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Detection of the Wenchuan aftershock sequence using waveform correlation with a composite regional network

Bulletin of the Seismological Society of America

Slinkard, Megan E.; Heck, Stephen H.; Schaff, David; Bonal, Nedra B.; Daily, David M.; Young, Christopher J.; Richards, Paul

Using template waveforms from aftershocks of the Wenchuan earthquake (12 May 2008, Ms 7.9) listed in a global bulletin and continuous data from eight regional stations, we detected more than 6000 additional events in the mainshock source region from 1 May to 12 August 2008. These new detections obey Omori’s law, extend the magnitude of completeness downward by 1.1 magnitude units, and lead to a more than fivefold increase in number of known aftershocks compared with the global bulletins published by the International Data Centre and the International Seismological Centre. Moreover, we detected more M >2 events than were listed by the Sichuan Seismograph Network. Several clusters of these detections were then relocated using the double-difference method, yielding locations that reduced travel-time residuals by a factor of 32 compared with the initial bulletin locations. Our results suggest that using waveform correlation on a few regional stations can find aftershock events very effectively and locate them with precision.

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TYPE Journal Article YEAR 2016
ScopusOSTIDOI

Assessment of Saturation Changes Around Cavities and the Impact on Seismic Waves

Bonal, Nedra B.; Desilets, Sharon L.

According to theory, seismic methods should be able to identify cavities. The high impedance contrast between the rock (fast velocity, high density) and the air (slow velocity, low density) should produce strong reflections and refractions off cavity walls. In practice, however, cavities are not always easily detected. Changes in pore saturation, fracturing, and stress near the wall of the cavity may decrease the impedance contrast and diffuse the rock/air interface. To determine the impact of these effects on seismic waves, we first aim to understand the hydrological processes in the medium surrounding the cavity, a tunnel in this case. We simulate flow through the vadose zone to provide insight into how a cavity alters saturation. Next we examine how this saturation affects seismic velocities. Finally, we use a wave propagation code to simulate seismic waves through the saturation models. Results are presented from hydrology models for three cases: the tunnel far above the water table, the tunnel just above the water table and within the capillary fringe, and the tunnel below the water table with the tunnel being drained. We also present the results of seismic wave propagation through these models to determine the impact these effects have on seismic waves.

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TYPE Journal Article YEAR 2015
OSTI

Density estimation using muon imaging

Near Surface Geoscience 2015 - 21st European Meeting of Environmental and Engineering Geophysics

Preston, Leiph A.; Bonal, Nedra B.; Dorsey, Daniel J.; Schwellenbach, D.; Dreesen, W.; Green, J.A.

Muons are subatomic particles capable of penetrating the earth's crust several kilometers. Muons have been used to image the Pyramid of Khafre of Giza, various volcanoes, and smaller targets like cargo. For objects like a volcano, the detector is placed at the volcano's base and muon fluxes for paths through the volcano are recorded for many days to weeks.

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TYPE Conference Poster YEAR 2015
ScopusOSTIDOI
Results 1–25 of 29
Results 1–25 of 29