Sheri Molnar

A Comparison of Site Response Techniques Using Weak-Motion Earthquakes and Microtremors

The applicability of the microtremor spectral ratio method is examined by comparing microtremor and weak-motion earthquake site responses at seven permanent strong-motion sites in Victoria, British Columbia. For each site, a weak-motion earthquake standard spectral ratio (bedrock reference), the average horizontal-to-vertical spectral ratio of up to five weak-motion earthquakes, and the average microtremor (Nakamura method) spectral ratio are compared. The geologic setting of Victoria is ideal for site response studies with a near-surface high impedance contrast between thin geologic layers of Victoria clay (about 11 m maximum in this study) and Pleistocene till or bedrock. Regardless of excitation source (weak-motion earthquakes or microtremors) and spectral ratio method, similar peak amplitudes and fundamental frequencies were found. Thicker material (>10 m) sites displayed higher peak amplitudes (up to six times amplification) at frequencies of 2–5 Hz compared to sites with a thin lens of material (<3 m) over bedrock that showed peak amplitudes at frequencies of >8 Hz.

Site Response in Victoria, British Columbia, from Spectral Ratios and 1D Modeling

This article examines site response in Victoria, British Columbia, using weak ground-motion recordings (<3.5%g) of the 2001 M w 6.8 Nisqually earthquake in Washington State, 150 km distant. Significant variations are observed in acceleration spectra across the city that can be largely attributed to local site conditions. Thin soil sites (<3 m) generally have flat-amplitude spectra at frequencies less than 10 Hz, whereas thicker soil sites (5–11 m) show peak amplitudes at 2–5 Hz. Standard spectral ratios (bedrock reference) and horizontal-to-vertical spectral ratios are computed for sites with varying geology. Thin soil sites show flat-site response (like bedrock) at frequencies <10 Hz, whereas the thicker soil sites show peak amplification of up to six times that of bedrock at frequencies of 2–5 Hz. There is good agreement between these standard spectral ratio site-response estimates with the horizontal-to-vertical spectral ratios at each site and spectral ratios from four other earthquakes between 1996 and 2002. Ground-motion spectra obtained from numerical 1D modeling indicate that <3 m of soft soil (National Earthquake Hazard Reduction Program [NEHRP] class E) generates a flat response at <10 Hz, whereas thick layers of either “grey Victoria clay” (NEHRP class E) or Pleistocene till (NEHRP class C) produce peak amplitudes at 2–5 Hz. The correlation of near-surface soft soil conditions on crystalline bedrock with observed weak ground motions provides an important baseline toward future seismic hazard studies in the Victoria area.

Ambient Vibration Testing of the Eureka-Samoa Channel Bridge

The Eureka-Samoa Channel Bridge is a 20-span pre-stressed concrete I-girder bridge, built in 1971 with a 225 ft navigable span. It is the North-eastern-most part of three bridges collectively known as the Samoa Bridge, which carry two lanes of California State Route 255 between Eureka and the Samoa Peninsula. In June 2013, a detailed ambient vibration test was performed on the bridge. This was done using a set of nine GPS-timed velocity/acceleration recorders. Along the length of the 2,500 ft structure, every pier and every midspan on the west side of the bridge was measured; on the east side every midspan was measured. In addition, the navigable span was measured in detail on both sides; span 5 and 16 were measured with four points along the length on each side, and a measurement was taken on either side of each expansion joint. During the testing, one lane was closed at a time allowing for the test team to have access to the outside of the lanes; traffic crossed with a lead vehicle each time at 25 mph. This created a unique situation of no-vehicle excitation followed by a single line of traffic, moving in one direction, at a uniform speed. This paper presents the results of the ambient vibration tests.

Ambient-noise inversion in ocean geoacoustics and seismic-hazard assessment

This paper presents methodologies and results for the estimation of seabed and soil-column geophysical properties based on Bayesian inversion of ambient ocean-acoustic and seismic noise, respectively. In both marine and terrestrial applications, beamforming is applied to array-based measurements to select the direction of arrival of ambient-noise energy. For the ocean-acoustic application, wave-generated surface noise is recorded at a vertical line array and beamformed to extract up- and down-going energy fluxes from which bottom loss vs. angle can be computed and inverted for geoacoustic profiles. Results from ambient-noise measurements at the Malta Plateau are presented and compared to controlled-source inversions and core measurements. The terrestrial seismic application is aimed at earthquake-hazard site assessment, which requires knowledge of the shear-wave velocity profile over the upper tens of meters of the soil column. In this case, urban seismic noise is recorded on a geophone array and beamform...

Ambient Vibration Testing of the Painter St. Overpass

The Painter St Overpass is a continuous concrete box girder bridge with two-spans over Highway 101 in Rio Dell, California. The bridge has a 17-channel permanent monitoring system that has recorded several earthquakes including the 1992 Petrolia Magnitude 7.1, with a measured ground acceleration of 0.5 g at the site. An ambient vibration test was performed in 1993, which showed that the dynamic properties of the bridge are very sensitive to the level of shaking, in particular at the abutments. In June 2013, another detailed ambient vibration test was performed on the bridge. Both of the tests captured several points along both edges of the bridge deck. The largest earthquake since the 1993 ambient test was the 2010 Ferndale Magnitude 6.5, with ground accelerations of 0.36 g. This paper presents the results of the 2013 ambient vibration test and compares to a reanalysis of the 1993 test using the latest available tools.

Modal Testing of a Repaired Building After 2010 Chile Earthquake

A 24 storey reinforced concrete residential building in the city of Concepcion, Chile, was severely damaged during the 2010 MW 8.8 Maule earthquake. After the earthquake structural elements at the base of the building were repaired in an attempt to restore the structure to its original state. A modal test using ambient vibrations was conducted on this repaired building to determine its dynamic properties. Additional studies using ambient vibrations at near free field locations confirm that ground conditions may have contributed to seismic amplification of the ground shaking at frequencies that were dominant in the seismic response of this high-rise building. This amplification of ground shaking can be considered an important contributing factor to the damage suffered by this building.