The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary--from an environment of continental collision to an island arc ...subduction zone. On 11 April 2012, one of the largest strike-slip earthquakes ever recorded (seismic moment magnitude M(w) 8.7) occurred about 100-200 kilometres southwest of the Sumatra subduction zone. Occurrence of great intraplate strike-slip faulting located seaward of a subduction zone is unusual. It results from northwest-southeast compression within the plate caused by the India-Eurasia continental collision to the northwest, together with northeast-southwest extension associated with slab pull stresses as the plate underthrusts Sumatra to the northeast. Here we use seismic wave analyses to reveal that the 11 April 2012 event had an extraordinarily complex four-fault rupture lasting about 160 seconds, and was followed approximately two hours later by a great (M(w) 8.2) aftershock. The mainshock rupture initially expanded bilaterally with large slip (20-30 metres) on a right-lateral strike-slip fault trending west-northwest to east-southeast (WNW-ESE), and then bilateral rupture was triggered on an orthogonal left-lateral strike-slip fault trending north-northeast to south-southwest (NNE-SSW) that crosses the first fault. This was followed by westward rupture on a second WNW-ESE strike-slip fault offset about 150 kilometres towards the southwest from the first fault. Finally, rupture was triggered on another en échelon WNW-ESE fault about 330 kilometres west of the epicentre crossing the Ninetyeast ridge. The great aftershock, with an epicentre located 185 kilometres to the SSW of the mainshock epicentre, ruptured bilaterally on a NNE-SSW fault. The complex faulting limits our resolution of the slip distribution. These great ruptures on a lattice of strike-slip faults that extend through the crust and a further 30-40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates.
Subduction zone plate boundary megathrust faults accommodate relative plate motions with spatially varying sliding behavior. The 2004 Sumatra‐Andaman (Mw 9.2), 2010 Chile (Mw 8.8), and 2011 Tohoku ...(Mw9.0) great earthquakes had similar depth variations in seismic wave radiation across their wide rupture zones – coherent teleseismic short‐period radiation preferentially emanated from the deeper portion of the megathrusts whereas the largest fault displacements occurred at shallower depths but produced relatively little coherent short‐period radiation. We represent these and other depth‐varying seismic characteristics with four distinct failure domains extending along the megathrust from the trench to the downdip edge of the seismogenic zone. We designate the portion of the megathrust less than 15 km below the ocean surface as domain A, the region of tsunami earthquakes. From 15 to ∼35 km deep, large earthquake displacements occur over large‐scale regions with only modest coherent short‐period radiation, in what we designate as domain B. Rupture of smaller isolated megathrust patches dominate in domain C, which extends from ∼35 to 55 km deep. These isolated patches produce bursts of coherent short‐period energy both in great ruptures and in smaller, sometimes repeating, moderate‐size events. For the 2011 Tohoku earthquake, the sites of coherent teleseismic short‐period radiation are close to areas where local strong ground motions originated. Domain D, found at depths of 30–45 km in subduction zones where relatively young oceanic lithosphere is being underthrust with shallow plate dip, is represented by the occurrence of low‐frequency earthquakes, seismic tremor, and slow slip events in a transition zone to stable sliding or ductile flow below the seismogenic zone.
Key Points
Seismic radiation from megathrust earthquake rupture varies with depth
A 4‐domain model of radiation segmentation is introduced for megathrusts
Strong‐ground motions originate from the down‐dip region
The Pacific/North American plate boundary is undergoing predominantly right-lateral strike–slip motion along the Queen Charlotte and Fairweather transform faults. The Queen Charlotte Fault (QCF) ...hosted the largest historical earthquake in Canada, the 1949 MS 8.1 strike–slip earthquake, which ruptured from offshore northern Haida Gwaii several hundred kilometers northwestward. On January 5, 2013 an Mw 7.5 strike–slip faulting event occurred near the northern end of the 1949 rupture zone. Along central and southern Haida Gwaii the relative plate motion has ∼20% oblique convergence across the left-stepping plate boundary. There had been uncertainty in how the compressional component of plate motion is accommodated. The October 28, 2012 Mw 7.8 Haida Gwaii earthquake involved slightly (∼20°) oblique thrust faulting on a shallow (∼18.5°) northeast-dipping fault plane with strike (∼320°) parallel to the QCF, consistent with prior inferences of Pacific Plate underthrusting beneath Haida Gwaii. The rupture extended to shallow depth offshore of Moresby Island beneath a 25–30km wide terrace of sediments that has accumulated in a wedge seaward of the QCF. The shallow thrusting caused seafloor uplift that generated substantial localized tsunami run-up and a modest far-field tsunami that spread across the northern Pacific, prompting a tsunami warning, beach closure, and coastal evacuation in Hawaii, although ultimately tide gauges showed less than 0.8m of water level increase. The mainshock rupture appears to have spread with a ∼2.3km/s rupture velocity over a length of ∼150km, with slip averaging 3.3m concentrated beneath the sedimentary wedge. The event was followed by a substantial aftershock sequence, in which almost all of the larger events involve distributed intraplate normal faulting extending ∼50km oceanward from the QCF. The highly oblique slip partitioning in southern Haida Gwaii is distinctive in that the local plate boundary-parallel motion on the QCF may be accommodated either by infrequent large strike–slip ruptures or by aseismic creep, as seems to be the case for deeper oblique relative plate motion beneath Haida Gwaii, while the sedimentary terrace accumulates plate boundary-perpendicular compressional strain that releases in almost pure thrust faulting earthquakes, seaward of the QCF.
