Megathrust Earthquake Cycle Research Articles

Asthenosphere Flow Modulated by Megathrust Earthquake Cycles

AbstractSubduction megathrusts develop the largest earthquakes, often close to large population centers. Understanding the dynamics of deformation at subduction zones is therefore important to better assess seismic hazards. Here I develop consistent earthquake cycle simulations that incorporate localized and distributed deformation based on laboratory‐derived constitutive laws by combining boundary and volume elements to represent the mechanical coupling between megathrust slip and solid‐state flow in the oceanic asthenosphere and in the mantle wedge. The model is simplified, in two dimensions, but may help the interpretation of geodetic data. Megathrust earthquakes and slow‐slip events modulate the strain rate in the upper mantle, leading to large variations of effective viscosity in space and time and a complex pattern of surface deformation. While fault slip and flow in the mantle wedge generate surface displacements in the same, that is, seaward, direction, the viscoelastic relaxation in the oceanic asthenosphere generates transient surface deformation in the opposite, that is, landward, direction above the rupture area of the mainshock. Aseismic deformation above the seismogenic zone may be challenging to record, but it may reveal important constraints about the rheology of the subducting plate.

The 2016 Mw 5.9 earthquake off the southeastern coast of Mie Prefecture as an indicator of preparatory processes of the next Nankai Trough megathrust earthquake

Megathrust earthquakes have occurred repeatedly at intervals of 100 to 150 years along the Nankai Trough, situated in the southwest of Japan. Given that it has been 70 years since the last event, the occurrence of the next devastating earthquake is anticipated in the near future. On April 1, 2016, a moderate earthquake (Mw 5.9, MJMA 6.5) occurred off the southeastern coast of Mie Prefecture in the source region of the 1944 Tonankai earthquake (Mw 8.2). In this study, we investigated the influence of the 2016 earthquake on future megathrust earthquakes. We first determined the hypocenter distributions using a precise velocity structure obtained from seismic surveys in the source region. Using data obtained from the DONET ocean-bottom observation network, we found that this earthquake occurred along the plate boundary fault, which is also believed to have slipped during past megathrust earthquakes. We then performed a preliminary numerical simulation to reproduce the occurrence of a moderate earthquake in the middle of a megathrust earthquake cycle. We used a hierarchical asperity model, in which smaller asperities causing moderate earthquakes are embedded in a hyper-asperity that serves as the source region of megathrust earthquakes. The simulation shows that moderate earthquakes, caused by ruptures of a smaller asperity, occur as a result of shrinkage of strongly coupled areas in the hyper-asperity. This result is consistent with the observation that the hypocenter of the 2016 earthquake was located at the edge of a strongly coupled region along the plate boundary. The simulation also reproduced postseismic slip (afterslip/slow slip) along the plate boundary updip of the hyper-asperity, which is consistent with the observations of slow slip events and very-low-frequency earthquakes after the 2016 earthquake. Thus, the occurrence of the moderate earthquake offshore southeastern Mie Prefecture in the middle of the megathrust earthquake cycle implies that the shrinkage of the strongly coupled area along the plate boundary is occurring as a preparatory process for the next megathrust earthquake in the region.

Larger earthquakes recur more periodically: New insights in the megathrust earthquake cycle from lacustrine turbidite records in south-central Chile

Historical and paleoseismic records in south-central Chile indicate that giant earthquakes on the subduction megathrust – such as in AD1960 (Mw9.5) – reoccur on average every ∼300 yr. Based on geodetic calculations of the interseismic moment accumulation since AD1960, it was postulated that the area already has the potential for a Mw8 earthquake. However, to estimate the probability of such a great earthquake to take place in the short term, one needs to frame this hypothesis within the long-term recurrence pattern of megathrust earthquakes in south-central Chile. Here we present two long lacustrine records, comprising up to 35 earthquake-triggered turbidites over the last 4800 yr. Calibration of turbidite extent with historical earthquake intensity reveals a different macroseismic intensity threshold (≥VII1/2 vs. ≥VI1/2) for the generation of turbidites at the coring sites. The strongest earthquakes (≥VII1/2) have longer recurrence intervals (292 ±93 yrs) than earthquakes with intensity of ≥VI1/2 (139±69yr). Moreover, distribution fitting and the coefficient of variation (CoV) of inter-event times indicate that the stronger earthquakes recur in a more periodic way (CoV: 0.32 vs. 0.5). Regional correlation of our multi-threshold shaking records with coastal paleoseismic data of complementary nature (tsunami, coseismic subsidence) suggests that the intensity ≥VII1/2 events repeatedly ruptured the same part of the megathrust over a distance of at least ∼300 km and can be assigned to Mw≥8.6. We hypothesize that a zone of high plate locking – identified by geodetic studies and large slip in AD 1960 – acts as a dominant regional asperity, on which elastic strain builds up over several centuries and mostly gets released in quasi-periodic great and giant earthquakes. Our paleo-records indicate that Poissonian recurrence models are inadequate to describe large megathrust earthquake recurrence in south-central Chile. Moreover, they show an enhanced probability for a Mw 7.7–8.5 earthquake during the next 110 years whereas the probability for a Mw≥8.6 (AD1960-like) earthquake remains low in this period.

