| nisee |
National Information Service for Earthquake Engineering
University of California, Berkeley |
Seismicity in the Southern Santa Cruz Mountains in the 20-Year Period Before the Earthquake.
J. A. Olson, USGS, Menlo Park, California.
Examined are the spatial distribution of the well-located earthquakes in the twenty years preceding the Loma Prieta earthquake and their association with faults along a 100 km-long extent of the Southern Santa Cruz Mountains.
I. Summary
Several files are archived here. The principal data files are master event
files from which the other data files were derived. These master files, named
"archive" files (described below), are primarily earthquake and quarry blast
hypocenter and traveltime information for the period 1969 through 1991 located
by the U.S. Geological Survey's Northern California Seismic Network (Calnet).
All events in the Calnet catalog within a 100-km-long by 30-km-wide rectangle
which is oriented northwest-southeast and approximately centered on the Loma
Prieta main shock are included in the "archive" data files. The corner
coordinates of this rectangle are:
Latitude Longitude
deg (N) min deg (W) min
37 14.00 122 20.00
37 25.30 122 6.20
36 48.70 121 18.00
36 37.10 121 32.20
The Calnet catalog event recording, processing and location methods are
summarized in Oppenheimer, Klein and Eaton (1992), which includes many
references for further, specific information about the catalog data. The
events are located by Calnet using the FORTRAN computer program HYPOINVERSE
(Klein, 1989). The "archive" files archived here (files with suffix "ARC") are
in the HYPOINVERSE output "full archive" format (Klein, 1989; p. 44) and each of
the parameters listed in those files is described in detail in that report.
Briefly, the data in the "archive" files for each event consist of one line
which summarizes the hypocenter information (HYPOINVERSE "summary" output
format of Klein (1989; p. 42) including the event's date, time, magnitude,
location, etc., followed by many lines which list the information obtained
from recordings at individual stations including the station code name, arrival
times, first motion polarity, coda duration, etc. For convenience, these
"archive" files are divided into five-year periods except for the period 1989
through 1991. The file names are:
(1) LP69to73.ARC
(2) LP74to78.ARC
(3) LP79to83.ARC
(4) LP84to88.ARC
(5) LP89.ARC
(6) LP90to91.ARC
The corresponding hypocenter file with all the Calnet catalog data within the
rectangle described above is called:
LP69to91CN.SUM.
Other data archived here include hypocenter and fault plane solution data
for subsets of the Calnet catalog data (the master "archive" files) which are
reported by Dietz and Ellsworth (1993) and Olson and Hill (1993). The location
and magnitude determination methods and/or selection criteria used to obtain
these subsets are described below. The main difference between the hypocenter
data reported in those papers is that Olson and Hill (1993) used the Calnet
catalog hypocenters and Dietz and Ellsworth (1993) relocated those hypocenters
with a slighty different velocity model. The hypocenter data are in the
HYPOINVERSE output "summary" format (Klein, 1989; p. 42) and these files have
the suffix "SUM", the same format as the first line for each event in the
"archive" files. The fault plane solution data were determined with the
FORTRAN computer program FPFIT by Reasenberg and Oppenheimer (1985) using the
first-motion polarities from the Calnet catalog data in the "archive" files.
These data are in the HYPO71 "extended summary" format (Reasenberg and
Oppenheimer, 1985; p.13), which includes the dip angle, dip direction and rake
for one of the nodal planes and other parameters of the nodal plane
determinations in the columns following the hypocenter information. The
hypocenter and fault-plane solution data files archived here are:
(1) LP69to91DE.SUM (hypocenter data reported in Dietz and Ellsworth, 1993)
(2) LP89to91DE.FPS (fault-plane solution data reported in same)
(3) LP69to89OH.SUM (hypocenter data reported in Olson and Hill, 1993)
(4) LP88to89OH.FPS (fault-plane solution data reported in same)
II. Data Files from Dietz and Ellsworth (1993)
A. Location Method
Hypocenters from Dietz and Ellsworth (1993) were derived by relocating the
Calnet archive data (described above) with HYPOINVERSE using the velocity
model described in Dietz and Ellsworth (1993). To account for geologic
differences across the San Andreas fault, the velocity model uses a separate
1-dimensional velocity structure for the crust on each side of the fault.
