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Group 2 – Tectonic Geodesy | Poster Abstracts
may be critical to our ability to use CGPS time-series to investigate a wide class of potential nonperiodic
time-dependent crustal deformation processes. Toward this end, I have developed an
analysis method to investigate temporal variations in the amplitudes of sinusoidal signals. I
demonstrate the method using simulated coordinate time-series to numerically explore the
potential consequences of neglecting decadal variation in amplitude of annual motions on the
residual-error spectra of CGPS measurements, as well as potential bias in estimates for secular site
velocity. I show that secular velocity bias can be appreciable for shorter time-series, and that
residual-error time-series of longer duration may contain significant power in a broad band centred
on semi-annual frequency if temporal variation in the amplitude of annual motions is not
accounted for in the model used to reduce the observations to residuals. It may be difficult to
differentiate the bandpass filtered signature of mismodelled loading signals from power-law noise
using residual-error spectra for shorter time-series. I provide an example application to a ~9-yr
coordinate time-series for a CGPS station located in southern California at Carbon Creek Control
Structure (CCCS), which is known to experience large amplitude seasonal motions associated with
the Santa Ana aquifer system.
2-030
NEAR REAL-TIME PROCESSING OF RESULTS FROM THE 2008 IMPERIAL
VALLEY, CALIFORNIA, GPS SURVEY Crowell BW, Bock Y, Sandwell DT, Fialko Y, and
Lohman RB
In February of 2008, we surveyed with Global Positioning System (GPS) 38 geodetic monuments
established by Imperial College, London and 14 National Geodetic Survey (NGS) sites using the
rapid-static mode of surveying at a sampling rate of 1-Hz. We also installed and surveyed 10 new
monuments spanning the Imperial Fault trace along irrigation culverts. As base stations, 4 1-Hz
real-time stations in the California Real Time Network (CRTN) were used. Previous GPS surveys of
this dense array of monuments occurred in 1993, 1999 and 2000 [Genrich et al., 1997; Lyons et al.,
2002]. Data streamed over Bluetooth from Ashtech Z-XII3 receivers and was collected using
Geodetics, Inc. RTD Rover software installed on a Verizon XV6700 smartphone and processed in
real-time by connecting to SOPAC for ephemeris information. Raw data was also saved on the
phone’s memory card as well as on the SOPAC servers for use in post-processing with Geodetics,
Inc. RTD software (http://www.geodetics.com) [de Jonge et al., 2000]. Inclusion of site metadata into
the RINEX files and the conversion of raw files to RINEX files were performed using PGM Server,
which also archived the campaign and provides the associated RINEX and SINEX files
(http://sopac.ucsd.edu/projects/impvall2008.html). Crustal motion was computed for the periods
ranging from 1993 to 2008 and from 2000 to 2008 using both pre-processed and post-processed
positions. When North American plate motion is removed, the average velocity residual between
the pre-processed and post-processed positions is 1.62 mm/yr and 3.09 mm/yr for the periods
ranging from 1993 to 2008 and from 2000 to 2008 respectively. This results in roughly a 2 cm
difference between the pre-processed and post-processed positions, and part of the difference can
be accounted for by the 2007.0 epoch being used for the pre-processed positions and the epoch of
the survey was used for the post-processed positions. The azimuths and magnitudes of the velocity
vectors for the periods ranging from 1993 to 2008 and from 2000 to 2008 are fairly consistent,
indicating that both shallow and deep creep on the Imperial fault is essentially steady-state on a
time scale of several years.
2008 SCEC Annual Meeting | 153