Monitoring the Comprehensive Nuclear-Test-Ban Treaty: Surface Waves

241 Pages·2001·9.148 MB·English

by  Brian J. Mitchell (auth.), Anatoli L. Levshin, Michael H. Ritzwoller (eds.)

Monitoring
the Comprehensive
Nuclear-Test-Ban Treaty:
Surface Waves
Edited by
Anatoli L. Levshin
Michael H. Ritzwoller
Springer Basel AG
Reprint from Pure and Applied Geophysics
(PAGEOPH), Volume 158 (2001), No. 8
Editors:
Anato1i L. Levshin Michae1 H. Ritzwoller
University of Colorado University of Co10rado
Dep. ofPhysics / Seismology Group Dep. of Physics / Seismology Group
Campus Box 390 Campus Box 390
Boulder, Colorado 80309-0390 Boulder, Colorado 80309-0390
USA USA
e-mail: [email protected] e-mail: [email protected]
A CIP catalogue record for this book is available from the Library of Congress,
Washington D.C., USA
Deuische Bibliothek Cataloging-in-Publication Data
Monitoring the comprehensive nuclear test ban treaty. -Base1 ; Boston; Berlin : Birkhăuser
(Pageoph topica1 vo1umes)
Surface waves / ed. by Anatoli L. Levshin; Michael H. Ritzwoller. -2001
ISBN 978-3-7643-6551-6 ISBN 978-3-0348-8264-4 (eBook)
DOI 10.1007/978-3-0348-8264-4
This work is subject to copyright. All rights are reserved, whether the whole or part of
the material is concemed, specifically the rights of translation, reprinting, re-use of
illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and
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obtained.
© 2001 Springer BaseI AG
Originally published by Birkhauser Verlag BaseI, Switzerland in 2001
Printed on acid-free paper produced from chlorine-free pulp
987654321
Contents
1339 Prefaee: Monitoring the Comprehensive Nuc1ear-Test-Ban Treaty
B. J. Milchell
1341 Introduetion
M. H. Ritzwoller, A. L. Levshin
Surfaee Wave Tomography: Methods and Resu1ts
1351 A Fast and Re1iab1e Method for Surfaee Wave Tomography
M. P. Barmin, M. H. Rilzwoller, A. L. Levshin
1377 Global Models of Surfaee Wave Group Veloeity
E. W. F. Larson, G. Ekströin
1401 Optimization of Cell Parameterizations for Tomographie Inverse Problems
W. Spakman, H. Bijwaard
1425 Surfaee Wave Veloeities Aeross Arabia
T. A. Mokhtar, C. J. Ammon, R. B. Herrmann, H. A. A. Ghalib
1445 A Surfaee Wave Dispersion Study of the Middle East and North Afriea for
Monitoring the Comprehensive Nuc1ear-Test-Ban Treaty
M. E. Pasyanos, W. R. Walter, S. E. Hazler
1475 One-dimensiona1 Shear Veloeity Strueture ofNorthern Afriea from Rayleigh
Wave Group Veloeity Dispersion
S. E. Hazler, A. F. Sheehan, D. E. McNamara, W. R. Walter
Surfaee Wave Identifieation, Measurement, and Souree Charaeterizations
1497 Isotropie and Nonisotropie Components of Earthquakes and Nuc1ear
Explosions on the Lop Nor Test Site, China
B. G. Bukchin, A. Z. Mostinsky, A. A. Egorkin, A. L. Levshin, M. H. Ritzwoller
1517 Theoretieal and Observed Depth Correetion for Ms
M. Herak, G. F. Panza, G. Costa
1531 Automated Detection, Extraction, and Measurement of Regional Surface
Waves
A. L. Levshin, M. H. Ritzwoller
1547 Optimization of Surface Wave Identification and Measurement
J. L. Stevens, K. L. McLaughlin
© Birkhäuser Verlag, Basel, 2001
Pure appl. geophys. 158 (2001) 1339-1340 I
0033-4553/01/081339-02 $ 1.50 + 0.20/0 Pure and Applied Geophysics
Monitoring the Comprehensive Nuclear-Test-Ban Treaty
Preface
The first nuclear bomb was detonated in 1945, thus ushering in the nuclear age. A
few political leaders quickly saw a need to limit nuclear weapons through
international co operation and the first proposals to do so were made later in that
same year. The issue of nuclear testing, however, was not formally addressed until
1958 when the United States, the United Kingdom, and the Soviet Union, initiated
talks intended to establish a total ban on that testing (a Comprehensive Test-Ban
Treaty or CTBT). Those talks ended unsuccessfully, ostensibly because the
participants could not agree on the issue of on-site verification.
