THE
LOMA
PRIETA,
CALIFORNIA,
EARTHQUAKE OF
OCTOBER
17,1989:
EARTHQUAKE
OCCURRENCE
MAIN-SHOCK CHARACTERISTICS
STRONG-MOTION AND BROADBAND TELESEISMIC
ANALYSIS
OF THE
EARTHQUAKE FOR RUPTURE PROCESS AND HAZARDS
ASSESSMENT*
CONTENTS
By David
J.
Wald and Thomas H.
Heaton,
U.S.
Geological
Survey
and
Donald
V.
Helmberger,
California
Institute
of
Technology
Page
A235
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1
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1
ABSTRACT
We
have
used broadband
records from 18
teleseismic
stations and three-component records
from
16 local
strong-
motion
stations
in
a formal inversion
to
determine
the
temporal and spatial distribution
of
slip
during
the earth-
quake. Separate inversions
of
the
teleseismic (periods,
3-
Contribution
No.
4935,
Division
of
Geological and Planetary
Sciences, California Institute
of
Technology, Pasadena,
CA
91
125.
30
s)
and strong-motion
(periods,
1-5
s) data sets result
in
similar source
models.
The data require
bilateral
rupture,
with relatively
little
slip in
the
region
directly
updip
from
the hypocenter.
Slip is
concentrated
in
two patches: one
centered
6 km northwest
of
the hypocenter at 12-km
depth
with
an
average slip amplitude
of
250 cm, and the
other
centered
about 5
km
southeast
of
the hypocenter at 16-km
depth
with an
average slip amplitude
of
180
cm. This
bilateral
rupture results
in
large-amplitude
ground mo-
tions at
sites
both to
the
northwest and
southeast
along
the fault strike.
The
northwestern
patch, however, has a
larger seismic moment
and overall
stress
drop
and
thus is
the source
of
the highest
ground-motion
velocities,
a re-
sult consistent with observations.
The
bilateral rupture
also
results
in
relatively moderate ground motion directly
updip
from
the hypocenter, in
agreement
with
the ground
mo-
tions observed
at Corralitos, Calif. Furthermore, there
is
clear evidence
of
a foreshock
(M-4.5-5.0)
or
slow rup-
ture
nucleation
about
2
s before the
main
rupture;
the
origin time implied
by
strong-motion
trigger times is
sys-
tematically nearly
2 s later
than that
predicted from
the
high-gain
regional-network data.
The
seismic moment
ob-
tained
from
either or
both data sets
is
about
3.0~10~~
dyne-cm,
and the
seismic
potency
is 0.95
km3.
Our analy-
sis indicates
that the
rupture
model
determined from the
teleseismic
data set alone, independent
of
the
strong-mo-
tion
data set, is
adequate to predict many
characteristics
of
the
local-strong-motion recordings.
INTRODUCTION
In this study,
we
use
a least-squares linear inversion
of
strong-motion and teleseismic
data
to
solve for
the
spatial
and temporal distribution
of
slip during the 1989
Loma
Prieta earthquake
(Me7.1).
Although the
geometry
of
the
fault
plane
is
fixed in
the
inversion,
we
chose
it to
be
compatible
with the teleseismic waveforms and
the
after-
shock
distribution.
Our
estimates
of
the
spatial
and
tem-
A235
A236
MAIN-SHOCK CHARACTERISTICS
poral
distribution
of
slip should enhance studies
of
fault
segmentation
and
earthquake recurrence
(Working
Group
on
California Earthquake Probabilities,
1988; King and
others,
1990),
which depend
on
reliable estimates
of
the
rupture dimensions
and
slip amplitude.
Furthermore, the
variation in rake
angle
as
a function
of
position along
strike
and
downdip
on the
fault
plane
is
critical to analy-
ses
of
the
complex fault interactions
within the
Sargent-
San
Andreas
fault system (Dietz
and
Ellsworth,
1990;
Olson, 1990; Schwartz
and
others,
1990;
Seeber
and
Armbruster, 1990).
