Abstracts
2000 AGU Fall
Meeting
San Francisco, December 15-19, 2000
S71E-01 Invited
Review of Seismological Results from the 1999 Chichi, Taiwan, Earthquake
Mori, J., K.F. Ma
The
September 21, 1999 Chichi, Taiwan earthquake (Mw=7.6) produced the presently
best set of local and regional strong-motion records from a large earthquake.
This shallow thrust event ruptured about 85 km of the Chelungpu
fault in Central Taiwan with large displacements of 1 to 10 meters along most
of the surface trace of the fault. The dense strong-motion array operated by
the Seismology Center of the Central Weather Bureau recorded the earthquake on
more than 400 stations, at distances of less than 1 km to 150 km from the
fault. These data, along with locally recorded short-period data and teleseismic data from worldwide broadband stations provided
valuable information about the earthquake source process, three-dimensional
wave propagation, and regional tectonics. Using the strong-motion data, teleseismic data, and GPS displacement, several groups of
researchers have produced models of the spatial and time distribution of slip
and slip velocity for the earthquake. Common results of these studies show that
slip occurred over an area of about 80 km by 40 km with the hypocenter in the
southern region and rupture propagation mainly to the north. The seismic moment
was 2 to 4 x10**27 dyne-cm. All of the models show a large asperity in the
northern region of the fault with slip of over 10 m. This is in the region
where very large surface displacments (8-10m) were
measured on the fault. However, it difficult to model some of the strong-motion
records very close to the fault, suggesting that some of the very large shallow
slip did not radiate large amounts of elastic energy. Also, in the northern
region of the fault where displacements are very large, the level of ground
acceleration and associated building damage is relatively low. Estimates of the
slip velocity show that the fault moved very rapidly (1-3 m/sec) but slip was
"smooth" producing low levels of high-frequency radiation. There was
a strong aftershock sequence with 7 events over M6, and several M5-6 events
apparently triggered outside of the immediate region of the Chichi earthquake.
Aftershocks distributions from permanent and temporary short-period networks
show that most of the events are in the hanging wall of the fault and there are
relatively few aftershocks along the shallow portion of the fault between the
hypocenter (at about 7 km depth) and the surface trace of the fault. The
aftershock pattern suggests that the Chelungpu fault
is nearly horizontal at depths greater than 10 km, although there are large
variations along strike. There are numerous clusters of aftershocks off the Chelungpu fault and a large number of events to the east,
under the eastern edge of the Central Range. The pattern of aftershocks
resembles the pattern of regional seismicity before the earthquake.
S71E-02
Rupture Process of the 1999 Chi-Chi, Taiwan, Earthquake From Strong Motion, Teleseismic and GPS Data
Ma, K., J. Mori, S. Lee, S., S.Yu
We
investigated the rupture process of the 1999 Chi-Chi, Taiwan, earthquake, using
high-quality near-source strong-motion records, broadband teleseismic
displacement waveforms, and well-distributed GPS data. The near-source
strong-motion displacement waveforms recorded significant static offsets of up
to 8 m. The teleseismic displacement records show a significant pulse with duration of
about 18 to 20 sec. These data imply involvement of a large asperity during the
source rupture. Considering the surface breaks observed along the Chelungpu fault, we considered two fault geometries: a
single planar fault, and a two-segment fault with a northeast striking section
near the northern end. Using the finite-fault model with various slip vectors,
we derived the temporal and spatial slip distribution of the earthquake. The
GPS data provided good surface displacement constraints in the slip
distribution determination. The spatial slip distribution is generally
consistent with field observations. The results, for the simple fault model
show a large asperity located in the region about 25 km to 55 km north of the
hypocenter with maximum slip of about 12m. The asperity further extends to the
region where the fault bends to the northeast, which is consistent with GPS
data, as found in the two-segment fault model. The slip amplitude near the
hypocenter is about 3 to 6 m. The seismic moments determined from the various
data sets are within the range of 2 to 4 x 10$^{27}$
dyne-cm. Most of the slip concentrated at the shallow depths (less than 10 km).
The total rupture duration is about 28 sec and the rupture velocity is about
75\% to 80\% of the shear wave velocity. The slip vector shows a clockwise
rotation during the fault rupture. The static stress drop of the large asperity
region is comparable with the dynamic stress drop as observed directly from the
slip velocity at the station near the large slip region.
