Post-seismic Behaviour Research Articles

Mechanics of coseismic and postseismic acceleration of active landslides

Active slow-moving landslides exhibit very different coseismic and postseismic behaviour. Whereas some landslides do not show any postseismic acceleration, there are many that experience an increased motion in the days to weeks following an earthquake. The reason for this behaviour remains debated and the underlying mechanisms are only partially understood. In recent years, it has been suggested that postseismic acceleration is caused by excess pore water pressures generated outside of the shear zone during seismic shaking, with their subsequent diffusion into the shear zone. Here we show that this hypothesis is indeed plausible and hydro-mechanically consistent by using a basic rate-dependent physical landslide model. Our simulations provide insight into the landslide behaviour in response to seismic shaking and its main controlling parameters: preseismic landslide velocity, rate-dependency of soil strength in the shear zone, hydro-mechanical characteristics of the adjacent soil layers and the earthquake intensity.

Classification of transient triggering mechanisms of aftershocks in the post-seismic phase of the 2017 Pohang earthquake, South Korea

SUMMARY The 2017 Mw 5.5 Pohang earthquake occurred near an enhanced geothermal system site and generated thousands of aftershocks, the largest of which, a Mw 4.6 earthquake, occurred 87 d after the mainshock. Redistribution of the groundwater pressure perturbed by the mainshock has been suggested as a cause of the post-seismic stress changes triggering several aftershocks, including the time-delayed event. However, to date, possible effects of variations in pore pressure on the aftershock occurrence have not been quantified in this region. Therefore, we conducted poroelastic modelling to evaluate this contribution to spatiotemporal distribution of the aftershocks, including the delayed event, using a fully coupled hydromechanical code. To construct a poroelastic model, a segmented fault geometry and a layered lithological structure were used. In addition, we utilized a kinematic slip model, a split-node algorithm and in-situ properties to simulate reliable coseismic and post-seismic behaviour. Our reference model successfully reproduced coseismic surface deformation in a line-of-sight direction, comparable to the corresponding observation from interferometric synthetic aperture radar, and was calibrated using groundwater measurement in a well. In addition to constructing the reference model, a series of numerical simulations were conducted to explore the effects and sensitivities of various hydraulic conductivities. Finally, the modelled Coulomb stress changes and spatiotemporal distribution of the aftershocks were analysed to elucidate the transient triggering mechanisms based on conditional statements to classify the mechanisms into several subsets. The classification showed that the poroelastic effect driven by depth/conductivity-dependent fluid diffusion is more critical to aftershock occurrence than the diffusion in the entire simulation time, and we propose that the delayed earthquake of Mw 4.6 could be correlated with poroelastic triggering rather than diffusion triggering. Furthermore, we inferred that this poroelastic effect could contribute to decay of aftershocks, particularly relatively small-magnitude aftershocks, as well as slow this decay in bedrocks. However, the proposed model does not explain all of the observed aftershocks, and other driving forces or triggering mechanisms need to be considered.

Failure Mode Recognition of Columns Using Artificial Neural Network

Columns are one of the most vital segments in bridgessince its post-seismic behaviour is of much importance. The retrofitting methods and rehabilitation strategies of bridges mainly rely on the identification of the failure mode of columns. It has been witnessed in various studies on columns that the mode of failure highly depends on section and material properties and there is no specific boundary between the modes, which makes their identification more sophisticated. This paper uses an artificial neural network to predict the modes of failure by analysing the effects of such soft computing methods. In this study, machine- learning models were generated from the experimental data of 253 columns of rectangular cross-section and its accuracy of failure mode prediction was evaluated by considering failure modes mainly flexure, flexure-shear, and shear. The optimal input parameters have also been evaluated for the machine-learning algorithm that enhances the efficiency of failure mode prediction.

