The Comprehensive Nuclear Test Ban

Treaty (CTBT), opened for signature in

1996, has now been signed by 165

countries and in the words of then UN Secre-

tary General Boutros Boutros-Ghali, it is “a

major milestone on the road to making the

world a safer and more secure place for gener-

ations to come”. The treaty is the culmination

of 40 years of negotiations, characterized by

political and scientific concerns over whether it

could be adequately verified. From the start,

most scientific questions centred on seismology

– vital for verifying a treaty which bans under-

ground nuclear explosions, the predominant

type of nuclear test. This meeting, held in

London on 7–8 February 2002, addressed both

the scientific issues behind the Treaty and the

scientific advances that have resulted.

The first day of the meeting reviewed the pio-

neering research that has allowed a verifiable

CTBT, emphasizing the work of UK seismolo-

gists and particularly those of the Atomic

Weapons Establishment (AWE) at Blacknest,

Berkshire. The Blacknest group was formed

after the initial Geneva 1958 discussions on

nuclear test limitation, to work on “forensic

seismology” – seismology applied to the verifi-

cation of arms control treaties. This small

group has produced significant advances and

published work in this field out of all propor-

tion to its size. Prof. Alan Douglas led the

group from 1982 to his “retirement” in 2001,

and the meeting recognized his considerable

contributions to forensic seismology over

nearly 40 years. Peter Marshall OBE, Alan’s

successor and chief seismological adviser to the

UK side during the treaty negotiations, intro-

duced the meeting on the first day.

R Blandford (AFTAC, USA) gave an Ameri-

can perspective and noted the different, but

complementary approaches US and UK seis-

mologists have taken in solving treaty verifi-

cation problems. He suggested that the most

significant AWE contribution was the recogni-

tion that explosion yield estimates from seis-

mic P waves are influenced by zones of high

and low attenuation near source and receiver,

and related proving studies. These results,

indicating high attenuation of P waves propa-

gating to and from the area of the Nevada

Test Site, were fiercely debated in the 1970s as

they suggested that the US was overestimating

the yield of Soviet tests, but later verified by

jointly conducted experiments at the US and

Soviet test sites.

Seismogram modelling

UK seismologists have long promoted seismo-

gram modelling as an aid to interpretation

and understanding of seismograms in terms of

source processes and wave propagation

through attenuative, inhomogeneous media.

J Hudson (Cambridge) demonstrated how

studies of seismic scattering had developed

from early work on modelling the effects of a

deep river valley near the Eskdalemuir array

to, recently, explanation of the effects of an

explosion cavity on surface-reflected body

waves. The modelling theme was extended by

R Pearce (CTBTO) who showed how a search

for a systematic way of explaining observed

amplitudes of direct P and surface reflected pP

and sP waves led to a powerful method for

seismic source mechanism determination from

only a few well-recorded seismograms. Pearce

showed how negative evidence of pP and sP

may be the most valuable in determining the

source mechanism, and how discrimination

between earthquakes and anomalous artificial

events, such as closely separated explosions, is

possible. Comparison of body and surface

wave magnitudes, mb and Ms, provides a

robust discriminant, but a sparse monitoring

network may generate few Ms measurements,

and these may be biased due to path effects

such as focusing. N Selby (AWE) showed how

station and path corrections for Ms improve

its reliability.

Unusual seismic events provide an incentive

for forensic seismologists to develop and refine

techniques. D Bowers (AWE) examined seismic

signals associated with the sinking of the sub-

marine Kursk and showed evidence of two

underwater explosions of magnitudes 1.5 and

3.5 ML, separated by 135 s. Spectral features

indicative of bubble pulse formation and inter-

ference of direct and surface reflected waves,

indicate that the first explosion (estimated

2–80 kg TNT) was contained at a depth of

60 m or less, whereas the second explosion

(estimated 2–8 tonnes TNT) at a greater depth

(~120 m), opened the hull. Oceanic seismic

events also figured prominently in the review

by B Massinon (LDG, France) of French work

towards verification of the CTBT, which also

included infrasound to detect atmospheric

tests. French studies on the use of seismic

“T”-phases recorded at island seismic stations

have provided an effective, cheaper alternative

to expensive hydrophones for detecting and

locating events at sea. P Marshall (AWE) noted

that the synergy between the seismic, hydro-

acoustic and infrasound technologies in the

global CTBT International Monitoring System

(IMS) of sensors had significantly helped in

making the CTBT possible. C Guralp (Guralp

Systems) provided a fascinating review of how

rapid advances in seismometer development,

including the use of mechanical and electrical

feedback, have allowed the development of

borehole instruments providing data – vital for

analysis of source-signal characteristics and

path attenuation effects.

