Abstracts
2004 AGU Fall
Meeting
San Francisco, California December 13-17, 2004
T23A-0566
Temperature
Measurements and Active Faulting
Mori, J. and H. Ito
Temperature measurements associated with active faults can be useful for
studying the total energy balance, and especially the dynamic frictional levels
during faulting of large earthquakes. However, temperature anomalies across
faults or temperature changes associated with earthquakes are relatively rare.
We make some simple calculations to estimate the temperature changes that
should be observed across a fault for large earthquakes. For example, a
temperature profile at 500 m depth across a fault that slipped 2 meters, at a
time 6 months following the earthquake, shows a temperature anomaly of about
0.2 degrees, assuming an apparent coefficient of friction of 0.6. For an
apparent coefficient of friction of 0.3, the anomaly reduces to about 0.05
degrees. The differences in the apparent coefficient of friction should be resolvable
with the current temperature sensor instruments we are developing for borehole
measurements. Estimating the apparent coefficient of friction is important for
understanding the mechanics of faulting. The level of friction, and thus the
amount of heat produced during an earthquake, has been a controversial issue in
seismology for several decades. Timely measurements of the temperature profile
across the fault following large earthquakes may be able to answer these
long-standing questions about the level of dynamic friction.
S11A-0993
Relation
between Seismicity and Strain Rate in Japan
Tanimura, T. and J. Mori
We examine the relationship between occurrence rates of small earthquakes and
the crustal strain rates for the Japan Islands region. Recent GPS data from
GEONET, operated by the Geographical Survey Institute, provide good
measurements of the crustal deformation rates and the rates of small
earthquakes are well recorded by the regional seismic networks. Therefore,
there is good regional coverage of strain and seismicity rates across most of
Japan. We want to look at the microearthquake
occurrences associated with the secular rates of strain, so we eliminate
locations of large earthquakes and high seismicity associated with volcanic
events. For the crustal deformation rates, we calculated strain rate in a grid
of even intervals of 0.1 degree. We used the method of Shen
et al. (1996) which estimates horizontal displacement rate, strain rate and
rotation rate at each grid point from observed displacement rates of the GPS
stations. Then, we calculated dilatation rate and maximum shear strain rate
using the estimated horizontal strain rate. For the seismicity rates, we used
the JMA (Japan Meteorological Agency) hypocenter catalogue and estimated the
number of earthquakes (M$>$2) that occurred within a 15km radius of each
grid point at depths shallower than 20km. There is a large scatter in the plot
of seismicity rate as a function of strain rate, but if values of seismicity
rates are averaged over a range of strain rate values, some clear trends can be
seen in the results. Comparing the numbers of earthquakes with the strain
rates, we obtain the following results. Maximum shear strain rate and the
number of earthquakes show a positive correlation for rates of 0 to 90 nanostrain/year. However, above 90 nanostrain/year,
the number of earthquakes decreases with increase of the strain rate.
Similarly, there is an increase in the number of earthquakes with increase of
dilatation rate from 0 to -110 (negative dilatation rate indicates
compression), and earthquake numbers decrease for values of compressive strain
rate greater than -110. Thus, for both shear strain rate and dilatation rate,
the number of earthquakes increased with increasing strain rate, but above some
threshold strain rate, the numbers of earthquakes then decrease. The threshold
above which seismicity rates decline, may be an important factor in the
regional stress conditions that control the rates of small earthquakes.
