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, ɓ et al., fwCMj^[̂߂̐xv). 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.