Abstracts
2003 AGU 2003
Fall Meeting
San Francisco,
California December 8-12 , 2003
T41E-08 INVITED
Source Process of the 1999 Chi-Chi, Taiwan Earthquake: Comparisons to
Shallow Faulting in Subduction Zones
Mori, J.
The 1999 Chi-Chi, Taiwan earthquake (Mw 7.6) occurred along a zone of rapid
convergence in central Taiwan. Since there were large amounts of slip at
shallow depths on the Chelungpu fault (8 to 10 meters
of thrust faulting at depths of a few kilometers to the surface), the
earthquake provides a first-hand opportunity to study many of the physical
properties associated with large slip on a fault. There has been a large
variety of geological and geophysical observations of this event, including, a
dense network of strong-motion instruments that recorded the earthquake, GPS
observations of the pre-, co- and post-seismic deformation rates, borehole
sampling into the fault, and surface geology studies. The combination of these
studies produces one of the most complete pictures of the source process of a
large thrust earthquake. Especially, the detailed imaging of the area of large
slip in the northern part of the fault provides information about the fault
dynamics that may control the large amounts of slip on a fault. The faulting
process appears very different between the large slip areas in the north and
the more moderate slip regions in the south. The large slip regions in the
north appeared to be associated with 'smooth' rupture, that
generated relatively low levels of high-frequency radiation and caused
relatively low levels of damage. This is in contrast to the more typical
high-frequency radiation produced by the fault in the south, which caused
considerably more damage, even though the amounts of fault slip were less. The
rather unusual characteristic of the large slip in the north may be associated
with low levels of dynamic friction on the fault. The thrust faulting of the
Chi-Chi earthquake may be similar to the shallow faulting in subduction zones, such as the Nankai
Trough. The geometry of the Chelungpu fault, which is
connected to more shallowly dipping fault structures at depth, is similar to
the splay faults that are mapped in the Nankai subduction zone. For the Nankai
trough, it is unclear if slip on the splay faults occurs during the great subduction earthquakes or in separate earthquakes. Part of
the answer to this question may come from the results of current projects, such
as the borehole sampling of the Nankai splay faults
and comparisons with similar borehole samplings of the Chelungpu
fault.
S52F-0176
Initiation Process of Moderate Earthquakes in the Northern Miyagi, Japan,
Earthquake Sequence.
Sato, K. and J. Mori
Scaling relationships between the durations of the beginning portion of P-waves
and final sizes of earthquakes have been reported by several studies (e.g. Iio
[1992]). These studies mainly targeted microearthquakes
and moderate to large earthquakes have been analyzed in only few studies. We
tried to determine whether such scaling relationships hold for larger events,
because it especially important for understanding the initiation processes of
large earthquakes. We analyzed several moderate (magnitude up to 5) earthquakes
and two large (magnitude 6 to 7) earthquakes and concluded that there may be a
scaling relationship for small to moderate events but the scaling probably does
not hold for the larger events. An earthquake sequence including several
moderate earthquakes (magnitude up to 6.2) began on July 26, 2003 (JST) in the
Northern Miyagi region, Japan. To study characteristics of initial phases of
the moderate earthquakes, we analyze 4 $<$ M $<$ 6.2 events in the
sequence. In this study, we employ the method which we used in the past study,
a source model of Sato and Kanamori [1999]. The
initial rupture of an earthquake is described with two parameters in this
model: trigger factor and initial crack size. The trigger factor represents the
contrast of surface energy on the edge of a crack and off of the crack. A large
value for the trigger factor means that very high surface energy exists only on
the edge and smaller surface energy off of the edge and rupture propagates
spontaneously. A smaller value means that similar values of
surface energy exists on and off of the edge and rupture propagates
gradually depending on the initial crack size. We can use clearly recorded data
from several near source borehole stations which are operated by Hi-net, NIED.
We analyze 11 events with magnitude between 4.0 and 6.2. Hypocenter locations
determined by the Japan Meteorological Agency and focal mechanisms determined
by F-net, NIED are used. For these events, we obtain the results that all
events can be explained by a small trigger factor and their initial crack sizes
are between 15 and 50m. We can see a scaling relationship that the largest
event (M6.2) has the largest initial crack size. However, we can also see that
similarly sized events began from various initial cracks.
