S41A-01
Density, Shear and Compressional Velocity Models of the Vrancea Seismogenic Zone
Additional constraints on the geodynamic models for the origin of the intermediate depth Vrancea Seismogenic Zone are given by three-dimensional P and S-wave velocity and density images. The reconstructed physical parameters aim to substantiate or eliminate two contrasting models which explain the Vrancea seismicity: the subduction model and the active continental lithospheric delamination model. For our goal, we apply the tomographic inversion method of sequential integrated inversion proposed by Tondi and de Franco (2006) to shot data collected during the VRANCEA99 (Hauser et al., 2001) and VRANCEA2001 (Landes et al., 2004) seismic refraction experiments, to local earthquake data collected during the CALIXTO (EOS, 1998) experiment and to recent gravity measurements of the studied area. We first locate P and S wave sources of local events with the NonLinLoc location program (Lomax et al., 2000) and then we consider these events as those originated from shots points. The mathematical formulation of the seismic travel time inversion algorithm, which regularizes the solution with the minimization of the first and the second partial derivatives of the functionals describing the velocity parameters, enables us to control the proliferation of caustics and arrivals during iterations, which is a common problem when using ray-tracing techniques with realistic and extensive heterogeneous velocity models. This increases the robustness and efficiency of the method and efficiently handles a seismic data set which is severely affected by scattering effects. Furthermore, the density model parametrization, which uses polyhedral bodies whose density is linearly dependent on the three coordinates (Pohànka, 1998), leads to a perfect match between the density and the velocity model parametrization and takes into account the presence of geological structures characterized by a gradual increase in density with depth. After each iteration, the events are relocated with the updated velocity model until the discrepancies between two subsequent localizations are sufficiently small. The reliability of the reconstructed models, which explain equally well both travel times and gravity data, is quantified through a restoring test and the estimation of travel times and gravity residuals.
S41A-02
Crustal Structure of the Pakistan Himalayas from Ambient Noise and Seismic Rayleigh Wave Inversion
The western Himalayan syntaxi is a unique feature resulted from the India-Asia collision and its formation remains poorly understood. To image crustal structure in the western syntaxi, we analyze Rayleigh waves from ambient seismic noise and earthquake data recorded during the Pakistan Broadband Seismic Experiment. The Pakistan experiment included 9 broadband stations with an aperture of ~200 km and operated from September to December in 1992. We compute cross-correlations of ambient noise data on an hourly base and stack all the cross-correlations for 70 days to produce the estimated Green functions. Power spectrum analysis shows that the dominant energy is from 0.15 to 0.25 Hz and from 0.05 to 0.07 Hz, consistent with the well-know background seismic noise. A phase with large amplitude appears at near zero time on almost all stacked cross- correlations and its origin is not clear to us at this moment. Rayleigh waves can be clearly observed for station pairs at the distance of 80 km and larger but are contaminated by the near zero time phase at shorter station spacing. Rayleigh wave phase velocities at periods of 4 to 15 s will be produced from the ambient noise data. Using regional and teleseismic earthquakes, we expect to obtain Rayleigh wave dispersions at periods from 15 to 50 s. The phase velocities from both datasets will be inverted for crustal thickness and shear-wave structure beneath the Pakistan Himalayas.
S41A-03
A Study of Dispersion and Directionality Using Ambient Seismic Noise in Pakistan
As suggested by recent theoretical and observational studies, the long-time average cross-correlation of the ambient noise field between two seismographs approximates the inter-station Green's function. Using data from a PASSCAL deployment of 9 broadband seismographs in NE Pakistan (1992), we examine estimates of the longitudinal, radial and transverse Green's functions between all station pairs. The seismic stations have separation distances ranging from 40 to 160 km, and in most cases 73 days of continuous data are cross- correlated for each pair. Dispersion characteristics of the time-averaged correlation functions are analyzed using Hilbert transform filtering. Preliminary results show group velocities between 1 and 4 km/s for the period band between 2 and 20 seconds. Asymmetry between the causal and acausal portions of the cross-correlation functions is interpreted as preferred directionality of the ambient noise field. By comparing peak energy at causal and acausal delay times, the field appears to propagate from the north-northeast over the time period of interest. This may indicate that noise field propagation is related to the regional tectonics at short periods.
