S. C. Jaume

Evolution of moderate seismicity in the San Francisco Bay region, 1850 to 1993: Seismicity changes related to the occurrence of large and great earthquakes

The rate of seismic activity of moderate-size (M > 5.5) earthquakes in the San Francisco Bay (SFB) region has varied considerably during the past 150 years. As measured by the rate of seismic moment release, seismic activity in the SFB region is observed to accelerate prior to M > 7.0 earthquakes in 1868, 1906, and 1989, and then to decelerate following them. We examine these seismicity changes in the context of the evolution of the stress field in the SFB region as a result of strain accumulation and release using a model of dislocations in an elastic halfspace. We use a Coulomb failure function (CFF) to take into account changes in both shear and normal stresses on potential failure planes of varying strike and dip in the SFB region. We find that the occurrence of a large or great earthquake creates a “stress shadow”: a region where the stress driving earthquake deformation is decreased. Interseismic strain accumulation acts to reverse this process, gradually bringing faults in the SFB region out of the stress shadow of a previous large or great earthquake and back into a state where earthquake failure is possible. As the stress shadow generated by a large or great earthquake disappears, it migrates inward toward the fault associated with that large or great event. The observed changes in the rate of occurrence of moderate earthquakes in the SFB region are broadly consistent with this model. In detail, the decrease in seismicity throughout most of the SFB region and a localized increase in the Monterey Bay region following the great 1906 earthquake is consistent with our predicted stress changes. The timing and location of moderate-size earthquakes when the rate of seismicity increases again in the 1950s is consistent with areas in which the 1906 stress shadow had been eliminated by strain accumulation in the SFB region. Those earthquakes that are most inconsistent with our stress evolution model, including the 1911 earthquake southeast of San Jose, are found to occur in regions where dip-slip faulting is common in addition to strike-slip. The 1906 earthquake brought that zone of dip-slip faulting closer to failure, suggesting that the 1911 event may have been a reverse faulting earthquake rather than a strike-slip one similar to the 1984 Morgan Hill earthquake. The occurrence of activity on faults very close to the San Andreas, such as the Lake Elsman earthquakes of 1988 and 1989, appear to be associated with the last disappearence of the stress shadow on the Loma Prieta segment of the San Andreas fault. Thus events of that type may represent an intermediate-term precursor to a large earthquake, such as the 1989 Loma Prieta event. Much of the moderate-size earthquake activity in the SFB region appears to be modulated in time by the buildup and release of stress in large and great earthquakes. A tensorial approach to earthquake prediction, i.e., taking into account changes in the components of the stress tensor, has several advantages over examining scalar changes such as those in seismic activity and moment release rates. This tensorial approach allows for both activation and quiescence (but in different subregions) prior to as well as after large earthquakes.

Evolution of Stress Deficit and Changing Rates of Seismicity in Cellular Automaton Models of Earthquake Faults

Abstract—We investigate the internal dynamics of two cellular automaton models with heterogeneous strength fields and differing nearest neighbour laws. One model is a crack-like automaton, transferring all stress from a rupture zone to the surroundings. The other automaton is a partial stress drop automaton, transferring only a fraction of the stress within a rupture zone to the surroundings. To study evolution of stress, the mean spectral density

Accelerating seismic energy release and evolution of event time and size statistics : Results from two heterogeneous cellular automaton models

\cal S

Evolving Towards a Critical Point: A Review of Accelerating Seismic Moment/Energy Release Prior to Large and Great Earthquakes

(kr) of a stress deficit field is examined prior to, and immediately following ruptures in both models. Both models display a power-law relationship between

Seismic activity on neighbouring faults as a long-term precursor to large earthquakes in the San Francisco Bay area

\cal S

Changes in earthquake size-frequency distributions underlying accelerating seismic moment/energy release.

(kr) and spatial wavenumber (kr) of the form

Lattice Solid Simulation of the Physics of Fault Zones and Earthquakes: the Model, Results and Directions

{\cal S}(k_r) \sim k^{- \beta}_r

Accelerating seismic energy release and evolution of event time and size statistics: Results from two heterogeneous cellular automaton models

. In the crack model, the evolution of stress deficit is consistent with cyclic approach to, and retreat from a critical state in which large events occur. The approach to criticality is driven by tectonic loading. Short-range stress transfer in the model does not affect the approach to criticality of broad regions in the model. The evolution of stress deficit in the partial stress drop model is consistent with small fluctuations about a mean state of high stress, behaviour indicative of a self-organised critical system. Despite statistics similar to natural earthquakes these simplified models lack a physical basis. Physically motivated models of earthquakes also display dynamical complexity similar to that of a critical point system. Studies of dynamical complexity in physical models of earthquakes may lead to advancement towards a physical theory for earthquakes.

Evolution of stress deficit and changing rates of seismicity in Cellular automaton models of earthquake faults

Abstract—The evolution of event time and size statistics in two heterogeneous cellular automaton models of earthquake behavior are studied and compared to the evolution of these quantities during observed periods of accelerating seismic energy release prior to large earthquakes. The two automata have different nearest neighbor laws, one of which produces self-organized critical (SOC) behavior (PSD model) and the other which produces quasi-periodic large events (crack model). In the PSD model periods of accelerating energy release before large events are rare. In the crack model, many large events are preceded by periods of accelerating energy release. When compared to randomized event catalogs, accelerating energy release before large events occurs more often than random in the crack model but less often than random in the PSD model; it is easier to tell the crack and PSD model results apart from each other than to tell either model apart from a random catalog. The evolution of event sizes during the accelerating energy release sequences in all models is compared to that of observed sequences. The accelerating energy release sequences in the crack model consist of an increase in the rate of events of all sizes, consistent with observations from a small number of natural cases, however inconsistent with a larger number of cases in which there is an increase in the rate of only moderate-sized events. On average, no increase in the rate of events of any size is seen before large events in the PSD model.