seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

, a supershear earthquake is an

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

in which the propagation of the

rupture Rupture may refer to: General * Rupture (engineering), a failure of tough ductile materials loaded in tension Anatomy and medicine * Abdominal hernia, formerly referred to as "a rupture" * Achilles tendon rupture * Rupture of membranes, a "wate ...

along the fault surface occurs at speeds in excess of the seismic

shear wave __NOTOC__ In seismology and other areas involving elastic waves, S waves, secondary waves, or shear waves (sometimes called elastic S waves) are a type of elastic wave and are one of the two main types of elastic body waves, so named because th ...

(S-wave) velocity. This causes an effect analogous to a

sonic boom A sonic boom is a sound associated with shock waves created when an object travels through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding similar to an explosion or a thunderclap to t ...

Rupture propagation velocity

During seismic events along a fault surface the displacement initiates at the focus and then propagates outwards. Typically for large earthquakes the focus lies towards one end of the slip surface and much of the propagation is unidirectional (e.g. the 2008 Sichuan and

2004 Indian Ocean earthquake An earthquake and a tsunami, known as the Boxing Day Tsunami and, by the scientific community, the Sumatra–Andaman earthquake, occurred at 07:58:53 local time (UTC+7) on 26 December 2004, with an epicentre off the west coast of northern Suma ...

s). Theoretical studies have in the past suggested that the upper bound for propagation velocity is that of

Rayleigh waves Rayleigh waves are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructi ...

, approximately 0.92 of the shear wave velocity. However, evidence of propagation at velocities between S-wave and

compressional wave Longitudinal waves are waves in which the vibration of the medium is parallel ("along") to the direction the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical longitudinal wav ...

(P-wave) values have been reported for several earthquakesArchuleta,R.J. 1984
A faulting model for the 1979 Imperial Valley earthquake
J. Geophys. Res., 89, 4559–4585. in agreement with theoretical and laboratory studies that support the possibility of rupture propagation in this velocity range.

Occurrence

Evidence of rupture propagation at velocities greater than S-wave velocities expected for the surrounding crust have been observed for several large earthquakes associated with strike-slip faults. During strike-slip, the main component of rupture propagation will be horizontal, in the direction of displacement, as a Mode II (in-plane) shear crack. This contrasts with a dip-slip rupture where the main direction of rupture propagation will be perpendicular to the displacement, like a Mode III (anti-plane) shear crack. Theoretical studies have shown that Mode III cracks are limited to the shear wave velocity but that Mode II cracks can propagate between the S and P-wave velocities and this may explain why supershear earthquakes have not been observed on dip-slip faults.

Initiation of supershear rupture

The rupture velocity range between those of Rayleigh waves and shear waves remains forbidden for a Mode II crack (a good approximation to a strike-slip rupture). This means that a rupture cannot accelerate from Rayleigh speed to shear wave speed. In the "Burridge–Andrews" mechanism, supershear rupture is initiated on a 'daughter' rupture in the zone of high shear stress developed at the propagating tip of the initial rupture. Because of this high stress zone, this daughter rupture is able start propagating at supershear speed before combining with the existing rupture. Experimental shear crack rupture, using plates of a photoelastic material, has produced a transition from sub-Rayleigh to supershear rupture by a mechanism that "qualitatively conforms to the well-known Burridge-Andrews mechanism".

Geological effects

The high rates of strain expected near faults that are affected by supershear propagation are thought to generate what is described as pulverized rocks. The pulverization involves the development of many small microcracks at a scale smaller than the grain size of the rock, while preserving the earlier

fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...

, quite distinct from the normal brecciation and cataclasis found in most fault zones. Such rocks have been reported up to 400 m away from large strike-slip faults, such as the San Andreas Fault. The link between supershear and the occurrence of pulverized rocks is supported by laboratory experiments that show very high strain rates are necessary to cause such intense fracturing.

Examples

Directly observed

1999 Izmit earthquake File:1999 Events Collage.png, From left, clockwise: The Death and state funeral of King Hussein, funeral procession of King Hussein of Jordan in Amman; the 1999 İzmit earthquake kills over 17,000 people in Turkey; the Columbine High School massac ...