S42C-0177
TI: Determination of Fault Planes and Rupture Velocities of Small
Earthquakes in a South African Gold Mine: Constraints on Radiation Efficiency
Yamada,
T., J. Mori, H. Ogasawara, Y. Iio, H. Kawakata, S. Ide
Analyses of source processes of small earthquakes are important for
investigating whether or not there are dynamic differences between small and
large earthquakes. However, it is difficult to resolve details of the source of
small earthquakes because close station spacing near the hypocenter and data
with high sampling rates are necessary. Such observations of mining induced
earthquakes are being carried out in a South African gold mine. Nine tri-axial
borehole accelerometers were installed within 200 m along a 2,650-m-deep
haulage tunnel in the Mponeng gold mine. Many seismic
events (-2.7 $<$ M $<$ 3.3) were recorded with a sampling frequency of 15
kHz from February to October, 1996. In this study we focused on the rupture
velocity, which is important for investigating characteristics of initiations,
arresting mechanisms, and radiation efficiency of earthquakes. We carried out
kinematic wave-form inversions for three larger events (M1.4, 1.1, and 0.8)
that occurred within 200 m of the stations. First, we determined the velocity
structure using arrival time data. Velocities of P and S waves were estimated
to be 6.00 km/s and 3.83 km/s, respectively. Next, we determined focal
mechanisms from amplitudes of P, SH, and SV waves. Finally we carried out
kinematic wave-form inversions for both nodal planes of focal mechanisms
assuming various rupture velocities to distinguish the fault plane and the
best-fitting rupture velocity. We could determine the fault plane for all
events because the model fit to the data for the fault plane was significantly
better than for the auxiliary plane. Also, the deduced fault planes were
consistent with the results of sub-event locations for these events by Yamada
et al. (2002). On the other hand, we could not determine the rupture velocities
with complete confidence. One problem is that in general, residuals are likely
to be smaller by assuming higher rupture velocities. However, we can conclude
that rupture velocities were not less than 50% of the S-wave velocity, on the
basis that the slower rupture velocities could not explain wave-forms very
well. Therefore, we conclude that rupture velocities of small earthquakes in
the South African gold mine are almost the same as those of larger natural
earthquakes. The radiation efficiency can be written as a function of the
rupture velocity and becomes greater with increase of the rupture velocity.
This study indicates that radiation efficiencies of small earthquakes in the
South African gold mine are almost equal to those of larger natural
earthquakes.
T51D-0188
Rupture Areas of Large Earthquakes In Papua New
Guinea in 1971 and 2000
Park,
S. and J. Mori
Two large earthquakes with magnitudes greater than 7.0 (Mw) occurred near New
Ireland and New Britain, Papua New Guinea, on November 16-17, 2000. Both
earthquakes were thrust events with shallow dip angles in the New Britain subduction zone. Also, another two large earthquakes (both
Ms 7.9) occurred in the same area on July 14 and July 26 in 1971, almost 30
years before the 2000 events. Aftershock zones of the 1971 events covered large
areas, including the fault zones of the 2000 events. In this study we estimate
the rupture areas of the 1971 and 2000 events using teleseismic
waveform inversion and investigate their inter-relationships. To understand the
rupture processes of the 1971 events, waveform inversions using Pdiff waves was carried out. Paper recordings of the Pdiff waveform data at seismic stations for distance
between 100 and 140 degree were scanned from microfilm and digitalized.
Synthetic seismograms were calculated with theoretical Green's Functions and
the best fit between observed and synthetic seismograms was obtained by a
least-square method. Fault mechanisms and rupture processes were obtained by
the waveform inversion. Results of the waveform inversion showed that both
earthquakes were thrust events having shallow dip angles. The July 14 event has
two areas with large slip and the July 26 event has a single area of large
slip. Teleseismic P wave inversions were previously
carried out to understand the rupture processes of the 2000 events. P-wave data
from stations at distance between 30 and 90 degree were retrieved from the IRIS
data center and used for waveform inversion. As the result of the waveform
inversion, both events seem to have fairly simple rupture patterns with a
single area of large slip. Based on the result of the waveform inversions, we
will discuss the relationships between the rupture areas of the 1971 and 2000
events, and their rupture mechanisms.
