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California-Nevada Earthquakes

Index Map for CA-NV

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Real-time Shaking Maps
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 which have not pleased all of you, but which (we hope) will 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 the Northern and Southern California and Nevada seismograph networks and will expand to include others.
  • 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 We look forward to hearing from you and hope you will find the Recent Earthquake system useful.

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 index map on one of our sites such as on this server or see our list of mirror sites.

Once you have looked at the index map click on an area to zoom in. Or you can select one of the lists of earthquakes below the map.

If you click on the index 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 a list below the map. 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 such as fault plane solutions.

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.

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How can I zoom in further?

There are a variety of special maps listed on each page. These special maps cover selected areas in greater detail. We can't allow arbitrary zooming in because this would mean creating maps for each individual user and our servers could not handle that load.

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What pages should I bookmark?

You may want to bookmark the index page on the various mirror sites as well as the 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.

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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 that makes them transfer more slowly.

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.

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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.

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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.

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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.

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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.

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How do the earthquakes get here?

Another page describes how the earthquake information gets onto the web server.

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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.

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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.

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