We
use the method
of
Hartzell
and
Heaton
(1983),
which
has been
shown
to provide valuable insight
into
the rup-
ture
history
of
other California earthquakes
(Hartzell and
Heaton,
1986;
Mendoza
and
Hartzell,
1988; Wald and
others,
1990),
as
have
other finite-fault approaches
(Olson
and
Apsel, 1982; Archuleta, 1984;
Beroza and
Spudich,
1988). In
addition
to
providing
an
estimate
of
the
rupture
history
for individual earthquakes, these studies also give
new
insight
into
the general characteristics
of
the
rupture
process
that
are common
to many
events.
After studying
slip models
for
several
earthquakes,
Mendoza and Hartzell
(1988) suggested
that large
gaps
in aftershock patterns
commonly
coincide
with regions
of
relatively high
slip.
From the distribution
of
slip,
we
can also
constrain the
location and
depth extent
of
significant
energy release
and
characterize the distribution
of
stress changes
on
the
fault. These results
provide
a starting
point
for calculating
ground motions in
future events comparable in size
to
the
1989
Loma
Prieta earthquake. Such
ground-motion
calcu-
lations
are
important
for
augmenting the sparse data
base
of
near-source strong-motion
recordings
of
Af>7
crustal
earthquakes.
The
1989
Loma
Prieta earthquake
was well recorded at
both
local-strong-motion
and teleseismic broadband sta-
tions.
The strong-motion
velocity recordings used here
are dominated
by
energy
in
the
range
1-5
s, whereas the
broadband teleseismic recordings show energy in the range
3-30
s.
This
wealth
of
data
provides an opportunity to
compare rupture models
that
are derived
independently
from either strong-motion
or
teleseismic
data sets
with
those derived from combined data sets
and
over a
wide
range
of
frequencies.
Our
results give insight
into
the limi-
tations
of
previous studies
that used
less extensive data
sets.
DATA
Ground motions from
the
1989
Loma
Prieta
earthquake
were
recorded
over
a wide
range
of
frequencies
and dis-
tances,
from
high-frequency waveforms observed
on
local
accelerometers
and
regional seismic
networks to very low
frequency waveforms observed
in teleseismic surface
waves and
geodetic line-length changes. Deterministic
waveform
inversion
of
high-frequency
(>3
Hz) motion,
however,
requires
an
accurate
and
detailed knowledge
of
the
wave propagation
in
the geologically complex struc-
ture
of
the
Loma
Prieta
region. Furthermore, inversion
of
high-frequency waveforms
requires a proliferation
of
free
variables that significantly
increases
computation
time
and
decreases the
stability
of
the inversion
process. Therefore,
we
chose
to
concentrate
our study on
the lower-frequency
part
of
the
rupture
history. Near-source, low-pass-filtered
strong-motion
and teleseismic body waves
seem
to
be the
most
suitable data sets
to
study
the general characteristics
of
the slip
history. Although
geodetic data can also
pro-
vide important constraints
on
an
earthquake
slip-distribu-
tion
model, they
can
be
overly sensitive
to
the geometry
of
the
inferred fault
plane
and
so
are not
always suitable
for determining detailed variations in slip.
TELESEISMIC WAVEFORMS
The
teleseismic
stations
chosen
for
this
study
are
listed
in table 1.
The data are digital recordings obtained from
Chinese
Digital
Seismograph
Network
(CDSN), Institut
National
des
Sciences
de
l'univers,
France
(GEOSCOPE),
and Incorporated
Research Institution
for
Seismology
(IRIS)
broadband
components
and
Global Digital Seismo-
graph Network
(GDSN) intermediate-period components.
These
stations
provide
a uniform
azimuthal coverage
of
the focal
sphere
and contain
several near-nodal observa-
tions
for
both
P-
and SH-wave source radiation
(fig. 1). In
this analysis, instrument responses were deconvolved from
the original
recordings
to
obtain
true ground
velocities.