•The October 28, 2012 Haida Gwaii earthquake involved underthrusting.•The slip was located offshore from the Queen Charlotte Fault.•Transpression on the plate boundary accounts for the thrusting.•Tsunami recordings require shallow slip under a sedimentary terrace.•The Queen Charlotte Fault may locally be aseismic or may have large events.
Teleseismic short‐period (0.5–5 s) P waves from the 27 February 2010 Chile earthquake (Mw 8.8) are back projected to the source region to image locations of coherent short‐period seismic wave ...radiation. Several receiver array configurations are analyzed using different P wave arrivals, including networks of stations in North America (P), Japan (PKIKP), and Europe (PP), as well as a global configuration of stations with a broad azimuthal distribution and longer‐period P waves (5–20 s). Coherent bursts of short‐period radiation from the source are concentrated below the Chilean coastline, along the downdip portion of the megathrust. The short‐period source region expands bilaterally, with significant irregularity in the radiation. Comparison with finite fault slip models inverted from longer‐period seismic waves indicates that the regions of large slip on the megathrust are located updip of the regions of short‐period radiation, a manifestation of frequency‐dependent seismic radiation, similar to observations for the great 2011 Tohoku earthquake (Mw 9.0). Back projection of synthetic P waves generated from the finite fault models demonstrates that if the short‐period energy had radiated with the same space‐time distribution as the long‐period energy, back‐projection analysis would image it in the correct location, updip. We conclude that back‐projection imaging of short‐period signals provides a distinct view of the seismic source that is missed by studies based only on long‐period seismic waves, geodetic data, and/or tsunami observations.
Key Points
Short‐period energy was radiated downdip of the major slip release
Rupture velocity was faster for the short‐period energy release
Back projection provides information that is complementary to slip models
Delineating Deep Faults. Most large, damaging earthquakes initiate in Earth's crust where friction and brittle fracture control the release of energy. Strong earthquakes can occur in the mantle too, ...but their rupture dynamics are difficult to determine because higher temperatures and pressures play a more important role. Ye et al. (p. 1380) analyzed seismic P waves generated by the 2013 Mw 8.3 Sea of Okhotsk earthquake-the largest deep earthquake recorded to date-and its associated aftershocks. The earthquake ruptured along a fault over 180-kilometer-long and structural heterogeneity resulted in a massive release of stress from the subducting slab. In a set of complementary laboratory deformation experiments, Schubnel et al. (p. 1377) simulated the nucleation of acoustic emission events that resemble deep earthquakes. These events are caused by an instantaneous phase transition from olivine to spinel, which would occur at the same depth and result in large stress releases observed for other deep earthquakes.
The Pacific/North American plate boundary is undergoing predominantly right-lateral strike-slip motion along the Queen Charlotte and Fairweather transform faults. The Queen Charlotte Fault (QCF) ...hosted the largest historical earthquake in Canada, the 1949 M S 8.1 strike-slip earthquake, which ruptured from offshore northern Haida Gwaii several hundred kilometers northwestward. On January 5, 2013 an M w 7.5 strike-slip faulting event occurred near the northern end of the 1949 rupture zone. Along central and southern Haida Gwaii the relative plate motion has 20% oblique convergence across the left-stepping plate boundary. There had been uncertainty in how the compressional component of plate motion is accommodated. The October 28, 2012 M w 7.8 Haida Gwaii earthquake involved slightly (20 degree ) oblique thrust faulting on a shallow (18.5 degree ) northeast-dipping fault plane with strike (320 degree ) parallel to the QCF, consistent with prior inferences of Pacific Plate underthrusting beneath Haida Gwaii. The rupture extended to shallow depth offshore of Moresby Island beneath a 25-30km wide terrace of sediments that has accumulated in a wedge seaward of the QCF. The shallow thrusting caused seafloor uplift that generated substantial localized tsunami run-up and a modest far-field tsunami that spread across the northern Pacific, prompting a tsunami warning, beach closure, and coastal evacuation in Hawaii, although ultimately tide gauges showed less than 0.8m of water level increase. The mainshock rupture appears to have spread with a 2.3km/s rupture velocity over a length of 150km, with slip averaging 3.3m concentrated beneath the sedimentary wedge. The event was followed by a substantial aftershock sequence, in which almost all of the larger events involve distributed intraplate normal faulting extending 50km oceanward from the QCF. The highly oblique slip partitioning in southern Haida Gwaii is distinctive in that the local plate boundary-parallel motion on the QCF may be accommodated either by infrequent large strike-slip ruptures or by aseismic creep, as seems to be the case for deeper oblique relative plate motion beneath Haida Gwaii, while the sedimentary terrace accumulates plate boundary-perpendicular compressional strain that releases in almost pure thrust faulting earthquakes, seaward of the QCF.
