Abstracts
2009 AGUFall Meeting
San Francisco,
California Dec. 14-18, 2009
S22A-04
Real-time Estimation of
Fault Rupture Extent for Recent Large Earthquakes
Yamada, M. and Mori, J. J.
Current earthquake early warning systems assume point source models for the rupture. However, for large earthquakes, the fault rupture length can be of the order of tens to hundreds of kilometers, and the prediction of ground motion at a site requires the approximated knowledge of the rupture geometry. Early warning information based on a point source model may underestimate the ground motion at a site, if a station is close to the fault but distant from the epicenter. We developed an empirical function to classify seismic records into near-source (NS) or far-source (FS) records based on the past strong motion records (Yamada et al., 2007). Here, we defined the near-source region as an area with a fault rupture distance less than 10km. If we have ground motion records at a station, the probability that the station is located in the near-source region is; P = 1/(1+exp(-f)) f = 6.046log10(Za) + 7.885log10(Hv) - 27.091 where Za and Hv denote the peak values of the vertical acceleration and horizontal velocity, respectively. Each observation provides the probability that the station is located in near-source region, so the resolution of the proposed method depends on the station density. The information of the fault rupture location is a group of points where the stations are located. However, for practical purposes, the 2-dimensional configuration of the fault is required to compute the ground motion at a site. In this study, we extend the methodology of NS/FS classification to characterize 2-dimensional fault geometries and apply them to strong motion data observed in recent large earthquakes. We apply a cosine-shaped smoothing function to the probability distribution of near-source stations, and convert the point fault location to 2-dimensional fault information. The estimated rupture geometry for the 2007 Niigata-ken Chuetsu-oki earthquake 10 seconds after the origin time is shown in Figure 1. Furthermore, we illustrate our method with strong motion data of the 2007 Noto-hanto earthquake, 2008 Iwate-Miyagi earthquake, and 2008 Wenchuan earthquake.
S42B-08
The Slapdown
Phase in the Strong Motion Record for the 2008 Iwate-Miyagi Nairiku
Earthquake
Mori, J. J., Yamada, M. , Heaton, T. H.
The 2008 Iwate-Miyagi Nairiku
earthquake (Mw 6.9, Mjma 7.2) produced strong shaking
throughout northern Honshu, Japan with severe damage of buildings and extensive
landslides. The shallow event occurred in southwestern Iwate prefecture
(39.03N, 140.88E, depth 8 km) on June 13, 2008 at 23:43:45 GMT (JMA, 2008).
This earthquake produced relatively high-frequency ground motions, which
resulted in large values of PGA (peak ground acceleration). Station IWTH25 of KiK-net, located 3 km southwest of the epicenter, produced
one of the largest strong-motion values of PGA (4278 cm/s/s for the vector sum
of the three components) ever recorded. The surface acceleration record at the
station IWTH25 shows an asymmetric amplification in the vertical components (Aoi et al., 2008). The upward vertical acceleration is much
larger than the downward direction, although in the borehole record at a depth
of 260 m at the same site, the upward and downward accelerations have symmetric
amplitudes. On the other hand, the horizontal components do not show this
asymmetric effect. This difference between the surface and borehole recordings
for the vertical component implies a strong non-linear amplification. In this
presentation, we will analyze these records and provide an explanation for the
asymmetric amplification in the vertical acceleration. We interpret the large
upward spikes in acceleration as slapdown phases,
which are also typically observed in near-field recordings of nuclear explosion
tests. The large upward acceleration is produced when a near-surface layer
separates from the sublayer then returns, striking
the separation surface. This effect is seen in a number of strong-motion
records that have larger upward than downward accelerations. If we assume the near-surface layer
returns to the original level, the separation gap is roughly 1 to 12 mm.
S43C-02
Preliminary Results of
the Time-Dependent Earthquake Forecast Model MARFSTA Applied to Mainland Japan
Smyth, C. W. and Mori, J. J.
