The thrust faults responsible for megathrust earthquakes often lie at the bottom ofoceanic trenches; in such cases, the earthquakes can abruptly displace the sea floor over a large area. As a result, megathrust earthquakes often generatetsunamis that are considerably more destructive than the earthquakes themselves.Teletsunamis can cross ocean basins to devastate areas far from the original earthquake.
Diagram of asubduction zone. The megathrust fault lies on the top of the subducting slab where it is in contact with the overriding plate.
The termmegathrust refers to an extremely largethrust fault, typically formed at the plate interface along a subduction zone, such as theSunda megathrust.[4][5] However, the term is also occasionally applied to large thrust faults in continental collision zones, such as theHimalayan 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 ofreverse fault, in which the rock above the fault is displaced upwards relative to the rock below the fault. This distinguishes reverse faults fromnormal faults, where the rock above the fault is displaced downwards, orstrike-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 theEarth's crust is being compressed by tectonic forces.[11]
Megathrust faults occur where twotectonic plates collide. When one of the plates is composed ofoceanic lithosphere, it dives beneath the other plate (called theoverriding plate) and sinks into theEarth's mantle as aslab. 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 thePacific andIndian Oceans.[5] These subduction zones are also largely responsible for thevolcanic activity associated with the PacificRing of Fire.[12]
Since these earthquakes deform theocean floor, they often generate strongtsunami 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 theSouth China Sea lies theManila 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, theJuan de Fuca plate subducts under theNorth American plate, creating theCascadia subduction zone from mid Vancouver Island, British Columbia down to Northern California. This subduction zone was responsible for the1700 Cascadia earthquake.[21] TheAleutian Trench, of the southern coast ofAlaska and theAleutian Islands, where the North American plate overrides thePacific plate, has generated many major earthquakes throughout history, several of which generated Pacific-wide tsunamis,[22] including the1964 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 theHimalayan region, where theIndian plate subducts under theEurasian plate, the largest recorded earthquake was the1950 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]
The largest recorded megathrust earthquake was the1960 Valdivia earthquake, estimated between magnitudes 9.4–9.6, centered off the coast of Chile along thePeru-Chile Trench, where theNazca plate subducts under theSouth American plate.[26] This megathrust region has regularly generated extremely large earthquakes.
A study reported in 2016 found that the largest megathrust quakes are associated with downgoing slabs with the shallowest dip, so-calledflat 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.ISBN978-1-107-05764-7.
^"Tsunami Terminology".The National Tsunami Hazard Mitigation Program History, 1995–2005. Pacific Marine Environmental Laboratory. Archived fromthe 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.
^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 fromthe original(PDF) on 2011-09-27. Retrieved2009-11-14.