S24A-01
Triggered
Events due to the 2003 Tokachi-oki Earthquake (Mw
8.1), Japan
Miyazawa,
M. and J. Mori
The 2003 Tokachi-oki earthquake, (Mw 8.1), one of the
largest recent earthquakes in Japan, affected the seismicity of small shallow
earthquakes within a few hundred kilometers and remotely triggered deep low
frequency tremors at farther distances of 1000 km or more. The earthquake
occurred offshore of southeast Hokkaido, where the Pacific plate subducts beneath the North American plate and large
earthquakes have recurrently occurred. After the main event in 2003, shallow
seismicity increased in the central area and decreased along the western coast
of the Hokkaido region. The changes of seismicity could be explained by Coulomb
failure stress changes ($\Delta CFF$). In the central area, where active
volcanoes are located, $\Delta CFF$ values were 0.1--0.2 MPa
if triggered earthquakes had mechanisms consisting of NW-SE compression on
right lateral strike-slip faults. Off the western coast, seismicity in the
aftershock area of the 1993 south-western Hokkaido earthquake (Mw 7.6)
decreased, where $\Delta CFF$ had negative values of about -0.005 MPa for the thrust mechanism of the aftershocks. In order
to detect the dynamically triggered events due to the seismic waves, we used
continuous velocity waveform data at 642 stations of the High Sensitivity
Seismograph Network (Hi-net) that covers most of Japan. The waveform data were
filtered with pass-bands of 5--20 Hz so that the local signals in the vicinity
of the observation stations could be detected. We constructed RMS envelopes of
the filtered waveforms for three components. Two statistical parameters were
adapted to quantitatively evaluate changes of amplitudes of 1000 sec envelopes
before and after arrivals of the body waves. We examined the commonly used
$z$-value and $\beta$-value to see the variant seismicity, substituting the
amplitude of the envelope for the earthquake frequency. These obtained values
indicated increases of the amplitude in three regions of western Japan, where
the travel distances were 1000 km or more. The increased tremors are thought to
be deep low frequency events, which occur at depths of 30--60 km near the subducting Philippine Sea plate. The transient stress
perturbations in the furthest region were on the order of 10**-3 MPa or more.
S54A-05
Relocations
and 3-D Velocity Structure for Aftershocks of the 2000 W. Tottori (Japan)
Earthquake and 2001 Gujarat (India) Earthquake, Using Waveform
Cross-correlations
Enescu, B. and J. Mori
The newly developed
double-difference tomography method (Zhang and Thurber,2003) makes use of both
absolute and relative arrival times to produce an improved velocity model and
highly accurate hypocenter locations. By using this technique, we relocate the
aftershocks of the 2000 Western Tottori earthquake (Mw 6.7) and 2001 Gujarat
(Mw 7.7) earthquake and obtain a 3D-velocity model of the aftershock region.
The first data set consists of 1035 aftershocks recorded at 62 stations during
a period of about a month following the mainshock
(Shibutani et al.,2002). In order to get the best arrival times a
cross-correlation analysis was used to align the waveforms. The epicentral distribution of the relocated events reveals
clear earthquake lineations, some of them close to the
mainshock, and an increased clustering. The
aftershocks' depth distribution shows a mean shift of the hypocenters' centroid of about 580m; a clear upper cutoff of the seismic
activity and some clustering can be also seen. The final P-wave velocity model shows
higher-value anomalies in the vicinity of the mainshock's
hypocenter, in good agreement with the results of Shibutani et al.(2004). The
second data set consists of about 1300 earthquakes, recorded during one week of
observations by a Japanese-Indian research team in the aftershock region of the
Gujarat earthquake (Sato et al.,2001). Using the double-difference algorithm
and waveform cross-correlations, we were able to identify a more clear
alignment of hypocenters that define the mainshock's
fault and an area of relatively few aftershocks in the region of the mainshock's hypocenter. Both studies demonstrate that the
cross-correlation techniques applied for events with inter-event distances as
large as 10km and cross correlation coefficients as low as 50% can produce more
accurate locations than those determined from catalog phase data. We are going
to discuss briefly the critical role of frequency filtering and of the time
window used for cross-correlation on the relocation results. To facilitate the data
processing tasks we have developed a GUI oriented, Matlab-based
toolbox.
S42B-04
Fluid
Activity Around the Downward Extension of the Seismogenic
Fault of the 2000 Western Tottori Earthquake Inferred From Deep Low-Frequency
Earthquakes
Ohmi, S., I. Hirose, J.
Mori
Low-frequency tremors were newly detected in the forearc
region of the Nankai and Cascadia
subduction zones recently. They are associated with
the subduction of the young plates and attributed to
the fluid activity around the plate boundary. On the other hand, there is
another example of low-frequency events in the backarc
region in southwest Japan that is associated with active faults. One example is
the western Tottori area, where we had a Mw=6.7 earthquake in 2000. It is an
unusual example because the seismogenic fault is
outlined by an intense aftershock activity, beneath which many deep
low-frequency (DLF) earthquakes were observed. DLF earthquakes were observed at
depths of around 30 km beneath the aftershock activity. A fault model derived
from the coseismic crustal movements (Sagiya et al., 2002) indicates that the DLF earthquakes are
located around the downward extension of the fault. The DLF events are
classified into three groups in features of the waveform. Type-1 are the most
commonly observed ones. One of them shows a single-force type source mechanism
(Ohmi and Obara, 2002).