The
Gutenberg-Richter frequency magnitude distribution (Gutenberg and Richter,
1944) plays a major role in earthquake forecasting and hazards analysis. The
parameters of this distribution are often assumed to be stationary. As
earthquake forecasts are specified over a long period of time, short term
irregularities in the parameter values are not of interest. However, we show
that over a particular study region these parameters are in fact
non-stationary, and that models with temporally variant parameter values
describe the data significantly better than models assuming stationarity
(Smyth and Mori, 2009). We attempt to model these temporal fluctuations in the
Gutenberg-Richter distribution with an autoregressive technique to forecast the
short term rate of earthquakes within a region of Japan. Over time, our model
can be trained to detect increasing or decreasing rates based over a long
history. The predicted forecast rate and frequency magnitude distribution are
overlaid on a density map of the area obtained using a multivariate mixture
model. The mixture model clusters historical earthquakes with no restraints
placed on the size or shape of clusters. Overlaying the two models gives a
spatially and temporally variant forecast of seismicity. We repeat the
procedure for many regions to obtain a short term forecast for the Japanese
mainland. The forecast model differs from currently proposed models by its
assumption of non-stationarity and its density
estimation. We also incorporate a further time-dependency component in our
model by assuming that as time passes since the last large earthquake the
probability of another larger earthquake increases. This requires the knowledge
of the repeat times of earthquakes along a fault in order to estimate the
probability distribution of repeat times. We cannot estimate the repeat times
empirically owing to the long history required, so here we use a simulation
approach based on the available historical data. The overall product is an
earthquake forecast algorithm, MARFSTA, ready for real time testing. We present
preliminary results of the model applied to mainland Japan. These results show
that recent large earthquakes in Japan fall into regions of high probability
indicated by our forecast.
S51B-1423
Is There a Relation
Between b-values and Asperities?
Yui, Y. and Mori, J. J.
b-value is an important parameter, representing a statistical
characteristic of the size distribution of earthquakes. Low b-values are
sometimes reported to be associated with areas of large slip (asperities) and
might be used to identify where the slip occurs prior to an earthquake. The
1999 Chi-chi, Taiwan earthquake is one of the best examples to study this
issue, because there is a clear region of large slip which can be compared to
the seismicity of the past 30 years. In this study, we look at the spatial
relation between the b-value before and after the Taiwan Chi-chi earthquake
using the seismicity data recorded in Taiwan from January 1973 to August 2003,
a total number of 278,102 earthquakes. For estimating b-values, we determine
the minimum magnitude and use the maximum likelihood method. We divide the
aftershock region of the earthquake into smaller areas to compare the local
b-values with the slip during the mainshock. We test
several size regions from 230 to 3400 km2. We also look at the effect of depth
by using grouping events using only epicentral
locations and by grouping events by hypocenters that are close to the dipping
fault plane of the mainshock. The results show that
there are generally high b-values (1.37 to 1.80) in the northern part of the
aftershock area where the slip was large during the earthquake. The b-value
near the epicenter appears to be significantly lower (0.52 to 1.18). These
results show that the area of the asperity had relatively high b-values during
10 to 20 years prior to the mainshock. Also, the
spatial distribution of b-value is similar between the seismicity prior to the mainshock and the aftershocks.
T21D-1849
Seismic Travel Time
Tomography around the Yamasaki Fault System of Southwest Japan
Nugraha, A. D., Ohmi, S. , Mori, J. J., Shibutani,
T.