2003 Seismological Society of Japan Fall Meeting
Kyoto, October
6-8, 2003
A059
Swarm Activity of DLF Earthquakes
Beneath Western Tottori Region
Ohmi, S. and J. Mori
B030
Scaling of Radiated Energy for
Intermediate Depth Earthquakes
Mori, J. and A. Kim
P028
Estimate of Fault Planes and
Rupture Veloclties of Small Earthquakes in a South
African Gold Mine:
Constraints on Radiation Efficiency
Yamada, Y., J. Mori, H. Ogasawara, Y. Iio, H.
Kawakata, S. Ide,
The
Research Group for Semi-Controlled Earthquake-Generation Experiments in
Deep African Gold Mines
P030
Rupture Areas of Large
Earthquakes in Papua New Guinea in 1971 and 2000
Park, S. and J. Mori
P189
Initiation Process of Earthquakes
in Northern Miyagi
Sato, K. and J.Mori
23rd
General Assembly of the International Union of Geodesy and Geophysics
Sapporo, Japan,
June 30 - July , 2003
SS02/03A/A03-006
Frictional Heat and Radiated Energy Budget for 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 1.0x1016 joules. The static
stress drop from the total moment and the fault area is about 3 MPa. 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 x 106 joules per square
meter in the north and 4.5 x 106 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 1017 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 1017 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 3x1016 joules
and the seismic efficiency for the asperity is about 15%, which is much higher
than the average value for the whole earthquake.
JSS02/02A/D-006
Image of the Fault Plane and Crustal Structure for the 2001 Gujarag, India Earthquake from Aftershock Observations: A
Deep Crustal Event with High Stress Drop
Negishi, H., J. Mori, T.
Sato, S. Kumar, R. Singh, P. Bodin, B.K. Rastogi
A large earthquake (Mw 7.7) occurred in the state of Gujarat in northwestern
India on January 26, 2001, causing severe damage over a widespread area.About 20,000 people were killed and the total damage
amounted to about US$ 5.2 billion. Surprisingly, this crustal earthquake did
not have obvious surface displacements for the main fault, although the
magnitude is large. It is important to know the orientation and size of the
fault for evaluations of seismic hazards and understanding the tectonics of
this region.
We deployed 7 seismographs around the
damaged area from February 28 through March 6, 2001, and recorded waveforms
from over 1,300 events. Hypocenters were determined using the Joint Hypocenter
Determination program, which simultaneously calculates hypocenters, station
corrections and a one-dimensional velocity structure (e.g., Kissling
et al., 1994). Our aftershock locations show a trend that dips toward the south
at about 50 degrees which is interpreted as the fault plane of the mainshock. The depth range of the aftershocks is from 10 to
35 km, which somewhat deeper than other crustal earthquakes, and indicates that
the faulting did not reach the surface. The area of the fault is about 40 km x
40 km, which is small for a Mw 7.7 earthquake and
results in a high static stress drop of 13 to 25 MPa.
There are no mapped faults or obvious topographic features along the surface
projection of this fault. These findings show that very large damaging
earthquakes can occur without producing surface faulting.
A
tomographic inversion for the 3-D velocity structure
was also carried out. Because the resolution of the inversion depends on the
number of data and ray coverage, we used the merged data set collected by the
National Geophysical Research Institute of India, Center for Earthquake
Research and Information, Memphis University and our data in this analysis. We
tried to determine optimal damping parameters quantitatively by using the
Cross-Validation Technique. Our 3-D velocity model shows that the aftershock
distribution corresponds to the high velocity anomalies. Low Vp/Vs anomalies are generally found at depths of 10 to 35
km, i.e. the depth range of the aftershock distribution. However, relatively
high Vp/Vs and low Vs characterize the deeper region
below the hypocenter of the mainshock, at depths of 30
to 40 km. This anomaly may be due to a weakly fractured and fluid filled rock
matrix, which might have contributed to triggering this earthquake. This
earthquake occurred on a relatively deep and steeply dipping reverse fault with
a large stress drop. Theoretically, it is difficult to slip such steep faults.
Our tomographic investigation may provide
information, which helps explain why this thrust event could occur at such
depths in the crust.
