Aftershocks due to property variations in the fault zone: A mechanical model (original) (raw)
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International Journal of Earth Sciences, 2016
largest aftershocks of the mainshocks studied tend to occur inside the stress-increased lobes that were also stressed by the background earthquakes and not to occur inside the stress-increased lobes that fall into the stress shadow of the background earthquakes. We suggest that the stress shadows of the previous mainshocks may persist in the crust for decades to suppress aftershock distribution of the current mainshocks. Considering active researches about use of the Coulomb stress change maps as a practical tool to forecast spatial distribution of the upcoming aftershocks for earthquake risk mitigation purposes in near-real time, it is further suggested that the background mainshocks along the neighbouring faults should be taken into account in producing the stress change maps for commenting on aftershock occurrences.
Stress triggering of normal aftershocks due to strike slip earthquakes in compressive regime
Journal of Asian Earth Sciences, 2008
A few cases of occurrence of normal aftershocks after strike slip earthquakes in compressive regime have been reported in the literature. Occurrence of such aftershocks is intriguing as they occurred despite the apparent stabilizing influence of compressive plate tectonic stresses on the normal faults. To investigate the occurrence processes of such earthquakes, we calculate change in static stress on optimally oriented normal and reverse faults in the dilational and compressional step over zones, respectively, due to slip on a vertical strike slip fault under compressive regime. We find that change in static stress is much more pronounced on normal faults as compared to that on reverse faults, for all values of fault friction. Change in static stress on reverse fault is marginally positive only when the fault friction is low, whereas for normal faults it is positive for all values of fault friction, and is maximum for high fault friction. We suggest that strike slip faulting in compressive regime creates a localized tensile environment in the dilational step over zone, which causes normal faulting in that region. The aftershocks on such normal faults are considered to have occurred as an almost instantaneous response of stress transfer due to strike slip motion.
A physical model for aftershocks triggered by dislocation on a rectangular fault
Arxiv preprint physics/0505033, 2005
We find the static displacement, stress, strain and the modified Coulomb failure stress produced in an elastic medium by a finite size rectangular fault after its dislocation with uniform stress drop but a non uniform dislocation on the source. The time-dependent rate of triggered earthquakes is estimated by a rate-state model applied to a uniformly distributed population of faults whose equilibrium is perturbated by a stress change caused only by the first dislocation. The rate of triggered events in our simulations is exponentially proportional to the Coulomb stress change, but the time at which the maximum rate begins to decrease is variable from fractions of hour for positive stress changes of the order of some MP a, up to more than a year for smaller stress changes. As a consequence, the final number of triggered events is proportional to the Coulomb stress change. The model predicts that the total number of events triggered on a plane containing the fault is proportional to the 2/3 power of the seismic moment. Indeed, the total number of aftershocks produced on the fault plane scales in magnitude as 10 M. Including the negative contribution of the stress drop inside the source, we observe that the number of events inhibited on the fault is, at long term, nearly identical to the number of those induced outside, representing a sort of conservative natural rule. Considering its behaviour in time, our model doesn't completely match the popular Omori law; in fact it has been shown that the seismicity induced closely to the fault edges is intense but of short duration, while that expected at large distances (up to some tens times the fault dimensions) exhibits a much slower decay.
Effects induced by an earthquake on its fault plane: a boundary element study
2000
Mechanical e¡ects left by a model earthquake on its fault plane, in the post-seismic phase, are investigated employing the`displacement discontinuity method'. Simple crack models, characterized by the release of a constant, unidirectional shear traction are investigated ¢rst. Both slip componentsöparallel and normal to the traction directionöare found to be non-vanishing and to depend on fault depth, dip, aspect ratio and fault plane geometry. The rake of the slip vector is similarly found to depend on depth and dip. The fault plane is found to su¡er some small rotation and bending, which may be responsible for the indentation of a transform tectonic margin, particularly if cumulative e¡ects are considered. Very signi¢cant normal stress components are left over the shallow portion of the fault surface after an earthquake: these are tensile for thrust faults, compressive for normal faults and are typically comparable in size to the stress drop. These normal stresses can easily be computed for more realistic seismic source models, in which a variable slip is assigned; normal stresses are induced in these cases too, and positive shear stresses may even be induced on the fault plane in regions of high slip gradient. Several observations can be explained from the present model: low-dip thrust faults and high-dip normal faults are found to be facilitated, according to the Coulomb failure criterion, in repetitive earthquake cycles; the shape of dip-slip faults near the surface is predicted to be upward-concave; and the shallower aftershock activity generally found in the hanging block of a thrust event can be explained bỳ unclamping' mechanisms.