We carried out a high-resolution tomographic
inversion to determine crustal structures around the Yamasaki fault system. The
Yamasaki fault system is one of the seismically most active onshore faults in
southwestern Japan, although large earthquakes around the fault system have not
been occurred for about 1100 years. We used a combination of data from a dense
seismic network operated by DPRI Kyoto University and the Japan Meteorological
Agency catalog. Totally, 16,800 events with 174,271 P and 154,406 S arrival
times from 91 stations were used for the tomographic
inversion in the study area. We set grid nodes with horizontal spacing of 10 x
10 km2 and vertical intervals of 2- 5 km. We determined three-dimensional Vp and Vp/Vs simultaneously with hypocentral adjustments. We show our results for the Vp, Vs, Vp/Vs ratio, and
Poissonfs ratio structures of the crustal heterogeneity along the Yamasaki
fault system. The low velocities and high Poissonfs ratio anomalies are imaged
in the lower crust at depths of 15 - 25 km along the central portion of the
northwestern part of the Yamasaki fault. These anomalies may be associated with
fluid-rich regions.
2009
Seismological Society of Japan Fall Meeting
Kyoto, October
21-23, 2009
A22-10
Subsurface
Velocity Changes during Strong Shaking as Seen from Seismic Interferometry
Yamada,
M., J. Mori, S. Ohmi
B22-07
Short
History of the Rebeur-Paschwitz type Tiltmeters Used in Kamigamo
Observatory, Kyoto
Takemoto, S., J. Mori, L. Rivera, J. Frechet
P2-06
Modeling
Temporal Variations of Seismicity Parameters to Forecast Earthquake Rates in
Japan
Smyth,
C. and J. Mori
P2-20
Finding the Rebeur-Paschwitz tiltmeter at Kamigamo Observatory
Mori, J.,
L. Rivera, J. Frechet, S. Takemoto
P2-53
Crustal
Structure around the Yamasaki Fault System in Southwestern Japan as Recorded
from High-Resolution Travel Time Tomography
Nugraha,
A.D., S. Ohmi, J. Mori, T. Shibutani
P2-55
Improvement of
the 3D Seismic Velocity Structure in Southwestern Japan Using Pronounced sP Arrivals Observed for Intra-Slab Earthquakes
Hayashida, T., F. Tajima, J. Mori
P3-70
Is there a
Relation between b-value, Earthquake Generation Processes and Heterogeneous
Structure
Yui, Yuichi
and J. Mori
P3-89
Slip
Distribution of Two Large Solomon Islands Earthquakes in 1975 Estimated Using
WWSSN Pdif Waveforms
Park, S.-C.
and J. Mori
P3-90
Joint
Inversion using Teleseismic Waveform Data and inSAR Data for the 2009 Papua Indonesia Earthquakes (Mw7.6
and Mw7.3)
Norimatsu, K., J. Mori, M. Hashimoto
Japan Geoscience Union Meeting 2009
Makuhari,
Chiba, May 16-21, 2009
S149-P004
Analysis of the 2009 Papua Indonesia Earthquakes (Mw 7.6 and 7.3)
Norimatsu, K. and J. Mori
The Mw 7.6 earthquake of 3 January 2009 occurred at 19:43 (UTC) as thrust
faulting on a plate boundary along the northwest coast of the New Guinea
Island. and the Mw 7.3 earthquake occurred at 22:33 (UTC) slightly to the east
with almost same mechanisms. This is a region of complex tectonics where with
convergence between the Pacific and Australian plates. The Pacific plate is
likely being subducted southward underneath New
Guinea on the Australian plate. Assuming southwestward dipping fault planes,
these two events are consistent with this subduction
process. The 2009 Papua earthquakes represent a doublet sequence. Doublet
earthquakes are characterised by pairs of events
within a short amount of time occurring on nearby portions of the same fault or
no separate faults. It is important to understand the relationship between the
earthquakes by estimating the slip distributions, distributions of the aftershocks
and induced stress around the hypocenter area of mainshocks.
