Background
Why are we doing earthquake risk analysis? Firstly it is motivated by the fact that when earthquake damages are demonstrated people are motivated to take mitigative actions; the second motif is more engineering oriented: when we can quantify damages and losses we can also quantify the cost/benefit ratio of strengthening actions.
When all earthquake disasters are depicted with collapsed homes and structures it is because these collapses are the real killers (not the earthquake shaking alone). The combination of shaking vulnerability and earthquake hazard is called Earthquake Risk. When the vulnerability is expressed as probability of a specified degree of damage from a specified shaking and the hazard is expressed as the probability of the specified shaking, then earthquake risk is expressed as the combined probability. This simple relation provides a mathematical framework for assessing damage and losses from an earthquake quantitatively.
Risk analysis software - The SELENA-RISe Open Risk Package
Being aware of the importance of proper seismic risk estimation, the International Centre for Geohazards ICG, through NORSAR (Norway) and the University of Alicante (Spain), are developing an open source software tool in order to compute the seismic risk of urban areas using capacity-spectrum methodologies. The tool named SELENA (SEimic Loss EstimatioN using a logic tree Approach), it is open for any user-defined data and it can therefore be applied to any part of the world. The user has to supply a number of input files that contain the necessary input data (e.g., building inventory data, demographic data, definition of seismic scenario etc.) in a simple ASCII format.
It should be explicitly stated that the core of the HAZUS methodology (FEMA, 1999, 2003) was adapted for SELENA. However, one of the main differences between both tools is that SELENA works independently of any Geographic Information System, while HAZUS is connected to the ArcGIS software (ESRI, Inc.). In addition, a logic tree-computation scheme has been implemented in SELENA which allows the user to define weighted input parameters and thus to properly account for epistemic uncertainties. Consequently, all output is provided with corresponding confidence levels.
Recently, NORSAR in cooperation with the University of Madrid (Spain) and INETER (Managua, Nicaragua) developed RISe, a tool that allows the generation of GoogleTM Earth maps of all geo-referenced input and output files of SELENA.
Both tools are provided free of charge and are distributed under the GNU Public License (GPL). Since July 2010 the SELENA–RISe Open Risk Package is hosted on the web-based open source repository SourceForge. Please visit http://selena.sourceforge.net for detailed information on both tools. The software and their corresponding source codes can be downloaded from: http://sourceforge.net/projects/selena/files/.
Vulnerability
Probabilistic vulnerability relationships are usually expressed either in matrix form (e.g. as in Woo, 1999), or in terms of probability curves that describe the probabilities that a given structure sustain a certain degree of damage when exposed to a certain ground motion. Such probability curves are often termed vulnerability or fragility functions.
Vulnerability functions are described probabilistic or deterministic. A typical vulnerability curve depicts the cumulative probability of damages (in defined damage states) for a given structure or type of structure.
NORSAR can provide consulting and research into seismic vulnerability, and provide results based on deterministic methods and empirical data.
Click here for more information on vulnerability.
Soil response
The amplification of seismic waves as they propagate through less consolidated sediments and soils is, as already mentioned, a major factor behind earthquake damages. The thick clay deposits in and around Oslo lends this region to such studies. Although Oslo is not a place with a significant seismic activity, a few earthquakes with noticeable intensity have occurred in the past, including a magnitude 5.4 earthquake in the outer Oslofjord in 1904. This earthquake caused masonry building walls to crack as well as chimneys and roof tiles to fall off the houses in the city of Oslo. The observed significant shaking (and the associated damages) was caused by wave amplification through the thick layers of sediments underlying Oslo.
To investigate this phenomenon in more detail we have conducted special studies both in 2004 and 2006, visiting 35 different sites within the city. The technique used insisted in recording ambient seismic noise and to infer the soil response on the basis of such data using the so-called Nakamura technique, based on ambient noise H/V (horizontal-to-vertical) ratios. The results are displayed in the figures below (site 28) and are largely explaining the distribution of damages from the 1904 earthquake.
Results from Site 28 in the Oslo soil response study, showing recorded data.
Time histories and windows selected for the analysis.
Instrumentation used at site 28.
Statistical analysis of spectral H/V-ratios, median and standard deviations.
The peak at around 6 Hz indicated the main resonant frequency in the soils.
Site characterization
The simple Nakamura technique presented above is complemented by more elaborate methods which have also been tested and used under ICG Project 3, notably an array technique and a technique based on spectral analysis of surface waves (SASW). The former technique has been used in a conceptual study at Sogn Hagekoloni in Oslo, using two different array diameters as shown in the figure below. The data have been analyzed in terms of H/V ratios on individual stations, and a subsurface shear-wave velocity profile was established through joint analysis of the two array geometries. Concordant results were found between the H/V analysis and the array analysis leading to the subsurface shear-wave velocity model.
For the analyzed site, the obtained results indicate a first layer with a thickness of 15.6 m and a Vs of 171 m/s; and a second layer (halfspace) with Vs 955 m/s. Moreover, these values provide a resonance frequency of 2.7 Hz, which is in accordance with the expected value from the initial H/V analysis.
Principles of the ambient vibration technique: (top left) ambient seismic noise is
recorded by two arrays with different aperture, (top right) surface-wave dispersion
curves are computed from the different arrays in different frequency bands and
connected; (bottom) a non-unique forward modeling of the velocity profile that
matches the observed dispersion curves is the final step.
The SASW method is based on the same principles as the ambient vibration technique, the main difference being that the SASW method uses ground vibrations from active impact sources (sledge hammer, explosives or alike). Due to energy limitations of such sources (in particular the sledge hammer which is often the only option in cities), SASW is mainly suitable for characterizing the shear-wave velocity structure down to about 30-40 m. On the other hand, this method can be designed to provide more accurate information of the shallow subsurface properties.
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