Seismology

The detection and location of low magnitude earthquakes in northern Norway using multi-channel waveform correlation at regional distances

by Steven J. Gibbons

The detection and location of low magnitude earthquakes in northern Norway using multi-channel waveform correlation at regional distances☆
by: S. Gibbons, M. B. Sorensen, D. Harris, F. Ringdal
Physics of The Earth and Planetary Interiors, Vol. 160, No. 3-4. (16 March 2007), pp. 285-309, doi:10.1016/j.pepi.2006.11.008

A fortuitous sequence of closely spaced earthquakes in the Rana region of northern Norway, during 2005, has provided... more A fortuitous sequence of closely spaced earthquakes in the Rana region of northern Norway, during 2005, has provided an ideal natural laboratory for investigating event detectability using waveform correlation over networks and arrays at regional distances. A small number of events between magnitude 2.0 and 3.5 were recorded with a high SNR by the Fennoscandian IMS seismic arrays at distances over 600 km and three of these events, including the largest on 24 June, displayed remarkable waveform similarity even at relatively high frequencies. In an effort to detect occurrences of smaller earthquakes in the immediate geographical vicinity of the 24 June event, a multi-channel correlation detector for the NORSAR array was run for the whole calender year 2005 using the signal from the master event as a template. A total of 32 detections were made and all but 2 of these coincided with independent correlation detections using the other Nordic IMS array stations; very few correspond to signals detectable using traditional energy detectors. Permanent and temporary stations of the Norwegian National Seismic Network (NNSN) at far closer epicentral distances have confirmed that all but one of the correlation detections at NORSAR in fact correspond to real events. The closest stations at distances of approximately 10 km can confirm that the smallest of these events have magnitudes down to 0.5 which represents a detection threshold reduction of over 1.5 for the large-aperture NORSAR array and over 1.0 for the almost equidistant regional ARCES array. The incompleteness of the local network recordings precludes a comprehensive double-difference location for the full set of events. However, stable double-difference relative locations can be obtained for eight of the events using only the Lg phase recorded at the array stations. All events appear to be separated by less than 0.5 km. Clear peaks were observed in the NORSAR correlation coefficient traces during the coda of some of the larger events; the local stations confirm that these are in fact aftershocks exhibiting very similar waveforms to the main events. Many of the more marginal correlation detections are not made when the calculations are repeated using shorter signal segments, fewer sensors or more distant stations. We demonstrate in addition how these almost repeating seismic sources have been exploited to detect and measure timing anomalies at individual sites within the arrays and network.

Analysis and simulation of chemical explosions in nonspherical cavities in granite

by Steven J. Gibbons

Analysis and simulation of chemical explosions in nonspherical cavities in granite
by: Jeffry L. Stevens, Steven Gibbons, Norton Rimer, Heming Xu, Conrad Lindholm, Frode Ringdal, Tormod Kvaerna, John R. Murphy
Journal of Geophysical Research - Solid Earth, Vol. 111, No. B4. (25 April 2006), B04306, doi:10.1029/2005JB003768 

We analyze data from 15 decoupled chemical explosions conducted from 1986 to 2002 in Älvdalen, Sweden, recorded on... more We analyze data from 15 decoupled chemical explosions conducted from 1986 to 2002 in Älvdalen, Sweden, recorded on regional seismic stations NORES, HFS, and NORSAR. The explosions were conducted in three approximately rectangular granite chambers at a depth of 100 m in chambers with volumes of 200, 300, and 1000 m3 and yields ranging from 500 to 10,000 kg. The smallest explosions in the largest chamber are fully decoupled, while the other explosions are partially coupled, overdriven by up to a factor of 25. The data show that decoupling remains fairly constant for overdrive up to about a factor of 10 then decreases rapidly at higher yields. Several 1000 kg explosions were conducted in the two smaller chambers. These events did not appear in seismic bulletins; however, we were able to identify the waveform and to estimate the origin time using a waveform correlation procedure. For the more recent explosions, near-field data were recorded on pressure gauges in the chamber and adjacent tunnel and on velocity gauges in boreholes at several locations near the chamber. We have modeled these data using three-dimensional finite difference calculations. The calculations show enhanced signals in the direction along the small axis of the chamber and reduced signals near the long end of the tunnel. Although we do not have free-field data from all directions, the data for the available locations are consistent with these calculations.