We calculated the slip distributions for the Mw 7.6 and Mw 7.3 earthquakes
using the teleseismic waveform modeling. From our
results, the depth of Mw 7.6 earthquake is approximately 15 km shallower than
the reported depth from USGS (30 km), and the slip area is about 135 km along
strike and 60 km along dip. The maximum slip is located west of the epicenter
near the location of the largest aftershock, before the occurrence of the Mw 7.3
earthquake. The Mw 7.3 earthquake was apparently triggered by the Mw 7.6
earthquake. We study the details of the source process using seismic and
geodetic data to estimate the relation between Mw 7.6 and Mw 7.3 earthquake.
S151-P001
Prediction
of the Earthquakes in the Wenchuan Aftershock
Sequence
Smyth, C. and J. Mori
S152-P007
The Slapdown Phase in the Strong Motion Record for
the Iwate-Miyagi Nairiku Earthquake
Yamada, M. and J. Mori
The 2008 Iwate-Miyagi Nairiku earthquake (Mw 6.9, Mjma 7.2) produced strong shaking throughout northern
Honshu, Japan with severe damage of buildings and extensive landslides. The
shallow event occurred in southwestern Iwate prefecture (39.03N, 140.88E, depth
8 km) on June 13, 2008 at 23:43:45 GMT (JMA, 2008). This earthquake produced
relatively high-frequency ground motions, which resulted in large values of PGA
(peak ground acceleration). Station IWTH25 of KiK-net,
located 3 km southwest of the epicenter, produced one of the largest
strong-motion values of PGA (4278 cm/s2 for the vector sum of the three
components) ever recorded. The surface acceleration record at the station
IWTH25 shows an asymmetric amplification in the vertical components (Aoi et al., 2008).The upward vertical acceleration is much
larger than the downward direction, although in the borehole record at a depth
of 260 m at the same site, the upward and downward accelerations have symmetric
amplitudes. On the other hand, the horizontal components do not show this
asymmetric effect. This difference between the surface and borehole recordings
for the vertical component implies a strong non-linear amplification. In this
presentation, we will analyze these records and provide an explanation for the
asymmetric amplification in the vertical acceleration. We interpret the large
upward spikes in acceleration as slapdown phases,
which are also typically observed in near-field recordings of nuclear explosion
tests. The large upward acceleration is produced when a near-surface layer
separates from the sublayer then returns, striking
the separation surface. This effect is seen in a number of strong-motion
records that have larger upward than downward accelerations. If we assume the
near-surface layer returns to the original level, the separation gap is roughly
1 to 12 mm.
S156-P007
Seismic Attenuation and Thermal Structures in Western Shikoku and Bungo channel: Relationship to Low-frequency Earthquakes
Nugraha, A.D., J. Mori, S. Ohmi,
We conducted 3-D attenuation tomography using our previous 3-D velocity
structure in western Shikoku and Bungo channel
region. We then estimated thermal structure by applying parameters about
relation of temperature to P-wave attenuation data in order to investigate
source region of low-frequency earthquakes (LFEs). We analyzed clear waveform
for 973 small earthquakes and 128 Hi-net stations to calculate t₯Λ attenuation operators through
spectral fitting procedure. We have successfully obtained 19,000 P-wave t*
values and 18,500 S-wave t* values to invert for 3-D frequency independent
attenuation tomography. We used this P-wave attenuation to estimate thermal
structure by following high temperature background method. The obtained
structures show the subducting Philippine Sea (PHS)
slab is clearly imaged as low attenuation (Qp and Qs
of 400-800) and low temperature features (660-680 C), respectively. We look at
relationship between obtained structures and LFEs. The LFEs occur in a region
of high attenuation (Qp and Qs of 200-250) and
moderate high temperature (680-700 C), respectively.
S157-003
Numerical Simulation of Remarkable Later Phases for Events Occurred Within
the Subducting Philippine Sea Slab
Hayashida, T., F. Tajima, J. Mori,
Later phase arrivals help us to construct more realistic 2D/3D seismic velocity
models. Several significant later phases are recorded at many stations in
southwest Japan for events occurring within the subducting Philippine Sea slab
(e.g. Okada et al, 1990; Ohkura, 2000; Miyoshi and Ishibashi, 2007). A distinct later phase, has been detected
in seismograms between P- and Swave arrivals at epicentral distances of about 70 km or more for aftershocks
of the 2001 Geiyo earthquake. Characteristics of the
phase are as follows;
(1) The dominant period is about 1 Hz (nearly same or slightly slower than that
of the P wave).
