Policy Statement: NGSD Response to an Earthquake.
Background:
The National Geodetic Survey (C&GS,NOS,NOAA) performs geodetic
measurements to measure deformation resulting from earthquakes.
The NGS response to hazardous earthquakes is coordinated with FEMA
(Federal Emergency Management Agency). NGS has the responsibility
for re-positioning geodetic control points that have been offset
horizontally and vertically by the earthquake. The geodetic
determination of the extent and magnitude of deformation, and
subsequent mathematical modeling, provides scientists with
knowledge that can be correlated with geophysical information
concerning faulting, stratigraphy, state of unreleased strain,
previous non-geodetic measurements, etc. This information is
synthesized in many different ways to improve warning systems,
zoning regulations, insurance rating, and the integrity and safety
of infrastructure engineering.
The timing and strategy of a NGS response to a particular
earthquake depends on several factors such as the geometry of
faulting, magnitude of earthquake, location of earthquake, geometry
and type of pre-existing geodetic monumentation in the area of
suspected deformation, and funding.
Geometry of Faulting:
The geometry of faulting is critical for determining the type
of survey response. Preliminary indications about the type of
fault slip can be derived from seismic records obtained while the
earthquake was occurring, or from field inspection of scarps and
offsets. If a fault slips in a direction which is mostly parallel
to the direction of the fault (strike), the resulting deformation
will be primarily horizontal, although a small amount of vertical
motion may also occur. If a fault slips in the direction normal
to strike, the deformation will be mainly vertical. If the dip
angle of a fault is significantly non-vertical, the deformation is
likely to have both vertical and horizontal components. If the
deformation is primarily vertical, then releveling may produce the
most accurate assessment of monument shifts. If the deformation
is primarily horizontal, EDM or GPS measurements will provide
accurate shifts. In the event that deformation is a mix of
significant vertical and horizontal motions, then GPS or a
combination of leveling and GPS can give a clear result.
Earthquake Location:
The earthquake location will also influence the type of survey
response. Earthquakes in coastal regions will normally require a
very accurate assessment of vertical changes because of the
possibility of shifting flood plains. NGS will coordinate its
response to coastal earthquakes with components of NOS responsible
for tide levels. Earthquakes near population centers, rivers,
dams, nuclear power plants, military installations, and other
critical facilities may require a response with special
considerations for future safety and commerce. Earthquakes in the
proximity of volcanoes are likely to be indicators of major
volcanic activity which may worsen and pose a greater threat. The
focus of survey efforts may temporarily be directed towards
monitoring surface deformation associated with volcanic activity,
rather than in assessing the amount of seismic deformation.
Pre-existing Geodetic Control:
The spatial distribution, types, and history of pre-earthquake
geodetic measurements govern the potential for deriving a good
mathematical description of crustal deformation by resurveying.
For example, horizontal motions will usually not be well detected
if only leveling surveys predated the earthquake. Very sparse or
poorly located configurations of pre-earthquake measurements have
little potential to reveal deformation by re-surveying.
Earthquake Magnitude:
The magnitude and depth of an earthquake primarily determine
the regional extent of deformation, and therefore the ultimate cost
of resurveying. Small earthquakes (M < 6.0) do not generally
cause significant deformation, and therefore seldom require a
response from NGS. Deeper earthquakes cause less deformation at
the earth's surface. Great earthquakes, such as the 1964 Prince
William Sound event (M=8.4), caused deformation over hundreds of
miles, with maximum horizontal shifting of 20 m, and vertical
changes of 10 m. Large earthquakes (M > 7) normally are followed
by a period of rapid deformation. During this period, which may
last a decade or more for a great earthquake, the direction of
motion is often opposite to that which occurred during the
earthquake. This type of postseismic motion decreases
exponentially with time, although postseismic motion can have
several causes - with part of the post-earthquake deformation being
a much slower long term reaction. Therefore the geodetic survey
response after a great earthquake should be well planned,
considering both the determination of the coseismic as well as the
postseismic effects. To determine total postseismic motion, and
to clearly distinguish it from coseismic motion, a comprehensive
survey of the region should be undertaken within a few weeks after
the earthquake.
Timing of Response:
A great earthquake will damage infrastructure over a wide
area. For this reason, immediate attempts to resurvey after an
earthquake are likely to be impeded, but also might interfere with
restoration of power and transportation facilities. NGS will use
the period immediately after an earthquake to research how to make
the best survey response possible. This will be done in
consultation with other interested and responsible agencies, and
will include a search and evaluation of pre-existing monumentation
and measurements, formulation of a geodetic resurvey plan, and
estimation of costs. The period immediately after the earthquake
will also be used to coordinate personnel, equipment and monetary
resources to support the survey project.
Funding of the Survey Response:
NGS will normally have very limited funds which can be
redirected to an unscheduled survey to determine crustal motions
caused by an earthquake. For this reason funding of such a survey
is usually provided by one or more agencies which have a mission
involving public safety or seismology. NGS provides expertise in
planning and execution of these surveys, and in the cataloging of
the resulting measurements and descriptive information for future
purposes. NGS enters into cooperative funding agreements to
support projects which are of critical importance to the National
Geodetic Reference System.
