Event Page Terms

[emartinez-usgs]

Make this page. Previously available at /earthquakes/eventpage/terms.php. Verify links, prefer https. etc...

<div class="page-content">
  <a id="magnitude"></a>
  <h2>Magnitude</h2>

  <p>
    Earthquake magnitude is a measure of the size of an earthquake at its
    source.
    It is a logarithmic measure. At the same distance from the earthquake, the
    amplitude of the seismic waves from which the magnitude is determined are
    approximately 10 times as large during a magnitude 5 earthquake as during a
    magnitude 4 earthquake. The total amount of energy released by the
    earthquake
    usually goes up by a larger factor: for many commonly used magnitude types,
    the total energy of an average earthquake goes up by a factor of
    approximately 32 for each unit increase in magnitude. There are various
    ways
    that magnitude may be calculated from seismograms. Different methods are
    effective for different sizes of earthquakes and different distances
    between
    the earthquake source and the recording station. The various magnitude
    types
    are generally defined so as to yield magnitude values that agree to within
    a
    few-tenths of a magnitude-unit for earthquakes in a middle range of
    recorded-earthquake sizes, but the various magnitude-types may have values
    that differ by more than a magnitude-unit for very large and very small
    earthquakes as well as for some specific classes of seismic source. This is
    because earthquakes are commonly complex events that release energy over a
    wide range of frequencies and at varying amounts as the faulting or rupture
    process occurs. The various types of magnitude measure different aspects of
    the seismic radiation (e.g., low-frequency energy vs. high-frequency
    energy).
    The relationship among values of different magnitude types that are
    assigned
    to a particular seismic event may enable the seismologist to better
    understand the processes at the focus of the seismic event. The various
    magnitude-types are not all available at the same time for a particular
    earthquake. Preliminary magnitudes based on incomplete but
    rapidly-available
    data are sometimes estimated and reported. For example, the Tsunami Warning
    Centers will calculate a preliminary magnitude and location for an event as
    soon as sufficient data are available to make an estimate. In this case,
    time
    is of the essence in order to broadcast a warning if tsunami waves are
    likely
    to be generated by the event. Such preliminary magnitudes are superseded by
    improved estimates of magnitude as more data become available. For large
    earthquakes of the present era, the magnitude that is ultimately selected
    as
    the preferred magnitude for reporting to the public is commonly a moment
    magnitude that is based on the scalar seismic-moment of an earthquake
    determined by calculation of the seismic moment-tensor that best accounts
    for
    the character of the seismic waves generated by the earthquake. The scalar
    seismic-moment, a parameter of the seismic moment-tensor, can also be
    estimated via the multiplicative product rigidity of faulted rock x area of
    fault rupture x average fault displacement during the earthquake.
  </p>

  <p>
    <a href="/learn/topics/mag-intensity/magnitude-types.php">Magnitude Types</a>
  </p>

  <a id="eventTime"></a>
  <h2>Event Time</h2>

  <p>
    We indicate the date and time when the earthquake initiates rupture, which
    is
    known as the "origin" time. Note that large earthquakes can continue
    rupturing
    for many 10's of seconds. We provide time in UTC (Coordinated Universal
    Time).
    Seismologists use UTC to avoid confusion caused by local time zones and
    daylight savings time. On the individual event pages, times are also
    provided
    for the time at the epicenter, and your local time based on the time your
    computer is set.
  </p>

  <a id="location"></a>
  <h2>Location</h2>

  <p>
    An earthquake begins to rupture at a hypocenter which is defined by a
    position
    on the surface of the earth (epicenter) and a depth below this point (focal
    depth). We provide the coordinates of the epicenter in units of latitude
    and
    longitude. The latitude is the number of degrees north (N) or south (S) of
    the
    equator and varies from 0 at the equator to 90 at the poles. The longitude
    is
    the number of degrees east (E) or west (W) of the prime meridian which runs
    through Greenwich, England. The longitude varies from 0 at Greenwich to 180
    and the E or W shows the direction from Greenwich. Coordinates are given in
    the
    <a href="http://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350_2.html">
      WGS84</a> reference frame. The position uncertainty of the hypocenter
    location
    varies from about 100 m horizontally and 300 meters vertically for the best
    located events, those in the middle of densely spaced seismograph networks,
    to
    10s of kilometers for global events in many parts of the world.
  </p>

  <a id="depth"></a>
  <h2>Depth</h2>

  <p>
    The depth where the earthquake begins to rupture. This depth may be
    relative
    to mean sea-level or the average elevation of the seismic stations which
    provided arrival-time data for the earthquake location. The choice of
    reference depth is dependent on the method used to locate the earthquake.
    Sometimes when depth is poorly constrained by available seismic data, the
    location program will set the depth at a fixed value. For example, 33 km is
    often used as a default depth for earthquakes determined to be shallow, but
    whose depth is not satisfactorily determined by the data, whereas default
    depths of 5 or 10 km are often used in mid-continental areas and on
    mid-ocean
    ridges since earthquakes in these areas are usually shallower than 33 km.
  </p>

