Heather Crow

Probabilistic Liquefaction Hazard Analysis for Four Canadian Cities

Changes to building codes in the last decade, lowering the probability at which design ground motions for geotechnical applications are defined, have led to an urgent need for a probabilistic approach/tool for liquefaction potential assessment. We propose a consistent approach for probabilistic liquefaction hazard analysis (PLHA) that is based on probabilistic seismic hazard analysis and incorporates a reliability- based liquefaction potential evaluation method based on shear-wave velocity data. The method directly takes the joint probability distribution of peak ground accelera- tion and moment magnitude into account. We demonstrate the method for four Canadian cities, employing our interim updated seismic hazard models for eastern and western Canada. Using the developed method and representative site profiles, PLHA is implemented for four major cities across Canada with the aim of investigating the impact of regional seismic characteristics on liquefaction hazard assessment. Sensitivity analysis indicates that different magnitude ranges of dominant contributing seismic events have significant impact on the extent of liquefaction hazard. More specifically, for a given seismic excitation level, the relatively high hazard contribu- tions from small-to-moderate earthquakes in eastern Canada leads to less significant liquefaction potential, in comparison with similar sites in western Canada.

Seismic Site Response Analysis for Ottawa, Canada: A Comprehensive Study Using Measurements and Numerical Simulations

The surficial geology of the city of Ottawa primarily consists of soft soil sediments with low shear‐wave velocities (![Graphic][1] ) underlain by hard bedrock with very large shear‐wave velocities (![Graphic][2] ). Earthquake recordings show unusually large seismic amplification values for weak motion. These unusually large seismic amplification factors were reconfirmed with the earthquake spectral ratio method using two stations, the horizontal‐to‐vertical earthquake spectral ratio method using a single station, and the horizontal‐to‐vertical spectral ratio technique using background noise. These findings were the motivation for carrying out an extensive site response analysis, using finite element modeling (FEM), as a part of the seismic microzonation studies for the city of Ottawa. The FEM results confirmed the large amplification ratios for weak‐motion recordings. FEM analysis was also carried out using a selection of strong‐motion time series for the study area. The combined effect of the soil–bedrock acoustic impedance contrast and the level of ground shaking on the variation of soil amplification factors for the fundamental frequency were investigated. The maximum value of the soil amplification factor for the fundamental frequency increased with increasing impedance contrast ratios until the soil/bedrock acoustic impedance contrast ratio reached values that were usually greater than 12; however, the change in peak amplification was much less with subsequent increases in the contrast ratio beyond that value. As expected, the value of the soil amplification factors for the fundamental frequency decreased with increasing peak ground acceleration (PGA) of the input motion due to nonlinear soil damping. Finally, for the Ottawa region, a mathematical model is suggested for soil amplification factors at the fundamental frequency, as a function of the soil/bedrock acoustic impedance contrast ratio and the PGA of the input motion. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Near-surface geophysical techniques for geohazards investigations: Some Canadian examples

Over the last 40 years, there has been an expansion of activity in applications of near-surface geophysical techniques for various types of geohazards investigations in Canada; numerous national and international research groups have been working with the Near Surface Geophysics Section of the Geological Survey of Canada to develop techniques for specific Canadian engineering and environmental geohazards problems. A few of the more interesting examples from widespread parts of the country are discussed in this paper.

Multicomponent vibroseismic profiling over high velocity glacial ground: an example from Southern Ontario

