Post Falls High School, Physical Geology. PLATES BOUNDARIES Earthquakes occur along plate boundaries SEISMIC ACTIVITY. EARTHQUAKES IN PLATE MARGINS. Stars indicate earthquake foci. Divergent –They are Shallow and weak. B. Subduction – Deep (down to 700 km) and very strong.
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PLATES BOUNDARIES Earthquakes occur along plate boundaries SEISMIC ACTIVITY
EARTHQUAKES IN PLATE MARGINS • Stars indicate earthquake foci. • Divergent –They are Shallow and weak. • B. Subduction – Deep (down to 700 km) and very strong. • C. Convergent – Down to 300 km and sometimes strong. • D. Transform Fault – Down to 100 km and often strong.
An Earthquake is caused by sudden release of elastic energy stored in rocks. Elastic deformation: Any change of shape or size that disappears when the deforming forces are removed.
The focus of an earthquake is the site of first movement on a fault and the center of energy release. The epicenter of an earthquake is the point of the Earth's surface that lies vertically above the focus.
When an earthquake occurs, the elastically stored energy is carried outward from the focus to other parts of the Earth by vibrations. These vibrations are called seismic waves and spread out spherically in all directions, just as sound waves do.
SEISMIC WAVES Body Waves: Travel outward in all directions from the focus and have the capacity to travel through the Earth's interior. P (primary) waves -compression/expansion S (secondary) waves - shear Surface Waves: Travel around but not through the Earth; they are guided by the Earth's surface.
TRAVEL TIME OF SEISMIC WAVES
REFRACTION AND REFLECTION OF BODY WAVES
P waves are the fastest waveS waves are slightly slowerL waves are the slowest of the three waves
A highly simplified simulated recording of earthquake waves (a seismogram). This seismogram is a simulation. The actual records of earthquake waves are far more complicated than what is presented here
The speed differences between waves are used to determine the distance away an earthquake is from the recording station. A seismogram can not tell in what direction the earthquake occurred. Seismologist use simple triangulation (math) to determine the epicenter of an earthquake.
Measuring Earthquake Magnitude The Richter magnitude scale is used to measure the strength of an earthquake. It assigns a single number to quantify the size of an earthquake. It is a base-10 logarithmic scale. Developed in 1935 by Charles Richter in collaboration with Beno Gutenberg.
Magnitudes Effects Less than 3.5 Generally not felt, but recorded 3.5-5.4 Often felt, but rarely causes damage Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions. 6.1-6.9 Can be destructive in areas up to about 100 kilometers across where people live 7.0-7.9 Major earthquake. Can cause serious damage over larger areas 8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across
Measuring Earthquake Magnitude The Modified Mercalli Intensity Scale is commonly used in the United States by seismologists seeking information on the severity of earthquake effects. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end. The Intensity Scale differs from the Richter Scale in that the effects of any one earthquake vary greatly from place to place, so there may be many Intensity values (e.g.: IV, VII) measured from one earthquake
People do not feel any Earth movement. • II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. • Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring. • IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock. • Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.
VII Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well- built buildings; considerable in poorly built buildings. • Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.
Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage. • X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. • XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. • XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.
Moment Magnitude • Richter's original methodology is no longer used because it does not give reliable results when applied to M >= 7 earthquakes and it was not designed to use data from earthquakes recorded at epicentral distances greater than about 600 km. It is, therefore, useful to separate the method and the scale in releasing estimates of magnitude to the public
Seismic moment of an earthquake is given byThe actual moment magnitude is calculate using
Earthquake hazards exist throughout the United States
GREAT U.S. EARTHQUAKES • New Madrid 1811 and 1812 Earthquakes • The Great 1906 San Francisco Earthquake - April 18, 1906 5:12 AM • The great Alaska earthquake of 1964 – March 27,1964 5:36 p.m.
Although most Americans associate earthquakes with California, the tremors that shook the Mississippi valley in southeast Missouri from December 16, 1811, through February 7, 1812, are among the most violent quakes ever to hit the North American continent in recorded history. Collectively known as the New Madrid earthquakes, these quakes affected more than 1 million square miles.
1811-1812 New Madrid Earthquakes Roman numerals indicate estimated Modified Mercalli intensities for a 6.5 magnitude earthquake. The New Madrid seismic zone is so named because the town of New Madrid, Missouri was the closest settlement to the epicenters of the 1811-1812 quakes.
Isoseismal map for the earthquake of December 16,1811 (first of the 1811-1812 New Madrid series). Felt in Washington & Boston (causing church bells to ring).
Severe shaking accompanied the powerful New Madrid earthquakes. By winter's end, few houses within 250 miles of the Mississippi River town of New Madrid (Missouri) remained undamaged.
A Detailed Narrative of the Earthquakes which occurred on the 16th day of December, 1811 Samuel L. Mitchill “On the morning of Monday, the 16th of December, 1811, several shocks of earthquakes were felt at the city of Washington. The first of these happened at three o'clock; and in some houses was considerable enough to shake the doors and windows, and wake persons from their sleep. There were successive tremors. Tassels of curtains were seen to move; and pitchers of washing-stands were heard to rattle upon their basins. The sound was very distinguishable, and was believed by many to pass from southwest to northeast. The alarm was so great in some families, that searches were made from room to room, to discover the robbers who were imagined to have broken into the houses.”
The Great 1906 San Francisco Earthquake The quake lasted only a minute but caused the worst natural disaster in the nation's history. Modern analysis estimates it registered 8.25 on the Richter scale. The greatest destruction came from the fires the quake ignited. These ravaged the city for three days before burning themselves out.
This photograph by Arnold Genthe shows Sacramento Street and approaching fire.
Refugees search for souvenirs on sidewalk in front of the Stanford mansion on Powell Street at California.
This photograph, taken from a tethered balloon five weeks after the great earthquake
The quake awoke G.A. Raymond as he slept in his room at the Palace Hotel. This is an excerpt from his description of escape. “How I reached the ferry I cannot say. It was bedlam, pandemonium and hell rolled into one. There must have been 10,000 people trying to get on that boat. Men and women fought like wildcats to push their way aboard.”
16 Views of the 1906 San Francisco Earthquake and Fire From the Archives of The Museum of the City of San Francisco
Fire around the Call Building at Third and Market Streets. Emporium, at Powell and Market, is at right.
“Ham and Egg” fire burns at Grove and Laguna streets. View is along Grove Street. City Hall dome can be seen in the smoke cloud.
Close-up view of refugees fleeing along Grove Street. The large clusters of refugees along the street suggest they are involved in rescuing trapped persons.
Crowds gather at Market and Laguna streets to flee the Great Fire. Building at lower center right still survives as do along Laguna. Almost all others pictured here burned.
View from Laguna and Market streets of the Great Fire burning through the Mission District.
Dreadful earthquake damage, along Grant Ave., in Chinatown.