Recent Earthquakes - Frequently Asked Questions (FAQ)

The Recent Earthquake information system was developed through a long process that included input from seismologists, graphic artists, and users. Our goal is to provide users with as much information as possible in a timely fashion. Because the number of users continues to grow, we have been forced to make some design choices to maximize the number of users getting information over the web, especially after felt earthquakes. We consider this system a major step forward for the following reasons: It currently combines data from Advanced National Seismic System (ANSS) seismograph networks from around the country. It is mirrored on multiple sites for robust service during earthquake sequences and network problems. It offers complete coverage with no gaps because of an overlapping system of maps. It provides text information on each earthquake and auxiliary information on some of them. To meet our goal, we had to make many decisions and compromises. We realize that not everyone will agree with our decisions but we do hope that you will read this file and consider why we made them. If you have comments or suggestions please send them to Web_Team. We look forward to hearing from you and hope you will find the Recent Earthquake system useful. How do I find what I want? What pages should I bookmark? Why are squares used for the earthquakes? How were the symbol sizes chosen? How were the colors chosen? Why don't the maps show topography? What does "last hour", "last day" and "last week" mean? How do the earthquakes get here? Why do some earthquakes disappear? Why isn't the distance to the nearest fault provided? How did you decide which places to use for computing the distance to individual earthquakes? How did you decide which places toplot on the maps? How do I find what I want? NOTE: Our information changes with time. Sometimes when you look at a page your browser will show you an old copy that it has cached. Check the time on the page and in the map. If it was produced over an hour ago, or if you think something has happened since it was produced, then use the reload feature of your browser to get a new version of the file. To navigate through these maps you should start at the USA map. Once you have looked at the USA map click on an area to zoom in. Or you can select the list of earthquakes below the map. If you click on the USA map and zoom in you can get more information on any earthquake by either clicking on its map symbol or by clicking on its summary information presented in the list of earthquakes. Or, to move to an adjacent map you can click on one of the blue arrows near the edges of the map. If you click on an earthquake (either on a map or in a list) and get a page of detailed information you should look at the bottom of this page for auxiliary information. At any time you can use your back button to return to a previous page, but always remember the note above about reloading if a page seems old. Return to top of faq What pages should I bookmark? You may want to bookmark the USA map as well as more detailed maps for the area where you live and work and others that you find interesting. This may help you get fast access after significant earthquakes when many people are trying to use the Recent Earthquakes system. Return to top of faq Why are squares used for the earthquakes? Some users prefer to see earthquakes drawn as circles. We have chosen to use squares because computer screens are based on a square grid of dots or rasters. This makes it easier to draw a clear square than a circle. To draw a good looking circle requires a technique called anti-aliasing, however this introduces shades of grey into the image which makes the map files larger and slower to transfer. We agree that circles are prettier than squares, and we anticipate that as software and hardware get better and faster, it will be more feasible to use circles. Right now we have opted for the simplicity and visual clarity of squares. Return to top of faq How were the symbol sizes chosen? Some users have noted that the magnitude 1, 2, and 3 earthquakes are drawn with fairly small symbols. This was done because after a large earthquake there will be many small aftershocks. If the aftershocks have large symbols they may obscure the mainshock on the map. To help prevent this we have made the small earthquakes have small symbols. Return to top of faq How were the colors chosen? Our goal was to highlight the earthquakes while also showing background information such as faults, roads, towns, and bodies of water. We limited our color choices by selecting from the approximately 200 "browser safe" colors that are consistently displayed without dithering by a variety of web browsers. We also attempted to select colors that could be easily distinguished by users with red-green color blindness. Return to top of faq Why don't the maps show topography? Some of our older earthquake maps showed topography in the background as either a range of colors or shades of gray. Unfortunately, this popular feature makes the map files about three times larger. These larger files take longer to transfer and at times of heavy usage can prevent us from serving many of those who want the earthquake information. While we would like to show the topography, we have concluded that it is more important to provide fast access to the earthquake data to as many people as possible. Return to top of faq What does "last hour", "last day" and "last week" mean? Each map shows the time it was created. The phrases "last hour", "last day", and "last week" are with respect to that time. Any earthquakes that occurred within one hour of the creation time are in the "last hour" and are colored red. Those that occurred between 1 and 24 hours before the map was created are in the "last day" and are colored blue. Those that occurred between 24 hours and 7 days (168 hours) before the map was drawn are in the "last week" and are colored yellow. Return to top of faq How do the earthquakes get here? Another page describes how the earthquake information gets onto the web server. Return to top of faq Why do some earthquakes disappear? The earthquake data shown here is automatically generated and despite our best efforts some glitches will create bogus earthquakes. When we find a bogus event, usually by studying the seismograms, we delete it and careful observers may notice that an earthquake has disappeared. This often happens after a large earthquake when our systems don't realize that all of the seismograms were created by a single event. In this case, one earthquake will turn into multiple "events" on the maps. In other cases problems in our telemetry systems that bring the data from our seismometers to our computers create glitches that also can create bogus events. For these reasons it is very important to remember that this data is preliminary and when events disappear they weren't real to begin with. Return to top of faq Why isn't the distance to the nearest fault provided? Seismologists evaluate the hypocenter location and the focal mechanism of an earthquake to decide if the earthquake occurs on a named fault. Research shows that many earthquakes occur on small, un-named faults located near well known faults. For example, most of the aftershocks of the 1989 M6.9 Loma Prieta earthquake occurred on small, subsidiary faults within a few hundred meters of the mainshock rupture plane. On other fault segments like the San Andreas fault near Parkfield, most of the earthquakes occur on the San Andreas fault. It is difficult to automate this decision process, and it would be misleading to imply on the basis of only distance that an earthquake occurs on a named fault. Return to top of faq How did you decide which places to use for computing the distance to individual earthquakes? The list of "populated places" comes from the U.S. Census Bureau Census 2000 gazetteer of places. We sort the list based on population, then choose the four places closest to epicenter. We check to see that at least one of the names is shown on the map, or that one place on the list has a population of at least 100,000 people. Return to top of faq How did you decide which places to plot on the maps? The list of places comes from the U.S. Census Bureau Census 2000 gazetteer of places. The name-selection scheme places priority on population while requiring locations to be separated by a minimum specified distance. The algorithm attempts to find at least 4 names on a map, with one in each quadrant of the map. It begins with a requirement for a minimum population (16,000/64,000, respectively, for the 2/10 deg maps) and a minimum location separation of 25/130 km, respectively, within each map. For those maps in which the algorithm could not find one populated place above the minimum population within each quadrant of the map, it progressively relaxed the population criteria to a minimum of 100/25,000, respectively, or until it found a place in the quadrant. In Alaska where the population is sparse, any place was accepted if it didn't violate the separation rule. Almost all places with a population greater than 350,000 are guaranteed a place on the map. Return to top of faq