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Ithas proven difficult to uniquely untangle the source and propagation effects on the observed seismic data from underground nuclear explosions, even when large quantities of near-source, broadband data are available for analysis. This leads to uncertainties in our ability to quantify the nuclear seismic source function and, consequently the accuracy of seismic yield estimates for underground explosions. Extensive deterministic modeling analyses of the seismic data recorded from underground explosions at a variety of test sites have been conducted over the years and the results of these studies suggest that variations in the seismic source characteristics between test sites may be contributing to the observed differences in the magnitude/yield relations applicable at those sites. This contributes to our uncertainty in the determination of seismic yield estimates for explosions at previously uncalibrated test sites. In this paper we review issues involving the relationship of Nevada Test Site (NTS) source scaling laws to those at other sites. The Joint Verification Experiment (JVE) indicates that a magnitude(m b )bias(6m b )exists between the Semipalatinsk test site (STS) in the former Soviet Union (FSU) and the Nevada test site (NTS) in the United States. Generally thisSmbis attributed to differential attenuation in the upper-mantle beneath the two test sites. This assumption results in rather large estimates of yield for largem b tunnel shots at Novaya Zemlya. A re-examination of the US testing experiments suggests that thisSm b bias can partly be explained by anomalous NTS (Pahute) source characteristics. This interpretation is based on the modeling of US events at a number of test sites. Using a modified Haskell source description, we investigated the influence of the source Reduced Displacement Potential (RDP) parameters LKand Bby fitting short-and long-period data simultaneously, including the near-field body and surface waves. In general, estimates of Band Kare based on the initial P-wave pulse, which various numerical analyses show to be least affected by variations in near-source path effects. The corner-frequency parameter Kis 20% lower at NTS (Pahute) than at other sites, implying larger effective source radii. The overshoot parameter Bappears to be low at NTS (although variable) relative to other sites and is probably due to variations in source conditions. For a low B,the near-field data require a higher value of to match the long-period M s and short-periodmbobservations. This flexibility in modeling proves useful in comparing released FSU yields against predictions based onm b andM s .

Regional crustal waveguide calibration is essential to the retrieval of source parameters and the location of smaller (M < 4.8) seismic events. This path calibration of regional seismic phases is strongly dependent on the accuracy of hypocentral locations of calibration (or master) events. This information can be difficult to obtain, especially for smaller events. Generally, explosion or quarry blast generated travel-time data with known locations and origin times are useful for developing the path calibration parameters, but in many regions such data sets are scanty or do not exist. We present a method which is useful for regional path calibration independent of such data, i.e. with earthquakes, which is applicable for events down to Mw = 4 and which has successfully been applied in India, central Asia, western Mediterranean, North Africa, Tibet and the former Soviet Union. These studies suggest that reliably determining depth is essential to establishing accurate epicentral location and origin time for events. We find that the error in source depth does not necessarily trade-off only with the origin time for events with poor azimuthal coverage, but with the horizontal location as well, thus resulting in poor epicentral locations. For example, hypocenters for some events in central Asia were found to move from their fixed-depth locations by about 20 km. Such errors in location and depth will propagate into path calibration parameters, particularly with respect to travel times. The modeling of teleseismic depth phases (pP, sP) yields accurate depths for earthquakes down to magnitude Mw = 4.7. This Mwthreshold can be lowered to four if regional seismograms are used in conjunction with a calibrated velocity structure model to determine depth, with the relative amplitude of the Pnl waves to the surface waves and the interaction of regional sPmP and pPmP phases being good indicators of event depths. We also found that for deep events a seismic phase which follows an S-wave path to the surface and becomes critical, developing a head wave by S to P conversion is also indicative of depth. The detailed characteristic of this phase is controlled by the crustal waveguide. The key to calibrating regionalized crustal velocity structure is to determine depths for a set of master events by applying the above methods and then by modeling characteristic features that are recorded on the regional waveforms. The regionalization scheme can also incorporate mixed-path crustal waveguide models for cases in which seismic waves traverse two or more distinctly different crustal structures. We also demonstrate that once depths are established, we need only two-stations travel-time data to obtain reliable epicentral locations using a new adaptive grid-search technique which yields locations similar to those determined using travel-time data from local seismic networks with better azimuthal coverage.

