origin in the cycle of stress accumulation and release that characterizes the earthquake cycle. The working group assumed that, following an earthquake large enough to rupture an entire segment of a fault (termed a characteristic earthquake), the potential for a future large earthquake along that fault segment is initially small and increases as a function of time as the motion of the earth's crust again builds the stress on the fault toward the limit for failure. These properties of recurring earthquakes are qualitatively expressed in the seismic gap hypothesis, which states that the potential for a future earthquake is greater along those active fault segments having had long periods of time since the last charac

teristic earthquake. To determine timedependent probabilities, the faults were divided into their recognizable segments; the potential for a future large earthquake on each segment was calculated on the basis of the time that has elapsed since the most recent large earthquake and fault parameters such as slip rate and amount of displacement. The time interval chosen for the probability calculations was 30 years-1988 to 2018; similar calculations using the same models were performed for 5-year, 10-year, and 20-year intervals, as well.

• The 30-year probability of large earthquakes is highest in southern California. The 62-mile-long (100-km-long) Coachella Valley segment has the highest

probability (0.4 or 4 in 10) of producing an earthquake of magnitude 7.5 in the next 30 years. A major earthquake has not occurred there since about A.D.

1680. The Mojave segment, part of the source region of the great 1857 earthquake, has a 30-year probability of 0.3. • Evaluation of the earthquake probability for the southern San Andreas fault depends on the future behavior of the San Bernardino Mountains segment of the fault. If the San Bernardino Mountains segment slips independent of the adjacent segments, the expected magnitude of earthquakes on the southern San Andreas fault would be about 7.5, with a 0.7 probability of at least one such event in the next 30 years. If the San Bernardino Mountains segment slips along with either the Mojave segment to the north or the Coachella Valley segment to the south, then the resulting earthquake would approach the size of the 1857 earthquake and would have a 30-year probability of 0.6.

The probability of large earthquakes within the next 30 years along fault segments in the San Francisco Bay area is also significant. The total probability for all fault segments evaluated is 0.5. The Hayward fault has produced two earthquakes in historical time, in 1836 and 1868; both had estimated magnitudes approaching 7. The Northern East Bay segment of the Hayward fault, the Southern East Bay segment of the Hayward fault, and the San Francisco Peninsula segment of the San Andreas fault each have a probability of 0.2 of an earthquake of magnitude 7 in the next 30 years. The 30-year probability of a great earthquake along the North Coast segment, extending north from the San Francisco Peninsula, is less than 0.1. • Fewer data are available about the recurrence of large earthquakes along five separate segments of the San Jacinto fault. During the course of the probability study, the late November 1987, magnitude 6.6 Superstition Hills earthquake more than 100 miles south of Whittier, Calif., occurred on one of these segments. The USGS estimated a probability of 0.5 for the four remaining segments combined, for the occurrence of earthquakes of about magnitude 7 within the next 30 years. The segment of the San Jacinto having the highest probability

is the Anza segment (0.3). The others are San Bernardino Valley segment, 0.2; San Jacinto Valley segment, 0.1; and the Borrego Mountain segment, less than 0.1. • The Imperial fault, spanning the Mexico-U.S. border, has produced two magnitude 6.6 or greater earthquakes in the past half century. A 50-percent probability of a 6.5 or greater earthquake in the next 30 years is estimated.

The 30-year probability of large earthquakes is highest in southern California.

Recent moderate earthquakes in Whittier and Coalinga, Calif., serve as a reminder that not all active faults have been recognized everywhere in California. Although almost all well-studied California earthquakes of magnitude 7 and larger have, in fact, occurred on faults having clear surface expressions, some faults capable of events of this size have not been identified. This is particularly true in the Transverse Ranges of southern California, where shallowly dipping thrust faults and major folds dominate the tectonic environment and where the configuration of faults at depth is known to be exceedingly complicated. Thus, not all potential sources of large California earthquakes have been identified in these probabilistic analyses.

