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Revision of NCSN Catalog

April 29, 1997

On April 29, 1997, the USGS Northern California Seismic Network (NCSN) revised the format and relocated the entire earthquake catalog at the Northern California Earthquake Data Center. We undertook the catalog revision for the following reasons:

Because we did not change the location procedure or velocity models, the new locations and magnitudes are nearly identical to the old locations. Minor changes result from the new station coordinates and traveltime corrections. An analysis of changes in horizontal and vertical location, origin time, rms, and magnitude for the more than 200,000 earthquakes occurring in the sample period 1980-1989 (Figure 1) shows the median changes are nearly zero. Information on the change in location or magnitude for any earthquake is available upon request from David Oppenheimer at oppen@usgs.gov.

The following discussion provides more detail on why we made the changes to the catalog and provides supporting documentation. For more information on the NCSN network and processing procedures, see Oppenheimer et. al. (1992).

Universal station format

A new station name format was implemented because the old format did not uniquely define a station name and component. Consequently, it was difficult to combine phase information from other seismic networks with NCSN data and fully describe the type of instrumentation deployed by the NCSN. The old format designated a station by a 3-letter name, a one-letter seismic network code, and a 1-letter component code. The Universal code designates a station and component with a combination of a 5-letter site name, a 2-letter network code, and a 3-letter component code. The new phase format based on the Universal format is quite similar to the old format, and most users are unlikely to be affected by this change. A complete description of the NCSN format is available.

GPS survey of station coordinates

In April 1994 the NCSN began a systematic effort to survey all sites in the network using GPS. The effort was an adjunct to normal site maintenance visits, so the effort took more than 3 years to complete. The datum is WGS-84, versus the 1927 North American datum previously used. Uncertainty in the GPS coordinates is approximately 3 m. The mean horizontal shift is about 100 meters, though there were several sites that were mislocated by more than 500 meters (Table 1). However, the shift in datum produces a change in epicentral position of 84 - 101 m to the west and an elevation change of -15 to -20 m. Greater shifts are likely due to incorrect determination of the original site coordinates.

We obtained GPS coordinates only for NCSN sites operating since 1994. We do not anticipate obtaining GPS coordinates for discontinued sites and portable installations for previous special studies. Site coordinates are available.

Sites with GPS coordinates can be recognized in this file as those sites for which the "latitude/longitude minutes" filed has a precision of 4 significant digits to the right of the decimal place. As shown in Figure 1, the GPS coordinates do not greatly change the earthquake locations. Histograms of earthquake location differences

Figure 1. Histograms of earthquake location differences (old - new). Median values are shown above each plot. dt is the difference in origin time in seconds. dx is the absolute difference in epicentral (horizontal) position in km. dz is the absolute difference in vertical position in km. dm is the difference in magnitude drms is the difference in solution rms in seconds (positive values indicate improvement in solution fit).

Suspect coda durations

The NCSN computes a duration magnitude (Md) for almost every earthquake in the catalog. We discovered that an algorithm in one of our processing systems computed coda durations for seismograms in which there were no P or S arrival times or any amplitude observations. We consider it unlikely that a seismogram would have a reliable coda duration if the arrival time information was unreliable. Further examination showed that the suspect coda durations from these seismograms were inconsistent with durations from seismograms that had traveltime observations. In particular, the suspect durations tended to cause the computed (Md) to be too large. Consequently, we removed these suspect readings from the catalog. Their removal affected only the magnitude of a small number of earthquakes.

Assignment of new event identification numbers

Event identification numbers were re-assigned to the class of 218,026 earthquakes without associated digital seismograms. This class of data includes all earthquakes in the catalog with origin times before 1984, earthquakes from other seismic networks, and earthquakes from the NCSN Real-Time-Picker earthquake system that operated from 9/81 to 3/97. Event identification numbers of earthquakes processed by the CUSP system remain unchanged. These new numbers are negative and decrease (i.e., increase in the negative direction) with increasing origin time. A mapping of old-to-new event identification numbers is available upon request from David Oppenheimer at oppen@usgs.gov.

