abstracts

Abstracts

5^th KMA/METRI 2005
International Workshop on the Fundamental Research for Mitigating Earthquake Hazards
Jeju Island, Korea, December 19-20, 2005

The Current Earthquake Prediction Program in Japan (Invited)

Mori、J.

The earthquake prediction research in Japan is an extensive program that includes many aspects of earthquake observations, data analyses and theoretical studies. The program was largely reorganized following the 1995 Kobe earthquake, which caused severe damage in Japan and was not predicted. Since that time, there has been more emphasize within earthquake prediction studies to understand the physical mechanisms of earthquakes and their effects. There were also installations of dense seismometer and GPS arrays that make Japan the best seismically monitored country in the world. Some of the exciting new results of these obervations include various types of slow slip events on the subduction plate boundary and the discovery of deep low-frequency tremor. There has also been a greater effort in evaluting the likely shaking damage from future earthquakes, with scenario studies of potential large earthquakes and a nation-wide probabilisitic ground motion map.

The goal of actually predicing earthquakes remains elusive. There have been some recent large earthquakes in Japan, such as the M8.0 2003 Tokachi-oki earhthquake and the M6.8 2004 Niigata Chuestu earthquake, which have been well recorded on the high-quality seismic and GPS arrays. However, there were no obvious signs of precursors that could be used to predict these events. Previous studies have suggested that there might be some large precursory slip on the fault zone before large subduction earthquakes, however no such movements were observed prior to the Tokachi-oki earthquake.

Another issue of earthquake prediction research is investigating how similar are the earthquakes that recur on the same portion of a fault. Many efforts in long-term earthquake forecasting are based on the idea that repeating earthquakes are similar and have similar regions of large slip (asperities). However, recent data from the Miyagi Prefecture region of Japan and other areas of the world indicate that the simple idea of characteristic earthquakes may not be appropriate for estimating the future occurrences of large earthquakes.

The 2004 Sequence of Triggered Earthquakes off the Kii Peninsula, Japan
Park, S.-C. J. Mori

2005 AGU Fall Meeting
San Francisco, California, December 5-9, 2005

S43A-1060
Using Near-Field Seismogras to Estimate the Slip Weakening Distance
Mori, J.

Accurate estimates of the slip weakening distance, Dc, are usually difficult to make because a reliable determination of the slip time history is needed. Usually these time histories are obtained for numerous subfaults from the inversion of local seismograms. Instead of using the model results of finite fault inversion, I make measurements directly on near-field seismograms that are located very close to the fault. Recently, there are have been several large earthquakes with good recordings within a few kilometers of shallow faults, such as the 1999 Chi-Chi, Taiwan earthquake. I assume for stations that are located very close to the fault, the waveforms are dominated by the near-field terms from the portion of the fault closest to the station, if that portion has significant slip. The slip weakening distance is identified as the slip corresponding to the maximum slip velocity on the fault. Results from the Chi-Chi earthquake for stations in the north where the slip was large, show that the peak velocity is reached within about 2 seconds of the beginning of slip, and corresponds to a slip of about 230 cm. This estimate of the slip weakening distance is significantly less than some previous determinations using results from the model slip functions. For stations in the south, where there was considerable less slip on the fault, the slip weakening distance was about 100 cm. For cases where near-field data is recorded very close to faults with large amounts of slip, this method may provide a reliable way to measure the slip weakening distance.

S13B-0192
What excites deep low-frequency earthquakes?
Miyazawa, M. and J. Mori

We observed that large surface waves from the 2004 Sumatra-Andaman earthquake (M9.2) triggered deep low-frequency (DLF) tremors beneath Shikoku, the Kii peninsula and the Tokai regions in western Japan, where Philippine Sea plate subducts beneath Eurasian plate. Since the source distances are more than about 5000 km, this is a clear example of dynamic triggering. We investigate the relationship between the surface waves and high-frequency components (4-16Hz) of the observed records. During the arrivals of the surface waves, pulse-like features were observed in the high-frequency waveforms, which are identified as DLF tremors. To investigate what phase has triggered the tremor, we determined the hypocenters of the triggered events by use of a modified envelope method. Accurate locations of the triggered earthquakes are important so that we can compare the strain changes in the immediate source area of the triggered events. The tremors are located at depths of about 30 km, where DLF events have been observed. Also, the relationship between the amplitudes of Rayleigh waves and the event magnitudes shows a good correlation. We found the triggering can be correlated with both the phase and amplitude of the arriving Rayleigh waves, but could not find significant excitation during arrivals of S-, SS-, and Love waves. This observation indicates that the triggering is due to volumetric strain changes, and the triggering amplitudes are larger than 10-9 in the source region.