Less comprehensive treaties did, however, place some constraints on nuclear
testing. The Uni ted States, the United Kingdom, and the Soviet Union, in 1963,
negotiated the Limited Test-Ban Treaty (L TBT) which prohibited nuclear explosions
in the atmosphere, outer space and under water. The Threshold Test-Ban Treaty
(TTBT), signed by the Uni ted States and the Soviet Union in 1974, limited the size,
or yield, of explosions permitted in nuclear tests to 150 kilotons.
Seismological observations played an important role in monitoring compliance
with those treaties. Many of the world's seismologists set aside other research
projects and contributed to that effort. They devised new techniques and made
important discoveries ab out the Earth's properties that enhance our ability to detect
nuclear events, to determine their yield, and to distinguish them from earthquakes.
Seismologists are rightfully pro ud of their success in developing methods for
monitoring compliance with the LTBT and TTBT.
Although seismologists have also worked for many years on research related to
CTBT monitoring, events of recent years have caused them to redouble their efforts
in that area. Between 1992 and 1996 Russia, France and the United States all placed
moratoria on their nuclear testing, though France did carry out a few tests at the end
of this period. In addition, the United States decided to use means other than testing
to ensure the safety and reliability of its nuclear arsenal, and all three countries, as
weil as the United Kingdom, agreed to continue moratoria as long as no other
country tested. Those developments, as weil as diplomatic efforts by many nations,
led to the renewal of multilateral talks on a CTBT that began in January 1994. The
talks led to the Comprehensive Nuclear-Test-Ban Treaty. It was adopted by the
United Nations General Assembly on 10 September 1996 and has since been signed
1340 Brian J. Mitchel1 Pure appl. geophys.,
by 160 nations. Entry of the treaty into force, however, is still uncertain since it
requires ratification by all 44 nations that have some nuclear capability and, as of
30 April 2001, only 31 of those nations have done so.
Although entry ofthe CTBT into force is still uncertain, seismologists and scientists
in related fields, such as radionuclides, have proceeded with new research on issues
relevant to monitoring compliance with it. Results ofmuch ofthat research may be used
by the International Monitoring System, headquartered in Vienna, and by several
national centers and individual institutions, to monitor compliance with the CTBT.
New issues associated with CTBT monitoring in the 21st century have presented
scientists with many new challenges. They must now be able to effectively monitor
compliance by several countries that have not previously been nuclear powers. Effective
monitoring requires that we able to detect and locate substantially smaller nuclear
events than ever before and to distinguish them from small earthquakes and other types
of explosions. We must have those capabilities in regions that are seismically active and
geologically complex, and where seismic waves might not propagate efficiently.
Major research issues that have emerged for monitoring a CTBT are the precise
location of events, and discrimination between nuclear explosions, earthquakes, and
chemical explosions, even when those events are relatively smalI. These issues further
require that we understand how seismic waves propagate in the solid Earth, the
oceans and atmosphere, especially in regions that are structurally complex, where
waves undergo scattering and, perhaps, a high degree of absorption. In addition, we
must understand how processes occurring at the sources of explosions and
earthquakes manifest themselves in recordings of ground motion.
Monitoring a CTBT has required, and will continue to require, the best efforts of
some the world's best seismologists. They, with few exceptions, believe that methods
and facilities that are currently in place will provide an effective means for
monitoring a CTBT. Moreover, they expect that continuing improvements in those
methods and facilities will make verification even more effective in the future. This
topical series on several aspects of CTBT monitoring is intended to inform readers of
the breadth of the CTBT research program, and of the significant progress that has
been made toward effectively monitoring compliance with the CTBT.
The following set of papers, edited by Drs. A.L. Levshin and M.H. Ritzwoller
presents research results on surface waves that are applicable for monitoring a
CTBT. It is the fourth of eight topics addressed by this important series on
Monitoring the Comprehensive Nuclear-Test-Ban Treaty. Previously published topics
are Source Location, Hydroacoustics, and Regional Wave Propagation and Crustal
Structure. The topics to appear in ensuing issues are Source Processes and Explosion
Yield Estimation, Infrasound, Data Processing, and Source Discrimination.
