The China Digital Seismograph Network

By Jon R. Peterson

Introduction

Long earthquake histories in both countries prompted the United States and China to develop the means by which to cooperatively study earthquakes and seek ways to mitigate their effects. In 1980 the State Seismological Bureau (SSB) of the People's Republic of China, the U.S. Geological Survey, and the National Science Foundation signed the Earthquake Studies Protocol that initiated research programs to benefit the earthquake hazards reduction program in both nations. In October 1987, a panel of experts was convened in the People's Republic of China (PRC) to review the project, a policy required by the PRC Government to insure that design goals have been met. The panel, which included seismologists from France, Norway, and West Germany as well as the United States and China, visited facilities in Beijing, Lanzhou, and Shanghai.

During China's long history, earthquakes have repeatedly destroyed major urban areas. According to the SSB, in the 20th century, 104 earthquakes of magnitude 7 or larger have stricken 21 of the 30 Chinese administrative provinces, autonomous regions, and municipalities. In the past 37 years, earthquakes in China have killed 237,000 people and injured 763,000 others. In comparison, there have been 20 earthquakes of magnitude 7 or greater in the United States in this century, causing 1,380 deaths and more than $5 billion in property damage.

High-quality observational data are essential to earthquake studies. Thus, a high priority in planning cooperative activities was the modernization of data acquisition systems in China. Their purpose is twofold: to provide high-quality digital data for investigations of earthquakes in China and to supplement the data collected from the global seismograph network for the use of research scientists throughout the world. The global seismograph network operated by the

U.S. Geological Survey comprises stations in more than 60 countries and islands and is the major source of freely exchanged seismological data. Extending the global network into China has long been an important goal of the scientific community.

Discussions between the USGS and the SSB led to an agreement-in-principle, signed in May 1983, to cooperatively establish the China Digital Seismograph Network (CDSN). Funding for the network was provided by the SSB and, in this country, by the USGS and the Defense Advanced Research Projects Agency. The task of implementing the cooperative agreement was assigned in China to the SSB Institute of Geophysics and in the United States to the USGS Seismological Laboratory in Albuquerque, N. Mex. Work on the CDSN began soon after the agreement was signed and was completed three and one-half years later. The network was operated for a one-year trial period then officially accepted and inaugurated in October 1987. The CDSN is an important cooperative effort that was highlighted as a "success story" by the U.S.-PRC Joint Commission on Science and Technology. It is one of many projects in recent years that have brought American and Chinese scientists and engineers together for the benefit of both nations.

Network Plan

The CDSN was planned as a national network consisting of nine digital seismograph stations, a data management center, and a network maintenance center. The USGS agreed to design and develop the seismograph systems and to provide equipment for four of the stations, software for the data management system, parts and equipment for the network maintenance center, and training for Chinese engineers and technicians. The SSB agreed to provide equipment for five of the stations and hardware for the data management center, prepare all of the sites, and operate the network. Responsibility for assembly and installation of the equipment was shared.

The four U.S.-supplied seismograph systems were installed at Baijatuan (near Beijing), Kunming, Mudanjiang, and

Lanzhou; the five PRC-supplied seismograph systems were installed at Enshi, Hailar, Sheshan, Qiongzhong, and Urumqi. The data management and network maintenance centers were both installed in Beijing.

All of the new station equipment was installed at observatories that have been operated as part of the China national network. Thus, experienced personnel were available to assist with the installation and operation of the instruments. Data are recorded at the stations on high-density tape cartridges that are replaced at two-week intervals. The station tapes and operator logs are then sent by mail or other means to the data management center located in Beijing.

The functions of the data management center are to collect the network tapes, examine the quality of the data, assemble network-day tapes, and archive the data. A network-day tape contains the data from all stations in the network for a specific Julian day (the Julian Calendar is a numerical calendar system that operates from day 1 through day 365 beginning on January 1), which is a much more convenient form than station tapes for data users. The day tapes are the most important products of the center and are distributed to research scientists and other organizations.

Network Instrumentation

The type of digital seismograph system installed in China consists of seismometers that produce electrical signals in proportion to Earth motion, digital encoders that sample and convert the signals to a stream of digital words, and tape cartridges or other media used for recording the data. Other critical components include a clock so that the earthquake signals can be accurately timed, a radio to synchronize the clock to transmitted timing signals, a calibrator used to check the sensitivity of the seismometers, visual drum recorders to display signals at the station, a terminal for the operator, a microcomputer to control the data processing, and a backup power system that keeps the equipment operating if line power fails.

