Different Fault Response to Stress during the Seismic Cycle

Seismic prediction was considered impossible, however, there are no reasons in theoretical physics that explicitly prevent this possibility. Therefore, it is quite likely that prediction is made stubbornly complicated by practical difficulties such as the quality of catalogs and data analysis. Earthquakes are sometimes forewarned by precursors, and other times they come unexpectedly; moreover, since no unique mechanism for nucleation was proven to exist, it is unlikely that single classical precursors (e.g., increasing seismicity, geochemical anomalies, geoelectric potentials) may ever be effective in predicting impending earthquakes. For this reason, understanding the physics driving the evolution of fault systems is a crucial task to fine-tune seismic prediction methods and for the mitigation of seismic risk. In this work, an innovative idea is inspected to establish the proximity to the critical breaking point. It is based on the mechanical response of faults to tidal perturbations, which is observed to change during the “seismic cycle”. This technique allows to identify different seismic patterns marking the fingerprints of progressive crustal weakening. Destabilization seems to arise from two different possible mechanisms compatible with the so called preslip patch, cascade models and with seismic quiescence. The first is featured by a decreasing susceptibility to stress perturbation, anomalous geodetic deformation, and seismic activity, while on the other hand, the second shows seismic quiescence and increasing responsiveness. The novelty of this article consists in highlighting not only the variations in responsiveness of faults to stress while reaching the critical point, but also how seismic occurrence changes over time as a function of instability. Temporal swings of correlation between tides and nucleated seismic energy reveal a complex mechanism for modulation of energy dissipation driven by stress variations, above all in the upper brittle crust. Some case studies taken from recent Greek seismicity are investigated.

Keyboard Model of Seismic Cycle of Great Earthquakes in Subduction Zones: Simulation Results and Further Generalization

Catastrophic megaearthquakes (M > 8) occurring in the subduction zones are among the most devastating hazards on the planet. In this paper we discuss the seismic cycles of the megathrust earthquakes and propose a blockwise geomechanical model explaining certain features of the stress-deformation cycle revealed in recent decades from seismological and satellite geodesy (GNSS) observations. Starting with an overview of the so-called keyboard model of the seismic cycle by L. Lobkovsky, we outline mathematical formalism describing the motion of seismogenic block system assuming viscous rheology beneath and between the neighboring elastic blocks sitting on top of the subducting slab. By summarizing the GNSS-based evidence from our previous studies concerning the transient motions associated with the 2006–2007 Simushir earthquakes, 2010 Maule earthquake, and 2011 Tohoku earthquake, we demonstrate that those data support the keyboard model and reveal specific effect of the postseismic oceanward motion. However, since the seismogenic blocks in subduction systems are mostly located offshore, the direct analysis of GNSS-measured displacements and velocities is hardly possible in terms of the original keyboard model. Hence, the generalized two-segment keyboard model is introduced, containing both frontal offshore blocks and rear onshore blocks, which allows for direct interpretation of the onshore-collected GNSS data. We present a numerical computation scheme and a series of simulated data, which exhibits the consistency with measured motions and enables estimating the seismic cycle characteristics, important for the long-term earthquake forecasting.

The stop-start control of seismicity by fault bends along the Main Himalayan Thrust

The Himalayan megathrust accommodates most of the relative convergence between the Indian and Eurasian plates, producing cycles of blind and surface-breaking ruptures. Elucidating the mechanics of down-dip segmentation of the seismogenic zone is key to better determine seismic hazards in the region. However, the geometry of the Himalayan megathrust and its impact on seismicity remains controversial. Here, we develop seismic cycle simulations tuned to the seismo-geodetic data of the 2015 Mw 7.8 Gorkha, Nepal earthquake to better constrain the megathrust geometry and its role on the demarcation of partial ruptures. We show that a ramp in the middle of the seismogenic zone is required to explain the termination of the coseismic rupture and the source mechanism of up-dip aftershocks consistently. Alternative models with a wide décollement can only explain the mainshock. Fault structural complexities likely play an important role in modulating the seismic cycle, in particular, the distribution of rupture sizes. Fault bends are capable of both obstructing rupture propagation as well as behave as a source of seismicity and rupture initiation.

