UOA7-04: Seismological techniques for improved UK monitoring of seismic events with the aim of identifying potential underground nuclear explosions
Submitting InstitutionUniversity of Oxford
Unit of AssessmentEarth Systems and Environmental Sciences
Summary Impact TypeTechnological
Research Subject Area(s)
Earth Sciences: Geology, Geophysics
Summary of the impact
The Comprehensive nuclear Test Ban Treaty (CTBT) is the leading tool for
minimizing the high-magnitude threats posed by nuclear weapons to the
environment and to world peace. A major impediment to monitoring
compliance with the CTBT is the difficulty of distinguishing natural
earthquakes from man-made explosions: the seismic discrimination problem.
Seismological research at Oxford University into fundamentals of
theoretical and computational seismology led to the development of new
methods and algorithms that are now integrated into discrimination for
CTBT monitoring. The resulting impact has been a major upgrade in the UK's
capability in seismic discrimination, and a significant contribution to
global capabilities in this area.
A major thrust of Oxford seismological research is to image the Earth's
interior in three dimensions; a research field known as global seismic
tomography. Spatial variations in the wave speeds in the earth act as a
distorting lens when one seeks to image seismic earthquakes and
explosions. Armed with better knowledge of wave speed variations in the
Earth's interior, it is possible to obtain a better image of seismic
Key developments in understanding the nature of the earth's interior have
come from large-scale measurement campaigns which provide seismological
data required to construct and improve earth models (e.g. Trampert and
Woodhouse, 1996 ). Theoretical and computational methods, pursued by
Woodhouse and collaborators, built three-dimensional models of earth
structure (e.g. Ritsema et al. 2011 ) from unprecedentedly large and
diverse observational datasets (e.g. Trampert and Woodhouse, 2001 )
In a related line of research, new methods and algorithms were developed
at the University of Oxford to assess the influence of earth structure on
received seismic waveforms, with reference to the nature of the source and
its location. For example, this work assessed the influence of the source
on received seismic waves given variable earth models (Ferreira and
Woodhouse 2006 ), including those produced by Woodhouse and colleagues.
It also assessed the role of upper-mantle attenuation on received wave
amplitudes, with particular reference to the nature of the seismic source
(e.g. Selby and Woodhouse 2002 ). This latter work was developed
further by scientists at AWE Blacknest to assess the geographical
variation of Ms magnitudes (e.g. Selby et al, 2003. Empirical path and
station corrections for surface-wave magnitude, Ms, using a global
network. Geophys. J. Int., 155, 379-390). Coupled to the improved earth
models developed by Woodhouse and collaborators, this work identified
novel approaches that could be used to more accurately locate the source
of the events generating seismic waves.
Source characterization, location, and depth determination are of high
utility in the seismic discrimination problem. Determining with confidence
that the depth of an event is greater than 10 km may be used to rule out
an anthropogenic source. Similarly, the geographical location, and good
understanding of location uncertainties, can help to define the region to
be investigated during an on-site inspection to support the CTBT.
Understanding the effect of three-dimensional earth structure on the
propagation of seismic waves can improve the capability to estimate source
mechanisms and discriminate between natural and man-made seismic sources.
References to the research
The three asterisked outputs best indicate the quality of the
1. * Trampert, J., Woodhouse, J.H., 1996. High resolution global phase
velocity distributions. Geophysical Research Letters 23, 21-24.
Presents a new large set of dispersion measurements and uses them to
constrain earth structure.
2. Ritsema J, Deuss A, van Heijst, H. J. and J. H. Woodhouse, S40RTS: a
degree-40 shear velocity model for the mantle from new Rayleigh wave
dispersion, teleseismic traveltime, and normal-mode splitting function
measurements, Geophys. J. Int., 184, 1223-1236, doi:
10.1111/j.1365-246X.2010.04884.x , 2011.
3. Trampert, J. and J. H. Woodhouse, Assessment of global phase velocity
models, Geophys. J. Int., 144, 165-174, 2001.
4. Selby, N.D., Woodhouse, J.H., 2002. The Q structure of the upper
mantle: Constraints from Rayleigh wave amplitudes. Journal of Geophysical
Research B: Solid Earth 107, ESE 5-1 — ESE 5-12.
5. * Ferreira, A.M.G., Woodhouse, J.H., 2006. Long-period seismic source
inversions using global tomographic models. Geophysical Journal
International 166, 1178-1192.
Assesses the influence of source on received seismic waves given
variable earth models.
6. * Fox, B.D., Selby, N.D., and Woodhouse, J.H, 2012. Shallow seismic
source parameter determination using intermediate-period surface wave
amplitude spectra. Geophys. J. Int., 191, 601-615.
