Abstracts
2002
AGU
Fall Meeting
San
Francisco, California, December
6-10, 2002
S71E-09
Energy Budget of the 1999 Chichi, Taiwan Earthquake
Mori, J and H. Tanaka
We
examined the energy balance of the 1999 Chichi, Taiwan earthquake (Mw 7.6)
using several estimates of radiated and thermal
energy. Estimates of radiated energy from regional seismograms give a value of
about 1.0x10**16 joules. The static stress drop from the total moment and the
fault area is about 3MPa. Temperature measurements from 2 shallow boreholes in
the northern and southern sections of the fault show temperature profiles that
increase across the narrow fault zone. If we assume this temperature increase
was caused by frictional heating during faulting of the earthquake, thermal
modeling gives the results that the fault generated 2.5 x10**6 joules per
square meter in the north and 4.5 x 10**6 joules per square meter in the south.
If these frictional values are extrapolated to depth, using
higher normal pressure, we estimate that the earthquake produced a total
of about 2 x 10**17 joules of frictional heat. Adding the radiated and thermal
energy gives a total energy of the earthquake (neglecting the fracture energy)
of about 2.1 x 10**17 joules. This implies an average seismic efficiency is
about 5%. The average energy values for the earthquake can be quite different
from the energy balance on smaller portions of the fault. For example, most of
the radiated energy is generated by a large asperity on the northern part of
the fault, which has an area that is about 20% of the whole fault surface. For
this region of large slip, it has been suggested that the dynamic friction may
be very low. If we use a value of 0.2 for the
coefficient of friction, which is consistent with the borehole temperature
data, the thermal energy for region of the asperity will be about 3x10**16
joules and the seismic efficiency for the asperity is about 30%, which is much
higher than the average value for the whole earthquake.
S71B-1096
Relationship Between Regional Strain and Microseismicity in Japan
Yoshikawa, K and J. Mori
It
is generally thought that the level of activity of small earthquakes is related
to the regional stress or strain field. In this study, we compare the observed
strain field in Japan with the rates of shallow crustal earthquakes, to see how
well the microearthquake activity correlates with the
regional strain field. With the installation of over 1000 continuous GPS
stations of GEONET in Japan, we are able to observe the deformation field to a
resolution of several tens of kilometers. We used the average horizontal
displacement rates measured over the last 5 to 7 years at each station. The
average rates were calculated by fitting linear trends to the data with periods
of large earthquakes or other large deviation removed.
These displacement rates were interpolated onto a 50 km grid and horizontal
strains were calculated for the region of the Japanese Islands. Using
earthquake locations from the Japan Meterological
Agency catalog, we calculated yearly rates of shallow earthquake activity for
the same grid and same time period that was used for the GPS observations. For
the comparisons with the strain field, we tested several depth ranges from 10
to 30 km and magnitude thresholds of M1.0 to M3.0. We compared the rates of earthquake activity with the rates of maximum
shear strain for the region covering the 4 major islands of Japan. Areas
that had the very highest strain rates 1x10**-6, such as the region of inferred
dike injection near Kozushima
had the highest rates of seismicity. However, in the strain rate range of 10**-8 to 10**-7, which includes over 90% of the
data, there were no clear correlations between strain rate and earthquake
activity. Using different depth ranges and magnitudes of earthquakes or varying the grid the size did not produce
significantly different results.
S72B-1143
Rupture Velocities of Small Earthquakes (0.0 < M < 1.5) in a South
African Gold Mine:Constraints on Fracture Energy
Yamada, T, J. Mori, H. Kawakata, H. Ogasawara, S. Ide, S. Tanbo
Analyses
of rupture velocities of earthquakes are important to investigate
characteristics of fracture energies, initiations, and arresting mechanisms.
