The thrust faults responsible for megathrust earthquakes often lie at the bottom of oceanic trenches; in such cases, the earthquakes can abruptly displace the sea floor over a large area. As a result, megathrust earthquakes often generate tsunamis that are considerably more destructive than the earthquakes themselves. Teletsunamis can cross ocean basins to devastate areas far from the original earthquake.
Terminology and mechanism
Diagram of a subduction zone. The megathrust fault lies on the top of the subducting slab where it is in contact with the overriding plate.
The term megathrust refers to an extremely large thrust fault, typically formed at the plate interface along a subduction zone, such as the Sunda megathrust.[4][5] However, the term is also occasionally applied to large thrust faults in continental collision zones, such as the Himalayan megathrust.[6] A megathrust fault can be 1,000 kilometers (600 mi) long.[7]
Cross-sectional illustration of normal and reverse faults
A thrust fault is a type of reverse fault, in which the rock above the fault is displaced upwards relative to the rock below the fault. This distinguishes reverse faults from normal faults, where the rock above the fault is displaced downwards, or strike-slip faults, where the rock on one side of the fault is displaced horizontally with respect to the other side. Thrust faults are distinguished from other reverse faults because they dip at a relatively shallow angle, typically less than 45°,[8] and show large displacements.[9][10] In effect, the rocks above the fault have been thrust over the rocks below the fault. Thrust faults are characteristic of areas where the Earth's crust is being compressed by tectonic forces.[11]
Megathrust faults occur where two tectonic plates collide. When one of the plates is composed of oceanic lithosphere, it dives beneath the other plate (called the overriding plate) and sinks into the Earth's mantle as a slab. The contact between the colliding plates is the megathrust fault, where the rock of the overriding plate is displaced upwards relative to the rock of the descending slab.[5] Friction along the megathrust fault can lock the plates together, and the subduction forces then build up strain in the two plates. A megathrust earthquake takes place when the fault ruptures, allowing the plates to abruptly move past each other to release the accumulated strain energy.[7]
Megathrust earthquakes are almost exclusive to tectonic subduction zones and are often associated with the Pacific and Indian Oceans.[5] These subduction zones are also largely responsible for the volcanic activity associated with the Pacific Ring of Fire.[12]
Since these earthquakes deform the ocean floor, they often generate strong tsunami waves.[13] Subduction zone earthquakes are also known to produce intense shaking and ground movements that can last for up to 3–5 minutes.[14]
In the South China Sea lies the Manila Trench, which is capable of producing Mw 9.0 or larger earthquakes,[17] with the maximum magnitude at Mw 9.2 or higher.[18]
In North America, the Juan de Fuca plate subducts under the North American plate, creating the Cascadia subduction zone from mid Vancouver Island, British Columbia down to Northern California. This subduction zone was responsible for the 1700 Cascadia earthquake.[21] The Aleutian Trench, of the southern coast of Alaska and the Aleutian Islands, where the North American plate overrides the Pacific plate, has generated many major earthquakes throughout history, several of which generated Pacific-wide tsunamis,[22] including the 1964 Alaska earthquake; at magnitude 9.1–9.2, it remains the largest recorded earthquake in North America, and the third-largest earthquake instrumentally recorded in the world.[23]
In the Himalayan region, where the Indian plate subducts under the Eurasian plate, the largest recorded earthquake was the 1950 Assam–Tibet earthquake, at magnitude 8.7. It is estimated that earthquakes with magnitude 9.0 or larger are expected to occur at an interval of every 800 years, with the highest boundary being a magnitude 10, though this is not considered physically possible. Therefore, the largest possible earthquake in the region is a magnitude 9.7, assuming a single rupture of the whole Himalayan arc and assuming standard scaling law, which implies an average slip of 50 m.[24]
A megathrust earthquake could occur in the Lesser Antilles subduction zone, with a maximum magnitude of 9.3, or potentially even 10.3 through recent evaluations, a value not considered impossible.[25]
The largest recorded megathrust earthquake was the 1960 Valdivia earthquake, estimated between magnitudes 9.4–9.6, centered off the coast of Chile along the Peru-Chile Trench, where the Nazca plate subducts under the South American plate.[26] This megathrust region has regularly generated extremely large earthquakes.
The largest possible earthquakes are estimated at magnitudes of 10 to 11, most likely caused by a combined rupture of the Japan Trench and Kuril–Kamchatka Trench, or individually the Aleutian Trench or Peru–Chile Trench.[27][28][29][30] Another possible area could be the Lesser Antilles subduction zone.[25]
A study reported in 2016 found that the largest megathrust quakes are associated with downgoing slabs with the shallowest dip, so-called flat slab subduction.[31]
Compared with other earthquakes of similar magnitude, megathrust earthquakes have a longer duration and slower rupture velocities. The largest megathrust earthquakes occur in subduction zones with thick sediments, which may allow a fault rupture to propagate for great distances unimpeded.[5]
^Park, J.; Butler, R.; Anderson, K.; et al. (2005). "Performance Review of the Global Seismographic Network for the Sumatra-Andaman Megathrust Earthquake". Seismological Research Letters. 76 (3): 331–343. Bibcode:2005SeiRL..76..331P. doi:10.1785/gssrl.76.3.331. ISSN0895-0695.
^Fossen, Haakon (2016). Structural geology (Second ed.). Cambridge, United Kingdom: Cambridge University Press. pp. 485, 488, 491. ISBN9781107057647.
^"Tsunami Terminology". The National Tsunami Hazard Mitigation Program History, 1995–2005. Pacific Marine Environmental Laboratory. Archived from the original on 2011-02-25.
^Megawati, K.; Pan, T.-C. (1 April 2009). "Regional Seismic Hazard Posed by the Mentawai Segment of the Sumatran Megathrust". Bulletin of the Seismological Society of America. 99 (2A): 566–584. Bibcode:2009BuSSA..99..566M. doi:10.1785/0120080109.
^Sieh, Kerry (March 2007). "The Sunda megathrust: past, present and future". Journal of Earthquake and Tsunami. 01 (1): 1–19. doi:10.1142/S179343110700002X.
^Hirahara, K.; Kato N.; Miyatake T.; Hori T.; Hyodo M.; Inn J.; Mitsui N.; Sasaki T.; Miyamura T.; Nakama Y.; Kanai T. (2004). "Simulation of Earthquake Generation Process in a Complex System of Faults"(PDF). Annual Report of the Earth Simulator Center April 2004 – March 2005. pp. 121–126. Archived from the original(PDF) on 2011-09-27. Retrieved 2009-11-14.
^Ojeda, Javier; Ruiz, Sergio; del Campo, Francisco; Carvajal, Matías (1 May 2020). "The 21 May 1960 Mw 8.1 Concepción Earthquake: A Deep Megathrust Foreshock That Started the 1960 Central-South Chilean Seismic Sequence". Seismological Research Letters. 91 (3): 1617–1627. Bibcode:2020SeiRL..91.1617O. doi:10.1785/0220190143. S2CID216347638.
