To the causes of earthquakes, Ancient cultures offered a variety of explanations for seismicity (earthquake activity), most of which involved the action or mood of a giant animal or god. Scientific study suggests that seismicity instead occurs for several reasons, including:
- the sudden formation of a new fault (a fracture or rupture on which sliding occurs)
- sudden slip on an already existing fault
- a sudden change in the arrangement of atoms in rock minerals
- movement of magma in, or explosion of, a volcano
- a giant landslide
- a meteorite impact
- an underground nuclear-bomb test
Of these various reasons, faulting related to plate movements is by far the most significant. In other words, where do most earthquakes occur are along faults slip.
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| Earthquake hypocenters and epicentres. |
The place within the Earth where rock ruptures and slips, or the place where an explosion occurs, is the hypocenter or focus of the earthquake. Energy radiates from the focus. The point on the surface of the Earth that lies directly above the focus is the epicentre, so maps can portray the position of epicentres (figure above a, b). Since slip on faults causes most earthquakes, we focus our discussion on faults.
How earthquakes manifest? Where do most earthquakes arise? Why do earthquakes appear? How do earthquakes show up? Where are earthquakes maximum possibly to arise? Why do earthquakes take place?
Faults inside the Crust
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| Examples of fault displacement at the San Andreas fault in California. |
At first glance, a fault may look simply like a fracture or break that cuts across rock or sediment. But on closer examination, you may be able to see evidence of sliding that occurred on a fault. For example, the rock adjacent to the fault may be broken up into angular fragments or may be pulverized into tiny grains, due to the crushing and grinding that can accompany slip, and the surface of a fault may be polished and grooved as if scratched by a rasp. In some localities, a fault cuts through a distinct marker (a sedimentary bed, an igneous dike, or a fence); where this happens, the end of the marker on one side of the fault is offset relative to the end on the other side. The distance between two ends of the marker, as measured along the fault surface in the direction of slip, is the fault’s displacement (figure above a, b). Many faults are completely underground, and will be visible only if exposed by erosion of overlying rock. But some faults intersect and offset the ground surface, producing a step called a fault scarp (figure below a). The ground surface exposure of a fault is called the fault line or fault trace.
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| The basic sorts of fault. Fault kinds are distinguished from each other by the path of slip relative to the fault surface. |
19th-century miners who encountered faults in mine tunnels referred to the rock mass above a sloping fault plane as the hanging wall, because it hung over their heads, and the rock mass below the fault plane as the footwall, because it lay beneath their feet. The miners described the direction in which rock masses slipped on a sloping fault by specifying the direction that the hanging wall moved in relation to the footwall, and we still use these terms today. When the hanging wall slips down the slope of the fault, it’s a normal fault. When the hanging wall slips up the slope, it’s a reverse fault if steep, and a thrust fault if shallowly sloping (figure above a–c). Strike-slip faults are near-vertical planes on which slip occurs parallel to an imaginary horizontal line, called a strike line, on the fault plane no up or down motion takes place on such faults (figure above d).
Faults are found in many locations but don’t panic! Not all of them are likely to be the source of earthquakes. Faults that have moved recently or are likely to move in the near future are called active faults (and if they generate earthquakes, news media sometimes refer to them as “earthquake faults”). Faults that last moved in the distant past and probably won’t move again in the near future are called inactive faults.
Generating Earthquake Energy: Stick-Slip
What is the connection among faulting and earthquakes? Earthquakes can appear either when rock breaks and a new fault forms, or when a pre-current fault suddenly slips once more. Let?S look extra closely at these two causes.