S13B-0206
Rupture Velocity of the 2002 East China Deep Earthquake (Mw 7.3)
Park, S., and J. Mori

Recent studies on deep earthquake have suggested that deep earthquake source properties vary with the temperature of the subducting plate. Earthquakes in cold slabs have high aftershock activity, high rupture velocity and high seismic efficiency, while events in warm slabs have low aftershock activity, low rupture velocity and low seismic efficiency. These contrasts of the source properties seem to be distinguishable in very low (~2000 km, warm slab) and very high (~10000 km, cold slab) thermal parameters (The thermal parameter is the product of the velocity of subduction and the age of the plate). The thermal parameter of the Pacific slab (~6000 km) is in the middle of the two representative values of warm and cold slabs, and the source properties of deep earthquakes do not show clear characteristics. As first step toward understanding the source properties of deep earthquakes in the Pacific slab, we investigated the rupture velocity of the 2002 East China deep earthquake which occurred at about 560 km depth on June 28, 2002. The moment magnitude (Mw) is 7.3. This earthquake shows a strong directivity from northeast to southwest. Using waveforms of F-net stations of NIED (National Research Institute for Earth Science and Disaster Prevention), Japan with azimuths of 86 ~ 188°, the time differences between the P arrival and a large later arrival were measured. Then we calculated the relative azimuth, dip and distance to the hypocenter. The relative azimuth, dip and distance were 248°, -18° (or 18°) and 16.3 km. With these results, the rupture velocity was obtained as 1.6 km/s. This result of rupture velocity will be a good constrain for investigating the source process and the source properties of this deep earthquake.

S13B-0205
Fault Plane Determination for the 1964 Niigata Earthquake Using Tsunami Simulations
Morita, M. and J. Mori

The 1964 Niigata earthquake (Ms 7.5) occurred off the Japan Sea coast of Honshu, Japan, with a 4 m tsunami that caused significant damage. The earthquake is important for understanding the seismic hazards along the Japan Sea coast and also for understanding the tectonic boundary between the Eurasian and North America plates. This earthquake was located fairly close to the coast, yet the orientation of the fault plane has not been clearly determined. The focal mechanism has been well constrained to be a thrust solution striking approximately north, with one nodal plane dipping steeply toward the west and the other dipping at a low angle toward the east (Hirasawa, 1965; Abe, 1975; Mori and Boyd, 1985). In this study, we calculate sea bottom deformation for assumed models with fault plane dipping toward the west and toward the east, and simulate tsunami wave caused by these deformations. We used teleseismic P waveforms to determine the slip distribution for the earthquake. We assumed two fault planes for the earthquake. Fault 1 has a strike of 9°E and a dip of 25°to the east, and Fault 2 has a strike of 189°E and a dip of 65°to the west. To estimate slip distributions on these assumed fault planes, we inverted the waveforms from 10 stations, well distributed in azimuth around the earthquake. Using the derived slip distribution for each fault plane, we calculated the ground (sea bottom) deformation by the method of Okada (1992). This deformation was used for the tsunami simulation. Our results show that the large area of subsidence, fairly close to the Honshu coast, in the model for Fault 2 (westward dip) is very similar to the tsunami generation area obtained by back-projecting the tsunami arrival times (Iida, 1968). Also, the relative arrival times of the tsunami for the close stations along the Honshu coast compared to the arrivals at Sado island, match the model for Fault 2 better than for Fault 1. There is limited tsunami waveform data for the close stations, but we were able to examine the polarity of the initial tsunami wave at the station, Matsugasaki. The observed data shows a positive first motion which is consistent with the model for Fault 2. The model for Fault 1 (eastward dipping) shows a downward first motion. These results indicate that the fault plane for the 1964 Niigata earthquake dips toward the west.

S53B-1100

Seismicity Cycle of Large Earthquake Occurrence Zones
Itaba, S., K. Watanabe, K., J. Mori