S41A-04
Seismic-Wave Attenuation and Source Excitation in La Paz-Los Cabos, Baja California Sur, Mexico
We present results from a regional study of seismic-wave attenuation and source excitation from small- magnitude earthquakes recorded at distances from 6 to 180 km in the La Paz-Los Cabos region, at the south end of the Baja California Peninsula. Data were recorded using 32 strong-motion seismic stations from the La Paz network (LAP). A least squares regression separating the excitation, site, and propagation effects was carried out. We performed the analyses in the time and frequency domains, and we compared these results with results from a coda-normalization method. The propagation term was parameterized to represent a geometrical spreading function and a frequency-dependent Q(f) at a reference distance of 40 km. We estimated the regional attenuation by measuring the maximum amplitude of the S- or Lg-waves as a function of frequency, defining a continuous piecewise propagation term, D(r,f), after separating the excitation and site terms. Our results show that the attenuation is lower compared to that of central or northern Mexico. Recorded data were of remarkably good quality in spite of the fact that the strong-motion network recorded only small-size earthquake.
S41A-05
Chicxulub Peak Ring Characteristics from 2D Reflection Seismic Survey
Since 1980's research interest over Chicxulub crater located SE Gulf of Mexico, has grown not only because its
relationship with the K-P(Cretaseous -Paleogene) extinction but because of its size (diameter ~ 200 km) and
grade of preservation.
Based on results from several surveys using different geophysical methods, Chicxulub has been classified as a
multiring crater. A topographic high rising from crater floor was first recognized as the Chicxulub peak ring on four
1996 reflection seismic profiles but the low density of this data set made impossible to describe on detail this
structure. Recently, during 2005 we carried out a marine survey acquiring 29 profiles. A grid located over the
central marine portion of the crater was conformed by eleven profiles 80 km long oriented WSW-ENE and ten 25
km long NW-SE. Data was recorded on 480 channels spaced 12.5 cm on a 6 km streamer and air guns were
shot every 50 m allowing us to image the earth up to 14 s TWTT.
This new data set along with the 1996 profiles allow us to build up the first 3D image of Chicxulub peak ring as
well as to analyze some important features of this ring. Results show that the peak ring lays down closer to the
surface and the crater rim on its NW portion where it rises more abruptly from the crater floor reaching up to 430
m. Based on the information of the radial lines this characteristics change in clockwise direction being opposite
on the NE. The relationship between the peak ring and other Chicxulub structures,such as the slump blocks and
the dipping reflector, change as well in the same direction indicating that the peak ring is displaced to the NW.
These asymmetries could be related to the process of formation of the peak ring as a result of: a)an asymmetric
collapse of the central uplift which has been proved not to be related to impact direction, b)
displacement of the central uplift towards the transient cavity rim or c)heterogeneities on impact surface predating
the impact.
http:www.ig.utexas.edu/people/staff/sean/
S41A-06
Depth Seismic-Migration Modeling Offshore `Tierra Del Fuego', Argentina (54° 25' S)
Within the framework of the TESAC Project (Tectonic Evolution of the South America-Scotia plate boundary during the Cenozoic), about 900 km of multichannel seismic reflection profiles were acquired off the Atlantic coast of the Tierra del Fuego Island. The profiles cut across the South America-Scotia plate boundary, a transform margin which traverses in an E-W direction the Island. Data processing and interpretation of a seismic reflection profile is presented in this contribution. A robust post-stack technique involved depth-migration of the seismic section, using an interval-velocity model of the upper Crust adjusted by iterative processing. An interpreted seismic- velocity section (which trends roughly NW-SE), shows a complex superposition of different tectonic structures, with presence of extensional, compressional and transtensional features in the area located to the north of Isla de los Estados. The profile, which crosses the offshore part of the Magallanes fold-and-thrust belt, images the deep structural framework of part of this tectonic province. The identification of acoustic fabrics and seismic discontinuities allowed us to recognize four main units. Overlaying the acoustic basement (Seismic unit 1), there is another unit (Seismic unit 2) which exhibits some reflector packages of high amplitude; this unit must be related to the volcanic and volcaniclastic sequences of Tobífera/Lemaire Fms. The Seismic Unit 3 displays internal reflector configurations of moderate amplitude and continuity and low-to-moderate frequency; the Yaghán/Beauvoir Fms must be the onshore equivalent of this unit. An uppermost seismic layer (Unit 4) may be correlated with the Tertiary sediments of the Magallanes foreland basin which were involved in the fold and thrust belt. A major structure identified in the studied seismic profile is a SE structural high (which involves the units 1, 2 and 3) and a NW down-faulted area. The latter display folds of kilometric size (3-4 km). Steeply dipping (mainly to the south) reverse faults cut through the folds. Some of these faults represent old extensional faults of the Middle- Mesozoic Rocas Verdes marginal basin rifting, which have been subsequently inverted by compressional stress fields. In addition, a noticeable flower fault system is recognized in the seismic profile, which involves both the sedimentary cover and the acoustic basement. This fault is interpreted as the result of shear stresses produced along the transcurrent South America-Scotia plate boundary.
