Landslide Discussion at the Kick-off Meeting on Sunday 3 February 2002
The discussion was in two parts. The first part was briefly touching on specific issues in regard to the Cambridge interaction with the Azores. These included the following.
1
Availability of digital topography data: Current digital topography is
at approximately 25m spacing, derived from topographic maps. Data at 1-10m
spacing needs to be acquired for the landslide studies. A possible source is
air photos (see below) – Cambridge needs to look into this.
2
Air photos: The Portuguese airforce has full coverage of the islands
flown in 1974, partial coverage in 1995, coverage specific to the 1997 and 1998
landslides flown after the events.
3
Climate and precipitation data: The spatial coverage of this data is
thought to be poor – Azores to check with their meteorological people.
4
Wave height data near the coasts of the islands. This is thought to
available, and erosional undercutting of the shoreline is a significant cause
of landsliding –Niels Hovius to discuss this directly with Paulo Valadao.
The second part was a more general discussion of landsliding, and in
particular the relationship between the sizes of earthquakes and the areas
where landslides are triggered by earthquakes. For example, magnitude 6.0
earthquakes typically have source dimensions of the order of 10km, and the
landslides generally occur over areas of much the same dimension directly above
the source. The correspondence between the earthquake source area and the area
where landslides occur is well known, and in some cases has been used to
estimates the sizes of earthquakes. Surface topography can amplify the
earthquake ground motion by up to a factor of 2-3. Further and some times more
dramatic amplification of seismic accelerations can occur if near surface
materials are poorly consolidated, particularly where the poorly consolidated
material is in contact with much stronger rock. Such an example would be where
ash or pyroclastic material is in contact with lava flows.
The French Alps were briefly discussed following Philippe Bardey’s
earlier presentation.
There is a good “static” model of the landslides there, with and
abundance of hydrological information. Amy Clifton told us a little about the
landslides in her study region in Iceland, which appear to be in accord with
the standard relationship between earthquake source area and area of
landsliding. Philippe drew our attention to Engineering Geology Special Issue Vol. 58 Nos. 3-4 December 2000, which has many pertinent papers,
including a very nice summary of this relationship, plus data showing the
influence of topography.