Description: The event search mapping tool includes the capability to enter user-defined data for display of up to 4 station locations and the location (epicenter) of a selected event. This capability can be used to display event location information from the S minus P epicenter location method for the user-defined event. Associated with the station locations, an inferred epicenter-to-station distance can be entered for each station. If entered, the station locations will be displayed with a triangle symbol, the 3- or 4-letter station code (for the station name) will be displayed adjacent to the triangles, the event location will be displayed with a star symbol, and the epicenter-to-station distances will be used to display circles around each station indicating the possible location of the epicenter based on each station’s data. Because of the map projection, the circles around the station will not appear to be perfect circles (they would be displayed as circles on a spherical globe; and the distortion will be more pronounced for maps showing large areas) and the circles may extend outside of the range of the map. When the user-defined station and event location data are used to illustrate the results of the S minus P method, the circles (showing epicenter-to-station distances interpreted from the observed S minus P arrival times on seismograms from those stations) should approximately intersect at the epicenter location of the event. The map illustrates the earthquake location information determined from the S minus P data and provides a check on the accuracy of the location. Because the circles and event location can be displayed using the background seismicity (earthquake locations from previous time periods from the event search tool) on the base map, the location of the user-defined event can be interpreted in relation for the historical earthquake information in the area. The display can even be used for single-station estimates of earthquake location to determine if the inferred epicenter-to-station distance circle is consistent with the actual location (if known) or passes through regions of earthquake activity.
Station Data: Optional station data (one to four stations and related information can be entered) are the station code (usually a 3- or 4-letter code), the latitudes and longitudes of the station locations (given in degrees and decimal degrees; south latitudes and west longitudes are negative), and the calculated distances (in kilometers; to calculate distance in km from distance in degrees, multiply by 111.19 km/degree) from S minus P times. Station information, including the latitude and longitude of seismograph stations, can be found from the IRIS Seismic Monitor page (www.iris.edu, click on “Seismic Monitor”; click on an area of interest such as North America; click on stations [triangles] to open a window with station information). An extensive list of stations can also be found at: http://neic.usgs.gov/neis/gis/station_comma_list.asc.
Event Data: Optional event data are the latitude and longitude (given in degrees and decimal degrees; south latitudes and west longitudes are negative) of the event of interest (if known). After an S minus P event location is inferred by interpreting the seismograms from three or four stations, the location of the epicenter that was calculated by the US Geological Survey from data from many stations can be entered into the “User Defined Event” data entry boxes. Official earthquake location information can be found at the USGS earthquake site or the IRIS Seismic Monitor site, or using the IRIS event search tool. Detailed instructions for performing an Internet catalog search, and illustrated examples, are given at: http://web.ics.purdue.edu/~braile/edumod/eqdata/eqdata.htm. Event searches can be performed for selected dates, locations (latitude-longitude range), magnitudes and depths of events. An example of user-defined data is illustrated below:
|Optional user defined data entered to produce the map below. Location and S minus P distance information for zero to four stations can be entered. An optional event location can also be entered. Click Make Map to generate a new map.|
|Map produced using Event Search and the optional user supplied data for an S minus P earthquake location. The Event Search map was produced by searching for M6+ earthquakes from January 1, 1990 to December 31, 1999 within the area -15 to 55 degrees latitude and -165 to -65 degrees longitude. The dot sizes were reduced to a maximum of 0.25 using the optional inputs, the plate boundaries were selected, and a map title was entered before clicking on Make Map.|
Map Boundaries: The latitude and longitude range (boundaries) of the map can be entered to modify the area of the map. To change the map area, for example to “zoom in” on a specific region surrounding the event of interest, the northern (top), southern (bottom), western (left) and eastern (right) boundaries of the desired map can be entered in the “Map Legend” area.
Make Map: To make a map with the user-defined data, enter the optional station and event data and the map range data and click on the “Make Map” button.
Example: An example of the results of an S minus P event location displayed on an event search map is shown above for four stations (TUC = Tucson, AZ; CCM = Cathedral Caves, MO; KIP = Kipapa, HI; and NNA = Nana, Peru), their interpreted epicenter-to-station distances from S minus P arrival times, and the location of the earthquake (obtained from earthquake catalogs). The event is the September 30, 1999 Oaxaca, Mexico earthquake. An example seismogram from this earthquake (station NNA; displayed using the AmaSeis software, http://bingweb.binghamton.edu/~ajones/) is shown below. The difference between the S arrival time (at about 16:47:33 UTC) and the first arrival (at about 16:42:05) indicates an epicenter-to-station distance of about 3891 km. Seismograms (in SAC format which can be viewed and analyzed using the AmaSeis software) can be downloaded from http://web.ics.purdue.edu/~braile/edumod/as1lessons/UsingAmaSeis/UsingAmaSeis.htm (section 6).
Sample seismogram (from station NNA for the Oaxaca earthquake) plotted using the AmaSeis software