geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, a piercing point is defined as a feature (usually a geologic feature, preferably a linear feature) that is cut by a fault, then moved apart. Reconfiguring the piercing point back in its original position is the primary way

geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...

s can find out the minimum slip, or displacement, along a fault. This can be done on a large scale (over many kilometers), a small scale (inside a single outcrop or fault trench) or even a single hand sample/rock (see image). Items that are usually used in a piercing point study include large geologic formations or other rock units that can be matched either stratigraphically, geochemically, or by age dating. Features that are linear or planar, like a stratigraphic unit, are much better for use in a piercing point study than rounds or irregular-shaped objects, such as a pluton, because the reconstruction is always more precise with a more predictable shape (because of the Principle of lateral continuity). Of course, it is important to keep in mind that piercing points only give a minimum amount of offset that fault could have taken. In certain situations, rock units can be created as fault movement occurs, making the piercing point measurement even less than a minimum value. Mason Hill and Thomas Dibblee were the first to use piercing points along the San Andreas Fault, notably the Pelona schist in the

San Gabriel Mountains The San Gabriel Mountains () are a mountain range located in Los Angeles and San Bernardino counties, California, United States. The mountain range is part of the Transverse Ranges and lies between the Los Angeles Basin and the Mojave Desert ...

and Orocopia schist in the Orocopia Mountains, in 1953; they showed at least of slip using that piercing point. Another famous example of San Andreas fault piercing points include the unique rocks at Point Lobos State Reserve and Point Reyes National Seashore. Though 180 km apart, the rocks match exactly: they were cut and separated by the fault. A complete, detailed analysis shows that the movement, while uncertain because of the various piercing points used, is over since the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

. Piercing points are used on faults other than the San Andreas, like the Hilina fault system in

Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...

and the Lake Clark fault system in

Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

.26 km of Offset on the Lake Clark Fault Since Late Eocene Time
/ref> In rare situations, even human structures built across a fault can be used, like an

Ottoman Empire The Ottoman Empire (), also called the Turkish Empire, was an empire, imperial realm that controlled much of Southeast Europe, West Asia, and North Africa from the 14th to early 20th centuries; it also controlled parts of southeastern Centr ...

-era canal berm that was offset along the North Anatolian Fault zone in a 1754 earthquake and the 1999 Izmit earthquake in

Turkey Turkey, officially the Republic of Türkiye, is a country mainly located in Anatolia in West Asia, with a relatively small part called East Thrace in Southeast Europe. It borders the Black Sea to the north; Georgia (country), Georgia, Armen ...

. The berm showed of movement in the 1999 earthquake.

References

{{Reflist Structural geology Seismic faults Measurement