It is known from trench excavation surveys that large earthquakes rupture the whole brittle zone (earthquake occurrence zone) of the crust occur repeatedly [e.g. Ohnaka et al., 2002] on active faults. For inland active faults, after the occurrence of a large earthquake, the aftershocks usually continue for several to tens of years, decreasing with time [Watanabe, 1989]. In the region of the 1891 Nobi Earthquake, it is known that aftershock activity has been continuing for more than a hundred years. Aftershock activity generally fits very well the Omori formula [Omori, 1894] which describes the aftershock occurrence in time [e.g. Yamashita, 1987]. Following the high level of aftershock activity, the earthquake cycle moves into a phase of much lower seismic activity [Toda, 2002], and then the next major earthquake occurs. For the inland active faults, the seismicity cycle is thousands to tens of thousands of years. For plate boundary, it is usually tens to hundreds of years. The duration of a cycle widely varies between various faults and regions. The period of observations for modern seismology is about a hundred years., and is only a small portion of one cycle. Therefore, it is difficult to recognize what portion of the earthquake cycle the current seismic activity corresponds to. For many active faults, however, the current stage of the cycle can be estimated from geological data. By combining both geological data and current seismicity for many different faults, we can view a complete seismicity cycle. We evaluated quantitatively the seismic activity of 98 major active faults in Japan and 17 segments of the San Andreas Fault System from the relation between the lapsed time from the last large earthquake, and the present seismic activity. Although there is large variation in the scales and recurrence times of faults, we developed a method to correct for these factors. We find a clear correlation between the lapsed time from a large earthquake and the present seismic activity. After the occurrence of large earthquakes, the seismic activity decays smoothly inversely proportional to the lapsed time for over a thousand years. Since this decay follows the modified Omori formula, we interpret the observations as indicating that the aftershock decay lasts for almost the entire earthquake cycle.

S43A-1052

Apparent Stress and Rupture Speed of Small Earthquakes in a South African Gold Mine : Constraints on Fracture Energy

Yamada, T., J. Mori, S, Ide, H. Kawakata, Y. Iio, H. Ogasawara

Nine tri-axial borehole accelerometers were installed within 200 m along a 2,650-m-deep haulage tunnel in the Mponeng gold mine in South Africa. We analyzed the high sample rate recordings (15 kHz) to determine source parameters of small earthquakes in the mine. We analyzed radiated seismic energies and static stress drops of 28 earthquakes (0.0 < M < 1.4) that occurred within 200 m of the stations to investigate their apparent stresses. Apparent stresses of the 28 events were from 0.2 to 3 MPa and static stress drops were 0.71 to 29 MPa. These values are similar to those for larger (M > 6) earthquakes. To study the source processes, we also carried out multiple time-window waveform inversions for the five largest events (0.8 < M < 1.4) among the 28 earthquakes. From the inversion results, we could determine the fault planes for all five events and estimate the range of rupture speed. We can conclude that rupture speeds were faster than 2.5 km/s (65 % of the shear wave velocity). The radiation efficiency is written as a function of the rupture speed and becomes greater with increasing rupture speed. 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 found that the source parameters (apparent stress, rupture speed, radiation efficiency, and static stress drop) did not largely differ from values for larger earthquakes. This suggests that the dynamic rupture processes of these small events were similar to those of the larger earthquakes.

T51A-1322

Fault zone Temperature Measurements to Search for a Thermal Anomaly in the Chelungpu Fault, Taiwan

Fujio, R., J. Mori, H. Ito, T. Yanagidani, Y.Kano, S. Nakao, K. Nishimura, M. Toma

The energy released when earthquakes occurs can be divided into three types, fracture energy, radiated energy, and frictional heat. It is important to know the amount of frictional energy, when we try to understand how earthquakes occur. So, we are measuring the temperature near the fault zone of a recent large earthquake, to try to find a heat anomaly. A heat anomaly will enable us to estimate the frictional energy produced by the earthquake. This is one of the first efforts to directly measure the heat produced by a large earthquake. 2.Observations We are investigating the 1999 Chichi earthquake in Taiwan. The moment magnitude of this event is 7.6 and there was about 8 m slip on the fault near the measurement site. We installed thermometers in a borehole that penetrates the Chelungpu fault at a depth of about 1110m, and are currently measuring the temperature near the fault zone starting from early March 2005. In the observation, we use 5 platinum resistance thermometers and 2 quartz crystal oscillator thermometers. The temperature sensors are spread over a distance of about 30 m in the vicinity of the fault. 3.Measurement Results The data from the platinum resistance thermometers are connected to a computer on the surface so that we can monitor the temperature measurements. When the system works well, the accuracy of the data is about 0.02 degrees. We are continuously monitoring the temperature over time to estimate the stability of the temperature and look for any temporal effects that remain from the drilling. We should be able to observe a frictional heat residual from the earthquake, if it is larger than about 0.1 degrees. Theoretical calculations indicate that 5 years after the earthquake, a temperature anomaly of about 0.2 to 0.6 degrees; should remain, assuming a range of values for the apparent coefficient of friction. Therefore, the resolution of our measurements should be accurate enough to detect a temperature anomaly.