We present a new three-dimensional inventory of the southern San Francisco Bay area faults and use it to calculate stress applied principally by the 1989 M ϭ 7.1 Loma Prieta earthquake and to compare fault seismicity rates before and after 1989. The major high-angle right-lateral faults exhibit a different response to the stress change than do minor oblique (right-lateral/thrust) faults. Seismicity on oblique-slip faults in the southern Santa Clara Valley thrust belt increased where the faults were unclamped. The strong dependence of seismicity change on normal stress change implies a high coefficient of static friction. In contrast, we observe that faults with significant offset (Ͼ50-100 km) behave differently; microseismicity on the Hayward fault diminished where right-lateral shear stress was reduced and where it was unclamped by the Loma Prieta earthquake. We observe a similar response on the San Andreas fault zone in southern California after the Landers earthquake sequence. Additionally, the offshore San Gregorio fault shows a seismicity rate increase where right-lateral/oblique shear stress was increased by the Loma Prieta earthquake despite also being clamped by it. These responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones. We can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup; coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes, whereas in more limited offset faults, fluids could rapidly escape. The difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes, since many of these events occur both inside and outside major fault zones.
Research on static earthquake triggering has been carried out widely in the world, and achieved remarkable results. But it is still unclear whether this model is effective to all large earthquakes. In this paper, we investigated the Coulomb stress changes of 3 megathrust earthquakes along subduction zones (the 2011 Mw9.1 Tohoku earthquake, the 2010 Mw8.8 Chile earthquake, and the 2004 Mw9.0 Sumatra-Andaman earthquake) to test the triggering effects by examining the correlation between Coulomb stress increases and spatial distribution of the following aftershocks. The calculated results suggest that there is no obvious evidence that the Coulomb stress changes caused by the 3 megathrust earthquakes promoted the occurrence of the aftershocks. There are only 47% of the encouraged aftershocks following the Tohoku earthquake. And there are 47.6% and 49.8% for the Sumatra-Andaman and the Chile earthquake, respectively. We also calculated the Coulomb stress changes with different focal models and parameters. It is still less than 60% of the promoted aftershocks in the optimal case. However, the static triggering model is good for the Wenchuan earthquake and the Chi-Chi earthquake which have enhanced more than 85% of the subsequent aftershocks. This model may be not reasonable for large subduction earthquakes. Therefore, other model should be introduced in studying earthquake triggering in subduction zone and further studies will be performed.
