Recent Megathrust Earthquakes Research Articles

Seismic collapse performance of high-rise RC dual system buildings in subduction zones

The satisfactory ‘collapse prevention’ performance level of reinforced concrete (RC) buildings has been widely recognized during recent earthquakes in Chile. However, there is limited research on the actual level of seismic collapse protection. In this study, the seismic collapse behavior of high-rise RC dual wall-frame buildings representative of the Chilean inventory is quantitatively evaluated. A suite of four 16-story structural archetypes was carefully selected and code-based designed assuming two different locations (i.e., high and moderate seismicity zones) and two different soil types (i.e., very stiff and moderately stiff soils). The archetypes were analyzed considering the latest developments in performance-based earthquake engineering implementing incremental dynamic analyses for 3D nonlinear models with sets of Chilean subduction ground motions. Results, expressed in terms of the probability of collapse conditioned on the Maximum Considered Earthquake (MCE) hazard level (<10%) and the collapse probability in 50 years (<1%), showed that all archetypes fully met the targets specified by ASCE 7 for an acceptable ‘life safety’ risk level. These results indeed explain why a very small number of RC building collapses was observed in the recent megathrust earthquakes (Mw>8.0) in Chile. Nevertheless, it was also found that the seismic collapse performance is not uniform, due mainly to the soil type. This observation suggests that the design spectra indicated by the Chilean seismic design code for buildings might need to be revised.

Imaging the source region of recent megathrust earthquakes along the Chile subduction zone: A summary of results from recent experiments

In addition to the largest documented earthquake, the Mw 9.5 Valdivia earthquake of 1960, the Chilean subduction zone has experienced three great (>Mw 8) earthquakes since 2010 for which detailed models of deformation prior to, during and after the earthquake are available. The 1960 and 2010 Mw 8.8 Maule earthquake affected the south-central Chilean margin, where the trench is sediment filled, whereas the 2014 Mw 8.2 Iquique and 2015 Mw 8.3 Illapel earthquakes occurred farther north where there is little sediment in the trench. We summarize recent results from three marine seismological projects designed to elucidate the impact that geologic structure may have had on slip during these events. Because analysis of these datasets is ongoing, this report should be considered as an interim summary rather than a comprehensive synthesis. None-the-less, some patterns can be discerned. That we can resolve 5 m of margin uplift by differencing swath bathymetric data highlights the value of obtaining ground truth marine geophysical data from potential seismic gaps. Controlled source seismic imaging indicates that subduction of nearly all incoming sediment in the sediment-filled section of the trench is a widespread characteristic of the subduction zone segment that ruptured in the two largest events and is likely a generalizable characteristic that can be used to anticipate whether a subduction zone is likely to rupture in great tsunamigenic earthquakes. While this is not a new idea, our data provide additional insights into plate boundary evolution at depth in this scenario. We also find that subduction of anomalously rough basement topography ∼2 million years ago may have led to indentation, large scale internal deformation, and erosion of the overlying wedge north of the 2010 rupture zone and that the process of wedge recovery may have resulted in an abrupt offset along strike of the plate boundary that served as a barrier to slip propagation in 2010. In general, structures generating wedge disruption may no longer be detectable following their subduction deep beneath the forearc wedge, but their passage can be inferred from residual structures mapped within the wedge, and their impact on slip during large earthquakes may be long lasting. We anticipate that further analysis of the data from these experiments, and from future experiments that build on these results, will further elucidate the relationship between geologic structure and slip such that images of subsurface structure become an additional tool in the toolbox of observations used to evaluate future hazard.