KAGI21 3rd International Symposium
Wuhan, China, November 8-10. 2005

Source Inversion of the 1976 Tangshan Earthquake
Mori, J. and S. Park

We study the source process for the destructive 1976 Tangshan earthquake (Ms7.6). This surface wave magnitude seems slightly small, so we will determined the seismic moment by looking at long-period P waves. We use teleseismic P waves and Pdiff waves to observe the earthquake and look for the subevents and complexity in the rupture process. Since many of the paper records are clipped for the P wave, we use the core diffraction wave, Pdiff. This wave has smaller and longer period amplitudes, but still can be used to observe the source.　These waves are especially useful for observing large earthquakes, since the amplitudes are small and there is a long time window before the arrival of other phases, such as PP.The amplitude and frequency attenuation for the Pdiff waves are calibrated using recent deep earthquake that are well recorded on broadband stations in Japan. The calibration study shows that the shape of the waveforms is consistent over a distance range from 110 to 120 degrees. We have used Pdiff waves in previous studies to determine the slip distribution of other large earthquakes. The results are similar to results obtained for the direct P waves, but there is less spatial and time resolution.
For the 1976 Tangshan earthquake, we digitize the paper seismograms recorded at teleseismic distances on long-period instruments of the World Wide Standard Seismogram Network (WWSSN). The slip distribution is determined with a least-squares inversion of the waveforms, using green functions calculated for the teleseismic distances. For the estimates of slip of a shallow earthquake, there is usually good resolultion of the depth that comes from the depth phases pP and sP. Also, the spatial resolution for the slip is usually on the order of about 10 to 20 km. We will show the results of the source inversion for this large earthquake.

2005 Seismological Society of Japan Fall Meeting

Sapporo, October19-21, 2005

C041

Estimates of Slip-Weakening Distance from Near-field Seismograms
Jim Mori

Accurate estimates of the slip weakening distance, Dc, are usually difficult to make because a reliable determination of the slip time history is needed. Usually these time histories are obtained for numerous subfaults from the inversion of local seismograms. Instead of using the model results of finite fault inversion, I make measurements directly on near-field seismograms that are located very close to the fault. Recently, there are have been several large earthquakes with good recordings within a few kilometers of shallow faults, such as the 1999 Chi-Chi, Taiwan earthquake. I assume for stations that are located very close to the fault, the waveforms are dominated by the near-field terms from the portion of the fault closest to the station, if that portion has significant slip.
The slip weakening distance is identified as the slip corresponding to the maximum slip velocity on the fault. Results from the Chi-Chi earthquake for stations in the north where the slip was large, show that the peak velocity is reached within about 2 seconds of the beginning of slip, and corresponds to a slip of about 230 cm. This estimate of the slip weakening distance is significantly less than some previous determinations using results from the model slip functions. For stations in the south, where there was considerable less slip on the fault, the slip weakening distance was about 100 cm.
For cases where near-field data is recorded very close to faults with large amounts of slip, this method may provide a reliable way to measure the slip weakening distance.

P038

Fault Model for the 1964 Niigata Earthquake Using Tsunami Simulation
Morita, M. and J. Mori

P055

Occurence Patterns of Foreshocks to Large Earthquakes in Japan
Hiura, H. and J. Mori

P107

Probable Mechanism of Deep Low-Frequency Tremors Suggested by Periodical Triggering from the 2004 Sumatra-Andaman Earthquake
Miyazawa, M. and J. Mori

P108

Rupture Velocity of the 2002 East China Deep Earthquake (Mw7.3)
Park, S.C. and J. Mori

P134

Fault Zone Temperature Measurements to Search for a Thermal Anomaly in the Chelungpu Fault,Taiwan
Fujio, R., J. Mori, H. Ito, T. Tanagidani, Y. Kano, S. Nakao, K. Nishimura, M. Toma

PM09

Comparison of the 1936 and 2005 Earthqukaes off shore Miyagi Prefecture from seismograms
Kanamori, H., M. Miyazawa, J. Mori

Geological Society of Japan
Kyoto, Sep. 2005

S5- Invited Talk
Seismological Issues for the Taiwan Chelungpu Fault Driling Project (TCDP)
Mori, J.

The Taiwan Chelungpu Fault Drilling Project (TCDP) provides one of the first opportunities to directly study a fault with large displacement from a recent earthquake (1999 Chi-Chi, Taiwan earthquake, Mw 7.6). The borehole penetrates the fault at a depth of about 1100 m in a region where there was 8-10 m of fault slip. Since the earthquake was well recorded by a dense network of strong-motion instruments, the overall rupture process for this earthquake has been well determined. Combining the seismological observations with the direct sampling of the fault region will provide new information about the faulting dynamics of large earthquakes.