Station traveltime corrections

As a result of the new site locations, we derived traveltime corrections for all stations in the NCSN catalog. To derive these corrections, we first relocated the entire catalog with the GPS station coordinates. We used the same location procedure, velocity models, and station traveltime corrections described in Oppenheimer et. al. (1992). We then adjusted the existing station traveltime correction with the mean traveltime residual for each station. If no correction existed for a station, the mean residual became the new station correction. The latter situation most often occurred for stations installed subsequent to the time when the velocity model was developed. We then relocated the catalog again with the new traveltime corrections. The new traveltime corrections are available from David Oppenheimer at oppen@usgs.gov.

Table 3. NCSN COORDINATE SHIFTS

             OBSERVED                 EXPECTED

        DX     AZIMUTH   DZ     DX     AZIMUTH   DZ
        km       deg      m     km       deg      m
____________________________________________________
AAR    0.129      79     -19   0.093     84      -18
AAS    0.095      99     -34   0.092     84      -18
ABJ    0.164     107     -23   0.093     83      -18
ABR    0.348     161     -25   0.094     83      -18
ADW    0.109      77     -23   0.092     84      -18
AFD    0.369     308     -44   0.093     84      -18
AFH    0.381      57     -25   0.092     84      -18
AFR    0.068      79     -21   0.093     84      -18
AHR    0.145     113      -5   0.093     84      -18
ALA    0.082     126     -30   0.091     84      -18
ALN    0.091     311     -27   0.093     84      -18
AOD    0.150      85     -16   0.091     84      -18
AOH    0.178     126     -47   0.095     83      -18
APR    0.094      92     -20   0.093     84      -18
ARJ    0.113      76     -23   0.091     84      -18
ARR    0.067      55     -27   0.093     84      -18
ASM    0.153     109     -54   0.092     84      -18
AVR    0.094      77     -19   0.093     84      -18
BAP    0.532     163     -26   0.094     87      -16
BAV    0.031     114     -32   0.092     86      -17
BBG    0.074      56     -32   0.092     86      -17
BBN    0.082      93     -30   0.092     86      -17
BCG    0.066     220     -55   0.094     86      -16
BCW    0.081      75     -13   0.093     87      -16
BEH    0.110      98      -8   0.093     86      -17
BEM    0.075      67     -24   0.093     86      -17
BHR    0.245      81     -38   0.093     86      -16
BJC    0.109      99     -34   0.094     86      -16
BJO    0.063     105     -31   0.093     86      -16
BLR    0.077     130     -43   0.094     86      -16
BMS    0.098      95     -31   0.092     86      -17
BPC    0.173     154     -10   0.094     86      -16
BPI    0.097      94     -28   0.092     86      -16
BPO    0.098      71     -24   0.095     87      -16
BPR    0.095      99     -30   0.095     87      -16
BRM    0.118      93     -29   0.093     86      -17
BRV    0.072      77     -16   0.093     86      -17
BSG    0.132      90     -31   0.092     86      -16
BSL    0.133     150     -13   0.093     86      -16
BSM    0.132     109     -55   0.093     87      -16
BSR    0.180     125     -20   0.094     86      -16
BVL    0.101      73     -31   0.092     86      -16
BVY    0.406      32      62   0.093     86      -16
CAD    0.162      88     -55   0.094     85      -16
CAI    0.047      89     -36   0.098     85      -16
CAL    0.103      47     -14   0.095     86      -16
CAO    0.201      62      -6   0.094     85      -16
CBR    0.096     111     -30   0.095     85      -16
CBS    0.071     100     -30   0.094     85      -17
CBW    0.085      84     -20   0.094     84      -16
CBZ    0.148      66     -30   0.094     85      -16
CCO    0.193     108     -37   0.095     86      -16
CCY    0.020      51     -46   0.095     85      -16