Locking-derived tsunami scenarios for the most recent megathrust earthquakes in Chile: implications for tsunami hazard assessment

The ever-increasing data from continues geodetic networks has allowed to reveal first-order spatial relations between pre-seismic highly locked zones on the interface and regions of large coseismic slip. Here we use a distribution of locking degree and recurrence of historical earthquakes along the Chilean subduction zone to estimate the slip deficit accumulated since previous tsunamigenic earthquakes and use these constrains to simulate tsumani signals. We generate tsunami models for the recent five major tsunamigenic earthquakes in Chile during the last decade: Maule (2010), Pisagua (2014), Illapel (2015), Melinka (2016) and Valparaiso (2017). Results were compared with tsunami records to evaluate the use of locking degree as an estimate of displacement of future earthquakes and as a tool for tsunami hazard assessment. Our tsunami simulations for the Maule (2010) and Illapel (2015), earthquakes that filled a seismic gap, reproduce well the tsunami observations, for the case of Pisagua (2014), Valparaiso (2016) and Melinka (2017) although it exists a correspondence in the areas of greater degree of locking with the coseismic slip, there is also an imbalance between the energy accumulated and released in the interseismic period considered according to the historical seismicity. This indicate that the rupture of small asperities (areas that release high slip during earthquakes) may have a complex pattern of recurrence and do not fully affect a locked patch. Locking-derived tsunami scenarios represent well tsunami observations of earthquakes that ruptured a locked seismic gap, and they can be used in addition to the actual methodologies to improve the estimation of tsunami hazard.

Enhancement of interplate coupling in adjacent segments after recent megathrust earthquakes

Landward increase of surface velocity has been found for segments adjacent along-strike to megathrust faults after the 2003 Tokachi-oki and the 2011 Tohoku-oki earthquakes, NE Japan. A similar increase of landward velocities was reported for the segments to the north of the rupture of the 2010 Maule earthquake, Chile. We utilize available GNSS data to find such changes for six megathrust earthquakes in four subduction zones, including NE Japan, central and northern Chile, Sumatra, and Mexico to investigate their common features. Our study showed that such increase, ranging from a few mm/yr to ~1 cm/yr, appeared in adjacent segments following the 2014 Iquique (Chile), the 2007 Bengkulu (Sumatra), and the 2012 Oaxaca (Mexico) earthquakes in addition to the three cases. The region of the increased landward movements extends with spatial decay and reach the distance comparable to the along-strike fault length. On the other hand, the temporal decay of the increased velocity is not clear at present. The degree of increase seems to depend on the earthquake magnitude, and possibly scales with the average fault slip in the earthquake. This is consistent with the simple two-dimensional model proposed earlier to attribute the phenomenon to the enhanced coupling caused by accelerated subduction. However, these data are not strong enough to rule out other possibilities.

Formation of the frontal thrust zone of accretionary wedges

The combination of recent megathrust earthquakes that have ruptured the frontal portions of accretionary prisms and historical evidence for large wedge-front, fold-and-thrust belt events has sparked interest in the development of wedge-front fault systems. Here we explore the formation of these faults using two-dimensional finite-difference models of spontaneous thrust fault formation and wedge accretion. A reference model with uniform material strength parameters predicts wedge-front thrusts to nucleate near the surface of the sedimentary layer and propagate down towards the basal décollement. The formation of a new frontal thrust is preceded by incipient faulting over a zone several kilometers wide forward of the wedge toe, resembling the protothrust zone (PTZ) imaged in seismic reflection data from the front of submarine accretionary prisms. Similar behavior, with a new frontal thrust forming from top to bottom, occurs if the fraction (λ) of pore-fluid pressure relative to the total (dynamic) pressure is uniform and <0.8 throughout the sediment section, or if λ increases with depth and is <0.7 at the base of the décollement. For greater values of λ, models predict more frontal thrusts to form from the bottom to top. These results are reasonably well explained by a simple elastic stress analysis. Models that simulate cohesive brittle strength that is negligible in the shallow sediments and increases with depth also predict new thrust faults to form from top to bottom. Models in which viscous flow at shallow depths represents creep of sediments predict bottom-up thrust nucleation when viscous stresses due to plate convergence remain lower than the brittle yield stress in the top ∼15–50% of the sediment section. Top-down nucleation is otherwise favored when viscous stresses exceed the brittle yield strength throughout most of the sediments. Our study illuminates key controls on the mechanical and temporal links between the PTZ, the forward propagation of slip along the underlying décollement, and the formation of a new frontal thrust.