SS01/03A/D-003
Deep Low-frequency Earthquake Associated with Active Faults in
Central-Southwest Japan
Ohmi, S. and J. Mori
Deep low-frequency (DLF) earthquakes have been discussed in association with
fluid (magma) activity around volcanoes. In this paper, however, we will show
several examples of DLF activity beneath active faults in central - southwest
Japan which do not have volcanoes in their near vicinity.
On October 6, 2000, a
Mw6.7 crustal earthquake occurred in western Tottori prefecture, southwest
Japan. Beneath the focal region of the earthquake, DLF earthquakes were
observed at depths of around 30 km. Five DLF earthquakes were detected during
the 3 years before the mainshock and one occurred 9
hours before the mainshock. The focal mechanism of
the DLF earthquake that occurred 9 hours before the mainshock
was analyzed by using amplitude ratios of the S-waves to the P-waves and
polarization patterns of the S-waves (Ohmi and Obara, 2002).The result indicated that a single-force
source mechanism is more preferable than a double-couple source mechanism,
which suggests the transport of fluid, such as water or magma. More than 100
DLF earthquakes, most with magnitudes up to about M1.5, were observed during
the 2 years after the mainshock. There are a few
larger DLF earthquakes (M>2.0), that have source time durations considerably
longer than those expected from scaling relations between source dimension and
seismic moment for normal earthquakes. This indicates that the source
mechanisms of these DLF events are quite different from that of ordinary
tectonic earthquakes.
In central Kyoto prefecture, southwest Japan, DLF earthquakes have been
observed since 1978. They are located below the Mitoke
Fault system. Continuous shallow swarm activity is also observed in the region.
There are no volcanoes in the area. In central Japan, DLF earthquakes were
mainly concentrated in the Hida Mountain range. One
prominent area of activity is near the northeastern part of the Atotsugawa fault, where the fault enters the region of
volcanoes in the Hida Mountain range. Mt. Tateyama, which is an active volcano, is located 10 km from
the focal region of the DLF′s. In all the regions, the maximum magnitudes of
the DLF events are slightly larger thanM2.0, and the focal depths ranges from
25 km to 35 km.
Seismic
tomography analysis (e.g. Zhao et al., 2000) indicates the existence of a
low-velocity body at depths from the lower-crust to the upper-mantle in the focal
region of these DLF′s. The distribution of DLF activities, described above,
corresponds well to the low-velocity region rather than to the distribution of
volcanoes. The occurrence of DLF events is probably direct evidence of fluid
activity below the seismogenic zone, where fluids
might be injected from the lower-crust beneath active faults.
SS02/04A/D-013
Triggering of Large Earthquakes in the 2000 New Ireland, Papua New Guinea
Sequence
Park,
S. and J. Mori
A major earthquake (Mw8.0), followed by two more large events (magnitudes
greater than Mw7.0), occurred near New Ireland, Papua New Guinea, on November
16-17, 2000. Since the events happened within two days and their hypocenters
are located within 250 km, this is one of the best opportunities to study
triggering of large earthquakes. The occurrence of the first event has a large
rupture and significantly alters the stress field in the local area. In this
study we calculate the static Coulomb stress changes resulting from these
earthquakes and investigate how these stress changes may trigger the following
large earthquakes.
Teleseismic P
wave inversions were carried out for the three events to understand the source
processes and to obtain the slip distributions. P-wave data retrieved from the
IRIS data center were used for the inversion. Using the data and synthetic
seismograms, we carried out an inversion process to search for the best fitting
fault mechanism, slip distribution, and total seismic moment for each event.
Using the slip distribution of the first event, Coulomb stress changes for the
location and orientation of the fault plane of the second event were
calculated. We also calculated the Coulomb stress changes for the fault plane
of the third event using the slip distributions of the first two events.
The rupture process of the first event
shown by the slip distribution seems very complex according to the teleseismic P wave inversion. The fault does not appear to
be a pure strike-slip event, but has some component of dip-slip. There is an
area of large shallow slip that is consistent with the observed surface
displacements of about 5 meters on southern New Ireland. Details of the slip
distribution of the first event and the depth of the second event make it
difficult to say clearly if the second event initiated in an area of stress
increase or decrease caused by the first event. The third event clearly
occurred in an area which showed a decrease of Coulomb stress change from the
first and second events. So static stress changes do not
provide a good explanation for the triggering of this third event.
