Geology And Geophysics

Understanding Strike Slip Faults

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"Understanding Strike Slip Faults"
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Before explaining the title as a whole, the three words in the title need to be understood.

Strike refers to the direction along a bed, or stratum, of rock.  Many rock strata lie at an angle to the horizontal.  The angle at which the strata dip down is called the dip.  The strike is the direction perpendicular to the dip.

Slip relates to the movement of one area of rock in relation to another.  This may be a vertical movement or a horizontal movement.

A fault is a line along which rocks have cracked and moved as a consequence.

Together, the three words refer to an earth movement along a line (fault) which corresponds to the strike of the rocks.   An alternative word for a strike slip fault is a tear fault.

Pressures within the earth’s crust, mainly in the horizontal, can exert so much strain on the rock strata that they bend or fold, and if there is a serious weakness anywhere in those rocks, they will fault or break.   Considering the different result of bending a plastic as opposed to a wooden ruler, illustrates the point.   The plastic ruler will bend considerably, as folding rocks can do, but then will suddenly break if too much pressure is exerted.   The wooden ruler will bend only slightly before breaking.  Rocks behaving like the wooden ruler are more likely to produce faults than folds.

A strike slip fault, or tear fault, relates to the line along which the faulting takes place.  In this case it is parallel to the strike of the rocks, hence the name.

There are several well known examples of tear faults, but a consideration of two will help to explain the formation.

In northern Scotland there is a long valley, called the Great Glen.  It runs in a south-west to north-east direction and contains a very deep lake known as Loch Ness.  The line of the Great Glen is the line of a series of badly shattered rocks.  Along this line rocks have been displaced horizontally to a distance of 100 kms (62.5 miles), as is shown by outcrops of the same granite which occur on each side of the glen.

The part of Scotland to the north of the glen slipped 100 kms to the south-west at the end of the Devonian geological period, about 350 million years ago, yet earth tremors still occur along the line of the fault.

The San Andreas fault in California, which is 1300 kms (812.5 miles) long, contains an area of shattered rock about 2 kms wide.  These rocks are susceptible to sudden horizontal movement and there has been movement here since the late Jurassic, about 140 million years ago.

The fault marks the line along which two tectonic plates meet, the Pacific plate to the west and the American plate to the east.  As the Pacific plate has moved northwards along the tear fault it has displaced rocks by up to 500 kms, and caused the massive invasion of the southern Californian valley which is the Gulf of California.  Needless to say, along this line there are earthquakes on a regular basis, some of which are very serious, particularly if their epicentre is directly below an urban area.

A strike slip fault then is otherwise known as a tear fault, and occurs where there is faulting of rocks along a line parallel to the line of the strike.

More about this author: Keith Redfern

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