Charles A. Langston

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

[1]xa0Crustal 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 Poissons 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 Poissons 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 Poissons ratios vary from 0.23 to 0.28. Within the Main Ethiopian Rift, Moho depths vary from 27 to 38 km, and Poissons ratios range from 0.27 to 0.35. A crustal thickness of 25 km and a Poissons 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.

Spatial Gradient Analysis for Linear Seismic Arrays

I incorporate the spatial gradient of the wave field recorded from one- dimensional arrays into a processing method that yields the horizontal-wave slowness and the change of geometrical spreading with distance. In general, the model for seismic-wave propagation is enough to be appropriate for body and surface waves propagating from nearby seismic sources but can be simplified into a plane-wave model. Although computation of the spatial gradient requires that array elements be closer than 10% of the horizontal wavelength, seismic-array apertures, in the usual sense, may extend over many horizontal wavelengths and illuminate changes within the wave field. Array images of horizontal slowness and the relative geometrical- spreading changes of seismic waves are derived using filter theory and used to interpret observed array wave fields. Errors in computing finite-difference spatial gradients from array nodes are explicitly considered to avoid spatial aliasing in the estimates. I apply the method to interpret waves in strong ground motion and small- scale refraction data sets. Use of the wave spatial gradient accentuates spatial differences in the wave field that can be theoretically exploited in fine-scale tomographic studies of structure and is complementary to frequency/wavenumber or beam- forming array-processing techniques.

Ambient seismic noise tomography and structure of eastern North America

[1]xa0The time derivative of cross-correlation functions (CCF) of ambient noise fields recorded by two stations can be approximated as the Greens Function (GF) between the stations. The CCFs are thus used as Peudo-GFs (dominated by surface waves) to invert for group velocity structure in eastern North America. Stations from two regional networks deployed to monitor the New Madrid Seismic Zone and eastern Tennessee seismic zone, together with stations of the US National Seismic Network, greatly improve tomographic ray coverage. The short period (T = 5 s) group velocity map shows strong correlations with the depth to Precambrian basement. Many subtle local structures can be clearly identified from the velocity map, including the Ozark uplift, Cincinnati Arch, Nashville Dome and the Blue Ridge province of the Appalachians showing relatively high group velocity. The long period (T = 15 s) group velocity map shows strong correlations with regional geology. Ancient rift basins, such as the Mid-Continent Rift (MCR) system, the Reelfoot rift, the Oklahoma Aulacogen and the Eastern Continent Rift, are associated with low velocity belts along their rift axes. We also find that all major seismic zones in eastern North America, such as the New Madrid seismic zone, Eastern Tennessee seismic zone as well as Ouachita Orogen seismic zone, are approximately located at transition zones separating velocity highs and lows. This observation suggests that those seismic zones may reflect the reactivation of ancient faults associated with continental rift and collision zones.

Wave Gradiometry in Two Dimensions

The spatial displacement gradient of a seismic wave is related to dis- placement and velocity through two spatial coefficients for any one dimension. One coefficient gives the relative change of wave geometrical spreading with distance and the other gives the horizontal slowness and its change with distance. The essential feature of spatial gradient analysis is a time-domain relation between three seismo- grams that yields information on the amplitude and phase behavior of a seismic wave. Filter theory is used to find these coefficients for data from 2D areal arrays of seismometers, termed gradiometers. A finite-difference star is used to compute the displacement gradient for irregularly shaped gradiometers, and a relation for the frequency-dependent error in the displacement gradient is obtained and applied to ensure accurate estimates. This kind of array analysis is useful for gradiometers at any distance from a source and yields a variety of time-domain and frequency-domain views of wave-amplitude changes and horizontal phase velocity estimates across the gradiometer. For example, time-dependent horizontal slowness and wave-azimuth plots are natural results of the analysis. These time-domain maps may be used in conjunction with time-distance and horizontal slowness-distance models to locate seismic sources or may be used directly to study earth structure. These methods are demonstrated by using data from a small-aperture (40 m) seismic gradiometer.