Dynamic rupture activation of backthrust fault branching

We perform dynamic rupture simulations to investigate the possible reactivation of backthrust branches triggered by ruptures along a main thrust fault. Simulations with slip-weakening fault friction and uniform initial stress show that fast propagation speed or long propagation distance of the main rupture promotes reactivation of backthrust over a range of branch angles. The latter condition may occur separately from the former if rupture speed is limited by an increasing slip-weakening distance towards the junction direction. The results suggest a trade-off between the amplitude and duration of the dynamic stress near the main rupture front for backthrust reactivation. Termination of the main rupture by a barrier can provide enhanced loading amplitude and duration along a backthrust rooted near the barrier, facilitating its reactivation especially with a high frictional resistance. The free surface and depth-dependent initial stress can have several additional effects. The sign of the triggered motion along the backthrust can be reversed from thrust to normal if a deeply nucleated main rupture breaks the free surface, while it is preserved as thrust if the main rupture is terminated by a barrier at depth. The numerical results are discussed in relation to several recent megathrust earthquakes in Sumatra, Chile, and Japan, and related topics such as branch feedbacks to the main fault. The dynamic view on backthrust fault branching provided by the study fills a gap not covered by quasi-static models or observations. A specific examined case of antithetic fault branching may be useful for indicating a barrier-like behavior along the main fault.

Short-lived tectonic switch mechanism for long-term pulses of volcanic activity after mega-thrust earthquakes

Abstract. Eruptive rates in volcanic arcs increase significantly after subduction mega-thrust earthquakes. Over short to intermediate time periods the link between mega-thrust earthquakes and arc response can be attributed to dynamic triggering processes or static stress changes, but a fundamental mechanism that controls long-term pulses of volcanic activity after mega-thrust earthquakes has not been proposed yet. Using geomechanical, geological, and geophysical arguments, we propose that increased eruption rates over longer timescales are due to the relaxation of the compressional regime that accompanies mega-thrust subduction zone earthquakes. More specifically, the reduction of the horizontal stress σh promotes the occurrence of short-lived strike-slip kinematics rather than reverse faulting in the volcanic arc. The relaxation of the pre-earthquake compressional regime facilitates magma mobilisation by providing a short-circuit pathway to shallow depths by significantly increasing the hydraulic properties of the system. The timescale for the onset of strike-slip faulting depends on the degree of shear stress accumulated in the arc during inter-seismic periods, which in turn is connected to the degree of strain-partitioning at convergent margins. We performed Coulomb stress transfer analysis to determine the order of magnitude of the stress perturbations in present-day volcanic arcs in response to five recent mega-thrust earthquakes; the 2005 M8.6, 2007 M8.5, and 2007 M7.9 Sumatra earthquakes; the 2010 M8.8 Maule, Chile earthquake; and the 2011 M9.0 Tohoku, Japan earthquake. We find that all but one the shallow earthquakes that occurred in the arcs of Sumatra, Chile and Japan show a marked lateral component. We suggests that the long-term response of volcanic arcs to subduction zone mega-thrust earthquakes will be manifested as predominantly strike-slip seismic events, and that these future earthquakes may be followed closely by indications of rising magma to shallower depths, e.g. surface inflation and seismic swarms.

The seismic cycle at subduction thrusts: Insights from seismo‐thermo‐mechanical models

The underestimation of the size of recent megathrust earthquakes illustrates our limited understanding of their spatiotemporal occurrence and governing physics. To unravel their relation to associated subduction dynamics and long‐term deformation, we developed a 2‐D continuum viscoelastoplastic model that uses an Eulerian‐Lagrangian finite difference framework with similar on‐ and off‐fault physics. We extend the validation of this numerical tool to a realistic subduction zone setting that resembles Southern Chile. The resulting quasi‐periodic pattern of quasi‐characteristic M8–M9 megathrust events compares quantitatively with observed recurrence and earthquake source parameters, albeit at very slow coseismic speeds. Without any data fitting, surface displacements agree with GPS data recorded before and during the 2010 M8.8 Maule earthquake, including the presence of a second‐order flexural bulge. These surface displacements show cycle‐to‐cycle variations of slip deficits, which overall accommodate ∼5% of permanent internal shortening. We find that thermally (and stress) driven creep governs a spontaneous conditionally stable downdip transition zone between temperatures of ∼350°C and ∼450°C. Ruptures initiate above it (and below the forearc Moho), propagate within it, interspersed by small intermittent events, and arrest below it as ductile shearing relaxes stresses. Ruptures typically propagate upward along lithological boundaries and widen as pressures drop. The main thrust is constrained to be weak due to fluid‐induced weakening required to sustain regular subduction and to generate events with natural characteristics (fluid pressures of ∼75–99% of solid pressures). The agreement with a range of seismological, geodetic, and geological observations demonstrates the validity and strength of this physically consistent seismo‐thermo‐mechanical approach.