The assessment of long-term seismic hazard on California's major faults is an active and rapidly developing field. New data and improvements in the model on which the assessments are based will probably lead to revision and refinement in the probabilities assigned here to segments of the San Andreas system. The total regional values, however, support the main conclusion that the probability of a major earthquake on the San Andreas in southern California within the next 30 years is high, about 0.6, and that the probability approaches 0.5 for both the San Francisco Bay area in northern California and the San Jacinto fault in southern California.

Developing a Climate Record for the Past 3 Million Years

By David P. Adam

Global climatic change has attracted increasing attention in recent years as human activities have grown to a scale that could affect the climate. The carbon dioxide content of the atmosphere has increased by about 20 percent since the Industrial Revolution; consumption of fossil fuel and deforestation have contributed, possibly substantially, to this increase. In addition, deforestation threatens tropical rain forests and can alter local and regional weather patterns. Manmade chlorofluorocarbon gases have made their way into the upper atmosphere and are affecting the ozone layer.

U.S. Geological Survey scientists are studying these and other effects of human activities by studying computer models of climate. These models can offer predictions of the behavior of future climates under various conditions. In order to verify the accuracy of these models, however, scientists must test them to see how well they can reproduce the climates of the past.

Historical records provide one type of climatic data against which to judge the results of models but have the disadvantage that they span at most only a few hundred years. In order to evaluate model results for conditions outside the range documented in the historical instrumental records, it is necessary to study the evidence of past climatic conditions preserved in the extensive geologic record.

Many lake, bog, and alluvial sediments have yielded records of past events and environmental conditions through their physical properties and the biological remains preserved in them. However, most of these records span no more than the past few tens of thousands of years. Studies of these records have provided a relatively good understanding of the dynamics of climate change during the present interglacial period and during the transition from the last glacial period to the present interglacial. By contrast,

much less is known about how climate changes when the world passes from an interglacial climate into a glacial climate, or from a climate like the present interglacial into an even warmer mode. Therefore the USGS has sought to locate longer records of past climates.

Studies by the USGS Climate Change Program in northeastern California and south-central Oregon since 1983 have produced a detailed record of regional climatic changes that spans the past 3 million years. The most important record is from a 334-meter (1,095-foot) sediment core from an ancient lake at the town of Tulelake, Siskiyou County, Calif. Paleontologic studies have described changes in the frequencies of pollen grains, diatoms and other algae, fish remains, and ostracodes at various depths within the core; examples of these changes are shown in figure 5. These changes can be interpreted in terms of regional events, including not only climate changes but also volcanic, tectonic, and geomorphic changes. Such changes are also observed in the geochemical record extracted from the core.

In addition to the fossils found in the core, numerous volcanic ash layers (tephra) are well preserved. Because each ash layer identifies the same time horizon, wherever it occurs, studies of the major- and trace-element composition of volcanic glass, using electron microprobe techniques, provide a means by which to correlate the Tulelake record with records at other localities where the same ash layers have been found. The ages of many of the tephra layers are already known from prior age analyses done at other localities, and dates determined elsewhere can thus be reliably correlated with particular horizons in the Tulelake core. For example, the Rockland ash bed (fig. 5) was erupted from Mt. Lassen about 0.4 million years ago, is found in many locations in California, and has been used to interpret changes in the drainage history of central California. The DSDP-173-3/4 ash bed, also shown on figure 5, has been found both at Tulelake and in a sediment core from the northeastern Pacific Ocean off the coast of Oregon. That ash layer thus provides a firm link between the continental climate record at Tulelake and the more thoroughly studied marine climatic record.

