Taiwan, 1999. (Sathiyam.tv)
This document was modified and supplemented by BYU-Idaho faculty. All illustrations are from USGS unless otherwise noted. Learning Objectives Your goals in studying this chapter are to: Understand exactly what an earthquake is and the related terminology. Understand the types of faults and seismic waves Understand how earthquakes are measured. Understand the kinds of damage earthquakes can cause. Understand earthquake mitigation measures, including basic principles of seismic engineering. Understand earthquake risk in the United States. Understand the limitations of earthquake prediction. Understand earthquake preparedness.
What Is An Earthquake? On April 18, 1906, the earth moved. Not only did the ground shake on the day of the Great San Francisco earthquake, but land on both sides of the San Andreas fault permanently shifted. Precise measurements of the amount of motion led scientists to discover why earthquakes happen. Fences across the San Andreas fault ripped apart, and it was no longer clear who owned the land nearby. Surveyors went to mountain peaks to relocate the property boundaries. While the fences showed that ground had moved near the fault, the surveyors also discovered that much of northern California had moved and distorted during the earthquake. The movement followed a pattern with most of the motion near the fault and less motion far away. At the time, nobody knew what caused earthquakes. The survey measurements led a scientist named H. F. Reid to propose one possible explanation (in the Lawson Report). He hypothesized that strain built up in the earth s crust like the stretching of a rubber band. At some point, the earth would have to snap, sending shockwaves through the earth in an earthquake. This process is called the elastic rebound theory. The problem was that Reid didn t know what caused the strain to build up. Scientists continued to survey after the earthquake and saw that motion continued throughout California, providing an important piece of evidence that the Earth s tectonic plates are in constant motion. This plate motion is Reid s missing cause of strain. Two plates can get stuck together where they meet (at boundaries called faults), but forces deep within the earth drag and pull the plates in different directions. Faults remain stuck together for many years as the nearby crust deforms and stretches, but eventually the strain is too much and the two plates shift suddenly in an earthquake. Today, scientists monitor the buildup of strain near locked faults using satellite observations, and have discovered that the pattern is much like Reid hypothesized 100 years ago. elastic rebound movie 1 Elastic rebound elastic rebound movie 2
Faults and Basic Terminology Earthquakes are the result of relentless forces deep within the Earth that continuously stress the lithosphere and crus. The energy from these forces is stored as strain (deformation or bending) in the rocks. When this energy is released suddenly, an earthquake results. The area underground on the fault where the sudden rupture takes place is called the focus or hypocenter of the earthquake. The point on the Earth's surface directly above the hypocenter is called the epicenter of the earthquake. (see illustration on following page) A fault scarp is a rupture of the earth s surface along a fault. It is typically manifest as an unusually steep step in ground level. Fault scarps at Borah Peak, Idaho (left) formed in the M6.9 earthquake in 1983, and at the Red Canyon fault, Montana (above) formed during the M7.3 Hebgen Lake earthquake in 1959. The Culligan Ranch above had just been built as a safe haven in case of nuclear war, but as they say in the real estate business, location is everything. The building on the upthrown side (right) was completely flattened.