Velocities within each layer are constant. The focal mechanisms described below
(section II.D) were calculated by FPFIT from Calnet first motions having
takeoff angles derived from this model.
B. Magnitude Determination
Magnitudes (MF) reported in Dietz and Ellworth (1993) were calculated by
HYPOINVERSE from measurements of record duration using the method and site
corrections developed by Eaton (1992).
C. Selection Criteria for Hypocenters
1. Area: Polygon with coordinates:
Latitude Longitude
deg (N) min deg (W) min
37 20.00 122 10.00
37 20.00 122 2.00
36 57.20 121 29.40
36 45.00 121 29.40
36 45.00 121 43.00
37 3.70 122 10.00
2. Time Period: January 1, 1969 through October 17, 1991 (G.m.t.)
3. Magnitudes: duration magnitudes (MF) 1.0 and greater
4. RMS (root mean square traveltime residual): 0.15 s and less
5. ERH (horizontal error): 1.0 km and less
6. ERZ (vertical error): 2.0 km and less
7. Exclusion of known and suspected quarry blasts:
All events labeled as quarry blasts by Calnet ("Q" in col. 77) were excluded
from LP69to91DE.SUM. Additionally, all hypocenters near two known quarry
sites were screened carefully for unlabeled blasts. The area screened
around each site encompasses the epicenters of labeled blasts from that
site. Those events which fell into the range of depth, magnitude, and
time-of-day of known quarry blast hypocenters were labeled as suspected
quarry blasts and were excluded from the summary file. The screened areas
are defined by these coordinates:
Latitude Longitude
deg (N) min deg (W) min
(1) 37 23.00 122 1.00
37 16.00 122 1.00
37 16.00 122 10.00
37 23.00 122 10.00
(2) 36 54.50 121 36.00
36 53.00 121 36.00
36 53.00 121 38.50
36 54.50 121 38.50
D. Selection Criteria for Hypocenters with Fault-Plane Solutions:
1. Area: same as section II.C.1.
2. Time Period: October 18, 1989 through October 17, 1991 (G.m.t.)
3. Magnitude: MF 1.5 and greater
4. RMS, ERH, ERZ: same as sections II.C.4-7.
5. Number of First-Motion Readings: 30 and greater
6. Fj (FPFIT misfit value): 0.15 and less
7. Exclusion of multiple fault-plane solutions:
In the cases where FPFIT returned multiple solutions for one event, all
mechanisms for that event were examined visually. A single "best" solution
was chosen and saved in LP89to91DE.FPS for each event. The decision about
which mechanism to save was based on comparisons of factors such as the
distribution of arrivals about the nodal planes, the weights of misfit
arrivals, the estimated errors in the nodal plane orientations, etc.
III. Data Files from Olson and Hill (1993)
A. Location Method
Hypocenters reported in Olson and Hill (1993) are those in the Calnet
catalog as described in Oppenheimer et al. (1992). These data, located in
the southern Santa Cruz Mountains area, were located with a model (Olson
and Hill, 1993, fig. 3) based on the model of Dietz and Ellsworth (1990).
The main difference between these models is that the Calnet model has a
linear velocity gradient within each layer and the Dietz-Ellsworth model has
homogeneous layer velocities.
B. Magnitude Determination
The determination of magnitudes for Calnet catalog hypocenters is described
in Eaton (1992). The hypocenter data archived for Olson and Hill (1993)
uses the Calnet magnitude determinations with one exception: the magnitudes
for hypocenters before the Loma Prieta main-shock have values for U.C.
Berkeley amplitude magnitudes (MX) substituted in the columns for duration
magnitude (MF) values (cols. 68-69) for events with MF 4.0 and greater; in
these cases, the MX values more closely approximate ML (J.P. Eaton, oral
commun.). These substitutions were made merely to facilitate plotting.
