Inversion of phase velocity of long-period microtremors to the S-wave-velocity structure down to the basement in urbanized areas.

Abstract In preparation for the next phase of the Source Physics Experiments, we acquired an active‐source seismic dataset along two transects totaling more than 30 km in length at Yucca Flat, Nevada, on the Nevada National Security Site. Yucca Flat is a sedimentary basin which has hosted more than 650 underground nuclear tests (UGTs). The survey source was a novel 13,000 kg modified industrial pile driver. This weight drop source proved to be broadband and repeatable, richer in low frequencies (1–3 Hz) than traditional vibrator sources and capable of producing peak particle velocities similar to those produced by a 50 kg explosive charge. In this study, we performed a joint inversion of P‐wave refraction travel times and Rayleigh‐wave phase‐velocity dispersion curves for the P‐ and S‐wave velocity structure of Yucca Flat. Phase‐velocity surface‐wave dispersion measurements were obtained via the refraction microtremor method on 1 km arrays, with 80% overlap. Our P‐wave velocity models verify and expand the current understanding of Yucca Flat’s subsurface geometry and bulk properties such as depth to Paleozoic basement and shallow alluvium velocity. Areas of disagreement between this study and the current geologic model of Yucca Flat (derived from borehole studies) generally correlate with areas of widely spaced borehole control points. This provides an opportunity to update the existing model, which is used for modeling groundwater flow and radionuclide transport. Scattering caused by UGT‐related high‐contrast velocity anomalies substantially reduced the number and frequency bandwidth of usable dispersion picks. The S‐wave velocity models presented in this study agree with existing basin‐wide studies of Yucca Flat, but are compromised by diminished surface‐wave coherence as a product of this scattering. As nuclear nonproliferation monitoring moves from teleseismic to regional or even local distances, such high‐frequency (>5 Hz) scattering could prove challenging when attempting to discriminate events in areas of previous testing.

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Long-Period Ground Motions Observed in the Northern Part of Kanto Basin, During the 2011 off the Pacific Coast of Tohoku Earthquake, Japan

Journal of Disaster Research ◽

10.20965/jdr.2013.p0781 ◽

2013 ◽

Vol 8 (sp) ◽

pp. 781-791 ◽

Cited By ~ 2

Author(s):

Seiji Tsuno ◽

◽

Andi Muhamad Pramatadie ◽

Yadab P. Dhakal ◽

Kosuke Chimoto ◽

...

Keyword(s):

Wave Velocity ◽

Love Wave ◽

Pacific Coast ◽

Velocity Structure ◽

Ground Motions ◽

Tohoku Earthquake ◽

S Wave ◽

Long Period ◽

The Pacific ◽

Kanto Basin

During the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0), strong ground motions were observed at many seismic stations in the Tokyo Metropolitan Area located about 200 km away from the southern edge of the earthquake source fault. Large earthquake responses in high-rise buildings having long natural periods of several seconds were also observed. The largest ground responses for a period of 4 to 5 seconds were observed locally in Oyama (K-NET TCG012) and Koga (K-NET IBR009) on the border between Tochigi and Ibaraki Prefectures in the northern part of Kanto basin. Geophysical information in these areas was not accurate enough, however, to evaluate these ground motions. To understand S-wave velocity structures, we performed array microtremors observations at TCG012 seismic station in Oyama. We applied the Spatial Autocorrelation (SPAC) method to array microtremors data for vertical components. Rayleigh wave phase velocity from 0.3 to 1.6 km/s was obtained for a period of 0.25 to 3 seconds. We inverted phase velocity to a S-wave velocity structure reaching to bedrock at a depth of 1.6 km, using a Genetic Algorithm. The estimated structure explained the first peak of the H/V spectral ratio of microtremors well by the ellipticity of fundamentalmode Rayleigh wave. To evaluate long-period ground motions observed around Oyama during the main shock, we estimated earthquake ground motions by 1-D analysis, showing agreements with and the differences from those observed. As a result, velocity calculated at IBR008 located midway between the Tsukuba Mountains and Oyama, explained that observed for main phases and later phases. However, velocity calculated at TCG012 did not explain that observed for later phases. According to the emphasis of airy phases for group velocity of Love wave using the estimated S-wave velocity structure and the principal axis for later phases obtained by PCA corresponding to the vibration direction of Love wave propagating from the earthquake source fault and through the Tsukuba Mountains, long-period ground motions of a period of 3 to 5 seconds observed at TCG012 lasting for 200 seconds after the arrival of main phases, consist of Love wave.

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