Click for next page ( 58


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 57
Appendix C A Revitalization of Regional Seismic Networks: Implementation Strategies David W. Simpson Lamont-Doherty Geological Observatory Palisades, NY 10964 September 1988 INTRODUCTION The decision of the U.S. Nuclear Regulatory Commission to phase out its support of regional networks in the eastern United States, and to support the establishment of a National Seismic Network by the USGS, has led to both a short-term crisis for the support of regional earthquake studies in the East and a long-term opportunity for revitalization of regional network data gathering. It is not the purpose here to make the case for the breadth of scientific or practical opportunities provided by regional networks. Various documents from IRIS, the National Academy of Sciences and the USGS have described in detail the range of new opportunities that can be explored with data from modern seismographic instrumentation. A group led by Tom Heaton is developing a Science Plan for a National Seismic System, which concen- trates on the specific contributions that can be made by networks on a regional scale. From discussions in various forums over the past two years, the follow- ing have emerged as some of the main areas of consensus on the develop- ments necessary to improve the state of regional seismology in the United States: . The U.S. National Seismic Network (US~SN) provides, both in con- cept and implementation, a model that has wide ranging implications for regional seismic studies. Broadband waveform data from the network will be used in the analysis of regional earthquakes. The satellite communica- 57

OCR for page 57
58 APPENDIX C tion system that forms the backbone of the USNSN will have capacity to handle regional network data as well. . Even with the establishment of an enhanced National Seismic Net- work, there will continue to be an essential role for regional-scale networks in monitoring low-magnitude seismicity and defining earthquake patterns with a scale and resolution that are geologically significant. ∑ If regional seismology is to survive, both financially and intellectu- ally, major changes are required. The financial future depends on decreas- ing operating costs and establishing new funding sources, especially at the state level, for stable support of operation. The intellectual future depends on making major changes in the way regional networks are operated in order to provide data that are sufficient to meet the challenges of new interests and techniques in analysis of the complete seismogram. ∑ There continues to be a wealth of opportunity for research in seismol- ogy at the regional scale. Fundamental problems in geotectonics and earth- quake prediction remain to be solved. There is increasing interest in monitoring nuclear test ban treaties with seismic stations at regional distances. ∑ There are no technical limitations to acquiring data that satisfy current and anticipated research needs. Broadband sensors with digital conditioning and telemetry are now capable of reproducing earthquake motions over the complete amplitude and spectral range of interest in regional seismology. ∑ A reassessment of the mode in which regional network data are processed, archived, and distributed should be carried out in concert with any major changes in field equipment for data collection. A funding strategy should be developed that provides for the capitali- zation of new equipment for regional networks, stabilizes the long-term support for routine network operation (maintenance, processing, and catalog generation) and encourages the growth of funding for research based on data from an improved and integrated national/regional network system. REGIONAL NETWORKSóCAPABILITIES AND LIMITATIONS Regional telemetered networks, of the type now in operation in many parts of the United States, were originally installed with the primary pur- pose of locating large numbers of small-magnitude events as a mapping tool in defining active faults and in determining modes of deformation based on fault plane solutions. As an increasingly detailed picture of seismicity along major active zones emerged, it has become obvious that regional seismic networks are also important tools in monitoring intermediate-term regional strain (through changes in seismicity). Deviations from the stable background seismicity, both in space and time, are extremely important in unraveling the details of regional tectonic processes and may play an essen- tial role (through the detection of foreshocks and other short-term changes

