S U M M A R YIn order to understand the causal relation between postglacial rebound and earthquakes, a simple disc load model is used to: (1) calculate stresses induced in the lithosphere and mantle by glacial loading, melting and postglacial rebound; and (2) evaluate the effect of glacial loading/rebound on the failure potential for earthquakes in the upper crust. The dependence of the failure potential and the actual mode of failure on the coefficient of friction, the ambient tectonic stress magnitude/direction, the stress due to the overlying rocks, and lithospheric thickness are investigated. Prominent features of this paper are the inclusion of: (1) a viscoelastic mantle and thus the migration of stress; and (2) the ambient tectonic stress and overburden stress contributions in the calculation of the total stress field.It is assumed that, throughout the Earth, there are optimally oriented pre-existing virtual faults that are initially close to but not at failure; thus, a time-dependent quantity called dFSM (related to the Coulomb-Mohr failure criterion) can be defined such that a negative value of dFSM would advocate faulting or earthquake activities whereas a positive value of dFSM would promote stability.The results indicate that, under all combinations of tectonic stress magnitude and overburden stress, crustal loading promotes fault stability directly underneath the load.Upon the removal of the load, thrust faulting is predicted within the ice margin if the horizontal stress (S,) induced by the overburden is greater than or equal to the vertical component (S,) of the overburden stress (121, where (=SJS,). Under this condition, theory predicts that faulting or earthquake activity should have reached a maximum immediately after deglaciation.If the horizontal stress induced by the overburden is less than the vertical component of the overburden stress (1 < l), then theory predicts fault stability within the ice margin.The theory predicts fault instability both north and south of the ice margin. The mode of failure, however, is completely determined by the value of i.The trade-off between the tectonic stress magnitude and the overburden stress parameter (1) is also investigated. It is shown that a larger tectonic stress magnitude can be used to compensate a smaller value of 1.The results of this analysis show that variations in the coefficient of friction, lithospheric thickness and a ductile zone below the upper crust do not significantly affect the above conclusions.
S U M M A R YIn order to understand the causal relation between postglacial rebound and earthquakes, a realistic ice and water load model is used to (1) calculate stresses induced in the lithosphere and mantle by glacial loading, melting and postglacial rebound and (2) evaluate the effect of glacial loading/rebound on the failure potential for earthquakes in the upper crust. The dependence of both the failure potential and the actual mode of failure on the ambient tectonic stress magnitude, the overburden stress, and lithospheric properties are investigated. Prominent features of this analysis are the inclusion of (1) a viscoelastic mantle and thus the migration of stress, and (2) the ambient tectonic stress and overburden stress contributions in the calculation of the total stress field.The spatio-temporal calculations, by a finite-element technique, of upper-crustal stresses and the failure potential for earthquakes indicate that fault stability is invariably enhanced directly beneath the load. For the case where stresses induced by the overburden are such that the horizontal component (S,) is greater than or equal to the vertical component (S,) (i21, where i=S,/S,), the model predicts the onset of thrust faulting and maximum earthquake activities soon after deglaciation is complete (when rebound rates are at a maximum). Observational data support this prediction. Since that time, rebound stresses have been decreasing in magnitude, but they continue to act as a trigger mechanism for optimally oriented pre-existing faults that are otherwise on the verge of failure. If one limits the existence of such faults to lie within the preweakened zones of eastern Canada, then the spatial distribution of current earthquakes can also be explained.Perturbations to the magnitude of the tectonic stress components or lithospheric properties do not affect, to any significant extent, the above conclusions.
Diversified phenomena are contributing to our knowledge of various aspects of the stress field and the origin of the crustal stresses in eastern Canada. Both in situ stress measurements (to 2100 m) and earthquake fault plane solutions in the upper crust (• 5-20 km) indicate that the maximum horizontal stress is greater than the vertical. Absence of earthquakes in the lower crust (>20 km) implies a comparatively lower deviatoric horizontal stress in this layer. Stress directions as determined from both in situ measurements and earthquake fault plane solutions indicate an overprint confined to the ENE octant, which correlates with the direction predicted by modeling of plate tectonic stresses. Thus spreading (Mid-Atlantic) ridge stress is considered to be one of the important contributors to the stress field in eastern Canada. Viscoelastic relaxation of this stress is considered to enhance the deviatoric stress level in the upper crust, while simultaneously reducing that in the lower crust. The crust acts as a stress guide within which stresses in the upper crust that are lost through brittle failure (i.e., earthquake stress drop) can be replenished from the lower crust; moreover, the directionality predicted by modeling of spreading (Mid-Atlantic) ridge stress is maintained. Incomplete postglacial rebound also contributes to deviatoric compression. Other possible contributors to the upper crustal stress field are basal drag and membrane stress, but the relative contributions from these sources are not properly understood.
Although Arctic earthquakes have been recorded since 1908, detailed study of them has been hampered due to the lack of seismograph stations and the infrequent occurrence of large earthquakes north of the Arctic Circle. Detailed analysis of Arctic earthquakes began during the International Geophysical Year (IGY, 1957–1958), and subsequent studies have been facilitated by the development of the World-Wide Standardized Seismograph Network (WWSSN) starting in 1963. Many authors have published summaries of Arctic seismicity. The pre-IGY state of knowledge is summarized by Hodgson and others (1965), and epicentral coordinates and magnitude estimates of pre-WWSSN seismicity are given in Gutenberg and Richter (1954), Linden (1961), Hodgson and others (1965), and Rothe (1969). Overview summaries of the distribution and magnitude of Arctic seismicity are presented by Sykes (1965) and Wetmiller and Forsyth (1978). Numerous maps of Arctic seismicity have been published (e.g., Veis-Ksenofontova, 1962; Sykes, 1965; Barazangi and Dorman, 1970; Tarr, 1970; Wetmiller, 1978; Avetisov and Sokolova, 1980). Additional details about Arctic seismicity are given in international bulletins, national seismicity summaries, and annual reports. In this chapter we summarize the development of seismograph stations in the Arctic, the distribution of seismicity in the Arctic, focal mechanisms that have been determined for the Arctic seismic zone, including northeastern Siberia and Baffin Bay, and the implications of the seismic data for plate tectonic models of the region. In addition, we summarize inferences on crustal structure of the Arctic region based on the propagation characteristics of earthquake waves.
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