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