Determination of 2D and 3D velocity structure in and near Reno: Development of the Western Basin and Range Community Velocity Model and the Reno-Carson urban hazard map
Abstract
Estimation of shallow and deep shear velocity is a key element in the assessment of sites for potential earthquake ground shaking, damage, and the calibration of recorded ground motions. We assess shear-wave velocities across the deepest portion of the Reno-area basin as defined by gravity, as well as at twelve seismic stations in the near-field region of the 26 April 2008 Mw 5.0 Mogul, Nevada earthquake 12 km west of Downtown Reno. To date, no permanent seismic instrumentation is located over this thickest portion of the Reno-area basin and little structural and velocity data is available. Existing velocity models for the region are limited in resolution to intervals of 1 km to 3 km. As a result, 3D basin details are currently insufficient for scenario modeling and ShakeZoning, essential components of seismic hazard evaluation. These efforts contribute towards development of the Western Basin and Range Community Velocity Model and the Reno-Carson urban hazard map. As part of a NEHRP-IMW grant, we measured shear velocities to depths of 500 m using refraction-microtremor (ReMi) arrays across this portion of the basin with 50 m depth resolution. This was achieved through the deployment of 30 stand alone wireless instruments in arrays 2.9 to 5.8 km long, to record ambient urban noise. Data were obtained along two parallel east-west arrays and one shorter north-south array. The ReMi technique was employed to obtain 1D velocity profiles as a function of depth across each array from these noise records. To further characterize and map lateral velocity heterogeneity beneath the arrays, subsets of instruments were used to obtain a series of 1D soundings that were then interpolated to obtain a 2D structural representation of shear-wave velocities. The 1D and 2D velocity soundings along each array are combined produce a 3D shear-velocity volume delineating the deep velocity structure of the basin. Observed lateral velocity variations were identified and correlated with the locations of hypothesized faults in an attempt to constrain their locality. The 26 April 2008 Mw 5.0 Mogul, Nevada earthquake, located at a shallow depth of 3.1 km, was the largest event during a shallow earthquake swarm that began in February 2008 and persisted for several months. The largest peak acceleration vector during this mainshock was recorded at station MOGL (1.19g; 1164 cm/s2), which is among the 25 largest recorded earthquake accelerations worldwide. Strong ground motions were observed at the four closest stations installed at the time of the mainshock, which exceed accelerations of 300 cm/s2 and velocities of 14 cm/s. Both shear-wave (ReMi) and P-wave (refraction) techniques were used to characterize and map lateral velocity heterogeneity beneath the twelve near-field seismic sites. 2D shear-velocity sections together with 2D P-wave refraction tomography sections show complex site conditions beneath these stations. Some of these sites may be crossed by local faults, and the velocity sections reflect this possibility. These resultant site condition assessments at these near-filed sites will contribute towards quantification of local site response and calibration of the high peak ground motions recorded during both the mainshock event and the foreshock/aftershock swarm events.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.S41A2358P
- Keywords:
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- 0935 EXPLORATION GEOPHYSICS / Seismic methods;
- 7200 SEISMOLOGY;
- 4344 NATURAL HAZARDS / Microzonation and macrozonation