We present a study of regional earthquakes in the western Mediterranean geared toward the development of methodologies and path calibrations for source characterization using regional broadband stations. The results of this study are useful for the monitoring and discrimination of seismic events under a comprehensive test ban treaty, as well as the routine analysis of seismicity and seismic hazard using a sparse array of stations. The area consists of several contrasting geological provinces with distinct seismic properties, which complicates the modeling of seismic wave propagation. We started by analyzing surface wave group velocities throughout the region and developed a preliminary model for each of the major geological provinces. We found variations of crustal thickness ranging from 45 km under the Atlas and Betic mountains and 37 km under the Saharan shield, to 20 km for the oceanic crust of the western Mediterranean Sea, which is consistent with earlier works. Throughout most of the region, the upper mantle velocities are low which is typical for tectonically active regions. The most complex areas in terms of wave propagation are the Betic Cordillera in southern Spain and its north African counterparts, the Rif and Tell Atlas mountains, as well as the Alboran Sea, between Spain and Morocco. The complexity of the wave propagation in these regions is probably due to the sharp velocity contrasts between the oceanic and continental regions as well as the the existence of deep sedimentary basins that have a very strong influence on the surface wave dispersion. We used this preliminary regionalized velocity model to correct the surface wave source spectra for propagation effects which we then inverted for source mechanism. We found that this method, which is in use in many parts of the world, works very well, provided that data from several stations are available. In order to study the events in the region using very few broadband stations or even a single station, we developed a hybrid inversion method which combines P_(nl) waveforms synthesized with the traditional body wave methods, with surface waves that are computed using normal modes. This procedure facilitates the inclusion of laterally varying structure in the Green's functions for the surface waves and allows us to determine source mechanisms for many of the larger earthquakes (M > 4) throughout the region with just one station. We compared our results with those available from other methods and found that they agree quite well. The epicentral depths that we have obtained from regional waveforms are consistent with observed teleseismic depth phases, as far as they are available. We also show that the particular upper mantle structure under the region causes the various P_n and S_n phases to be impulsive, which makes them a useful tool for depth determination as well. Thus we conclude that with proper calibration of the seismic structure in the region and high-quality broadband data, it is now possible to characterize and study events in this region, both with respect to mechanism and depth, with a limited distribution of regional broadband stations.

Twelve moderate-magnitude earthquakes (m_b 4–5.5) in the Pamir-Hindu Kush region are investigated to determine their focal mechanisms and to relocate them using their regional waveform records at two broadband arrays, the Kyrgyzstan Regional Network (KNET), and the 1992 Pakistan Himalayas seismic experiment array (PAKH) in northern Pakistan. We use the “cut-and-paste” source estimation technique to invert the whole broadband waveforms for mechanisms and depths, assuming a one-dimensional velocity model developed for the adjacent Tibetan plateau. For several large events the source mechanisms obtained agree with those available from the Harvard centroid moment tensor (CMT) solutions. An advantage of using regional broadband waveforms is that focal depths can be better constrained either from amplitude ratios of Pnl to surface waves for crustal events or from time separation between the direct P and the shear-coupled P wave (sPn + sPmP) for mantle events. All the crustal events are relocated at shallower depths compared with their International Seismological Centre bulletin or Harvard CMT depths. After the focal depths are established, the events are then relocated horizontally using their first-arrival times. Only minor offsets in epicentral location are found for all mantle events and the bigger crustal events, while rather large offsets (up to 30 km) occur for the smaller crustal events. We also tested the performance of waveform inversion using only two broadband stations, one from the KNET array in the north of the region and one from the PAKH array in the south. We found that this geometry is adequate for determining focal depths and mechanisms of moderate size earthquakes in the Pamir-Hindu Kush region.

