Evolution of Stress Fields and Faulting in Seismic Zones

Abstract

Measurements indicate that stress magnitudes in the crust are normally limited by the frictional equilibrium on pre-existing, optimally oriented faults. Fault zones where these limitations are frequently reached are referred to as seismic zones. Fault zones in the crust concentrate stresses because their material properties are different from those of the host rock. Most fault zones are spatially relatively stable structures, however the associated seismicity in these zones is quite variable in space and time. Here we propose that this variability is attributable to stress-concentration zones that migrate and expand through the fault zone. We suggest that following a large earthquake and the associated stress relaxation, shear stresses of a magnitude sufficient to produce earthquakes occur only in those small parts of the seismic zone that, because of material properties and boundary conditions, encourage concentration of shear stress. During the earthquake cycle, the conditions for seismogenic fault slip migrate from these stress-concentration regions throughout the entire seismic zone. Thus, while the stress-concentration regions continue to produce small slips and small earthquakes throughout the seismic cycle, the conditions for slip and earthquakes are gradually reached in larger parts of, and eventually the whole, seismogenic layer of the seismic zone. Prior to the propagation of an earthquake fracture that gives rise to a large earthquake, the stress conditions in the zone along the whole potential rupture plane must be essentially similar. This follows because if they were not, then, on entering crustal parts where the state of stress was unfavourable to this type of faulting, the fault propagation would be arrested. The proposed necessary homogenisation of the stress field in a seismic zone as a precursor to large earthquakes implies that by monitoring the state of stress in a seismic zone, its large earthquakes may possibly be forecasted. We test the model on data from Iceland and demonstrate that it broadly explains the historical, as well as the current, patterns of seismogenic faulting in the South Iceland Seismic Zone.

Fault interaction and stresses along broad oceanic transform zone: Tjörnes Fracture Zone, north Iceland

“…Similar results were obtained by analytical calculation [Pollard and Aydin, 1984] and finite elements modeling . Gudmundsson and Homberg [1999] demonstrated that shear stress increases as plate separation continues. When the shear stress reaches the strength of the crust, faults develop and then relax the shear stress.…”

“…Large efforts have been done to understand the necessary conditions for development of transform faults. Several mechanisms has been advanced involving old continental weakness zones [ Wilson , 1965], upper mantle negative thermal anomalies [ Bonatti , 1996] or stress concentration [ Pollard and Aydin , 1984; Gudmundsson and Homberg , 1999].…”

Stress changes induced at neighbouring faults by the June 2000 earthquakes, South Iceland Seismic Zone

“…This morphological and structural difference suggests that the Skard Fault developed earlier, or was much more active before the Holocene, than the Leirubakki Fault. This complex fracture pattern resulting from a longer faulting history can interact in various ways and give rise to complex stress fields as expressed by stress rotation in pre-and postseismic periods (Homberg et al, 1997;Gudmundsson and Homberg, 1999).…”

“…In numerical modelling of the SISZ (Gudmundsson and Brynjolfsson, 1993; Gudmundsson, 1995; Gudmundsson and Homberg, 1999; Gudmundsson and Brenner, 2003), it was proposed that the eastern rift zone (Fig. 1) is thought to propagate southwards with shear stresses concentrating within the SISZ and participating in the arrangement of fracturing.…”

Stress Changes Modelled for the Sequence of Strong Earthquakes in the South Iceland Seismic Zone Since 1706

“…Even though there is no clear expression of an E-W trending fault and the angle between the SISZ and the neighbouring ridges is far from 90°, it is considered as a transform zone (cf. and ANGELIER, 2000;BJARNASON et al, 1993;GUðMUNDSSON, 1995GUðMUNDSSON, , 2000GUðMUNDSSON and HOMBERG, 1999;SIGMUNDSSON et al, 1995). Following this hypothesis, left-lateral shear stress is expected along the E-W striking zone.…”