The strike-slip Denali fault in Alaska (both photos) ruptured for more than 200 miles in a M7.9 earthquake in 2002. In the photo above, the fault (between the arrows) offset the canyon streams. The geologists at right are examining the scarp. (USGS)
The aftershock pattern of this moderate earthquake in Hawaii is typical aftershocks die out with time, and magnitudes decrease. (USGS) Click to see an animation of aftershocks. (IRIS) An aftershock is a quake in the same volume of crust as the main shock, often on the same fault. They occur when the displaced rock stabilizes in its new position, and they relieve the last bits of stress. Aftershocks diminish in both magnitude and frequency with time, as shown on the chart above. Some aftershocks have occurred more than a year after the main shock. A seismogram (above) showing a main shock, foreshock, and aftershock. The map at right shows aftershocks of the great M9.0 Tuhoku earthquake in northern Japan in March, 2011. (USGS)
Fact: 80% of all earthquakes take place at or near tectonic plate boundaries. We will learn about some important exceptions. epicenters (generalized) hypocenters (generalized)
The rocks get narrower At/near transform boundaries animation The rocks are in compression animation The rocks are in extension The rocks get wider animation
Normal faults break the crust into horsts (uplifted blocks) and grabens (valleys). Extension of the crust has broken the western United States into horsts and grabens across western Utah, all of Nevada, and west to the Sierra Nevada mountains, which are also a horst. This region is known as the Basin and Range geological province. (NPS)
Basin and Range province, United States
This diagram shows the Teton Range horst and the Jackson Hole graben. The block of rock above a fault is called the hanging wall, and the block of rock below the fault is called the footwall. Miners gave them these names because they frequently tunneled along faults to find the valuable minerals deposited there by hot waters one block hung over their heads, and the other was under their feet. The Teton Range is the northeastern margin of the Basin and Range province. To the east is the northern Rocky Mountains province. (NPS)
The Teton fault in western Wyoming is one of the major seismic hazards nearest to BYU-Idaho. The Tetons are a horst, and the Jackson Hole valley is a graben. The fault scarp is shown at left, the result of several earthquakes that ruptured the surface. (NPS)
Recognizing Active Faults A fault is defined as a fracture in earth s crust along which displacement has taken place. Most faults shown on maps have not moved in a very long time, up to millions of years. Active faults, however, are defined as the ones that have had displacement more recently and have potential to slip again. Faults are most often stuck tight, but at times they lurch several feet in a great earthquake. Living near faults is a fact of life for many, but how do you recognize an active fault? Some faults creep, which means they move very slowly all the time. Structures like bridges, sidewalks, and buildings built astride these faults will be offset as the faults slowly move (up to a half inch each year). You can find these faults by looking for bent or offset curbs and sidewalks in cities like Hollister, California (see the photo at top right). Not every offset curb is a fault, but if you find several features that all line up, you may have found a fault. Most faults don t creep, however, so geologists look for the effects faults have on the landscape. Natural features like streams, valleys, and ridges can be offset from repeated earthquakes if they cross the fault (Photo 2). Active faults also make their own landscape features. If one side of the fault moves up or down, it creates a long, straight step called a scarp. As faults move along in repeated earthquakes, the rock along the fault is broken and ground down. This shattered zone is more easily eroded than the surrounding rocks, so long valleys can form along the fault (Photo 3). So faults can cause both ridges and valleys to form. Faults also can disrupt the movement of underground water, forcing it to the surface to form springs and ponds. You ve seen all of these features when you took the Google Earth tour of the San Andreas in a previous week. A lot of these features are easiest to spot from the air. Our newest tool to find faults is Laser Imaging Detection And Ranging (LIDAR), which uses laser light from an airplane to make a detailed image of the ground surface that can even see through trees in a forest. Being able to read the landscape allows us to pinpoint the exact location of dangerous faults. This sidewalk is offset by over one foot due to creep on the Calaveras Fault. The arrows show the direction of movement. Crystal Springs Reservoir lies within the long, straight valley broken up by the San Andreas Fault, several miles south of San Francisco. BYUI The stream in this photo is offset by displacement along the San Andreas Fault. As the fault continues to move, the two parts of the stream will get farther apart. A straight step, or scarp runs along the fault.
When you take these tours in Google Earth, make sure the faults item with each one is checked so the faults will be displayed. You should also check the populated places layer (under borders and labels and the labels subfolder). Tour the Wasatch fault Tour faults in southern California landslides Tour faults in the BYU- Idaho region landslides BYUI
Seismic Waves Every earthquake generates a suite of seismic waves. The waves generated at the hypocenter are primary or p-waves, and secondary or s-waves. P-waves are compressional waves, like sound waves. S-waves are shear waves, and travel about a third slower than p-waves hence they are secondary, or the second to arrive at a seismometer. When p- and s-waves reach the earth s surface, some of their energy is converted into surface waves, which include Rayleigh and Love waves (see the diagram and video). Surface waves travel slower than s-waves, and have higher amplitudes, especially in soft soils. Surface waves do most of the damage in earthquakes, and are the memorable ones most people describe after a quake. seismic waves video How seismic waves travel
animation Watch the seismic waves from a quake on a normal fault travel through earth s crust and along the surface. waves This animation compares travel of seismic waves to waves from a drop hitting water. The data are from the Wells, Nevada quake in 2005.