Preseismic rupture progression and great earthquake instabilities at plate boundaries
Journal of Geophysical Research: Solid Earth, 1983
We present a procedure for modeling the initially quasi-static upward progression of a zone of slip from some depth in the lithosphere toward the earth's surface, along a transform plate margin, culminating in a great crustal earthquake. Stress transmission in the lithosphere is analyzed with a generalized Elsasser model, in which elastic lithospheric plates undergo plane stress deformation and are coupled by an elementary foundation model to a Maxwellian viscoelastic asthenosphere. Upward progression of rupture over a finite length of plate boundary, corresponding to a seismic gap along strike, is analyzed by a method based on the 'line-spring' concept, whereby a two-dimensional antiplane analysis of the upward progression provides the relation between lithospheric thickness-averaged stress and slip used as a boundary condition in the generalized Elsasser plate model. The formulation results in a nonlinear integral equation for the rupture progression as a function of time and distance along strike. A simpler approximate single degree of freedom analysis procedure is described and shown to lead to instability results that can be formulated in terms of the slipsoftening slope at the boundary falling below the elastic unloading stiffness of the surroundings. The results also indicate a delay of ultimate (seismic) instability due to the stiffer short versus long time asthenospheric response and predict a final period of self-driven creep toward instability. The procedures for prediction of rupture progression and instability are illustrated in detail for an elastic-brittle crack model of slip zone advance, and parameters of the model are chosen consistently with great earthquake slips and stress drops. For example, an effective crack fracture energy of the order 4 x 10 6 J/m 2 at the peak, 7 to 10 km below surface, of a Gaussian bell-shaped distribution of fracture energy with depth, with variance of the order 5 km, simulating strength build-up in a seismogenic layer, leads to prediction of nominal seismic stress drops of 30 to 60 bars and slips of 2 to 5 m in great strike slip earthquake ruptures breaking 100 to 400 km along strike. Precursory surface straining in the self-driven stage is predicted to proceed at a distinctly higher rate over time intervals beginning 3 to 10 months before such an earthquake, this interval being greater for longer distances along strike over
On the origin of diverse aftershock mechanisms following the 1989 Loma Prieta earthquake
Geophysical Journal International, 1997
We test the hypothesis that the origin of the diverse suite of aftershock mechanisms following the 1989 M 7.1 Loma Prieta, California, earthquake is related to the postmain-shock static stress field. We use a 3-D boundary-element algorithm to calculate static stresses, combined with a Coulomb failure criterion to calculate conjugate failure planes at aftershock locations. The post-main-shock static stress field is taken as the sum of a pre-existing stress field and changes in stress due to the heterogeneous slip across the Loma Prieta rupture plane. The background stress field is assumed to be either a simple shear parallel to the regional trend of the San Andreas fault or approximately fault-normal compression. A suite of synthetic aftershock mechanisms from the conjugate failure planes is generated and quantitatively compared (allowing for uncertainties in both mechanism parameters and earthquake locations) to wellconstrained mechanisms reported in the US Geological Survey Northern California Seismic Network catalogue. We also compare calculated rakes with those observed by resolving the calculated stress tensor onto observed focal mechanism nodal planes, assuming either plane to be a likely rupture plane.
Earth, Planets and Space, 2013
Coulomb stress triggering is examined using well-determined aftershock focal mechanisms and source models of the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake. We tested several slip distributions obtained by inverting onshore GPS-derived coseismic displacements under different a priori constraints on the initial fault parameters. The aftershock focal mechanisms are most consistent with the Coulomb stress change calculated for a slip distribution having a center of slip close to the trench. This demonstrates the capability of the Coulomb stress change to help constrain the slip distribution that is otherwise difficult to determine. Coulomb stress changes for normal-fault aftershocks near the Japan Trench are found to be strongly dependent on the slip on the shallow portion of the fault. This fact suggests the possibility that the slip on the shallow portion of the fault can be better constrained by combining information of the Coulomb stress change with other available data. The case of normal-fault aftershocks near some trench segment which are calculated to be negatively stressed shows such an example, suggesting that the actual slip on the shallow portion of the fault is larger than that inverted from GPS-derived coseismic displacements.
Coulomb pre-stress and fault bends: ignored yet vital factors for earthquake triggering
2018
Successive locations of individual large earthquakes (Mw>5.5) over years to centuries can be difficult to explain with simple Coulomb stress transfer (CST), because seismicity can miss out nearest-neighbour along-strike faults where coseismic CST increases are greatest. We show that “Coulomb pre-stress” may explain this, because magnitudes are >±50 bars if interseismic loading and local stress amplification at fault bends are included, so coseismic CST, in the range of ±2 bars, will rarely overwhelm the Coulomb pre-stress. To illustrate this, we calculate the Coulomb pre-stress prior to 34 earthquakes from 1349-2016 A.D. in central Italy and use this to discuss the location of subsequent earthquakes. We show that earthquakes tend to occur where the cumulative coseismic and interseismic CST is positive. Ruptures propagate both across faults that are positively stressed, and in a few examples, from positions where highly stressed patches associated with along-strike fault bends ...