OCR for page 57
APPENDIX C in seismicity) in earthquake prediction. Both the magnitude range (m 59 3) and spatial resolution necessary to observe these processes demand moni- toring on a regional scale with a station density that is beyond the scope of a national network. The use of networks telemetering relatively low-dynamic-range, narrow- bandwidth data to a central recording facility has served well the purpose of studying the spatial and temporal distribution of seismicity. The low cost per station has allowed for a relatively large number of sites. The high magnification at high frequencies and recording at a central station with common time base have allowed for accurate timing of body phases. Both of these factors a dense network and common timing are necessary to provide accurate location and a sensitive detection threshold in seismicity studies. There have been trade-offs, however. Inherent in the low cost and the analog telemetry available in the 1960s, when these systems were origi- nally designed, are severe restrictions on dynamic range (less than 50 dB) and bandwidth (1-20 Hz). The major factor limiting the quality of data recorded is the continuous analog telemetry, often by telephone. With rising telemetry costs, the maintenance of this weakest link in the system has also become one of the major costs in operating the networks. At the same time, techniques for the analysis of seismic data have developed to the level where the quality of the data obtained now lags behind the expectations of those wishing to use them. Broader bandwidth and higher dynamic range are required for a wide variety of new studies in earthquake source mechanics, seismic wave propagation, and structure of the earth. THE U.S. NATIONAL SEISMIC NETWORK The USNSN was designed to locate earthquakes above magnitude 2.5 throughout the United States to serve the monitoring purposes of the Na- ~ional Earthquake Information Center (NEIC) and the Nuclear Regulatory Commission (NRC). As such its primary goals are to provide relatively uniform coverage throughout the United States and rapid data telemetry to a central facility in Golden for near-real-time hypocenter determination. As a monitor of seismicity at the national level, the USNSN will provide for the consistent and stable location of earthquakes above magnitude 2.5, both serving the immediate reporting purpose of the NEIC and providing the foundation for eventually producing a long-term stable catalog of U.S. earthquakes, which will have broad application in studies of seismicity and hazard assessment. As a facility for collecting and distributing waveform data from a conti- nent-scale array of first-order broadband stations, the USNSN will make it possible to carry out studies of earthquake sources, wave propagation at regional distances and the structure of the continental lithosphere and earth's . . deep Interior.

OCR for page 57
60 APPENDIX C As a national communication network for seismological data, the USNSN holds perhaps the greatest potential for improving regional earthquake stud- ies. The USNSN satellite telemetry system can provide a low-cost means of collecting and distributing a significant component of regional network data. The existence of a national system for data distribution will also help to encourage the interchange of data among various networks. Details of the instrumentation and organization of the USNSN are de- scribed in various documents from the Branch of Global Seismology. For the purposes of the discussion here, the following points are highlighted: ∑ The USNSN instrumentation provides data adequate for full wave- form analysis of significant regional, national and global earthquakes (i.e. On-scale recording of three-component ground motion in the bandwidth 100 s-30 Hz). ∑ The density of stations within the USNSN (relatively uniform spacing of 110 stations over the continental United States) is a reasonable compro- mise between cost and quality for monitoring on a national scale. The USNSN, through the NEIC, will have the facility for rapid location of significant earthquakes. . The USGS will be responsible for the archiving and timely distribu- tion of all USNSN- data. ∑ The USNSN will have the facility to provide near-real-time access to waveform data of interest in regional earthquake studies. The USNSN telemetry system will have a capacity sufficient to handle significant amounts of regional network data. . NATIONAL-REGIONAL NETWORK INTERACTIONS The USNSN will provide data for a variety of new approaches to study- ing earthquake sources and seismic wave propagation that will have direct application to problems on a regional scale. While it may meet some of the current requirements placed on regional networks, it will not replace the key characteristic of regional networks, namely the close station spacing and thus the capability for high spatial resolution of earthquake hypocenters. The USNSN must be seen, therefore, as a complement to the current activi- ties of regional networks and not a replacement. Centers for regional seismic studies, with regional networks as an impor- tant component, will continue to play an important role in providing a complement to a national program to: ∑ provide denser coverage to focus on areas of special interest (higher seismicity, significant tectonic problems, higher seismic risk, critical engi- neering structures); ∑ act as a regional center for coordinating response to earthquakes and interaction with the public on questions of regional seismicity;

OCR for page 57
APPENDIX C 61 provide a regional focus for seismicity studies and broad-based seis- mological research; and ∑ provide for the cataloging, archiving, and distribution of data on a regional basis. In terms of network operation, the primary goals of an integration be- tween the USNSN and regional networks should be to: ∑ remove the special barriers inherent in the current collection of earthquake data, allowing data with appropriate bandwidth to be applied to regional, national, and global earthquake studies; make use of modern digital signal conditioning, processing, and te- lemetry to decrease the cost of data collection and at the same time improve data quality; and ∑ develop a system of data collection and distribution that allows near- real-time access to data at regional centers and coordinate the access to all levels of network data by all interested users. The design route that the USGS has taken has involved a number of decisions that result in a system that has potential for development well beyond the original monitoring purpose of the network. This applies not only in the inherent capabilities of the network itself, but especially in the area of interaction of the USNSN with regional networks. The network design throughout involves digital signal conditioning and telemetry of broadband three-component ground motion, providing high-quality data suitable for many research applications on regional, national, and teleseismic earthquakes. The satellite telemetry systems provide a vehicle for national communica- tion of seismic data, which has the capacity to extend well beyond the demands of the USNSN itself. - Beyond the complementary roles that they can play in data collection, the importance of regional and national networks in stimulating and focus- ing research on problems of different scale is also important. While argu- ments of scale and efficiency might be used to make the case for centralized control of a complete national recording system, the reality is that local participation in data gathering is essential in the stimulation of research on regional problems. A regional focus in seismicity studies is also important for increasing public awareness of earthquake problems and in interactions with the public and news media following felt earthquakes. DATA REQUIREMENTS FOR REGIONAL NETWORKS The fundamental requirements for regional networks are the same as those for a national network. As a monitoring tool: near-real-time access to arrival times and ampli- tudes from all stations As a research tool: full waveform recording of all events of interest