But it is especially difficult to resolve rupture velocities of small earthquakes because close station spacing near
the hypocenter and high sampling rates are necessary. Such observations
are being carried out in a South African gold mine for mining induced earthquakes. Nine tri-axial borehole accelerometers
were installed within 200 m along a 2,650-m-deep
haulage tunnel in the Mponeng gold mine. More
than 25,000 seismic events (-2.7 < M < 3.3) were
recorded with a sampling frequency of 15 kHz from February to October,
1996. We carefully picked 10 events with
magnitudes between 0.0 and 1.5 having good azimuthal
coverage and analyzed the waveforms to try to determine rupture
velocities. The events studied have rather complicated waveforms and individual subevents
could be identified. Arrival times of the subevents
were picked relative to the initial arrival. These differential arrival times
were used to locate the subevents relative to the
initial hypocenter. Approximate rupture
velocities could be obtained by dividing the distance to the subevent by the delay time.
We obtained results that showed rupture velocities ranging from 2.34 to 2.70
km/s for earthquakes of magnitude from 0.0 to
1.5. These values are about 70 % of the shear-wave velocity and consistent with those of larger natural earthquakes.
This result suggests that if the static stress drops of these earthquakes
are the same as those of natural earthquakes, the ratios of fracture energies
to radiated energies of small earthquakes in a South African gold mine
are not particular large and almost the same as those of larger natural
earthquakes.
S71C-1115
The Initial Rupture of the 2000 Western Tottori Earthquake
Hirata, M, Y. Umeda, J. Mori, H. Kawakata
Two
clear P phases were identified on the seismograms for the 2000 Western Tottori
earthquake (M7.3). Following the initial P phase (P1), a larger arrival (P2)
several seconds later indicates that the rupture did
not grow smoothly, but has at least 2 subevents.
Hypocenters corresponding to these two P arrivals were determined using 14
stations within hypocentral distances of 60 km. The
initial rupture started from the edge of a
region that has had swarm activity, including M5 events, since 1989. The source
of the second larger P phase was located 1km
deeper and 5km southeast from the initial rupture. The hypocenter of the second rupture was off the fault plane estimated from
the mechanism solution of the initial rupture using the P1 phase first
motions. The average time interval between the two phases was 2.5 second, which
is consistent with an empirical relation between
the magnitude and duration time for initial ruptures. According to rupture
model of this earthquake determined by Sekiguchi and
Iwata, little slip occurred near the starting
point, and 3 seconds after, larger slip occurred 5km southeast of the starting
point, corresponding to our location of the source of second rupture. In
this study, we tried deriving the rupture process
for the spatiotemporal slip distribution of the first few seconds of the
rupture of this earthquake by using a waveform inversion. We divided the
area near the initial hypocenter into 10 x 6 subfaults
with dimensions of 250 x 250 m2. For the
inversion, we used 3-component data from 12 KiK-net
and K-net stations data integrated from
acceleration to velocity and bandpassed filtered from
0.1 to 3.0 Hz. Green_fs functions were
calculated using a 1-dimensional velocity structure that was determined from
the temporary aftershock observations. We used 12 time windows spaced at 0.05
sec intervals Our results show that the area of
the initial rupture area was about 0.5 km2 and extended toward the southeast
from the initial hypocenter. There does not appear to be much slip in the
area between the location of the initial rupture and the location of the sudden
increase in slip 2.5 seconds later.
The
2002 APEC Symposium on Confronting Urban Earthquakes and Seismic Early Warning
Academia Sinica,
Taipei, Taiwan,
November 28-29, 2002
Strong Ground Motions in Urban
Areas
Mori, J.
