Each year, tens of thousands of small earthquakes occur throughout the United States, reflecting the brittle deformation of the North American plate along its edges and within its interior. Although not damaging, these smaller earthquakes provide a wealth of information that enables seismologists and engineers to better assess the distribution, frequency, and severity of seismic hazards throughout the country. Seismograph networks supply earthquake parameter and waveform data that are essential for the real-time evaluation of tectonic activity for public safety (e.g., volcanic eruptions, tsunamis, earthquake mainshocks and aftershocks), the development of earthquake hazard maps and seismic design criteria used in building codes and land-use planning decisions (e.g., characterization of seismic sources, ground failure, strong ground motion attenuation), and basic scientific and engineering research.

National and regional earthquake hazard maps published by the U.S. Geological Survey (USGS) and state geological surveys involve the collection and integration of seismograph network data with other geologic and geophysical data, including paleoearthquake chronologies, locations of active faults, determinations of three-dimensional velocity and geologic structure, and wave propagation and attenuation parameters. These earthquake hazard data and maps help define the level of earthquake risk throughout the United States and provide input to risk management decisions at both the national and local levels (see Chapter 2). Efforts to reduce the uncertainties in these data help to clarify the level of seismic hazard and risk and to identify the appropriate mitigation and response strategies for different parts of the country.

Earthquake Monitoring

Seismic monitoring provides a wealth of critical information for earthquake hazard assessment and for improved understanding of the earthquake process. The basic product of earthquake monitoring is the seismicity catalog, a listing of all earthquakes, explosions, and other seismic disturbances (both natural and manmade). Parametric data, such as earthquake origin times, locations, and magnitudes, are used to characterize the frequency and size of earthquakes in a particular region and help identify active faults. Earthquake catalogs play a key role in probabilistic seismic hazard assessment, especially in the eastern and central United States where there is generally insufficient detailed information on active faults and their tectonic causes (NRC, 1996).

The importance of earthquake catalogs for seismic hazard assessment underscores the need for consistent and reliable long-term recording and

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