Monitoring of seismic events from a specific source region using a single regional array: A case study

by Steven J. Gibbons

Monitoring of seismic events from a specific source region using a single regional array: A case study
by: S. Gibbons, T. Kværna, F. Ringdal
Journal of Seismology, Vol. 9, No. 3. (1 July 2005), pp. 277-294, doi:10.1007/s10950-005-5746-7

In the monitoring of earthquakes and nuclear explosions using a sparse worldwide network of seismic stations, it is... more In the monitoring of earthquakes and nuclear explosions using a sparse worldwide network of seismic stations, it is frequently necessary to make reliable location estimates using a single seismic array. It is also desirable to screen out routine industrial explosions automatically in order that analyst resources are not wasted upon detections which can, with a high level of confidence, be associated with such a source. The Kovdor mine on the Kola Peninsula of NW Russia is the site of frequent industrial blasts which are well recorded by the ARCES regional seismic array at a distance of approximately 300 km. We describe here an automatic procedure for identifying signals which are likely to result from blasts at the Kovdor mine and, wherever possible, for obtaining single array locations for such events. Carefully calibrated processing parameters were chosen using measurements from confirmed events at the mine over a one-year period for which the operators supplied Ground Truth information. Phase arrival times are estimated using an autoregressive method and slowness and azimuth are estimated using broadband f-k analysis in fixed frequency bands and time-windows fixed relative to the initial P-onset time. We demonstrate the improvement to slowness estimates resulting from the use of fixed frequency bands. Events can be located using a single array if, in addition to the P-phase, at least one secondary phase is found with both an acceptable slowness estimate and valid onset-time estimate. We evaluate the on-line system over a twelve month period; every event known to have occured at the mine is detected by the process and 32 out of 53 confirmed events were located automatically. The remaining events were classified as “very likely” Kovdor events and were subsequently located by an analyst. The false alarm rate is low; only 84 very likely Kovdor events were identified during the whole of 2003 and none of these were subsequently located at a large distance from the mine. The location accuracy achieved automatically by the single-array process is remarkably good, and is comparable to that obtained interactively by an experienced analyst using two-array observations. The greatest problem encountered in the single array location procedure is the difficulty in determining arrival times for secondary phases, given the weak Sn phase and the complexity of the P-coda. The method described here could be applied to a wide range of locations and sources for which the monitoring of seismic activity is desirable. The effectiveness will depend upon the distance between source and receiver, the nature of the seismic sources and the level of regional seismicity.

On the Identification and Documentation of Timing Errors: An Example at the KBS Station, Spitsbergen

by Steven J. Gibbons

On the Identification and Documentation of Timing Errors: An Example at the KBS Station, Spitsbergen
by: Steven J. Gibbons
Seismological Research Letters, Vol. 77, No. 5. (1 September 2006), pp. 559-571, doi:10.1785/gssrl.77.5.559

(no abstract) (no abstract)

Challenging development priorities: The case for earthquake hazard mitigation as a policy imperative in developing countries

by Gah-Kai Leung

Undergraduate paper submitted to the Department of Politics and International Studies, University of Warwick, May 2012.

I intend to submit this paper to Volume 5, Issue 2, of "Reinvention: A Journal of Undergraduate Research". I also plan to present this paper at the British Conference of Undergraduate Research 2013, to be held at the University of Plymouth.

The substantive mission of this essay is to provide a case for earthquake hazard mitigation as a policy priority for... more The substantive mission of this essay is to provide a case for earthquake hazard mitigation as a policy priority for earthquake-prone developing countries. I explain why seismic hazard should constitute an issue for development by showing how developing countries suffer disproportionately from the effects of severe earthquakes compared to more advanced economies. I go on to offer reasons for why earthquake mitigation measures have been marginalised within the development community. Finally a series of policy solutions are proposed, casting the earthquake resistance project in terms of a coherent network of knowledge-sharing from the international to the local levels.

Development of a three-dimensional velocity model for the crust and upper mantle in the greater Barents Sea region

by Nils Maercklin

H. Bungum, N. Maercklin, O. Ritzmann, J. I. Faleide, C. Weidle, A. Levshin, J. Schweitzer, W. D. Mooney, S. T. Detweiler (2005). 27th Seismic Research Review: Ground-based nuclear explosion monitoring technologies, LA-UR-05-6407, pages 13-22