Erosion rates of the New Zealand Southern Alps reflect long-term tectonics and transient climate

<p>The Southern Alps of New Zealand are the expression of the oblique convergence between the Pacific and Australian plates, which move at a relative velocity of nearly 40 mm/yr. This convergence is accommodated by the range-bounding Alpine Fault, with a strike-slip component of ~30-40 mm/yr, and a shortening component normal to the fault of ~8-10 mm/yr. While strike-slip rates seem to be fairly constant along the Alpine Fault, throw rates appear to vary considerably, and whether the locus of maximum exhumation is located near the fault, at the main drainage divide, or part-way between, is still debated. These uncertainties stem from very limited data characterizing vertical deformation rates along and across the Southern Alps. Thermochronology has constrained the Southern Alps exhumation history since the Miocene, but Quaternary exhumation is hard to resolve precisely due to the very high exhumation rates. Likewise, GPS surveys estimate a vertical uplift of ~5 mm/yr, but integrate only over ~10 yr timescales and are restricted to one transect across the range.</p><p>To obtain insights into the Quaternary distribution and rates of exhumation of the western Southern Alps, we use new <sup>10</sup>Be catchment-averaged erosion rates from 20 catchments along the western side of the range. Catchment-averaged erosion rates span an order of magnitude, between ~0.8 and >10 mm/yr, but we find that erosion rates of >10 mm/yr, a value often quoted in the literature as representative for the entire range, are very localized. Moreover, erosion rates decrease sharply north of the intersection with the Marlborough Fault System, suggesting substantial slip partitioning. These <sup>10</sup>Be catchment-averaged erosion rates integrate, on average, over the last ~300 yrs. Considering that the last earthquake on the Alpine Fault was in 1717, these rates are representative of inter-seismic erosion. Lake sedimentation rates and coseismic landslide modelling suggest that long-term (~10<sup>3</sup> yrs) erosion rates over a full seismic cycle could be ~40% greater than our inter-seismic erosion rates. If we assume steady state topography, such a scaling of our <sup>10</sup>Be erosion rate estimates can be used to estimate rock uplift rates in the Southern Alps. Finally, we find that erosion, and hence potentially exhumation, does not seem to be localized at a particular distance from the fault, as some tectonic and provenance studies have suggested. Instead, we find that superimposed on the primary tectonic control, there is an elevation/temperature control on erosion rates, which is probably transient and related to frost-cracking and glacial retreat.</p><p>Our results highlight the potential for <sup>10</sup>Be catchment-averaged erosion rates to provide insights into the magnitude and distribution of tectonic deformation rates, and the limitations that arise from transient erosion controls related to the seismic cycle and climate-modulated surface processes.</p><p> </p><p> </p>

Relationships between deformation and morphology of forearc wedges and earthquake ruptures

<p>Over the last decade, we have accumulated evidence that, along subduction zones, a significant part of the seismic cycle deformation is permanently acquired by the medium and reflects the variation of rupture properties along the megathrust. Assuming a persistence of the megathrust segmentation over several hundred thousand years, this permanent deformation and the forearc topography could thus reveal the mechanics of the megathrust. Numerous recent studies have also shown that the megathrust effective friction appears to differ significantly between aseismic or seismic areas. From mechanical modelling, I will first discuss how such differences in effective friction are significant enough to induce wedge segments with varying morphologies and deformation patterns. I will present examples from different subduction zones characterized by either erosive or accretionary wedges, and by different seismic behaviors. Secondly, I will present how this long-lived deformation can in turn control earthquake ruptures. I will show, that along the Chilean subduction zone, all recent mega-earthquakes are surrounded by basal erosion and underplating. Therefore, the deformation and morphology of forearcs would both be partly linked to the megathrust rupture properties and should be used in a more systematic manner to improve earthquake rupture prediction.</p>

Source identification of Middle Age extreme event deposits in the Lesser Antilles using forward numerical modeling of tsunami