Local Earthquake Wave Propagation through Mississippi Embayment Sediments, Part I: Body-Wave Phases and Local Site Responses

P and S body waves from microearthquakes in the New Madrid Seismic Zone (NMSZ) are investigated at selected sites in an effort to understand wave propagation from future large earthquakes. Earthquake body waveforms display distinctive features that constrain the nature of P - and S -wave local site responses and wave propagation within the unconsolidated Mississippi embayment sediments. Modeling of the waveforms demonstrates that a near-surface low-velocity zone is characteristic of structure within the upper 60 m of the sedimentary column and produces large P - and S -wave resonance effects that can be used to infer near-site conditions. Site resonance effects change because of velocity heterogeneity between individual receivers but imply that embayment sediments will substantially amplify ground motions at high frequencies. Site resonance affects P - and S -wave amplitude spectra and can bias estimates of source and anelastic attenuation parameters. Travel times of observed body-wave phases such as P, PpPhp (the first P -wave reverberation within the entire sedimentary column), Ps, Sp, S , and SsPhp can be used to estimate the average wave slownesses and Poissons ratio within the embayment sediments; an average Poissons ratio of 0.44 is obtained for the central NMSZ under station PEBM. Detailed S -wave velocities are derived for a Nafe-Drake sediment model using acoustic well logs and the travel-time constraints of observed seismic phases. V p/ V s ratios vary from 5.5 near the surface to approximately 2.4 at the base of the sediments. Use of the well-log data in wave calculations also explains much of the nature of P - and S -wave coda within the waveforms and shows that 1D heterogeneity is a first-order influence on seismic-wave propagation within the Mississippi embayment.nnManuscript received 11 March 2003.

Review: Progress in Rotational Ground-Motion Observations from Explosions and Local Earthquakes in Taiwan

Rotational motions generated by large earthquakes in the far field have been successfully measured, and observations agree well with the classical elasticity theory. However, recent rotational measurements in the near field of earthquakes in Japan and in Taiwan indicate that rotational ground motions are 10 to 100 times larger than expected from the classical elasticity theory. The near-field strong-motion records of the 1999 Mw 7:6 Chi-Chi, Taiwan, earthquake suggest that the ground motions along the 100 km rupture are complex. Some rather arbitrary baseline corrections are necessary in order to obtain reasonable displacement values from double integra- tion of the acceleration data. Because rotational motions can contaminate acceleration observations due to the induced perturbation of the Earths gravitational field, we started a modest program to observe rotational ground motions in Taiwan. Three papers have reported the rotational observations in Taiwan: (1) at the HGSD station (Liu et al., 2009), (2) at the N3 site from two TAiwan Integrated GEodynamics Research (TAIGER) explosions (Lin et al., 2009), and (3) at the Taiwan campus of the National Chung-Cheng University (NCCU )( Wuet al., 2009). In addition, Langston et al. (2009) reported the results of analyzing the TAIGER explosion data. As noted by several authors before, we found a linear relationship between peak rotational rate (PRR in mrad=sec) and peak ground acceleration (PGA in m=sec 2 ) from local earthquakes in Taiwan, PRR 0:002 1:301 PGA, with a correlation coefficient of 0.988.

Seismic ground motions from a bolide shock wave

[1]xa0An intensely brilliant bolide accompanied by an audible sonic boom occurred on the night of 3 November 2003 (∼2150 LT or 4 November 2003, 0350 UT) in northeastern Arkansas. The sonic boom was well recorded by 22 three-component stations of the Center for Earthquake Research and Information Cooperative Seismic Network. Arrival times and wave polarizations were used to reconstruct the acoustic wave front propagating across the network and to estimate trajectory parameters of the bolide. A grid search location technique constrains the trajectory and gives a reference height of 50–100 km above the southern edge of the network, an azimuth of propagation of 270°–290° toward the northwest and a plunge of 38°–48° downward. The trajectory and date of the bolide event are consistent with a Taurid shower meteor. Transient ground motions from the sonic boom were studied using plane wave, Cagniard-deHoop, and propagator matrix theory for an incident pressure pulse impinging on a layered elastic Earth model. General characteristics of the vertical and radial polarization and wave frequency content require acoustic coupling within a thin (∼10 m) near-surface layer with low P wave (∼0.25 km/s) and S wave (∼0.125 km/s) velocities. Ground motions are generally confined to this near-surface layer, are not sensitive to deeper Earth structure within the Mississippi embayment unconsolidated sediments, and display both “leaky” mode PL (a type of dispersed, P-type seismic wave) and “locked” mode Rayleigh wave propagation. Acoustic wave/ground motion coupling studies may be a useful way to analyze near-surface site responses to determine velocity structure in earthquake shaking hazard studies.