, magnitude ''M''_w 7.6 associated with strike-slip movement on the North Anatolian Fault Zone
Bouchon, M., M.-P. Bouin, H. Karabulut, M. N. Toksöz, M. Dietrich, and Ares J. Rosakis, A. J. Rosakis (2001)
How Fast is Rupture During an Earthquake ? New Insights from the 1999 Turkey Earthquakes
Geophys. Res. Lett., 28(14), 2723–2726.] * 1999 Düzce earthquake, magnitude ''M''_w 7.2 associated with strike-slip movement on the North Anatolian Fault Zone * 2001 Kunlun earthquake, magnitude ''M''_w 7.8 associated with strike-slip movement on the Kunlun fault * 2002 Denali earthquake, magnitude ''M''_w 7.9 associated with strike-slip movement on the

Denali Fault The Denali Fault is a major intracontinental dextral (right lateral) strike-slip fault in western North America, extending from northwestern British Columbia, Canada to the central region of the U.S. state of Alaska. Location The Denali Fault i ...

* 2010 Yushu earthquake, magnitude ''M''_w 6.9 associated with strike-slip movement on the Yushu Fault * 2012 Indian Ocean earthquakes, magnitude ''M''_w 8.6 associated with strike-slip on several fault segments - the first supershear event recognised in oceanic lithosphere. *

2013 Craig, Alaska earthquake The 2013 Craig, Alaska earthquake (also known as the Queen Charlotte Fault earthquake) struck on January 5, at 12:58 am ( UTC–7) near the city of Craig and Hydaburg, on Prince of Wales Island. The 7.5 earthquake came nearly three months afte ...

, magnitude ''M''_w 7.6 associated with strike-slip on the

Queen Charlotte Fault The Queen Charlotte Fault is an active transform fault that marks the boundary of the North American plate and the Pacific plate. It is Canada's right-lateral strike-slip equivalent to the San Andreas Fault to the south in California. The Queen Ch ...

- the first supershear event recognised on an oceanic plate boundary. * 2014 Aegean Sea earthquake, magnitude ''M''_w 6.9, supershear was recognised during the second sub-event. * 2015 Tajikistan earthquake, magnitude ''M''_w 7.2, supershear slip on two segments, with normal slip at the restraining bend linking them. * 2016 Romanche fracture zone earthquake, magnitude 7.1, westwards-directed supershear rupture following an initial easterly-travelling phase on the Romanche ocean transform fault in the equatorial Atlantic * 2017 Komandorsky Islands earthquake, magnitude ''M''_w 7.7, supershear transition followed a rupture jump across a fault stepover. * 2018 Swan Islands earthquake, 7.5 earthquake consisted of three sub-events with a compact rupture area and large cosesimic slip. * 2018 Sulawesi earthquake, magnitude ''M''_w 7.5, associated with strike-slip movement on the

Palu-Koro Fault The Palu-Koro Fault or Palu-Koro Fault System is a major active NNW-SSE trending left-lateral strike-slip fault zone on the island of Sulawesi in Indonesia. It caused the 2018 Sulawesi earthquake and tsunami. Geometry It extends from near Dondow ...

* 2020 Caribbean Sea earthquake, magnitude ''M''_w 7.7, unilateral rupture propagation westward from the epicenter along a 300 km section of the Oriente transform fault with two episodes of supershear rupture * 2021 Maduo earthquake, 7.4 earthquake in the Tibetan Plateau. This earthquake ruptured bilaterally for a length of 170 km within the

Bayan Har block The Bayan Har block or Bayan Kola block is an elongate wedge-shaped block that forms part of the eastern Tibetan Plateau. It is bounded to the southeast by the Longmenshan Fault, a major thrust fault zone, which forms the active tectonic boundary b ...

Inferred

1906 San Francisco earthquake At 05:12 Pacific Standard Time on Wednesday, April 18, 1906, the coast of Northern California was struck by a major earthquake with an estimated moment magnitude of 7.9 and a maximum Mercalli intensity of XI (''Extreme''). High-intensity sha ...

, magnitude 7.8 associated with strike-slip movement on the

San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...

1979 Imperial Valley earthquake The 1979 Imperial Valley earthquake occurred at 16:16 Pacific Daylight Time (23:16 UTC) on 15 October just south of the Mexico–United States border. It affected Imperial Valley in Southern California and Mexicali Valley in northern Baja Calif ...

, magnitude 6.4 associated with slip on the

Imperial Fault The Imperial Fault Zone is a system of geological faults located in Imperial County in the Southern California region, and adjacent Baja California state in Mexico. It cuts across the border between the United States and Mexico. Geology The I ...

* 1990 Sakhalin earthquake, 7.2 earthquake at over 600 km depth inferred to have ruptured at supershear speeds. * 2013 Okhotsk Sea earthquake magnitude 6.7 aftershock was an extremely deep (640 kilometers (400 miles)) supershear as well as unusually fast at "eight kilometers per second (five miles per second), nearly 50 percent faster than the shear wave velocity at that depth."

References

External links

Eric Dunham's webpage on Supershear Dynamics
Seismology Types of earthquake Strike-slip earthquakes