Abstracts
1999
AGU Fall Meeting
San Francisco, California, December 13-17, 1999
T41B-06
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.
S22B-04
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
(http://www2.rcep.dpri.kyoto-u.ac.jp/~sato/taiwan/index.html)
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
Si-029
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.