Wave gradiometry for USArray: Rayleigh waves

[1]xa0Wave gradiometry (WG) is a new array data processing technique to extract phase velocity, wave directionality, geometrical spreading, and radiation pattern from spatial gradients of waveforms. A weighted inversion method and a reducing velocity method are introduced to compute spatial gradients accurately for irregular arrays. Numerical experiments are conducted to test techniques and to evaluate the parameters determined from the WG method. We apply this method to USArray data for the western United States. In this study, Rayleigh waves from nine earthquakes with varying azimuths are analyzed. The stability of this method is shown by the similarity between the results from two nearly collocated earthquakes from the Kurile Islands. The error check shows the WG results are stable for ambient noise level as high as 10%. Phase velocities determined by WG and two station (TS) methods are statistically consistent, while these determined from beam forming method are systematically higher for wavelength larger than one quarter of the array diameter. Our results show that, first, the average phase velocities of Rayleigh waves range from 3.8 to 4.1 km/s for periods from 60 s to 150 s. This is consistent with average earth models. The prominent feature on the phase velocity map is that the Basin and Range province is dominated by velocity lows while the west coast of the United States and the north and northeastern Snake River plain are dominated by velocity highs. The Snake River plain appears to be a primary tectonic boundary. Second, azimuthal variations represent the accumulated wave directionality changes along the raypath. A velocity contrast of 0.25 km/s across the oceanic-continental lithosphere boundary along the west coast of the United States is needed to explain the negative azimuth variations. Third, geometrical spreading is slightly anticorrelated with phase velocity, which may suggest that amplitude variations in radial directions are subject to surface wave focusing and defocusing. Fourth, similar to the wave directionality, radiation pattern variations also exhibit strong path dependence. Further theoretical and experimental studies will be conducted to understand the two amplitude parameters: geometrical spreading and radiation pattern and their relations with the local geophysical properties.

Wave Gradiometry in the Time Domain

A time-domain approach for solving for the change in geometrical spreading and horizontal wave slowness in wave gradiometry is presented based on the use of the analytic signal. The horizontal displacement gradient of a wave is linearly related to the displacement and its time derivative. The coefficients of this relationship give the change of geometrical spreading, the change in radiation pattern, and horizontal slowness. The new time-domain technique incorporates estimates of the instantaneous amplitude and frequency of the three time series to solve uniquely for the wave-field coefficients. The analysis is simpler and more suited to fast array processing of displacement gradient data sets compared with a spectral ratio method.

Wave-Field Continuation and Decomposition for Passive Seismic Imaging Under Deep Unconsolidated Sediments

Abstract The coastal plains of the central and eastern United States contain deep sections of unconsolidated to poorly consolidated sediments. These sediments mask deeper crustal and upper-mantle converted phases in teleseismic receiver functions through large amplitude, near-surface reverberations. Thick sediments also amplify ambient noise levels to generally reduce data signal-to-noise ratios. Removing shallow-sediment wave-propagation effects is critical for imaging deep lithospheric structures. A propagator matrix formalism is used to downward-continue the wave field for teleseismic P waves into the midcrust and then to separate the upgoing S -wave field from the total teleseismic response of the P wave to expose deep Sp conversions. This method requires that the Earth model from the surface to the reference depth be known. Teleseismic P -wave data for the Memphis, Tennessee, station (MPH) are analyzed using a reference-station deconvolution technique to produce vertical and radial P -wave transfer functions. These transfer functions are modeled using a simple model parameterization for sediment structure through grid inversion. The inverted Earth model is incorporated into the wave-field continuation and decomposition technique to estimate the upgoing S -wave field at 10xa0km depth in the crust. Moho and possible deeper Ps conversions are identified with this process.