One main target of the research is to investigate the very fast (high fault slip velocity) and ‘smooth’ (lack of high frequency radiation) for the very large fault displacements in the region of the borehole. This is an important subject that is applicable to the general understanding of how fault slip occurs during large earthquakes. To explain the rupture process for large slip during earthquakes, we need to understand the frictional properties of the fault. At present there is very little observational data which can address this issue. From TCDP we hope to obtain estimates of physical parameters, such as the fault width, fault roughness, fluid permeability, which will influence the dynamic frictional during the earthquake.

Another approach to estimating the frictional level during the earthquake, is to measure the heat generated from the faulting. Even several years after the earthquake, there may be a temperature anomaly close to the fault, which represents the residual heat from the frictional heating (Figure 1). We are currently carrying out temperature observations near the fault in one of the boreholes, to look for this temperature anomaly. Estimates of the frictional levels during the earthquake are the key information for understanding the faulting mechanisms, e.g. fault melting, thermal pressurization, fault lubrication.

In general, the results from fault-zone drilling (Tawian, Nankai, San Andreas) will enable better understanding of the earthquake process. The role of fluids in earthquake faulting is one new field that can now be studied with these observations. Detailed investigations of the physical properties of faults, combined with seismological observations of earthquakes, should tell provide information about the initiation process and dynamic rupture process for large earthquakes.

APRU/AEARU Research Symposium 2005 Earthquake Hazards around the Pacific Rim - Prediction and Disaster Prevention -
Kyoto, August 31-September 2, 2005
　

S2 (Invited)

Temperature Measurements and Earthquake Heat
Mori, J.

Faulting during large earthquakes should produce significant amounts of heat and raise the temperature in the vicinity of the fault. The amount of heat depends on the coefficient of friction and the levels of stress. This issue about the amount of heat generated by large faults has long been the discussed for faults such as the San Andreas fault. For the San Andreas fault, there appears to be no strong heat flow anomaly, and there is still debate about what this implies for the value of the coefficient of friction and the level of stress. Although the frictional heat generated by the earthquake is thought to be about 80 to 90 percent of the total energy, there has never been a direct estimate of the heat for a large earthquake.

The 1999 Chi-Chi, Taiwan earthquake (Mw7.6) produced large displacements on shallow portions of the Chelungpu fault and provides the rare opportunity to actually measure the amount of frictional heat produced on the fault. 15 months following the earthquake, a borehole was drilled into the fault at a depth of about 325m. A temperature profile in the borehole shows a small temperature increase across the fault of about 0.1oC. We interpret this temperature anomaly as the residual heat generated during the earthquake. Using a simple heat conduction model, we can estimate the amount of heat produced in this region of the fault at the time of the earthquake that would cause this temperature increase. If this value is extrapolated to the whole fault, the total frictional heat generated by the earthquake is estimated to be 3.7 x 10**16 joules.

In addition to the frictional heat, we determine the total energy for the earthquake. The energy partition of the earthquake can be divided into three main parts, radiated energy, fracture energy, and frictional heat. From seismic waves a radiated energy of 0.7x1016 joules was determined by Venkataraman (2002). From an estimate of the rupture velocity, we obtain a ratio of the radiated energy to the sum of the fracture energy plus the radiated energy (radiated efficiency) of 0.6. This ratio with the estimate of radiated energy, gives a fracture energy of 0.4x1016 joules. Adding the three energy values together gives a total energy of 4.8x10**16 joules for the earthquake.

P113

Quantifying the Early Aftershock Activity of the 2004 Niigata Chuetsu Earthquake (Mw6.6)

Enescu, B., J. Mori, T. Shibutani, K. Ito, Y. Iio, M. Miyazawa, T. Matsushima, K. Uehira

The occurrence rate of aftershocks is empirically well described by the Modified Omori formula: n(t) = k/(t+c)p, where n(t) is the frequency of aftershocks per unit time, at time t after the mainshock, and k, c and p are constants. The parameter c relates to time when the aftershock power-law decay begins, and typical values range from 0.5 to 20 hours. Often the c-value is attributed to difficulties in counting the number of aftershocks immediately following the mainshock.

To determine the c-value for the 2004 Niigata Chuetsu sequence, we first examined the JMA catalog, but it is clear that there are many events missing during the first hour immediately following the mainshock, so it is difficult to reliably estimate the c-value. To estimate the number of early aftershocks, we examined the continuous seismograms recorded at six Hi-Net stations located close to the aftershock distribution. These stations have a large dynamic range so it is possible to extract the occurrences of the early aftershocks. The waveforms were high-pass filtered at 7Hz to attenuate the low-frequency mainshock coda. Clear aftershocks can be identified on the filtered waveforms, starting at about 35 sec. after the mainshock. Most of these early events are not recorded in the JMA catalog. We then select two representative stations, situated on opposite sides and close to the aftershock distribution, and count the events with amplitudes above some threshold value which can be clearly seen on their continuously recorded waveforms. This threshold corresponds to about a magnitude 3.3 earthquake. The number of identified events satisfies well the Modified Omori law, with a p-value close to 1.0. The c-value is estimated to be about 100 sec, i.e. this is the delay following the mainshock when the power-law decay of the aftershocks begins.

