What type of earthquakes happen at transform boundaries

Almost all earthquakes occur at the edges of the crustal plates. The constant bumping, grinding, and lateral movement along crustal boundaries can create sudden movements that result in earthquakes. Each of the three types of plate boundaries—convergent, divergent, and transform—has a distinctive pattern of earthquakes.

There are two kinds of convergent boundaries : subduction and collision. A subduction boundary is marked by the oceanic crust of one plate that is being pushed downward beneath the continental or oceanic crust of another plate. A collision boundary separates two continental plates that are pushed into contact; the suture zone is the line of collision.

Both types of boundaries have distinctive earthquake patterns. Earthquakes associated with a collision boundary define shallow, broad zones of seismic activity that form in complex fault systems along the suture zone. The magnitude of the earthquake depends on the extent of the area that breaks the area of the rupture surface and the average amount of displacement sliding.

Most earthquakes take place near plate boundaries, but not necessarily right on a boundary, and not necessarily even on a pre-existing fault.

The distribution of earthquakes across the globe is shown in Figure 4. It is relatively easy to see the relationships between earthquakes and the plate boundaries. Along divergent boundaries like the mid-Atlantic ridge and the East Pacific Rise, earthquakes are common, but restricted to a narrow zone close to the ridge, and consistently at less than 30 km depth.

Shallow earthquakes are also common along transform faults , such as the San Andreas Fault. Along subduction zones earthquakes are very abundant, and they are increasingly deep on the landward side of the subduction zone. Earthquakes are also relatively common at a few intraplate locations. Along much of the boundary, the bulk of the motion occurs along the San Andreas Fault. Other parks in the region, namely Pinnacles, Channel Islands and Joshua Tree national parks, Cabrillo National Monument and Santa Monica Mountains National Recreation Area, reveal evidence of the shearing, rotation, and uplift that occurs within the broad zone of deformation between the two plates.

Deformation along the transform plate boundary in California can be visualized by placing a deck of cards between your hands in a praying position. Imagine that your left hand is the undeformed Pacific Plate, your right hand the intact North American Plate.

Notice what happens as you move your left hand away and slide your right hand toward you. The cards slip along their faces, forming a broad zone of shearing between your unaffected hands. For western California, each slipping card face would be a fault surface.

The broad zone of transform motion between the Pacific and North American plates formed numerous slivers of mountain ranges with narrow valleys in between.

The valleys are commonly due to erosion along individual fault lines. Lillie, Wells Creek Publishers, 92 pp. The broad zone of shearing at a transform plate boundary includes masses of rock displaced tens to hundreds of miles, shallow earthquakes, and a landscape consisting of long ridges separated by narrow valleys. The San Andreas Fault is just one of many active earthquake faults in a broad zone of shearing along the transform plate boundary in the San Francisco Bay Area. These forces also create a sheared-up landscape that includes spectacularly beautiful coastlines and economically important harbors.

Thousands of earthquakes over millions of years have built this landscape not only along the major fault line—the San Andreas Fault—but also on other faults within the broad zone of shearing between the Pacific and North American plates.

For example, rocks found today in Point Reyes National Seashore north of San Francisco were originally part of the line of granite rocks formed beneath ancient subduction zone volcanoes. The plate motion has plucked the rocks from their original position and moved them more than miles north-northwestward to their current position at Point Reyes. Other rocks in the San Francisco Bay Area were originally part of an accretionary wedge, similar to rocks found today in the coastal ranges of the Cascadia Subduction Zone in northern California, Oregon, and Washington.

The transform plate boundary is a broad zone forming as the Pacific Plate slides northwestward past the North American Plate. It includes many lesser faults in addition to the San Andreas Fault. Parks near the coast, including Point Reyes National Seashore, Golden Gate National Recreation Area, and Pinnacles National Park, contain volcanic and plutonic rocks that were plucked from the edge of the North American Plate and transported tens to hundreds of miles northwestward as part of the Pacific Plate.

Parks in the Sierra Nevada, including Yosemite, Kings Canyon, and Sequoia national parks, contain granite-type rocks that cooled within magma chambers beneath ancient subduction zone volcanoes. National Park Service sites along the transform plate boundary in California contain rocks formed during the earlier subduction that occurred in western North America. Like modern subduction zones, the region had an accretionary wedge Coast Range , a forearc basin Great Valley , and a volcanic arc Sierra Nevada.

Rocks have been disrupted by shearing and other forces associated with the transform plate motion and, in some instances, transported northward a long distance from where they originally formed. The magnitude 7. It caused extensive damage to the city, including fires that lasted for several days, and killed an estimated 3, people. The Earthquake Trail at Point Reyes weaves back and forth across the fault line.

Exhibits along the trail include the reconstruction of a fence that was offset 16 feet 5 meters during the earthquake. Doing some quick math, one can appreciate how dramatically plate-tectonic forces can affect the landscape, even in our lifetimes. The average movement of the Pacific Plate past the North American Plate in California is about 2 inches 5 centimeters per year. The 16 feet about inches, or 5 meters of offset along the fence line thus carries a powerful message.

Every century or so a large earthquake is necessary to release stress accumulated along large segments of the San Andreas Fault that lock rather than slip smoothly. This type of knowledge helps us better design and site infrastructure, and develop disaster preparedness plans so that our families and communities are less at risk when earthquakes do strike. Here, as we saw in Chapter 10, the segments of the mid-Atlantic ridge are offset by some long transform faults.

Most of the earthquakes are located along the transform faults, rather than along the spreading segments, although there are clusters of earthquakes at some of the ridge-transform boundaries. Some earthquakes do occur on spreading ridges, but they tend to be small and infrequent because of the relatively high rock temperatures in the areas where spreading is taking place.

The distribution and depths of earthquakes in the Caribbean and Central America area are shown in Figure In this region, the Cocos Plate is subducting beneath the North America and Caribbean Plates ocean-continent convergence , and the South and North America Plates are subducting beneath the Caribbean Plate ocean-ocean convergence. In both cases, the earthquakes get deeper with distance from the trench. In Figure There are also various divergent and transform boundaries in the area shown in Figure The distribution of earthquakes with depth in the Kuril Islands of Russia in the northwest Pacific is shown in Figure This is an ocean-ocean convergent boundary.

The small red and yellow dots show background seismicity over a number of years, while the larger white dots are individual shocks associated with a M6. The relatively large earthquake took place on the upper part of the plate boundary between 60 km and km inland from the trench.