CDA    0.179     106     -23   0.094     85      -16
CDO    0.086      95     -29   0.094     85      -16
CDU    0.076     109      -8   0.095     85      -16
CDV    0.083     257      12   0.093     85      -16
CGP    0.098      94     -29   0.094     85      -16
CGP1   0.098      94     108   0.094     85      -16
CLC    0.106      95     -26   0.097     85      -16
CMC    0.087      81     -38   0.095     85      -16
CMH    0.118      66     -15   0.095     85      -16
CMJ    0.259      75     -19   0.095     85      -16
CMK    0.113      51     -48   0.093     85      -16
CML    0.129      99     -21   0.095     85      -16
CMN    0.113     245     -36   0.094     84      -16
CMO    0.108      72     -47   0.095     85      -16
CMP    0.864      90     -39   0.093     85      -17
CMR    0.094     112     -30   0.096     85      -16
CNI    0.051     104     -31   0.094     85      -16
COS    0.107      61     -48   0.093     85      -17
CPI    0.083      96     -23   0.096     85      -16
CPM    0.074      95     -22   0.097     84      -16
CRP    0.088     102     -28   0.094     85      -16
CSA    0.180     322     -38   0.094     85      -16
CSL    0.086      77     -27   0.095     85      -16
CSP    0.069      53     -37   0.096     85      -16
CSV    0.325     103     -39   0.094     85      -16
CVA    0.154      83     -27   0.096     85      -16
CVL    0.311      12      68   0.094     86      -16
CVP    0.108      90     -41   0.095     85      -16
CYB    0.087      80     -30   0.097     85      -16
GAC    0.066      81     -17   0.098     84      -16
GAX    0.196      85     -12   0.098     84      -16
GBD    0.137     111     -23   0.098     83      -16
GBG    0.289      98     -22   0.098     84      -16
GCB    0.845     170      39   0.100     83      -16
GCR    0.090      46     -38   0.098     84      -16
GCV    0.041     175     -25   0.098     84      -16
GCW    0.106      89     -39   0.098     83      -16
GDC    0.114     127     -36   0.097     84      -16
GDX    0.080      98     -21   0.098     84      -16
GGP    0.066      80     -31   0.098     84      -16
GGU    0.031      68     -16   0.099     84      -15
GHC    0.082      88     -41   0.097     84      -16
GHG    0.022     354     -32   0.097     84      -16
GHL    0.068     102     -13   0.098     84      -16
GHM    0.086      74     -30   0.097     83      -16
GHO    0.079      87     -16   0.099     84      -16
GHV    0.136     105     -11   0.096     83      -16
GMC    0.182     263      -5   0.098     84      -16
GMK    0.063      95      -2   0.096     84      -16
GNA    0.103      90     -26   0.100     83      -16
GPM    0.279       1     -71   0.097     84      -16
GRT    0.124      90     -29   0.098     84      -16
GSG    0.121     320     -23   0.098     84      -16
GSN    0.068      81     -21   0.099     83      -16
GSS    0.068      95      -8   0.097     84      -16
GTS    0.076      85     -34   0.096     83      -17
GWK    0.122      98     -17   0.096     84      -16
GWR    0.192      32      -4   0.100     83      -16
HAZ    0.104      77     -24   0.095     86      -16
HBT    0.111      91     -30   0.094     87      -16
HCA    0.077      93     -32   0.094     86      -16
HCO    0.342      71     -14   0.095     86      -16
HCR    0.116       5     -37   0.095     86      -16
HDL    0.110      79     -25   0.093     86      -16
HER    0.081      78     -30   0.095     86      -16
HFE    0.161      72     -33   0.094     86      -16
HFP    0.105      83     -47   0.093     86      -16
HGS    0.105      68      -1   0.093     86      -16
HJG    0.086     116     -50   0.094     86      -16
HJS    0.063     190     -53   0.093     86      -16
HLT    0.104      98      -3   0.093     86      -16
HMO    0.029      22     -14   0.096     86      -16
HOR    0.683     107     -59   0.095     86      -16
HPH    0.033     341     -52   0.094     86      -16