By checking the completeness levels of the events, we think that the non-zero c-value that we obtained is not due to detection problems of the early aftershocks. The delay of about 100 sec. before the ’normal’ aftershock sequence begins may be an important observation for understanding the occurrence of aftershocks.

P110

Fault model for the 1964 Niigata earthquake Using Tsunami Simulations

Morita, M. and J. Mori

The Niigata earthquake (Ms = 7.5) occurred on June 16, 1964, off the Japan Sea coast of Honshu, Japan. This is a significant earthquake that caused 26 deaths and caused a large amount of shaking and liquefaction damage in the Niigata region. There was also a 4 m tsunami along the Honshu coast that caused significant damage. The earthquake is important for understanding the seismic hazards along the Japan Sea coast and also for understanding the tectonic boundary between the Eurasian and North America plates, which passes through this region. This earthquake was located fairly close to the coast where seismic stations were operating and ground deformation was measured, yet the orientation of the fault plane has not been clearly determined. The focal mechanism has been well constrained to be a thrust solution striking approximately north, with one nodal plane dipping steeply toward the east and the other dipping at a low angle toward the west (Hirasawa, 1965; Abe, 1975; Mori and Boyd, 1985). In this study, we calculate sea bottom deformation for several assumed models with fault plane dipping steeply toward the east and shallowly toward the west, and simulate tsunami wave caused by these deformations. Comparing results of simulation with the observed tsunami data, we try to determine which the orientation of the fault plane for the earthquake.

P128

Fault zone Temperature Measurements to Search for a Thermal Anomaly in the Chelungpu Fault, Taiwan
Fujio, R., J. Mori, H. Ito, T. Yanagidani, Y. Kano, S. Nakao, K. Nishimura, M. Toma

1. Objective

2. Observations

We are investigating the 1999 Chichi earthquake in Taiwan. The moment magnitude of this event is 7.6 and there was about 8 m slip on the fault near the measurement site. We installed thermometers in a borehole that penetrates the Chelungpu fault at a depth of about 1110m, and are currently measuring the temperature near the fault zone starting from early March 2005. In the observation, we use 5 platinum resistance thermometers and 2 quartz crystal oscillator thermometers. The temperature sensors are spread over a distance of about 30 m in the vicinity of the fault.

3. Measurement Results

The data from the platinum resistance thermometers are connected to a computer on the surface so that we can monitor the temperature measurements. When the system works well, the accuracy of the data is about 0.02℃. We are continuously monitoring the temperature over time to estimate the stability of the temperature and look for any temporal effects that remain from the drilling. We should be able to observe a frictional heat residual from the earthquake, if it is larger than about 0.1 ℃. Theoretical calculations indicate that 5 years after the earthquake, a temperature anomaly of about 0.2～0.6℃ should remain, assuming a range of values for the apparent coefficient of friction. Therefore, the resolution of our measurements should be accurate enough to detect a temperature anomaly.

P107

Noncharacteristic Rupture of the Asperities of Repeating Large Earthquakes along the New Britain Trench
Park, S.-C. and J. Mori

Some previous research has suggested that the size and location of an asperity remain the same over repeated earthquakes. In order to investigate the locations of asperities of large earthquakes that rupture repeatedly the same fault, we derived slip distributions of five large earthquakes along the New Britain Trench in 1971, 1995 and 2000. We obtained slip distributions by Pdiff waveform inversions for the two 1971 earthquakes and teleseismic P wave inversions for the 1995 and 2000 events. We then compared the slip zones and show the asperity distribution. The asperities did not show significant overlap although the slip zones suggest that the same portions of the subduction boundary ruptured in several of the earthquakes. The hypocenters can be located in the area of asperity or far from asperity. Also the sizes of asperities seem to vary. These facts support the idea that asperity is noncharacteristic from cycle to cycle.