A 3-D Quaternary mapping project conducted by the Ontario Geological Survey (OGS) in the southern part of Simcoe County involves borehole drilling, airborne geophysics, such as TDEM and magnetics and ground gravity surveys. Geophysical surveys are necessary to define the top of bedrock, including buried bedrock valleys and the architecture of overlying sediments for evaluating groundwater resources. In support of this project, the Geological Survey of Canada (GSC) carried out a three-line 21.2 km seismic reflection survey. Geophysical logging in two deep boreholes was undertaken to assist with the calibration of the seismic sections. The seismic survey was performed using an IVI “Minivib 1” source with a “landstreamer” three-component geophone array built by the GSC. The landstreamer consists of 72 - 3 kg metal sleds, spaced at 1.5 m, towed using low-stretch belts. Data were acquired with shot points every 4.5 m. The source vibrates a 140 kg mass in in-line (H1) horizontal mode, using a 7 second nonlinear logarithmic sweep of -2 DB/Oct from 20 to 300 Hz. This type of sweep increases the time spent in the low end of the sweep which has the effect to increase the low frequency energy to enhance shear body wave energy. Data were recorded using seven 24- channel Geometrics Geode engineering seismographs operated in the cab of the Minivib. Only the vertical component of the 24 geophones, furthest from the source, was recorded in order to obtain a better coverage of the P-wave data acquisition window. Uncorrelated records were collected to allow pre-whitening of the data and careful choice of the correlating function was the first step in the data processing sequence. P-wave sections were derived from processing the first 0.5 sec. (after correlation) of data acquired on the vertical geophones, while S-wave sections were produced using the in-line, H1, component over a correlated window of 2 seconds. Seismic sections were then correlated with borehole geophysical data. Interpretation of the equivalent compressional (P-) wave section permits delineation of seismic facies sequences. The P-wave velocity is an order of magnitude higher than the shearwave velocity and as a result, the vertical resolution of the section is lower. However, the acoustic impedance contrast with underlying materials (coarser sediments, tills or bedrock) is lower than in the case of shear-wave. The shear-wave data produce remarkably detailed sections over buried valleys down to 150 m.

Near Surface Seismic Reflection and Refraction

Every reliable interpretation method of shallow refraction seismic traveltime data (also known as first arrivals or head waves) has to include criteria for distinguishing between vertical and horizontal variations of velocities. The Common Refractor Element -CREmethod presents a simple approach to interpret shallow refraction seismic data especially in cases of piecewise lateral changes along the refracting interfaces. In this method, a common linear traveltime element, (or part of it), which corresponds to a linear common refractor element is used for inversion of these first arrivals. Linear traveltime elements are defined as linear parts of the traveltime curve with equal slopes and consequently equal apparent velocities. Traveltime parameters such as the layer reciprocal time, the apparent refractor velocity and the intercept time values are used to distinguish between vertical and horizontal variations. Layer reciprocal time means that the forward and reversed traveltime refracted from one layer must be equal at the two ends of the traveltime curve. Lateral changes in dip and/or velocity along the refracting interface create a traveltime curve of different linear segments and the extrapolation of corresponding traveltime elements for reversed profiles are not equal at both ends of the traveltime curve, except for the last two refractor elements that represent the last two elements from each side of the refractor. Apparent refractor velocity can be used in several ways as another parameter for lateral variation or structure identification. In-line reversed profiling technique is essential for the required data.

Empirical Geophysical/Geotechnical Relationships in the Champlain Sea Sediments of Eastern Ontario

Geophysical and geotechnical data are presented from different sites in eastern Ontario where variable geotechnical properties of Champlain Sea sediments (‘Leda Clays’) are found. Sites range from thick “undisturbed” silts and clays, to “disturbed” geologically similar soils (earthquake triggered landslides and other deformed materials). High-resolution seismic profiles provide stratigraphic context for some of the boreholes drilled in the study area. Downhole geophysical logs from 14 boreholes are compared to core sample measurements of porosity, sensitivity, and porewater conductivity to develop useful empirical relationships. According to these relationships, silt and clay sediments can be sensitive or quick when formation conductivity drops below 100 mS/m. Conversely, silts and clays with elevated conductivities (>250 mS/m) are rarely sensitive. Salinity values calculated from porewater conductivity indicate sensitive or quick behaviour may be expected in leached soils when salinity drops below 2 g/l.

Some Applications of Near Surface Geophysics to Earthquake Geohazards Investigations: Examples from Eastern Ontario, Canada

The nature of seismic shaking is dependent on source characteristics, travel path and near-surface site conditions. Many years of observations of earthquake damage have indicated that the presence of thick soil is a major contributing factor to the shaking response of structures. As well, seismic-induced changes in soil parameters can lead to other effects such as loss of resistance to shear (liquefaction) and landsliding. In the last several years, many national building codes have recognized the importance of soil effects, including shear strength, damping, amplification and resonance. Many of these insitu geotechnical parameters can now be measured or estimated using modern near-surface geophysical techniques. Indeed, current building codes indicate that the preferred measurement technique for seismic zonation is based on shear wave velocity structure of soil and bedrock. Other active and passive, surface or invasive techniques, also contribute valuable ancillary data leading to assessment of soil strength and liquefaction potential in granular materials.