Subject: [IGSMAIL-0466] Northridge Earthquake
From: Ken Hudnut and Mark Murray
Date: Wed, 26 Jan 1994 11:03:24 PST

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IGS Electronic Mail      Wed Jan 26 11:03:24 PST 1994      Message Number 0466
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Author: Ken Hudnut and Mark Murray
Subject: Northridge Earthquake

January 25, 1994

To: Distribution on [email-masked] and GPS colleagues by e-mail

>From:	Ken Hudnut				Mark Murray
	U. S. Geological Survey			U. S. Geological Survey
	525 South Wilson Ave.			345 Middlefield Rd. MS #977
	Pasadena, CA  91106			Menlo Park,  CA  94025
	(818)405-7232				(415)329-4850
	FAX: 405-7827				FAX: 329-5163
	[email-masked]		[email-masked]

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The GPS response to the Northridge earthquake has basically involved several components;
1) The PGGA continuously operating line between Palos Verdes (PVEP) and JPL (JPL1) - Y. Bock
2) The JPL pre- and post-earthquake response, headed by Andrea Donnellan
3) The USGS & SCEC pre- and post-earthquake response, headed by myself    
4) Post-seismic studies that have been initiated, including;
	JPL efforts by Ken Hurst and Frank Webb
	SCEC efforts (UCLA and others) in cooperation with USGS on non-"Profile Array" points
	USGS efforts on establishing a N/S profile array ~60 km long, crossing the fault

This is a first effort to distribute the initial results of a large group of people.
See below for participants.

The GPS field work and processing efforts of the first week have now been
processed preliminarily, and both Mark Murray and I here present a couple
of models for the slip in the event, based on different modeling approaches.
If you have questions, please let us know.  Also, we are going to be posting
plots of the results onto the andreas.wr.usgs.gov anonymous ftp site in
/pub/northridge directory.  The files we post will be of the form "gps*.ps"
for postscript files or "gps*.eps" for macintosh files.

I know many of you will be interested in hearing the scientific results first, so
here goes.  Those in the GPS community will find additional notes on our plans for
data availability below as well.

PRELIMINARY DISPLACEMENT RESULTS:

Summary of Preliminary GPS Pre- and Post-Earthquake GPS Solutions:
******************************************************************

 NAME      EDAT    NDAT    VDAT  LATIT   LONGIT    ESIG    NSIG    VSIG	    Result
 ****      ****    ****    **** ******* *********   ***     ***     ***     ********
 PVER      0.09    1.04    0.50 33.7438 -118.4035   0.4     0.3     0.6	    YB/PGGA
 JPLM      0.65   -0.51   -0.20 34.2051 -118.1733   0.4     0.3     0.6	    YB/PGGA
 CALA      2.10    8.48   -0.02 34.1400 -118.6457   1.0     1.0     2.0	    MM/USGS  
 PACO      7.98   -0.58    6.72 34.2635 -118.4083   1.0     1.0     2.0	    MM/USGS  
 GLEN      1.38   -0.59    4.92 34.1611 -118.2825   1.0     1.0     2.0	    MM/USGS  
 MULH      2.03    6.77    3.37 34.1300 -118.5600   1.0     1.0     2.0	    MM/USGS  
 NORT      2.79   -7.11   47.26 34.2167 -118.5547   1.0     1.0     2.0	    MM/USGS  
 RESE     18.36    0.07   21.46 34.2917 -118.4883   1.0     1.0     2.0	    MM/USGS  
 PICO    -13.53   16.25   43.12 34.3295 -118.6007   1.0     1.0     2.0	    MM/USGS  
 SAFR    -11.78   16.50   44.14 34.3295 -118.6007   1.0     1.0     2.0	    MM/USGS  
 2131     -3.43   -1.54    2.08 34.3510 -118.4250   1.0     1.0     2.0	    MM/USGS  
 LOVE    -10.80   -3.90   -9.00 34.4833 -118.6667   1.0     1.0     2.0	    AD/JPL      

Notes:
******
   In the above table is shown the summary of preliminary GPS displacement
results that I now know of.  The references for these results are given below
This table has only rough estimates of the approximate repeatibility for errors.
Data and errors are in centimeters.

	YB/PGGA = Yehuda Bock, results from GLOBK analysis of PGGA solutions, already
		distributed on IGS mail by that group.
	MM/USGS = Mark Murray, results from GLOBK analysis of solutions based on
		K. Hudnut and Y. Feng pre-earthquake solutions for the Inter-County '93
		data and USGS & SCEC data collected since the earthquake 
	AD/JPL  = Andrea Donnellan, results from GIPSY/OASIS analysis (also, this group
		obtained similar displacement estimate to ours at Oat Mtn., aka PICO & SAFR)
		Results for stations SAFE and LOVE already distributed publicly by Andrea.

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PRELIMINARY MODELING STUDIES:

Mark Murray's Preliminary Model Solution for Rupture Plane:
***********************************************************

  I estimated a uniform-slip rectangular-fault dislocation model 
from the observed surface displacements given in the Section A 
table. I did not include LOVE and assumed JPLM was fixed.
I used a random-cost optimization technique to search a broad
range of model parameters to find a best-fitting fault geometry:

  Dip   Depth(km) Width(km)   Az   Length(km)    Lat         Lon
  39.22   7.77      12.19   109.13   7.11     34.40944   -118.56083

  Slip(m)    Rake     Moment (N-m)
  4.5213     89.1674  1.176E+19

where depth is to the top edge, azimuth is clockwise from north,
and the origin is the projection of the west edge of the fault
to the surface. The nrms of the residuals is about 2, with horizontal
residuals generally < 2 cm, and vertical residuals generally < 5 cm.