OCR for page 57
62 APPENDIX C The primary difference is in the number of stations required; regional studies require a station density that is at least an order of magnitude higher than for national monitoring. To instrument regional networks to the re- quired density entirely with stations of USNSN quality is financially impractical. Thus, there is the need to develop a less expensive means of providing the special data needed to satisfy the additional requirements of regional net- works. Some compromises are required. Regional studies concentrate on data from earthquakes at relatively short distances and often from source zones that are known a priori. Therefore, the frequency range of interest is higher and narrower than for a national network and the distribution of stations need not be uniform, but can concentrate on regions of known seismicity. In most networks, sufficient experience has been gained with the character of local seismograms to allow for considerable automation in the identifica- tion of events and extraction of parameters (arrival times, amplitudes etc.~. Thus, at the lowest level, it may be possible to develop one class of regional stations which provide only limited parameter data and short waveform segments from events. One model for how a national and regional network might interact is shown in Figure C1. Types of data and telemetry links are summarized in Figure C2 and Table C1. Within a given region, the national network produces data from a relatively small number of broadband first-order sta- tions, and the national network satellite link provides real-time telemetry to the national center and back to the regional center. In those areas where station density and communication links make it feasible, regional nodes (in some cases co-located with a national network station) gather data from a dense cluster and use the same USNSN satellite telemetry link back to the regional center. Regional network nodes would be capable of automatic event detection, parameter determination, hypocenter location, coordination of communication protocol, and backup recording. These nodes might be located at cooperating institutions (e.g., a local college), providing support for maintenance and local recording. For most networks, satellite telemetry might provide the only data con- tinuously received at the regional network center. This data stream would consist or: continuous long-period data, broadband events, continuous short-period monitor plus from each USNSN station within the region short-period events, derived parameters, selected continuous monitors from all nodes in areas of high . station density

OCR for page 57
APPENDIX C National Network Station I NATIONAL 1 I CENTER I 1 __ ~ At/ I'd,': / 'A ~ /\ National Network Stations and Regional Clusters ~ //f \ / 1 // \\ 63 / Regional Network Alit '~ Node // \ ~ "Smart" Regional Stations STATIONS ,5b National Network Broadband 3-component Regional Network ShorVintermediate Period 3-comp (or vertical only) Regional NehNork Node COMMUNICATIONS Satellite - hNo-way Dial up or computer net I ~ Radio or microwave REGIONAL I CENTER I Figure C1. An integrated national/regional network system. National network sat- ellite telemetry link provides telemetry of all event data and some continuous short- period data from regional clusters to regional centers in near real time. Regional centers have access to data from all stations via dial-up or computer network. These data sources would form the backbone of the regional monitoring system. Additional "smart" regional stations would be located where re- quired to provide the necessary station density. Since the backbone network is intended to provide the basic monitoring, these additional remote stations need not have continuous telemetry. They would be provided with suffi- cient intelligence to detect and store event parameters and waveforms, to be regularly or automatically accessed, decreasing communication costs. After implementation of the USNSN, the major components of the sys- tem that need development are the hardware for regional nodes and smart stations (both processors and communication) and the software to control the data flow. Many of the concepts for the smart station and regional node hardware are in various stages of development by regional network groups. Sufficient experience should now be available to set specific guidelines for the development of both node and smart station processors that would be acceptable by most networks. An immediate task for regional network operators should be to initiate a concrete plan for development of these components.

OCR for page 57
64 NATIONAL NETWORK SATELLITE TELEMETRY National Network Station data: Continuous bng period Broadband event waveforms Continuous short period monitor Regional Network Node data: PamrT eternal Short period event data Additional derived data (QED etc) APPENDIX C ~ . National Net REGIONAL Processor ~ NETWORK ~ ~ NODE Telemetry \ - W Ned meter data (P,S, Amp etc) Bed waveform data ) Regional ~ r REGIONAL \ : Network Node ~ ~ Continuous Pi NETWORK Dial-up Processor \~t Short Period J g~CENTER ~ TO backup \ ~~& I\ p Processor DO (Derived parameters:/ ~ 1-comp short period Short period evens J .~. 3-comp short period .5b 3-comp broadband | 45b 3-comp strong motion ~P~-J'~1 "SMART" REGIONAL STATION Figure C2. Data sources and telemetry for regional seismic networks. RECOMMENDATIONS USNSN Deployment ~ . In those parts of the country where regional networks now exist, the development of an integrated national/regional network system will depend on close coordination between regional network operators and the USGS. ∑ Continue development of a plan for siting USNSN stations. Take into consideration possible sites for regional network nodes in choosing USNSN station locations. Involve regional network operators in site selection, installation and maintenance of USNSN stations.

OCR for page 57
APPENDIX C Table C1. National Seismic Sytem: Data Sources 65 US National Seismic Network 100+ stations at 100-300 km spacing Long Penod High Frequency Broadband Short Penod Monitor QED Regional Clusters lO's of stations per cluster at ~10 km spacing Multiple clusters per network Short Penod Events Short Penod Monitor Parameter Data Short Penod Monitor "Smart" Regional Stations Remote dial-up stations As required to fill gaps in regional nets 1 sps 0.3 hz 80 30 40 13.3 100+ sps 50 hz 13.3 5 Short Penod Events 100+ sps 50+ hz events Parameter Data USNSN satellite telemetry - continuous near-real-time to Regional Centers - possible broadcast mode Eventual goal - All event waveforms recorded from three component stations - Digital signal conditioning from seismometer to recorder - Two-way communication and automatic polling of remote stations Parameter Data - Single station - phase times, arnpliudes, duration, first motion etc Clusters - -ditto- plus preliminary event location State of health, system parameters continuous strong events events continuous event parameter data, ~ , Satellite telemetry events continuous ~ Satellite, dial-up &/or tape continuous Local helicorder dial-up, packet radio Implementation Experiment To a large extent the use of the USNSN telemetry for regional network data is more an experiment in data communication than seismology. Accelerate the initial deployment of the USNSN by providing satellite telemetry to universities or regional networks with existing broadband sta- tions to start experimentation with telemetry and data collection. Start with those networks that have regional node configurations to experiment with the concept of national/regional network integration. Develop specific protocols for interaction between the USNSN, re- gional networks, and other university groups.

OCR for page 57
66 APPENDIX C Hardware Development New station and processor hardware for regional networks will be re- quired to see the development of a completely integrated National Seismic System. ∑ Network operators and engineers should meet to draw up specifica- tions of additional hardware components required to complete the integra- tion of a national/regional network. ∑ Cost and integration with existing new development programs (USNSN and IRIS) should be major design factors. ∑ One or two groups should be identified to lead a national project in the development of these systems. ∑ Special funding should be found to support this development. Software Development The transition to a new system of data collection provides the opportu- nity to carefully reevaluate the ways data are processed, archived, and dis- tributed. Considerable standardization in computers for data processing already exists between regional networks (primarily Unix). Data collection and distribution through the USNSN satellite link will impose one level of standardization in data formats and communication. ∑ Serious consideration should be given to the advantages of developing standard software for the initial processing and cataloging of regional net- work data (i.e., below the "research" level) to simplify data exchange and improve quality control. ∑ One or two groups should be identified to lead a national project in the development of new software. ∑ Special funding should be found to support this development. Funding Strategy A new National Seismic System, combining a national network with regional programs, will require capitalization beyond that available from . . existing programs. A Science Plan should continue to be developed that clearly identifies the unique contributions of regional networks and is aimed at those agen- cies that can benefit from regional studies. The Committee on Seismology's Panel on Regional Networks should prepare a realistic profile of the funding required to establish an integrated National Seismic System of national and regional networks.

OCR for page 57
APPENDIX C 67 The funding profile should be broken down into capital investment and ongoing operational costs. Consideration should be given to what fund- ing sources are appropriate for each of these areas. A specific short-term plan should be developed for the capitalization of the new hardware required for regional networks. Efforts should be made to rapidly decrease recurring telemetry costs and to apply the savings to capitalization of modern telemetry equipment. A long-term plan should be developed for continued operation of regional networks. Special emphasis should be given to means of stimulating state funding for operational support for monitoring of regional seismicity. Strategies should be developed to take advantage of the improved data that will be available to generate new initiatives for support of research in regional seismology.

OCR for page 57