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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geocoded Did You Feel It? responses for a sonic boom off the coast of New Jersey on January 28, 2016
Steps to identification of a sonic boom: The USGS sees either nothing on our seismic records or a fairly short high-frequency signal  that doesn't look like an earthquake. On rare occasions, we see the event on multiple stations, and the time difference between stations matches the speed of sound in air, which is slower than the speed of...
world map showing time zones
Since we and other seismic network agencies record earthquakes around the globe in all the various time zones, using a single standard time reference is best for record-keeping and exchange of data.  Also, we tried converting UTC to the local time zones for epicenters and reporting them that way for a while on our website, and it confused our web...
Large umbrella shaped cloud of volcanic ash viewed from a distance
Deadliest Volcanic Eruptions Since 1500 A.D.      Eruption                                     Year               Casualties                 Major Cause Nevado del Ruiz, Colombia        1985              25,0001,3                      Mudflows3 Mont Pelée, Martinique              1902              30,0001(29,025)2        Pyroclastic flows2...
1975 earthquake in HVNP
Sometimes, yes. A few historic large regional earthquakes (>M 6) are considered by scientists to be related to a subsequent eruption or to some type of unrest at a nearby volcano. The exact triggering mechanism for these historic examples is not well understood, but the volcanic activity probably occurs in response to a change in the local...
Chaitén Volcano in Chile
Volcano eruptions have occurred shortly after earthquakes and they may be linked, but scientists are still debating the topic. Notably, an Andean volcano (Cordon Caulle) began erupting 2 days after the magnitude 9.6 1960 Chile earthquake. Eruptions of mud volcanoes have occurred in the Andaman Islands following the recent magnitude 9.0 megathrust...
Old Faithful Geyser in eruption
Earthquakes cannot be predicted yet, but modern surveillance conducted with seismographs (instruments that measure earthquake locations and magnitudes) and Global Positioning System (GPS) instruments that measure slow ground movements help scientists understand the state of stress in the Earth's crust that could trigger earthquakes as well as...
snow-covered Mt. Hood volcano on the horizon overlooking tall building in city of Portland
Felt earthquakes on Mount Hood (Oregon) occur every 2 years on the average. Seismic monitoring, in effect since 1977, indicates a generalized concentration of earthquakes just south of the summit area and 2-7 kilometers below sea level. A seismic swarm in July 1980, during which nearly 60 earthquakes (mostly 5-6 kilometers deep with a maximum...
Image: Mw6.6 Lushan China Earthquake, April 20 2013
Phenomena such as sheet lightning, balls of light, streamers, and steady glows, reported in association with earthquakes are called earthquake lights (EQL).  Geophysicists differ on the extent to which they think that individual reports of unusual lighting near the time and epicenter of an earthquake actually represent EQL:  some doubt that any of...
Image: Natural Entrance at Carlsbad Caverns
There is nothing different about a cave that would make it immune to the shaking from an earthquake.  Just as there are safer and less safer places to be on the surface of the earth during an earthquake, there are also various characteristics inside caves that make some cave locations safer or less safe than others.  First of all, whether or not...
Image: Liquefaction in Subsurface Layer of Sand
Liquefaction takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking. Liquefaction occurring beneath buildings and other structures can cause major damage during earthquakes. For example, the 1964 Niigata earthquake caused widespread liquefaction in Niigata,...
Devastation of 1906 San Francisco Earthquake
Two sources for photographs that show earthquake damage are: Earthquake Hazards Program - Earthquake Photo Collections U.S. Geological Survey Photographic Library (see 'earthquakes' in the categories left column)  
Image: Damage in Residential Area
Groundwater levels in wells may oscillate up and down while seismic waves pass, and in some cases, the water level may remain higher or lower for a period of time after the seismic wavetrain has ended.