  <a id="nearbyCities"></a>
  <h2>Nearby Cities</h2>

  <p>
    We provide distances and directions from nearby geographical reference
    points
    to the earthquake. The reference points are towns, cities, and major
    geographic features derived from US Census data, such as from
    <a href="https://www.census.gov/geo/www/gazetteer/places2k.html">
      http://www.census.gov/geo/www/gazetteer/places2k.html</a>. International
    places were gathered from a specially created USGS catalog. Selected places
    were based on minimum population values that were specified for each
    particular region.
  </p>

  <p>
    We realize that these distances are uncertain both because of the errors
    inherent in locating earthquake (typically one or more kilometers) and
    because
    of the impossibility of describing the location of a city by a single
    longitude-latitude entry in a table. For places in the US, rather than
    rounding off distances to, say, the nearest 10 kilometers, we chose to
    trust
    the user's common sense in interpreting the accuracy of these distances.
    For
    places outside the US, distances are rounded depending on the
    <a href="#locationUncertainty">location uncertainty</a>. If the computed
    location is close to an operating quarry which is known to use explosives
    in
    its operations, we indicate that the event may be a quarry explosion. We
    try
    to always provide at least one widely recognized reference point in the
    list
    on the event page, even if the earthquake occurs in a remote location.
  </p>

  <a id="magnitudeUncertainty"></a>
  <h2>Magnitude Uncertainty</h2>

  <p>
    The estimated standard error of the magnitude. The uncertainty corresponds
    to
    the specific magnitude type being reported and does not take into account
    magnitude variations and biases between different magnitude scales. We
    report
    an "unknown" value if the contributing seismic network does not supply
    uncertainty estimates.
  </p>

  <a id="locationUncertainty"></a>
  <h2>Location Uncertainty</h2>

  <p>
    The horizontal location error, in km, defined as the length of the largest
    projection of the three principal errors on a horizontal plane. The
    principal
    errors are the major axes of the error ellipsoid, and are mutually
    perpendicular. The horizontal and vertical uncertainties in an event's
    location varies from about 100 m horizontally and 300 meters vertically for
    the best located events, those in the middle of densely spaced seismograph
    networks, to 10s of kilometers for global events in many parts of the
    world.
    We report an "unknown" value if the contributing seismic network does not
    supply uncertainty estimates.
  </p>

  <a id="depthUncertainty"></a>
  <h2>Depth Uncertainty</h2>

  <p>
    The depth error, in km, defined as the largest projection of the three
    principal errors on a vertical line.
  </p>

  <a id="azmithulGap"></a>
  <h2>Azimuthal Gap</h2>

  <p>
    The largest azimuthal gap between azimuthally adjacent stations (in
    degrees).
    In general, the smaller this number, the more reliable is the calculated
    horizontal position of the earthquake. Earthquake locations in which the
    azimuthal gap exceeds 180 degrees typically have large location and depth
    uncertainties.
  </p>

  <a id="nst"></a>
  <h2>Number of Stations Used</h2>

  <p>
    Number of seismic stations which reported P- and S-arrival times for this
    earthquake. This number may be larger than the Number of Phases Used if
    arrival times are rejected because the distance to a seismic station
    exceeds
    the maximum allowable distance or because the arrival-time observation is
    inconsistent with the solution.
  </p>

  <a id="nph"></a>
  <h2>Number of Phases Used</h2>

  <p>
    Number of P and S arrival-time observations used to compute the hypocenter
    location. Increased numbers of arrival-time observations generally result
    in
    improved earthquake locations.
  </p>

  <a id="dmin"></a>
  <h2>Minimum Distance</h2>

  <p>
    Horizontal distance from the epicenter to the nearest station (in km). In
    general, the smaller this number, the more reliable is the calculated depth
    of
    the earthquake.
  </p>

  <a id="rmss"></a>
  <h2>Travel Time Residual</h2>

  <p>
    The root-mean-square (RMS) travel time residual, in sec, using all weights.
    This parameter provides a measure of the fit of the observed arrival times
    to
    the predicted arrival times for this location. Smaller numbers reflect a
    better fit of the data. The value is dependent on the accuracy of the
    velocity
    model used to compute the earthquake location, the quality weights assigned
    to
    the arrival time data, and the procedure used to locate the earthquake.
  </p>

  <a id="reviewStatus"></a>
  <h2>Review Status</h2>

  <p>
    Review status is either automatic or reviewed. Automatic events are
    directly
    posted by automatic processing systems and have not been verified or
    altered by a human. Reviewed events have been looked at by a human. The
    level
    of review can range from a quick validity check to a careful reanalysis of
    the event.
  </p>

  <a id="eventID"></a>
  <h2>Event ID</h2>

  <p>
    A combination of a 2-letter
    <a href="http://www.iris.edu/stations/networks.txt">Seismic Network Code</a>
    and a number assigned by the contributing seismic network.
  </p>

  <a id="momentTensors"></a>
  <h2>Moment Tensors</h2>

  <p>
    A mathematical representation of the movement on a fault during an
    earthquake.
    The tensor depends on the source strength and fault orientation. See also
    <a href="/learn/topics/beachball.php">Focal Mechanisms</a>
  </p>
</div>