Postseismic deformation following the 2 July 2013 Mw 6.1 Aceh, Indonesia, earthquake estimated using GPS data

GPS data after the 2 July 2013 Mw 6.1 Aceh earthquake provide valuable information for understanding the postseismic behavior of a strike-slip fault in northern Sumatra, Indonesia. The analysis of postseismic deformation following the 2004 Sumatra-Andaman earthquake and the 2012 Indian Ocean earthquake suggest that the ongoing deformation affected northern Sumatra between July and December 2013. The postseismic deformation of the 2013 Aceh earthquake is herein investigated, with the contribution of poroelastic rebound and viscoelastic relaxation evaluated. The results suggest that the model displacement of those two mechanisms failed to estimate the data. The afterslip analysis of the 2013 Aceh earthquake, on the contrary, fits the data very well with a seismic slip occurring at multiple fault segments of the Sumatran fault in northern Sumatra, Indonesia.

Transient poroelastic stress coupling between the 2015 M7.8 Gorkha, Nepal earthquake and its M7.3 aftershock

The large M7.3 aftershock occurred 17 days after the 2015 M7.8 Gorkha earthquake. We investigate if this sequence is mechanically favored by the mainshock via time-dependent fluid migration and pore pressure recovery. This study uses finite element models of fully-coupled poroelastic coseismic and postseismic behavior to simulate the evolving stress and pore-pressure fields. Using simulations of a reasonable permeability, the hypocenter was destabilized by an additional 0.15 MPa of Coulomb failure stress change (∆CFS) and 0.17 MPa of pore pressure (∆p), the latter of which induced lateral and upward diffusive fluid flow (up to 2.76 mm/day) in the aftershock region. The M7.3 location is predicted next to a local maximum of ∆p and a zone of positive ∆CFS northeast of Kathmandu. About 60% of the aftershocks occurred within zones having either ∆p > 0 or ∆CFS > 0. Particularly in the eastern flank of the epicentral area, ~83% of the aftershocks experienced postseismic fluid pressurization and ~88% of them broke out with positive pore pressure, which are discernibly more than those with positive ∆CFS (71%). The transient scalar field of fluid pressurization provides a good proxy to predict aftershock-prone areas in space and time, because it does not require extraction of an assumed vector field from transient stress tensor fields as is the case for ∆CFS calculations. A bulk permeability of 8.32 × 10−18 m2 is resolved to match the transient response and the timing of the M7.3 rupture which occurred at the peak of the ∆CFS time-series. This estimate is consistent with the existing power-law permeability-versus-depth models, suggesting an intermediately-fractured upper crust coherent with the local geology of the central Himalayas. The contribution of poroelastic triggering is verified against different poroelastic moduli and surface flow-pressure boundaries, suggesting that a poroelastic component is essential to account for the time interval separating the mainshock and the M7.3 aftershock.

Co-seismic and post-seismic behavior of a wall type breakwater on a natural ground composed of liquefiable layer

The tsunami disaster countermeasures such as breakwaters might be damaged or lose their capacity to resist tsunami after a strong earthquake. Therefore, not only the co-seismic behaviors of a breakwater and its foundation system during earthquake loading, but also the post-seismic behavior after the earthquake loading should be investigated and estimated for future tsunami preparation, especially when the foundation ground is composed of liquefiable layer, which may cause large amount of displacement to the breakwater. In this study, the co-seismic and post-seismic behavior of an existing wall type breakwater on a natural ground which is composed of nonuniform liquefiable layer and thick cohesive layer with low permeability is investigated using an effective stress-based soil–water coupling numerical method. In the calculation, the complicated nonlinear dynamic behavior of the foundation soil is described by an advanced elasto-plastic soil constitutive model. A real recorded seismic wave, which consists of a major shock and two aftershocks from the 2011 Great East Japan earthquake, is adopted as the input earthquake loading. The calculation results indicate that the used numerical method is capable of capturing the progressive liquefaction and consolidation process of the foundation soil, as well as the subsidence and differential settlement of the breakwater during and after earthquake loading. The influence of earthquake loading can significantly reduce the capacity of breakwater to resist tsunami which may arrive within 1 h after earthquake; therefore, anti-seismic design should be taken into consideration in future tsunami preparation.

