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 webmaster@quake.wr.usgs.gov.
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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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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What does the color of the fault represent on the fault maps?
On the California-Nevada fault map, the color of each fault indicates
when the fault last slipped (see map legend). However, on the zoomed-in and
special maps, the colors do not indicate recency of fault movement. On
these maps, different colors were assigned to help visually distinguish
nearby and adjoining faults from each other.
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Why do some earthquakes appear where no fault is shown?
There are several possible reasons. (1) The earthquake occurred on a fault
shown on the map, but the location of its epicenter is in error. All
epicentral locations have some error. (2) The earthquake occurred at depth
on a nearby fault that is shown on the map, but the fault is not vertical.
The epicenter will then appear offset relative to the trace of the fault at
the surface. (3) The earthquake occurred on a fault not shown on the map.
Many unnamed and unmapped faults underlie the San Francisco Bay region.
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