Renata Dmowska

International Geophysics Series

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Interaction of the San Andreas Fault Creeping Segment with Adjacent great rupture zones and earthquake recurrence at Parkfield

Three-dimensional finite element calculations are employed to study interactions in space and time between the creeping segment of the San Andreas fault in central California and the adjacent currently locked zones of the 1857 and 1906 great earthquakes. Vertically, the model consists of an elastic upper crust over a Maxwell viscoelastic region, representing the entire lower crust or a narrower horizontal detachment layer, and a stiffer and more viscous upper mantle. The crust has a single vertical fault extending to the top of the mantle at 25 km depth. In zones along strike corresponding to the 1857 and 1906 events, the top 12.5 km of the fault is locked against slip, except in great earthquakes. Below the locked zones and everywhere along the creeping region between them, the fault is freely slipping. The model parameters are compatible with seismological and geological observations, and with a ratio of Maxwell relaxation time to the relaxing layer thickness in the range 1 to 2 yr/km, as established by Li and Rice (1987) and Fares and Rice (1988) based on fits to geodetic data along the San Andreas fault. An imposed constant far field shear motion and periodic 1857- and 1906 - type earthquakes generate slip rates along the creeping fault segment that evolve in time throughout the entire earthquake cycle. Shortly after an adjacent great earthquake, slip rates in the creeping zone are higher than the far field velocity, while later in the cycle they are lower. Hence, time dependency should be accounted for when measurements of fault slip are used to estimate the plate motion. If Parkfield earthquakes are a response to a time dependent loading of the type simulated here, their recurrence interval would tend to lengthen with time since the 1857 event. Thus, the hypothesis of characteristic periodic earthquakes at Parkfield may not provide the best estimate of the occurrence time of the next event. Using, for example, the statistics of past events and assuming that Parkfield earthquakes are a response to a slip deficit near Middle Mountain, and that the elastic crustal layer is 17.5 km thick, we find that the next event is predicted for about 1992 ± 9 years if the lower crust is a 7.5 km thick layer having a material relaxation time of 15 years, and 1995 ± 11 years if the 7.5 km thick lower crust is characterized by a relaxation time of 7.5 years. These values may be compared to the 1988 ± 7 years estimate based on periodicity in time. The modeling results also indicate that the interaction between the 1857 and 1906 rupture zones is small.

Cyclic stressing and seismicity at strongly coupled subduction zones

We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long-term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench-normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate starts in the middle or late part of the cycle, depending on position. This result suggests that the negligible or small contraction measured on the Shumagin Islands, Alaska, during 1980 to 1991, may not invalidate an interpretation of that region as being a moderately coupled subduction zone. In contrast, mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate, even when the Maxwell time is an order of magnitude less than the recurrence interval.

Seismicity and deformation at convergent margins due to heterogeneous coupling

Studies of the mechanics of subduction as inferred from earthquake cycle observations suggest that the distribution and style of seismicity in the seafloor, between the trench and the outer rise, and in the slab at intermediate depth, can in some cases serve to identify asperity locations along the thrust interface [Dmowska and Lovison, 1992]. Such asperities, identified from seismic wavefield modeling, are the zones of highest seismic moment release in large underthrusting events. To the extent that asperity locations are relatively stationary from one event to the next, their locations provide the zones of highest expected moment release in future large earthquakes, and rupture often nucleates at the border of an asperity. The region of the thrust interface outside such asperities is, apparently, less well coupled and releases moment throughout the great earthquake cycle in some combination of aseismic creep and moderate seismicity. Thus it is reasonable that stress and deformation rates associated with the earthquake cycle should be most pronounced near asperities, and that this should have seismic and geodetic consequences. Three-dimensional finite element modeling is used here to understand such stress and deformation patterns and their variation in time, in relation to heterogeneity of coupling along thrust interfaces. The stress field helps to explain the observed clustering of seafloor seismicity along the strike of the convergent margin. In cases of convergence at approximately normal incidence, like for the region of the Valparaiso, Chile, 1985 thrust event, the modeling is consistent with the observation that areas of large earthquakes in the seafloor toward the outer rise and in the slab tend to lie within corridors through thrust zone asperities, running perpendicular to the line of the trench. We seek to learn if such model stress fields are consistent with observations, for the strongly oblique subduction margin of the Rat Islands, western Aleutians, 1965 event, that active areas of the outer rise and slab at intermediate depth are offset along strike from asperity locations. Modeling results here for the stress in the seafloor raise the possibility that to explain this offset, the asperity zones along the thrust interface may have to be strung out along the direction of oblique slip, perhaps reflecting the contact path of subducting seamounts or geometric irregularities along the interface. Shear stress patterns created in the upper plate, when there is oblique subduction, suggest that favorable areas for back-arc strike slip activity following underthrusting, as in the Adak Island, central Aleutians, region of the 1986 Andreanof Island earthquake [Ekstrom and Engdahl, 1989], will also be shifted along strike from asperity locations. Our analyses show how deformation patterns on the earths surface above asperities differ from patterns above nonasperities, and hence provide tools to identify inhomogeneous coupling from geodetic observations. We discuss possible bathymetric, topographic, and structural signals of strength of coupling, and of asperities, particularly noting that the density and extent from the trench of seafloor normal faults correlates with seismically inferred zones of strongest coupling in the central Aleutians.