S128-14
Large Long-Period Earthquakes from Miyakejima
Volcano Recorded at Kouzu Island
Mori, J., K. Sato, T. Shibutani
We
studied several of the long-period events that are associated with eruptive
activity from Miyakejima volcano using continuous
strong-motion recordings from Kouzu Island and
broadband Freesia data. We recorded about 15 large long-period events between
July 1 and 14 at Kouzu Island, which is located about
40 km northwest of the volcano. At Kouzu Island, the
seismograms of these events have long durations that last about 150 sec. The
recordings have large amplitudes in the period range of 1 to 20 sec. Waveforms
for the various long-period events look similar suggesting that they have
similar mechanisms. Moment estimates for these events from the National
Institute of Earth Science and Disaster Prevention, Science and Technology
Agency are in the range of 10**23 to 10**24 dyne-cm (Mw 5.0 to 5.6). We carried
out moment tensor inversions for several of these events using data in the
period range of 10 to 100 sec. The waveforms are largely composed of P and S
waves at Kouzu Island and surface waves at the
farther Freesia stations. There were large non-double couple components in the
moment tensor. The diagonal components of the moment tensor have similar values
indicating a large isotropic component. The moment tensors can be interpreted
as the sum of an isotropic component (expansion) and double couple of about
equal size. The source time functions were also very long with durations of 10
to 40 sec. We estimate the depths to be about 2 km. Some of the long-period
events occurred during times of eruptive activity at the volcano, while others
occurred when the volcano was relatively quiet. The long time duration and
isotropic component in the moment tensor suggest that the mechanism of these
events involves movement
S61A-25
Estimates of Radiated Energy for Moderate Shallow Earthquakes in Japan
Kobayashi, H, J. Mori, K.Sato
We
estimated the seismic energy radiated by earthquakes in Japan using strong
motion recordings from K-NET operated by the National Institute for Earth
Science and Disaster Prevention, Science and Technology Agency. We analyzed
data from 78 shallow events (depth less than 50 km) of magnitude 4.0 or greater
that occurred from March 1997 through October 1999. Each event was recorded on
10 - 184 stations within 200 km of the epicenter. The method we used consisted
of the time integration of the squared ground-motion velocity from the three
components of ground motion, with a distance attenuation function and the
station correction. The time window length is 50 sec from the first
P-arrival. We used a 0.1Hz highpass filter for
the recordings. We separated rock and stiff soil sites from the other sites
that had softer soil conditions and computed radiated energy for the two types
of sites. Amplitudes for the soft sites averaged several times larger than for
the hard sites. We consider the energy values from the hard sites to be a
better estimate of the radiated energy. We computed station corrections by
assuming that the average value of energy from the hard sites was correct, and
calculated the average amplitude residual for each station using all the events.
We also tested the effect of including the radiation pattern and using various
attenuation relations. The results show that the log of the energy increases as
a function of the log of the seismic moment with a slope slightly larger than
1.0. This relation is larger then expected for simple
constant stress drop scaling and may indicate a trend in dynamic friction or
driving stress as a function of earthquake size.
S72D-01
Strong Motion Recordings in the Izu Islands,
Japan: Observations of Earthquake Initiations
Sato, K. and J. Mori
We
recorded the strong earthquake swarm that was occurring during July and August
2000 in the Izu Islands on several strong-motion
instruments deployed on Kouzu and Shikine
Islands. We used 20 sec velocity sensors with both triggered and continous data loggers. The epicentral
distances to the events were 5 to 40 km. We recorded on Kozu
Island from July1 to 18 and on Kozu and Shikine Islands from August 9 to September 15. During these
periods there were two M6, twelve M5, and numerous M4 events that were well
recorded on our instruments. We use these data to look at details of the
rupture initiation of moderate size events. For events recorded at close
distances (less than 15 km) there are many clear waveforms that show the details
of the beginning of the P-wave. We compare these observations to theoretical results from crack models to estimate the size of the
initiation zone.
T51E-04
Fault Plane Determination of the M7.6 Volcano Islands Earthquake
Kim, A and J. Mori
We
determined the fault plane for the March 28, 2000 Volcano Islands earthquake
(Mw7.6) in the Izu-Bonin subduction
zone. This event at 100 km depth, is important for
understanding the stresses within the down-going slab. The focal mechanism
shows down-dip extension on either a nearly vertical or a nearly horizontal
fault plane. If the nearly vertical nodal plane is identified as the actual
fault plane, the slip along the fault plane is consistent with a sinking slab.
