North American Plate - Wikipedia
ten Brink, Uri S.; Miller, Nathaniel; Andrews, Brian; Brothers, Daniel; Haeussler, Peter J. The relative plate motion between the Pacific and North American plates To meet these goals, PBO will install continuous GPS stations, Pacific Ocean crust west of southwest North America was formed by Cenozoic distributed Pacific and North America relative plate motion strain that could . Shepard & Emery () were the first to document and provide .. are not well understood, and interpretations vary (Legg ; Miller ; Fig. Ben Miller. they are moving as well. The whole of continental North America, the northeastbound Pacific Plate and the southwestbound North American Plate; the When two plates meet, one made of granite and one of basalt, the basalt.
Research results that provide information about seismic risk for a given area are incorporated into local building codes used by architects and engineers. Figure Map of earthquake risk.
KGS Pub. Inf. Circ. Earthquakes
The amount of damage that occurs during an earthquake is closely related to the geology of an area. This relationship was first scientifically observed in the damage pattern of the San Francisco earthquake.
The most severe damage occurred on "made land," where people used fill to raise land in areas that were formerly below sea level. Similarly, the major damage in San Francisco from the Loma Prieta earthquake was on made land, showing people's unfortunate tendency to ignore the lessons of history.
Recording and Measuring Earthquakes Earthquakes generate vibrations called seismic waves that travel through the earth in all directions from the focus, the point beneath the earth's surface where the earthquake begins. The point on the earth's surface directly above the focus, where the strongest shaking occurs, is called the epicenter. Shaking decreases with distance from the epicenter. Seismologists use sensitive instruments called seismometers to record the waves.
Seismometers can electronically amplify seismic waves more than 10, times and are sensitive enough to detect strong earthquakes originating anywhere in the world. The time, location, and magnitude of an earthquake can be determined from a graphical plot of the data called a seismogram. To measure the strength of an earthquake, seismologists use two different types of scales: The Modified Mercalli Intensity scale gauges earthquakes by their effect on people and structures.
It was originally developed in in Italy and relies on newspaper and eyewitness reports.
This scale also is used to estimate the size, or magnitude, of earthquakes that occurred before sensitive instruments existed to measure them. It has 12 levels designated by roman numerals, ranging from imperceptible shaking I to catastrophic destruction XII fig. Figure Approximate comparison for the U. Midcontinent of Modified Mercalli and Richter scales at locations very near the epicenter.
The most readily recognized magnitude scale is the Richter magnitude scale. It was developed in by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes.
Since then, other magnitude scales based on different mathematical formulas that overcome some of the Richter scale's limitations have been developed. In the United States, the most commonly used scale today is the moment magnitude scale, which is based on a formula that more accurately measures medium and large earthquakes than Richter's. Because the numbers on the Richter scale have become so universally familiar, all measurements calculated with the different formulas are converted into numbers comparable to those on the Richter scale.
Using records of seismic waves plotted on a seismogram, seismologists determine magnitude mathematically based on the size of the recorded waves and the calculated distance between the earthquake focus and the seismometer.
Magnitude scales express magnitude in whole numbers and decimal fractions. Each increase in magnitude by one whole number represents a tenfold increase in measured wave size. In terms of energy, each whole-number increase represents 31 times more energy released.
For example, a magnitude 5. The world averages about 20 earthquakes each year of magnitude 7. The largest earthquake ever recorded in the world was a magnitude 9. Sensitive seismometers are capable of recording nearby earthquakes of magnitudes A person with a sledgehammer can generate the equivalent of a magnitude An earthquake's magnitude does not necessarily express the damage it caused. In a densely populated area, an earthquake may do far more damage than one of greater magnitude that occurs in a remote area.
For example, the magnitude 6. Though it was the deadliest earthquake inits magnitude was lower than 25 other earthquakes recorded that year. Although magnitude scales and the Modified Mercalli Intensity scale are not strictly comparable, they can be roughly correlated for locations near the epicenter of an earthquake fig. Earthquakes in Kansas Many Kansans have never felt so much as a tremor, but at least 25 earthquakes were felt in the state between and fig.
The Kansas Geological Survey KGS also measured more than earthquakes during a study of Kansas seismic activity between and fig. Most of the ones the KGS detected were microearthquakes, or earthquakes too small to feel.
The largest recorded Kansas earthquake was centered near Wamego east of Manhattan in Estimating the magnitude based on the intensity near the epicenter, it probably measured between magnitude 5.
Figure Historical earthquakes in Kansas, prior to Figure Microearthquakes recorded by the Kansas Geological Survey between August and August are size-coded by local magnitude. The largest event had a magnitude of 4. Some Kansas earthquakes are associated with the Nemaha Ridge, a buried mountain range that extends from roughly Omaha, Nebraska, to Oklahoma City fig. The ridge was formed about million years ago and is bounded by faults that are still slightly active today, especially the Humboldt fault zone that forms the eastern boundary of the Nemaha Ridge and passes near Wamego, east of Manhattan, and El Dorado, east of Wichita.
About 50 miles 80 km west of the Nemaha Ridge is the Midcontinent rift, a zone of the earth's continental crust that was ripped apart and filled with oceanic-type crust basaltic rocks about 1. This zone of rifting extended from central Kansas near Salina, northeastward across Nebraska, Iowa, and Minnesota, and into the Lake Superior region.
For unknown reasons, the rifting stopped after spreading only about 30 to 50 miles km ; if it had not stopped, eastern and western Kansas would likely be on different continents today. Figure Major regional tectonic features that are apparently related to earthquake activity.
Nemaha County is the locality where the Nemaha Ridge was discovered by drilling in the early s Kansas Geological Survey,Bulletin Recording Kansas Earthquakes To better understand earthquakes in Kansas, seismologists at the KGS maintained a network of seismometers throughout the state from December to June to monitor sesmic activity. The North American plate is moving to the west-southwest at about 2. This may seem like small and slow motion but over geologic time scales these movements add up to hundreds and thousands of kilometers, and can reform parts of the surface of Earth.
Vancouver Island to Cape Mendicino California and a large tract of sea floor real estate in between. It was subducted beneath California leaving the San Andreas fault system behind as the contact between the North America and Pacific plates. The Juan de Fuca Plate is still actively subducting beneath N. Its motion is not smooth, but rather sticky; strain builds up until the fault breaks and a few meters of Juan De Fuca slips under North America in a big earthquake.
This action takes place along the interface between the plates from the Juan de Fuca Trench off shore down-dip until the fault is too weak to store up any elastic stress.
The locked zone varies in width from a few tens of kilometers km along the Oregon coast to perhaps a hundred km or more off of Washington's Olympic Penninsula, and is about 1, km long. These plate motions are the primary source of strain in the lithosphere that lead to earthquakes in our region. In California, much of the strain generated by the grinding of the Pacific Plate against North America is taken up in earthquakes on the San Andreas Fault and related structures.
British Columbia however is part of rigid North America and moves with it. Shoveling off all the sedimentary deposits from the basement rocks underlying the Puget Lowland would certainly be one way to reveal this pattern.We're The Millers - Scotty P