We have compiled a 3D seismic velocity model for the crust and upper mantle in the greater Barents Sea region... more We have compiled a 3D seismic velocity model for the crust and upper mantle in the greater Barents Sea region including northern Scandinavia, Svalbard, Novaya Zemlya, the Kara Sea, and the Kola-Karelia regions. While the general motivation for developing this model is basic geophysical research, a more specific goal is to create a model for research on the identification and location of small seismic events in the study region, and for operational use in locating and characterizing seismic events in the study region.
The observational basis for the velocity model are previous, crustal-scale 2D seismic reflection and refraction profiles, and passive seismological recordings, supplemented by potential field data to provide additional constraints on the crustal structure. The model is defined at grid tiles spaced every 50 km, and each tile is represented by up to two sedimentary and three crystalline crustal layers (plus water and ice). For crustal regions not constrained by primary velocity data, we developed an interpolation scheme based on several defined geological provinces that are characterized by individual tectono-sedimentary histories. The interpolation utilizes the observed strong correlation between sediment and crystalline crustal thickness within continental provinces. For comparison, an alternative interpolation approach applies a continuous curvature gridding algorithm within each of the provinces.
To provide a complete lithospheric model, we complemented the crustal model with an upper mantle velocity model based on surface wave inversion, thereby covering depths essential for Pn and Sn travel time modeling. As an extension to the previously existing data set, we recently retrieved a large amount of surface wave data recorded or excited in the European Arctic during the last three decades. The merged surface wave data set will enable us to refine the upper mantle velocity structure in the study region significantly. Preliminary group velocity maps for Rayleigh and Love waves reflect large-scale geological structures and demonstrate lateral velocity variations in the mantle.
Validation of our velocity model includes travel time modeling and relocation of seismic events. For this purpose we compiled a set of Ground Truth (GT) events comprising chemical and nuclear explosions, and natural earthquakes. Phase arrival times of multiple events at some sites provide timing error estimates at some stations. With the GT events we obtain a rather good Pn and Sn ray coverage in the main target region. Besides the comparison of observed and modeled travel times along selected transects, we have computed source-specific station corrections (SSSCs) from our 3D model.
The crustal velocity models are also evaluated by comparison of predicted gravity fields with the observed free-air gravity. To model the gravity field, we used standard velocity-density relationships for crustal rock types and the density structure of the upper mantle from previous studies. The inferred gravity fields both reflect and exaggerate the basic geological features. Accomplishments so far have been concerned with implementation of a forward modeling procedure and software development needed to support the complex 3D model structure. The forward modelling is done in order to reduce the misfit between observed and modelled gravity and finally to supplement our crustal velocity model with a density distribution.

Detection and characterization of seismic phases using continuous spectral estimation on incoherent and partially coherent arrays

by Steven J. Gibbons

Detection and characterization of seismic phases using continuous spectral estimation on incoherent and partially coherent arrays
by: Steven J. Gibbons, Frode Ringdal, Tormod Kværna
Geophysical Journal International, Vol. 172, No. 1. (January 2008), pp. 405-421, doi:10.1111/j.1365-246X.2007.03650.x

Seismic arrays are employed in the global monitoring of earthquakes and explosions because of their superior ability... more Seismic arrays are employed in the global monitoring of earthquakes and explosions because of their superior ability to detect and estimate the direction of incident seismic arrivals. Traditional beamforming and f–k analysis require waveform semblance over the full array aperture and cannot be applied in many situations where signals are incoherent between sensors. The NORSAR and MJAR arrays are two primary IMS stations where this is the case for high-frequency regional phases. Large intersite distances and significant geological heterogeneity at these arrays result in waveform dissimilarity which precludes coherent array processing in the frequency bands with optimal SNR. Multitaper methods provide low variance spectral estimates over short time-windows and seismic arrivals can be detected on single channels using a non-linear spectrogram transformation which attains local maxima at times and frequencies characterized by an energy increase. This detection procedure requires very little a priori knowledge of the spectral content of the signal. The transformed spectrograms can be beamformed over large-aperture arrays or networks according to theoretical time-delays resulting in an incoherent detection system which does not require waveform semblance at any frequencies. We outline a real-time automatic detection system for regional phase arrivals on the NORSAR array and demonstrate how stable and accurate slowness and azimuth estimates can be obtained for quite marginal signals. In the case of partially coherent arrays, the procedure described may provide stable, if low resolution, estimates which can subsequently be refined using coherent processing over subsets of sensors. In particular, we illustrate how the spectrogram beamforming method facilitates a stable and accurate slowness estimate for the incoherent high-frequency Pn arrival at the MJAR array in Japan from the 2006 October 9 underground nuclear test in North Korea.

The detection of low magnitude seismic events using array-based waveform correlation

by Steven J. Gibbons

Geophysical Journal International, Vol. 165, No. 1. (April 2006), pp. 149-166, doi:10.1111/j.1365-246X.2006.02865.x
by Steven J. Gibbons, Frode Ringdal

