomena called elastic rebound • Vibrations (earthquakes) occur as rock elastically returns to its original shape 1 TERMINOLOGY • When an earthquake occurs, the sudden movement of the rock causes seismic waves to radiate out from the area where the movement occurred (the earthquake hypocenter or focus) at a speed of several kilometers per second. • EPICENTER: directly above the hypocenter, is the location of earthquakes projected at the surface (in latitude and longitude. • Main Shock: largest and generally First earthquake in a sequence. • Aftershock: smaller earthquakes after first main shock. Can last as much as a month afterward. Can be almost as large as main shock, generally smaller. Decrease in magnitude with time. • Foreshock: an earthquake that occurs prior to a large one. EARTHQUAKE WAVES Earthquake waves • Types of earthquake waves • Body waves • generated at the focus when an earthquake occurs. • travel in the interior of the Earth (3-7 km/sec) • P waves - Primary • S waves - Secondary • Surface waves • Produced when body waves “hit” the surface • Complex motion • Slowest velocity of all waves • Study of earthquake waves is called seismology • Earthquake recording instrument (seismograph) • Records movement of Earth • Record is called a seismogram PRIMARY (P) WAVES • Push-pull (compressional) motion: particles are displaced parallel to the direction of wave propagation. • Travel through solids, liquids, and gases • Greatest velocity of all earthquake waves (up to 7 km/sec) • Because P waves are like sound waves, when they reach the surface they can create sound waves in the air that are audible to humans and animals. SECONDARY (S) WAVES • "Shake" motion: they propagate by laterally displacing the medium through which they move • Travel only through solids • Slower velocity than P waves (about 3.5 km per second.) • However, S waves, because of their shearing motion, are far more damaging to structures than P waves. SURFACE WAVES Surface waves are slower than body waves. Because their motion is restricted to the surface of the earth, they generally have a longer distance to travel to reach a particular point than do body waves. Rayleigh (R) waves - make the surface of the ground to go up and down, like ripples on the surface of a pond. Love (L)waves - are a horizontal displacement at the surface, that is, they cause the surface of the ground to shear sideways. The horizontal shaking of Love waves is particularly damaging to building foundations. RECORDING EARTHQUAKES: SEISMOGRAPHS What we want to do to record an earthquake is to measure the shaking of the earth. •However, everything attached to the earth, including our measuring instruments, will move with the earth. •We need a stationary frame of reference from which we can measure the shaking without being a part of it. •Although we cannot easily detach our instruments from the earth, we can take advantage of inertia to isolate them from earth movements. •Inertia is the tendency of an object at rest to remain at rest. Rayleigh (R) waves Love (L) waves • The key to a seismograph is the presence of a weight that stays fixed in space while everything else moves around it. 2 HOW A SEISMOGRAPH WORKS • Let’s consider a mechanical vertical-motion seismograph consisting of a heavy weight (like a pendulum) suspended from a spring. • When an earthquake wave arrives and causes the ground surface to move up and down, it makes the seismograph frame also move up and down. • The weight, however, remains fixed in space. As a consequence, the revolving paper roll moves up and down under the pen, which traces out the waveforms representing the up-and-down movement. • On a real seismograph record, one revolution of the paper cylinder corresponds to an hour; a single paper roll holds the record for a whole day. Horizontal (N-E) Figure 7.10 Vertical (Z) SEISMOGRAMS The seismic trace recorded on a seismograph contains a variety of information useful for analyzing the intensity, distance from the epicenter, and location of the earthquake. One can recognize the different kinds of earthquake waves on the seismic trace. The first pulse of waves are the fast moving primary waves (P) waves. As they begin to fade, the second large pulse records the arrival of the slower secondary (S) waves. Finally, a mishmash of surface waves and reflected p and s waves arrive. Seismograms: the written record of an earthquake Seismic waves reach a recording station at different times producing identifiable sets of waves (pattern). P→ emerge at steep angle producing mainly vertical ground motion S→ last somewhat longer than P trains (E-N components of motion) Earthquake coda→ the dying end of an earthquake composed of a mixture of surface waves (L-R) and scattered P and S late arrivals (deeper structures) Wave Dispersion See textbook figure 7.12 WHAT CAN WE LEARN FROM SEISMOGRAMS ? 1. Distance: we know how fast the fasted P and S waves travel, so we can use the difference in their arrival times (time lag) at the seismograph to determine how far away the earthquake was. 2. Origin Time: once we know how far away an earthquake was, we can determine the exact time that it happened. 3. Location of Epicenter: If we have the distances to the earthquake epicenter calculated from three or more seismic stations, then we can use triangulation to find the exact location of the epicenter. 4. Magnitude: the strength of the earthquake is indicated by the amplitude or size of the spikes on the seismograph. However, the farther away the seismic station is, the more attenuated the waves become and the smaller are the spikes produced by a given earthquake. Fortunately, since we know how far away the earthquake was, we can compensate for distance to determine how large the Earthquake was where it happened. FINDING THE EPICENTER By accumulating a tremendous amount of data, seismologists have determined the average times of S and P waves for any specific distance. These travel times are published as time-distance graphs (or traveltime curves), illustrating the difference between the arrival times of P and S waves. The farther away a seismograph station is from the focus of an earthquake, the longer the interval between the arrivals of the P and S waves, and hence the distance between the two curves on the graph. 3 TRIANGULATION The epicenter of any earthquake can be determined by using a timedistance graph and knowing the arrival times of the P and S waves at three seismograph locations. 