A plan for seismic location calibration of 30 IMS stations in Eastern Asia
In March 2000, a collaborative academic-industry research consortium comprised of five institutions started an integrated series of projects, all with the goal of improving the capability to locate seismic events based on data acquired by International Monitoring System (IMS) stations in Eastern Asia.
The focus of this effort is to develop and deliver validated high-resolution travel time grids for operational use, in support of the location estimates made by the International Data Centre (IDC) of the Comprehensive Test Ban Treaty Organization, for on the order of a hundred events per day at locations around the world. For background information on IMS and IDC, see the paper by Paul G. Richards, Building the Global Seismographic Network for Test Ban Monitoring, EARTHmatters, pp 37 – 40, Fall 1999 (text and graphics, or link to EARTHmatters layout).
In the first project, the Lamont-Doherty Earth Observatory of Columbia University will contribute numerous newly-obtained ground truth locations in Eastern Asia, whose errors are thought to be of the order of five km or better (so-called GT5 events), and that are expected to be large enough for detection at IMS stations, and in most cases recently enough for inclusion in the Reviewed Event Bulletin of the PIDC since 1995. These events in Central Asia, China, Mongolia, Korea, the Indian subcontinent, and Russia, have been located principally on the basis of large numbers of signals recorded by regional networks. Most of these GT5 events will be provided in the first year of work. Lamont personnel on this project include Paul Richards, Won-Young Kim, Vitaly Khalturin, John Armbruster, two research staff yet to be hired, and a graduate student.
In the second project, the University of Wyoming will contribute observed travel times for about 3000 three-component recordings at stations widely deployed in the Soviet era to detect regional waves from 21 nuclear explosions carried out during the Deep Seismic Sounding program. This dataset is an invaluable resource for thorough calibration of major aseismic regions in Russia and Central Asia. We expect to be able to find analog seismograms for several of these 21 nuclear explosions, as recorded at Eastern Asia stations now identified as part of the IMS, or at stations that were operated at sites close to the IMS station locations. Our personnel at the University of Wyoming are Scott Smithson, Elena Morozova, and Igor Morozov.
In the third project, Mission Research Corporation will derive and test travel time surfaces, for IMS stations, that fit the GT data and Calibration Event Bulletin data. The lead person at MRC for us is Mark Fisk.
URS Greiner Woodward Clyde will contribute some ground truth data for India, Nepal, Pakistan, and much 1D and 2D modeling experience; the University of Connecticut will contribute 3D modeling experience. Both these organizations, and Wyoming and Lamont, will work together in Project 4 to provide expected travel times to 30 IMS station locations in Eastern Asia. In this fourth project, detailed studies of a small percentage of our claimed GT5 events will be carried out for purposes of validation of their location quality. The lead person at URS Greiner is Chandan Saikia, and at the University of Connecticut is Vernon Cormier.
Mission Research Corporation will package all of the products of the consortium for delivery to the Center for Monitoring Research in the fifth project, including quantitative evaluation of location improvements.
In the sixth project, an Experts Group Review process will provide overall guidance. In this work, a group of consultants will meet at Lamont for a few days each year, with consortium members, to give advice on GT events, and on how to use phase picks of GT events to provide improved location estimates, for events recorded regionally at IMS stations in East Asia.
Download here a pdf file for the two-day program of the consortium's first "Experts Group Review" meeting, held at Lamont Feb 15 and 16, 2001.
This is a three-year program of work, heavily tilted toward delivery of most GT events and 50% of PNE travel times in the first year, then turning toward modeling and other validation and evaluation efforts in years two and three, as additional GT events continue to accumulate.
Major users of seismic data include:
Although the most basic data in seismology for all these users are seismograms, in practice the great majority of those who work with seismic data do not use seismograms directly. Instead they mostly use data products derived from seismograms.
The most important of these products, are bulletins of seismicity.
In the last 20 years there have been enormous improvements in the quality and quantity of seismograms, associated with the development of broadband feedback sensors and techniques of digital recording to permit high dynamic range across wide bands of frequency.
There is ongoing revolutionary improvement in access to seismogram data, as satellite communications and the internet spread even to remote locations.
It has therefore been frustrating to find that the quality of the principal data product derived from seismograms acquired internationally, the global bulletin of seismicity, has not yet seen the types of radical improvement needed by any of the user communities, 1 through 3, above.
The US Geological Survey (USGS) and the International Seismological Centre (ISC) publish their bulletins months to years in arrears, using volunteered data, and methods of analysis that essentially have not changed for sixty years. These are very useful bulletins, and their quality has greatly improved because of the increased number of reported signal detections.
