Subject: [IGSMAIL-0457] Northridge Earthquake
From: Yehuda Bock, Peng Fang, Keith Stark, Jie Zhang and Shelley Marquez
Date: Sun, 23 Jan 1994 9:20:55 PST

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IGS Electronic Mail      Sun Jan 23  9:20:55 PST 1994      Message Number 0457
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Author: Yehuda Bock, Peng Fang, Keith Stark, Jie Zhang and Shelley Marquez
Subject: Northridge Earthquake

Subject: Northridge Earthquake Update # 2: 
         Coseismic compression detected by the PGGA across the Los Angeles Basin
Date: January 23, 1994

The Northridge Earthquake (surface wave magnitude 6.6) struck the San Fernando
Valley on January 17, 04:30 AM Pacific Standard Time, 12:30 UT. The epicenter
is located at 34 13 N (dm) and 118 33 W (dm) at a depth of 14.6 km
(ref: 17 January memo by Lucy Jones, Caltech/USGS).

The closest PGGA stations to the epicenter are JPLM and PVEP, at the Jet
Propulsion Laboratory in Pasadena and on the Palos Verdes Peninsula,
respectively.  Both sites were displaced significantly during the earthquake,
indicating 12.1 +- 4.1 mm of coseismic compression across the Los Angeles Basin.
(Interseismic compression across the L.A. Basin has been computed from PGGA data
to be 5.5 mm/yr -- work in progress with K. Hudnut, which compares well with
earlier geodetic estimates).

Vertical displacements are insignificant.

The details of the analysis are as follows:

We analyzed daily solutions before the earthquake for January 2-8,10,12,13-17.
In the pre-earthquake analysis of day 17 we used only the pre-earthquake data
collected on Jan. 17 at JPLM and PVEP (<12:31 UT).  Daily solutions were computed with the GAMIT software.

We analyzed daily solutions after the earthquake for January 17,18,19.
In the post-earthquake analysis of day 17 we used only the post-earthquake data
collected on Jan. 17 at JPLM and PVEP (>12:31 UT).  Daily solutions were
computed with the GAMIT software.

The individual GAMIT solutions were analyzed with a forward Kalman filter
solution using the GLOBK software.  The JPLM and PVEP sites were renamed
for the post-earthquake solutions.  The displacements are essentially a 
result of a weighted mean of the positions of these two sites before and
after the earthquake.

The displacements of the individual sites are computed with respect to the
ITRF92 system adopted by the International GPS Service for Geodynamics
(Z. Altamimi IGS mail 430).

The displacements at JPLM:

North: -5.1 mm +- 3.2 mm
East:   6.5 mm +- 3.9 mm
Up:    -2.0 mm +- 6.4 mm

The displacements at PVEP:

North: 10.4 mm +- 3.2 mm
East:   0.9 mm +- 3.8 mm
Up:     5.0 mm +- 6.4 mm

For the baseline length JPLM => PVEP:

Before the earthquake: 54470.2184 +- 0.0015 m
After the earthquake:  54470.2063 +- 0.0039 m

The coseismic baseline length displacement: 12.1 mm +- 4.2 mm

The SIO facility is currently analyzing the January 9,11,20-22 data and will
update this report as soon as results are available.

Note1: Uncertainties above are based on goodness of fit to the data.
We are currently computing uncertainties based on repeatability.

Note2: The JPLM displacements agree within a millimeter with the results of the
JPL IGS Analysis Team (J. Zumberge, IGS Mail 0455, 21 Jan. 94).
 
The PGGA data collection for the period of the earthquake was nearly 100%
complete.  The only problem was an 80% loss of data from JPLM on 15 January.
All data (raw and RINEX data, and solutions) can be retrieved from the SIO 
archives. As a reminder:

To access SIO GARNER archive:
 ftp [email-masked] (132.239.152.80)
 login: anonymous
 password: your name
 directories: rinex, raw, solutions, products
Direct problems to: [email-masked], [email-masked]

To reiterate our earlier update, our response to the earthquake has been aided
immensely by the rapid availability of global data from the IGS network.  

Please contact us for any additional information.