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Martynov, VG, Vernon FL, Kilb DL, Roecker SW.  2004.  Directional variations in travel-time residuals of teleseismic P waves in the crust and mantle beneath northern Tien Shan. Bulletin of the Seismological Society of America. 94:650-664.   10.1785/0120030015   AbstractWebsite

We study the directional variation in travel-time residuals using 13,820 P-wave arrivals from 1,998 teleseismic events (15degrees less than or equal to Delta less than or equal to 98degrees, 4.1 less than or equal to m(b) less than or equal to 7.3) recorded in 1991-1997 by the Kyrgyz Digital Seismic Network (KNET). Based on a modified version of the iasp91 model that accounts for the Kyrgyz crustal thickness beneath KNET, we convert P-wave travel times to travel-time residuals deltat. The dependence of deltat on backazimuth is modeled as one-, two-, and four-lobed variations in a horizontal plane (Backus, 1965). A least-squares fit of the azimuthal variation of deltat indicates that the crust in the northern Tien Shan is about 11-15 km thicker than it is in the Kazakh Shield and the Chu Depression. From nine KNET stations, the one-lobe model estimates that the slowest P-wave travel-time direction is - 5.0degrees +/- 4.8degrees (almost directly north) and the magnitude of variation is 1.71 +/- 0.13 sec. This result is consistent with an upwelling lower mantle plume. For the two-lobe model, the slowest P-wave travel-time directions (anisotropy term) are 89.7degrees and 269.7degrees +/- 4.7degrees (i.e., trending east-west). We find P-wave velocity anisotropy of 2.0%-2.9% associated with a layer with a thickness of 440 km at the top of the lower mantle. The fast direction of the P-wave travel-time (north-south) azimuthal anisotropy at the top of the lower mantle is (1) parallel to the absolute motion of the India plate and (2) close to the direction of the upwelling hot mantle flow. The last result suggests that the azimuthal anisotropy of the travel-time residuals is due to the shape-preferred orientation of middle-mantle material that results from plume intrusion. Shear-wave splitting studies (Makeyeva et al., 1992; Wolfe and Vernon, 1998) estimated the fast polarization direction to be parallel to the strike of the geological structures of the northern Tien Shan (71degrees +/- 29degrees). Thus, the fast polarization direction determined from these shear-wave splitting studies using KNET data contradicts (differs by >90degrees) the fast travel-time direction (-0.3degrees and 179.7degrees +/- 4.7degrees) we determine here using P-wave travel-time residuals using KNET data. This suggests that the azimuthal anisotropy determined from P-wave travel-time variations and from shear-wave splitting in SKS and SKKS have different sources.

Martynov, VG, Vernon FL, Mellors RJ, Pavlis GL.  1999.  High-frequency attenuation in the crust and upper mantle of the northern Tien Shan. Bulletin of the Seismological Society of America. 89:215-238. AbstractWebsite

We analyze 243 three-component broadband digital seismograms of aftershocks from the M-s = 7.3 Suusamyr, Kyrgyzstan, earthquake to determine seismic attenuation in the northern Tien Shan. P-g, S-g, and SmS body waves and 5' coda waves were used to estimate the frequency-dependent (f = 1.2-30 Hz) attenuation and scattering parameters of the crust and upper mantle as a function of depth. Using the equation Q(f) = Q(0)f(n) multiple layers, we find that Q(0) increases with depth from 76 (upper crust) to 1072 (upper mantle), and the value of n decreases from 0.99 to 0.29 over the same ranger The Q coda results also demonstrated an azimuthal dependence: Q(0) = 736 in the north-south direction and Q(0) = 494 in the east-west direction. There is a strong 2 phi dependence on azimuth for high frequencies (>1.2 Hz). The depth and azimuthal dependence of the quality factor show that the Q is complicated and three dimensional. Estimates of the inhomogeneity scale a show two types of multiple scatterers (Wu and Aki, 1985): a velocity perturbation with a = 0.5-3.0 km and an impedance perturbation with a = 0.052-0.413 km. It appears that a increases with depth. The coefficient of scattering decreases from [g(0)](f) = 0.0055 km(-1) at h = 0-6 km to 0.0020 km(-1) at h = 6-11 km and 0.0048 km(-1) at h = 11-15 for forward scattering and [g(pi)](f) = 0.0006 km(-1) at depth of low crust and upper mantle for backscattering. Estimates off seismic albedo B-0 show that the primary source of attenuation is intrinsic anelasticity, and the variation of Q(0) and n indicates that the level of heterogeneity varies with depth, possibly due to pressure. The lateral variation of Q(0) can be connected with azimuthal anisotropy in the upper mantle related to current deformation under the Tien Shan.