  This model shows good agreement with seismological estimates in 
orientation, dip and moment. The fault extends from 7.8 to 15.5 km 
depth, and lies close to the main shock hypocenter. The small
(7x12 km) fault plane suggests that the slip was concentrated at 
depth. The location of the plane corresponds to a region with
relatively few aftershocks. 

  A figure showing the observed and model-estimated horizontal 
displacements and the fault plane location with respect to the 
aftershocks is in /pub/northridge/gps_murray_model1.ps

Ken Hudnut's Preliminary Model Incorporating Variable Slip:
**********************************************************
	Used data and errors as shown in above table as input
	(GINV code developed by Shawn Larsen, LLNL)

Assumed fault geometry based partly on Mark's result above and on aftershock plane:

TYPE SLIP  LENGTH    TOP  WIDTH  DIP LONGITUDE LATITUDE AZIMUTH FAULT    NX  NZ
---- ----  ------    ---  -----  --- --------- -------- ------- -----    --  --
t   -33.0   30.00   0.00  30.00 40.0  34.3917 -118.5583 160.0   NT_T      3   3
r     0.0   30.00   0.00  30.00 40.0  34.3917 -118.5583 160.0   NT_S      3   3
                                     (UpDip Ctr of Flt) (Azim. c-cl from east)

NRMS Residual = 3.13    :  abs. val. (Obs. - Model)/Sigma   (for all components & all data modeled)
			:  some (Obs.-Model) residuals are up to ~22 cm in vertical and ~11 cm horiz.

Total Moment  = 1.88 x 10^26 dyne-cm  (assumes 3 x 10^11 for rigidity)

                       top
     NW_____________(surface)____________SE
       |          |          |          |		Information on the diagram and model:
   dip>|    14    |   -23    |    10    |
       |          |          |          |		Projection viewed from a point above the
strike>|     4    |    53    |     1    |		fault looking down orthogonal to the plane.
       |          |      ....|...       |		The fault plane is 30 x 30 km, based on
       |__________|______.___|__._______|		the aftershock distribution.
       |          |      .   |  .       |
   dip>|  -108    | -142 .   |  . -60   |		Each sub-fault is 10 x 10 km and both the
       |          |      .   |  .       |		dip-slip and strike-slip component of slip
strike>|    -2    |  -11 .   |  . -28   |		is solved for in the inversion.  These slip
       |          |      .   |  .       |		estimates are given in each sub-fault box.
       |__________|______.___|__._______|		Thrust and left-lateral are negative, and
       |          |      ....|...       |		normal and right-lateral are positive.
   dip>|   -94    |  -117    |   -61    |		Dip-slip # and strike-slip # indicated at
       |          |          |          |		left side of subfaults.
strike>|    -9    |   -26    |   -29    |            
       |          |          |      @<----------- Approximate location of mainshock hypocenter.
       |__________|__bottom__|__________|		(depth of bottom of model plane @19.3 km)

  The dotted box in this diagram roughly represents the location of Mark's model slip patch where
he obtains ~450 cm of slip, for comparison.

  The model residuals are still substantial (should be ~1.0), and moment seems too high, but 
remember these results are preliminary.  Also, remember that these data will contain
the slip contribution of the aftershocks that were occurring as the data were collected.
We will have additional GPS displacements for an additional 20 GPS stations spanning
the area affected by displacements within the next week or so, probably.  These additional
results will help greatly in constraining the model.

  I am solving for the dip-slip and strike-slip component on each subfault, employing
smoothing and truncating at 5 singular values.  I am not constraining slip sense or
amount in the inversion parameterization.  So, there are 18 parameters solved for and
12 GPS displacements, each comprising 3 components for 36 'data'.

  In this model, the fault plane is meant to be approximately co-planar with Mark Murray's
initial result, but solving for slip on the entire plane that has been defined by the
aftershock relocations of Jim Mori and Egill Hauksson and others.

********************************************************************************************************
  The following people have been working on this effort to my knowledge right now
varying from field work to data processing and logistics.

USGS NEHRP funds, the NSF via the Southern Calif. Eq. Center (SCEC), NASA and others support these groups

	SIO/UCSD	Y. Bock, E. Calais, H. Johnson, P. Fang, K. Stark
	UCLA		X. Ge, Z-K. Shen, D. Potter, Y. Feng
	Caltech		M. Cline, B. Zajac, Louie, Mark
	MIT		B. Hager, T. Herring, Bob King, and 6? students
	USGS		K. Hudnut, M. Murray, J. Savage, K. Gross, J. Sutton, C. Stiffler, G. Hamilton,
			K. Wendt, several others as well
	JPL		A. Donnellan, K. Hurst, F. Webb, G. Franklin, others
	LLNL		S. Larsen

********************************************************************************************************
DATA AVAILABILITY:

  Starting very soon, we will have data access available by ftp.  Our method of
doing so is that we will establish two main data repositories.  First, all of
the immediate response data for the coseismic will be placed on ftp at the
SCEC facility at UCLA, including our first observations of the USGS "Profile
Array."  We will place RINEX files into that location.  This should be set
up very soon, but is not yet ready.  In the meantime until this is set up, we
can make data available but it is more work for us.  The second site will be
on line at Menlo Park, and will be our main repository for the "Profile Array"
data set.  For our convenience, we prefer to maintain that data set separate
from the SCEC archive at UCLA.  Again, however, people may call us to discuss
their interests in the data and ask for that password.

********************************************************************************************************