Earthquakes and Landslides
Let us see some examples of landslides and earthquakes and their impacts.
Landslides are often triggered by the shaking of earthquakes. These ground failures are of two principal types:
= Disrupted slides, falls and flows - landslides with highly jumbled materials that start on steep slopes and move at rela-tively high speeds, such as soil or rock slides, rock falls and avalanches, and debris flows; and
= Coherent slides - blocks of unjumbled materials that move on a discrete slide surface such as slumps, block slides and earth flows.
Much effort was made to document the location, shape, and severity of the landslides triggered by the October 1989 Loma Prieta earthquake and the January 1994 Northridge earthquake. Approximately 1,500 earthquake-triggered landslides were mapped, and up to 4,000 slides may have moved, in the Loma Prieta earthquake. Over 11,000 landslides occurred in the Northridge earthquake. Significantly, both earthquakes occurred when the ground was exceptionally dry. Extensive research on the distribution and causes of these slides shows that failure rates can be correlated with (1) shaking severity; (2) slope steepness; (3) strength and engineering properties of geological materials; (4) water saturation (which varies with precipitation and by season);
(5) existing landslide areas; and (6) vegetative cover.
Shaking Intensity and Building Damage
How does ground shaking intensity relate to damage to various types of building construction?
The likelihood of building damage is radically different for different types of buildings. After the Northridge earthquake, the Superior Apartments were heavily damaged. However, a group of single family homes behind the apartments experienced little damage. These apartments were constructed to comply with modern
building codes. The damage to buildings can be depicted using two separate measures of damage:
1. The percentage of buildings of a particular construction type, defined by use, construction materials, height and age, 'red-tagged' by the local government building inspector as 'unsafe for human occupancy,' that is, uninhabitable, or
2. The average money loss, expressed as a percentage of the replacement value, for each construction type.
It is relatively easy to generate data on the percentage of housing units and commercial buildings typically 'red tagged' for several construction types.
The Turkey Earthquake 1999
The 1999 Turkey earthquakes resulted in far more extensive damage and casualties than occurred in California's 1989 Loma Prieta or 1994 Northridge earthquakes. They provide several lessons for local governments that should not be ignored.
Principal Lessons of Turkey's Earthquake
LESSON 1 - Mitigation guidelines may be developed centrally, but implementation of building codes and land use planning for new construction are the responsibility of local governments.
LESSON 2 - Human needs services are delivered in the context of other damage.
LESSON 3 - Local government staff are first to respond to disasters, not social service agencies or the central government.
LESSON 4 - Local governments need to plan to distribute data initially after the disaster, as well as for weeks and months as data are compiled.
Several themes emerge from the lessons of the 1999 Turkey earthquakes. They are:
= Training of local government employees is essential.
= Local governments are the first to respond and must lead recovery.
= The key local government role, and the need for leadership of local elected officials, is not unique to earthquakes, as the recent tragedies on September 11, 2001, illustrated.