Rate and state of background stress estimated from the aftershocks of the 1989 Loma Prieta, California, earthquake

Abstract

Abstract. Estimates of the tectonic stress state including loading rate and magnitude of background stress are derived from the spatial and temporal distribution of Loma Prieta aftershocks. This technique was previously applied to the Landers aftershock sequence [Gross and Kisslinger, 1997] and is based upon the seismicity model of Dieterich [1994]. Dieterich's theory suggests that background seismicity should be proportional to stress rate and the number of aftershocks in an area should be proportional to the stress step experienced in that area. We used two independently derived source models to compute the stress step from the mainshock and to determine how effective that stress step was in triggering aftershocks. A background stress state is then chosen which makes the stress steps at the aftershock locations most distinct from the stress steps at hypocenters of background seismicity. The best fitting background stress state has its greatest compressive stress plunging • 17 ø to N13øE, and an intermediate stress very close in magnitude to the least principal stress. The small shear stresses at depth and low coefficient of friction suggest that high-pressure pore fluids may be present inside active faults. The estimated stress rate of •-70 Paid (0.25 bar/yr) is comparable to the stress rate found for the southern San Andreas Fault system. We found a best fitting "effective" coe•cient of friction/• • 0.2 for the Loma Prieta area, significantly less than • • 0.6 estimated for Landers in previous work.Variations in aftershock decay rate within the Loma Prieta aftershock zone are correlated with static stresses caused by postseismic slip as modeled by •irgmann et al. [1997]. Some of the postseismic slip occurred on structures that did not slip during the mainshock, so the postseismic and coseismic stress step fields have different spatial distributions. The effectiveness of slowly accumulated postseismic static stresses in triggering aftershocks is especially interesting because dynamic stresses are insignificant in this case. IntroductionThe 1989 Mw = 7.0 Loma Prieta earthquake was a very educational event for the seismological community, illustrating the significance of seismic hazard from events that do not break the surface. Loma Prieta also raised some important questions relating to earthquake source mechanics because the aftershock focal mechanisms were so diverse that they suggested that slip on the mainshock had completely relieved homogeneous resolved shear stresses on the mainshock fault plane Copyright 1998 by the American Geophysical Union. Paper number 97JB03010.0148-227 / 98 / 97 JB-03010509.00 [Michael et al., 1990]. Effectively zero shear stresses following the mainshock were unexpected because they imply very low dynamic coefficients of friction, which had not been observed in the laboratory. In combination with the modest stress drop, the low residual stresses on the fault plane also suggest that either the static coefficient of friction was quite low or the effective normal stress was much les...

Aftershock Statistics

“…DIETERICH (1994) has derived Omori's law from the equations of rate-and-state friction. GROSS and BURGMANN (1998) have applied this result to the Loma Prieta earthquake and its application has been discussed by ZIV and RUBIN (2003). There is clearly a close association between this work and the damage mechanics approach.…”

“…In the simplest dislocation model of an earthquake, the strain accumulation and release produces regions of stress increase (DAS and SCHOLZ, 1981a,b). These stress ''halos'' certainly contribute to the occurrence of aftershocks (TODA et al, 1998;GROSS and BURGMANN, 1998;RYBICKI, 1973;MENDOZA and HARTZELL, 1988;KING et al, 1994;MARCELLINI, 1995;HARDEBECK et al, 1998;STEIN, 1999). However, the actual stress fields prior to and after an earthquake are undoubtedly very complex.…”

Evolution of Stress Fields and Faulting in Seismic Zones

“…2), with large-scale and small-scale heterogeneity in the physical properties and associated stress fields, where continuous aseismic creep takes place (e.g., SIBSON, 1986;RICE, 1992;GUDMUNDSSON, 1993;FJÄ DER et al, 1994). Seismic zones are also subject to changes in fluid pressure and permeability, which affect the associated seismicity (e.g., SIBSON, 1981SIBSON, , 1994SIBSON, , 1996BYERLEE, 1993;WOLF et al, 1997;GROSS and BÜ RGMANN, 1998). These complexities in the infrastructure and stress fields of seismic zones significantly decrease the probability of successful prediction of most earthquakes.…”

“…If the fault plane is subject to high fluid pressure during faulting, of which there is abundant evidence (e.g., RICE, 1992;BYERLEE, 1993;GROSS and BÜ RGMANN, 1998), then v s $0 and the condition of eq. (2) becomes c :2T 0 .…”

Deep coseismic slip of the 2008 Wenchuan earthquake inferred from joint inversion of fault stress changes and GPS surface displacements

“…Dieterich's aftershock model has been used to assess probabilistic seismic hazards (Toda et al, 1998(Toda et al, , 2002, estimate the tectonic stressing rate from the spatiotemporal evolution of Loma Prieta aftershocks (Gross and Burgmann, 1998) and infer stress change due to dike intrusion in the Kilauea volcano in Hawaii (Dieterich et al, 2000). These studies have demonstrated that this model has (Huang et al, 2008;Zhu et al, 2008;Zhao et al, 2010).…”