Type-2 events have larger P-wave onsets compared to those of type-1 events.
Magnitudes of the type-2 events are slightly larger than those of type-1 events.
They have been observed since mid 2002. Assuming that type-2 events are caused
by shear faulting, we estimated the seismic moment and source dimension from
the source pulse. Relation between the source dimension and moment indicates
that the stress drop of the type-2 events are extremely low compared to those
of ordinary earthquakes. It suggests the existence of soft materials such as
fluid saturated gauge zone at the fault interface. Type-3 event is a
tremor-like event observed in April 2003. We examined the tilt data in the
region if the associated slip of the fault is observed. However, it was
difficult to detect the tilt change more than 1.0 \times $10^{-7}$ radian,
which is apparently equal to 1.3 cm slip on the fault model of Sagiya et al. (2002). As we described, observed features
suggest the fluid activity in the focal region of the DLF events and is also
supported by the seismic tomography analysis (e.g. Zhao et al., 2004). It shows
the existence of low velocity bodies in the focal region of the DLF events,
that reflects the fluid related to the dehydration process of the subducting Philippine Sea plate. Recent studies (e.g. Iio
and Kobayashi, 2002) proposed that the seismogenic
faults have downward extension in the lower crust, whose aseismic
slip accumulate stress on the seismogenic faults in
the upper crust and controls the occurrence of the earthquake. Hypocenters of
the DLF earthquakes discussed in this paper are distributed around the deeper
extension of the shallow aftershock distribution and probably located on the
downward extension of the seismogenic fault of the
Western Tottori earthquake. It is important to understand the nature of DLF
events beneath active faults, in relation to the behavior of fluids in the
lower crust that might affect the aseismic slip of
the downward extension of the seismogenic faults and
control the occurrence of the shallow crustal earthquakes.
2004
Seismological Society of Japan Fall Meeting
Fukuoka, October 9-11, 2004
A019
Temperature and Earthquake Faulting
Mori, J., H. Ito, O. Matsubayashi, Y. Kano, R. Fujio, S. Nakao, M. Touma
Temperature measurements
associated with active faults can be useful for studying the total energy
balance, and especially the dynamic frictional levels during faulting of large
earthquakes. However, temperature anomalies across faults or temperature
changes associated with earthquakes are relatively rare.We
make some simple calculations to estimate the temperature changes that should
be observed across a fault for large earthquakes. For example, the figure shows
a temperature profile at 500 m depth across a fault that slipped 2 meters, at a
time 6 months following the earthquake. The different curves are for various
values of normal stress, which correspond to values for the apparent
coefficient of friction. The differences in the curves should be easily
resolved with the current temperature sensor instruments. (See poster in this
session, Ι‘ et al., fwCMj^[Μ½ίΜΈ§·xvͺ). Estimating the
apparent coefficient of friction is important for understanding the mechanics
of faulting. The level of friction, and thus the amount of heat produced
during an earthquake, has been a controversial issue in seismology for several
decades. Timely measurements of the temperature profile across the fault
following large earthquakes may be able to answer these long-standing questions
about the level of dynamic friction.
A083
Relation between seismicity and
strain rate in Japan
Tanimura, T. and J. Mori
P031
A high precision temperature monitoring
system for an accurate estimate of the heat generated by faulting during a
large earthquake
Ito, H., O. Matsubayashi,
Y. Kano, R. Fujio, S. Nakao,
M. Touma, T. Yanagidani, J.
Mori
P118
Relocation and 3D velocity
structure for aftershocks of the 2000 W. Tottori earthquake using waveform
cross correlations
Enescu, B. and J. Mori
P149
Making recorded maximum intensity
maps (1560-2004)
Miyazawa, M. and J. Mori
P154
Source Parameters of May 29, 2004
South Korea Earthquake (ML5.2)
Park, S.-C. and J. Mori
P160
Deep Seismic Zone and Low-Frequncy Sources of Kyushu Region Redetermined
by 3-D Inversion
Morita,M. and J. Mori
P163
b-value and Asperity of the 1999
Chi-Chi, Taiwan earthquake
Fujio, R. and J. Mori
International
Conference in Commemoration of the 5th Anniversary of the 1999 Chi-Chi
Earthquake, Taiwan
Taipei, Taiwan, September 8-9, 2004
Energy
Budget of the 1999 Chichi, Taiwan Earthquake
Mori, J. and H. Tanaka
We use seismic data and borehole
temperature data to infer characteristics of the faulting process and the total
energy budget of the 1999 Chichi, Taiwan earthquake. A temperature profile from
a shallow borehole that penetrates the fault at about 300 m, shows an increase
across the fault zone of about 0.1oC. If we assume that this temperature
anomaly is a result of the frictional heat generated at the time of the
earthquake, we can calculate the amount of heat and the level of friction
during the faulting. Using a simple one-dimensional heat diffusion model, we
estimate the amount of heat that would locally produce the observed temperature
change, which was measured 16 months after the earthquake. Extrapolating the
local value to the entire fault plane, we estimate that earthquake produced a
total of 3.7x1016 J of frictional heat. These values of heat generation are
rather low and indicate a low value for the coefficient of friction (0.4).