<p><span>The arc of the Lesser Antilles is one of the most quiet subduction zone in the world. In this region, the convergence of the Atlantic and the Caribbean plates is low (</span><span>few </span><span>mm/year) and most of the seismicity is a</span><span>n</span><span> intraplate and crustal seismicity. Among the Mw>7 earthquakes recorded in the historical catalog (1690 near Barbuda, 1843 near Guadeloupe, 1867 near the Virgin Islands, 1839 offshore Martinica, 1969 offshore Dominica, 1974 near Antigua), only the 1839 and 1843 events are suspected to be interplate earthquakes. The 1867 Virgin Island earthquake generated an important tsunami with waves of 10m that devastated the closest islands. A tsunami followed the 1843 earthquake but without much damage. These two events are the only known damaging tsunami in this region, but another older one might be added to the list. Indeed, an increasing number of tsunami deposits have been identified in the recent years on several islands of the arc, all of them being around 500 years old (~1450 AD). These deposits are all located in the northern segment of the arc, between Antigua and Puerto-Rico, in Anegada, St-Thomas (Virgin Islands), Anguilla </span><span>and</span><span> Scrub islands. There is </span><span>unfortunately</span><span> no record and no testimonies of an extreme event at that time.</span></p><p><span>The northern segment of the arc is particularly complex because located at the transition </span><span>between</span><span> the Greater Antilles </span><span>and the Lesser Antilles</span><span>. </span><span>It</span><span> is crossed by the Anegada Passage, a series of faults and basins cutting through the arc, which defines the limit between the Puerto-Rico micro-plate and the Caribbean plate. This passage and the numerous intra-arc fault systems present between the islands are active and likely compensate for the plates motion. The very low slip deficit detected with GPS measurements at the subduction contacts of Puerto-Rico and the Lesser Antilles indicates that the interface from Guadeloupe to Puerto-Rico can be considered as totally uncoupled or holding the characteristics of a very long seismic cycle. A tsunami generated by an extreme event 500 years ago in this region could be related to </span><span>intra-arc, outer-rise,</span><span> intraplate </span><span>or</span><span> interface fault rupture. The identification of the source </span><span>would</span><span> enable a better understanding of the seismic cycle and the dynamic of this part of the arc.</span></p><p><span>This study lists </span><span>and set models of</span><span> all the potential faults that could trigger an earthquake in the area encompassing the three islands : Anguilla, Anegada and StThomas. </span><span>We have created high-resolution bathymetric grids and</span><span> performed tsunami simulations </span><span>for each fault model</span><span>. </span><span>W</span><span>e uses run-up models to compare the simulated wave heights </span><span>and run-up distance</span><span> to all the deposits heights </span><span>and positions</span><span>. The magnitudes of our fault models range between 7 and </span><span>9,</span><span> but very few of them generate a strong enough tsunami t</span><span>o</span> <span>match</span><span> the observ</span><span>ed deposits</span><span>.</span></p>

Roles of poro-elastic compressibility, rate-dependent strength and strain-stress dependent dilation for spontaneous generation of seismicity along fluid-bearing fault structures

<p>We present a newly developed marker in cell staggered finite difference poro-visco-elasto-plastic numerical model for spontaneous seismic cycle along fluid-bearing fault structures. The fully coupled hydro-mechanical multi-physics model includes poro-elastic compressibility of the solid matrix together with experimentally calibrated rate-dependent strength laws and strain-stress dependent dilation. Localised brittle/plastic deformation is treated accurately through global Picard iterations. To simulate deformation on both long- and short-time scale, an adaptive time stepping is used allowing the resolution of large seismic events with time steps on the order of milliseconds.</p><p>Our new numerical modelling tool allows to explore how the presence of pressurised fluids in the pore space of subduction interface and strike-slip zones triggers poro-elastic stress accumulation and release in form of various seismic cycles. The model is capable of simulating spontaneous quasi-periodic seismic events along self-consistently forming highly localized self-pressurised ruptures accommodating shear displacement between the plates. The generated elastic rebound events show slip velocities ranging from the order of Nm/s to m/s, covering the entire range of seismic and slow slip phenomena. The governing strength decrease along the propagating fracture is related mainly to the significant increase of fluid pressure generated by deformation induced plasto-elastic collapse of pores. The reduction of the effective pressure decreases the brittle/plastic strength of fluid-bearing rocks along the rupture, thus providing a dynamic feedback mechanism for the accumulated elastic stress release at the fault interface.  It is remarkable that the seismic behaviours for both slow slip and ordinary earthquakes can be generated within the same self-consistent poro-visco-elasto-plastic rheological framework without any involvement of rate- and state-dependent friction commonly used for seismicity modelling. We furthermore analyse how this process and the seismic cycle are affected by poro-elastic, rate weakening and dilation parameters.</p>

