Planet X Module 4
“Earthquakes do not kill people but buildings do.” This statement is the idea behind the building codes that are in place in countries throughout the world. These codes set minimum standards needed for new buildings to withstand the internally generated forces caused by the vibration of the building mass during an earthquake. But, ground failure can increase the likelihood of damage to a building.
Ground failure changes the characteristics of the surface beneath a structure during an earthquake. There are several classifications of ground failure: settlement, slope failure and liquefaction. Of these, liquefaction is the major cause of building damage during an earthquake.
Liquefaction occurs when a solid acts as a liquid. Soil that is made up of fine, uniform grains of sand and has water within its layers is likely to liquefy when shaken. Sandy soil is capable of supporting structures because the sand grains touch and support each other. There are spaces between the grains that are filled with air or in the case of a shallow water table, with water. Pore pressure is the pressure of this water filling the void between sand particles. When loose sand is shaken it tends to settle and become denser. When water is present the compaction of the sand during shaking increases the pore pressure and forces the water to the surface. This breaks the contact of the sand grains and the soil flows like a liquid allowing structures on it to sink or tilt. Other factors affecting liquefaction are vibration characteristics, porosity, permeability, depth of water table, and initial relative density of the soil.
Figure 1 Diagram of the response of sandy, loose soil with a shallow water table during an earthquake. (University of Bristol web site)
The vibration characteristics are the duration, intensity, and direction of the shaking. Horizontal shaking causes larger settlement than vertical shaking. Pore pressure builds up more rapidly when shaking is in more than one direction as in an earthquake. Longer duration shaking is more likely to cause liquefaction. Porosity is the percentage of the soil’s volume that is open space and is a measure of its ability to hold water. Sand ranges from 30 – 50 % porosity while clay is in the 35 – 80 % range. Permeability is the ability of the soil to drain. When soil does not drain well the pore pressure increases, thus increasing the chance of liquefaction during an earthquake. Low density or soft soils and shallow water tables also contribute to liquefaction. Low-lying, water saturated areas with fine grained sands are widespread especially in agricultural regions.
Site response is the way local soil, ground water and rock conditions change the incoming seismic waves. In sandy soil, waves are amplified, increase in duration, and their frequency content may be altered by absorption. Site and soil conditions greatly influence the degree of shaking that buildings experience during an earthquake.
Figure 2 The geological cross section (Right) shows waves propagating through an alluvial basin. Below are seismic traces recorded above and to the right of the basin. The waves in the basin and recorded above the basin are noticeably amplified.
On September 19th, 1985, an earthquake of magnitude 8.1 caused much damage and loss of life in Mexico City. Most of the damaged buildings were concentrated in the city center. This area of Mexico City is built on top of a dried-out lakebed and is underlain by a thick deposit of very soft, high-water-content sands and clays. There was only moderate ground shaking near the epicenter on the west coast, amounting to 16% of gravity in a horizontal direction. As the seismic waves traveled the 350 km to Mexico City they decreased in amplitude. The horizontal ground shaking on the higher, firm part of the city was only 4% of gravity. But, in the lake zone the softer sediments amplified surface waves. As a result peak horizontal accelerations of up to 40% of gravity were recorded in this area.
The importance of evaluating sites has been recognized and soil engineers now test soil to see if it is capable of liquefaction. If the results are positive then special restrictions are placed on the type of buildings that are allowed. This zoning restriction applies to new construction. There are many seismically active areas in the world that are at risk for liquefaction that already have buildings on them. The metropolitan area of Los Angeles is built on a deep sedimentary basin, as is the Marina district of San Francisco. Many coastal areas of Japan have buildings on recently reclaimed land, including the port city of Kobe. Earthquakes have and will continue to occur in these areas.
In this exercise you are a member of a team of scientists on Planet X. From recent seismic data you will evaluate two sites for seismic hazard. Based on your evaluation of sites for active faults and type of foundation soil you will choose the site of the capital city of Planet X.
1. Examine Map 1. and compare the two marked sites.
2. Examine figure 5 (below) and figure 2. in the above introduction. Based on this data what do you think is the foundation soil at each site?
3. Evaluate each site for seismic hazard: - active faults, earthquake occurrence, and soil prone to ground failure. Use this evaluation to choose the best site of the two for the location of the capital city.
Figure 3 Map showing location of recent earthquakes (red and yellow), tectonic plate boundary (black line), and site 1. and 2. on Planet X.
PLANET X GENERATED GROUND DISPLACEMENT.
Figure 4 Generated data for Planet X showing ground displacements from a seismic event located an equal distance from both sites.