Subject: [IGSMAIL-43] Seicmic Slip and Satellite Signals
From: Geoffrey Blewitt
Date: 15 JUL 1992 18:58:40

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IGS Electronic Mail       15-JUL-1992 18:58:40       Message Number 43
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From:    Geoffrey Blewitt
Subject: Seicmic Slip and Satellite Signals
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PRELIMINARY CO-SEISMIC DISPLACEMENT RESULTS IN CALIFORNIA
FOLLOWING THE LANDERS EARTHQUAKE, MAGNITUDE=7.4, JUNE 28, 1992, 11:57 UTC

or "Seismic Slip and Satellite Signals"
(inspired by Tom Clark's 1989 message, "Quakes and Quasars")

Geoffrey Blewitt, Frank Webb, Jim Zumberge, Ken Hurst
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif.

Step functions are clearly seen in a time-series of daily estimates of
longitude and latitude for Californian GPS sites.  Preliminary estimates
of absolute displacement are given in the following table:

                    East (mm)       North (mm)
    GOLD(STONE)     -10 +- 3         - 9 +- 2
    JPL             - 9 +- 3         + 8 +- 2
    PINY(ON FLAT)   +18 +- 4         +46 +- 3

No co-seismic signal was apparent in the vertical component (at the 1 cm-level).
This information is presented here to warn analysis centers who are
constraining station locations as fiducial points. Our estimates will be
updated as we analyze more data, apply the "free-network" method, and
apply global-scale ambiguity resolution.

This displacement solution is based on 7 routine daily solutions preceding the
quake, and 6 routine daily solutions following the quake. "Absolute" motions
were inferred by estimating baselines to PENT(ICTON) in British Columbia,
Canada. Note that the relative displacements between these stations are
even more significant than the quoted absolute displacements; for example, the
southern motion of GOLD with respect to JPL is a clear signal. The estimated
relative vertical displacements for these 3 sites are no larger than 11 mm,
and are consistent with zero.

The error bars are 1-standard deviation errors, based on the rms scatter
of daily estimates. Daily baseline repeatability over the ~1600 km baselines
to PENT(ICTON) were approximately 5 mm for horizontal components.  The
reference frame was defined by holding the following stations fixed in our GPS
realization of the ITRF'90 reference frame:  FAIR(BANKS), Alaska;
ALGO(NQUIN), Canada; MADR(ID), Spain. (See our IGS mail message 37, or the
description of "Model B" in the May 4 issue of Geophysical Research Letters,
Vol. 19, No. 9, pp. 853-856)

These results are in reasonable agreement with our elastic-half space models
of the sequence of ruptures near Landers, California, 11:57 UTC, June 28, 1992;
however, the motion of GOLD is on the low-end of our expectations.