We also hope that you will be able to report your experiences to the USGS Community Internet Intensity Map from any possible earthquake. These reports are of great value to seismologists attepting to piece together the effects of earthquakes, and to forecast the possible effects of future events.
Earthquakes, caused by
plate-tectonic forces in the
Earth's interior, always
produce seismic waves
that can be recorded on sensitive instruments called
seismographs.
The NSL has a seismograph station of its
seismic
network at Donnelly, about 100 miles north of
Reno,
labeled DNY.
This instrument separately records motions in three directions: up-and-down
(labeled the DNYZ component); east-west (labeled DNYE); and north-south
(labeled DNYN).
Continuous radio and microwave
telemetry brings these recordings to
our building on the
UNR campus in Reno.
Our camera is mounted above the
earthquake helicorder display, where the signals from several stations
are traced on heat-sensitive paper wrapped around
horizontally-rotating drums.
The camera looks at the top two drums of the
stack of three on the right-hand side of the display.
From top to bottom, the three drums display the signals from components
DNYZ, DNYE, and DNYN, respectively. So the camera image shows up-and-down
ground motions at the top and east-west motions on the drum below.
The top of each drum rotates slowly toward the camera, turning once each hour.
A heated stylus records the motions on the sensitized paper, producing a
graphical trace of ground motion relative to time.
View a Java movie showing the drums after an earthquake on Nov. 27, 1996; or an
MPEG version.
The ``tic marks'', just under an inch apart, are made each minute.
The stylus also moves to the right as the day goes on, so each hour's trace
appears offset to the right from the previous hour's.
Each morning we change the paper on all the helicorders and crank the
styli over to the left side, so right after that the drums
look empty. As the day goes on the
styli progress to the right, and just before the paper is changed the
drums look full.
It is often difficult for even a trained seismologist to verify from the helicorder traces that an earthquake has really occurred. Certainly the appearance of similar disturbances on all three DNY drums is good evidence for a real earthquake. Records of earthquakes in our region appear compact and high-frequency (below, left), while those of earthquakes elsewhere in the world can show slow vibrations that last for several hours, wrapping several times around the drums (below, right).
To find additional stills of camera pictures from earthquakes, take a look at our press releases page. You can click there to view press releases from individual months, and down below some of them are links to camera pictures of events.
There can also be disturbances resulting from
electronic ``glitches'' during telemetry, or lightning strikes, that
are hard to distinguish from records of real earthquakes.
Noise from a host of sources also impedes
interpretation of small events.
Visit our UNRNET Seismogram Data Server.
About one day after the occurrence of most events, you can visit our Earthquake Information page to see what NSL analysts have discovered about an earthquake's location and magnitude. An event in Nevada or eastern California will appear on our ``finger'' map of recent events, or list, once an analyst has been able to interpret it by hand. Notable or large events anywhere in the world will appear within a week on the NSL Record of the Day (or Week), where you can see detailed maps of the earthquake's location, plots of the digital earthquake traces, and download binary seismogram data if you wish to interpret it yourself. Since 1995 we have had automatic, rapid dissemination of preliminary locations and magnitudes; a sample of this Nevada Broadcast of Earthquakes is available.
Some other facilities that may post the locations and magnitudes of earthquakes more quickly than we do are the U.S. Geological Survey, for recent events worldwide, the Southern California Earthquake Center for events in So. Calif., the U.S.G.S. in Menlo Park, Calif. for events in No. Calif., the University of Utah for events in Utah, and the University of Washington for events in Washington and Oregon.
After a week or so, you can put an event into context with other events occurring in the region by looking at Charlie Watson's Seismo-Watch for Reno, or at our weekly maps of recent earthquakes located by NSL, or recent earthquakes throughout the western U.S.
Steve Malone of the University of Washington maintains a list of earthquake-related Internet resources called Seismo-Surfing the Internet.
J. Louie, 10 June 1996
