Bulletin
of
the
Seismological
Society
of
America,
Vol.
76,
No.3,
pp.
649-674,
June
1986
RUPTURE
HISTORY
OF
THE
1984
MORGAN
HILL,
CALIFORNIA,
EARTHQUAKE
FROM
THE
INVERSION
OF
STRONG
MOTION
RECORDS
BY
STEPHEN
H.
HARTZELL
AND
THOMAS
H.
HEATON
ABSTRACT
Near-source
strong
motion
velocity
records
and
teleseismic
short-period
P
waveforms
are
modeled
to
obtain
the
spatial
and
temporal
distribution
of
slip
for
the
1984
Morgan
Hill
earthquake.
Both
forward
modeling
and
constrained,
least-
squares
inversion
techniques
are
used
to
interpret
the
strong
motion
velocity
waveforms
in
the
frequency
range
of
approximately
0.2
to
2.0
Hz.
These
data
support
a
nearly
unilateral
rupture
to
the
southeast
with
a rupture
propagation
velocity
of
nine-tenths
of
the
local
S-wave
velocity.
The
majority
of
the
slip
occurs
over
a
fault
length
of
25
km
and
to
a first
approximation
can
be
interpreted
as
two
main
source
regions,
each
with
an
extent
of
about
5 km
with
their
centers
separated
by
about
12
km.
However,
each
of
the
sources
has
detailed
structure
of
its
own, and
a
simple
two-point-source
model
is not
an
accurate
representation
of
the
Morgan
Hill
earthquake.
The
second
source
occurs
about
4.5
sec
after
the
first
and
is approximately
3 times
larger.
The
maximum
dislocation
on
the
fault
plane
is about
1
m.
The
total
moment
of
the
earthquake
is estimated
to
be
2.1
x
10
25
dyne-em.
The
Morgan
Hill
earthquake
offers
convincing
evidence
for
very
inhomogeneous
slip
and
stress
distributions
on
shallow
strike-slip
faults.
INTRODUCTION
The
Morgan
Hill
earthquake
of
24
April
1984
(21:15:18.8
UTC)
is a
moderate-
sized
event
(ML
6.2
USGS)
which
occurred
on
the
Calaveras
fault
approximately
17
km
southeast
of
San
Jose,
California.
The
epicenter
is
located
at
37.18.56'N,
121
•40.68'W
with
a hypocentral
depth
of
8.5
km
(Cockerham
and
Eaton,
1984).
This
earthquake
produced
a significant
set
of
near-source
strong
motion
records;
12
ground-level,
mostly
free-field
stations
with
accelerations
greater
than
0.1
g
were
recorded
within
40
km
of
the
epicenter.
One
of
these
stations
(Coyote
Lake
Dam)
recorded
a peak
acceleration
of
1.3
g
and
a peak
velocity
of
79
em/sec
on
a horizontal
component.
Other
records
of
the
strong
motion
data
set
suggest
that
this
earthquake
consisted
of
two
main
sources,
or
alternatively,
a single
source
which
produced
a
prominent
"starting
phase"
and
"stopping
phase."
In
this
paper,
the
strong
motion
velocity
records
and
teleseismic
short-period
P
waveforms
are
modeled
to
determine
the
distribution
of
coseismic
slip.
This
slip
distribution
is then
used
to
determine
which
of
the
above
source
models
is
correct,
as
well
as,
increase
our
general
understanding
of
the
build-up
and
release
of
stress
on
shallow
strike-slip
faults.
Recent
seismicity
along
this
section
of
the
Calaveras
fault
is
discussed
by
Bakun
(1980).
Larger
recent
events
are
the
6 August
1979
Coyote
Lake
earthquake
(ML
5.9),
located
just
south
of
the
Morgan
Hill
rupture,
the
29
August
1978
Halls
Valley
earthquakes
(ML
4.2
and
ML
3.9),
and
the
8 May
1979
Halls
Valley
earthquake
(ML
4.5).
DATA
The
near-source
strong
motion
velocity
records
are
shown
in
Figure
1,
a to
c
(Shakal
et
al.,
1984;
Brady
et
al.,
1984).
Unfortunately,
most
of
the
stations
are
not
naturally
rotated
with
respect
to
the
fault
orientation,
with
the
majority
of
stations
649
a
Morgan
Hi
II
Earthquake
Ground
Velocity
Perpendicular
to
the
Fault
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HAL
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FIG.
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Strong
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velocity
components
parallel
to
the
Calaveras
fault
zone.
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Hill
Earthquake
Vertical
Ground
Velocity
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sec
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Hill
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strong
motion
velocity
components.
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