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Figure 5. Selected variables plotted against depth for a 1,095-foot core from Tulelake, Siskiyou County, Calif. Age increases with depth; the oldest deposits shown are about 3 million years old. Pollen types are expressed as percent of total pollen; diatoms are expressed as percent of total diatoms. Paleomagnetic timescale is shown at right (ka 1,000 years; Ma 1 million years). The black boxes indicate intervals during which the Earth's magnetic field was oriented as it is today, and the unshaded boxes indicate intervals during which the Earth's magnetic field was reversed. Solid arrows indicate positions of volcanic ash layers (tephra) used to correlate the core with other areas; hollow arrows indicate ages of paleomagnetic boundaries as determined by other studies. The pollen curves reflect variations in the regional vegetation in response to climate. High frequencies of pine pollen indicate times of relatively moist conditions. High frequencies of juniper pollen at the expense of pine reflect more open, less heavily forested conditions, and hence less available moisture. High

frequencies of sagebrush pollen represent cold, dry conditions that were most common during glacial times. The lightly shaded boxes shown on the pollen curves indicate parts of the section where no pollen was recovered. A major change in the behavior of the regional vegetation that took place about 1.6 million years ago (about the top of the Olduvai Event) corresponds to the onset of continental glaciation in the Northern Hemisphere. The frequencies of the various diatoms reflect changing conditions within the lake and also evolutionary changes in the diatoms. For example, the diatom Anomoeoneis costata prefers saline waters, and the peak in its frequency from 50-90 meters depth reflects the relatively high salinity of the lake about 500,000 years ago. The peaks in frequency of "other algae" (note the very compressed scale for the curve) reflect a well-developed lake; when "other algae" are scarce, the lake was probably shallower or seasonal in nature.

The Tulelake cores also preserve a record of changes in the Earth's magnetic field, which is known to reverse itself at irregular intervals. The ages of the various reversals have been carefully studied, and they provide another way to date the Tulelake section and to refine age estimates for volcanic ashes that lie between reversals. For example, the Bear Gulch ash bed (fig. 5) is estimated to be about 1.94 million years old, because of its position between the base of the Olduvai Event (age 1.87 million years) and the Gauss-Matuyama boundary (age 2.48 million years). The tephra and paleomagnetic records together provide a means of dating the fossil record that is independent of the fossil record itself; such independent corroboration is an essential part of the scientific method.

Although long climatic records like the Tulelake core are still only rarely accessible to researchers, there are many suitable deposits in other areas. Recent drilling by the USGS to the northeast of Tulelake has yielded a core from Summer Lake, Oreg., estimated to span the past 1 million years, and a core from the Upper Chewaucan Marsh that extends back at least 750,000 years. These cores await detailed study.

Much less is known about how climate changes when the world passes from...a climate like the present interglacial into an even warmer mode.

Future studies of these and other long, continuous sediment records will provide better understanding of how the Earth's climate has behaved in the past, better constraints against which to evaluate climate models, and thus a better understanding of how human activities may affect climate and what the longrange impact of climate change on the environment may be.

Using Geology to Map and Understand Radon Hazards in the United States

By James K. Otton

Indoor radon continues to be a major concern for public health officials in the United States. A 1988 review of the available health data by the National Academy of Sciences supports the Environmental Protection Agency's 1986 estimates that 5,000 to 20,000 lung cancer deaths annually may be attributed to long-term exposure to radon and its decay products. Radon is derived from the radioactive decay of uranium and radium.

The principal sources of indoor radon are the rocks and soils that surround building foundations. How much of the radon in rocks and soils is available to enter buildings is controlled by the uranium (or radium) content and the soil permeability. House construction practices, the structural integrity of the foundation, regional and local climatic factors, and the use patterns of the homeowner control how much of the radon available in the soils and rocks actually enters the home.

USGS scientists are investigating radon generation and migration in the ground and developing assessment techniques for estimating the radon potential of rocks and soils across the United States. USGS expertise in the geology of radon comes from decades-long investigations of uranium and its radioactive decay products in ore deposits and other natural and manmade settings.

Current concerns began with the discovery of high radon levels in a home near Boyertown, Pa. USGS geologists mapped the rocks and measured the soil gas radon concentrations in the neighborhood near the house and found three distinct rock types: a biotite-hornblende gneiss, a quartz-feldspar gneiss, and a uranium-rich quartz-feldspar gneiss; the uranium enrichment was caused by shearing of the gneiss. All the very high readings of indoor radon in the neigh