Seismograph systems have improved dramatically during the past decade. New

broadband force-balance seismometers produce usable signals over a frequency bandwidth from 0.00002 Hz to 10 Hz, a range sufficient for recording diurnal solid Earth tides, free oscillations of the Earth, long-period surface waves, and short-period body waves, all in a single channel of data. The dynamic recording range of a seismograph has been extended as well. Dynamic recording range is the ratio of the largest signal that can be recorded to the smallest signal that can be detected. The typical drum type recorder used for many years to record earthquakes has a width of about 300 millimeters and a resolution of about 0.5 millimeter, so its dynamic range is 600 to 1 (assuming that the pen or light beam can travel the full width of the recorder). In contrast, the dynamic range achieved in digital recording is now approaching 10,000,000 to 1. To match this, a drum recorder would have to be several kilometers wide. The much greater bandwidth and dynamic range of modern seismographs permits seismologists to use more powerful analytical techniques, especially in the study of earthquake source mechanisms and Earth

surface noise. The seismometer signals are filtered, digitized, and formatted, then recorded on high-density tape cartridges together with a time code and other information needed to process the data. Long-period and very-long-period signals derived from the broadband signals are recorded continuously; broadband and short-period signals are recorded in a triggered mode, that is, only when an earthquake signal is detected by an automatic signal detector in the microcomputer.

The data management center is equipped with a minicomputer, several tape drives and disk drives, and a variety of terminals and plotters used to display and monitor the station data as the tapes are being processed. Much of the time and effort expended on the assembly of the data management system went into the development of the highly specialized application software needed for its operation. The design of the data management center was modeled on the data collection center at the USGS Albuquerque Seismological Laboratory, which is used to process data from the global seismograph network.

The network maintenance center, which is located next to the data management center, is equipped with a complete seismograph system used for testing and training, a wide range of test equipment needed to diagnose and repair electronic boards, and a large stock of spare parts and boards. One program objective has been to make the CDSN as self-sustaining and self-sufficient as possible. This objective has been successfully met; only a few electronic boards and components have been returned to the United States for repair.

Network Deployment

Because of the desire of the Chinese to participate in design and assembly of the CDSN systems, the decision was made to develop and assemble the CDSN systems at the Albuquerque Seismological Laboratory. Most of the equipment for the stations was purchased during 1984. and 1985. Eleven data systems were assembled: nine for the network stations, one for the network maintenance center, and one to remain at the Albuquerque

Laboratory to serve as a test bed for future hardware and software changes. Development of data management system hardware and software was completed in late 1985, and the assembly and testing of the data systems were completed in early 1986.

As equipment was being assembled in the United States, the stations were being prepared in China under the supervision of the Institute of Geophysics. Modifications were needed at most of the stations to support the new equipment. At some stations it was necessary to construct roads and new power facilities; most needed backup generators and air conditioning. Boreholes were drilled and cased at two of the stations to accommodate special borehole seismometers. New facilities were also needed to house the data management and network mainte

nance centers.

Extending the global network into China has long been an important goal of the scientific community.

Because training is the key to successful, self-sustained operation of the network, Chinese engineers and technicians have spent a total of 4 months at Albuquerque in two training programs, and in China there have been training programs for the station operators. The training has been especially effective because of the intense interest and dedication shown by the Chinese in this program.

The first station equipment was installed at Baijatuan in February 1986 to be used as a demonstration and training system. In May 1986, USGS and Chinese personnel began deploying equipment at the remaining eight sites. During an 8week period, the installation team made a complete circuit of China installing the station equipment.

Installation of the data management system took place in February with additional work in June 1986. The first network-day tapes were assembled on

October 1, 1986; this is considered to be the date when the network became operational and at which the one-year trial period began.

Results and Conclusions

The panel of experts that met in China in October 1987 to review the CDSN project expressed their evaluation of the project's success as follows:

The CDSN represents a unique contribution to seismology. It will provide new data of fundamental importance to the study of earthquake activity in China. The data will enable scientists to obtain fundamental new insight in the structure of the crust and upper mantle. Furthermore, the CDSN will be a key contribution from China to the international network of digital seismic stations and will thus be instrumental in a wide variety of studies making use of global seismic data.

Since its inauguration, the CDSN has consistently produced reliable, highquality earthquake data that are being used for a variety of research applications in China, the United States, and many other countries that share the data. Information from the network is providing a new look at the structure and composition of the crust of eastern Asia. It also will be useful in determining the geological evolution of the Tibetan plateau and in studying the collision boundary of the Indian and Asian crustal plates. Yet the establishment of the CDSN is a milestone, not a final objective. Plans are already being made to expand the network into southwest China. Experiments are being planned for telemetering data from the stations to Beijing by satellite, and there are plans to continue updating the technology to insure that the CDSN remains a stateof-the-art scientific facility.

The deployment of the CDSN has been a particularly rewarding experience for those who were fortunate enough to take part in the project. The Chinese, Americans, and one New Zealander worked together to solve a myriad of technical and logistical problems while striving to learn and appreciate each other's customs and cultures-they were successful in both endeavors.