STRONG MOTION
The
distribution
of
near-source ground
velocities
used
in this study
is mapped in
figure
2;
station abbreviations,
station
geometries with
respect
to
the epicenter, and
trig-
ger
times (where
available)
are
listed
in
table
2.
The
ve-
locity waveforms were
obtained
by
integrating corrected
acceleration
recordings
provided
by
the California
Divi-
sion
of
Mines
and Geology
(CDMG) (Shakal
and
others,
1989) and the U.S. Geological
Survey (USGS)
(Maley
and others,
1989),
and
uncorrected recordings
from
the
University
of
California,
Santa
Cruz
(UCSC).
The
veloc-
ity waveforms were
bandpass
filtered
between
0.1
and
1.0
Hz,
using
a zero-phase, third-order
Butterworth filter. The
horizontal
components are rotated
with
respect
to
the epi-
center
to
obtain "radial"
and
"tangential"
components.
Although this rotation
is correct for
energy
originating
near the
epicenter, it is only approximate for source re-
gions
farther northwest
and southeast along the fault.
STRONG-MOTION
AND BROADBAND TELESEISMIC ANALYSIS
OF
THE EARTHQUAKE
FOR
RUPTURE
PROCESS
A237
Table
1
.-Teleseismic
stations
used in this study
Station
Distance
Azimuth
Backazimuth
Phases
(fig.
1)
(O)
(O)
used
A
FI
ARU
CAY
COL
HIA
HON
HRV
MDJ
NNA
OBN
PPT
RPN
SCP
SSB
TOL
WFM
Two
criteria
were
used
to
select stations
for inclusion
in
the inversion:
The
observations
should be both
close
to
the
aftershock zone and well distributed in azimuth. Within
the epicentral region,
peak ground
motions
are
relatively
independent
of
surface geology (Benuska, 1990). Care was
also
taken
to
avoid stations that
seemed
to
have
unusual
site responses. For this reason, the CDMG
station Agnew
was not used,
although
it is
at a similar distance
and
azi-
muth
to
station
LEX
(fig.
2).
UCSC
stations
BRN, LGP,
UCS,
and WAH were included
to
provide
important
sta-
tion
coverage
to
the
west and
southwest
of
the epicenter.
These stations, however, did not record
absolute time
and
required
additional processing
to
remove a few random
spikes in the
raw
acceleration data. Although
the
despiking
process that
we
used may
be
inadequate at high frequen-
cies, it provides useful velocity
recordings at the frequen-
cies
of
interest in this analysis
(0.1-1
Hz).
The
station
LGP
acceleration
recording exhibited a
permanent
step
on
the
vertical
component
that
does
not carry through in our
bandpassed
data; the
horizontal
components
were appar-
ently unaffected.
Station
BRN
was
set
for
0.5
g
maximum
amplitude, and because
amplitude reached close
to
that
value, the accuracy
of
the response
is
unknown.
We
ad-
dress
the issue
of
estimating absolute
time
for these
sta-
tions
in
the
section below
entitled
"Inversion Method."
FAULT-RUPTURE
MODEL
The
fault
parametrization and
modeling procedure that
we
employ was
described
by
Hartzell
and
Heaton
(1983)
in their study
of
the 1979 Imperial Valley, Calif., earth-
quake. Faulting
is represented as slip
on
a planar surface
that
is
discretized into
numerous
subfaults.
The
ground
motion
at a
given station
can
be
represented as a linear
sum
of
subfault
contributions, each appropriately delayed
in
time
to
simulate fault rupture. Formal inversion
proce-
dures are
then used
to
deduce the
slip
distribution on these
subfaults that minimizes
the difference
between
the
ob-
served and synthetic waveforms.