HPL    0.103      77     -27   0.094     86      -17
HSF    0.113      69     -27   0.094     86      -16
HSL    0.053      78     -32   0.093     86      -17
HSP    0.126      82     -24   0.094     86      -16
JAL    0.126      59     -30   0.095     86      -16
JBC    0.592     147       1   0.095     86      -16
JBG    0.135     150     -23   0.096     86      -16
JBL    0.117      48     -42   0.096     86      -16
JBM    0.169      40     -59   0.096     85      -16
JBZ    0.108     109     -34   0.095     86      -16
JCB    0.097      69     -31   0.095     86      -16
JCH    0.116      92     -33   0.097     85      -16
JCP    0.109      99     -33   0.096     85      -16
JEC    0.097      82     -39   0.094     86      -16
JEG    0.099      76     -33   0.097     85      -16
JEL    0.083      92     -30   0.095     86      -16
JHL    0.122      94     -67   0.095     85      -16
JHP    0.221      88      -6   0.096     86      -16
JJR    0.112      80     -30   0.097     85      -16
JLX    0.112      96     -26   0.095     85      -16
JMG    0.195     106     -40   0.096     85      -16
JMP    0.081      84     -16   0.096     85      -16
JPL    0.156     109     -66   0.095     86      -16
JPP    0.145      45     -47   0.096     85      -16
JPR    0.042     147     -24   0.097     85      -16
JPS    0.072      11     -40   0.096     85      -16
JRG    0.137     130      -6   0.095     86      -16
JRR    0.089      78      -9   0.094     86      -16
JSA    0.257      84      14   0.097     85      -16
JSB    0.544      18     -14   0.096     85      -16
JSC    0.052      63     -28   0.096     86      -16
JSF    0.169     292     -31   0.096     85      -16
JSG    0.088     129     -32   0.096     85      -16
JSJ    0.087      86     -30   0.095     85      -16
JSM    0.097      91      -9   0.096     86      -16
JSS    0.103      90     -25   0.095     85      -16
JTG    0.124      88     -49   0.095     86      -16
JUC    0.051      73     -21   0.096     86      -16
JUM    0.119      81     -21   0.095     86      -16
KBB    0.130      96     -38   0.100     82      -16
KBN    0.094      69     -34   0.098     82      -16
KBR    0.136     314     -58   0.099     82      -16
KBS    0.107      49     -14   0.099     83      -16
KCP    0.179      58     -10   0.099     83      -16
KCR    0.181      94     -23   0.098     82      -16
KCS    0.112     104       0   0.098     82      -16
KEB    0.108      73     -25   0.100     79      -17
KFP    0.102      72     -28   0.098     83      -16
KGM    0.045     276     -11   0.099     82      -16
KHB    0.103      73     -21   0.098     82      -17
KHM    0.075      75     -33   0.099     82      -16
KIP    0.074      67     -26   0.099     83      -16
KJJ    0.125      78     -32   0.100     82      -16
KKP    0.112      88     -18   0.099     82      -16
KMP    0.043     118     -26   0.101     82      -16
KOM    0.083      82     663   0.097     81      -17
KPP    0.098      77     -24   0.098     82      -16
KRK    0.134     105     -21   0.098     83      -16
KRM    0.097     106     -33   0.099     81      -17
KRP    0.147     353     -18   0.100     81      -16
KSC    0.070     100     -19   0.101     80      -17
KSM    0.138     141     -60   0.100     82      -16
KSP    0.102     173     -60   0.098     83      -16
KSX    0.077      90     -23   0.099     81      -17
KTR    0.089      76     -31   0.098     80      -17
LAS    0.088      91     -13   0.093     81      -19
LBF    0.053      98     -21   0.094     82      -18
LBK    0.184      78     -37   0.098     81      -17
LBP    0.188      16     -32   0.098     82      -17
LCF    0.195      85     -19   0.092     82      -18
LCM    0.131      94     -23   0.095     83      -18
LDB    0.156      64     -52   0.095     82      -18
LGB    0.133     137      -9   0.095     81      -18
LGM    0.102      68      47   0.095     81      -19
LGP    0.073      71     -25   0.097     82      -17