On the other hand, if the orientation of fault plane is nearly horizontal,
other stresses may be acting within the plate. To determine the actual fault
plane, we analyzed the P and P depth phases at teleseismic
distances (between 30 and 90 degrees) from 17 IRIS/GSN network stations. We
used a least-squares inversion method to determine the slip distribution on the
fault plane. We divided the fault plane into 135 subfaults
and multiple time windows to solve for the slip. Various
rupture velocities from 3.0 to 4.5 km/sec were tested to see which rupture
velocity and which fault plane produced the best fit to the data. It was
difficult to resolve the rupture velocity. For the range of rupture velocites we tested, slip on the horizontal plane fit the
data better than slip on the vertical plane. This result suggests that there
are regional stresses near this kink in the Izu-Bonin
trench that are conducive to horizontal faulting.
Furthermore, in the Volcano Island region the Pacific plate is subducting obliquely beneath the overlying plate. This may
cause bending in the slab which generates a complicated regional stress field.
2000
Seismological Society of Japan Fall Meeting
Tsukuba, November 20-22, 2000
B79
Initiation of the
Chichi, Taiwan Earthquake
Mori, J. and K.-f.
Ma
P064
Strong motion recordings in the Izu Islands, Japan: Observations of Earthquake Initiations
Sato, K. and J. Mori
2000
Western Pacific Geophysics Meeting
Tokyo, June 27-30, 2000
S51D-03
Slip Velocity Estimates of the 1999 Chi-Chi, Taiwan, Earthquake: New Observations
of Fault Dynamics
Mori, J and K.-F. Ma
We used the extensive near-field
strong-motion data along with teleseismic data
recorded for the 1999 Chichi, Taiwan earthquake (Mw 7.7) to study the
distribution of slip velocity on the fault. Using three-component data from 23
near-field stations and P waves from 22 teleseismic
stations we carried out a finite fault inversion to obtain the distribution of
slip and slip velocity on the fault plane. The dense distribution of stations
and close proximity to the fault allows us to make some of the best estimates
of slip velocity ever done for a large earthquake. We find that the near the
hypocenter the slip velocities are relatively low in the range of 0.3 to 0.7
m/sec. In the region of very large slip on the northern part of the fault we
find very high slip velocities up to several meters per second. Since slip
velocity is directly proportional to the dynamic stress drop, this region of
large slip has a large dynamic stress drop. We suggest that as the rupture
propagates toward the north, heat generated by the slip melts the fault zone or
pressurizes fluids, which reducing the friction stress, increasing the slip
velocity, and promoting the extensive sliding. Due to the dynamic rupture
behavior such as melting or fluid pressurization, the large slip lubricates the
fault zone, which is initially rather heterogeneous. This behavior smoothes the
roughness of the fault zone, allowing a fast and smooth
S21A-07
Rupture Process of the 1999 Chi-Chi, Taiwan Earthquake From Direct
Observations and Joint Inversion of Strong Motion, GPS and Teleseismic Data
Ma, K-F. and J. Mori
The strong motion stations along
the surface ruptures of the fault directly recorded the dynamic
rupture behavior of the 1999 Chi-Chi (Mw7.7), Taiwan earthquake.
The complete data set of this earthquake, including strong motion, GPS and teleseismic provide us with important information to understand
the rupture process of the earthquake from several viewpoints. The observations from the near-fault strong motion data provides
the most direct evidence on the rupture behavior of the fault. The joint
inversion of the combined data provides insight into the temporal and spatial
rupture process of the faulting. The results reveal the variations of spatial rotation
of rake angle and slip velocity over the fault area, which implies the possible
non-uniform pre-stress distribution and the evolvement of the rupture process.
The GPS and near source strong motion data reveal similar slip distribution
patterns; while the teleseismic data and velocity
records of strong motion data show similar slip distribution patterns. These
results imply that the large slip observed near the northern end of the fault
is rather localized and might not radiate as much energy as expected. The large
slip observed near the northern end of the fault has a large dynamic stress drop,
equivalent to its static stress drop. We suggest that there may have been fault
melting in this region to reduce the friction on the fault to very low levels.
S21A-08
Teleseismic and GPS Data Analysis of the
1999 Chi-Chi, Taiwan, Earthquake
Lee, S, K.-F.