Brian J. Mitchell
Saint Louis University
Series Editor
© Birkhäuser Verlag, Basel, 2001
Pure appl. geophys. 158 (2001) 1341-1348 I
0033-4553/01/081341-8 $ 1.50 + 0.20/0 Pure and Applied Geophysics
Introduction
MICHAEL H. RITZWOLLER1 and ANATOLI L. LEVSHIN1
This is one of several volumes planned to appear in Pure and Applied Geophysics
covering a range of topics related to monitoring the Comprehensive Nuclear-Test
Ban Treaty (CTBT). This volume concentrates on the measurement and use of
surface waves and the papers fall into two general categories: The development and/
or application of methods to summarize information in surface waves (e.g., surface
wave tomography) or the use of these summaries to improve capabilities to monitor
and verify the CTBT by advancing the art of surface-wave identification, measure
ment, and source characterization. Because of the emphasis here on a type of wave
rather than on a specific application, the papers in this volume overlap those in the
other volumes appreciably. Readers interested in the application of surface waves are
encouraged also to investigate the contents of the other volumes, after thoroughly
digesting the results in this volume, of course.
Surface waves compose the longest and largest amplitude parts of broadband
seismic waveforms generated both by explosions and shallow earthquakes. In
addition, they contain most of the low frequency information ra dia ted by seismic
sources. Measurements of the properties of surface waves have been important for
evaluating source mechanisms, estimating yields, and helping to discriminate nuclear
explosions from naturally occurring earthquakes, and have been widely used by
national and international organizations charged with monitoring and verifying
various nuclear test treaties. Under the CTBT, concentration has shifted from
teleseismic monitoring of a threshold yield targeted on a few well-defined locations to
identifying and characterizing signals from weak nuclear explosions and earthquakes
using potentially very noisy and incomplete regional data following events that may
be distributed widely in space. Concentration is no longer on yield estimation, but
rather on being able to discriminate explosions from naturally occurring earthquakes
and to locate small events using sparse regional networks in complex tectonic
environments with the accuracy and precision demanded by the CTBT.
1 Center for Imaging the Earth's Interior, Department of Physics, University of Colorado at Boulder,
Boulder, Colorado, 80309-0390, U.S.A.
E-mail: [email protected], [email protected]
1342 Michael H. Ritzwol1er and Anatoli L. Levshin Pure appl. geophys.,
Within the context of the CTBT, the use and interpretation of information from
surface waves has grown in significance. There are two general uses of surface waves
under the CTBT. First, the comparison of the amplitudes of surface waves and body
waves remains the most reliable regional discriminant, an example of which is the
well known mb : Ms discriminant (e.g., STEVENS and DAY, 1985). Second, broadband
surface-wave dispersion provides important information used in estimating 3-D
seismic models of the crust and uppermost mantle which are necessary to obtain
accurate locations of small events for which only regional data may be available. The
success of both applications depends on obtaining reliable surface-wave dispersion
measurements and representing these measurements in a useful form, usually as
group- or phase-velocity maps.
The measurement of the group velocity of Rayleigh and Love waves is performed
on the envelope of the surface-wave packet and can be robustly measured across a
broad frequency band, from several seconds to hundreds of seconds period (e.g.,
DZIEWONSKI et al., 1969; LEVSHIN et al., 1972; CARA, 1973; KODE RA et al., 1976;
RUSSELL et al., 1988; LEVSHIN et al., 1989, 1992; RITZWOLLER et al., 1995). Arecent
experiment by a number of research groups in the V.S. revealed general agreement
among the various methods and codes used to measure group velocities (WALTER and
RITZWOLLER, 1998). Phase-velocity measurements are typically obtained by wave
form fitting (e.g., WOODHOUSE and DZIEWONSKI, 1984) or by differencing phase
spectra obtained at adjacent stations or from nearby events. There are three key
reasons why group velocities have been considered more useful in nuclear monitoring
than phase velocities. First, absolute phase-ve1ocity measurements are strongly
affected by initial source phase (e.g., KNOPOFF and SCHWAB, 1968; MUYZERT and
SNIEDER, 1996), which may be poorly known or completely unknown for small
events. Group velocities are much less sensitive to source characteristics (e.g.,
LEVSHIN et al., 1999). Second, phase velocities are difficult to measure unambigu
ously below about 30 s period. Finally, although multi-station and multi-event
differential phase measurements are largely unaffected by source phase, they are
typically too sparsely distributed to be of general use in constructing tomographie
maps. With a few notable exceptions surface-wave data processing for use in nuclear
monitoring has concentrated on estimating velocities rather than wave amplitudes,
polarizations, or scattering. If the emphasis on constructing 3-D models to improve
regionallocation capabilities continues, it is likely that a larger share of future efforts
will be devoted to short-period phase-velocity estimation and the use of more
complicated wavefield effects to constrain 3-D models, such as polarization anomalies
(e.g., LEVSHIN et al., 1994; LASKE, 1995) and scattering (e.g., POLLITZ, 1994).