Dynamic Slip Transfer from the Denali to Totschunda Faults, Alaska: Testing Theory for Fault Branching

We analyze the observed dynamic slip transfer from the Denali to Totschunda faults during the Mw 7.9 3 November 2002 Denali fault earthquake, Alaska. This study adopts the theory and methodology of Poliakov et al. (2002) and Kame et al. (2003), in which it was shown that the propensity of the rupture path to follow a fault branch is determined by the preexisting stress state, branch angle, and incoming rupture velocity at the branch location. Here we check that theory on the DenaliTotschunda rupture process using 2D numerical simulations of processes in the vicinity of the branch junction. The maximum compression direction with respect to the strike of the Denali fault near the junction has been estimated to range from approximately 73 to 80. We use the values of 70 and 80 in our numerical simulations. The rupture velocity at branching is not well constrained but has been estimated to average about 0.8 cs throughout the event. We use 0.6 cs, 0.8 cs, 0.9 cs, and even 1.4 cs as parameters in our simulations. We simulate slip transfer by a 2D elastodynamic boundary integral equation model of mode II slip-weakening rupture with self-chosen path along the branched fault system. All our simulations except for 70 and 0.9 cs predict that the rupture path branches off along the Totschunda fault without continuation along the Denali fault. In that exceptional case there is also continuation of rupture along the Denali fault at a speed slower than that along the Totschunda fault and with smaller slip.

Influence of asperities along subduction interfaces on the stressing and seismicity of adjacent areas

Abstract Dmowska, R. and Lovison, L.C., 1992. Influence of asperities along subduction interfaces on the stressing and seismicity of adjacent areas. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics , 211: 23–43. We have investigated the influence of large-scale fault inhomogeneities in large subduction earthquakes on the style of deformation and seismic behavior of the incoming oceanic plate and slab at intermediate depths during the earthquake cycle. The zones of the large subduction events of Rat Islands 1965, Alaska 1964 and Valparaiso 1985 have been searched for earthquakes with m b ⩾ 5.0, if available, and for time periods as long as possible. It has been found that in general the seismicity in the incoming oceanic plate clusters in front of asperities ( areas of highest seismic moment release and strongest locking) and is positioned relative to them in the direction of plate motion. It is usually lacking in areas adjacent to non-asperities, that is to zones that slip during the main event but with appreciably smaller seismic moment release, and possibly slip seismically/aseismically during the whole cycle. Similar behavior occurs in the downgoing slab at intermediate depths, where seismicity during the cycle clusters (but less strongly than in the oceanic crust) next to asperities and down-dip from them. We infer that the locking of asperities causes higher stresses associated with the earthquake cycle itself to occur in areas adjacent to asperities, both up-dip and down-dip from them along the direction of plate motion, and that such stressing is much less pronounced in the areas adjacent to non-asperities. This opens the possibility of identifying the areas of highest seismic moment release in future subduction earthquakes, and carries implications for where the highest deformation and, possibly, precursory phenomena and/or nucleation of a future event might occur.

Modeling earthquake cycles in the Shumagin subduction segment, Alaska, with seismic and geodetic constraints