In his closing presentation, A Douglas (AWE)

noted some unsolved problems, and asked

what had been learned about the Earth from

forensic seismology. In his opinion this was not

as much as expected, considering the potential

for examination of the seismic response close

to large controlled sources, albeit at some cost.

Effects of anelastic attenuation, geometric

spreading, spalling at the surface, reactivation

of existing faults and distribution of down-

ward radiated energy have never been proper-

ly measured, nor an observed time delay in

surface-reflected pP wave satisfactorily

explained. Even now, relatively few test epi-

centre and origin times have been released to

allow the examination of station and network

bias due to heterogeneity in the crust and

upper mantle. He noted how varied research

conducted on a consistent theme invariably

turns out to be useful, and illustrated this with

reference to path-dependent wave attenuation

by describing how observations and modelling

Meeting report

2.33April 2002 Vol 43

The CTBTand advancesin earth scienceDavid Booth and Peter Maguire report on a two-day meeting organized by

the Royal Astronomical Society and the British Geophysical Association,

that showed how scientific challenges generated by the Comprehensive

Nuclear Test Ban Treaty have been addressed, and how they continue to

stimulate advances in earth science.

have explained many apparent anomalies in

observed seismograms, as well as providing

evidence that seismologists possess the capabil-

ity to verify the CTBT by discriminating

between earthquakes and explosions.

The second day of the meeting showed how

the requirement for accurate location of events

for CTBT monitoring requires detailed knowl-

edge of the deep structure of the British Isles,

and how this is being advanced using past and

present research. Accurate location is vital for

verifying the CTBT. A suspicious event cannot

be identified unambiguously as a nuclear

explosion without an on-site inspection, which

is permitted by the CTBT. The search area is

restricted to 1000 km2, and thus the epicentre

must be accurately determined using a dataset

which will be sparse for small events. Source

location is determined from the records of

monitoring networks by determining the point

at which wave travel times best match theoret-

ical travel times derived from the best earth

models available. Treaty signatories, including

the UK, have been asked to provide data which

will allow global structural models to be

refined for local and regional effects at source

and receiver.

Calibrating seismic stations

Calibration of the 170 global IMS seismic

stations is an immense task and several con-

sortia have been formed to meet the challenges

that it creates. K McLaughlin (SAIC/CMR,

USA) outlined the work his consortium is

doing to improve locations in North Africa

and the Mediterranean, Middle East, and

Western Asia. Initial surface-wave models are

being refined using higher-resolution P-wave

studies, and emphasis is placed on statistical

analysis to quantify the improvements made.

Joint Hypocentre Determination, pioneered by

Alan Douglas, is vital for calibration studies, as

is the use of accurate locations from dense

networks and aftershock studies.

Most information on the velocity structure in

the region of the UK has been gained through

controlled-source wide-angle seismic refrac-

tion and reflection profiles by UK and Irish

academic and government institutions over the

past 30 years. The late Brian Jacob and col-

leagues at DIAS, Ireland, have contributed

much to the success of several deep seismic

profiles in and around the British Isles, and

B O’Reilly (DIAS) reviewed this work, show-

ing how successive experiments improved the

resolution of the velocity structure. R England

(Leicester) has compiled results from existing

UK and Irish profiles, most of which are off-

shore, to produce contoured maps of Moho

depth and average crustal P velocity. Interest-

ingly, computer-generated “unbiased” velocity

contours show no obvious correlation with

tectonic terrane boundaries. However, evi-

dence of magmatic underplating is suggested

by high-velocity zones in the lower crust,

which correlate with areas of maximum uplift

in the early Tertiary, later noted also by

N White (Cambridge). Ample evidence for 3-D

heterogeneity requires the capability to model

it, and R Hobbs (Cambridge) demonstrated an

economical, effective “Phase-Screen” method

for modelling wave propagation at narrow

angles through 3-D models, by reducing the

model to a series of slabs. The method has

been used to show the focusing and defocusing

effects of interface topography, and the sensi-

tivity of receiver function analysis to assump-

tions of a local 1-D structure.

