S151-P007

アイソクロン・バックプロジェクション法による２００５年福岡県西方沖の震源過程のイメージング
Nelson Pulido, Shin Aoi and Hiroyuki Fujiwara
National Research Institute for Earth Science and Disaster Prevention (NIED), Japan (nelson@bosai.go.jp)

Conventional approaches to investigate the source rupture process of earthquakes are based on inversion of near-fault ground motions. Because these methods rely on the calculation of alarge number of Green’s functions, slip images of the earthquake are only obtained several days after the earthquake.In this study we introduce a technique for imaging the rupture process of anearthquake shortly after the records from the earthquake become available. We study the source process of the 2005 Fukuoka-ken Seiho Oki Earthquake by using an Isochrons Back-Projection
Method (IBM). The idea of the procedure is to back-project amplitudes of seismograms envelopes around the source into a time-space image of the earthquake rupture.We set a grid of possible
sources across the fault plane of the Fukuoka earthquake and calculate the theoretical travel times from every grid to every station. We assume a rupture initiation point, that in this casecorresponds to the Hi-net hypocenter location, and calculate the rupture time of every grid within the fault plane by assuming some arbitrary rupture velocity value. Then we obtain theisochrones distribution within the fault plane for every station. We choose 55 stations (KiK-net boreholes and K-NET) with distances ranging from 40km to 100km from the epicentre. Wecalculate the waveforms envelopes using the root-mean-square of the original waveforms and their Hilbert transform and start them at the origin time. We calculate a grid “luminosity” byadding all the envelope amplitudes corresponding to every grid isochron time for all stations. In this way we scan for all possible isochrons time contributions to the grid luminosities from theshortest to the longest possible isochrons time values. The present method is an extension of the source imaging method by Festa et. al.(2006).
We obtained an image of the total luminosity at every grid within the fault plane. We also search for an optimum rupture velocity value by exploring the model with the largest total grid
luminosity within the fault plane. We obtained that a rupture velocity of 2.1 km/s maximizes the total fault plane luminosity for the Fukuoka earthquake. Our results show that the grids with thelargest luminosity correspond to a region approximately 8 km above and slightly south of the hypocenter. This high luminosity region corresponds approximately to the large slip region(asperity) obtained by a kinematic inversion of the Fukuoka earthquake (Sekiguchi et. al. 2006). Our results show so far that the IBM has the capability to identify asperities within the faultplane. The present method has the advantage that it does not require calculation of Green’s functions and therefore is able to identify asperities within the fault plane, soon after the earthquake.
Future developments of the method will address the issue of how to relate the outputs of the model with quantitative images of fault slip, as well as extending the search domain to a volume
around the hypocenter in order to study the capability of IBM to identify the causative fault plane without any a-priori constraint.

I Method and DataII Stations used for source imaging-We use 31 K-NET and 24 KiK-net stations of NIED, with distances ranging from 40km to 100km fromthe epicenter, for source imaging.
-We calculate all travel times between subfaults and stations using a velocity model obtained for theFukuoka earthquake source region from a temporal seismic network (Uehira et. al., 2006). All travel times
are adjusted by a station correction factor.
-We calculate the rupture time of every grid within the fault plane by a constant rupture velocity and add
subfaults rupture times and the corresponding travel times for every station (isochrons times).
-We select waveforms that have clear P and S wavelets, extract P-wave windows between the origin timeof the earthquake and the theoretical arrival of the S-wave, and taper 1s of the waveforms at the end.
-We band-pass filter the data between 1Hz and 30Hz, and calculate the waveforms envelopes using theroot-mean-square of the original waveforms and their Hilbert transform.
-We calculate a grid “brigthness” by adding all the envelope amplitudes
corresponding to every grid isochron time for all stations. Envelopevalues are obtain by taking the average of a +/- 1.5 sec window aroundeach isochron time. The final result is a distribution of the brightness
across the fault plane, which gives us an idea of the location ofasperities within the fault plane.

Aftershocks information in the figure
to the left by Uehira et al. (2006)

III Isochrons and velocity envelopes at K-NET stations
Normalized P-wave velocity
envelopes at 31 K-NET sites for
the EW component. Envelopes
were obtained as follows:Ε[u& (t )] = (u& (t ) 2 + Η 2 [u& (t )])1/ 2.
where u(t) is the velocity and His the Hilbert transform.

Isochrons were obtained for aconstant rupture velocity of 2.0km/s and a multilayered 1D
velocity model obtained for theFukuoka earthquake source
region (Uehira et. al 2006).

III Effect of local geology on Source Brightness imaging

In order to explore the effect of local site conditions into the source imaging result we analyzed three different data sets including
recordings of KiK-net and K-NET surface sites, as well as KiK-net boreholes recordings. From this analyses we can conclude that thesite effects have a small influence on the source imaging result. Source model by Sekiguchi et. al. (2006) is shown as contour lines andvectors.

IV Conclusions-We obtained a source imaging of the Fukuoka Seihou-Oki earthquake by a non-conventional approach that do
not requires calculation of Greens functions and inversion.
-Our source model shows a shallow asperity towards the South-East of the hypocenter. Comparison with asource model by an inversion approach, suggests that the fault brightness of the Fukuoka earthquake
aproximately corresponds to large slip regions.
-We obtained that a rupture velocity of 2.0 km/s yields the maximum fault plane brigthness for the Fukuoka
earthquake. However this result is highly dependent on the assumption of a uniform rupture velocity across thefault plane.
-We found that local site condition at surface sites has a small effect on the result of the IBM source imaging,from a comparison between models obtained from surface and borehole data.

We explored the optimum rupture
velocity defined for the value thatmaximixes the total fault planebrigthness.

V References-Sekiguchi, H. S. Aoi, R. Honda, N. Morikawa, T. Kunugi,and H. Fujiwara, 2006. Rupture process of the 2005 West
Fukuoka Prefecture earthquake obtained from strong motion
data of K-NET and KiK-net, Earth Planets Space, 58, 37-43.
-Festa, G., and A. Zollo. 2006. Fault slip and rupture velocity
inversion by isochrone backprojection, Geophys. J. Int.,166,745-756.
-Uehira, K. et al., 2006. Precise aftershock distribution of the2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0)
using a dense onshore an ofshore seismic network, EarthPlanets Space, 58, 1605-1610.