S41A-07
Analyzing the Resolvability of Crustal and Mantle Velocity Anomalies beneath Tibet
The Tibetan Plateau is a product of the largest active continent-continent collision event on Earth. A variety of models have been proposed that explain its Cenozoic tectonic evolution [Harrison et al., 1992], such as continental underthrusting, delayed underthrusting, distributed shortening, mantle lithosphere detachment and so on. Each of the models makes specific predictions about the lithosphere structure in the India-Asia collision zone that may be verifiable by seismic imaging. For instance, a recent study [Zhou and Murphy, 2005] using re- processed ISC traveltimes provided evidence for wholesale underthrusting of Indian slab beneath much of the Tibetan plateau. However, due to the lack of seismologic stations over the Tibetan region, it is necessary to quantify the resolvability of the tomographic methods and data before drawing conclusions on the velocity anomalies. As an example, the demean process (removal of the layer velocity averages or removing traveltime means) that is used in most tomography studies will hinder the ability to resolve sub-horizontal features such as the possible underthrusting Indian slab anomaly. We attempt to quantify the resolvability of the crustal and mantle velocity anomalies by conducting tomographic resolution tests along a series of 2D dip lines across the Tibetan plateau, using the actual station and event positions of the ISC dataset. For each dip line we will make several velocity models based on the lithosphere structure predicted by the tectonic models. In each resolution test, an interpreted tectonic model will be translated into a synthetic true model, traveltimes generated from the synthetic model between real event and station positions will be used to quantify the resolution of major tectonic features. The above process may allow us identifying some dip lines with the best resolvability, which will guide our further studies to better evaluate the likelihood of different tectonic models of the Tibetan region.
S41A-08
3D Shear-Wave Structure Beneath Central Tien Shan
The Tien Shan is the world's largest and most active intracontinental orogen with the actual shortening rate of 20 mm/year and earthquake magnitude up to 8.0. The tectonic reactivation is usually attributed to the India-Eurasia collision. However the mechanism responsible for the actual tectonic setting of the Tien Shan remains debated. The aim of this study is to construct 3D shear-wave structure beneath the central Tien Shan and to understand the geodynamic mechanism for the intraplate mountain building. We have analyzed Rayleigh waves recorded at the CHENGIS and KNET seismic networks, which consist of 41 broadband seismic stations. We extract fundamental mode Rayleigh wave trains from 100 teleseismic events at central frequencies from 7.5 mHz to 50 mHz with a 10 mHz frequency interval. A two-plane-wave tomography technique is used to solve for 2-D phase velocities by inverting all the Rayleigh wave phases and amplitudes. We have generated phase velocity maps at the periods of 30-150 s. Our preliminary results of phase velocity maps at periods of 40, 50, 67 and 87 s, based on 10 events, show that a low velocity structure is present beneath the Tien Shan range and that the lateral variations of phase velocities increase with depth. 3-D shear-wave structure in the crust and upper mantle will be computed from the 2-D phase velocities. We will present our findings and discuss their implications.