A Chadwick (British Geological Survey)

examined Moho two-way travel time (TWTT)

images derived from seismic reflection profiles,

and considered TWTT perturbations in terms

of lateral variations of the velocity–density

relationship in the crust, and of density in the

mantle. It is difficult to differentiate between

these effects without additional data from

shear-wave velocities and 3-D gravity model-

ling, but an upward perturbation of Moho

TWTT under granite appears consistent with a

locally hotter, less dense upper mantle. G Kim-

bell’s (BGS) broad geophysical perspective

examined crustal thickness variation in the NE

Atlantic continental margin, estimated from

topography and an initial model for sedimen-

tary cover, under different assumptions of

isostasy and lithospheric flexure. Allowance

for density variation between continental and

hotter oceanic lithosphere, gives a good match

between theoretical and seismically observed

Moho depth across the continent–ocean

boundary. The morning concluded with

N White’s absorbing study of sedimentary

basin subsidence analysis, demonstrating an

uplift event beneath the British Isles at about

60–55 Ma, centred on the Irish Sea. The results

are consistent with vitrinite reflectance and fis-

sion track data and suggest a major thermal

event occurring beneath this region during the

early Cenozoic.

Three PhD students showed the way ahead in

the afternoon session. L Brana (Bristol) has

mapped the locations of crustal scatterers in

the UK, and near the former Kazakh test site,

using BGS records of teleseismic P waves, and

showed that P to Rayleigh (Rg) wave scattering

predominates. Where P to shear wave scatter-

ing is present, it correlates with topography,

whereas faults generate mainly Rg scattering.

New UK crust and upper-mantle velocity struc-

tures have been generated from teleseismic

receiver functions using three-component UK

seismograms. P Denton, reporting for J Tom-

linson (both Leicester), showed that while

these generally agree with results from nearby

seismic profiles, there is evidence for a local

subcrustal anomalous velocity zone near the

Isle of Man, possibly associated with the

Iapetus Suture. Recent tomographic images of

upper-mantle velocity structure under most of

the UK down to 400 km (see figure 1) were

displayed by S Arrowsmith (Leeds), who has

inverted teleseismic P-wave delay times at UK

seismic stations. There is dramatic correlation

between mantle low velocity anomalies and the

Tertiary dyke swarms and plutonic centres in

the UK, while fast velocity anomalies in North-

ern Scotland are interpreted as signatures of

Caledonian subduction. In east and central

southwestern Britain, high velocities may be

related to possible Caledonian subducted

lithospheric plates, or the presence of a micro-

craton which has remained relatively unde-

formed throughout the Caledonian and

Variscan orogenies.

Finally, I Smith (BGS) noted that stored data

and models should be accessible, understand-

able and protected against the departure or

retirement of staff and storage media. He

described the framework of the BGS Digital

Geoscientific Spatial Model for the UK, which

also addresses such thorny issues as storage of

textual description of model quality and uncer-

tainty, and description of best practice. There is

not so much useful data around, that we can

afford to lose any of it! �

David Booth (British Geological Survey)and PeterMaguire (Leicester Uni.) were joint organizers ofthis meeting. Meeting programme, abstracts, BGSlinks: www.gsrg.nmh.ac.ukCTBT organization: www.ctbto.org AWE Blacknest: www.blacknest.gov.uk

Meeting report

2.34 April 2002 Vol 43

1: 3-D perspective view of volumes from 50 to600 km depth enclosing high (blue) and low (red)P-wave velocity anomalies beneath the BritishIsles, where the surfaces enclose regions of over0.50% higher and lower velocity. The underlyinggrey sphere denotes the 660 km discontinuity.