2002
Seismological Society of Japan Fall Meeting
Yokohama, November 11-13, 2002
B45
Energy Balance of the 1999 Chichi Taiwan Earthquake
Mori
J. and H. Tanaka
P120
Possibility
of Triggering Mechanism in Three Major Papua New Guinea Earthquakes in 2000
Park, S. and J. Mori
P124
Rupture Velocities of Small Earthquakes in
a
South African Gold Mine: Constraints on Rupture Velocity
Yamada,
T., J. Mori, H. Kawakata, H. Ogasawara, S. Ide, S. Tanbo, The Research Group
for Semi-controlled Earthquake Generation Experiments
in South African Deep Gold Mines
P129
The Initial Rupture of the 2000 Western
Tottori Earthquake - The Source Process of Initial Rupture-
Hirata,
M., Y. Umeda, J. Mori, H. Kawakata
P136
Relationship between Microseismicity and Regional
Strain in Japan
Yoshikawa,
K. and J.
Mori
5th
Taiwan-Japan Joint Seminar on Natural Hazards Mitigation
Tainan, Taiwan,
October 28-29, 2002
Seismological Results from the
1999 Chichi Earthquake: New Insights into Large-scale Faulting
Mori,
J.
2002
Japan Earth and Planetary Science Joint Meeting
Tokyo, Japan, May 27-31, 2002
S042-006
Estimates of the Attenuation in the Philippine Sea Plate slab Using Strong
Motion data from the 2001 Geiyo Earthquake
Mori, J.
The intensity distribution of the
2001 Geiyo earthquake shows an asymmetric pattern
that is elongated toward the east. This effect may be due to the lower values
of attenuation for waves that travel through the Philippine Sea plate. In this
study we look at profiles of strong-motion records of the mainshock
to estimate the Q for a path through the subducting
slab and a path that does not pass through the slab. We use the borehole data
from KiKNet. The amplitudes of the strong motion data
for the path along the subducting plate decay with
distance much slower that the other path, indicating a higher value of Q. From
the spectral amplitudes at 0.2 to 10 hz,
we estimate that the average Q of the subducting slab
is three times than the surrounding mantle.
S040-P006
Estimates of Radiated Energy for Shallow and Intermediate-depth Earthquakes
beneath Northeastern Japan
Kim A.I. and J. Mori
I studied radiated seismic energy
for shallow (5-21km) and intermediate-depth earthquakes (64-156km) to
understand the differences of source parameter
scaling. In this study, I analyzed 32 shallow (MJMA 3.6 - 5.4) and 37
intermediate-depth events (MJMA 3.6 -6.5), that
occurred in northeastern Japan from June 1996 to December 2001, and which were
recorded by at the K-net and Freesia network
stations. Because the Q structure is different at shallow and
intermediate-depths, propagation effects were obtained
separately for the two depth ranges. Also site response is considered to be a
function of incident angle,so
it was also examined separately for the two depth ranges. The results of the
estimation of radiated energy show that intermediate-depth earthquakes have
nearly constant energy to moment ratios. This is different from shallow events,
which from previous studies are reported to have increasing energy to moment
ratios as a function of moment. Comparing radiated energies for the two depth
ranges at large moments, those of the shallow events are somewhat higher than
intermediate-depth events, but the apparent stresses are almost the same. This
implies that the seismic efficiency of intermediate-depth events is lower. For
deep events more energy may go into heat and fracture formation.
