Seismic waves from earthquakes can produce violent ground shaking. These wave vibrations produce several different effects on the natural environment that also can cause tremendous damage to the built environment (buildings, transportation lines and structures, communications lines, and utilities).
Earthquakes can also affect people through secondary effects. Secondary hazards include landslides, liquefaction, tsunamis, seiches, and fires. The following hazards and secondary effects can occur from an earthquake:   
The first main earthquake hazard (danger) is the effect of ground shaking. Ground shaking is both a hazard created by earthquakes and the trigger for other hazards such as liquefaction and landslides. Ground shaking describes the vibration of the ground during an earthquake.
Most earthquake damage results from the shaking caused by seismic waves passing beneath buildings, roads, and other structures. For example, ground shaking may cause a store’s exterior building walls to crumble, injuring people, blocking sidewalks and streets and bringing down utility lines.
Ground rupture generally occurs only along the fault zone that moves during the earthquake, and are thus a primary effect. Thus structures that are built across fault zones may collapse, whereas structures built adjacent to, but not crossing the fault may survive.
Earthquakes may cause both uplift and subsidence of the land surface. During the 1964 Alaskan Earthquake, some areas were uplifted up to 11.5 meters, while other areas subsided up to 2.3 meters.
Liquefaction describes the way in which soil liquefies during ground shaking. Liquefaction can undermine the foundations and supports of buildings, bridges, pipelines, and roads, causing them to sink into the ground, collapse or dissolve.
Landslides and avalanches are secondary effects. In mountainous regions subjected to earthquakes ground shaking may trigger landslides, rock and debris falls, rock and debris slides, slumps, and debris avalanches. They also can block roads and disrupt utility lines.
Fires Fires are a secondary effect of earthquakes. Because power lines may be knocked down and because natural gas lines may rupture due to an earthquake, fires are often started closely following an earthquake. The problem is compounded if water lines are also broken during the earthquake since there will not be a supply of water to extinguish the fires once they have started. In the 1906 earthquake in San Francisco more than 90% of the damage to buildings was caused by fire.
Flooding is a secondary effect of earthquakes that may occur due to rupture of human made dams and levees, as a result of ground rupture or subsidence. The water from the river or the reservoir would then flood the area, damaging buildings and maybe sweeping away or drowning people.
Tsunamis and seiches are secondary effects but they can cause a great deal of damage.
A tsunami is what most people call a tidal wave, but it has nothing to do with the tides on the ocean. It is a huge wave caused by an earthquake under the ocean. Tsunamis can be tens of feet high when they hit the shore and can do enormous damage to the coastline.
Seiches are like small tsunamis. They occur on lakes that are shaken by the earthquake and are usually only a few feet high, but they can still flood or knock down houses, and tip over trees.
- National Atlas.gov – Seimic Hazards: http://www.nationalatlas.gov/mld/seihazp.html
- U.S. Geological Survey General Information Product 15 2005 – Putting Down Roots in Earthquake Country – Your Handbook for the San Francisco Bay Region: http://pubs.usgs.gov/gip/2005/15/gip-15.pdf
- California Geological Society – Earthquake Hazards:http://www.conservation.ca.gov/cgs/information/outreach/Documents/Discovery_hazards.pdf
- City of Seattle.gov – Hazard Identification & Vulnerability Analysis: http://www.seattle.gov/emergency/library/SHIVA.pdf
- Image Source: http://libraryphoto.cr.usgs.gov/htmllib/btch105/btch105j/btch105z/btch105/whg00003.jpg [Accessed: March 15, 2014]