Ma, J. Mori, S. Yu
We investigated 22 broadband teleseismic records and and
well-recorded near-field GPS data to understand the rupture process of the 1999
Chi-Chi, Taiwan earthquake. The teleseismic waveforms
are examined to determine the temporal and spatial slip distribution of the
1999 Chi-Chi, Taiwan earthquake. The analysis of the teleseismic
waveform inversion through different models consistently reveals an anomalous
large slip region centered about 40 to 50 km the north of the hypocenter at a depth
from 0 to 10 km. The largest slip is about 6 - 10 m. The large slip region
corresponds to the area where large surface displacements were observed on the
fault and large displacements were measured from GPS observations. The slip
near the vicinity of the hypocenter hypocenter is relatively
smaller. The seismic- moment obtained from the teleseismic
analysis is about 5-7x10**27 dyne-cm. The results also reveal significant
variations of rake angle during the rupture. Some strike-slip component to the
rupture was found in the middle portion of the fault. The slip distribution results
from GPS data shows a larger slip region and larger amounts of slip compared
with the teleseismic results. The difference between
the GPS and teleseismic data might imply that the energy
radiated by this earthquake is different for different frequencies. GPS data
reflect the static displacements, which are not recorded by the teleseismic data. However, the overall patterns of the slip
distribution and rake angle variations of these two data sets are quite
comparable.
S32B-07
Drilling the Chelongpu Fault, Taiwan: Observations
and Measurements of a Thrust-fault with Very Large Displacements and
Velocities in a Recent Earthquake
Ando, M , J. Mori, H. Tanaka, H. Ishii, M. Yamano, C.
Lee, K.-F. Ma, C. Wang, B. Huang, C. Lin, J. Wang, M.D. Zoback,
R. Ikeda
We propose to drill the northern
part of the Chelonpu fault to investigate the fault
properties and rupture characteristics of the recent large Chi-chi, Taiwan (M7.7)
earthquake of 1999. This earthquake presents the rare opportunity to probe an
active fault that has just had large fault movements, up to 8 m of vertical
surface displacement. Analyses of cores from the fault zone and heat flow
measurements will provide new insights into the important physical properties,
especially friction, that control the rupture of large
earthquakes. Such direct observations of a recent large earthquake have never
been made before. This project to drill the Chelongpu
fault will lead to results in the following research areas. (1) Improved
understanding of the whole fault zone structure and rock distribution for an
active thrust-fault. (2) Estimates of fault surface velocity at the time of earthquake
from core analyses. (3) Study of the mode of strain localization which results
in high velocity motion and thus heat generation by mechanical wearing along the
fault surface. (4) Study of fluid paths and permeability distributions in the
fault zone. (5) Study of thermal properties and constraints on friction for the
recent faulting process from heat-flow measurements. (6) High-quality seismic
recordings of aftershocks from installation of borehole seismic and deformation
instrumentation. (7) Study of earthquake triggering by carrying out a
water-injection experiment.
S51D-07
Estimating Source Dimensions of the 1998 Earthquake Swarm in the Hida Mountains Region, Central Japan, Using
Near-field Waveforms
Sato, K. and J. Mori
We estimated source durations for
larger events in a strong swarm that occured from
August 7 1998 in the Hida mountains region, central
Japan. We used broad band seismograms observed at the Kamitakara
observatory at distances of about 30 km. The events had seismic moments between
10**14 Nm to 10**17 Nm. To determine the source durations, we systematically
searched for the rise times of the source time functions which minimized the
difference between synthetic waveforms and observed data. In the data we could
clearly see the near-field contributions to the waveforms, so it seemed that
the durations were longer than expected for typical events, considering their
size and the distance to the station. We estimated the source durations (rise
time of source time functions) and source dimensions to study the source parameters
of these event. For calculating synthetic waveforms,
we used focal mechanisms determined by the FREESIA project and we assumed
source time functions of symmetric triangles. Source dimensions are determined
from these source durations under the assumption of circular faults and seismic
moments are estimated from maximum amplitudes. These results show that source durations
are longer and source dimensions are larger compared to most earthquakes with
the same seismic moments.
2000 Japan Earth
and Planetary Sciences Joint Meeting
Tokyo,
June 25-28, 2000
Sa-002
Observations on the Fault Dynamic
Rupture of the 1999 Chi-Chi (Mw7.7), Taiwan Earthquake
James Mori, K-F., Ma
Sa-015
Estimating Earthquake Source
Dimensions in the Hida Mountains Region Swarm using Near-Field
Waveforms
Sato, K. and J. Mori