1-Determine the distance of the epicenter from each of the seismographs. 2-For each seismograph, draw on a map a circle whose radius equals the distance from the epicenter. 3-The intersection of the three circles is the location of the earthquake’s epicenter. A minimum of THREE locations is necessary because two locations will provide two possible epicenters, and one location will provide an infinite number of epicenters. EARTHQUAKES Earthquake intensity and magnitude • Intensity • A measure of the degree of earthquake shaking at a given locality based on the amount of damage • Most often measured by the Modified Mercalli Intensity Scale • Magnitude • Concept introduced by Charles Richter in 1935 MEASURING THE MAGNITUDE •Earthquake strength is measured against a scale developed by the seismologist Charles Richter in 1935. •The magnitude of an earthquake is a measure of the amount of energy released. Each earthquake has a unique magnitude assigned to it. •The magnitude is calculated as the logarithm of the amplitude of waves recorded by seismographs. •The magnitude is determined by measuring the maximum amplitude of the largest seismic wave (usually a surface wave) and the difference between the arrival times of the P and S waves. •Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. THE RICHTER SCALE •The Richter scale is logarithmic, that is an increase of 1 magnitude unit represents a factor of ten times in amplitude. The seismic waves of a magnitude 6 earthquake are 10 times greater in amplitude than those of a magnitude 5 earthquake. •However, in terms of energy release, a magnitude 6 earthquake is about 31 times greater than a magnitude 5. •The total amount of energy released in the largest earthquakes ever recorded is 9.5 (Chile, 1960) (roughly equal to the energy of 10,000 Hiroshima sized atom bombs). Probably rocks are not able to store the energy necessary to generate earthquakes of higher magnitude. 1886 4 WHERE AND WHY DO EARTHQUAKES OCCUR? •The majority of earthquakes (~80%) occur in the circum-pacific belt. •The second major seismic belt (~15%) is the Mediterranean-Asiatic belt. •The remaining 5% of earthquakes take place mostly in the interior of plates and along oceanic spreading ridges (divergent plate boundaries). EARTHQUAKES AT PLATE BOUNDARIES • TRANSFORM MARGINS => shallow focus; • DIVERGENT MARGINS AND CONTINENTAL RIFTS => shallow focus; • INTRAPLATE => shallow focus • CONVERGENT MARGINS => shallow to deep. CLASSIFICATION Seismologists recognize three categories of earthquakes: •Shallow-focus-focal depth less than 70 km. These earthquakes usually generate along transform or divergent plate boundaries. In general these are the most destructive. •Intermediate-focus- foci between 70 and 300 km. •Deep-focus-foci deeper than 300 km. •Approximately 90% of all earthquake foci occur at a depth of less than 100 km. •Intermediate and deep earthquakes occur along convergent plate boundaries, especially along the circum-pacific belt. •Benioff zones => convergent margins. EARTHQUAKES Earthquake prediction • Short-range – no reliable method yet devised for short-range prediction • Long-range forecasts • Premise is that earthquakes are repetitive • Region is given a probability of a quake EARTHQUAKES DAMAGE Factors that determine structural damage: •Intensity of the earthquake •Duration of the vibrations •Nature of the material upon which the structure rests •The design of the structure Destruction results from •Ground shaking •Liquefaction of the ground (saturated material turns fluid, underground objects may float to surface •Tsunami, or seismic sea waves •Landslides and ground subsidence •Fires Denali Fault Earthquake, M7.9, Nov. 3, 2002 5 Aerial view of the Trans-Alaska Pipeline and Richardson Highway, looking north. Rupture along the fault resulted in approximately 2.5 meters (8 feet) displacement of the highway, with the north side moving east relative to the south side. Photo by Patty Craw, DGGS. An aerial photo of the Trans-Alaska Pipeline System (TAPS) line near the Denali fault, looking west. This is where the line is supported by rails on which it can move freely in the event of fault offset. Here the line has moved toward the west end of the rails. Alyeska Pipeline Service Company reported no breaks to the line and therefore no loss of oil. Note the transverse crack on the Richardson Highway in lower left. STRUCTURAL DAMAGE • Most buildings and bridges are constructed to withstand the downward force of gravity. Construction materials such as brick and concrete are very strong in compression and can support great weight. • Unfortunately, these same materials are brittle and incapable of resisting tensional forces introduced by bending. Side to side or upward motion introduces bending forces and these materials fail and collapse. Most people killed in earthquakes die from trauma caused by building collapse and object falling from walls. • The principle goal of earthquake engineering is to prevent loss of life from building collapse. Even if a building is damaged beyond repair, if it does not collapse on the occupants then it will not cause loss of life. LIQUEFACTION •In addition to the collapse of buildings, the shaking from earthquakes can cause ground composed of loose soil and sand to liquefy. •The most impressive example of liquefaction was seen at Turnagain Heights in Anchorage, during the Good Friday earthquake of 1964, where 60 foot high soft clay beach cliffs collapsed causing the slumping of developed land up to 900 feet inland along more than a mile of coastline. TSUNAMI TRAVEL TIMES TO HONOLULU EARTH'S LAYERED STRUCTURE Most of our knowledge of Earth’s interior comes from the study of P and S earthquake waves • Travel times of P and S waves through Earth vary depending on the properties of the materials • S waves travel only through solids 6 EARTH'S LAYERED STRUCTURE SHADOW ZONE •Absence of P waves from about 105 degrees to 140 degrees around the globe from an earthquake •Explained if Earth contained a core composed of materials unlike the overlying mantle Earth's layered structure Discovering Earth’s major layers • Discovered using changes in seismic wave velocity • Mohorovicic discontinuity • Velocity of seismic waves increases abruptly below 50 km of depth • Separates crust from underlying mantle 7