The mb:MS ratio determined by teleseismic observations has proven to be an effective discriminant, for explosive sources tend to be significantly richer in short-period energy than are earthquakes. Unfortunately, this method is limited by the detection threshold of teleseismic surface waves. However, recent advances in instrumentation allowing low amplitude surface wave measurements coupled with new analytical techniques make it feasible to use regional waveform data to determine the long-period source excitation level of low magnitude events. We propose using the ratio of ML (local magnitude) to M0 (scalar seismic moment) as an analogous regional discriminant. We applied this criterion to a data set of 299 earthquakes and 178 explosions and found that this ratio seems to be diagnostic of source type. For a given M0, the ML of an explosion is more than 0.5 magnitude units larger than that of an earthquake. This separation of populations with respect to source type can be attributed to the fact that ML is a short-period (1 Hz) energy measurement, whereas seismic moment is determined from long-period body wave phases (period > 4 s) and surface waves (10 to 40 sec). Using regional stations with sources 200 to 600 km away, the effective threshold for magnitude measurements for this discriminant is found to be ML = 3.1 for earthquakes and ML = 3.6 for explosions. This method does require the determination of regional crustal models and path calibrations from master events or by other means.

We have examined broadband regional waveforms of recent (since 1988) Nevada Test Site (NTS) underground explosions and earthquakes throughout the southwestern United States and Baja, Mexico, recorded by TERRAscope and other IRIS stations in order to characterize seismic sources for the purposes of event identification. As expected, earthquakes tended to be richer in long-period surface-wave and short-period shear-wave energy relative to explosions of comparable P-wave strength. Also, explosions, in general, were found to be richer in 1- to 6-sec surface-wave (Rg) energy and other late-arriving coda energy than were earthquakes. Most earthquakes show relatively little long-period (T > 6 sec) Rg and surface-wave coda energy, which we attribute to their deeper source depths, whereas known shallow earthquakes do exhibit these phases. We have developed several seismic discriminants based on our observations. The most promising discriminant is the ratio of short-period (f ≧ 1.0 Hz), vertical component, Pnl wave-train energy (EspPz) to long-period (0.05 to 0.167 Hz), three-component, surface-wave energy (Elp−3). For this ratio, explosions tend to have a higher value than do earthquakes. This discriminant works on the same premise as the teleseismic mb:MS ratio, for which earthquakes are richer in long-period surface-wave energy relative to explosions with the same body-wave magnitude. The long-period passband was chosen to limit the effect of longer-period noise and to remove the effect of the coda surface waves. Another potential discriminant examined is the ratio of short-period (f ≧ 1.0 Hz), vertical-component, P-wave to S-wave energy (EspPz:EspSz). We find that this criterion only yields marginal separation of the source populations but becomes more effective at higher frequency bands (f ≧ 4.0 Hz) or when looking at single-station observations. It does, however, help to quantify significant short-period waveform differences between the three test subsites, with Pahute Mesa shots generating relatively little S-wave energy compared to those of Yucca Flat for which the S wave (or Lg) is often the largest phase, while Rainier's shots are intermediate in character with distinct but less prominent S waves. This S-wave generation is thought to be caused by near-source scattering to converted phases and appears to be highly dependent on the near-source geology. These two discriminants are useful in that they are simple and fast to calculate. Using regional stations for sources 200 to 1300 km away, the magnitude threshold for the EspPz:Elp−3 discriminant is roughly ML ≧ 4.0, the limiting factor being the signal level of the Airy phase, while that for the EspPz:EspSz discriminant is roughly ML ≧ 3.0 for the same distance ranges.

We re-examine the use of surface-wave magnitudes to determine the yield of underground nuclear explosions and the associated magnitude-yield scaling relationship. We have calculated surface-wave magnitudes for 190 Nevada Test Site (NTS) shots using regional long-period seismograms from a combined super-network of 55 North American stations. Great effort went towards making the data set comprehensive and diverse in terms of yield, source location and shot medium in order to determine the portability of surface-wave magnitude scales. In particular, we examine Pahute Mesa, Rainier Mesa and Yucca Flat explosions detonated above and below the water table, and which range over three orders of magnitude in yield. By observation we find a low-yield measure threshold of approximately one kiloton (kt) for (assumedly) moderately well-coupled explosions recorded at near-regional (

With the spread of nuclear weapons technology, more regions of the world need to be monitored in order to verify nuclear nonproliferation and limited test-ban treaties. Seismic monitoring is the primary means to remotely sense contained underground explosions “Bolt, 1976; Dahlman and Israelson, 1977”. Both underground explosions and earthquakes generate seismic energy, which propagates through the Earth as elastic waves. The crux of the verification problem is to differentiate between the seismic signatures of explosions and earthquakes. Such identification is most difficult in countries with seismically active areas, where bombs might be detonated to blend in with the region's natural seismicity.