1999 AGU Fall Meeting
San Francisco, California, December 13-17, 1999

Slant-Stacking Space-time Seismicity in the Region of the Hyogo-ken Nanbu (Kobe) Earthquake
Mori, J  and H. Katao

We have developed a new technique to identify migrations of earthquake epicenters. Using space-time seismicity data, we use this method to systematically search for linear trends in the occurrence of earthquakes. Space-time seismicity data are converted into a set of time series, where each time series is the number earthquakes that occurred in a small region as a function of time. These time series are slant-stacked over a range of "slownesses" (days/km) to search for correlations that indicate linear migrations of earthquake locations. The method was applied to the region of the 1995 Hyogen-ken Nanbu (Kobe) earthquake, which includes the aftershock zone and the Tamba area. Tamba, located north of the aftershock zone, is an area where numerous small earthquakes occurred following the mainshock. We converted the epicentral data into time series of earthquake counts for the period from April 1, 1995 through June 30, 1999. For the spacial intervals, we used 1 km spacings along a trend that roughly parallels the strike of the Kobe mainshock. We tested slownesses of -100 to +100 days/km. The results show a migration of epicenters toward the north during the four years following the mainshock.

Fine Structure of the Rupture Zone of the April 26th and 27th, 1997 Northridge Aftershocks
Venkataraman, A, J. Mori, H. Kanamori, H, L. Zhu,

It is often assumed that a planar distribution of small earthquakes approximately delineates an earthquake slip plane. However, a high resolution analysis of the rupture pattern using close-in broad-band records reveals that an earthquake fault zone can have a very complex rupture geometry. We analyzed the details of the rupture pattern of two events (E1:4/26/97; E2:4/27/97 with Mw4.5) which occurred on the western edge of the 1994 Northridge aftershock area. The background seismicity in the epicentral region suggests the existence of a north dipping fault plane, similar to that of the 1971 San Fernando earthquake. However, the spatial trend of the two events and their aftershocks reveals tightly clustered seismicity on a steep plane dipping south. Relative relocation of the April 26th and 27th events shows that the April 27th event ruptured about 1.4km N70E of the April 26th event at a slightly shallower depth. Moreover, the directivity of each of these events determined using a small aftershock (Mw3) as the empirical Green's function suggests that both events ruptured on this steep plane. These observations suggest that the two events ruptured on a plane which is almost perpendicular to the trend of the regional background seismicity. Hence, the deformation beneath the Transverse Ranges occurs on a complex array of rupture planes rather than on one simple plane. The rupture length for the April 26th event is about 1.3km and preliminary calculations indicate a stress drop of about 100 bars.


Report of a Quick Investigation of the September 21, 1999 Chichi Taiwan Earthquake
Katao, H, J. Mori, M. Ando, S. Ohmi, S, K. Sato, K,-F.Ma, C.-T. Lee

Following the September 21, 1999 Chichi, Taiwan (Mw 7.7) earthquake, a team of researchers from the Disaster Prevention Research Institute, Kyoto University visited the epicentral area and talked to Taiwanese scientists from September 24 -27. The purpose of the trip was to gather information about the earthquake faulting, damage patterns, and performance of the earthquake monitoring systems. We saw the large vertical offsets (1-8 m) at various locations along 85 km of the Chelongpu fault. The largest displacements were in the northern region, where the north-south trending fault curved toward the northeast. We observed the damage to structures near the fault and had a general impression that the shaking damage in the northern part of the rupture area was not as severe as might be expected, given the very large fault displacements (4-8 m) that were seen. At the Central Weather Bureau in Taipei we saw the monitoring system that was able to quickly report the location, magnitude, and intensity distribution within 2 minutes following the mainshock. Information and photographs gathered from this investigation were quickly posted on a webpage


providing some the first widely distributed English and Japanese information about the earthquake.


Teleseismic and Near Source Strong Motion Investigation of the September 20, 1999 Chi-Chi Taiwan Earthquake
Ma, K-F, T. Song, J. Mori

We investigated the teleseismic and near source strong motion waveforms of the September 20, 1999 (Mw=7.6) Chi-Chi Taiwan earthquake. For teleseismic waveform analysis, a deconvolution technique was applied to estimate the possible slip distribution along the fault. 13 stations were used in the analysis. The distribution of the amplitudes and waveform shapes reflect the directivity of the rupture toward the north. The source time function thus obtained has a total duration of about 40 sec. The slip distribution is quite consistent with the field observation, which has an average slip of about 2 to 3 m along the fault and reaches a maximum slip of about 7 to 8 m at about 40 km to the north of the epicenter. The near-field strong-motion displacement waveforms at the stations along the fault reveal significant static displacements. The maximum static displacement is about 6 m for the station near the northern end of the fault. The near-source strong-motion displacement waveforms were inverted to investigate the spatial slip distribution during the fault rupture. A significant asperity was obtained with a maximum displacement of up to about 10 m at the depth of about 10 km and a slip of about 8 m at surface, relatively consistent with the location where the maximum slip was observed. The slip distribution thus obtained can be used to calculate the slip velocity on the fault.


Dynamic Stress Drop Estimates for the 1999 Chichi Taiwan Earthquake: Indications of Low Dynamic Friction
Mori, J., K.-F. Ma

There are near-field strong-motion records from the 1999 Chichi, Taiwan earthquake (Mw 7.7) that recorded large ground motions close (within 2 km) to the Chelongpu fault. The ground velocities were in the range of 100-300 cm/sec and displacements of 1 to 6 meters. These data can be used for robust estimates of the slip velocity of the fault since they are directly proportional to the near-field velocities. We use the results of a finite fault inversion to estimate the distribution of slip velocities on the fault. The slip velocities are then used to infer the distribution of dynamic stress drop. If one assumes that the driving stress is constant for various parts of the fault, the changing dynamic stress implies changing dynamic friction. For the region of large slip that is identified in the fault inversion, we speculate that increased fluid pressure or fault melting may cause a large drop of dynamic friction in the region of the large fault slips. Energy estimates for the earthquake are consistent with the trend of crustal earthquake in California that shows that the larger earthquakes radiate relatively more energy that smaller earthquakes. This can also be explained by a significant decrease of the dynamic friction during large earthquakes.


International Conference on Science Frontier Tsukuba 999 (SFT999)
Tsukuba, November 17-19, 1999

Prediction of Earthquakes and Ground Motions in the US
 Mori, J.


1999  Japan Earth and Planetary Sciences Joint Meeting
Tokyo, June 8-11, 1999

Stress Drops and Radiated Energies for Southern California earthquakes
Mori, J


We used empirical Green function deconvolutions to obtain source time  functions for 49 of the larger (M 3 4.0) aftershocks of the 1994 Northridge,  California earthquake.Combining the Northridge results with data from other southern California  earthquakes, we see a clear increase in the amount of relative radiated  energy as a function of earthquake moment. The results also show that there
is not an increase in the static stress drop over the same moment range.  This systematic change in earthquake scaling between small and large  earthquakes suggests differences in rupture properties that may be attributed to differences of dynamic friction on the fault.