S086-006
Tomographic velocity model for the
aftershock region of the 2001 Gujarat, India Earthquake
Negishi, N., S. Kumar, J. Mori, T. Sato
We conducted filed seismological
observations in the damaged area of the 2001 Gujarat, India Earthquake during
the period from February 28 to March 6. 8 temporary stations were operated and
totally 1434 hypocenters were determined. Aftershocks were distributed as a
plane dipping to south, and the depth range is 10 km to 35 km. It indicates
that the fault plane does not reach to surface. A tomographic
investigation has been applied to the aftershock data, both P and S arrivals,
to clear the fine velocity structure of this region. We used the resultant
hypocenters obtained by the Joint Hypocenter Determination Method as initial
hypocenters. The velocity model that used in National Geophysical Research
Institute of India was modified and used as an initial model. The analysis
technique is based on a grid-modeling tomography method by Zhao et al. (1992). Vp, Vs and hypocenters are detemermined simultaneously. It is very important to
investigate structure and rheology of seismogenic zone in this area, because this earthquake
occurred at very deep, as lower crust. Therefore we tried to use the Extended
Information Criterion (EIC) incorporation the bootstrap statistics for
determining an optimum model in the seismic tomography. This approach has been
applied to some tomographic study (e.g., Nishizawa and Kei, 1995; Aoike et
al., 1998). The resultant Vp
and Vs structure show similar pattern at the all depths. It seems that the
aftershock distribution is corresponds to the boundary between high- and
low-velocity heterogeneities. Relatively small Vp/Vs is found generally at the depth of 10 to 40 km. In Japan, Vp/Vs is relatively larger at
middle to lower crust, and it is considering that earthquake can occur hardly
at the depth range. The fault plane of this earthquake is, however, at the
middle to lower crust. Our tomographic investigation
becomes a piece of
information for explaining why this event occurred at such depth
in the crust.
T043-P012
Rotations of the 0T24 Mode Observed in Japan
Nakanishi,
T. and J. Mori
It has been known that the local
structure can affect high-frequency surface waves. On the other hand, it is
expected that the structure will not have a large effects on the lower
frequency (less than 0.003Hz) waves, and large changes of the arrival direction
will not be observed. If this is true, the direction of oscillations of the
0T24 mode should always correspond to the transverse direction at the station.
In this study, We look for 0T24 rotations by examining
data sets aligned in various directions using a spectral analysis. 24 hours of
data beginning at the time of large earthquakes are used. The rotation of the
waveforms is done clockwise and in 10 degrees increments from 0 (N/S) to 170
degrees. The peaks of the 0T24 mode are clearly identified in the spectrum of
each direction, and the direction that has the maximum amplitude of the 0T24
mode is determined. The direction of the 0T24 oscillations is often shifted
from the transverse as a result. This fact indicates that the long-period
(about 300 sec) component of the seismic waves arrived at the station from a
path off the great circle. The direction of the particle-motion for a single
Love wave packet also shows the same rotation that is observed in the spectral
analysis of 24 hours of data at the same station. The pattern of the shifts of
the 0T24 directions across Japan is complicated, but there are clear regional
trends. According to the results for several large events at three stable
stations, FUJ, SGN, and TTO in the Tokai region, the shifts depend on the paths
from the epicenters. For instance, when an epicenter is to the south (or the
north) of these stations, the 0T24 directions hardly show any shift, but there
are large rotations for earthquakes arriving from the northeast. The large
shifts in the 0T24 polarization are likely due to the regional structure.
2002
Japan-Taiwan Joint Seminar on Earthquake Mechanisms and Hazards
Nagoya University, January 27-28, 2002
Estimate
of Absolute Stress from Rotations of Focal Mechanisms
Mori, J. and K. Yoshikawa
We use a new technique to
estimate the level of absolute stress before an earthquake from the changes in
the orientation of focal mechanisms. This method was first applied to the
2000 Tottori-ken Seibu earthquake (Mw6.6) in western Japan. The
aftershocks of this event showed a clear bi-modal distribution for the focal
mechanisms. There is one set of aftershocks that show mechanisms similar to the
strike-slip faulting of the mainshock. The other set
shows a clockwise rotation of 10o to 20o. We interpret this bi-modal pattern as
an indication of the rotation of the local stress field caused by the mainshock. Adapting a technique presented by Spudich,
(1992), we can use the rotation of the
focal mechanisms along with the calculated stress change from the earthquake to
estimate the initial value of shear stress before the earthquake. The
results we obtain indicate that the stress levels near the fault prior to the
earthquake were about 20 MPa.
We would like to apply this
method to the focal mechanism data before and after the Chichi, Taiwan
earthquake to estimate the stress levels in Central Taiwan.