The frequency-dependent rupture process of the 11 March 2011
M
w
9.0 off the Pacific coast of Tohoku Earthquake is examined using backprojection (BP) imaging with teleseismic short-period (~1 s)
P
...waves, and finite faulting models (FFMs) of the seismic moment and slip distributions inverted from broadband (>3 s) teleseismic
P
waves, Rayleigh waves and regional continuous GPS ground motions. Robust features of the BPs are initial down-dip propagation of the short-period energy source with a slow rupture speed (~1 km/s), followed by faster (2–3 km/s) rupture that progresses southwestward beneath the Honshu coastline. The FFMs indicate initial slow down-dip expansion of the rupture followed by concentrated long-period radiation up-dip of the hypocenter, then southwestward expansion of the rupture. We explore whether these differences correspond to real variations in energy release over the fault plane or represent uncertainties in the respective approaches. Tests of the BP results involve (1) comparisons with backprojection of synthetic
P
waves generated for the FFMs, and (2) comparisons of backprojection locations for aftershocks with corresponding NEIC and JMA locations. The data indicate that the down-dip environment radiates higher relative levels of short-period radiation than the up-dip regime for this great earthquake, consistent with large-scale segmentation of the frictional properties of the megathrust.
Great earthquakes (having seismic magnitudes of at least 8) usually involve abrupt sliding of rock masses at a boundary between tectonic plates. Such interplate ruptures produce dynamic and static ...stress changes that can activate nearby intraplate aftershocks, as is commonly observed in the trench-slope region seaward of a great subduction zone thrust event. The earthquake sequence addressed here involves a rare instance in which a great trench-slope intraplate earthquake triggered extensive interplate faulting, reversing the typical pattern and broadly expanding the seismic and tsunami hazard. On 29 September 2009, within two minutes of the initiation of a normal faulting event with moment magnitude 8.1 in the outer trench-slope at the northern end of the Tonga subduction zone, two major interplate underthrusting subevents (both with moment magnitude 7.8), with total moment equal to a second great earthquake of moment magnitude 8.0, ruptured the nearby subduction zone megathrust. The collective faulting produced tsunami waves with localized regions of about 12 metres run-up that claimed 192 lives in Samoa, American Samoa and Tonga. Overlap of the seismic signals obscured the fact that distinct faults separated by more than 50 km had ruptured with different geometries, with the triggered thrust faulting only being revealed by detailed seismic wave analyses. Extensive interplate and intraplate aftershock activity was activated over a large region of the northern Tonga subduction zone.
The diffuse deformation zone between the Indian and Australian plates has hosted numerous major and great earthquakes during the seismological record, including the 11 April 2012 Mw 8.6 event, the ...largest recorded intraplate earthquake. On 2 March 2016, an Mw 7.8 strike‐slip faulting earthquake occurred in the northwestern Wharton Basin, in a region bracketed by north‐south trending fracture zones with no previously recorded large event nearby. Despite the large magnitude, only minor source finiteness is evident in aftershock locations or resolvable from seismic wave processing including high‐frequency P wave backprojections and Love wave directivity analysis. Our analyses indicate that the event ruptured bilaterally on a north‐south trending fault over a length of up to 70 km, with rupture speed of ≤ 2 km/s, and a total duration of ~35 s. The estimated stress drop, ~20 MPa, is high, comparable to estimates for other large events in this broad intraplate oceanic deformation zone.
Key Points
The 2016 earthquake has little seismic wave directivity for an Mw 7.8 strike‐slip faulting event
Aftershocks, surface waves, and finite fault inversions indicate north‐south rupture ≤70 km long
The static stress drop is ~20 MPa, a high value similar to other events in the intraplate deformation zone
PDF
