The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at approximately 2-kilometer depth on planes parallel to the San Andreas fault.

After a strong earthquake, the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures. A stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock. The probabilities depend strongly on the model parameters, which are estimated with Bayesian statistics from both the ongoing aftershock sequence and from a suite of historic California aftershock sequences. Probabilities for damaging aftershocks and greater mainshocks are typically well-constrained after the first day of the sequence, with accuracy increasing with time.

The MW (moment magnitude) 7.9 Denali fault earthquake on 3 November 2002 was associated with 340 kilometers of surface rupture and was the largest strike-slip earthquake in North America in almost 150 years. It illuminates earthquake mechanics and hazards of large strike-slip faults. It began with thrusting on the previously unrecognized Susitna Glacier fault, continued with right-slip on the Denali fault, then took a right step and continued with right-slip on the Totschunda fault. There is good correlation between geologically observed and geophysically inferred moment release. The earthquake produced unusually strong distal effects in the rupture propagation direction, including triggered seismicity.

The Landers earthquake, which had a moment magnitude (M(w)) of 7.3, was the largest earthquake to strike the contiguous United States in 40 years. This earthquake resulted from the rupture of five major and many minor right-lateral faults near the southern end of the eastern California shear zone, just north of the San Andreas fault. Its M(w) 6.1 preshock and M(w) 6.2 aftershock had their own aftershocks and foreshocks. Surficial geological observations are consistent with local and far-field seismologic observations of the earthquake. Large surficial offsets (as great as 6 meters) and a relatively short rupture length (85 kilometers) are consistent with seismological calculations of a high stress drop (200 bars), which is in turn consistent with an apparently long recurrence interval for these faults.

Foreshocks occur before a large fraction of the world's major (M ≥ 7.0) earthquakes. Teleseismically located events before major earthquakes from 1914 to 1973 were considered together to examine possible average temporal and spatial patterns of foreshock occurrence. Several days before the main shocks and apparently near the epicenters of them (Δ ≲ 30 km) the activity begins to increase, culminating in a final rapid acceleration of activity in the last day. The acceleration continues up to the time of the main shocks, except for a possible temporary decrease about 6 hours before them. The seismicity increases approximately as the inverse of time before main shock. This relationship is essentially unrelated to the magnitude of the main shock. The magnitude of the largest foreshock is also unrelated to the magnitude of the main shock. In addition, pairs of major events are common. Ten percent of the world's major events are preceded by other major events within 100 km and 3 months. For foreshocks within each of three sequences studied, the ratio of the amplitudes of the P and S waves were approximately the same, suggesting that the faulting mechanisms are the same for events in each sequence. By assuming an inhomogeneous fault plane on which asperities fail by static fatigue, we derived an equation for accelerating premonitory slip as a function of time, which agrees with the observed time dependence of foreshocks.

Despite a lack of reliable deterministic earthquake precursors, seismologists have significant predictive information about earthquake activity from an increasingly accurate understanding of the clustering properties of earthquakes. In the past 15 years, time-dependent earthquake probabilities based on a generic short-term clustering model have been made publicly available in near-real time during major earthquake sequences. These forecasts describe the probability and number of events that are, on average, likely to occur following a mainshock of a given magnitude, but are not tailored to the particular sequence at hand and contain no information about the likely locations of the aftershocks. Our model builds upon the basic principles of this generic forecast model in two ways: it recasts the forecast in terms of the probability of strong ground shaking, and it combines an existing time-independent earthquake occurrence model based on fault data and historical earthquakes with increasingly complex models describing the local time-dependent earthquake clustering. The result is a time-dependent map showing the probability of strong shaking anywhere in California within the next 24 hours. The seismic hazard modelling approach we describe provides a better understanding of time-dependent earthquake hazard, and increases its usefulness for the public, emergency planners and the media.

The temporal behavior of 39 aftershock sequences in southem California, 1933California, -1988, was modeled by the modified Omori relation. Minimum magnitudes for completeness of each sequence catalog were determined, and the maximum likelihood estimates of the parameters K, p, and c, with the standard errors on each, were determined by the Ogata algorithm. The b value of each sequence was also calculated. Many of the active faults in the region, both strike slip and thrust, were sampled. The p values were graded in terms of the size of the standard error relative to the p value itself. Most of the sequences were modeled well by the Omori relation. Many of the sequences had p values close to the mean of the whole data set, 1.11 + 0.25, but values significantly different from the mean, as low as 0.7 and as high as 1.8, exist. No correlation of p with either the b value of the sequence or the mainshock magnitude was found. The results suggest a direct correlation of p values is with surface heat flow, with high values in the Salton Trough (high heat flow) and one low value in the San Bernardino Mountains and on the edge of the Ventura Basin (both low heat flow). The large fraction of the sequences with p values near the mean are at locations where the heat flow is near the regional mean, 74 mW/m 2. If the hypothesis that aftershock decay rate is controlled by temperature at depth is valid, the effects of other factors such as heterogeneity of the fault zone properties are superimposed on the background rate determined by temperature. Surface heat flow is taken as an indicator of crustal temperature at hypocentral depths, but the effects on heat flow of convective heat transport and variations in near-surface thermal conductivity invalidate any simple association of local variations in heat flow with details of the subsurface temperature distribution. The interpretation is that higher temperatures in the aftershock source volume caused shortened stress relaxation times in the fault zone materials, leading to a faster decay rate (higher p value). In 1967, Mogi proposed such a relation on the basis of Japanese data. Seismic velocity distributions published for southern California generally support the hypothesis, with low velocities (higher temperatures)corresponding to high p values, and vice-versa. mates of the three parameters K, p, and c in the relation n(t)=K(t +c) -p , where n(t) is the number of events per unit time at time t, were calculated following Ogata's [1983] approach for fitting the cumulative number of aftershocks as a function of time after the mainshock. The program also provides estimates of the standard errors of the values of the three parameters. Because the emphasis in this study is on the factors controlling the rate at which activity decays, only the p values are presented and discussed. K is dependent on the total number of events in the sequence and c on the rate of activity in the earliest part of the sequence.A few of the sequences are not described well by the Omori relation, as evidence...