Aftershocks of the 1989 Loma Prieta, California Earthquake and Their Tectonic Implications.

Lynn Dietz & William Ellsworth, USGS, Menlo Park, California.

Seismic velocity model and station corrections were developed for the aftershock area and all seismicity from 1969 through October 17, 1991 were relocated. The temporal and spatial patterns in the aftershock hypocenters and focal mechanisms were studied to help characterize the Loma Prieta earthquake.

• contents of file lp8910.sum

  The file lp8910-9110.sum includes all recorded earthquakes from Oct 18, 1989 thru Oct 17, 1991 which locate in the polygon defined by these corners:

  A 37 20.00 122 10.00

  A 37 20.00 122 2.00

  A 36 57.20 121 29.40

  A 36 45.00 121 29.40

  A 36 45.00 121 43.00

  A 37 3.70 122 10.00

  A 37 20.00 122 10.00

  I have removed known and suspected quarry blasts from this listing. A detailed look at the early aftershock catalog shows a completeness level of ~M4.0 for the first hour, M2.5-3.0 for hours 1 to 6, and ~M1.5 for hours 6 to 13. After hour 13 of the aftershock sequence, the catalog is complete down to M1.0.

  There are 10373 events in the file (1380 kbytes). The data is in HYPOINVERSE format, described below in a document from Fred Klein:

  -----------------------------------------------------------------------------

                                                 February 13, 1991
  
  HYPOINVERSE SUMMARY CARD FORMAT   
  ------- ------ ------
  Cols.  Format    Data
  _____  ______    ____
  
  1-10   5I2       Year, month, day, hour and minute.
  11-14  F4.2      Origin time seconds.
  15-16  F2.0      Latitude (deg).
  17     A1        S for south, blank otherwise.
  18-21  F4.2      Latitude (min).
  22-24  F3.0      Longitude (deg).
  25     A1        E for east, blank otherwise.
  26-29  F4.2      Longitude (min).
  30-34  F5.2      Depth (km).
  35-36  F2.1      Primary amplitude magnitude XMAG1. 
  37-39  I3        Number of P & S times with final weights greater than 0.1.
  40-42  I3        Maximum azimuthal gap.
  43-45  F3.0      Distance to nearest station (km).
  46-49  F4.2      RMS travel time residual.
  50-52  F3.0      Azimuth of smallest principal error (deg E of N).
  53-54  F2.0      Dip of smallest principal error (deg).
  55-58  F4.2      Magnitude of smallest principal error (km).
  59-61  F3.0      Azimuth of intermediate principal error.
  62-63  F2.0      Dip of intermediate principal error.
  64-67  F4.2      Magnitude of intermediate principal error (km).
  68-69  F2.1      Primary coda magnitude FMAG1. 
  70-72  A3        Event location remark. (See table 7 below).
  73-76  F4.2      Magnitude of largest principal error (km).
  77-78  2A1       Auxiliary remarks (See note below).
  79-80  I2        Number of S times with weights greater than 0.1.
  81-84  F4.2      Horizontal error (km).
  85-88  F4.2      Vertical error (km).
  89-90  I2        Number of P first motions.
  91-93  F3.1      Total of amplitude magnitude weights.
  94-96  F3.1      Total of duration magnitude weights.
  97-99  F3.2      Median-absolute-difference of amplitude magnitudes.
  100-02 F3.2      Median-absolute-difference of duration magnitudes.
  103-05 A3        3-letter code of crust and delay model. (See table 8 below).
  106    A1        Crust model type code (H or T).
  107    A1        Most common P & S data source code. (See table 1 below).
  108    A1        Most common FMAG data source code.
  109    A1        Most common XMAG data source code.
  110    A1        Primary coda magnitude type code (from FC1 command).
  111-13 I3        Number of valid P & S readings (assigned weight > 0).
  114    A1        Primary amplitude magnitude type code (from XC1 command).
  
  
                 The following are  written  only  if secondary magnitudes are
                 present.  Secondary magnitudes may appear in either position
                 and use the label codes given by the FC2 and XC2 commands.
                 The code L is for ML calculated by the USGS from Berkeley amp-
                 litudes, and B is for ML from the Berkeley catalog.
  
  115    A1        Secondary magnitude label or type code.
  116-18 F3.2      Secondary magnitude.
  119-21 F3.1      Total of the secondary magnitude weights.
  122    A1        Secondary magnitude label or type code.
  123-25 F3.2      Secondary magnitude.
  126-28 F3.1      Total of the secondary magnitude weights.
  

  MAGNITUDES

  The magnitudes and their label codes are:

  E   Primary coda magnitude. FMAG of Eaton (1991). Uses all components.
  X   Primary amplitude magnitude. Jerry Eaton's XMAG. Uses all components. 
                 Secondary magnitudes:
  L   Local magnitude computed from UC Berkeley Wood Anderson amplitudes.
  B   Local magnitude from UC Berkeley's catalog.
                 Magnitudes no longer used in CALNET catalog:
  T   Lapse time coda magnitude of Michaelson (1990) from high gain verticals. 
  Z   Low gain (Z component) magnitude of Hirshorn and Lindh (1989?). 
  

  AUXILIARY EVENT REMARKS (Summary card cols. 77-78)

  Assigned by analyst (col. 77):

  Q  Quarry blast (or NTS explosion)
  N  NTS blast
  F  Felt
  

  Assigned by HYPOINVERSE (col. 78):

  *  Location had convergence problems such as maximum number of iterations or
     failure to reach a minimum RMS.
  -  Depth was poorly constrained and was held fixed at its current value.
  

  ---

  
  Any questions about the catalog can be addressed to me at:
   
      E-mail:   dietz@andreas.wr.usgs.gov      
  
  
      Address:  Lynn Dietz
  
                U.S.G.S. MS977
  
                345 Middlefield Rd.
  
                Menlo Park, CA 94025               
  
  
  
      Phone:    415-329-5520           
  
  

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