Combined logarithmic and exponential function model for fitting postseismic GNSS time series after 2011 Tohoku-Oki earthquake

The time series of a postseismic deformation is commonly fitted by a logarithmic or exponential decay function. However, the high-quality postseismic Global Navigation Satellite System (GNSS) time series of the 2011 Mw 9 Tohoku-Oki earthquake indicates that a single decay function cannot be used to represent the postseismic behaviour. We therefore combined the logarithmic (log) and exponential (exp) decay functions and developed methods for obtaining global solutions using nonlinear least squares calculations for such complex functions. Our models significantly improved the fitting performance of the postseismic time series and the prediction performance of the evolution of postseismic deformation. The solutions obtained by the proposed models and methods enabled distinction between the contributions of the log and exp functions, and explanation of characteristic phenomena such as the subsidence that occurs immediately after an earthquake is reversed to an uplift. The analysis of the solutions may suggest that there has been a continuous increase in the contribution of viscoelastic relaxation to postseismic deformation in eastern Japan, whereas the contribution of afterslip has rapidly decreased. The short-term prediction performance and the universal applicability of the proposed models to the Tohoku-Oki earthquake have contributed to the detection of a slow-slip event in the Tokai region. Rather than the existence of a unique single relaxation time for each surface site, our results suggest a unique single relaxation time for each postseismic deformation mechanism at a given subsurface location. Although the predictions were highly dependent on the assigned steady velocities and the long-term relaxation time constants, they indicate that the coseismic subsidence of the Yamoto station in Miyagi prefecture will recover around the year 2020. The estimated relaxation time constants of the present models appeared to be uniform throughout eastern Japan.

Co-seismic and post-seismic behavior of an existed shallow foundation and super structure system on a natural sand/silt layered ground

Purpose – The purpose of this paper is to evaluate the co-seismic and post-seismic behaviors of an existed soil-foundation system in an actual alternately layered sand/silt ground including pore water pressure, acceleration response, and displacement et al. during and after earthquake. Design/methodology/approach – The evaluation is performed by finite element method and the simulation is performed using an effective stress-based 2D/3D soil-water coupling program DBLEAVES. The calculation is carried out through static-dynamic-static three steps. The soil behavior is described by a new rotational kinematic hardening elasto-plastic cyclic mobility constitutive model, while the footing and foundation are modeled as elastic rigid elements. Findings – The shallow (short-pile type) foundation has a better capacity of resisting ground liquefaction but large differential settlement occurred. Moreover, most part of the differential settlement occurred during earthquake motion. Attention should be paid not only to the liquefaction behavior of the ground during the earthquake motion, but also the long-term settlement after earthquake should be given serious consideration. Originality/value – The co-seismic and post-seismic behavior of a complex ground which contains sand and silt layers, especially long-term settlement over a period of several weeks or even years after the earthquake, has been clarified sufficiently. In some critical condition, even if the seismic resistance is satisfied with the design code for building, detailed calculation may reveal the risk of under estimation of differential settlement that may give rise to serious problems.

The effect of dynamic loading on the shear strength of pyroclastic Ash Deposits and implications for landslide hazard: The case of Pudahuel Ignimbrite, Chile

The co-seismic and post-seismic behaviour of pyroclastic ash deposits and its influence on slope stability remains as a challenging subject in engineering geology. Case studies in volcanic areas of the world suggest that soil structural changes caused by seismic shaking results in landslide activity. It is critical to constrain how this kind of soil behaves during coseismic ground shaking, as well as the effects of dynamic loading on shear strength parameters after shaking. Direct shear tests carried out on cineritic volcanic materials from the Pudahuel Ignimbrite formation in central Chile show a direct effect of cyclic loading on the shear strength and in a minor extent on the rheology. A high apparent cohesion found in monotonic shear tests, likely attributed to suction and cementation, is destroyed by dynamic loading. At the same time, the internal friction angle rises. This defines a differential post-dynamic behaviour depending on normal effective stress conditions, which favour the occurrence of shallow landslides. These results show how the use of shear strength parameters obtained from standard monotonic direct shear tests may produce misleading results when analysing seismic slope stability in this type of soils.