It has long been accepted that occurrences of a known signal are most effectively detected by cross-correlating the... more It has long been accepted that occurrences of a known signal are most effectively detected by cross-correlating the incoming data stream with a waveform template. Such matched signal detectors have received very little attention in the field of detection seismology because there are relatively few instances in which the form of an anticipated seismic signal is known a priori. Repeating events in highly confined geographical regions have been observed to produce very similar waveforms and good signals from events at a given site can be exploited to detect subsequent co-located events at lower magnitudes than would be possible using traditional power detectors. Even greater improvement in signal detectability can be achieved using seismic arrays; running correlation coefficients from single sensors can be stacked over an array or network to result in a network correlation coefficient displaying a significant array gain. If two events are co-located, the time separating the corresponding patterns in the wave train as indicated by the cross-correlation function is identical for all seismic stations and this property means that the correlation coefficient traces are coherent even when the waveforms are not. We illustrate the power of array-based waveform correlation using the 1997 August 16 Kara Sea event. The weak event that occurred 4 hr after the main event was barely detected using an STA/LTA detector on the SPITS array but is readily detected by signal matching on a single channel. The main event was also recorded by the far more distant NORSAR array but no conventional detection can be made for the second event. A clear detection is, however, made when the correlation coefficient traces are beamformed over all sensors of the array. We estimate the reduction in detection threshold of a test signal on a regional seismic array using waveform correlation by scaling down a master signal and immersing it into seismic noise. We show that, for this case, waveform correlation using a single channel detects signals of approximately 0.7 orders of magnitude lower than is possible using an STA/LTA detector on the array beam. Waveform matching on the full array provides an additional improvement of approximately 0.4 magnitude units. We describe a case study in which small seismic events at the Barentsburg coal mine on Spitsbergen were detected using the signals from a major rockburst as master waveforms. Many spurious triggers occurred in this study whereby short sections of signal exhibited coincidental similarity with unrelated incoming wave fronts. We demonstrate how such false alarms can almost always be identified and screened out automatically by performing frequency–wavenumber analysis upon the set of individual correlation coefficient traces.

S-wave identification by polarization filtering and waveform coherence analyses

by Nils Maercklin

O. Amoroso, N. Maercklin, A. Zollo (2012). Bulletin of the Seismological Society of America, 102(2), 854-861, doi:10.1785/0120110140

High-resolution imaging with microseismic events requires the use of large and consistent data sets of seismic phase... more High-resolution imaging with microseismic events requires the use of large and consistent data sets of seismic phase arrival times. In particular the S-phase is important to derive physical parameters of the subsurface. Typically this phase is identified on one of the horizontal seismogram components by a change of signal amplitude and frequency as compared to the previous P-phase. However, reliable S-phase identification can be difficult for local events because of a signal overlap with the P coda, the presence of converted phases, and possible S-wave splitting due to anisotropy. In this study we propose a new data processing technique aiming at uniquely identifying the S-phase arrival using all available records from a seismic network. The technique combines polarization analysis of single three components recordings of an event with analysis of lateral waveform coherence across the network. This makes it possible to construct seismic sections in which the first arrival is the S-phase. This graphical representation can support an operator in both the analysis of single events and in semi-automatic analyses of large datasets. In addition, an automated stacking velocity analysis provides S-wave velocities from these sections. We demonstrate the applicability of this technique using synthetic seismograms, and we evaluate the efficacy on a dataset of three-component velocimeter records from local earthquakes of the Campania-Lucania Apennines (southern Italy) recorded by the Irpinia Seismic Network (ISNet).

Improvements to Seismic Monitoring of the European Arctic Using Three-Component Array Processing at SPITS

by Steven J. Gibbons

S. J. Gibbons, J. Schweitzer, F. Ringdal, T. Kværna, S. Mykkeltveit, B. Paulsen
Bulletin of the Seismological Society of America, Vol. 101, No. 6. (01 December 2011), pp. 2737-2754, doi:10.1785/0120110109

The detectability of low magnitude seismic events in the European Arctic is determined primarily by the small-aperture... more The detectability of low magnitude seismic events in the European Arctic is determined primarily by the small-aperture International Monitoring System arrays ARCES and SPITS. In August 2004, the SPITS array was upgraded to a broadband array with an increase in the sampling rate from 40 to 80 Hz. Most important, however, for the detection and location of small-magnitude seismic events was the deployment of three-component instruments at six of the nine sites. Detection and correct classification of secondary phases are of paramount importance for events observed by only a small number of stations at regional distances; and, in the absence of the strong Lg phases typically observed for continental propagation paths, multiple three-component stations were deemed necessary to exploit the higher S-phase amplitudes anticipated on the horizontal sensors. We demonstrate improved signal-to-noise ratios (SNRs) for S phases on horizontal beams for several events close to Novaya Zemlya. Horizontal component f-k analysis improves direction estimates and phase classification for low-SNR signals. We demonstrate secondary phases that are misidentified by vertical-only f-k analysis but which are correctly classified by three-component array processing. A significant problem with array processing at SPITS is the overlap in slowness space of regional P and S phases. Phase identification is improved greatly by comparing the coherence between vertical traces with the coherence between horizontal traces. Considerations in the routine array processing of SPITS data are reviewed, including the need for elevation corrections in slowness estimation and the need to take into account azimuth-dependent variation of apparent velocity estimates for regional phases.