Combining the value of frictional heat with other estimates for the radiated
energy (0.66x1016 J.) and fracture energy (0.5x1016 J.), we obtain the total
energy of the earthquake (4.9x1016 J.). These values give an average seismic
efficiency of about 19%.
Inversion of strong-motion data
and associated observations of surface faulting show that the northern portion
of the Chelungpu fault had very large displacements
in excess of 10 m, while the southern part of the fault had much smaller
displacements of 1-2 m. In contrast the levels of accelerations and
damage show a very different pattern, with much more severe high frequency
ground motions in the south and extensive damage to small buildings. In the
north the levels of acceleration and associated damage are relatively low,
considering the very large fault displacements. These differences in observed
ground motions may be attributed to differences in dynamic fault behavior
during the earthquake, with low friction esmoothf slip occurring in the north.
Asia
Oceania Geosciences Society
Singapore, July 5-9, 2004
57-OSE-A1677
Energy Budget of
the 1999 Chichi, Taiwan Earthquake
Mori,
J.
We examined the energy balance of
the 1999 Chichi, Taiwan earthquake (Mw 7.6) using several estimates of radiated
and thermal energy. Temperature measurements from a a
shallow borehole in the northern section of the fault show a temperature
profile that increase across a narrow fault zone at about 325 meters. We assume
this temperature increase was caused by frictional heating during faulting of
the earthquake (about 6 meters in this location). Thermal modeling gives an
estimate of the coefficient of friction of about 0.45. Using this
frictional value extrapolated to depth with higher normal stresses and a slip
distribution model, we estimate that the earthquake produced a total of about
3.6 x 10**16 joules of frictional heat. Adding the radiated energy (0.9 x
10**16 joules) and thermal energy gives a total energy of the earthquake
(neglecting the fracture energy) of 4.5 x 10**16 joules. This implies an
average seismic efficiency is about 15 to 20 %.
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, as is indicated by
the low coefficient of friction.
57-IWG-A1680
Large Earthquakes and Volcanoes in the New
Ireland/New Britain Region of Papua New Guinea
Mori,
J.
The area of New Ireland and New
Britain Islands in Papua New Guinea is a very active tectonic area that
contains a trench-trench-transform triple junction between the Pacific, Solomon
Sea and Bismarck Sea plates. There are numerous large earthquakes on the plate
boundaries and volcanic eruptions on New Britain. Recent volcanic activity
includes the 1994 eruption at Rabaul Caldera that
destroyed the town of Rabaul. This eruption was
preceded by dramatic seismic and deformation activity in the 27 hours before
the eruption. Observations of this activity led to a successful evacuation of
the populated areas around the volcano. In November 2000, there was a sequence
of strong earthquakes (M8.2, M7.5, M7.4) along the Weitin
fault and the nearby New Britain subduction zone.
Using teleseismic data, slip distributions were
calculated for these events. The inversion results showed large strike-slip
displacement in the area of southern New Ireland, where surface displacements
of over 5 m were observed. From the slip distribution the amount of static
stress change was estimated to investigate triggering mechanisms. The 2nd and
3rd events occurred in regions where there were increases in the static
stress. However, the static stress changes were small (0.03 to 0.13 MPa) so there are likely other equally important factors
(e.g. dynamic effects, levels of initial stress) for the triggering of these
earthquakes.
