We present a new three-dimensional model of the major fault systems in southern California. The model describes the San Andreas fault and associated strikeslip fault systems in the eastern California shear zone and Peninsular Ranges, as well as active blind-thrust and reverse faults in the Los Angeles basin and Transverse Ranges. The model consists of triangulated surface representations (t-surfs) of more than 140 active faults that are defined based on surfaces traces, seismicity, seismic reflection profiles, wells, and geologic cross sections and models. The majority of earthquakes, and more than 95% of the regional seismic moment release, occur along faults represented in the model. This suggests that the model describes a comprehensive set of major earthquake sources in the region. The model serves the Southern California Earthquake Center (SCEC) as a unified resource for physics-based fault systems modeling, strong ground-motion prediction, and probabilistic seismic hazards assessment.

1.Drought is a natural disturbance that can cause widespread mortality of aquatic organisms in wetlands. We hypothesized that seasonal drying of marsh surfaces (i.e. hydrological disturbance) shapes spatio-temporal patterns of fish populations. 2. We tested whether population dynamics of fishes were synchronized by hydrological disturbance (Moran effect) or distance separating study sites (dispersal). Spatio-temporal patterns were examined in local populations of five abundant species at 17 sites (sampled five times per year from 1996 to 2001) in a large oligotrophic wetland. 3. Fish densities differed significantly across spatio-temporal scales for all species. For all species except eastern mosquitofish ( Gambusia holbrooki ), a significant portion of spatio-temporal variation in density was attributed to drying events (used as a covariate). 4. We observed three patterns of response to hydrological disturbance. Densities of bluefin killifish ( Lucania goodei ), least killifish ( Heterandria formosa ), and golden topminnow ( Fundulus chrysotus ) were usually lowest after a dry down and recovered slowly. Eastern mosquitofish showed no distinct response to marsh drying (i.e. they recovered quickly). Flagfish ( Jordanella floridae ) density was often highest after a dry down and then declined. Population growth after a dry down was often asymptotic for bluefin killifish and golden topminnow, with greatest asymptotic density and longest time to recovery at sites that dried infrequently. 5. Fish population dynamics were synchronized by hydrological disturbance (independent of distance) and distance separating study sites (independent of hydrological disturbance). Our ability to separate the relative importance of the Moran effect from dispersal was strengthened by a weak association between hydrological synchrony and distance among study sites. Dispersal was the primary mechanism for synchronous population dynamics of flagfish, whereas hydrological disturbance was the primary mechanism for synchronous population dynamics of the other species examined. 6. Species varied in the relative role of the Moran effect and dispersal in homogenizing their population dynamics, probably as a function of life history and ability to exploit dry-season refugia.

Models of community regulation commonly incorporate gradients of disturbance inversely related to the role of biotic interactions in regulating intermediate trophic levels. Higher trophic-level organisms are predicted to be more strongly limited by intermediate levels of disturbance than are the organisms they consume. We used a manipulation of the frequency of hydrological disturbance in an intervention analysis to examine its effects on small-fish communities in the Everglades, USA. From 1978 to 2002, we monitored fishes at one long-hydroperiod (average 350 days) and at one short-hydroperiod (average 259 days; monitoring started here in 1985) site. At a third site, managers intervened in 1985 to diminish the frequency and duration of marsh drying. By the late 1990s, the successional dynamics of density and relative abundance at the intervention site converged on those of the long-hydroperiod site. Community change was manifested over 3 to 5 years following a dry-down if a site remained inundated; the number of days since the most recent drying event and length of the preceding dry period were useful for predicting population dynamics. Community dissimilarity was positively correlated with the time since last dry. Community dynamics resulted from change in the relative abundance of three groups of species linked by life-history responses to drought. Drought frequency and intensity covaried in response to hydrological manipulation at the landscape scale; community-level successional dynamics converged on a relatively small range of species compositions when drought return-time extended beyond 4 years. The density of small fishes increased with diminution of drought frequency, consistent with disturbance-limited community structure; less-frequent drying than experienced in this study (i.e., longer return times) yields predator-dominated regulation of small-fish communities in some parts of the Everglades.

Studies were conducted to determine the effects of experimental manipulations of discharge on invertebrate drift in two regulated rivers in northwestern Montana, USA. During these studies the discharge regime in the Flathead River was characterized by frequent flow fluctuations, while in the Kootenai River high discharge was maintained for much longer periods before flow was reduced to minimum discharge. The magnitude of the response of invertebrates to disturbance was different in the two rivers, in part because of the different frequencies of flow changes. Midstream invertebrate drift increased an order of magnitude during increasing discharges in the Flathead River but was not substantially increased during decreasing discharges. When the prior discharge regime had been sustained at high levels in the Kootenai River, invertebrate drift densities as high as 300000/100 m 3 were measured along the shoreline following reductions in discharge, both immediately after flow began to decrease and after dark on the same day. There was also more recolonization of shoreline areas and more stranding of insects following dewatering of nearshore regions when there had been sustained high discharge levels prior to the flow reduction. More insect stranding occurred during a faster rate of decrease in discharge (50000 to 100000 organisms m-2).

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