Uses surface waves recorded by a global network to estimate the depth
and source mechanisms of an event, confirming that the event was an
Details of the impact
The research described in Section 2 has its impact through the operation
of AWE Blacknest, which has for over 50 years been a leading centre in the
world for research into techniques of detecting underground nuclear
explosions and is now responsible, under a contract from the Ministry of
Defence, for meeting the UK's requirement to determine whether seismic
events are violations of the Comprehensive nuclear Test Ban Treaty (CTBT).
An essential element of the CTBT regime is that any violation should be
detectable. The examination of seismic waves is one of the principal
methods by which this detection can be achieved. A major component of
detection is the ability to distinguish reliably between seismic waves
generated by naturally occurring earthquakes and those from man-made
explosions — the seismic discrimination problem. A closely related
operation is "event-screening", the process whereby those seismic events
that can be confidently identified as earthquakes are discounted as
potential violations of the Treaty.
The forensic seismology involved in seismic discrimination is a
continually evolving field. As more, and higher-quality data become
available, scientists seek to lower the threshold at which detection of
tests can be achieved. A fundamental part of this evolution is to discover
new discrimination techniques, key among which are the determination of
depth and geographical location of the events.
The potential for applying Woodhouse's fundamental research to
improvement of source location, thereby enhancing event screening, was
recognized by Oxford University (Woodhouse) and AWE Blacknest scientists
in the early 2000s. To quote the Team Leader of Forensic Seismology at AWE
"John conducted fundamental work in the late 1990s and early 2000s to
improve observational databases and 3-D models of earth structure.
Resulting earth models enable more accurate interpretation of seismic
waveforms and, through a number of novel algorithms developed in
Woodhouse's research group, have considerable relevance for the accurate
characterisation, depth determination and location of seismic events"
AWE Blacknest subsequently sponsored focused research at Oxford
University in the period 2003-2010, to modify the algorithms developed by
Woodhouse's group for specific use in the CTBT context. This work involved
a graduate studentship sponsored by AWE (Heyburn) and a NERC studentship
with AWE CASE support (Fox). Both students were supervised by Woodhouse
and worked closely with him. Heyburn subsequently joined AWE Blacknest as
a Junior Scientist and continues to use the algorithms developed at Oxford
University in event screening as part of AWE's work in support of the
CTBT. Fox also worked for some years at AWE following completion of his
Ph.D. He presented his joint Oxford-AWE work at several international
conferences and, together with Woodhouse, recently published the key
findings of his portion of the collaborative Oxford-AWE research on
identification of source location .
This collaborative work between the University of Oxford and AWE built on
the fundamental work of Woodhouse to achieve significant advances,
1) A technique based on intermediate seismic surface waves to investigate
the focal mechanism and depth of shallow earthquakes . This work
compares observed and theoretical amplitude spectra using state-of-the-art
methods and models for the calculation of theoretical seismograms.
2) An algorithm for constraining depth using body-wave data. The method
uses advanced signal processing techniques to detect correlated signals as
a tool for identifying depth phases and thereby constraining the depth of
a seismic event.
3) Studies of measurement and model uncertainties in seismic travel
times. The resulting method is in regular use at Blacknest for assessing
Blacknest routinely makes use of the methods developed from this research
and has done so throughout the REF period (2008-13), including the
adoption of new techniques developed from the collaborative work in
2008-2009. These methods are used for the analysis of specific events, and
to further understanding the background seismicity in areas of interest
for CBTB monitoring. A particular example was the use of research derived
from the University of Oxford to constrain uncertainties in the locations
of the North Korean explosion of 2009. Further research at Blacknest
continues to build on such University of Oxford research.
Quoting: "...techniques developed from John's fundamental research,
have been extensively used by AWE Blacknest. They were used, for
instance, to assess the location of the 2009 Korean nuclear test and the
uncertainty on that location. These techniques help us to fulfil our UK
obligation within the CTBT — a key role of AWE Blacknest." 
The primary beneficiaries of this work are those involved in
identification of explosions which may contravene the CTBT. In the UK,
this has specific relevance to AWE Blacknest who are tasked by the MoD to
fulfil national obligations for such event screening. Given AWE
Blacknest's international leadership in the field, this work also has
wider significance for the CTBT internationally. The full benefits of the
CTBT are to world security and the limiting of radionuclide contamination
of the environment.
Sources to corroborate the impact
- A letter from a Senior Scientist at AWE-Blacknest confirms the
importance of Woodhouse's fundamental work; the AWE sponsorship of
research to apply these findings to the CTBT, and the continued use by
AWE, during the period 2008-2013, of algorithms based on University of
Oxford research for source location as part of their CTBT remit.