Introduction - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 11

earthquakes. Geodesy can monitor the whole process of the preseismic, coseismic,
and postseismic strain accumulation and release and it is possible to establish the
mode of earthquake precursors, combining this with geophysical observational
results from the borehole strain meter, station extensometer, creepmeter, etc. For
example, the successful short-term prediction of the 1975 Haicheng (Liaoning
Province, China) earthquake used obvious short-term earthquake precursors. The
earthquake is related to global plate movements, so when the relative velocity
obviously deviates from the mean velocity, indicating that the strain accumulation
at the borders of plates is above average, an earthquake would probably occur.
Some countries such as the USA and Japan have established dense geodetic
deformation monitoring systems, including GPS, VLBI, and SLR stations on
seismic zones; for example, the USA has distributed GPS automatic monitoring
networks on the San Andreas Fault Zone. Of course, earthquake prediction is
extremely complex. We are almost sure that there will never be a precise earth-
quake forecast. We might know that an earthquake is due to occur, but no infor-
mation about date and time, position of the epicenter, depth of the hypocenter, or
any other important data will be predictable within the next 100 years.
Geodesy is equally important in preventing other kinds of geological disasters,
for instance, the monitoring of landslide and mudflow. In 1986, the Rockfall and
Landslide Research Institute of Hubei Province, China Three Gorges University
accurately predicted a destructive landslide near the Xintan Area of the Yangtze
River through geodetic monitoring, successfully preventing casualties and greatly
reducing the financial losses to residents.
El Ni˜o can cause disastrous climatic changes of long duration. The anomalies
of sea temperature distribution and ocean circulation cause an abnormal change in
atmospheric mass distribution through the interaction between ocean and atmo-
sphere, which has generated floods in some places and drought in others. Due to the
exchange of angular momentum, the change in atmospheric mass distribution
causes a change in the Earth's angular momentum and influences the Earth's
rotational velocity. When the 1982-1983 El Ni˜o occurred, the Earth's rotation
slowed down. With the VLBI and SLR techniques used today, the changes in the
Earth's rotational velocity can be accurately measured, which enables prediction of
El Ni˜o several years (say 3 years) ahead.
Disasters happen in the world every year—plane crashes, shipwrecks, traffic
accidents, people missing in severe environments, and so on. How to conduct
timely and effective rescue becomes people's primary concern. In the past, wireless
SOS distress signals were used to seek help, but often the exact position of the site
could not be determined so the speed of rescue would be affected. Now, a satellite
rescue system has already been established internationally and uses GPS rapid
positioning and satellite communication technology to allow international rescue
organizations to locate the site rapidly and organize rescue activities in time.