LHC    0.270      73      -8   0.093     82      -18
LHE    0.095      96     -38   0.096     81      -18
LHK    0.261       2     -46   0.092     82      -19
LME    0.100      77      19   0.093     82      -18
LMH    0.104      70     -27   0.095     81      -19
LMP    0.052     171     -16   0.096     81      -18
LPD    0.201      34     -20   0.094     81      -18
LPK    3.583      89     -28   0.097     81      -18
LRB    0.090      83     -33   0.097     83      -17
LRD    0.235     195      33   0.093     83      -18
LRR    0.626     270     -24   0.094     82      -18
LRS    0.050      91      -9   0.093     82      -19
LSF    0.088      91     -13   0.096     82      -17
LSH    0.149      97     -24   0.096     82      -18
LSL    0.055      46     -44   0.092     82      -18
LSM    0.407      16      -8   0.093     82      -18
LSR    0.110      78     -15   0.096     81      -18
LTC    0.112      95     -24   0.095     82      -18
LWH    0.414      47     -16   0.094     82      -18
MBE    1.902       3     -33   0.087     86      -19
MBU    3.627     270     -17   0.090     86      -18
MCD    0.149      85     -19   0.084     86      -20
MCL    0.097     100     -49   0.087     86      -20
MCM    0.052      23     -41   0.086     86      -20
MCS    0.113      83     -21   0.087     85      -20
MCU    0.222     104     -27   0.092     85      -18
MDC    0.039      28     -27   0.086     85      -20
MDP    0.064     103     -25   0.087     85      -19
MDR    0.086      23     -29   0.086     85      -20
MEM    0.062      59     -22   0.086     85      -20
MFB    0.055      68     -18   0.084     86      -20
MGP    0.104      85     -18   0.085     85      -20
MHD    0.638      13     -30   0.089     85      -18
MLC    0.184     195     -79   0.087     85      -20
MLH    0.251      60     -20   0.086     85      -20
MLK    0.138      78      24   0.086     85      -20
MLM    0.096      23     -39   0.086     86      -20
MMI    0.110      67      -7   0.090     85      -19
MMP    0.070      62     -52   0.086     85      -20
MMS    0.096      81     -16   0.087     85      -20
MMT    0.145      99     -37   0.086     86      -19
MNH    0.099      84     -30   0.093     85      -18
MOY    0.160      72     -28   0.092     85      -18
MPR    0.067      88     -25   0.089     85      -18
MRC    0.239     164      -7   0.085     85      -20
MRD    0.125      90     -33   0.087     85      -19
MRF    0.220     275     -51   0.092     85      -18
MSK    0.057      78      -3   0.086     86      -20
MSL    0.139      92     -51   0.086     85      -20
MST    0.207      74     -14   0.090     85      -18
MTC    0.095      82     -27   0.086     85      -20
MTU    0.029     311     -18   0.085     86      -20
MWB    0.143      66     -16   0.084     86      -20
MYL    0.107      79     -32   0.090     85      -18
NAD    0.066      84     -26   0.095     84      -17
NAP    0.093       5     -18   0.096     84      -16
NBO    0.122      96     -22   0.098     85      -16
NBP    0.120     107     -32   0.096     84      -17
NBR    0.102     100     -31   0.096     84      -16
NCF    0.062      71     -23   0.097     85      -16
NCP    0.158      78     -35   0.096     84      -17
NDH    0.040      80     -27   0.096     84      -17
NFR    0.106      79     -44   0.098     84      -16
NGV    0.091      69     -31   0.095     84      -16
NHF    0.088      87     -22   0.098     85      -16
NHM    0.355      94     -32   0.094     85      -17
NIM    0.091      98     -89   0.096     84      -16
NLH    0.352     160       3   0.096     84      -16
NLN    0.074      98     -24   0.096     84      -16
NMC    0.047      84     -29   0.098     84      -16
NMH    0.090      83     -12   0.097     84      -16
NMI    0.261      23     -30   0.096     85      -16
NMT    0.148      94     -10   0.097     84      -16
NMW    0.039      65     -15   0.097     84      -16