C. Selection Criteria for Hypocenters
1. Area: same as rectangle described for selection of archive files (section I)
2. Time Period: January 1, 1969 through December 31, 1989 (G.m.t.)
3. Magnitudes: all magnitudes in Calnet catalog
4. RMS (root mean square traveltime residual): 0.20 s and less
5. ERH (horizonal error): 1.0 km and less
6. ERZ (vertical error): 2.0 km and less
7. NST (number of stations): 6 and greater
8. Exclusion of hypocenters near quarry blast sites:
All events identified by Calnet to be quarry blasts (events with "Q" in
column 77) were omitted from "LP69to89OH.SUM". In addition, all remaining
events near quarry blast sites were excluded from that file, thus, if
earthquake hypocenters were near the site, these were excluded as well as
those of quarry blasts. The size of the area near each quarry site within
which hypocenters were excluded depended on the hypocenter locations of
known quarry blasts, as identified by Calnet readers (events with "Q" in
col. 77). There were five such sites in the 100-km-long rectangle described
above (section I)with hypocenters near them. In each case, hypocenters were excluded
within a polygon, the shape and size of which were chosen on the basis of
the location of hypocenters of known quarry blasts. The depths beneath
these polygons within which hypocenters were excluded were also chosen on
the basis of the depths of known quarry blast hypocenters. These depth
ranges were 0-5 km, with the exception of quarry number 2, below, in which
case there were no nearby faults or suspected earthquake hypocenters and all
events beneath the polygon were excluded. The area for each site within
which hypocenters were excluded are described by the following polygons:
Latitude Longitude
deg (N) min deg (W) min
(1) 37 9.31 121 59.80
37 9.51 121 51.84
37 17.96 121 44.30
37 19.23 121 53.89
(2) 36 42.05 121 39.64
36 42.65 121 32.52
36 44.98 121 32.50
36 48.59 121 35.11
36 47.76 121 40.13
36 45.23 121 41.20
(3) 36 52.36 121 40.05
36 52.34 121 34.76
36 55.73 121 35.01
36 55.75 121 39.77
(4) 37 17.10 122 8.81
37 16.89 122 2.41
37 20.94 122 1.61
37 20.94 122 9.07
(5) 36 45.81 121 29.86
36 46.01 121 26.96
36 48.96 121 26.67
36 49.41 121 30.88
D. Selection Criteria for Hypocenters with Fault-Plane Solutions
1. Area: Lake Elsman area - rectangle with coordinates:
Latitude Longitude
deg (N) min deg (W) min
37 8.98 121 58.93
37 11.28 121 57.07
37 8.50 121 51.58
37 6.18 121 53.45
2. Time Period: June 27, 1988 through September 30, 1989 (G.m.t.)
3. Magnitude: MF 1.5 and greater (see "magnitude determinations", section III.B)
4. Number of First-Motion Readings: 15 and greater
5. RMS, ERH, ERZ: same as section III.C.4-6.
IV. References Cited
Dietz, L.D., and Ellsworth, W.L., 1990, The October 17, 1989 Loma Prieta,
California, earthquake and its aftershocks; geometry of the sequence from
high-resolution locations: Geophysical Research Letters, v. 17, no. 9,
p. 1417-1420.
Dietz and Ellsworth, 1993, Aftershocks of the 1989 Loma Prieta, California
earthquake and their tectonic implications: U.S. Geological Survey
Professional Paper I - Earthquake Occurrence, Chapter on Aftershocks
and Postseismic Effects, P.A. Reasenberg, ed.
Eaton, J.P., 1992, Determination of amplitude and duration magnitudes and site
residuals from short-period seimsographs in northern California: Seismological
Society of America Bulletin, v. 82, no. 2, p. 533-579.
Klein, F.W., 1989, User's guide to HYPOINVERSE, a program for VAX computers
to solve earthquake locations and magnitudes: U.S. Geological Survey Open-
File Report 89-314, 58 p.
Olson, J.A., and Hill, D.P., 1993, Seismicity in the southern Santa Cruz
Mountains in the 20-year period before the earthquake, in The Loma Prieta,
California Earthquake of October 17, 1989: U.S. Geological Survey Professional
Paper I - Earthquake Occurrence, Chapter C. - Preseismic Observations, M.J.S.
Johnston, ed.
Oppenheimer, D.H., Klein, F.W., and Eaton, J.P., 1992, The first 20 years of
CALNET, The Northern California Seismic Network: U.S. Geological Survey Open-
File Report 92-209, 33 p.
Reasenberg, P., and Oppenheimer, D.H., 1985, FPFIT, FPPLOT, and FPPAGE, FORTRAN
computer programs for calculating and displaying earthquake fault-plane
solutions: U.S. Geological Survey Open-File Report 85-739, 109p.