S13E-01 INVITED
REGIONAL MOMENT TENSOR INVERSION FOR SOURCE TYPE IDENTIFICATION
With Green's functions from calibrated seismic velocity models it is possible to use regional distance moment tensor inversion for source-type identification. The deviatoric and isotropic source components for 17 explosions at the Nevada Test Site, as well as 12 earthquakes and 3 collapses in the surrounding region of the western US, are calculated using a regional time-domain full waveform inversion for the complete moment tensor. The events separate into specific populations according to their deviation from a pure double-couple and ratio of isotropic to deviatoric energy. The separation allows for anomalous event identification and discrimination between explosions, earthquakes, and collapses. Confidence regions of the model parameters are estimated from the data misfit by assuming normally distributed parameter values. We investigate the sensitivity of the resolved parameters of an explosion to imperfect Earth models, inaccurate event depths, and data with a low signal-to-noise ratio (SNR) assuming a reasonable azimuthal distribution of stations. In the band of interest (0.02-0.10 Hz) the source-type calculated from complete moment tensor inversion is insensitive to velocity models perturbations that cause less than a half-cycle shift (<5 sec) in arrival time error if shifting of the waveforms is allowed. The explosion source-type is insensitive to an incorrect depth assumption (for a true depth of 1 km), and the goodness-of-fit of the inversion result cannot be used to resolve the true depth of the explosion. Noise degrades the explosive character of the result, and a good fit and accurate result are obtained when the signal-to-noise ratio (SNR) is greater than 5. We assess the depth and frequency dependence upon the resolved explosive moment. As the depth decreases from 1 km to 200 m, the isotropic moment is no longer accurately resolved and is in error between 50-200%. However, even at the most shallow depth the resultant moment tensor is dominated by the explosive component when the data has a good SNR. Finally, the sensitivity investigation is extended via the introduction of the network sensitivity solution, which takes into account the unique station distribution, frequency band, and SNR of a given test scenario. An example of this analysis is presented for the North Korea test.
S13E-02
Regional Seismic Amplitude Modeling and Tomography for Earthquake-Explosion Discrimination
Empirically explosions have been discriminated from natural earthquakes using regional amplitude ratio techniques such as P/S in a variety of frequency bands. We demonstrate that such ratios discriminate nuclear tests from earthquakes using closely located pairs of earthquakes and explosions recorded on common, publicly available stations at test sites around the world (e.g. Nevada, Novaya Zemlya, Semipalatinsk, Lop Nor, India, Pakistan, and North Korea). We are examining if there is any relationship between the observed P/S and the point source variability revealed by longer period full waveform modeling. For example, regional waveform modeling shows strong tectonic release from the May 1998 India test, in contrast with very little tectonic release in the October 2006 North Korea test, but the P/S discrimination behavior appears similar in both events using the limited regional data available. While regional amplitude ratios such as P/S can separate events in close proximity, it is also empirically well known that path effects can greatly distort observed amplitudes and make earthquakes appear very explosion-like. Previously we have shown that the MDAC (Magnitude Distance Amplitude Correction, Walter and Taylor, 2001) technique can account for simple 1-D attenuation and geometrical spreading corrections, as well as magnitude and site effects. However in some regions 1-D path corrections are a poor approximation and we need to develop 2-D path corrections. Here we demonstrate a new 2-D attenuation tomography technique using the MDAC earthquake source model applied to a set of events and stations in both the Middle East and the Yellow Sea Korean Peninsula regions. We believe this new 2-D MDAC tomography has the potential to greatly improve earthquake-explosion discrimination, particularly in tectonically complex regions such as the Middle East.
S13E-03
Characteristics of source properties and seismic energy of underground nuclear explosions
Understanding the shear-wave excitation mechanism is a key issue for effective seismic monitoring of nuclear explosions. The shear-wave excitation mechanism has not been fully understood yet despite efforts for several decades. The shear-wave excitation mechanism can be understood by examining the source properties and phase composition of wavetrains from underground nuclear explosions (UNEs). The October 9, 2006 UNE in North Korea was well recorded by regional stations in South Korea, Japan and China. The dense regional observation allows us to study the regional source properties of the UNE. The source-spectral parameters and attenuation factors along ray paths can be inverted from the regional waveforms. High-frequency-rich energy from the UNE appears to be attenuated less during propagation across oceanic crust than a natural earthquake. The P/S amplitude ratio is observed to be useful for discriminating between the UNE and a natural earthquake. We also study the phase contents of regional and teleseismic wavetrains of various UNEs. We rarely observe shear waves at teleseismic distances, which suggests weak radiation of shear energy from the source at low takeoff angles.
