Frequently Asked Questions

Natural Hazards

The USGS monitors and conducts research on a wide range of natural hazards to help decision-makers prepare for and respond to hazard events that threaten life and property.

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Dry Valleys of Antarctica with dry. rocky soil in the foreground and snow-capped mountains and a glacier in the background
Earthquakes do occur in Antarctica, but not very often. There have been some big earthquakes--including one magnitude 8--in the Balleny Islands. The boundary between the Scotia Plate and the Antarctic Plate just grazes the north tip of the Antarctic Peninsula (look "northwest" from the Pole toward South America). There is also a hint of a line of...
Image: Geothermal Plant
The Geysers Geothermal Field is located in a tectonically active region of Northern California. The major seismic hazards in the region are from large earthquakes occurring along regional faults that are located miles away from the geothermal field, such as the San Andreas and Healdsburg-Rodgers Creek faults. However, activities associated with...
Image shows a diagram of a subduction zone
  Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles). The strength of shaking from an earthquake diminishes with increasing distance from the earthquake's source, so the strength of shaking at the surface from an earthquake that occurs at 500km deep is...
Image: 2014 South Napa CA M6 Earthquake - August 24
Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture.
Full moon appearing over red rocks in Grand Canyon.
Earthquakes are equally as likely to occur in the morning or the evening. Many studies in the past have shown no significant correlations between the rate of earthquake occurrence and the semi-diurnal tides when using large earthquake catalogs. Several recent studies, however, have found a correlation between earth tides (caused by the position of...
Map depicting more than 100 aftershocks that have occurred since the magnitude 7.8 earthquake in Nepal on April 25, 2015. To dat
"Foreshock" and "aftershock" are relative terms. Foreshocks are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs.  Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a...
Image shows an aerial view of the San Andreas Fault
An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel. In California there...
Image: Boulder Geomagnetic Observatory
Satellites and ground-based magnetometers are both important for making measurements of the Earth’s magnetic field. They are not redundant but are instead complementary: Satellites provide good geographical coverage for data collection. Ground-based magnetometers are much less expensive and much easier to install than satellites. An array of...
Giant Sunspot Erupts on October 24, 2014
Our technology-based infrastructure can be adversely affected by rapid magnetic-field variations. This is especially true during “magnetic storms." Because the ionosphere is heated and distorted during storms, long-range radio communication that relies on sub-ionospheric reflection can be difficult or impossible and global-positioning system (GPS...
A schematic depiction of coronal mass ejection headed toward the Earth and its surrounding magnetosphere. Graphic courtesy of NA
A magnetic storm is a period of rapid magnetic field variation. It can last from hours to days. Magnetic storms have two basic causes: The Sun sometimes emits a strong surge of solar wind called a coronal mass ejection. This gust of solar wind disturbs the outer part of the Earth's magnetic field, which undergoes a complex oscillation. This...
Earth (Mantle)
The Earth's magnetic field does not directly affect human health. Humans evolved to live on this planet. High-altitude pilots and astronauts can experience higher levels of radiation during magnetic storms, but the hazard is due to the radiation, not the magnetic field itself. Geomagnetism can also impact the electrically-based technology that we...
Moving Declination graphic
At most places on the Earth's surface, the compass doesn't point exactly toward geographic north. The deviation of the compass from true north is an angle called "declination" (or "magnetic declination"). It is a quantity that has been a nuisance to navigators for centuries, especially since it varies with both geographic location and time. It...