Structure and anisotropy of the Mexico subduction zone based on Rayleigh-wave analysis and implications for the geometry of the Trans-Mexican Volcanic Belt

by Caroline Beghein

Stubailo, I., Beghein, C., Davis, P.
J. Geophys. Res., 117, B5, B05303, doi:10.1029/2011JB008631, 2012

We develop a three-dimensional model of shear wave velocity and anisotropy for the Mexico subduction zone using... more We develop a three-dimensional model of shear wave velocity and anisotropy for the Mexico subduction zone using Rayleigh wave phase velocity dispersion measurements. This region is characterized by both steep and flat subduction and a volcanic arc that appears to be oblique to the trench. We give a new interpretation of the volcanic arc obliqueness and the location of the Tzitzio gap in volcanism based on the subduction morphology. We employ the two-station method to measure Rayleigh phase velocity dispersion curves between periods of 16 s to 171 s. The results are then inverted to obtain azimuthally anisotropic phase velocity maps and to model 3-D variations in upper mantle velocity and anisotropy. Our maps reveal lateral variations in phase velocity at all periods, consistent with the presence of flat and steep subduction. We also find that the data are consistent with two layers of anisotropy beneath Mexico: a crustal layer, with the fast directions parallel to the North American absolute plate motion, and a deeper layer that includes the mantle lithosphere and the asthenosphere, with the fast direction interpreted in terms of toroidal mantle flow around the slab edges. Our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure. Our anisotropy results favor a tear, which is also consistent with the geometry of the volcanic belt.

DESERT – Struktur und Dynamik der Dead Sea Transform

by Nils Maercklin

M. Weber, O. Ritter, N. Maercklin, C. Janssen, A. Mohsen, G. Rümpker, H.-J. Förster, S. Sobolev (2004). In: Zweijahresbericht 2002/2003, GeoForschungsZentrum Potsdam, Potsdam, Germany, pp. 1-18.

Wir leben auf einem dynamischen Planeten, dessen äußere Form durch Prozesse der Plattentektonik geformt und laufend... more Wir leben auf einem dynamischen Planeten, dessen äußere Form durch Prozesse der Plattentektonik geformt und laufend verändert wird. Drei Schlüsselelemente bei der Deformation und Bewegung der Erdplatten sind: (1) Das Aufbrechen von Platten (rifting), (2) Das Abtauchen von Platten in den Erdmantel (subduction) und (3) Das horizontale Vorbeigleiten/Schrammen der Platten (transform faulting); das Thema diese Beitrags.
Die wichtigsten Prozesse, die die großen Transformstörungen kontrollieren, sind trotz vieler Untersuchungen an verschiedenen Scherzonen, wie z.B. der San Andreas Fault (SAF), noch nicht vollständig verstanden. Die Dead Sea Transform (DST) ist deswegen neben der SAF eine der Schlüssel-Lokationen für die Untersuchung von Transform-Störungen. Die scheinbar einfache Struktur des DST-Systems und die relativ geringe Bewegungsrate von ca. 0,5 cm/Jahr unterscheidet die DST andererseits aber auch deutlich von der SAF, welche durch komplizierte Interaktion ozeanischer Mikroplatten an einem akkretierten Kontinentalrand gekennzeichnet ist und größere relative Plattenbewegungen (3,5 cm/Jahr) aufweist. Ein weiterer wichtiger Grund für unsere Studien ist neben grundlegenden Fragen, dass die Untersuchung historischer Erdbeben, paläoseismische Untersuchungen und die instrumentelle Seismologie der letzten hundert Jahre gezeigt haben, dass eine Reihe zerstörerischer Erdbeben entlang der DST aufgetreten sind. Die DST stellt somit für die Anlieger Palästina, Israel und Jordanien ein signifikantes seismisches Risiko dar.
Aufgrund der politischen Lage im Nahen Osten war allerdings eine Untersuchung der DST bisher nicht möglich. Das DEad SEa Rift Transect (DESERT) Projekt ist nun das erste geowissenschaftliche Projekt, das die DST überquerte. Es versucht folgende offene Fragen zu beantworten:
- Wie sieht die Struktur und die Dynamik der Kruste und des oberen Erdmantels im Bereich der DST aus?
- Wie sieht die kleinräumige Struktur und Dynamik der DST aus?
- Was sind die thermischen Bedingungen und wie lief die geodynamische Entwicklung ab?

Dynamic FE analysis of South Memnon Colossus including 3D soil-foundation-structure interaction

by Sara Casciati

Casciati S. and Borja R.I. (2004). “Dynamic FE analysis of South Memnon Colossus including 3D soil-foundation-structure interaction”. Computers & Structures, 82(20-21), 1719-1736. ISSN: 0045-7949.

DATE AND PLACE OF PUBLICATION: August 2004; Pergamon–Elsevier Science Ltd, Kidlington, Oxford OX5 1GB, Oxon, England.