Deformation associated with the earthquake cycle in the Shumagin Islands segment of the Alaska-Aleutian subduction zone is analyzed with the use of a two-dimensional finite element model. The model consists of an oceanic plate dipping under an upper plate, both of which respond elastically to stress fluctuations in the earthquake cycle, and these are underlain by asthenospheric mantle and mantle wedge regions which respond viscoelastically. It is tailored to the geometry of the Shumagin Islands region, by using seismicity to define the position of the interplate interface and (partially) coupled region along it. The model is preconditioned by forcing this interface to undergo periodically repeated slips up to (and including) the time of the May 31, 1917, event (Ms = 7.4) in that region, with each chosen to be consistent with the moment and estimated rupture area of that event. We investigated the dependence of model results for geodetic signals on the strength of seismic coupling between the plates and viscoelastic relaxation of deviatoric stresses in the mantle, including in the mantle wedge close to the plate junction and along the aseismic downdip continuation of the thrust interface. In models with significant relaxation in the wedge or downdip thrust zone, results show that as the intraseismic stage matures, there is a region of diminished compressional strain rates, and even of locally extensional rates, on the Earths surface above the downdip end of the seismically coupled zone. Based on the seismic estimates of the location of the coupled zone, this region is in the area of the Shumagin Islands. We find that if approximately 20% of the convergence takes place seismically (compatible with the previous seismic history), and if an extensive region of relaxed deviatoric stress is assumed to be present in the wedge and/or along the downdip interface, then deformations predicted by the model can be made consistent with the measured strain data from the Shumagin Islands geodetic network [Lisowski et al., 1988; Larson and Lisowski, 1994], as well as uplift and tilt data [Savage and Plafker, 1991; Beavan, 1992]. Our model simulations here suggest that the Shumagin segment is capable of large earthquakes. The hypothesis of totally aseismic subduction is not similarly consistent with all geodetic constraints.

Upper plate stressing and seismicity in the subduction earthquake cycle

We investigate upper plate stressing during the earthquake cycle in a subduction segment, using three-dimensional (3-D) elastic models to address the effects of strongly heterogeneous coupling along strike of the interplate interface. We show how heterogeneity controls the locations and mechanisms of seismicity in the upper plate. Oblique subduction segments, two from the Aleutians (Andreanof Islands 1986 and Rat Islands 1965) and one from Indonesia (Biak 1996) are studied. All examples of upper plate seismicity from the Aleutians represent events occurring toward the beginning of a new cycle, while in Biak, Indonesia, the examined events occur both toward the end of one cycle and the beginning of the next. In the majority of cases studied, the location and mode of the upper plate seismicity are consistent with space- and time-dependent stressing as predicted by modeling. This confirms earlier observations that seismicity in the vicinity of large/great subduction earthquakes (toward the outer rise, at intermediate depth, and now in the upper plate) depends, in an interpretable manner, on the stage in the earthquake cycle as well as on distribution of coupling along the interplate interface.

Intermediate-term seismic precursors for some coupled subduction zones

The paper discusses model results and then reviews observational data concerning some aspects of the mechanics of mature seismic gaps in coupled subduction zones. The concern is with space-and time-varying stresses, as signalled by the presence and mechanisms of earthquakes in the outer-rise zones adjacent to main thrust areas of large subduction events, and down-dip from such areas, in the downgoing slab. Observations are shown to be consistent with the expectation that in mature seismic gaps, as a result of interplate boundary locking in presence of sustained gravitational driving forces, at least the deeper portions of the ocean plate in the outer-rise zones are under increased compression, and the downgoing slab is under increased tension. The observational data cover two cases of closed seismic gaps, namely the region of the Chilean Valparaiso earthquake of March 3, 1985, and the earthquake of October 4, 1983. Four other cases concern still to-be-closed gaps in northern Chile and along the coast of Guatemala, and also the Kurile Islands Trench gap and the northern New Hebrides gap. It is concluded that the intermediate-term precursor, consisting of a combination of compressional outer-rise earthquake(s) and tensional intermediate-depth, intra-plate events in the downgoing slab, which mechanically signals the latter part of the earthquake cycle, could be useful in evaluating the maturity, and hence great earthquake potential of a seismic gap.

Finite Element Modeling of Branched Ruptures Including Off‐Fault Plasticity

Abstract Fault intersections are a geometric complexity that frequently occurs in nature. Here we focus on earthquake rupture behavior when a continuous planar main fault has a second fault branching off of it. We use the finite element (FE) method to examine which faults are activated and how the surrounding material responds for both elastic and elastic–plastic off‐fault descriptions. Compared to an elastic model, a noncohesive elastic–plastic material, intended to account for zones of damaged rock bordering maturely slipped faults, will inhibit rupture on compressional side branches and promote rupture of extensional side branches. Activation of extensional side branches can be delayed and is triggered by continued rupture propagation on the main fault. We examine the deformation near the branching junction and find that fault opening is common for elastic materials, especially for compressional side branches. An elastic–plastic material is more realistic because elevated stresses around the propagating rupture tip and at the branching junction should bring the surrounding material to failure. With an elastic–plastic material model, fault opening is inhibited for a range of realistic material parameters. For large cohesive strengths, opening can occur, but with material softening, a real feature of plastically deforming rocks, opening can be prevented. We also discuss algorithmic artifacts that may arise due to the presence of such a triple junction. When opening does not occur, the behavior at the triple junction is simplified and standard contact routines in FE programs are able to properly represent the physical situation.