The estimation of dispersion maps by tomography (e.g., DITMAR and YANOVS
KAYA, 1987; YANOVSKAYA and DITMAR, 1990) is now commonplace and new methods
such as kriging (e.g., SCHULTZ et al., 1998) have emerged. Dispersion maps on a
variety of scales have appeared in the last several years. For example, there are global
phase-velo city maps (e.g., LASKE and MASTERS, 1996; TRAMPERT and WOOD-
Vol. 158,2001 Introduction 1343
HOUSE, 1996; ZHANG and LAY, 1996; EKSTRÖM et al., 1997; VAN DER HEUST and
WOODHOUSE, 1999) as weIl as regional studies aeross Eurasia (e.g., Wu et al., 1997;
CURTIS et al., 1998; GRIOT et al., 1998; RITZWOLLER and LEVSHIN, 1998; RITZWOLLER
et al., 1998; YANOVSKAYA and ANTONOVA, 2000) and elsewhere (e.g., Antaretiea:
VDOVIN, 1999; South Ameriea: VDOVIN et al., 1999; Aretie: LEVSHIN et al., 2001).
Two papers in this volume deseribe the applieation of surfaee-wave tomography to
regions of interest for monitoring the CTBT. Pasyanos, Walter, and Hazler present a
study of the Middle East, North Afriea, southern Eurasia and the Mediterranean
using Rayleigh and Love waves at periods ranging from 10 s to 60 s. Mokhtar,
Ammon, Herrmann, and Ghalib present a tomographie inversion of Rayleigh and Love
group veloeities aeross the Arabian peninsula in the period range of 5~60 s. These and
other observational efforts exemplify the advanees that are emerging as data sets
aeeumulate and, in partieular, as the frequeney band of observation lowers.
Advanees in surfaee wave methodology eontinue to emerge both on regional (e.g.,
STEVENS and McLAUGHLIN, 1997) and global seales (e.g., WANG and DAHLEN, 1995;
WANG et al., 1998). In this volume, Barmin, Ritzwoller, and Levshin diseuss a
tomographie method for eonstrueting both isotropie and azimuthaIly anisotropie
surfaee-wave maps. Although their algorithm is based on a regular grid, it extends
naturaIly to irregular grids, and reeent advanees in the eonstruetion and use of
irregular grids in tomography (e.g., SAM BRIDGE et al., 1995; SPAKMAN and BUWAARD,
1998) are now being exploited in surfaee-wave tomography, as deseribed here by
Spakman and Bijwaard. Irregular grids are most useful when the spatial distribution of
data is inhomogenous, as is eommon in regional surfaee-wave tomography. Also in
this volume, Larson and Eksträm show that at periods above about 50 s group veloeity
maps eonstrueted direetly with regional tomography agree weIl with those eomputed
from phase-veloeity maps whieh were themselves eonstrueted globaIly. Thus,
information from disparate data types appears to provide eonsistent eonstraints on
the 3-D strueture of the earth. Other researehers have demonstrated that broadband
group- and phase-veloeity maps ean be simultaneously inverted for 3-D strueture on
both regional (e.g., VILLASENOR et al., 2001) and global seales (e.g., Stevens and
M cLaughlin in this volume). In addition, VILLASENOR et al. (2001) established that the
resulting model of the mantle agrees weIl with a reeent model eonstrueted with
teleseismie body wave travel times (e.g., SPAKMAN and BUWAARD, 1998).
Tomographie maps have four prineipal applieations: to detect and extract surface
waves from noisy records, to help discriminate nuclear explosions from other sources
of seismie energy, to characterize sources, and to be used as data in inversion for the
shear-veloeity strueture of the erust and uppermost mantle.
First, the focus of the CTBT on smaIl events makes the detection of seismic
signals and the extraction of useful information a crucial task. The detection and
extraction of surface waves is facilitated by using phase-matched filters (e.g., HERRIN
and GOFORTH, 1977; HERRMANN and RUSSELL, 1990), whieh are designed to
compensate for the dispersion of the surfaee wave-train. In this volume, Levshin and