The Reviewed Event Bulletin (REB) of the CTBT monitoring community, produced since 1995 January 01 by GSETT-3 and the PIDC and now by the IDC in Vienna, is vastly improved over the other global bulletins in its timeliness of publication. However, both the REB location estimates, and the estimates of their uncertainty (error ellipses), require improvement.
It appears that the principal reason for inaccuracies in the REB locations is lack of a sufficiently good model of Earth structure, and specifically of travel time information. It is desirable to calibrate each IMS station so that in effect the location of a new event can be located with reference to another event, whose location is known accurately and which, preferably, is not far from the new event. By using a sufficiently large number of calibration events, whose location is accurately known and whose signals are detected reliably at IMS stations, it is possible to generate a station-based travel time surface (a function of distance and azimuth), for each seismic phase.
As noted in the report of the first Oslo Workshop on IMS Location Calibration (January 1999, which led to the paper CTBT/WGB/TL - 2/18):
Our consortium project will carry out such an approach to calibration for 30 IMS stations.
SUMMARY OF METHOD
The IMS stations in East Asia which are the subject of this project, are listed in Table 1 with station coordinates as originally given in Annex 1 to the CTBT Protocol. Not all of these sites currently have operational IMS stations. However, is some such cases there are non-IMS stations which are operating at or near the IMS site, and in other cases stations have operated in the past, near the IMS site. In general we refer to such non-IMS stations as surrogate stations, and their data can potentially be used to assist in building up the necessary station-based travel-time data set for purposes of obtaining the types of travel-time surface needed at the IDC for every IMS station site.
Table 2 lists our present knowledge of the availability of data at each of the 30 IMS sites in Table 1, whether or not there is an IMS station currently operating at the site. In the case where no IMs station is operating, we list some appropriate surrogate stations.
Our basic approach, will be to acquire lists of reliably located seismic events in Eastern Asia, preferably occurring since the beginning of publication of the REB on January 1, 1995, and large enough to be included in the REB. From such events, preferably of GT5 quality or better, we shall obtain the picked arrival times at IMS stations and thus build up a set of station-based travel-times for events of accurately known location.
Code - IMS: station code listed on Annex 1, CTBT Protocol, September 1996
Phase data - ISC and PIDC (phase data are available from these stations of the ISC and PIDC).
Operators of digital stations, often part of joint programs, are:
Data sources are:
The AWRE operation of GBA has ended, with this station handed over to local operation, but much relevant data for this array is easily available.
For each major set of event locations that we plan to use for IMS station calibration, an extensive validation effort will be made. We plan to do this by acquiring waveform data and phase-pick data for a subset of the events, from as many stations as possible. From such data we shall make our own location estimates, including waveform studies of the depth, in order to validate our conclusions as to the quality of the locations.
For the major aseismic areas of Eastern Asia (for example, for much of the northern part of this region, which is in Russia), such an approach cannot be used. However, we are fortunate in that major reflection/refraction profiles were carried out in this region during the Soviet era, in the Deep Seismic Sounding program (DSS). Table 3 lists the DSS data which will be analysed in our consortium project by the University of Wyoming -- subject to the availability of the RUBY profiles (copies of the other data are in hand, and the RUBY data are likely to be acquired shortly). From such arrival time picks, and searches of the literature, it is possible to generate 2D and 3D regional models and hence travel-times to the IMS station sites.
We note that a sophisticated modeling effort is the only way to set up the required travel time grid for each IMS station, in two important cases: from large aseismic regions; and to IMS station locations where no station or nearby surrogate now exists. But we are well aware that it would be inappropriate for the IDC to rely for its monitoring operations on purely 3D calculations in a 3D model. Therefore we shall make great efforts to search for appropriate validation, to the extent possible, of any predicted effects on travel times caused by 3D structure. It is here that a search for data from old analog stations can play a key role. We are familiar with analog stations all over the former Soviet Union which operated during the period 1965 — 1990 at locations that were selected in 1996 to become the sites of modern instrumentation (IMS); and we plan to track down old data from these stations, and to work with scientists in Russia and Central Asia to analyse them. This work will bring together old analog data at fixed stations, the special DSS field data, modern waveform data, and the full sophistication of modeling in 1D, 2D, and 3D structures.
Download here a pdf file for a paper submitted July 2000 to the New Orleans CTBT monitoring symposium.
In this Lamont colloquium, an effort is made to explain model error, SSSCs, pick error, and cross-correlation for a general audience; and to show what can be achieved (with examples from David Schaff's Stanford Ph. D. thesis) when model error and pick error are minimized.
Your are here: