Broadband Seismic Data Research Articles

The 20 January 2007,M w 4.5, Odaesan, Korea, earthquake

Results of analyses for the source parameters and ground responses of the 20 January 2007 Odaesan earthquake, Korea, are presented. We use the ground velocity and acceleration data recorded at the regional seismic stations located in southern Korea. For the determination of hypocenter and origin time, we use the program HYPO71 which has been generally used for the hypocenter determination of local earthquake and perform a depth phase analysis using arrival time difference ofP n andsP n waves. The focal mechanism is obtained using a grid search algorithm with the polarities of the observedP g, Pn andSH waves. The moment magnitude (M w) and dynamic stress drop are estimated using an iterative integral method with broadband seismic data. The estimated source parameters of the Odaesan earthquake are as follows: origin time 11∶56∶53.48 in UTC, latitude 37.686°N, longitude 128.589°E, focal depth 12.1 km,M w 4.5, dynamic stress drop 77.4 bars, the fault plane 19.69°, 69.74° and −165.02°, the auxiliary plane 284.40°, 75.97° and −20.91° in strike, dip and rake, respectively. Considering geological characteristics around the epicenter, the fault plane solution with the strike angle of 19.69° is consistant with that of the existing Woljeongsa fault. The largest peak ground acceleration (PGA), 0.157 g, was recorded at the DGY station located 7.5 km east of the epicenter. The maximum instrumental intensity is estimated as VII in the Modified Mercalli Intensity (MMI) scale.

Characteristics and source modeling of broadband seismic signals associated with the hydrothermal system at Satsuma–Iwojima volcano, Japan

The broadband seismic data observed at Satsuma–Iwojima volcanic island in southwestern Japan show a wide variety of interesting features. One of them is the periodic and frequency-dependent amplitude modulation of volcanic tremors. Another interesting feature is the occurrence of very long period (VLP) seismic pulses synchronized with the tremor amplitude modulation. Both the tremor amplitude modulation and the VLP pulses have fairly regular intervals of 46–50 min, suggesting a spatially fixed, non-destructive repetitive source. We performed waveform inversions to reveal the source mechanism of the VLP pulses, assuming a point source embedded in a medium that realistically represented the topography of the island. Numerical tests showed that the Green's functions used in the inversion were significantly affected by the topography. The observed VLP pulses were well explained by the rapid expansion of an inclined crack 100 m beneath the floor of the summit crater, which had a seismic moment of 3 × 10 10 Nm. Both the characteristic amplitude modulation and the VLP pulses are explained systematically by a model consisting of a water-filled crack and a network of gas paths. The water-filled crack was heated above the boiling temperature of the water in it. A sudden break of the unstable superheated state caused sudden vaporization and a pressure increase, which was observed as the VLP pulse. The network of gas paths consisted of swollen portions and constricted portions, which worked as tremor generators when vapor from the water-filled crack passed through.

Investigation of upper mantle discontinuity structure beneath the central Pacific using SS precursors

Using a high‐quality broadband seismic data set of precursors to the phase SS, we investigate the structure of upper mantle discontinuities beneath the central Pacific including the Hawaiian hot spot. We image structure by stacking over 4000 records into geographic bins, retaining periods down to 5 s. We consider the effects of correcting for four separate tomographic models of mantle heterogeneity, excluding data at distances containing phases that potentially interfere with precursors. We find evidence for peak‐to‐peak topography of 15–20 km on the 670‐km discontinuity and 7–28 km of topography on the 400‐km discontinuity. Weak reflections are detected from discontinuities near 220‐ and 520‐km depth. The average transition zone thickness beneath our region is approximately 242 ± 3 km, very similar to previous estimates of the global average. Lateral transition zone thinning and thickening weakly correlate with reduced and increased transition zone shear velocity, respectively, consistent with a thermal origin to topographical variations on the discontinuities within our study region. The transition zone beneath Hawaii and to the east of the Line Island Chain is thinned by up to 20 km in a province spanning nearly 1000 km, suggesting an excess mantle temperature of ∼200 K. In the oldest crustal regions of our study area (>100 Ma), the 400‐km discontinuity is relatively shallow, and the transition zone is relatively thick (250–255 km); a possible explanation for this pattern includes small‐scale convection in the upper mantle bringing colder material into the transition zone.

Amplitude spectra moment tensor inversion of shallow earthquakes in Spain

SUMMARY The study of focal mechanisms for shallow earthquakes of low-to-moderate magnitude at regional distances requires a proper inversion technique. One of the major problems is the correct separation of path effects from source effects. The method used should take into account problems arising from errors in the definition of crustal structure and inclusion of the high-frequency content of the waveforms. In this study a method of frequency domain moment tensor inversion to determine deviatoric moment tensor solutions from body wave amplitude spectra is described. The method has been tested with synthetic data to define instabilities that are due to mismodelling of several parameters. The inversion technique is applied to broadband seismic data of shallow (10 km depth) earthquakes in Spain. Solutions are obtained for a range of depths and compared with solutions obtained with different methods, such as first motion polarities and moment tensor inversion in the time domain. A combined fit of amplitude spectra, first motion polarities and double-couple component is introduced as a method which resolves better the non-uniqueness of solution. The proposed method can be implemented in fast moment tensor inversion when a Green functions database has been created in advance for the studied area.

Initial-rupture fault, main-shock fault, and aftershock faults: Fault geometry and bends inferred from centroid moment tensor inversion of the 2005 west off Fukuoka prefecture earthquake

Abstract The 2005 west off Fukuoka prefecture earthquake (M JMA = 7.0) occurred on March 20, 2005 in northwest Kyusyu, Japan. The fault geometry and rupture propagation of the main shock are investigated by applying the centroid moment tensor (CMT) inversion method to densely-distributed broadband seismic network data. An accurate distribution of small aftershocks is also examined by a double-difference method. The source fault of the main shock whose moment magnitude was 6.6 had a strike of 123°, and the rupture propagated in the southeast direction. We obtain forty-four CMT solutions of the main shock and aftershocks with a moment magnitude greater than 3.5. For the main shock, the fault strike estimated using P-wave first-motion polarities differs from that of the moment tensor solution by 13° in the counterclockwise direction. Some aftershocks with a moment tensor solution similar to the focal mechanism of the main shock derived from the P-wave first motions are distributed north of the epicenter of the main shock. The largest aftershock with a strike of 8° in the clockwise direction, which differs from that of the CMT solution of the main shock, was located south of the main shock. These changes in the fault strike play an important role in the initiation and termination of the rupture process of the main shock.

Yield Estimation of the 11 May 1998 Indian Nuclear Test Using Relative Static Source Strength

The predicted total yield of the Indian nuclear tests at Pokhran on May 11, 1998 (POK2) ranges from 12 to 60kt according to the literature. A factor of about five in the uncertainties of yield estimation motivated this study. Based on the broadband seismic body wave data from the Global Seismic Network, we inverted for the effective source model parameters and estimated the seismic yield of event POK2. We found that the empirical equation between the relative static strength and yield in Nevada test site (NTS) obtained by Lay[1] is appropriate for estimating the Indian nuclear test. Our result of 36kt for event POK2 supports the point of view that the yield of POK2 is significantly less than 60kt.

2‐D or not 2‐D, that is the question: A northern California test

Reliable estimates of the seismic source spectrum are necessary for accurate magnitude and energy estimation. In particular, how seismic radiated energy scales with increasing earthquake size has been the focus of recent debate within the community and has direct implications on earthquake source physics studies as well as hazard mitigation. The 1‐D coda methodology of Mayeda et al. [2003] has provided the lowest variance estimate of the source spectrum when compared against traditional approaches that use direct S‐waves, thus making it ideal for networks that have sparse station distribution. The 1‐D coda methodology has been mostly confined to regions of approximately uniform complexity. For larger, more geophysically complicated regions, 2‐D path corrections may be required. The complicated tectonics of the northern California region coupled with high quality broadband seismic data provides for an ideal “apples‐to‐apples” test of 1‐D and 2‐D path assumptions on direct waves and their coda. Using the same station and event distribution, we compared 1‐D and 2‐D path corrections and observed the following results: (1) 1‐D coda results reduced the amplitude variance relative to direct S‐waves by roughly a factor of 8 (800%); (2) Applying a 2‐D correction to the coda resulted in up to 40% variance reduction from the 1‐D coda results; (3) 2‐D direct S‐wave results, though better than 1‐D direct waves, were significantly worse than the 1‐D coda. We found that coda‐based moment‐rate source spectra derived from the 2‐D approach were essentially identical to those from the 1‐D approach for frequencies less than ∼0.7‐Hz, however for the high frequencies (0.7 ≤ f ≤ 8.0‐Hz), the 2‐D approach resulted in inter‐station scatter that was generally 10–30% smaller. For complex regions where data are plentiful, a 2‐D approach can significantly improve upon the simple 1‐D assumption. In regions where only 1‐D coda correction is available it is still preferable over 2‐D direct wave‐based measures.

Crustal structure in Ethiopia and Kenya from receiver function analysis: Implications for rift development in eastern Africa

Crustal structure in Kenya and Ethiopia has been investigated using receiver function analysis of broadband seismic data to determine the extent to which the Cenozoic rifting and magmatism has modified the thickness and composition of the Proterozoic crust in which the East African rift system developed. Data for this study come from broadband seismic experiments conducted in Ethiopia between 2000 and 2002 and in Kenya between 2001 and 2002. Two methods have been used to analyze the receiver functions, the H‐κ method, and direct stacks of the waveforms, yielding consistent results. Crustal thickness to the east of the Kenya rift varies between 39 and 42 km, and Poisson's ratios for the crust vary between 0.24 and 0.27. To the west of the Kenya rift, Moho depths vary between 37 and 38 km, and Poisson's ratios vary between 0.24 and 0.27. These findings support previous studies showing that crust away from the Kenya rift has not been modified extensively by Cenozoic rifting and magmatism. Beneath the Ethiopian Plateau on either side of the Main Ethiopian Rift, crustal thickness ranges from 33 to 44 km, and Poisson's ratios vary from 0.23 to 0.28. Within the Main Ethiopian Rift, Moho depths vary from 27 to 38 km, and Poisson's ratios range from 0.27 to 0.35. A crustal thickness of 25 km and a Poisson's ratio of 0.36 were obtained for a single station in the Afar Depression. These results indicate that the crust beneath the Ethiopian Plateau has not been modified significantly by the Cenozoic rifting and magmatism, even though up to a few kilometers of flood basalts have been added, and that the crust beneath the rifted regions in Ethiopia has been thinned in many places and extensively modified by the addition of mafic rock. The latter finding is consistent with models for rift evolution, suggesting that magmatic segments with the Main Ethiopian Rift, characterized by dike intrusion and Quaternary volcanism, act now as the locus of extension rather than the rift border faults.

Application of near real‐time radial semblance to locate the shallow magmatic conduit at Kilauea Volcano, Hawaii

Radial Semblance is applied to broadband seismic network data to provide source locations of Very‐Long‐Period (VLP) seismic energy in near real time. With an efficient algorithm and adequate network coverage, accurate source locations of VLP energy are derived to quickly locate the shallow magmatic conduit system at Kilauea Volcano, Hawaii. During a restart in magma flow following a brief pause in the current eruption, the shallow magmatic conduit is pressurized, resulting in elastic radiation from various parts of the conduit system. A steeply dipping distribution of VLP hypocenters outlines a region extending from sea level to about 550 m elevation below and just east of the Halemaumau Pit Crater. The distinct hypocenters suggest the shallow plumbing system beneath Halemaumau consists of a complex plexus of sills and dikes. An unconstrained location for a section of the conduit is also observed beneath the region between Kilauea Caldera and Kilauea Iki Crater.

Anatomy of a lava dome collapse: the 20 March 2000 event at Soufrière Hills Volcano, Montserrat

A second extrusive phase of the currently ongoing 1995–2003 eruption of Soufrière Hills Volcano (SHV), Montserrat, commenced in mid-November 1999 following ∼19 months during which no fresh lava had reached the surface. By mid-March 2000, a new andesite lava dome constructed within a collapse scar girdled by remnants of the 1995–1998 dome complex had attained an estimated volume of ∼29±3 million m 3 (Mm 3). On 20 March 2000, during a period of heavy rainfall on the island, a significant collapse event ensued that removed ∼95% of the new lava dome (∼28±3 Mm 3) during ∼5 hours of activity that generated ∼40 pyroclastic flows and at least one magmatic explosion. The associated ash cloud reached an altitude of ∼9 km and deposited ash on the island of Guadeloupe to the southeast, and a number of lahars and debris flows occurred in valleys on the flanks of SHV. A large quantity of observational data, including contemporaneous field observations and continuous data from the broadband seismic network on Montserrat, allow a detailed reconstruction of this dome collapse event. In contrast to most of the large dome collapses at SHV, the 20 March 2000 event is distinguished by a lack of short-term precursory elevated seismicity at shallow depths beneath the lava dome. Broadband seismic amplitude data recorded during the event are used to infer the cumulative volume of collapsed dome as the collapse progressed. These data indicate that the high-velocity pyroclastic flows observed at the climax of the event removed by far the largest portion (∼68%) of the lava dome at peak discharge rates (estimated from the seismic record) of ∼2×10 4 m 3 s −1. Following the 20 March 2000 collapse, lava dome growth recommenced immediately and continued without significant interruption until another, larger dome collapse occurred on 29 July 2001. The 29 July 2001 event also coincided with heavy rainfall on Montserrat [Matthews et al. (2002) Geophys. Res. Lett. 29; DOI:10.1029/2002GL014863] and lacked precursory elevated seismic activity. We attribute the initiation of the 20 March 2000 collapse to a prolonged spell of heavy rainfall on the lava dome prior to and during the event. The precise causal mechanism remains controversial, though some combination of mechanical erosion and/or destabilization of a critically poised face of the lava dome, the action of pressurized steam or water on potential failure surfaces within the dome, rapid cooling of hot lava and small phreatic explosions seems likely. Anecdotal evidence exists for other rainfall-induced activity on Montserrat, and the triggering of explosive or pyroclastic flow activity by rainfall has been noted at dome-forming volcanoes elsewhere, including Merapi, Indonesia [Voight et al. (2000) J. Volcanol. Geotherm. Res. 100, 69–138], Unzen, Japan [Yamasato et al. (1997) Papers Meteorol. Geophys. 48], Santiaguito, Guatemala [Smithsonian Institution Global Volcanism Network Bull. 15 (1990)] and Mount St. Helens, USA [Mastin (1994) Geol. Soc. Am. Bull. 106, 175–185]. Hazard mitigation plans at dome-forming volcanoes would therefore benefit from the inclusion of meteorological forecasting and rain monitoring equipment, particularly in the tropics.

Lithospheric structure of the Pilbara Craton, Capricorn Orogen and northern Yilgarn Craton, Western Australia, from teleseismic receiver functions

The upper lithospheric structure from the Pilbara Craton, across the Capricorn Orogen, to the northern Yilgarn Craton is determined from high‐fidelity broadband seismic data. Three‐component temporary stations were deployed in a line running southwards from Marble Bar, Western Australia, at approximately 118°E longitude. They were in position between July and October 2000. Receiver functions are calculated from the recorded teleseismic earthquakes stacked to improve the signal to noise ratio and, finally, the stacked waveforms used to model the seismic velocity profile under each recording station. Data from the permanent Global Seismic Network station, MBWA, at Marble Bar, which was installed in August 2001, are also used. The method provides a means of probing the deep structure of the Earth's crust and the nature of the seismic Moho at an intermediate scale between that of detailed active‐source refraction techniques and the regional structure obtained from earthquake seismic tomography. The receiver functions and modelled seismic velocity structures along the north‐south profile show the crust‐mantle boundary under the Pilbara Craton to be shallow, at 30 km (±2 km), with a sharp Moho and high‐velocity crust beneath the exposed Pilbara granite‐greenstone terrane. The sharp Moho extends under the metasediments of the Hamersley Basin. Beneath the Capricorn Orogen the Moho is barely discernible, showing a small seismic velocity contrast and a broad zone of high‐velocity gradient. The northern Yilgarn Craton, which extends beneath the basins exposed on the surface, is deeper, at 40 km (±2 km), and again shows a sharply defined Moho.

Eruption-induced modifications to volcanic seismicity at Ruapehu, New Zealand, and its implications for eruption forecasting

Broadband seismic data collected on Ruapehu volcano, New Zealand, in 1994 and 1998 show that the 1995–1996 eruptions of Ruapehu resulted in a significant change in the frequency content of tremor and volcanic earthquakes at the volcano. The pre-eruption volcanic seismicity was characterized by several independent dominant frequencies, with a 2 Hz spectral peak dominating the strongest tremor and volcanic earthquakes and higher frequencies forming the background signal. The post-eruption volcanic seismicity was dominated by a 0.8–1.4 Hz spectral peak not seen before the eruptions. The 2 Hz and higher frequency signals remained, but were subordinate to the 0.8–1.4 Hz energy. That the dominant frequencies of volcanic tremor and volcanic earthquakes were identical during the individual time periods prior to and following the 1995–1996 eruptions suggests that during each of these time periods the volcanic tremor and earthquakes were generated by the same source process. The overall change in the frequency content, which occurred during the 1995–1996 eruptions and remains as of the time of the writing of this paper, most likely resulted from changes in the volcanic plumbing system and has significant implications for forecasting and real-time assessment of future eruptive activity at Ruapehu.

Location and mechanism of the Little Skull Mountain earthquake as constrained by satellite radar interferometry and seismic waveform modeling

We use interferometric synthetic aperture radar (InSAR) and broadband seismic waveform data to estimate source parameters of the 29 June 1992, Ms 5.4 Little Skull Mountain (LSM) earthquake. This event occurred within a geodetic network designed to measure the strain rate across the region around Yucca Mountain. The LSM earthquake complicates interpretation of the existing GPS and trilateration data, as the earthquake magnitude is sufficiently small that seismic data do not tightly constrain the epicenter but large enough to potentially affect the geodetic observations. We model the InSAR data using a finite dislocation in a layered elastic space. We also invert regional seismic waveforms both alone and jointly with the InSAR data. Because of limitations in the existing data set, InSAR data alone cannot determine the area of the fault plane independent of magnitude of slip nor the location of the fault plane independent of the earthquake mechanism. Our seismic waveform data tightly constrain the mechanism of the earthquake but not the location. Together, the two complementary data types can be used to determine the mechanism and location but cannot distinguish between the two potential conjugate fault planes. Our preferred model has a moment of ∼3.2 × 1017 N m (Mw 5.6) and predicts a line length change between the Wahomie and Mile geodetic benchmarks of ∼5 mm.

Polarization analyses of broadband seismic data recorded on Stromboli Volcano (Italy) from 1996 to 1999

[1] We analyze broadband seismic data recorded at a single station deployed on Stromboli island from 1996 to 1999 in order to determine basic parameters of the seismic wavefield and study their relationship with volcanic activity. In addition to a characterization in the frequency domain, we apply polarization analysis and study long-term variations of these parameters possibly reflecting also changes in source properties. Our analysis is twofold. First, we focus on a meaningful reduction of the data to selected parameters, i.e., azimuth and incidence angle, which can be correlated with volcanic activity. Secondly, we investigate if there is maintenance of the correlations over long time spans. We observe statistically significant long-term changes (from several months to year) in the wavefield parameters considered. We interpret such changes as possibly being caused by the enhanced strength of a deep source, potentially by supply of fluids from a low part of the magma feeder between the end of 1997 and the first months of 1998.

Application of Distributed Object Technology to Seismic Waveform Distribution

We develop a networked data distribution system for broadband seismic waveform data using Java RMI (Remote Method Invocation). Users can download data from various networks using a unified interface. There are two technical problems in building such a user-friendly system with a high capability for expansion: communication through firewalls and information synchronization between request interfaces. We solve these two problems using cgi (common gateway interface) filtering and the Directory Service, respectively. We are now distributing data from various Japanese broadband seismic networks (including PACIFIC21) together with data from BATS (Broadband Array in Taiwan for Seismology) and IRIS through the system. Digital seismic waveform data are being accumulated at an accelerating rate by various networks. Data analysis using these huge data sets is becoming increasingly common. For example, in the Western Pacific region, a large number of digital broadband seismographic stations have recently been installed by various institutions (Figure 1). If the data from these networks are appropriately combined, they effectively constitute a data set from a very dense broadband network. Such a data set should be very useful for determining seismic structure and earthquake source properties with high resolution. However, in the current waveform distribution system, such studies require somewhat tedious procedures for collecting the waveform data from different data centers, each of which has a different user interface. Looking at the field of computer science, distributed object technology (technology for distributing software modules to multiple computers on a network; The Object Management Group, 2000) is one of the most important technologies in modern system-programming practice, as it can handle a huge system in a straightforward manner. Java RMI (Gosling and Steele, 1996; Sun Microsystems, 1999) provides one of the most successful technologies to build a distributed object management system because of its high capability for expansion and security. …

Study of Regional Surface Waves and Frequency-dependent M s : m b Discrimination in the European Arctic

Accurate discrimination of seismic events with a regional network requires detailed knowledge of the propagation characteristics of seismic waves in the region. At present, such propagation characteristics are reasonably well known for P and S waves in the European Arctic, however much work remains to be done regarding surface wave propagation and magnitude estimation.Regional long-period or broadband seismic data in digital form has been available in the European Arctic for only a few years. In order to assess regional surface wave propagation, and in particular to evaluate the M s :m b discriminant at regional distances, it is therefore necessary to take advantage of the historic analog recordings. The station APA in Apatity forms a unique source of such data, with high-quality long-period seismic recordings of regional earthquakes and nuclear explosions dating back about 30 years.

Eurasia‐Africa Plate Boundary region yields new seismographic data

The tectonic plate boundary between Eurasia and Africa is complex, in that it cannot be characterized as a single discrete plate boundary Deformation near this plate boundary varies from trans‐tensional in the Azores archipelago, through strike‐slip in the eastern Atlantic basin, to overall compressional between the European and African continents, with extensional sub‐domains in the Mediterranean Sea. This complex pattern of deformation, related plate motion, and underlying driving forces leads to strong variations in seismic hazard throughout the region. A better understanding of the plate boundary processes requires knowing crust and upper mantle structure in the region, which is best investigated with three‐component, broadband seismic data. To investigate the region's three‐dimensional crust and upper mantle structure, we are carrying out a multiinstitutional project (MIDSEA) involving seismologists from 10 countries on the northern, southern, and western sides of the plate boundary.

Velocity structure of the upper mantle discontinuities beneath North America from waveform inversion of broadband seismic data using a genetic algorithm

We present a velocity structure of the mantle discontinuities beneath the United States from a full waveform inversion of broadband seismic data. The data consist of 10 moderate earthquakes in the western United States that were recorded on 13 broadband seismometers across the United States operated by the U.S. National Seismic Network. A total of 36 seismograms with distance ranging from 1150 to 2700 km were used. A full waveform inversion based on two‐dimensional modeling and one‐dimensional inversion methods using a genetic algorithm was adopted. The inversion method is objective, provides error bounds on the model, and does not require any a priori information about these discontinuities. We find that the upper discontinuity is ∼10 km thick, from 395 to 405 km, where the velocity increases from 8.80 to 9.15 km s−1 with a velocity gradient of 0.035 s−1. The lower discontinuity is ∼5 km thick, occurring over a depth range of 650–655 km, where the velocity increases from 10.20 to 10.70 km s−1. Our results are in agreement with previous studies of the lower discontinuity but do not agree for a sharp (<5 km) upper mantle discontinuity.

Seismic Source Characteristics of Soviet Peaceful Nuclear Explosions

— During the period 1965 to 1988, the former Soviet Union (FSU) conducted over 120 peaceful nuclear explosions (PNE) at locations widely dispersed throughout the territories of the FSU. These explosions sample a much wider range of source conditions than do the historical explosions at the known nuclear test sites and, therefore, seismic data recorded from these PNE tests provide a unique resource for use in deriving improved quantitative bounds on the ranges of seismic signal characteristics which may require consideration in global monitoring of the Comprehensive Test-Ban Treaty (CTBT). In this paper we summarize the results of a detailed statistical analysis of broadband seismic data recorded at the Borovoye Geophysical Observatory from 21 of these PNE tests at regional distances extending from about 7 to 19 degrees, as well as the results of theoretical waveform simulation analyses of near-regional (Δ < 25 km) seismic data observed from a selected sample of nine of these PNE tests. The results of these analyses have been found to be consistent with those of previous teleseismic investigations in that they indicate that the seismic source coupling efficiencies are very similar for explosions in a wide variety of hardrock and water-saturated media, while explosions in water-saturated clay are observed to have significantly higher coupling efficiencies. Moreover, the scaling of the seismic source function with explosion yield and depth of burial inferred from these analyses of the Soviet PNE data are shown to be generally consistent with the predictions of the Mueller/Murphy source model. These results suggest that the Mueller/Murphy source model can provide a reasonable basis for estimating the expected variation in regional phase spectral composition over a wide range of nuclear source conditions of potential interest in CTBT monitoring.

Seismic Properties of the Central Indian Shield

We use broadband seismic data at Hyderabad to investigate average crustal properties of the central Indian shield. Crustal receiver-functions ( P to SV conversions) based on data of excellent quality and azimuthal coverage show essentially no signal on the transverse component, indicating laterally homogeneous structures near this station. A joint analysis of receiver-functions and the dispersion of fundamental mode Rayleigh waves result in a simple model for the shear wave speed ( V s ) with a moderate gradient of 0.010–0.028 km/sec/km in the crust. The Moho is sharp, with a contrast of approximately 0.7 km/sec in V s across a small thickness of only 0–4 km. Measured from the middle of the Moho region, the crust is 30–34 km thick, with an average V s of 3.58 ± 0.10 km/sec. By modeling the timing and waveforms of Sp and SsPmp phases ( SV to P conversions), we estimate the average crustal Poisson9s ratio to be 0.26 ± 0.01. This value is comparable to that found in the Tanzanian craton which is also of Archean age, both values being somewhat lower than the global average of 0.29 ± 0.02 for shield regions that include both Proterozoic and Archean crust. A low Poisson9s ratio and a slow V s in the crust indicate that, on average, Archean crust beneath the central Indian shield is intermediate to felsic in composition, somewhat less mafic than globally averaged Precambrian crust.