Asia Oceania Geosciences Society
Singapore, June 20-24, 2005

SE15/2A-02-2/208
Temperature Measurements in the Taiwan Chelungpu-Fault Drilling Project
Mori, J., H. Ito, R. Fujio, Y. Kano, K.-F. Ma

Measurements of the temperature changes associated with earthquakes can be useful for studying the total energy balance, and especially the dynamic frictional levels during faulting of large earthquakes. However, such temperature anomalies across faults or temperature changes associated with earthquakes have rarely been observed. The Taiwan Chelungpu-Fault Drilling Project (TCDP) offers the opportunity to make temperature observations at about kilometer depth across a fault that recently had a large amount of slip (about 8 meters). We are using platinum resistance and quartz crystal oscillator thermometers that have an accuracy of about 0.01 degrees or better. The current temperature measurements are being carried out in Hole-A. Simple calculations indicate that there may still be a temperature anomaly of a few tenths of a degree, even 5 years after the earthquake. From an observed temperature anomaly, we hope to be able to determine the apparent coefficient of friction for the faulting process. 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.

Chapman Conference on Radiated Energy and the Physics of Earthquake Faulting
Portland, Maine, June 13-17, 2005

Energy Budget of the Chi-Chi, Taiwan Earthquake
Mori , J. and H. Tanaka

We estimate the total energy for the 1999 Chi-Chi, Taiwan earthquake (Mw7.6). The energy partition of the earthquake can be divided into three main parts, radiated energy, fracture energy, and frictional heat. From seismic waves we determine a radiated energy of 0.7e16 joules. From an estimate of the rupture velocity, we obtain a ratio of the radiated energy to the sum of the fracture energy plus the radiated energy (radiated efficiency) of 0.6. This ratio with the estimate of radiated energy, gives a fracture energy of 0.4e16 joules. In a temperature profile taken across the fault at a depth of 300m, 18 months after the earthquake, we see a residual temperature anomaly. We interpret this temperature anomaly as the residual of the frictional heat generated at the time of the earthquake. Simple modeling of this temperature profile gives an apparent coefficient of friction of 0.35. If we applying this value to the entire fault, we estimate the frictional heat generated by the earthquake was 3.7e16 joules. Adding the three energy values together gives a total energy of 4.8e16 joules.

2005 Japan Earth and Planetary Science Joint Meeting

Makuhari, Chiba, May22-26, 2005

J113-P015
Triggering deep low frequency tremors in Japan from the great Sumatra-Andaman earthquake
Miyazawa, M. and J. Mori

Large surface waves from the 2004 Sumatra-Andaman earthquake triggered deep low frequency (DLF) tremors beneath Shikoku, Kii peninsula and Tokai regions in Japan. Since the source distances are more than about 5000 km, this is a clear example of dynamic triggering. We investigate the relationship between the surface waves and high frequency components (4-16Hz) of the observed records. During the arrivals of the surface waves, pulse-like features were observed in the high-frequency waveforms, which are identified as DLF tremors. The excitations seem to occur periodically and can be associated with the phase and amplitudes of the surface waves. Such clear timing of the earthquake occurrences can give us information about the triggering mechanisms for these events.

S044-012

Apparent stress and rupture speed of small earthquakes in a South African gold mine

Yamada, T., J. Mori, S. Ide, H. Kawakata, Y. Iio, H. Ogasawara, S. Norihiko International Research Group for Semi-controlled Earthquake Generation Experiment at 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 in South Africa. We analyzed the high sample rate recordings (15 kHz) to determine source parameters of small earthquakes in the mine. We analyzed radiated seismic energies and static stress drops of 28 earthquakes (M=0.0-1.4) that occurred within 200 m of the stations to investigate their apparent stresses. Apparent stresses of the 28 events were from 0.05 to 1 MPa (Figure 1) and static stress drops were 0.71 to 29 MPa. These values are similar to those for larger earthquakes. To study the source processes, we also carried out multiple time-window waveform inversions for the five largest events (M=0.8-1.4) among the 28 earthquakes. From the inversion results, we could determine the fault planes for all five events and estimate the range of rupture speed. We can conclude that rupture speeds were faster than 2.5 km/s (65 % of the shear wave velocity). The radiation efficiency is written as a function of the rupture speed and becomes greater with increasing rupture speed. 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 found that the source parameters (apparent stress, rupture speed, radiation efficiency, and static stress drop) did not largely differ from values for larger earthquakes. This suggests that the dynamic rupture processes of these small events were similar to those of the larger earthquakes.

