<P> The Ring of Fire makes California and southern Alaska particularly vulnerable to earthquakes . Earthquakes can cause extensive damage, such as the 1906 San Francisco earthquake or the 1964 Good Friday earthquake near Anchorage, Alaska . California is well known for seismic activity, and requires large structures to be earthquake resistant to minimize loss of life and property . Outside of devastating earthquakes, California experiences minor earthquakes on a regular basis . </P> <P> There have been about 100 significant earthquakes annually from 2010 to 2012 . Past averages were 21 a year . This is believed to be due to the deep disposal of wastewater from fracking . None has exceeded a magnitude of 5.6, and no one has been killed . </P> <P> Other natural disasters include: tsunamis around Pacific Basin, mud slides in California, and forest fires in the western half of the contiguous U.S. Although drought is relatively rare, it has occasionally caused major economic and social disruption, such as during the Dust Bowl (1931--1942), which resulted in widespread crop failures and duststorms, beginning in the southern Great Plains and reaching to the Atlantic Ocean . </P> <P> The United States holds many areas for the use and enjoyment of the public . These include National Parks, National Monuments, National Forests, Wilderness areas, and other areas . For lists of areas, see the following articles: </P>

Explain how physical geography has affected the population density of the united states and canada