57-IWG-A1009
Investigating
Physics of Faulting: Taiwan Chelungpu fault Drilling
Project
Ma, K.-F. and J. Mori
2004 Japan Earth
and Planetary Science Joint Meeting
Makuhari,
Chiba, May 9-13, 2004
S044-010
Differences in the Initiation Area and the Large
Asperity of the 1999 Chi-Chi Taiwan Earthquake
Mori,
J., H. Ito, K.-F. Ma,
We study the relation between the
initiation and the area of the large slip (asperity) during the 1999 Chi-Chi,
Taiwan earthquake (Mw 7.6). The area of the initiation has relatively small
amounts of slip and the recorded strong-motion records show high levels of
high-frequency radiation. In contrast, the area of the largest slip (over 10
meters) occurs about 15 seconds later and is characterized by a smooth slip
that has a very fast slip velocity and generates much less high frequency radiation.
We use near-field strong-motion seismograms to model the slip behavior for the
two different regions and show that the differences can be explained by
differences in the level of dynamic friction. The area of large slip may have a
much lower level of dynamic friction during the rupture process. Such low
levels of dynamic friction may be characteristic of large asperities, and are
consistent with the observations of the fast, smooth slip observed on the
northern portion of the Chelungpu fault. Verification
of these inferences about the frictional properties of the fault may come from
observations collected by the Taiwan Chelungpu Fault
Drilling Project (TCDP). In this project a 1 km borehole is currently being
drilled into the area of large slip
on the Chelungpu fault to obtain physical samples of
the fault and measure its geophysical properties.
S044-003
Radiation
Efficiencies and Apparent Stresses of Small Earthquakes in a South African Gold
Mine
Yamada,
T., J. Mori,; S. Ide, H. Kawakata,
Y. Iio, H. Ogasawara, N. Sumitomo, International Research Group for
Semi-controlled Earthquake Generation Experiment at South African Gold Mine
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 (Figure 1). Many seismic events (M-2.7 to M3.3) 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 6 larger
events (M0.7 to M1.4) 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
the focal mechanisms, assuming various rupture velocities, in order to
distinguish the fault plane and the best-fitting rupture velocity. We could
determine the fault plane for five of the six events because the model fit to
the data for the fault plane was significantly better than for the auxiliary
plane (Figure 2). 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 not extremely low and 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.
We also calculated radiated
energies of the six events. They show that apparent stresses, or the ratios of
radiated energies to seismic moments, are constant compared with those of
larger natural earthquakes. Our result shows that radiation efficiencies and
apparent stresses of small earthquakes in the mine are equal to those of larger
natural earthquakes.
S045-P004
Dectection of Active Responses of the Crust
beneath Japan in 2003
M. Miyazawa and J. Mori
We investigated the active
response of the crust by examining the triggering of earthquakes from stress
changes of the Miyagi-oki earthquake (Mjma7.1) on 26
May and from stress perturbations due to wave propagation from the 2003 Tokachi-oki earthquake on 26 September.
1.
2003
Miyagi-oki earthquake (26 May)
The seismic activity
of shallow earthquakes in the northern part of Miyagi prefecture increased
immediately after the main event. Statistic-beta and z-value statistically
support the increases, which might be explained by the static triggering since
the Coulomb Failure Criterion (Delta_CFF) has postitive values on the order of 10e-3--10e-2MPa in these
regions. We can not discount the possibility that the
earthquakes were dynamically triggered in the region, where Delta_CFF
values are small. Subsequent earthquakes (Mjma5.6, 6.2, 5.4) occurred in the
area, where Delta_CFF had positive values, but there
was little increase in the seismic activity during the two months after the
event on 26 May. This suggests those earthquakes might have been statistically
triggered with a time delay.
2.
2003
Tokachi-oki Earthquake (September 26)
We studied active
response to the seismic waves from the Tokachi-oki
earthquake in 2003. We used Hi-net data recorded by NIED at about 700 stations,
and constructed RMS envelope waveforms, which were high-passed filtered to
detect radiated waves in the neighborhood of the station. We compared
amplitudes of the RMS envelope waveforms 1000 sec before and after the Tokachi-oki Earthquake. Large dynamic responses were found
around Shikoku, Kii peninsula, and the Tokai region,
where the epicentral distances are more than 1000km.
The increases in seismicity consist mainly of deep low frequency tremor (DLFT)
at depths of 30--40km. The stress changes produced by the teleseismic
waves were on the order of 10e-4--10e-3 MPa at
Shikoku. The seismic waves from the Tokachi-oki
earthquake in 2003 are shown to have remotely triggered the DLFT.