Geoscience Reference
In-Depth Information
quency, it's not possible to directly infer displacements from a seismometer. Furthermore, a
seismometer is limited by its mechanics and electronics to recording signals smaller than some
threshold; arbitrarily large displacements can be measured by GPS.
Bock et al. (2000) demonstrated that GPS receivers can measure ground motion in real
time as often as every few seconds. They tested the accuracy of these estimates over baselines
as large as 37 km and found that the horizontal components have accuracies no worse than
15 mm; they anticipated that the baselines could be extended to at least 50 km with no further
loss in accuracy. The vertical measurements were less useful with accuracies a factor 7-8 times
worse. The accuracies have improved over the past decade with the advent of new receivers,
new algorithms, and statistical analyses. GPS receivers of 10-50 Hz and methods are now practi-
cal and measurements routine (e.g., Genrich and Bock, 2006).
The application of near-real-time, continuous GPS measurements have made great strides
as well. For example, Song (2007) used coastal GPS stations (E-W and N-S horizontal measure-
ments) to infer displacements on the sealoor offshore using the location of the fault and infer-
ring the vertical uplift from conservation of mass. Song tested the method against geodetic
data from the 2005 Nias, 2004 Sumatra, and 1964 Alaska earthquakes. In the case of Nias and
Sumatra, both continuous GPS data as well as campaign GPS data were available. He tested
the model against satellite altimetry measurements of the tsunami wave using Topex, Jason,
and Envisat data (altimetry proiles included time epochs of 1:55-2:10, 1:48-2:03, and 3:10-3:22
[hr:min after the origin time]). The Nias and Alaska events were also tested against available
coastal tide gauge data. The methods were used again after the February 27, 2010, Chile earth-
quake and later veriied by satellite altimetry from JASON-1 & 2 satellites operated by National
Aeronautics and Space Administration (NASA) and the French Space Agency.
The successful use of GPS data for these four earthquakes makes a strong case for the use
of continuous GPS stations to measure coastal ground displacements to infer the correspond-
ing displacements offshore. In turn, these displacements can be used to predict tsunami gen-
eration including accurate wave heights as a function of time, range, and azimuth.
Near-ield tsunamis are generated by the rupture of hundreds of kilometers of an offshore
subduction fault. As in the case of the Sumatra earthquake this rupture can last as long as eight
minutes and more. During this period of time, GPS data will mimic seismic data with oscillatory
behavior that obscures the smaller, permanent displacements. The most distant part of the
fault from a station can be at least as large as eight minutes of propagation time away, and the
displacements generated by that distant source will take as long to propagate back to the sta-
tion. By that time, however, the static offsets will begin to be apparent, allowing the inference
of offshore displacements and realistic assignment of magnitudes (as little as 4-5 minutes after
the initiation of faulting). The tsunami associated with the earthquake will not come ashore
much earlier than 30 minutes following the beginning of the rupture, and technology-based
warnings could be made in time to provide useful warnings. None of these operations lie even
remotely outside the capabilities of modern networks, computational worklows, and comput-
ing capabilities.
Today there are thousands of GPS geodetic receivers located around the earth. Just in
southern California, there are more than 250 continuously recording GPS geodetic stations that