S13E-04
Shear Wave Generation by Decoupled and Partially Coupled Explosions
Decoupling is a means of evading detection by detonation of an explosion within a large cavity, which reduces the amplitude of the seismic waves. Such explosions are however still detectable with the current global seismic network, so their discrimination is important. A fully decoupled explosion detonated in the center of a spherical cavity will be a purely compressional seismic source, and so its discrimination should be straightforward. In practice however, decoupled explosions generate S waves, often identical to and sometimes even larger (relative to P) than S waves from comparable tamped explosions. If the source were purely compressional, the S waves must be the result of conversion from P and/or Rg. Asymmetries however, such as asphericity of the cavity or offset or asymmetry of the explosion, can lead to the direct generation of S waves even from a fully decoupled explosion. Fracturing or asymmetry of the nonlinear region about the cavity of a partially decoupled explosion could also result in direct generation of S waves. Most historical decoupling data have been studied extensively, but usually with the goal of quantifying P-wave decoupling. We identify S waves in the historical records, identify observations that can be used to distinguish their genesis, and model the observations to test the proposed mechanisms. Travel times and a bubble pulse peak in the P but not S spectra of water-filled cavity explosions in salt at the Soviet Azgir test site indicate that the S is generated at the source. The observed nearfield S radiation pattern of the US decoupled explosion Sterling is matched by source modeling that includes the flat floor (due to melted and recrystallized salt) of the cavity. The similarity of the Sterling coda waveforms with distance indicates their source is at or very near the cavity. Calculations of the extent and orientation of fracturing by both the Azgir and Sterling explosions predict minimal effects on the resulting waveforms. Both analytical and numerical models predict significant S wave generation by an off-center source within a spherical cavity, due to variation in the time and amplitude of the shock wave arrival at the cavity walls. We compare those predictions with records from a centered and an off-center explosion in a spherical cavity in limestone in Kirghizia.
S13E-05 INVITED
Properties of infrasonic signals due to the 2008 Iwate-Miyagi Nairiku Earthquake
We have started to make use of the infrasound data of CTBTO for geophysical researches with a cooperation of Japan Weather Association. An infrasonic signal due to the 2008 Iwate-Miyagi Nairiku Earthquake occurred on June 14, 2008 in Japan (Mw7.1, 39.03N, 140.88E, 7km depth) is detected in the infrasound data obtained at Isumi station (IS30) 417km south from the epicenter (Arai et al., 2008). This infrasound signal consists of two obvious phases. The first arrival phase appears ~ 1 minute after the origin time having peak-to-peak amplitude of ~ 3Pa, and this is considered to be a mixture of sensor movement and air oscillation generated by ground motion at the station (e.g., Watada et al., 2006). The second arrival phase appears ~ 25 minutes after the origin time having amplitude of ~ 1Pa, and this is considered to be acoustic waves propagated in the upper atmosphere directly from the epicentral region according to apparent velocity and azimuth determined by a semblance analysis. We found, in the secondary phase, remarkable three low-frequency pulses having travel times of ~ 1400, ~ 1500, and ~ 1600 seconds, respectively, and duration of ~ 1 minute. We carry out a ray-tracing of an acoustic wave that propagates in a stratified atmosphere, of which the acoustic velocity structure is derived from the NRLMSISE-00 model (Picone et al., 2002). It is found that the travel time and duration of the second pulse, which has the maximum amplitude among three pulses, can be explained by refracted waves launched continuously during the rupture time and going up to the lower thermosphere. It is also found that the remained two pulses can be explained by guide waves propagating between the stratopause and the lower troposphere reflected twice and three times at the boundary. An observation of such remarkable pulses is a rare case in the point that each ray path can be resolved clearly taking into consideration geometry and rupture time of the fault and wind field in the upper atmosphere. We believe that investigations of infrasound data for more events make it possible to clarify eventually such as propagation features of acoustic waves, temperature structure and wind field in the upper atmosphere, and coupling mechanism between solid earth and lower atmosphere in an epicentral region.
