fairly accurate measures of past maximum
temperatures. By far the most widely
used thermal maturity indicator is vitrinite
reflectance.

Vitrinite, one of several types of organic particles commonly found disseminated in sedimentary rock, is the fossil remains of woody plant material. Upon exposure to elevated temperatures, vitrinite becomes increasingly reflective. These changes in reflectivity are responsible for the increase in the brightness of coal, which consists primarily of vitrinite, with increasing coal rank: lignite coal is dull, bituminous coal is moderately bright, and anthracite coal is very bright.

Past temperatures can be determined by measuring the amount of light that is reflected off a polished surface of vitrinite, designated as R. and reported in percent. Typical values range from ~0.2 percent, for vitrinite that has never been subjected to temperatures higher than those found at the Earth's surface, to more than 5.0 percent, for vitrinite exposed to metamorphic conditions (generally exceeding 300°C). Oil is found in samples that give vitrinite reflectance values between 0.6 and 1.3 percent. At values higher than about 2.0 percent, generally only natural gas is present. Other useful thermal maturity indicators are based on color changes observed in fossil pollen, spores, and conodonts (teethlike microfossils) and structural changes in clay minerals.

Thermal Maturity Mapping in Alaska.During the last 15 years, the USGS has collected vitrinite reflectance data from many parts of Alaska in its study of oil and natural gas resources. These data, combined with data from the oil industry and other scientists, now provide information on thermal maturity at several thousand localities widely distributed over the State. These data have been combined with other measures of thermal maturity, such as conodont color, to produce a thermal maturity map of the surface rock of Alaska. The thermal maturity data are grouped into five categories: undermature (R.<0.6 percent), mature I (R=0.6-1.3 percent), mature II (R=1.3-2.0 percent), overmature (R-2.0-3.5 percent), and supermature (R>3.5 percent).

North Slope Example.-The northern part of the thermal maturity map of Alaska (the Brooks Range and North Slope) is shown in figure 1. This map is based on about 1,100 vitrinite reflectance determinations from surface localities, 3,000 determinations from 82 wells, and approximately 1,000 conodont color analyses. These data provide the most concentrated coverage of any area of the State. They show that the thermal maturity of rock at the surface generally increases from north to south. This increase indicates greater uplift to the south. Assuming an average geothermal gradient of 25°C per kilometer, the maximum reflectance values indicate

A'

Oil well

A, Geology.

North

A'

Tertiary

B, Thermal Maturity.

(Vitrinite reflectance, mean Ro, %)

North

Undermature (Ro < 0.6)1

0.34

Mature II (Ro 1.3-2.0)

uplift of 12 kilometers for the central Brooks Range. Uplift apparently decreases to the north, and the rock exposed along the coast east of Prudhoe Bay appears to show no uplift.

A salient feature of the regional thermal maturity pattern is a broad southward extension of thermally immature rock in the central portion of the North Slope. In comparison, rock in regions to the east and west is of higher thermal maturity, indicating that they were at one time buried to greater depths and have therefore undergone greater uplift than rock of the central North Slope. Such uplift in the eastern Brooks Range has previously been recognized, but the uplift in the western Brooks Range and North Slope is recognized only from thermal maturity data. These regional differences in uplift may indicate that mountain building continued longer in the east and west than in the central portion of the Brooks Range.

Vitrinite reflectance patterns within the Brooks Range indicate that maximum burial occurred after the early phases of mountain building. The pattern of vitrinite reflectance values just north of the Brooks Range, however, show a warping of the thermal maturity values (fig. 2)—an indication that the later stages of mountain building continued in this area after maximum heating and burial. Oil and natural gas are expected to have formed primarily at the time of maximum heating of the rock-the time when a thermal signature was imposed. The observed warping of the thermal maturity units means that late-stage deformation may have destroyed earlyformed oil and natural gas accumulations or shifted them to new locations. Thermal maturity data thus can be used to infer aspects of the timing of mountain development and have important implications for oil and natural gas exploration.

25 KILOMETERS

Devonian,

United States National

Seismograph Network

By Robert P. Massé

The frequency of occurrence, geographical distribution, and magnitude of earthquakes are important in assessing the seismic hazard of a region, for establishing the design and construction criteria for critical facilities, such as nuclear reactors, and for responding to large-magnitude events. This information defines the seismicity of a region and can be determined only through the operation of seismograph networks. For many years, scientists and government agencies have recognized the need for a high-quality national seismograph network in the United States.

