Vancouver Island to Cape Mendocino California and a large tract of seafloor real estate in between. But the central part of the old Farallon plate vanished beneath North America. 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 Megathrust earthquake. This action takes place along the interface between the plates from the Juan de Fuca Trench offshore 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 Peninsula, 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.
Tectonic speed limits from plate kinematic reconstructions. Earth and Planetary Science Letters , , Crust forming faster? But the result is controversial, since previous work seemed to show the opposite. Plate tectonics is driven by the formation and destruction of oceanic crust. This crust forms where plates move apart, allowing hot, light magma to rise from the mantle below and solidify. Where plates are being pushed together, the crust can either rise up to form mountains or one plate is shoved under the other and is sucked back into the mantle.
These zones are presently inactive, but the offsets of the patterns of magnetic striping provide evidence of their previous transform-fault activity. Not all plate boundaries are as simple as the main types discussed above.
In some regions, the boundaries are not well defined because the plate-movement deformation occurring there extends over a broad belt called a plate-boundary zone. One of these zones marks the Mediterranean-Alpine region between the Eurasian and African Plates, within which several smaller fragments of plates microplates have been recognized.
Because plate-boundary zones involve at least two large plates and one or more microplates caught up between them, they tend to have complicated geological structures and earthquake patterns.
We can measure how fast tectonic plates are moving today, but how do scientists know what the rates of plate movement have been over geologic time? The oceans hold one of the key pieces to the puzzle.
Because the ocean-floor magnetic striping records the flip-flops in the Earth's magnetic field, scientists, knowing the approximate duration of the reversal, can calculate the average rate of plate movement during a given time span. These average rates of plate separations can range widely. The Arctic Ridge has the slowest rate less than 2. Evidence of past rates of plate movement also can be obtained from geologic mapping studies. If a rock formation of known age -- with distinctive composition, structure, or fossils -- mapped on one side of a plate boundary can be matched with the same formation on the other side of the boundary, then measuring the distance that the formation has been offset can give an estimate of the average rate of plate motion.
This simple but effective technique has been used to determine the rates of plate motion at divergent boundaries, for example the Mid-Atlantic Ridge, and transform boundaries, such as the San Andreas Fault. Current plate movement can be tracked directly by means of ground-based or space-based geodetic measurements; geodesy is the science of the size and shape of the Earth.
Ground-based measurements are taken with conventional but very precise ground-surveying techniques, using laser-electronic instruments. However, because plate motions are global in scale, they are best measured by satellite-based methods. The late s witnessed the rapid growth of space geodesy, a term applied to space-based techniques for taking precise, repeated measurements of carefully chosen points on the Earth's surface separated by hundreds to thousands of kilometers.
The three most commonly used space-geodetic techniques -- very long baseline interferometry VLBI , satellite laser ranging SLR , and the Global Positioning System GPS -- are based on technologies developed for military and aerospace research, notably radio astronomy and satellite tracking. Among the three techniques, to date the GPS has been the most useful for studying the Earth's crustal movements. Twenty-one satellites are currently in orbit 20, km above the Earth as part of the NavStar system of the U.
Department of Defense. These satellites continuously transmit radio signals back to Earth. To determine its precise position on Earth longitude, latitude, elevation , each GPS ground site must simultaneously receive signals from at least four satellites, recording the exact time and location of each satellite when its signal was received.
By repeatedly measuring distances between specific points, geologists can determine if there has been active movement along faults or between plates. The separations between GPS sites are already being measured regularly around the Pacific basin.
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