What Causes Earthquakes?

What Causes Earthquakes?

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. 

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 happen? Where do most earthquakes occur? Why do earthquakes happen? How do earthquakes happen? Where are earthquakes most likely to occur? Why do earthquakes happen?

Faults in the Crust 

Examples of fault displacement on 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. 

The basic types of fault. Fault types are distinguished from one another by the direction 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 relationship between faulting and earthquakes? Earthquakes can happen either when rock breaks and a new fault forms, or when a pre-existing fault suddenly slips again. Let’s look more closely at these two causes. 

A model 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 during an Earthquake 

How much of a fault surface slips during an earthquake? The answer depends on the size of the earthquake: the larger the earthquake, the larger the slipped area and the greater the displacement. For example, the major earthquake that hit San Francisco, California, in 1906 ruptured a 430-km-long (measured parallel to the Earth’s surface) by 15-km-deep (measured perpendicular to the Earth’s surface) segment of the San Andreas fault. Thus, the area that slipped was almost 6500 km2. During the 2011 Tohoku earthquake an area 300 km long by 100 km wide (30,000 km2) slipped. 

The amount of slip varies along the length of a fault the maximum observed displacement during the 1906 earthquake was 7 m, in a strike-slip sense. Slip on a thrust fault that caused the 1964 Good Friday earthquake in southern Alaska reached a maximum of 12 m, and the maximum slip during the Tohoku earthquake was over 20 m. Smaller earthquakes, such as the one that hit Northridge, California, in 1994, resulted in only about 0.5-m slip even so, this earthquake toppled homes, ruptured pipelines, and killed 51 people. The smallest-felt earthquakes result from displacements measured in millimetres to centimetres. 

Although the cumulative movement on a fault during a human life span may not amount to much, over geologic time the cumulative movement becomes significant. For example, if earthquakes occurring on a strike-slip fault cause 1 cm of displacement per year, on average, the fault’s movement will yield 10 km of displacement after 1 million years.

Credits: Stephen Marshak (Essentials of Geology)

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