EXPLANATION
+
COMPRESSIONAL
Â
DILATIONAL
Â
DOWN
s
0
NODAL
s
0
NODAL
Figure
1.-Focal
spheres
with plot
of
takeoff
angles
of
P
(A)
and
SH
(B)
waves from 1989
Loma
Prieta
earthquake, showing global distribution
of
broadband teleseisrnic
stations
used in this
study. Radiation patterns
are for
a source mechanism with a
strike of
128O,
a dip
of
70°
and
a rake of
138O.
For
SH
waves,
"up"
refers to clockwise motion.
A23
8
MAIN-SHOCK CHARACTERISTICS
In
this study,
we
represent the
Loma
Prieta rupture
as a
40-km-long
plane
striking
N.
128'
E.
and dipping
70'
SW.
As a point
of
reference, the
northernmost
corner
of
our assumed
fault
plane in at lat
37.193'
N.,
long
122.020'
W.
The fault extends
from 1.5- to
20.3-km
depth and has
a
downdip
width
of
20
km
(fig.
3).
We chose the
overall
dimensions
of
the fault to enclose
the region
of
major aftershock activity (Dietz
and
Ellsworth, 1990);
possible vertical
strike-slip
faulting
on
a second plane extending past
the
south end
of
our in-
ferred
rupture
area
is discussed below. The
strike
and
dip
of
our fault plane
(128'
and
70°
respectively) were
cho-
sen
from
the broadband-inversion results
of
Kanamori and
Satake
(1990).
This fault
plane
is
also consistent with
the
aftershock lineation (Dietz
and Ellsworth,
1990),
the fo-
cal
mechanism
determined from first-motion
data
(Oppenheimer, 1990)
and
the
P-
and SH-wave teleseismic
waveforms plotted in figure 4. Slight discrepancies in strike
and
dip
would have
little effect
on
our
model results and
conclusions.
The fault-plane
geometry chosen
for
this study differs
somewhat from
that used
by
Lisowski and
others
(1990)
to model the geodetic
data. Although
they
also
used
a dip
of
70°
they found that
a strike
of
N.
136'
E.
(8'
more
northerly than
ours)
was needed
to
explain their data. Fur-
thermore, their
fault
plane was
shifted about 2 km
to
the
west
of
our
assumed plane, which was
chosen
to
coincide
with the aftershock
distribution.
In
general,
the geodetic
data
are more
sensitive
to fault geometry than
are the
waveform
data,
but they
are
not
as
powerful
in resolving
details
of
the slip
distribution.
Differences in the fault
geometry inferred from
static offsets,
in
comparison
with
waveform
studies,
may
reflect complexities in the rupture
process,
such as a
nonplanar
fault surface
or
multiple-
fault
rupture.
These complexities are
not
considered
fur-
ther
in
this
study.
Our
fault
plane
is
discretized
into
12
subfaults along
strike
and
8 subfaults
downdip,
each
2.5
km long
and
3.33
km
wide vertically
(fig.
3).
This
subfault
area
is
a
compromise chosen
to
give
sufficient freedom
so
as
to
allow the rupture variations needed to
successfully
model
the ground motions and yet
minimize computation
time.
The computation time
for
the
inversion is proportional
to
the
cube
of
the number
of
unknown
parameters, in
this
-
0
Sec
30
u
Figure
2.-Loma
Prieta
region, Calif.,
showing locations
of
strong-motion stations (triangles),
epicenter
of 1989
earthquake (star),
and surface
projection
of
model
fault plane used
in
this study (shaded rectangle). Curves represent seismograms
of
radial
(A)
and tangential (B) components
of
velocity recorded
at
each
station;
number to right
of
each curve is peak velocity (in centimeters per
second).
Irregular thin lines, faults (dashed where
inferred),
digitized
from
major Quaternary faults mapped
by
Jennings
(1975).
Crosses
(fig.
2B),
aftershocks.
Dashed
outline (fig.
20,
modified
Mercalli intensity
(MMI) contour
separating regions
of
MMI VII and VIII (from Stover and others,
1990).