On the Polar Motion Excitations by Recent Mega-thrust Earthquakes

On the Polar Motion Excitations by Recent Mega-thrust Earthquakes

Splitting function measurements for Earth's longest period normal modes using recent large earthquakes

Recent megathrust earthquakes, such as the 23 June 2001 Peru event, the Sumatra events of 2004 and 2005 and the 27 February 2010 Chile event, have given us the opportunity to measure splitting of the longest period normal modes. We use wave spectra to make robust measurements for modes 0S2, 0S3, 0S4, 2S1 and 1S2. Singlet frequencies of these modes have been measured previously using gravimeters, but here we use seismic records to observe splitting functions for 0S2 and 2S1 for the first time. Cross-coupling with nearby modes is included to account for ellipticity and rotation of the Earth and results in significantly improved splitting function measurements for 0S3, 0S4 and 1S2 compared with previous studies. The new splitting function measurements can easily be implemented in future tomographic modelling of aspherical velocity and, particularly, density structure.

The west Andaman fault and its influence on the aftershock pattern of the recent megathrust earthquakes in the Andaman‐Sumatra region

Distinctly different rupture patterns of December 2004 and March 2005 megathrust earthquakes occurred in the Andaman‐Sumatra region suggest strong influence of tectonic and structural elements. We have analysed the shipboard gravity, bathymetry and seismic data across the West Andaman Fault (WAF), a major tectonic feature in the Andaman Sea, to infer the crustal structure and to examine its influence in controlling the aftershock pattern. Our results support the idea of Singh et al. (2005) that WAF forms a lithospheric scale boundary and together with other tectonic elements modulate the occurrence of large earthquakes and their rupture pattern. The active strike‐slip motion along the WAF, presence of backarc spreading coupled with increased obliquity of subduction in the Andaman Sector reduce the probability of occurrence of major or great earthquakes north of 10°N.

Can electromagnetic disturbances related to the recent great earthquakes be detected by satellite magnetometers?

On December 26, 2004 the world's fourth largest earthquake since 1900 and the largest since the 1964 Prince William Sound, Alaska earthquake, occurred off the west coast of northern Sumatra with a magnitude of 9.3. On March 28, 2005 another event of magnitude 8.7 took place in the same region. The December 26, 2004 earthquake has prompted scientists to investigate possible electromagnetic signatures of this event, using ground magnetic observations. Iyemori et al. [Iyemori, T. et al., 2005. Geomagnetic pulsations caused by the Sumatra earthquake on December 26, 2004. Geophys. Res. Lett., 32, L20807, doi:10.1029/2005GL024083.] have suggested that a 3.6 min long geomagnetic pulsation, observed shortly after this event, was generated by the earthquake. They have speculated that a 30 s magnetic pulsation was also caused by the earthquake. Here for the first time, CHAMP satellite magnetic and electron density data are examined to find out if electromagnetic signatures which are possibly related to these recent megathrust earthquakes are observed in satellite magnetic data. We have shown that some specific features are observed after the two earthquakes, with periods of about 16 and 30 s. Our results favor an external source origin for the 30 s pulsation. Moreover, after more than 1 h, CHAMP magnetic data indicate the existence of a feature characterized by the same parameters (duration, amplitude, and frequency content), which could be associated with each earthquake, respectively. Further investigations are required in order to answer the question of whether these signals can be associated with earthquakes and to assign their possible usefulness with respect to earthquake development.