NOL    0.166      21     -20   0.096     84      -16
NPR    0.119      91     -32   0.099     85      -15
NPV    0.121     100     -26   0.096     84      -16
NRR    0.074     110     -16   0.096     84      -17
NSH    0.094      80     -21   0.097     84      -16
NSP    0.065      82     -29   0.096     84      -16
NTA    0.303      73     -22   0.098     85      -16
NTB    0.061      39     -13   0.097     84      -16
NTY    0.099      66     -32   0.096     84      -16
NVA    0.161     100     -63   0.096     84      -16
NVE    0.872     117      77   0.096     84      -16
NWR    0.067      97     -17   0.097     84      -16
OBH    0.242      64      -8   0.093     83      -18
OCM    0.054     147     -27   0.095     83      -18
OGO    0.108      98      -3   0.094     84      -18
OHC    0.110      64     -21   0.094     84      -18
ORA    0.163     122     -24   0.094     83      -18
ORD    0.089      19     -19   0.093     83      -18
OST    0.096     147     -27   0.094     83      -18
OSU    0.120      89     -33   0.095     83      -17
OWY    0.080     108     -34   0.094     83      -18
PAB    0.140     111     -57   0.091     88      -17
PAD    0.189     115     -41   0.093     87      -17
PAG    0.106     121     -46   0.090     88      -17
PAN    0.047     202     -25   0.093     87      -16
PAP    1.942      14      29   0.093     87      -16
PAR    0.070     100     -33   0.090     86      -17
PBI    0.118      94     -39   0.090     88      -17
PBP    0.109     107     -25   0.090     88      -17
PBW    0.241     140     -79   0.092     86      -17
PCA    0.116      80     -26   0.091     87      -17
PCB    0.121      83     -22   0.092     87      -16
PCH    0.165     101       0   0.091     87      -17
PCR    0.103      72     -35   0.090     87      -17
PDR    0.122      94     -60   0.091     87      -17
PFR    0.176      74     310   0.091     87      -17
PGH    0.138      63     -28   0.090     87      -17
PHA    0.023     154     -57   0.090     88      -17
PHB    0.144      84     -21   0.090     87      -18
PHC    0.110      90     -12   0.093     88      -16
PHF    0.097      77     -36   0.091     87      -17
PHO    0.266     327     -52   0.091     87      -17
PHP    0.701      38     -19   0.091     87      -17
PHR    0.054      93     -25   0.092     87      -17
PHS    0.093      64     -24   0.093     87      -16
PIR    0.515      87     -53   0.089     87      -17
PJL    0.245     107     -50   0.094     87      -16
PKE    0.077      48     -44   0.090     87      -18
PKY    0.094     102     -31   0.092     88      -17
PLO    0.044      64     -57   0.092     87      -16
PMC    0.063     170     -17   0.092     87      -17
PMG    0.024     281     -43   0.092     88      -17
PML    0.078      66     -26   0.091     88      -17
PMM    0.106      80     -34   0.090     87      -17
PMP    0.236     104     -30   0.091     87      -17
PMR    0.115      59     -14   0.090     87      -17
POP    0.119     134     -39   0.090     87      -17
PPB    0.162     108     -44   0.092     88      -16
PPC    0.092      91     -46   0.091     87      -17
PPG    0.070      55     -38   0.090     87      -18
PPT    0.289     108     -25   0.091     87      -17
PRC    0.192     300     -48   0.091     87      -17
PSA    0.111      47     -34   0.092     87      -17
PSC    0.145      91     -35   0.090     88      -17
PSM    0.167      55     -31   0.091     87      -17
PSR    0.179       6     -72   0.090     88      -17
PST    0.615     110     -57   0.091     87      -17
PTA    0.104      79     -27   0.091     88      -17
PTQ    0.235     105     -31   0.092     89      -17
PTR    0.202      74     -49   0.089     87      -17
PVC    0.060      97     -16   0.091     87      -17
PWK    0.070      90     -33   0.091     88      -17
PWM    0.124      82     -27   0.090     87      -18
VRC    0.000       0       0   0.095     80      -18