The discontinuous Galerkin method for sequences of earthquakes and aseismic slip

<p>Earthquakes and aseismic slip are typically modelled as a displacement discontinuity on a prescribed infinitesimally thin fault surface embedded in linear elastic or viscoelastic media. The fault slip behaviour can be described by laboratory-derived rate and state friction laws, which are suitable to model frictional sliding throughout the complete seismic cycle, i.e. interseismic, coseismic, and post-seismic phase. The governing time scales vary from years in the interseismic phase to seconds in the coseismic phase and the respective spatial scales vary from hundreds of kilometres of tectonic structures  to metres (or less) on-fault. Therefore, simulating the entire seismic cycle is computational challenging and as such mandates utilization of high performance computing (HPC).</p><p>We present the open-source code tandem which is designed to model quasi-dynamic sequences of earthquakes and aseismic slip (SEAS). In tandem we explore the usefulness of the symmetric interior penalty Galerkin (SIPG) method using unstructured simplicial meshes for the computation of the elastostatic response to a displacement discontinuity. The potential of the SIPG method for SEAS models lies in (i) its geometric flexibility, (ii) its high-order approximation spaces, (iii) and its natural ability to deal with discontinuities.</p><p>Using a number of 2D and 3D SCEC community benchmarks (Erickson et al., 2020) we verify the tandem SIPG implementation. Based on the same reference models, we demonstrate benefits of using highly refined unstructured meshes and a high-order geometric representation of the fault. We also explore whether using a high-order discretisation in space is advantageous. Lastly, we outline how tandem may leverage modern supercomputing resources.</p>

Two-Element Keyboard-Block Model Of Megathrust Earthquakes Generation For Central Kurils

<p>The strongest subduction earthquakes (M≥8) lead to the release of the huge amount of elastic stresses accumulated over hundreds or even thousands of years. Prediction of such earthquakes, causing significant socio-economic and environmental damage, is one of the most important and urgent tasks of geophysics.</p><p>To date, significant advances have been made in the field of earthquake prediction using models based on the concept of a continuous geophysical medium that ruptured coseismically along the main fault. As an alternative, models are proposed that take into account the fault-block structure of the continental margin, confirmed by seismological and oceanographic studies. In our study, we consider one of such models - a keyboard-block model (single-element) which combines the ideas of possible synchronous destruction of several adjacent asperities, mutual slip along a plane with variable friction depending on velocity, and subsequent healing of destructed portions of the medium under high-pressure conditions. This concept made it possible to simulate the displacement of surface points of frontal seismogenic blocks at all stages of the seismic cycle.</p><p>GNSS observations in subduction regions are carried out mostly on islands situated on the rear massif far from the seismogenic blocks. Strong multidirectional motion registered on GNSS stations during the seismic cycle, as well as seismological and geological data, clearly indicate that the rear part of the arc also has a complex structure and is divided into separate segments by large faults rooted into the contact zone of interacting lithospheric plates. We made a generalization (double-element) of the original model to consider the discontinuity of not only the frontal but also the rear part of the island arc.</p><p>We compared the earth's surface displacements during the seismic cycle in the Central Kurils, obtained within the framework of the continuous model, as well as the single-element and two-element keyboard models, to establish the influence of various configurations of the fault-block structure of the continental margin on the seismic cycle. We constructed the continuous model on the basis of our slip distribution model for the 2006 Simushir earthquake which indicates the interplate coupling patches prior to this earthquake.</p><p>This study was supported by the Russian Science Foundation (project 20–17-00140).</p>