Numerical simulations of seismic and post-seismic behavior of tailings

Several tailings impoundments have failed as a result of earthquakes in the last few decades. A majority of these failures were due to direct seismic loading, tailings liquefaction during shaking, or the post-seismic behavior of the tailings as it relates to the dissipation of excess pore-water pressures that were generated during shaking. Previous work has indicated that the UBCSAND model developed by Byrne et al. in 1995 is capable of simulating the cyclic simple shear testing response of low-plasticity tailings over a range of consolidation stresses and cyclic shear ratios. However, the ability of the model to simulate the dynamic and subsequent behavior of such tailings for other conditions, such those induced by shaking table tests, has not yet been evaluated. In this regard, previous work has shown that the main components of the UBCSAND model cannot realistically simulate some specific responses, including the post-seismic volumetric strains related to excess pore-water pressure dissipation. This paper presents numerical modeling results of the dynamic behavior of tailings from hard rock mines. It introduces a method for simulating their post-seismic behavior by including an updating scheme for the elastic moduli into the UBCSAND model. The results of cyclic simple shear testing, seismic table testing, and complementary experimental relationships were used to calibrate and validate the model with its new component. The simulated response of tailings during cyclic simple shear testing and for a complete seismic table test shows that the proposed approach simulates the experimental observations well. Level-ground, seismically induced liquefaction and post-seismic behavior of a 15 m thick tailings deposit are also simulated, leading to post-liquefaction settlements that are in agreement with empirical relationships.

Geodetic slip solutions for the Mw = 7.4 Champerico (Guatemala) earthquake of 2012 November 7 and its postseismic deformation

As the first large subduction thrust earthquake off the coast of western Guatemala in the past several decades, the 2012 November 7 Mw = 7.4 earthquake offers the first opportunity to study coseismic and postseismic behaviour along a segment of the Middle America trench where frictional coupling makes a transition from weak coupling off the coast of El Salvador to strong coupling in southern Mexico. We use measurements at 19 continuous GPS sites in Guatemala, El Salvador and Mexico to estimate the coseismic slip and postseismic deformation of the November 2012 Champerico (Guatemala) earthquake. An inversion of the coseismic offsets, which range up to ∼47 mm at the surface near the epicentre, indicates that up to ∼2 m of coseismic slip occurred on a ∼30 × 30 km rupture area between ∼10 and 30 km depth, which is near the global CMT centroid. The geodetic moment of 13 × 1019 N m and corresponding magnitude of 7.4 both agree well with independent seismological estimates. Transient postseismic deformation that was recorded at 11 GPS sites is attributable to a combination of fault afterslip and viscoelastic flow in the lower crust and/or mantle. Modelling of the viscoelastic deformation suggests that it constituted no more than ∼30 per cent of the short-term postseismic deformation. GPS observations that extend six months after the earthquake are well fit by a model in which most afterslip occurred at the same depth or directly downdip from the rupture zone and released energy equivalent to no more than ∼20 per cent of the coseismic moment. An independent seismological slip solution that features more highly concentrated coseismic slip than our own fits the GPS offsets well if its slip centroid is translated ∼50 km to the west to a position close to our slip centroid. The geodetic and seismologic slip solutions thus suggest bounds of 2–7 m for the peak slip along a region of the interface no larger than 30 × 30 km.