ABSTRACT. A full three-dimensional dynamic soil–foundation structure interaction (SFSI) analysis of a famous landmark... more ABSTRACT. A full three-dimensional dynamic soil–foundation structure interaction (SFSI) analysis of a famous landmark in Luxor, Egypt, the South Memnon Colossus, is performed to investigate the response of this historical monument to seismic excitation. The analysis is carried out using the finite element (FE) method in time domain. The statue comprising the upper structure is modeled using 3D brick finite elements constructed from a photogrammetric representation that captures important details of the surface and allows the identification of probable zones of stress concentration. The modeling also takes into account the presence of a surface of discontinuity between the upper part of the statue and its fractured base. FE models of the foundation and the surrounding soil deposit are constructed and coupled with the statue model to analyze the seismic response of the entire system incorporating dynamic SFSI effects. These studies are useful for future conservation efforts of this historical landmark, and more specifically for designing possible retrofit measures for the fractured base to prevent potential collapse of the monument from overturning during an earthquake.

KEYWORDS: Soil–foundation–structure interaction; Multi-body deformable contact; Non-linear dynamic finite element analysis

The DESERT passive seismic experiment 2000/2001 in the Middle-East

by Nils Maercklin

DESERT Group, M. Weber et al. (2002). ORFEUS Electronic Newsletter, 4(1), page 3

Energy partitioning for seismic events in Fennoscandia and NW Russia

by Nils Maercklin

H. Bungum, T. Kværna, M. Roth, S. Mykkeltveit, N. Maercklin, K. Åstebøl, D. B. Harris, S. Larsen (2006). 28th Seismic Research Review: Ground-based nuclear explosion monitoring technologies, LA-UR-06-5471, pages 570-579

In this three-year project we have been addressing the problem of energy partitioning at distances ranging from very... more In this three-year project we have been addressing the problem of energy partitioning at distances ranging from very local to regional for various kinds of seismic sources. On the local and regional scale (20-220 km) we have targeted events from the region offshore of Western Norway, where we have both natural earthquake activity and frequent underwater explosions carried out by the Norwegian Navy. On the small scale we have focused on analysis of observations from an in-mine network of 16-18 sensors in the Pyhäsalmi mine in central Finland. This analysis has been supplemented with 3-D finite difference wave propagation simulations in a realistic mine model to investigate the physical mechanisms that partition seismic energy in the near source region in and around the underground mine.
The results from modeling and analysis of local and regional data show that mean S/P amplitude ratios for explosions and natural events differ at individual stations and are in general higher for natural events and frequency bands above 3 Hz. However, the distributions of S/P ratios for explosions and natural events overlap in all analyzed frequency bands. Thus, for individual events in our study area, S/P amplitude ratios can only assist the discrimination between an explosion and a natural event. This observation is supported by synthetic seismograms calculated for simple 1-D models, which demonstrate that explosions generate shear-wave energy if they are fired close to an interface with a strong material contrast (as is the case for most explosions), e.g., free surface or the ocean bottom. The larger difference in S/P ratios between earthquakes and explosions for higher frequencies can be explained by the fact that at low frequencies (larger wavelengths), discontinuities and structural heterogeneities in the explosion source region are stronger generators of converted S energy. The S* phase, for example, is most efficiently generated whenever an explosion source is located close to (within one wavelength) a strong discontinuity.
High-frequency (50-400 Hz) S/P ratios for mine blasts (explosions) and rockbursts recorded at the Pyhäsalmi in-mine network do not show any significant dependency on the distance to the events, which ranges between 40 and 400 m. The Pyhäsalmi explosions have generally lower S/P ratios than the rockbursts for all frequencies, but the difference is far too small to be significant for classification purposes. S/P ratios for explosions and rockbursts located in the same small area of the mine show results very similar to those for the full data set. This indicates that the observed differences in S/P ratios between explosions and rockbursts are due to differences in the source characteristics, and not to propagation effects along paths in the mine.
Three-dimensional finite-difference simulations were used to model seismic events within the Pyhäsalmi mine. In particular, a January 26, 2003, rockburst was modeled at frequencies of 50 Hz (4 m grid) and 100 Hz (2 m grid). We were able to match the characteristics of the observed data at 50 Hz particularly well, and the characteristics of the 100 Hz data reasonably well. The simulations showed that significant shear-energy can be produced due to the geologic and structural heterogeneities within the mine. In fact, mode-converted shear-energy generated from mine heterogeneity can dominate the compressional energy from an explosive source. A strong correlation is observed between the distance of a source from a mine heterogeneity and the magnitude of generated shear-energy. The ratio of shear to compressional energy is about a factor of two larger when the source is located within one wavelength from a mine heterogeneity. The simulations also suggest that excavated mine volumes are significantly stronger contributors to shear-energy generation than are geologic heterogeneities.