Influence of the megathrust earthquake cycle on upper-plate deformation in the Cascadia forearc of Washington State, USA

Influence of the megathrust earthquake cycle on upper-plate deformation in the Cascadia forearc of Washington State, USA

The super‐interseismic phase of the megathrust earthquake cycle in Chile

AbstractAlong a subduction zone, great megathrust earthquakes recur either after long seismic gaps lasting several decades to centuries or over much shorter periods lasting hours to a few years when cascading successions of earthquakes rupture nearby segments of the fault. We analyze a decade of continuous Global Positioning System observations along the South American continent to estimate changes in deformation rates between the 2010 Maule (M8.8) and 2015 Illapel (M8.3) Chilean earthquakes. We find that surface velocities increased after the 2010 earthquake, in response to continental‐scale viscoelastic mantle relaxation and to regional‐scale increased degree of interplate locking. We propose that increased locking occurs transiently during a super‐interseismic phase in segments adjacent to a megathrust rupture, responding to bending of both plates caused by coseismic slip and subsequent afterslip. Enhanced strain rates during a super‐interseismic phase may therefore bring a megathrust segment closer to failure and possibly triggered the 2015 event.

Pre-, Co-, and Post-Seismic Deformation of the 2011 Tohoku-Oki Earthquake and its Implication to a Paradox in Short-Term and Long-Term Deformation

The 2011 Tohoku-oki earthquake caused large eastward displacement and subsidence along the Pacific coast of northeastern Japan. This earthquake partly solved a well-known paradox holding that sense and rate of deformation differ greatly between geologic and geodetic estimates. A paradox remains, however, in explaining long-term uplift along the Pacific coast on a geologic time-scale. Geodetic data show that coastal subsidence continued at a nearly constant rate of ∼5 mm/yr with small fluctuations associated with M7-8 interplate earthquakes for ∼120 years before the Tohoku-oki earthquake. In an area near the Oshika Peninsula where coseismic subsidence is largest, extrapolation of a logarithmic function fitting observed postseismic deformation suggests that coseismic subsidence may be compensated for by the postseismic uplift for several decades but it is difficult to expect the postseismic uplift exceeding 2 meters, so it is implausible that the observed rapid subsidence continued throughout an entire interseismic period in a great megathrust earthquake cycle. We propose a hypothetical model in which the sense of vertical deformation changes from uplift to subsidence during the interseismic period. Using simple elastic dislocation theory, this model is explained by the shallow coupled part of a plate interface in an early interseismic period and the deep coupled part of a late interseismic period.

Slow Slip Transients Before the 2011 Tohoku-Oki Earthquake

I review a spatiotemporal evolution of slow-slip transients on the plate interface of the subducting Pacific plate that happened in and around the mainshock rupture area prior to the 2011 Tohoku-Oki earthquake. Based on foreshock activity before the mainshock, two sequences of slow-slip transients were identified by earthquake migrations toward the initiation point of the mainshock rupture. These two slow-slip transients were also detected by geodetic measurement. The second sequence of slow-slip transients, which involved large slip rates, may have caused significant stress loading onto the hypocenter of the mainshock and prompted the initiation of unstable dynamic rupture. In addition, decadal slip-behavior on the plate interface revealed by geodetic measurement and small repeating earthquakes show that slow-slip transients occurred in the down-dip and up-dip portions of the mainshock rupture area. These slow-slip transients imply the reduction of coupling between the subducting and overlying plates, that could be interpreted as the late stage of mega-thrust earthquake cycle, although this notion remains conjectural.

Response of forearc crustal faults to the megathrust earthquake cycle: InSAR evidence from Mejillones Peninsula, Northern Chile

We report on a rare example of aseismic response of a creeping fault to the earthquake cycle of a nearby megathrust. Interferometric synthetic aperture radar (InSAR) is used to detect and analyze shallow creep of two crustal faults at Mejilones Peninsula, Northern Chile, located in the hanging wall of the 2007 Mw 7.7 Tocopilla subduction earthquake. We generate two independent time series of surface deformation spanning ∼3.5yr of late interseismic and ∼1.5yr early postseismic deformation associated with this event. The analysis reveals creep on the Mejillones fault as well as on a previously unmapped fault to the west of the Mejillones fault. The InSAR deformation maps and distributed slip models obtained from the data reveal that fault creep reversed between the interseismic and postseismic periods. Given the regional stress field perturbations due to interseismic and coseismic deformation, we argue that the observed shallow creep and its slip reversal are directly linked to the megathrust seismic cycle. Moreover, from similar eastward dips but opposite slip directions of the two faults, we infer that fault strength must be very low and that the kinematics is controlled by crustal flexure associated with the seismic cycle on the underlying megathrust.