U.S.-Canada Border Mapping Using Digital Cartographic Techniques

By Hedy J. Rossmeissl

An official series of U.S.-Canada border maps was published for the International Boundary Commission (IBC) between 1900 and 1930. The maps were prepared at various scales, depending on the detail and complexity of the information along the border. Because of the age and information content of this original series of maps, the IBC expressed an interest in obtaining a new, updated series of maps.

The U.S. Commission of the IBC provided funding to the U.S. Geological Survey for a cost-share border digital demonstration project. Officials of the USGS and Canada's Department of Energy, Mines and Resources agreed to produce a prototype border map as a derivative product from their established digital quadrangle mapping and data base programs. The IBC specified that graphic products should be produced for the series, that the international boundary line should be the focus of each map in the series, that each boundary monument and its designation should be shown, that the content of the new maps should be at least as detailed as the original boundary maps, and that the new maps should use standard topographic symbols.

Both countries use digital techniques, but they follow different procedures to collect cartographic information. Cartographers from each country addressed the technical challenges of establishing compatible standards, file formats, and coding schemes for the digital data and incorporating map design, symbology, and graphic specifications in order to generate a graphic product.

Each mapping agency collected the digital information for the pilot project from their standard topographic mapping series. For the United States, a 1:24,000-scale topographic map was used; for Canada, a 1:50,000-scale map

was used. The test area chosen for the project covered a part of the border between New York, Ontario, and Quebec near Cornwall, Canada. Because each agency uses different data collection procedures, coding schemes, and data standards, one existing coding scheme was adopted to limit the translation process to only one agency's data set. The Canadian coding scheme was chosen because it accommodated the requirement of IBC to produce a graphic product.

The USGS developed computer programs and translation tables to convert its digital data to the Canadian format. After the translation was complete, the Canadian data set was integrated with the U.S. data set, and the map edges were checked for alignment accuracy between the two sets. The data were then prepared for graphic production and shipped to Canada, where 1:25,000- and 1:50,000-scale prototype maps were produced. The prototype maps were forwarded to the International Boundary Commission in April 1988 and are presently undergoing review.

The project provided the opportunity for both mapping agencies to use data collected from standard digital methods to produce derivative graphic products. Working together, the agencies successfully overcame several technical challenges such as solving data compatibility issues, translating data codes from one scheme to another, and presenting graphic data digitally.

Saudi Arabian Gold
Investigations

By Arthur A. Bookstrom

The continued high price of gold on the international market, in contrast to depressed prices for many other mineral commodities, has prompted the Government of Saudi Arabia to put a high priority on gold exploration and development. Consequently, the USGS Saudi Arabian Mission, which provides support for cooperative U.S.-Saudi Arabian studies, is focusing its current gold investigations on exploration and evaluation of

gold prospects thought to have economic potential, mapping and sampling of mineral belts known to contain gold occurrences, and reconnaissance to identify new prospects and gather preliminary information needed to establish priorities for further work.

The gold deposits of Saudi Arabia are hosted by late Precambrian (more than 570 million years old) rocks of the Arabian Shield in the western part of the Arabian Peninsula (fig. 1). The Arabian and Nubian Shields are large areas along the sides of the Red Sea, where Precambrian rocks are exposed in the eroded core of a broad uplift around the younger Red Sea rift. Sedimentary cover rocks, which host oil fields in the eastern part of the Arabian Peninsula and contain phosphate deposits in northern Saudi Arabia, have been uplifted, tilted back away from the Red Sea rift, and stripped by erosion from the Precambrian basement rocks of the Arabian Shield.

Regional geological work by USGS geologists shows that the Arabian Shield consists of at least five geologically distinct terranes, or microplates, that meet along four suture zones. Gold and copper deposits (some of the copper deposits also contain gold) tend to be concentrated in mineral belts, many of which are within or near these suture zones.

Most known gold occurrences in the Arabian shield show evidences of ancient mining activities (pits, trenches, stopes, dumps, grindstones, slag piles, and former village sites). The largest ancient mine known to date is at Mahd adh Dhahab in the central part of the shield. Gold is present in gold and silver tellurides and as native gold in this mine. Inscriptions on rocks and carbon-14 dating of charcoal in slag indicate mining at about 800 A.D. Older tailings are of unknown age but may date to about 1,000 B.C. Waste dumps indicate that miners of ancient times extracted about 700,000 ounces of gold. From 1939 to 1954, another 700,000 ounces of gold was extracted by the Saudi Arabian Mining Syndicate.

In 1972, a USGS geologist recognized the potential for a hidden orebody south of the existing workings at Mahd adh Dhahab. Four core holes were drilled to test geochemical anomalies in the geologically favorable area. The first