The USGS has developed and is deploying a broadband digital seismograph network for the United States to meet this need. The network will consist of approximately 150 seismograph stations distributed across the contiguous 48 States and Alaska, Hawaii, Puerto Rico, and the Virgin Islands. Data transmission will be via two-way satellite telemetry from the network sites to a central recording facility at the USGS National Earthquake Information Center (NEIC) in Golden, Colo. The design goal for the network is the undistorted recording by at least five welldistributed stations of any seismic event of magnitude 2.5 or greater in virtually all areas of North America. The entire network, the U.S. National Seismograph Network (USNSN), is a cooperative effort between the USGS and the Nuclear Regulatory Commission (NRC).

The USGS will install and operate the USNSN, and the NRC is providing funds to the USGS for the completion of the network east of the Rocky Mountains. This network will increase the ability to characterize earthquakes in most parts of the United States. However, the USNSN will not, even when complete, eliminate the need for additional dense networks of seismograph stations in specific locations. Such dense local networks exist today in several critical areas within the United States. These dense local networks detect low-magnitude earthquakes (below the 2.5 threshold for USNSN) and achieve high location accuracy. The dense local networks are located in specific seismic risk areas to acquire important data for research in earthquake prediction and ground-motion estimation. The USNSN and the dense local seismograph networks are complementary. Data from USNSN will provide, for the first time, near-uniform high-quality coverage for the entire country.

The design of the USNSN will meet the following objectives:

⚫ Detect and locate all earthquakes of magnitude 2.5 or greater in the United States, within 30 minutes,

Minimize development risk, development cost, and operation cost of the network, Place seismograph stations at low-noise sites,

Acquire full seismic waveform data, and Provide rapid distribution of the data through the NEIC.

To ensure that a high-quality network is deployed within a few years, it is essential that the system design be feasible, that installation and implementation be timely, and that available funds cover all costs. For the USNSN, the USGS will minimize risk by using a stateof-the-art seismic processing system recently developed for the NEIC. All station hardware will be commercially available. Historically, the failure to maintain funding for seismograph networks and arrays has been high operating costs. Therefore, the design of the USNSN is such that operational costs will be minimized. The lower the nonearthquake background noise at the stations, the better the overall network detection will be. To the extent possible, seismograph stations are being located at low-noise sites. The USNSN will produce high-quality seismic data covering a wide dynamic range. Three-component data will be available from the network stations. Rapid access to data from USNSN will be provided by the USGS. Waveform data for earthquakes will be available in realtime through a broadcast mode satellite transmission from the NEIC and in near realtime through high-speed dial-up to the event waveform data base at the NEIC.

Waveform data for all earthquakes recorded by USNSN also will be provided on CD-ROM (compact disc, read-only memory). Satellite links to participating regional networks also will be provided. The NEIC has taken a leadership role in distributing national and global seismic data to the scientific community. For data from the USNSN, the NEIC is developing procedures to ensure rapid distribution and equal access to the event data for all interested users.

Each station will have three-component seismometers, which will acquire data over a very broad band and high dynamic range. The seismograph stations will have a microcomputer, a satellite transmitter, and an antenna. The event data will be digitized at the station. In addition, stations are being equipped with three-component strongmotion sensors, further extending the total dynamic range of the system. This larger

dynamic range will permit even large earthquakes to be recorded on scale.

Data will be transmitted from the station to the NEIC via satellite and a small satellite antenna. The NEIC will receive data from all seismograph stations using a large satellite dish having associated electronics located at the NEIC in Golden, Colo. This data transmission will allow the seismograph stations to be placed at sites that have low background noise and will eliminate dependency on the more costly long-distance telephone lines.

a

The system is being designed with sufficient capacity to telemeter all data simultaneously in the event of a great earthquake near North America. At NEIC in Golden, modular real-time seismic processing system can be easily expanded to meet all future USNSN requirements. Many future seismic studies of the United States, including those studies using data from the dense local networks, will be based on the data base obtained from the USNSN.

The functions of the processing system include refining signal detection, determining signal parameters, associating signals, determining preliminary earthquake location, archiving the signal waveform data with associated epicenter information, providing an interactive capability for reviewing all automatic event determinations, and producing final epicenter information for dissemination. Processed results for USNSN in the form of

122°15'

oceans will be used increasingly as a source of food and mineral and petroleum resources, as a place for recreation, and as a repository for waste products. To ensure that these human demands are met responsibly, the public and the public's representatives must have the best scientific data available to guide them in making choices about uses of the marine environment.

The USGS has conducted geologic research investigations in the marine environment for more than two decades. In 1983, President Reagan proclaimed the ocean area 200 nautical miles off the coast of the United States, its island territories, and territorial seas as the U.S. Exclusive Economic Zone (EEZ). The USGS is conducting systematic environmentally focused geologic investigations, called Geologic Inventory projects, on the