A 3D geophysical model of the crust the Barents Sea region: Model construction and basement characterization

by Nils Maercklin

O. Ritzmann, N. Maercklin, J. I. Faleide, H. Bungum, W. D. Mooney, S. T. Detweiler (2006). 28th Seismic Research Review: Ground-based nuclear explosion monitoring technologies, LA-UR-06-5471, pages 229-237

We here present BARENTS50, a new 3D geophysical model of the crust in the Barents Sea region. The target region of... more We here present BARENTS50, a new 3D geophysical model of the crust in the Barents Sea region. The target region of interest comprises northern Norway and Finland, parts of the Kola Peninsula and the East European lowlands. Novaya Zemlya, the Kara Sea and Franz-Josef Land terminate the region to the east, while the Norwegian-Greenland Sea marks the western boundary. In total, 680 one-dimensional seismic velocity profiles were compiled, mostly by sampling 2D seismic velocity transects, from seismic refraction profiles, every 25 km. Seismic reflection data in the western Barents Sea were further used for density modeling and subsequent density-to-velocity conversion. Velocities from these profiles were binned into two sedimentary and three crystalline crustal layers. The first step of the compilation comprised the layer-wise interpolation of the velocities and thicknesses. Within the different geological provinces of the study region, linear relationships between the thickness of the sedimentary rocks and the thickness of the remaining crystalline crust are observed. We therefore used the separately compiled (area-wide) sediment thickness data to adjust the crystalline crustal thickness according to the sedimentary thickness where no constraints from 1D velocity profiles existed. The BARENTS50 model is based on an equidistant hexagonal grid with a node spacing of 50 km. The P-wave velocity model was used for gravity modeling in order to obtain 3D density structure in the study region. A better fit to the observed gravity was achieved using a grid search algorithm which focused on the density contrast of the sediment-basement interface. The high resolution of 50 km is an improvement compared to older geophysical models.

Energy partitioning for seismic events in Fennoscandia and NW Russia

by Nils Maercklin

H. Bungum, T. Kværna, S. Mykkeltveit, N. Maercklin, M. Roth, K. Åstebøl, D. B. Harris, S. Larsen (2005). 27th Seismic Research Review: Ground-based nuclear explosion monitoring technologies, LA-UR-05-6407, pages 529-538

We address the problem of energy partitioning at distances ranging from very local to regional for various kinds of... more We address the problem of energy partitioning at distances ranging from very local to regional for various kinds of seismic sources, and are now in the last year of this three-year effort. On the small scale we have focused on analysis of observations from an in-mine network of 16-18 sensors in the Pyhäsalmi mine in central Finland. This analysis has been supplemented with 3D finite difference wave propagation simulations to investigate the physical mechanisms that partition seismic energy in the near source region in and around the underground mine. On the local and regional scale (20-220 km) we have targeted events from the region offshore Western Norway where we have both natural earthquake activity as well as frequent occurrence of underwater explosions carried out by the Norwegian Navy.
Since the previous reporting of this project at the 2004 Seismic Research Review (Bungum et al., 2004), we have extended the finite difference simulations in the 3D geological model of the Pyhäsalmi mine. This model, which encompasses a geologic volume 500 meters in each direction, includes 3-D representations of the ore bodies, excavated regions, tunnels, and voids. The model is discretized on both 2 and 4 meter grids making it possible to simulate seismic energy up to 100-200 Hz. We perform a variety of sensitivity tests to determine the mechanisms that produce shear energy in an underground mine environment. For example, we conduct a suite of 15000 (2-D) explosive source simulations to quantify the influence of source location on the amplitude of generated shear energy. We find that shear energy generation is particularly prevalent when the source is located near a geologic or structural boundary of the mine. In fact, most of the shear energy appears to be generated within 10-20 meters from the source (at frequencies of 50 Hz). Examination of waveforms reveals that both geologic heterogeneity and the structural influences of the mine are contributors to the near-source generation of shear energy. There is some suggestion that the geologic inhomogeneity is significant early in the wavetrain, whereas the mine structure is likely to produce scatter and be more significant later in the waveforms. As a validation measure, the synthetic waveforms are compared with observed data from single and multi-component instruments located in the mine. The simulated data match the amplitude and character of the observed waveforms particularly well, especially at frequencies at and below 50 Hz. This suggests that we can reliably infer energy partitioning phenomena based on these simulations.
A database of underwater explosions and earthquakes from the region offshore Western Norway, recorded at seven selected stations of the National Norwegian Seismic Network (NNSN), were analyzed for differences in the S/P amplitude ratios. In order to separate the path and source effects for the two event populations, we have investigated the station, distance and frequency dependencies of the recorded data in detail. The results indicate that the mean S/P amplitude ratios for both underwater explosions and natural events vary from station to station but are, in general, higher for natural events. For frequencies above 3 Hz, the difference in S/P ratios between explosions and natural events is higher than for lower frequencies. However, the distributions of S/P ratios for explosions and natural events overlap in all analyzed frequency bands. Thus, for individual events in our study area, S/P amplitude ratios can assist the discrimination between an explosion or a natural event, but other measures such as spectral analysis should be included in the interpretation.