S13E-06
Assessing the detection capability of the global IMS infrasound network
A global scale analysis based on available detection lists for all operating IMS infrasound stations confirms that the primary factor controlling signal detectability is the seasonal variability of the stratospheric wind circulation. At most arrays, near %80 of the detections in the 0.2 to 2 Hz bandpass are associated with propagation downwind of the dominant wind direction. The seasonal transition in the bearings and number of detections between easterly and westerly directions is presented. The observed detection capability of the IMS network is compared to the predicted one using near-real time atmospheric updates and station- dependent wind noise models. The influence of individual model parameters on the network performance is systematically assessed. At frequencies of interest for detecting atmospheric explosions (0.2 to 2 Hz), the simulations predict that explosions equivalent to ~500 t of TNT would be detected by at least two stations of the full IMS network at any time of the year. Comprehensive ground-truth databases provide a statistical approach for evaluating the potential of infrasound monitoring. Accidental explosions are analysed and used here as benchmark for validating the calculated threshold maps. Such studies would help to optimize the siting of infrasound arrays with respect to both the number and configuration in order to monitor infrasonic sources of interest. They are an important step to enable a successful monitoring regime for atmospheric or surface events to act as an effective verification tool in any future enforcement of the CTBT.
S13E-07
Effects of the Antarctic Circumpolar Current on hydroacoustic propagation and implications for Nuclear Explosion Monitoring
A series of small depth charges was detonated along a transect from New Zealand to Antarctica over a period of three days in late December of 2006. The hydroacoustic signals were recorded by a hydrophone deployed near the source, and at a sparse network of permanent hydrophone stations operated by the International Monitoring System (IMS), at distances up to 9600 km. Our purpose was to determine how well signal characteristics could be predicted by the World Ocean Atlas 2005 (WOA05) climatological database for sources within the Antarctic Circumpolar Current (ACC). Waveforms were examined in the 1-100 Hz frequency band, and it was found that, for clear transmission paths, the shot signals exceeded the noise only at frequencies above 20-30 Hz. Comparisons of signal spectra for recordings near the source and at the IMS stations show that transmission loss is nearly uniform as a function of frequency. Where recorded signal to noise ratios are high, observed and predicted travel times and signal dispersion agree to within two seconds under the assumption that propagation is adiabatic and follows a geodesic path. The deflection of the transmission path by abrupt spatial variations in sound speed at the northern ACC boundary is predicted to decrease travel times to the IMS stations by several seconds, depending on the path. Acoustic velocities within the ACC are predicted to vary monthly, hence the accuracy of source location estimates based only on arrival times at IMS stations depends on the monthly or seasonal database used to predict travel times, and on whether we account for path deflection. However, estimates of source locations within the ACC that are based only on observed waveforms at IMS hydrophones are highly dependent on the configuration of the IMS array; a set of shots observed only at an IMS station in the Indian Ocean and another in the South Pacific was located to within 10 km in longitude, but was unconstrained in latitude. Several sets of shots observed only at IMS hydrophones in the Indian Ocean were constrained to within 55 km in latitude but were unconstrained in longitude.
S13E-08
A Tribute to Hank Bass: The Past, Present, and Future of US Infrasound Research
The deployment of the International Monitoring System (IMS) global infrasound network at the turn of the 21st century inspired a renaissance in innovation, development, and application of infrasound technology. In the United States, Hank Bass (1944-2008) was responsible for defining and directing the scientific agenda. Under his leadership, US research teams in academia, industry, and government evaluated and developed the infrastructure to deploy, maintain, and make optimal use of infrasound arrays in the IMS and national networks. Case studies of national and international interest, such as the April 23, 2001 megabolide, the February 1, 2003 Columbia disaster, and the 2004 Sumatra earthquake and tsunami, integrated monitoring technologies and renewed the value of infrasound in covering observational gaps. Dramatic technological improvements have extended the use of infrasound to new natural hazard and national security applications, which will be discussed within the context of nuclear explosion monitoring.