Researcher: C. E. Rodriguez

Supervisors: Prof. R.J. Chandler, Dr. J.J. Bommer

Sponsor: : Universidad Nacional de Colombia, Colfuturo

Earthquake Geotechnical Hazards are related with phenomena such as amplification of ground motion, landsliding and liquefaction. Slopes failures during earthquakes have claimed a great number of casualties and have been a major cause of damage to structures and facilities constructed on or in the vicinity of the slopes. The scale of such landslides on natural slopes can be large enough to devastate entire villages or towns, such as the Huascaran Avalanche triggered by the Perú earthquake (1970, M_w = 7.8), or the complex slides caused by the Alaska earthquake in 1964 (M_s = 8.4).

Some attempts have been made to identify and appraise geotechnical hazards and to present them in the form of maps or inventories. However, at present, few methods have been developed for evaluating properly the triggering and susceptibility factors related to slope failures during seismic events.

Ground Motion factors such as topographical effects or site response can be considered by using hazard assessment instrumental seismic parameters, such as Arias’ intensity or the Power Spectral intensity. The mechanical material properties, such as stress–strain behaviour under dynamic conditions need to be included to improve the evaluation of the slope susceptibility. Finally, regional factors such as the precedent climatic conditions or the land-use need also to be included.

Existing zonation methods of earthquake-induced landslides must be reviewed and compared with historical data information in order to define the most important triggering and susceptibility factors and to establish a method to assess them to be applied in an improved zonation methodology.
 

The project:

Final result: A new seismic zonation methodology for landslide hazard

Data: The method will be based on data collected from worldwide historic earthquake cases. It requires a comprehensive review of historical data, reports and published papers in order to define as completely as possible the seismic event data and landslide characteristics.

These data will be used to develop a database of landslides triggered by earthquakes, which will include information from previous analyses, such as that carried on by Keefer (1984). Incorporating new factors, such as instrumental seismic parameters from strong–motion records or regional factors as the precedent climatic conditions, data will complement previous studies.

Methods: Based on the database knowledge of the following points will be improved:

1. Most common types of slides, including a review of the landslide classification commonly used in previous studies.
2. Relationship between the number of slides and seismic parameters.
3. Smallest earthquakes that cause landslides.
4. Seismic parameters and area affected by landslides relationships.
5. Seismic parameters and epicentral or fault distance relationships.
6. Landsliding and seismic shaking intensity thresholds, using different seismic parameters
7. Extension of the affected area and its relation with regional conditions such as the focal mechanisms and/or climatic conditions.
8. Landslide characteristics and their relationship with the seismic energy supplied by the seismic event.

A selection of the principal triggering and susceptibility factors will be made based on the analysis of historical data and a methodology for assessing the importance of each factor will be developed, and a new Zonation Methodology for slides will be proposed.
 

Results up to date:

A database of earthquake-induced landslides has been compiled which extends that of Keefer (1984) from 1980 to 1997. A total of 36 earthquakes world-wide are included, resulting in a combined database having about twice the number of landslides reported by Keefer. Correlations evolving from the new database are compared with those of Keefer. Generally the results are very similar, though the presence of extreme outliers in some of the correlations emphasises the need to be aware of special cases. Particularly, these latter include quick clay landslides. Seismological features, including multiple earthquakes and simultaneous arrival of different phases, also influence the outliers.

However, the correlations between earthquake magnitude and total landslide area differ somewhat from Keefer’s. For the intermediate magnitude range 5.3 - 7.0, a modified correlation has been suggested as shown in figure 1.

Figure 1. Area affected by landslides as a function of magnitude M_S (top) and M_W (bottom). The solid line is the upper bound as determined by Keefer (1984), and dashed line is the proposed modified bound.

The scatter of the data from which the correlations are derived is greater than found by Keefer. This is ascribed to the different geographic locations of the earthquakes in the two data sets.
 

JSSMFE (1993) Manual for zonation on seismic geotechnical hazards. Japanese Society for Soil Mechanics and Foundation Engineering, Tokyo.

Keefer, D.K. (1984) Landslides caused by earthquakes. Bulletin of the Geological Society of America 95, 406-421.

Varnes, D.J. (1984) Landslide hazard zonation: a review of principles and practice. UNESCO, Paris.

Wilson, R.C and Keefer, K.D (1985) Predicting areal limits of earthquake-induced landsliding. In Evaluating Earthquake hazards in Los Angeles Region. Ed. J.I. Ziony. U.S. Geological Survey, Professional Paper 1360, pp 317-345.