Seismic behaviour of a through-beam connection between concrete-filled steel tubular columns and reinforced concrete beams

Abstract This paper aims to investigate the seismic and post-seismic behaviour of a new and innovative type of through-beam connection between a concrete-filled steel tubular (CFST) column and a reinforced concrete (RC) beam. In this new connection system, the steel tube is completely or partially cut with the cut or openings being at the corresponding beam location, with the longitudinal steel reinforcing bars for the beam remaining continuous through the connection region. A strengthening ring beam is used to enlarge the connection zone in order to compensate for the possible decrease of the axial load carrying capacity as a result of the discontinuity of the steel tube that encases the column. Cyclic loading tests and subsequent axial compressive tests are reported on six beam–column specimens. Based on the experimental results, the hysteretic response, the skeletal and post-cyclic static load–deflection curves, the strain, ductility, strength and the stiffness degradation and energy dissipation are discussed. The favourable seismic performance of the connection is demonstrated in the tests. A finite element model is then developed and validated by a comparison with the experimental results. The relative effects of the reinforcement ratio of the frame beam and the ring beam, and the axial compressive force in the column on the seismic behaviour of the connection are elucidated through parametric studies.

A high-resolution, time-variable afterslip model for the 2010 Maule Mw = 8.8, Chile megathrust earthquake

The excellent spatial coverage of continuous GPS stations in the region affected by the Maule Mw = 8.8 2010 earthquake, combined with the proximity of the coast to the seismogenic zone, allows us to model megathrust afterslip on the plate interface with unprecedented detail. We invert post-seismic observations from continuous GPS sites to derive a time-variable model of the first 420 d of afterslip. We also invert co-seismic GPS displacements to create a new co-seismic slip model. The afterslip pattern appears to be transient and non-stationary, with the cumulative afterslip pattern being formed from afterslip pulses. Changes in static stress on the plate interface from the co- and post-seismic slip cannot solely explain the aftershock patterns, suggesting that another process – perhaps fluid related – is controlling the lower magnitude aftershocks. We use aftershock data to quantify the seismic coupling distribution during the post-seismic phase. Comparison of the post-seismic behaviour to interseismic locking suggests that highly locked regions do not necessarily behave as rate-weakening in the post-seismic period. By comparing the inter-seismic locking, co-seismic slip, afterslip, and aftershocks we attempt to classify the heterogeneous frictional behaviour of the plate interface.

Regression analysis for seismic slope instability based on a double phase viscoplastic sliding model of the rigid block

Semi-empirical models based on Newmark’s sliding block permit the estimation of expected co-seismic displacements in relation to one or more parameters which characterize the ground motion that theoretically caused them. Taking this into consideration, a regression analysis, based on a double-phase viscoplastic (DPV) model, was developed using 96 Italian ground motion accelerograms for a total of 1,448 combinations obtained for different parametric conditions of the indefinite slope model. Repeated stability analysis, performed by means of the DPV model, allows for the assessment of the seismic instability of a slope in relation to different reached behaviour levels, as well as seismically induced permanent displacements. At these behaviour levels, co-seismic increases and possible subsequent decreases of viscoplastic shear strengths are associated. This implies that the post-seismic persistent mobility (collapse) of the slope can be obtained from the computation. On the other hand, coherently with the increasing of shear resistances during fast sliding displacements in clay soils, the seismic-forced displacements result substantially lower than corresponding values obtained by means of the rigorous Newmark’s sliding block. In addition, in relation to some seismic ground motion parameters, regression and functional border and separation curves were obtained with the aim of providing an expeditious seismic slope stability evaluation in reference to the co-seismic and post-seismic behaviour of clayey slopes. Regarding this, the real behaviour of two historical landslide events is discussed in the light of the results of the regression analysis outlined in this work.

Co-Seismic and Post-Seismic Behavior of an Alternately Layered Sand-Clay Ground and Embankment System Accompanied by Soil Disturbance