Multinational geoscientific research effort kicks off in the Middle East

by Nils Maercklin

DESERT Group, K. Abu Ayyash et al. (2000). EOS Transactions, American Geophysical Union, 81(50), 609, 616-617, doi:10.1029/EO081i050p00609-01

The Dead Sea Rift Transect (DESERT 2000) is a multinational and interdisciplinary study of the Dead Sea Rift. The... more The Dead Sea Rift Transect (DESERT 2000) is a multinational and interdisciplinary study of the Dead Sea Rift. The project began field work in February 2000 and the first experiments were successfully completed in May. The seismic, seismological, and magnetotelluric experiments presented here, along with the future electromagnetic, gravity, magnetic, geodynamic, and geological studies, will provide the basic geophysical frame for further geoscientific research. DESERT 2000 should also help to address a fundamental question of plate tectonics: How do shear zones work and what controls them?

The crustal structure of the Dead Sea Transform

by Nils Maercklin

DESERT Group, Weber et al. (2004). Geophysical Journal International, 156(3), 655-681, doi:10.1111/j.1365-246X.2004.02143.x

To address one of the central questions of plate tectonics – How do large transform systems work and what are their... more To address one of the central questions of plate tectonics – How do large transform systems work and what are their typical features? – seismic investigations across the Dead Sea Transform (DST), the boundary between the African and Arabian plates in the Middle East, were conducted for the first time. A major component of these investigations was a combined reflection/refraction survey across the territories of Palestine, Israel and Jordan. The main results of this study are: (1) The seismic basement is offset by 3-5 km under the DST, (2) The DST cuts through the entire crust, broadening in the lower crust, (3) Strong lower crustal reflectors are imaged only on one side of the DST, (4) The seismic velocity sections show a steady increase in the depth of the crust-mantle transition (Moho) from ~26 km at the Mediterranean to ~39 km under the Jordan highlands, with only a small but visible, asymmetric topography of the Moho under the DST. These observations can be linked to the left-lateral movement of 105 km of the two plates in the last 17 Myr, accompanied by strong deformation within a narrow zone cutting through the entire crust. Comparing the DST and the San Andreas Fault (SAF) system, a strong asymmetry in subhorizontal lower crustal reflectors and a deep reaching deformation zone both occur around the DST and the SAF. The fact that such lower crustal reflectors and deep deformation zones are observed in such different transform systems suggests that these structures are possibly fundamental features of large transform plate boundaries.

Ground Truth Collection for Mining Explosions in Northern Fennoscandia and Northwestern Russia

by Nils Maercklin

D. B. Harris, F. Ringdal, E. Kremenetskaya, S. Mykkeltveit, D. Rock, N. Maercklin, J. Schweitzer, T. Hauk, J. Lewis (2005). 27th Seismic Research Review: Ground-based nuclear explosion monitoring technologies, LA-UR-05-6407, pages 31-41

We concluded comprehensive ground truth collection at the Khibiny, Olenegorsk, Kovdor and Zapolyarnyi mines, and have... more We concluded comprehensive ground truth collection at the Khibiny, Olenegorsk, Kovdor and Zapolyarnyi mines, and have basic information on 2052 explosions. In the past two years we used this ground truth information to extract waveform data from the ARCES array and a number of regional stations (KEV, LVZ, APA) as well as from six stations that we deployed along two lines stretching between the Khibiny Massif mines and the region around the ARCES array. We calculated P/S ratios using the ARCES array data for many of these events comprising several source types (compact underground explosions, underground ripple-fired explosions, surface ripple-fired explosions). We found that the P/S ratios of small compact underground explosions in mines of the Khibiny Massif are systematically lower than the P/S ratios of large ripple-fired surface explosions. We had anticipated that smaller underground shots would appear more like single well-coupled explosions, thus having higher P/S ratios than large ripple-fired explosions. A possible explanation for this phenomenon is that the compact underground explosions in these mines are designed to fracture and drop a large quantity of ore from the ceiling of a horizontal shaft. The potential energy released by the falling ore may express as shear wave energy, which may be considerably greater than the (P wave) energy released directly by the explosive.
We concluded the deployment of the six stations along the Khibiny-ARCES lines this past summer; this year we are examining the data from these stations to see how P/S ratios vary with range from the source. We expect to have an update on the P/S ratio analysis contrasting different source types for this years SRR meeting, with the addition of an analysis of range dependence using data from the temporary stations.
The portable stations were redeployed in the fall of 2004 to the Kiruna and Malmberget underground mines in northern Sweden. The stations deployed in Malmberget also record events from the surface mining operations at the Aitik mine, located some 15 km from Malmberget mine. The data from these stations will allow comparisons of seismic waveforms resulting from different types of shooting practices at different locations within the mines. These stations will provide ground truth on a large number of explosions at these mines allowing future analyses of the dependence of discriminants on source type, possibly assessing the portability of results obtained with the Khibiny explosion observations.