Fig. 6.4. Longitudinal and transversal waves (Földessy 2011)

Earthquake strength is measured as both magnitude and intensity. Magnitude measures the relative strength of an earthquake and is recorded with the Richter scale. Each earthquake only has one magnitude. People usually cannot feel earthquakes with magnitudes of 3.0 or less. Earthquakes have always seemed mysterious, sudden, and violent, coming without warning and leaving ruined cities and dislocated landscapes in their wake. Cities such as Kobe, Japan, or Mexico City, parts of which are built on soft landfill or poorly consolidated soil, usually suffer the greatest damage from earthquakes. Water-saturated soil can liquify when shaken. Buildings sometimes sink out of sight or fall down like a row of dominoes under these conditions. Earthquakes frequently occur along the edges of tectonic plates, especially where one plate is being subducted, or pushed down, beneath another. Earthquakes also occur in the centers of continents, however.

6.2.1.2. 6.2.1.3. Landslides

Gravity constantly pulls downward on every material everywhere on earth. Hillsides, beaches, even relatively flat farm fields can lose material to erosion. Often, water helps to mobilize loose material, and catastrophic landslides, beach erosion, and gully development can occur in a storm. They can also be started by earthquakes, volcanic activity, changes in groundwater, disturbance or change of a slope by man-made construction activities, or any combination of these factors. A variety of other natural causes may also result in landslides, and they may trigger additional hazards, such as tsunamis caused by submarine landslides. A landslide occurs when the force that is pulling the slope downward (gravity) exceeds the strength of the earth materials that compose the slope. Where houses are built on erodible surfaces, enormous loss of property can result. Landslides, also called mass wasting or mass movement, occur when masses of material move downslope. Slow and subtle landslips are common, but rockslides, mudslides, and slumping can be swift and dangerous. When unconsolidated sediments on a hillside are saturated by a storm or exposed by logging, road building, or house construction, slopes are especially susceptible to sudden landslides. Often people are unaware of the risks they face by locating on or under unstable hillsides. Sometimes they simply ignore clear and obvious danger. In Southern California, where land prices are high, people often build expensive houses on steep hills and in narrow canyons. Most of the time, this dry environment appears quite stable, but the chaparral vegetation burns frequently and fiercely (Fig. 6.6., 6.7.).