While most recorded earthquakes are consistent with expectations based on current understandings of plate tectonics, and with simplifying assumptions about the earthquake source, there are a number of very interesting exceptions. Earthquakes with non-double couple focal mechanisms, or which contain a non-zero isotropic component, may be the result of source processes other than slip on a planar fault surface. Earthquakes which occur outside of well-defined bands of seismicity, or which display focal mechanisms at odds with the stress regimes we believe to exist in a given region, may cause us to revise our understanding of regional tectonics, or of deformation processes in general. The Unusual Earthquakes research effort studies a small number of events in detail, by a variety of methods, in an effort to gain a more thorough understanding of source processes and tectonics which differ from accepted standard models.
Strong-motion recordings of damaging earthquakes in densely urbanized areas are critical for designing earthquake-resistant structures to reduce property loss and casualties from future earthquakes. The recordings also are fundamental for understanding and characterizing the physics of earthquake rupture, the generation and propagation of damaging ground motions, and the shaking performance of structures.
The USGS National Strong-Motion Project (formerly titled the National Strong Motion Program) has the primary Federal responsibility for acquiring strong motion records of significant earthquakes in the United States recorded by sensors placed in the ground and in man-made structures.
The USGS monitors and reports on earthquakes, assesses earthquake impacts and hazards, and conducts targeted research on the causes and effects of earthquakes. We undertake these activities as part of the larger National Earthquake Hazards Reduction Program (NEHRP), a four-agency partnership established by Congress.
On July 17, 1998, three huge waves – "tsunamis" – up to 15 meters high struck the north coast of Papua New Guinea, killing at least 2,200 people. A major earthquake, itself consisting of waves traveling through the Earth, triggered an underwater landslide that created the tsunamis. Radio stations reported the disaster by transmitting electromagnetic radio waves to listeners around the world. Listeners were able to hear the news transported by sound waves created by their radios.