S044--013
Rupture Initiation Sizes for Moderate to Large Earthquakes
Sato, K. and J. Mori

We studied the beginnings of the P waveforms for a wide range of earthquakes from M 3.5 to M7.9. Our analyses included earthquakes of the 2000 Izu Islands swarm (M3.8 to 4.9), 1999 Chi-Chi, Taiwan earthquake (Mw 7.6), 2000 Western Tottori earthquake (Mw 6.6), Northern Miyagi sequence (Mw 3.5 to 6.1), and 2003 Tokachi-oki earthquake (Mw 7.9). We used high-quality seismograms that were recorded usually within 20 km of the epicenters to examine the initiations of the ruptures. For the M7.9 Tokachi-oki earthquake we used data from an ocean-bottom seismometer at an epicentral distance of 36 km. Estimates of the beginning size of the seismic rupture were determined using the model of Sato and Kanamori (1999). We made corrections for the local attenuation and geometry of the fault. Our results indicate that for all the earthquakes, which had final sizes of kilometers to nearly 100 kilometers, the initial crack size was relatively small with dimensions of several tens of meters. For the larger earthquakes, we did not see any indication of a long smooth rupture initiation. Also, we could not see a clear dependence of the final size of the earthquake on the initial crack size. These results suggest a large earthquake
rupture is a cascade type of process, and does not depend on the initial size of the source of initiation.

S101-018
Triggering Sequence by Static Stress Changes of Large Aftershocks of the Niigata-Chuetsu, Japan Earthquake
Miyazawa, M., J. Mori, Y. Iio, T. Shibutani, S. Matsumoto, H. Katao, S. Ohmi, K. Nishigami

Following the 2004 Niigata-Chuetsu earthquake (M6.8), 4 large aftershocks (M6.3, 6.0, 6.5, 6.1) occurred: three within 40 minutes and one 4 days later. We examine the possibility for this triggering of these large aftershocks by static stress changes. For the close spatial triggering, it is important to have information about the fault geometries, slip distribution, and focal mechanisms. We determine the fault plane orientations from the aftershock distributions. Slip distributions of the main shock and largest aftershock are obtained by seismic waveform inversions of local strong-motion records. Mechanisms for the events are taken from CMT solutions. The temporal variations of Coulomb failure function changes (Delta-CFF) are calculated on the fault planes of the large aftershocks before their rupture. Positive Delta-CFF values (larger than 0.1 MPa) are obtained on the fault planes, indicating the possibility that static triggering from the main event and large aftershocks can explain the occurrence of aftershocks.

S101-P003
Detailed Image of Aftershock Activity of the 2004 Niigata Chuetsu Earthquake (M6.8)
Enescu, B., J. Mori,; T. Shibutani, K. Ito, Y. Iio, T. Matsushima, K. Uehira

The 23 October 2004 Chuetsu earthquake (M6.8) was followed by intense aftershock activity, which included the occurrence of four events with M larger or equal to 6.0. In order to analyse in detail the spatial and temporal characteristics of this earthquake sequence, we first determined accurate hypocenter locations using the double-difference algorithm (Waldhauser and Ellsworth, 2000). The relocated events show an increased spatial clustering. One can recognise at least three main fault-like structures, which are clearly defined by the aftershock distribution. About three days after the mainshock the seismic activity extended to the NW and SE. One of the large aftershocks (M6.1), which occurred to the SE, was preceded by few small events. We are presently cross-correlating the waveforms of events recorded at the same seismic station to obtain highly accurate relative arrival times. By using them as input for the double-difference relocation algorithm, we hope to obtain a very detailed spatio-temporal pattern of seismicity.
The seismic activity of each cluster of aftershocks is analysed for its frequency-magnitude distribution and decay rate of aftershocks. To have a more accurate estimate of the aftershock decay immediately after the mainshock (minutes-hours), we analyse the continuous Hi-Net waveform data at several stations located closely to the aftershock distribution and try to detect as many early aftershocks as possible.

International Symposium of Earthquake Engineering Commemorating Tenth Anniversary of the 1995 Kobe Earthquake
Awaji Island, January 13-16, 2005

Historical Maximum Seismic Intensity Maps for Japan from 1586 to 2004
Mori, J. and M. Miyazawa

We map the maximum seismic intensity for earthquakes in Japan from 1586 to September 2004 using compiled historical records (Usami, 2003) and Japan Metrological Agency (JMA) intensity data. We used a total of 336 events that had JMA intensity levels of 4 or greater. For each individual event, we interpolated the available data to make a smooth intensity distribution around the epicenter. Our use of the data makes no assumptions about magnitude-intensity or distance attenuation relationships, so it is one of the most direct ways of looking at the past history of strong shaking across Japan. Our composite results are plotted on a nationwide map that shows the maximum level of shaking that areas have experienced over the last 400 years. The regions with high intensities are located along the Pacific coast side, reflecting the recurrent large subduction zone earthquakes. Also onshore, we find high intensity regions due to the severe earthquakes on onshore active faults. We also make maps for one hundred year periods, which show considerable variations. This indicates that a one hundred year history is not adequate for characterizing the distribution of damaging earthquakes. During the last 400 years, about 90% of the regions in Japan have experienced JMA intensity equal to or greater than 5- and 30% of the regions have had intensities equal to or larger than 6-.