In the United States, earthquake monitoring is performed by the U.S. Geological Survey's National Earthquake Information Center (NEIC).6 NEIC reports the location and size of potentially damaging earthquakes. The information is given to state, federal, international emergency response organizations, and to parties operating critical facilities such as dams, power plants, and transportation networks.
The significance of earthquake location and size, however, extends beyond immediate disaster response. Small magnitude seismicity patterns often indicate areas of deformation where future large earthquakes are likely to occur. An accurate record of historical seismicity is thus important in assessing the future seismic activity of a region. Therefore, the NEIC not only issues earthquake alerts, but also compiles a comprehensive catalog of seismic events around the world. To accomplish this task, the USGS has developed the capability to retrieve or accept seismic data automatically from national and global seismic stations and to provide rapid event locations, magnitudes, depths, and other characteristics. In 1991, for example, NEIC published 16,515 event locations based on 531,097 readings received in reports from 2,150 seismic stations.7
Central to the NEIC are the telemetered stations of the U.S. NSN, a set of 22 existing and 48 planned, digital broadband seismic stations. The real-time capability of the U.S. NSN and its distribution throughout the U.S. allow the NEIC to detect and provide preliminary locations for most significant earthquakes. Once these preliminary locations have been calculated on the basis of data from the U.S. NSN, data from other sources are merged to improve the location. Automatic locations are determined within minutes of an event; publicly released and confirmed locations are available within a few hours; and final revised locations are obtained within a few days.
The IRIS "Gopher" system provides an example of how additional data can be accessed to improve the initial determination of the location of a seismic event. The preliminary alert by NEIC consists of an estimated location, time, and size. The alert triggers the Gopher system to retrieve data for the selected time period from GSN stations worldwide. The Gopher system takes the estimated location and size of the event, and calculates time windows at each of its accessible stations. Typically, the data are automatically retrieved within less than an hour of the event's origin time. This mode of data collection is referred to as an "external trigger" - i.e. one in which data are requested from remote stations based on the external determination (in this case by NEIC) that an event has occurred.
Rather than access all stations itself, the Gopher system parcels out many of its requests through computer communication networks that can interrogate the stations directly, as shown in the figure below.8 To accomplish this task, IRIS has developed a number of agreements with other operators of broadband seismic stations. For example, data from the French Geoscope network are accessed through the Institute de Physique du Globe in Paris. Data from other European stations are obtained through a data center in the Netherlands. Data from some seismic stations in the Western Pacific are obtained through the Japanese network. Similar arrangements with Australia and New Zealand provide access to seismic stations in the Southern Pacific. By delegating most of the station requests, the system is able to retrieve data from many stations concurrently and at low cost. In fact, it is now the relatively slow velocity of seismic waves (a few kilometers per second) that is the controlling factor for the speed at which data can be obtained.
The data are sent to the NEIC to assist them in improving their preliminary earthquake determination. The NEIC analyst is the first human to evaluate the seismic data. Up to this point, all triggers and data retrieval requests have been automatic. The analyst now has the benefit of looking at data from perhaps 50 additional stations to determine whether the trigger was a genuine seismic event. If the event is genuine, a U.S. Geological Survey alert is issued to the public.
The NEIC system is similar to that which is being proposed within the Conference on Disarmament. In the case of the NEIC monitoring, the preliminary "trigger" for the collection of data is the alert issued from the U.S. National Seismic Network. In the case of the Conference on Disarmament, the trigger might be a specialized network of arrays and stations termed the alpha network within the discussions of the Group of Scientific Experts.
There are, however, any number of triggers that could be developed. Last year, an experiment of tying the trigger to a nuclear explosion was conducted for the U.S. explosion code named "Hunter's Trophy" (See Box "The Hunter´s Trophy Experiment"). A location, magnitude, and yield estimate were determined for Hunter´s Trophy from open stations within 20 minutes of detonation.
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| Station Code | Location | Distance from Explosion |
|---|---|---|
| GSC | Goldstone, California | 219 km |
| PFO | Pinyon Flat, California | 312 km |
| BKS | Berkeley, California | 539 km |
| TUC | Tucson Arizona | 736 km |
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Return to Chapter III: Existing Networks and Arrays
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