Viscosity of the asthenosphere from glacial isostatic adjustment and subduction dynamics at the northern Cascadia subduction zone, British Columbia, Canada

Late glacial sea level curves located in the Cascadia subduction zone (CSZ) fore arc in southwestern British Columbia show that glacial isostatic adjustment (GIA) was rapid when the Cordilleran Ice Sheet collapsed in the late Pleistocene. GIA modeling with a linear Maxwell rheology indicates that the observations can be equally well fit across a wide range of asthenospheric thicknesses, provided that the asthenospheric viscosity is varied from 3 × 1018 Pa s for a thin (140 km) asthenosphere to 4 × 1019 Pa s for a thick (380 km) asthenosphere. Present‐day vertical crustal motion predicted by the GIA models shows rates of a few tenths of a millimeter per year, consistent with previous analyses. The model viscosities largely pertain to the viscosity of the oceanic mantle beneath the subducting Juan de Fuca slab but include a contribution from the mantle wedge above the slab. For comparison, effective viscosities for the upper mantle due to the tectonic regime (subduction) were computed using the strain rates and temperatures of an independent geodynamic model of the CSZ with a wet olivine power law rheology. The effective viscosities agree well with GIA model viscosities of 1019 Pa s or less, corresponding to an asthenosphere of 100 or 200 km thickness. The agreement suggests a significant role for power law flow in the GIA response. Regardless of the microphysical mechanisms responsible for the GIA response, the viscosity values inferred from GIA can be applied to studies of the megathrust earthquake cycle because both processes take place on comparable time scales.

Toward Advanced Earthquake Cycle Simulation

Recent earthquake cycle simulation based on laboratory derived rate and state friction laws with super-parallel computers have successfully reproduced historical earthquake cycles. Earthquake cycle simulation is thus a powerful tool for providing information on the occurrence of the next Nankai megathrust earthquake, if simulation is combined with data assimilation for historical data and recently ongoing crustal activity data observed by networks extending from the land to the ocean floor. Present earthquake cycle simulation assumes simplifications in calculation, however, that differ from actual complex situations. Executing simulation relaxing these simplifications requires huge computational demands, and is difficult with present supercomputers. Looking toward advanced simulation of Nankai megathrust earthquake cycles with next-generation petaflop supercomputers, we present 1) an evaluation of effects of the actual medium in earthquake cycle simulation, 2) improved deformation data with GPS and InSAR and of inversion for estimating frictional parameters, and 3) the estimation of the occurrence of large inland earthquakes in southwest Japan and of Nankai megathrust earthquakes.

Shocks in a box: An analogue model of subduction earthquake cycles with application to seismotectonic forearc evolution

We introduce and test an experimental approach to simulate elastoplastic megathrust earthquake cycles using an analogue model and apply it to study the seismotectonic evolution of subduction zones. The quasi‐two‐dimensional analogue model features rate‐ and state‐dependent elastic‐frictional plastic and viscoelastic material properties and is scaled for gravity, inertia, elasticity, friction, and viscosity. The experiments are monitored with a high‐resolution strain analysis tool based on digital image correlation (particle imaging velocimetry, PIV), providing deformation time series comparable to seismologic, geodetic, and geologic observations. In order to separate elastic and nonelastic effects inherent the experimental deformation patterns, we integrate elastic dislocation modeling (EDM) into a hybrid approach: we use the analogue earthquake slip and interseismic locking distribution as EDM dislocation input and forward model the coseismic and interseismic elastic response. The residual, which remains when the EDM prediction is subtracted from the experimental deformation pattern, highlights the accumulation of permanent deformation in the model. The setup generates analogue earthquake sequences with realistic source mechanisms and elastic forearc response and recurrence patterns and reproduces principal earthquake scaling relations. By applying the model to an accretionary‐type plate margin, we demonstrate how strain localization at the rupture peripheries may lead to a seismotectonically segmented forearc, including a tectonically stable shelf and coastal high (<20% plate convergence accommodated by internal shortening) overlying the area of large megathrust earthquake slip. Fifty to 75% of plate convergence is accommodated by internal shortening in the slope region where earthquake slip tapers out toward the trench. The inner forearc region remains undeformed and represents a basin.