ABSTRACT In the evaluation of the damage caused by earthquakes, particular attention has been paid until now to the unstable behaviour (the phenomenon of liquefaction) of sandy grounds during tremors. However, studies of post-seismic damage and damage to clayey grounds have also been reported. In this paper, the behaviour of an actual alternately layered sand-clay ground and embankment-coupled system before, during, and after an earthquake is investigated. The investigations were carried out by soil-water coupled finite element analysis (GEOASIA), which is capable of handling inertial forces and utilizes the SYS Cam-clay model as the elasto-plastic constitutive model of the soil skeleton. By accounting for the effects of the soil skeleton structure (structure, overconsolidation, and anisotropy) at work, the above constitutive equation is capable of expressing the reduction of shear modulus, etc. caused by disturbance in loose sands and naturally deposited clays. In the analysis, (1) the effect of the rigidity of the embankment and (2) the effect of the penetration depth of the sheet pile in a strengthened ground with retaining walls and sheet piles joined by tie rods were examined. The main results are as follows: (1) If the embankment is soft, liquefaction of the upper sandy soil layer during the earthquake causes the embankment itself to collapse. In contrast, if the embankment is relatively hard, the shape of the embankment remains stable. However, a decrease in shear rigidity occurs in the lower clayey soil layer due to degradation (disturbance) of the soil structure, resulting in increased settlement both during and after the earthquake. (2) In the case of grounds strengthened by the sheet pile method, if the tips of the sheet piles are located in a soft soil layer with a high degree of structure, large disturbance may occur at such locations during an earthquake and cause increased settlement during and after the tremors.

Afterslip and viscoelastic relaxation following the 1999M7.4 İzmit earthquake from GPS measurements

SUMMARY Intensive global positioning system (GPS) monitoring after the 1999 Izmit earthquake provides an opportunity to understand the postseismic behaviour of a strike-slip fault and the rheology below the brittle upper crust. Two data sets are available: displacements measured during the first 300 days after the Izmit earthquake and velocity measurements between 2003 and 2005. Using an inversion method and a forward modelling, respectively, we investigate two mechanisms: (1) afterslip on and below the coseismic rupture plane and (2) viscoelastic stress relaxation in the lower crust and upper mantle described by a Maxwell or a standard linear solid (SLS) rheology. The inversion results show that the first several months following the Izmit earthquake were dominated by afterslip at depths shallower than 30 km and the slip amount decayed with time; after that, apparent afterslip has a very different spatial distribution and is located much deeper. For viscoelastic relaxation, a model with an elastic upper crust and a Maxwell viscoelastic lower crust overlying a Maxwell mantle (E-M-M) fits the data measured in the first 300 days better in the far field than in the near field. However, the observed far-field, 300-day displacement and the long-term (2003–2005) displacement, which might be dominated by viscoelastic relaxation, cannot be described by a Maxwell rheological model with constant viscosity: the effective viscosity increases over time. Therefore, we have built a refined rheological model: an elastic upper crust and an SLS lower crust overlying a Maxwell viscoelastic mantle (E-SLS-M). Our best solution yields a viscosity for the lower crust of ∼2 × 1018 Pa s, a relaxation strength of 2/3 and a viscosity for the Maxwell mantle of 7 × 1019 Pa s. Finally, we explain the data using a composite model, consisting of the preferred E-SLS-M model and the afterslip model obtained from the residual displacement after correcting for viscoelastic relaxation. For the early time period, the residual displacements can be mainly explained by a shallow afterslip whose magnitude decays with time and whose spatial distribution is stable, whereas the residual displacements for the later time period require negligible afterslip. It indicates that the postseismic deformation in the later time period induced by a deep source can be almost entirely explained by the E-SLS-M model. The composite model can generally explain the data in the entire spatial and temporal space.

An aseismic slip pulse in northern Chile and along‐strike variations in seismogenic behavior

We use interferometric synthetic aperture radar, GPS, and seismic observations spanning 5 to 18 years to reveal a detailed kinematic picture of the spatiotemporal evolution of fault slip in a region corresponding to the 30 July 1995 Mw 8.1 subduction zone megathrust earthquake in northern Chile. In a single area, we document a complex mosaic of phenomena including large earthquakes, postseismic afterslip with a spatial distribution that appears to be tied to variations in coastal morphology, and a completely aseismic pulse that may have triggered a Mw 7.1 earthquake on 30 January 1998. In contrast to simple models of fault slip behavior, this spatial heterogeneity indicates that frictional parameters on the fault do not have a systematic transition with depth and also vary rapidly along strike. The low amount of afterslip from the Mw 8.1 earthquake relative to other similar events suggests that postseismic behavior may be modulated by the amount of sediment subducted.