2003 Japan Earth and Planetary Science Joint Meeting
Makuhari,
Chiba, May 26-29, 2003
S044-011
Absolute value of stress prior to the 2000 Tottori earthquake
Mori, J. and K. Yoshikawa
The well-determined focal mechanisms of aftershocks of the 2000 Western Tottori
earthquake show a bimodal distribution in the direction of P-axes. We interpret
these two directions as representing the local stress field before and after
the mainshock. At the time of the earthquake, the
slip on the fault causes a local change in the stress field, which is reflected
in the rotation of the focal mechanism. Using this change in direction along
with the calculated stress change due to slip during the mainshock,
we can estimate the absolute value of the local stress field before and after
the mainshock. For the direction of the pre-mainshock stress field, we assume east-west compression,
which is consistent with the focal mechanisms of the mainshock
and moderate earthquakes that occurred during the previous several years. For
the direction of the post-earthquake stress field, we use the focal mechanisms
of aftershocks that show a rotation of 15 to 20 degrees that is different from
the pre-earthquake direction. Our results show that the level of stress prior
to the mainshock was relatively low, on the order of
5 to 20 Mpa at the locations of the aftershocks. The
fact that we see any rotation of the stress field, is
an indication that the level of stress is rather low. For some of the
aftershocks, the level of stress after the earthquake,
is higher than the level before the earthquake. This is consistent with the
idea that some aftershocks occur in regions where stress is increased due to
the fault slip during the mainshock.
S044-P011
Complexity in Earthquake Initiation Process
Sato, K. and J. Mori
In
the recent years, earthquake initiation process and its scaling relationship
has been often studied. One of the studies performed by Iio [1992,1995] indicated that there is a scaling relationship in
duration of a slow initial phase that appeared at a beginning of the P-wave.
Sato and Kanamori [1999] showed a model to explain
the slow initial phase in which model initial crack size of an event scaled
with its final earthquake size. Using this model, some studies (e.g. Sato and
Mori [2002], Hiramatsu et al. [2002]) estimated
initial crack sizes and discussed their scaling relationships. Sato and Mori
[2002] analyzed moderate to large earthquakes and concluded that scaling
relationships appeared in microearthquakes continued
to M4 events, but it did not appear in larger events (M6-7). They removed
events with 'complex' waveforms (or chose 'simple' waveforms), although all of
the larger events have 'complex' waveforms. This may cause the break of the
scaling. In general, while larger earthquakes have complex rupture processes,
small events can have both simple and complex rupture processes, and we may
conclude that larger earthquakes than a certain size often have complex rupture
pattern. In this study, we analyze frequency distribution of multiple shock
events which have complex rupture pattern and its scaledependency.
We use Hi-net waveform data for events in the JMA catalog between June 2002 and
January 2003. In this data set, we can use 146 events with magnitudes larger
than 3. In addition, we calculate sizes of the first subevents
and initiation related parameters for multiple shock events and discuss scaling
relationship of earthquake initiation process using the first subevent sizes.
S044-P012
Rupture
Velocities of Small Earthquakes in a South African Gold Mine: Constraints on
Fracture Energy
Yamada,
T., J. Mori, H. Kawakata, H. Ogasawara, S. Ide, M. Yoshimura, International Research Group for
Semi-controlled Earthquake Generation Experiment at South African Gold Mine
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 with magnitudes between -2.7 and 3.3 were recorded with a sampling
frequency of 15 kHz from February to October, 1996. We carefully picked 3
events with magnitudes 1.4, 1.1, and 0.8 having good azimuthal
coverage and analyzed the waveforms to try to determine rupture velocities.
The
studied events 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 2.10 to 3.01 km/s. These values are
from 55 to 78 % of the shear-wave velocity and consistent with those of larger
natural earthquakes. This result suggests that the ratios of fracture energies
to radiated energies of small earthquakes in a South African gold mine are not
particularly large and almost the same as those of larger natural earthquakes.
S045-P011
The Mj5.3 Earthquake at Central Tottori
Prefecture (Sep. 16, 2002) and the Seismicity of the San-in District
Nakao, S., H. Katao, J.