(2)
The phase is predominantly observed on the radial and vertical components of
velocity seismograms.
(3)
The phase is clearly observed at borehole stations that are distributed of the
stations covers a wide range.
(4)
The apparent velocity is almost 6.1 km/s, corresponding to the P-wave velocity
in the uppermost crust.
(5)
The amplitude is slightly smaller or larger than that of the P wave.
(6)
The amplitude on the radial component is nearly the same or slightly larger than
that on the vertical component.
We
attempt to explain the characteristics of this phase using the e3d finite
difference code by Larsen and Shultz (1995). The constructed model covers a
region that extends 200 km in the horizontal and 100 km in depth, with a grid
interval of 0.2 km. Parameters of each layer are based on the models of Kakehi et al. (2004) and Iwata et al. (2008), and the
depths of the Moho and Conrad discontinuities are
inferred from the receiver function images of Shiomi
et al. (2004) and Ueno et al. (2008). The simulated waveforms can explain
characteristics of the later phase well and the snapshots of seismic wave
propagation indicate that this phase corresponds to a depth phase of sP, which is a P wave reflected from a direct S wave at the
Earthfs surface. The sP phase is clearly seen in the
simulated waveforms at epicentral distances of about
70 km or more. This is in good agreement with the features of the observed
data. We also explain the travel times of S conversions from the Moho and Conrad P refrections
which are clearly observed in recordings of the several stations by adjusting
the depths of the Moho and Conrad discontinuities.
S220-013
Assessing Temporal Variations in the Gutenberg-Richter
Distribution for a Short-term Forecast Model
Smyth, C. and J. Mori
2nd
China-Japan Science Forum
The 2008 Wenchuan Earthquake and Natural Disaster Mitigation
Beijing, China
March 9-10, 2009
Rupture Process of the 2008 Wenchuan, China Earthquake using a Back-Projection Analysis
Mori, J.
P waveforms from about
700 Hi-net stations in Japan were used in a back projection analysis of the
rupture for the May 12, 2008 Wenchuan, China
earthquake (Mw7.9). The stations in Japan are at a distance of about 25 to 30
degrees from the earthquake. The P waveforms have durations of about 100 sec
and these data were used to estimate the rupture propagation for the
earthquake.
The back-projection
method tests a grid of points to determine the best location for the source of
seismic radiation for a designated time window of the P wave. The initial
arrival of the first time window was assumed to come from the grid point
corresponding to the earthquake hypocenter. For each subsequent time window,
the data were stacked assuming a source at each grid
point. The grid point that corresponds to the stack with the highest amplitude
is determined to be the source of the P-wave energy. Relative time shifts for
each time series were calculated using the theoretical travel times from the
station to the grid point, using the IASPEI91 model. The grid of the 290 tested
source locations over a fault length of 400 km was used for each time window.
Time windows of 10 to 30 seconds were used and produced generally similar
results.
There may be some
significant complications in the waveforms from the depth phases for a shallow
strike-slip earthquake. Part of this problem may be overcome if large enough
time windows are used that include the direct arrivals and depth phases, and
the whole wavetrain is used.
The results show the
rupture progresses from the epicenter in the southwest toward the northeast for
a distance of over 300 km. The rupture velocity appears to be variable with an
average speed of about 3 km/second. There may be a relation between the areas
of large slip and rupture slip. Areas of large slip appear to have faster
rupture propagation speed across the fault plane for the Wenchuan
earthquake.
Model
Ensembles for Prediction of Wenchuan Aftershock
Activity
Smyth, C., J. Mori,