Earthquake swarms in the Vogtland/Western Bohemia region: Spatial distribution and magnitude–frequency distribution as an indication of the genesis of swarms?

Abstract Vogtland/Western Bohemia is a small intraplate earthquake region in Central Europe covering about 6750 km 2 . A brief discussion of the geological background is followed by analysis of the recurrence of earthquakes affecting the area. Earthquakes are concentrated in a few zones. The location of earthquakes in two of them, the Markneukirchen/Plesna and the Nový Kostel zone, is discussed in detail. A special feature of the Vogtland/Western Bohemia seismicity is the clustering of earthquakes in time and space. The location of earthquakes within the cluster on December 1994 is shown. The effect of clustering is investigated in detail. A total of 73 clusters are characterized by their magnitude–frequency distribution, especially by the slope ( b -value) of this distribution. In the plot of b -values versus the maximum magnitude of the cluster, the b -values are attracted by a few lines. Thus, clustering is regarded as a dynamic self-organized process. An existing model is improved by combining the existence of fluids and their post-seismic behaviour with the results of the earthquake clustering analysis.

Continuous borehole strain in the San Andreas fault zone before, during, and after the 28 June 1992, MW 7.3 Landers, California, earthquake

Abstract High-precision strain was observed with a borehole dilational strainmeter in the Devil's Punchbowl during the 11:58 UT 28 June 1992 MW 7.3 Landers earthquake and the large Big Bear aftershock (MW 6.3). The strainmeter is installed at a depth of 176 m in the fault zone approximately midway between the surface traces of the San Andreas and Punchbowl faults and is about 100 km from the 85-km-long Landers rupture. We have questioned whether unusual amplified strains indicating precursive slip or high fault compliance occurred on the faults ruptured by the Landers earthquake, or in the San Andreas fault zone before and during the earthquake, whether static offsets for both the Landers and Big Bear earthquakes agree with expectation from geodetic and seismologic models of the ruptures and with observations from a nearby two-color geodimeter network, and whether postseismic behavior indicated continued slip on the Landers rupture or local triggered slip on the San Andreas. We show that the strain observed during the earthquake at this instrument shows no apparent amplification effects. There are no indications of precursive strain in these strain data due to either local slip on the San Andreas or precursive slip on the eventual Landers rupture. The observations are generally consistent with models of the earthquake in which fault geometry and slip have the same form as that determined by either inversion of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake. Finally, there are some indications of minor postseismic behavior, particularly during the month following the earthquake.

A model of post-seismic recovery induced by a deep-focus earthquake

Post-seismic behavior induced by a deep-focus earthquake in subduction zone is closely investigated numerically on realistic upper-mantle models, taking into account the pressure and temperature conditions. For this purpose, the post-seismic stress changes expected from the deep event are calculated by a two-dimensional finite element method (2D FEM), assuming newtonian and power-law creep behaviors as justified by high P– T experiments. Several cases are investigated in detail for various fault depths and thermal structures. Two cases are considered for the location of the fault: one when an earthquake occurs within oceanic crust overlying a subducting plate, and the other when an earthquake is located within the plate. We also examine the case when the temperature distribution within the plate is lower by about 200°C on average than that in the upper mantle at the same depth. In addition, the post-seismic behavior due to a deep-focus event in a petrological thermal model of a mantle wedge is investigated. The results show that the post-seismic behavior associated with the deep event occurring within the oceanic crust indicates a pattern of relaxation of the coseismic stress changes. Lower temperatures inside the plate would suppress the post-seismic displacements and stress changes effectively. The petrological model also has some influence on the post-seismic flow pattern in the mantle wedge. We also study a possible correlation between deep-focus earthquakes and a shallow large event which has been detected in the northwestern Pacific region. However, effective mechanisms to explain the phenomenon could not be identified from the viewpoint of post-seismic behavior caused by power-law creep.