Mori, T. Shibutani, K. Watanabe, K. Ito
Major earthquakes (more than M6) along the coastline of the San-in district
have individually NW-SE trend of aftershock distribution. Only the 1943 Tottori
earthquake (M7.2) shows aftershock distribution of E-W direction. Precise
aftershock distribution is important to know the geometry of the mainshock rupture. During 1980-1989, seismic activities
along the San-in district migrated from east to west. During 1990-2000, active
seismicity was observed around western Tottori prefecture. Then, the Western
Tottori Earthquake (Mj7.3) occurred in 2000. After the Western Tottori
Earthquake, seismicity in the eastern Shimane prefecture, east side of Mt Daisen and the northern Hyogo prefecture are activated. On
September 16 in 2002, a Mj5.3 earthquake occurred at
central Tottori prefecture. The epicenter is located at east foot of Mt. Daisen. This area is western end of the aftershock zone of
the 1943 Tottori earthquake. Numerous number of aftershock occurred following
the mainshock. Aftershock distribution shows E-W
linear trend which is consistent with the focal mechanism of the mainshock. Also from the analysis of the directivity of the
waveform, it is confirmed that the mainshock rupture
is E-W direction. But 1 or 2 days after the mainshock,
the aftershock area extended to South, and formed another E-W trend line.
Finally entire aftershock distribution shows Z-shape. Total aftershocks are
located in the area of 4km x 3km. Depth range of the hypocenter is 3-11km.
Focal mechanisms of the major aftershocks are strike-slip, and the P-axis
directions are nearly NE-SW.
S053-P019
Seismic tomographic
imaging of subsurface structure of Rabaul volcano
Tanimura, T. and J. Mori
We carried out a tomographic inversion to determine
the three-dimensional P-wave velocity structure of Rabaul
Caldera, Papua New Guinea. This is an active volcano that has had two eruptions
during the 20th century, including the 1994 eruption that destroyed Rabaul Town. The three-dimensional velocity structure helps
to identify the magma system that underlies the
caldera. We used 3756 P-wave arrival times from 455 earthquakes recorded at the
Rabaul Volcanological
Observatory (RVO) permanent network of 12 stations. Added to this, 1854 P-wave
arrival times from marine seismic shots fired in the sea areas around Rabaul as part of the Rabaul
Earthquake Location and Caldera Stracture (RELACS)
program were used. The gird intervals were 1.5km in the horizontal direction
and 1km in the depth. Grids were selected to cover Rabaul
caldera (total 10x10x6 nodes). Checkerboard tests showed a good resolution
within the caldera to 4km depth. A low-Vp zone
stretching from north to south at center of caldera was found at 1-3km depths.
The location of this low-Vp zone is consistent with
ground uplift deformation area for the period 1973-1985. This suggests that the
low-Vp zone is a magma chamber. This low-Vp zone is located at the center of the caldera (not
beneath the active vents that produced recent eruptions). This implies there
are narrow pathways from the large magma reservoir in the center of the caldera
to the active vents on the outer ring fault.
S075-018
Activity of Deep Low-Frequency Earthquakes in the Western Tottori Region
Ohmi, S. and J. Mori
On October 6, 2000, a Mw6.7 crustal earthquake
occurred in western Tottori prefecture, southwest Japan. Beneath the focal
region of the earthquake, DLF earthquakes were observed at depths of around 30
km. Five DLF earthquakes were detected during the 3 years before the mainshock and one occurred 9 hours before the mainshock. The focal mechanism of the DLF earthquake that
occurred 9 hours before the mainshock was analyzed by
using amplitude ratios of the S-waves to the P-waves and polarization patterns
of the S-waves (Ohmi & Obara,
2002). The result indicated that a single-force source mechanism is more
preferable than a double-couple source mechanism, which suggests the transport
of fluid, such as water or magma. More than 100 DLF earthquakes, most with magnitudes
up to about M1.5, were observed during the 2 years after the mainshock. There are a few larger DLF earthquakes, whose
magnitudes are greater than 2.0, that have source time durations considerably
longer than those expected from scaling relations between source dimension and
seismic moment for normal earthquakes. This indicates that the source
mechanisms of these DLF events are quite different from that of ordinary
tectonic earthquakes.