Mellors, RJ, Camp VE, Vernon FL, Al-Amri AMS, Ghalib A.  1999.  Regional waveform propagation in the Arabian Peninsula. Journal of Geophysical Research-Solid Earth. 104:20221-20235.   10.1029/1999jb900187   AbstractWebsite

Regional waveform propagation is characterized in the Arabian Peninsula using data from a temporary network of broadband seismometers. Between November 1995 and March 1997, 332 regional (delta < 15 degrees) events were recorded from nine stations deployed across the Arabian Shield. Regional phase propagation was analyzed in two ways: by individual inspection of the waveforms and by stacking of waveforms. Inspection of the waveforms revealed consistent variations in individual seismograms according to the region of origin. Waveforms from events in the Gulf of Aqaba, northwest of the network, possess weak Pn, Pg, and Sn but show a prominent L-g phase. In contrast, clear Pn, Sn, and Lg are observed for events located in the Zagros, a region northeast of the network. Events near the Straits of Hormuz also display Pn and Sn but lack a strong high-frequency Lg. Southern Red Sea and African earthquakes have moderate-amplitude body phases with some Lg. For the stacks the data were high-pass filtered at 1 Hz, rectified, binned, and then stacked by time/distance or by time/slowness. The time/distance stacks show clear differences between regions that correspond to the variations observed in individual seismograms. The time/slowness stacks allow comparison of relative phase velocities and amplitudes. Pn velocity under the network was estimated to be 8.0 +/- 0.2 km/s, consistent with data from prior refraction profiles. The area of inefficient Pn and Sn propagation coincides with an area of Holocene volcanism and suggests that anomalous upper mantle underlies much of the Arabian Shield.

Mellors, RJ, Vernon FL, Pavlis GL, Abers GA, Hamburger MW, Ghose S, Iliasov B.  1997.  The M(s)=7.3 1992 Suusamyr, Kyrgyzstan, earthquake .1. Constraints on fault geometry and source parameters based on aftershocks and body-wave modeling. Bulletin of the Seismological Society of America. 87:11-22. AbstractWebsite

We investigated the Suusamyr, Kyrgyzstan, earthquake of 19 August 1992, using aftershock data, teleseismic body-wave modeling, and field observations. Aftershocks were recorded by the IRIS Kyrgyzstan broadband network, a temporary six-station aftershock network, and a regional network operated by the Kyrgyz Institute of Seismology. The aftershocks, which range in depth from the surface to 18 km, defined a 50 +/- 10-km-long rupture zone that dips 50 degrees +/- 13 degrees to the south and strikes roughly east-west. The base of the eastern end of the aftershock zone shallowed to the east along strike and may represent a lateral ramp. The surface ruptures also had an east-west strike and dipped south, but the total length (less than 4 km) was much shorter than the aftershock zone. A teleseismic body-wave inversion, using a point source and a directivity correction, yields a focal mechanism with a strike of 221 degrees, dip of 46 degrees, and a slip of 43 degrees. We obtained a moment of 4.1 x 10(19) N-m with a centroid depth between 5 and 21 km. The rupture propagated along an azimuth of 330 degrees +/- 60 degrees, which matches the relative location of the mainshock with respect to the aftershock zone. The results of the aftershock study and teleseismic inversion yield a clear picture of the fault geometry of a large-thrust earthquake.

Mellors, RJ, Camp VE, Vernon FL, Al-Amri AMS, Gharib AA.  2000.  Regional waveform propagation in the Arabian peninsula (vol 104, pg 221, 1999). Journal of Geophysical Research-Solid Earth. 105:6305-6305.   10.1029/2000jb900002   Website
Mellors, RJ, Vernon FL, Thomson DJ.  1998.  Detection of dispersive signals using multitaper dual-frequency coherence. Geophysical Journal International. 135:146-154. AbstractWebsite

We demonstrate the use of 'dual-frequency' coherence in defecting and characterizing dispersive waves, Using a multitaper method. we calculate the coherence between different frequencies of one or multiple signals, We test the algorithm both on a variety of synthetic signals and on broad-band seismic data. Dispersive waves such as seismic surface waves are easily identified, and we show that the method is robust in the presence of noise. Phase relationships between different frequencies can be extracted, allowing reconstruction of the original phase function. 'Dual-frequency' coherence is useful in identifying overtones and frequency shifts between signals, features that are undetectable by standard coherence measures. We construct a filter to extract only the coherent frequencies from a waveform, and show that it significantly increases the signal-to-noise ratio for dispersive waveforms.

Mikhalevsky, PN, Sagen H, Worcester PF, Baggeroer AB, Orcutt J, Moore SE, Lee CM, Vigness-Raposa KJ, Freitag L, Arrott M, Atakan K, Beszczynska-Moeller A, Duda TF, Dushaw BD, Gascard JC, Gavrilov AN, Keers H, Morozov AK, Munk WH, Rixen M, Sandven S, Skarsoulis E, Stafford KM, Vernon F, Yuen MY.  2015.  Multipurpose Acoustic Networks in the Integrated Arctic Ocean Observing System. Arctic. 68:11-27. AbstractWebsite

The dramatic reduction of sea ice in the Arctic Ocean will increase human activities in the coming years. This activity will be driven by increased demand for energy and the marine resources of an Arctic Ocean accessible to ships. Oil and gas exploration, fisheries, mineral extraction, marine transportation, research and development, tourism, and search and rescue will increase the pressure on the vulnerable Arctic environment. Technologies that allow synoptic in situ observations year-round are needed to monitor and forecast changes in the Arctic atmosphere-ice-ocean system at daily, seasonal, annual, and decadal scales. These data can inform and enable both sustainable development and enforcement of international Arctic agreements and treaties, while protecting this critical environment. In this paper, we discuss multipurpose acoustic networks, including subsea cable components, in the Arctic. These networks provide communication, power, underwater, and under-ice navigation, passive monitoring of ambient sound (ice, seismic, biologic, and anthropogenic), and acoustic remote sensing (tomography and thermometry), supporting and complementing data collection from platforms, moorings, and vehicles. We support the development and implementation of regional to basin-wide acoustic networks as an integral component of a multidisciplinary in situ Arctic Ocean observatory.