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| A version representing the development of a new fault. Rupturing can generate earthquake-like vibrations. |
- Earthquakes due to fault formation: Imagine that you grip each side of a brick-shaped block of rock with a clamp. Apply an upward push on one of the clamps and a downward push on the other. By doing so, you have applied a “stress” to the rock. (Stress refers to a push, pull, or shear.) At first, the rock bends slightly but doesn't break (figure above a). In fact, if you were to stop applying stress at this stage, the rock would return to its original shape. Geologists refer to such a phenomenon as elastic behaviour the same phenomenon happens when a rubber band returns to its original shape or a bent stick straightens out after you let go. Now repeat the experiment, but bend the rock even more. If you bend the rock far enough, a number of small cracks or breaks start to form. Eventually the cracks connect to one another to form a fracture that cuts across the entire block of rock (figure above b). The instant that this fracture forms, the block breaks in two and the rock on one side suddenly slides past the rock on the other side, and any elastic bending that had built up is released so the rock straightens out or rebounds (figure above c). Because sliding occurs, the fracture has become a fault. A fault can’t slip forever, for friction eventually slows and stops the movement. Friction, defined as the force that resists sliding on a surface, is caused by the existence of bumps on surfaces these bumps act like tiny anchors and snag on the opposing surface.
- Earthquakes due to slip on a pre-existing fault: Once a fault comes into being, it is a scar in the Earth’s crust that can remain weaker than surrounding, intact crust. When stress builds sufficiently, it overcomes friction and the pre-existing fault slips again. This movement takes place before stress becomes great enough to cause new fracturing of surrounding intact rock. Note that after each slip event, friction prevents the fault from slipping again until stress builds again. Geologists refer to such alternation between stress buildup and slip events (earthquakes) as stick-slip behaviour.
The breaking of rock that occurs when a fault slips, like the snap of a stick, generates earthquake energy. The concept that earthquakes happen because stresses build up, causing rock adjacent to the fault to bend elastically until slip on the fault occurs is called the elastic-rebound theory.
Of note, the major earthquake (or “mainshock”) along a fault may be preceded by smaller ones, called foreshocks, which possibly result from the development of the smaller cracks in the vicinity of what will be the major rupture. Smaller earthquakes, called aftershocks, occur in the days to months following a large earthquake. The largest aftershock tends to be ten times smaller than the mainshock, and most are even smaller. Aftershocks happen because slip during the mainshock does not leave the fault in a perfectly stable configuration. For example, after the mainshock, irregularities on one side of the fault surface, in their new position, may push into the opposing side and generate new stresses. Such stresses may become large enough to cause a small portion of the fault around the irregularity to slip again, or may trigger slip in a nearby fault.
The Amount of Slip at some point of an Earthquake
How an awful lot of a fault floor slips throughout an earthquake? The solution relies upon on the size of the earthquake: the bigger the earthquake, the larger the slipped area and the greater the displacement. For example, the essential earthquake that hit San Francisco, California, in 1906 ruptured a 430-km-lengthy (measured parallel to the Earth?S surface) by 15-km-deep (measured perpendicular to the Earth?S surface) section of the San Andreas fault. Thus, the region that slipped changed into nearly 6500 km2. During the 2011 Tohoku earthquake an area 300 km lengthy via a hundred km extensive (30,000 km2) slipped.
The amount of slip varies along the length of a fault the most determined displacement at some point of the 1906 earthquake was 7 m, in a strike-slip experience. Slip on a thrust fault that brought on the 1964 Good Friday earthquake in southern Alaska reached a maximum of 12 m, and the most slip all through the Tohoku earthquake was over 20 m. Smaller earthquakes, together with the one that hit Northridge, California, in 1994, ended in best about 0.5-m slip nonetheless, this earthquake toppled homes, ruptured pipelines, and killed 51 humans. The smallest-felt earthquakes end result from displacements measured in millimetres to centimetres.
Although the cumulative movement on a fault at some stage in a human existence span won't amount to a good deal, over geologic time the cumulative movement will become signi?Cant. For example, if earthquakes going on on a strike-slip fault reason 1 cm of displacement in line with 12 months, on average, the fault?S movement will yield 10 km of displacement after 1 million years.
Credits: Stephen Marshak (Essentials of Geology)
