ANSS Catalog Caveats
Note (4/9/2015): The ANSS Comprehensive Catalog (ComCat) contains earthquake source parameters (e.g. hypocenters, magnitudes, phase picks and amplitudes) and other products (e.g. moment tensor solutions, macroseismic information, tectonic summaries, maps) produced by contributing seismic networks. This comprehensive collection of seismic information will eventually replace the ANSS composite catalog hosted by the Northern California Data Center; however, historic regional seismic network catalogs have not yet been fully loaded.
The ANSS composite catalog is a valuable resource for seismologists and earthquake engineers, for teachers and the general public. For example, researchers may study such a catalog to determine the seismic hazard of a particular area.
As a composite catalog - comprised of contributions from member networks - the ANSS catalog is not uniform in its coverage. Many of the ANSS networks are funded by the USGS and began operations in the 1960s and 1970s. In other areas, however, catalogs are available for longer time periods. The catalog is also non-uniform in its magnitude completeness.
In the figures below, we provide information on the contributions from each seismic network in the time period from 1900-present. Note that the seismicity among the regions may vary by orders of magnitude. The most active areas generally update their catalogs daily, while less active areas update their catalogs less frequently.
The bottom portion of each figure indicates the monthly contributions from that network. An X indicates submitted data, no symbol indicates no data or no submitted data. The top part of the figure indicates the number of earthquakes in each submission. These vary as a function of time, due to issues such as network funding and background seismicity.
These figures are updated weekly and are based on this file of event counts for each network code. Click on a image to see the full-sized figure
As illustrated above, some of the earliest contributions to the ANSS catalog come from networks in California and Alaska. The earliest entry in the NEIC catalog is in 1920. Significant US effort in earthquake monitoring outside of California did not begin until the 1960s and 1970s, motivated by the concern over nuclear weapons testing and the advent of the NEHRP program.
The ANSS catalog contains a number of idiosyncrasies. For example, magnitudes for California earthquakes were not routinely reported until the 1940s. For global events, magnitudes are not routinely reported until the early 1960s. This means, for example, that the largest earthquake observed - the 1960 Chilean earthquake (Mw 9.5) - appears in this catalog without an associated magnitude.
As a further illustration, consider these histograms of the number of events in the ANSS catalog at various magnitude levels. These plots were generated for every 5 years between 1940 and 1995. One can see the relatively low numbers of events in 1940-1960, and the sudden increase in 1965. This represents the increase in USGS earthquake monitoring efforts as well as the routine reporting of magnitude from the NEIC. Another burst can be seen in 1975 as many of the NEHRP-funded networks began to come online. Beginning in the 1980s, events at the higher magnitude levels begin to show some stability. For example, here are year-by-year histograms for the 1980s and 1990s, showing the variations in numbers of events of magnitude 4 and higher. While the number of magnitude 4.0-4.9 events gradually evolves, the number of magnitude 5 and higher events has remained relatively stable on the year to year basis. The NEIC has a nice fact sheet on earthquake statistics
The ANSS brings together the earthquake catalogs of its member institutions in the composite catalog. Users should be aware of the potential problems due to variations in location thresholds and methodologies from agency to agency and due to temporal variations in network configurations. For users who are more interested in a uniform catalog, the Harvard Centroid Moment Tensor (CMT) catalog may be useful. For comparison, we generated histograms for the 1980s and 1990s from the CMT catalog, using Mw to bin the events. This catalog has been produced by the application of a consist methodology to determine the centroid location and seismic moment tensor from 1977 to the present to events of magnitude 5.0 and higher. The CMT catalog is a useful for the study of larger earthquakes, but does not have the detail of ANSS composite catalog for smaller events.