Fumiko Tajima

Dynamical modeling of trench retreat driven by the slab interaction with the mantle transition zone

We present the 2-D self-consistent dynamical model of interactions of a subducting slab with the 410-km and 660-km phase boundaries to further our understanding of the relation between the slab stagnation/penetration and the trench migration. Our model takes into account freely-movable plate boundaries and the difference between tensional and compressional yield strengths in the lithosphere. For the case in which the tensional strength is weaker than the compressional one, the negative buoyancy of the subducting slab produces extension of the overriding lithosphere and, accordingly, the trench retreats. Interactions with the 410-km and 660-km phasetransition boundaries further promote the trench retreat, and the dip angle of the slab is substantially decreased. This enhances the resistance of the 660-km phase boundary against the slab penetration. Slab weakening caused by the grain-size reduction in the transition zone may result in a horizontally-lying slab and trench retreat.

Seismic scaling relations and aftershock activity from the sequences of the 2004 mid Niigata and the 2005 west off Fukuoka earthquakes (MW 6.6) in Japan

[1]xa0We investigate seismic scaling relations and seismic activity using two Japanese earthquake sequences, the 2004 mid Niigata prefecture earthquake (MNPE) and the 2005 west off Fukuoka prefecture earthquake (WOFE) sequences. Although the mainshocks of the two sequences have the same MW of 6.6, the aftershock activities are different from each other in term of moment release. The MNPE produced a large number of aftershocks including six events with MW ≥ 5.5 over a period of about two weeks. On the other hand, the WOFE sequence did not produce aftershocks that exceeded MW 5.5. In the scaling relationship of seismic moment (M0) versus corner frequency (fc) obtained from the MNPE sequence, the fcs tend to decrease with decreasing M0 between MW 3.5 and 6.6 and the best fit line determined in a least squares sense is M0 ∝ fc−3.41. In the WOFE sequence, the best fit line is M0 ∝ fc−3.06 in the same MW range. The fc range of small aftershocks is lower in the MNPE sequence than in the WOFE sequence. Moreover, the M0 − fc relations for small events (3.5 ≤ MW ≤ 4.0) in the MNPE sequence are different between the events on the main faults (M0 ∝ fc−3.53) and the off-main fault events (M0 ∝ fc−4.44). These results suggest that there is difference (or variation) in the scaling relation even in the same MW range, probably reflecting different tectonic or seismogenic conditions.

Implications of seismic waveforms: Complex physical properties associated with stagnant slab

[1]xa0Anomalously broadened P waveforms of deep focus events were observed at some regional stations after sampling flattened high velocity zones. The source processes of these events are short and the P waveforms at other stations as well as the corresponding SH waves do not show such anomaly. The rays of the anomalous waves propagated in the vicinity of those whose waveforms can be synthesized with a relatively simple model M3.11 or M2.0 for a flattened cold slab in the transition zone. We suggest that the P waveform broadening is caused by SV converted waves at the heterogeneities within a narrow zone near the “660 km” discontinuity or slab boundary. The creation of such heterogeneities may involve in the complex structure of phase transformation with water and cold temperature anomaly associated with stagnant slab.

Calibration of amplification factors using KiK-net strong-motion records: toward site effective estimation of seismic intensities

Estimated seismic intensities using conventional relationships are not always in agreement with those observed at specific sites. We investigate subsurface amplification factors using the KiK-net strong-motion data recorded both in the borehole and on the ground surface and attempt to estimate more accurate intensities at a given site. Here the amplification factors are evaluated using the ratios of peak velocity amplitudes Vsurf/Vbor that are calculated from records on the ground surface and in the borehole. The ratios (amplification factors) decrease with hypocentral distance in general, but the attenuation characteristics vary from station to station. At stations where large amplifications are observed, the ratios show large scatter relative to the empirical amplification formulas. The variation is also affected by the incident direction of seismic waves as well as the hypocentral distance. We derived empirical amplification formulas of Vsurf/Vbed where Vbed is the peak velocity on the engineering bedrock as a function of hypocentral distance at individual stations (i.e. specific sites) in place of the conventionally used amplification constants. The estimated intensities using the site-effective formulas show substantial improvement at most of the stations in comparison with those calculated using conventional formulas.

Seismic scaling relation of the 2007 Off Mid Niigata, Japan, earthquake (Mw 6.6) sequence in comparison with two other earthquake (Mw 6.6) sequences

The relationship of seismic moment (M0) to corner frequency (fc) of the 2007 Off Mid Niigata prefecture (Chuetsu-oki) earthquake sequence is examined in comparison with that of two other earthquake sequences. The M0-f0 relation of this sequence (3.4 ≤ Mw ≤ 6.6) is M0 ∝ fc−3.87, which deviates from M0 ∝ fc−3. The relatively large deviation from fc−3 may reflect the influence of fluids identified in tomographic studies around the source region of the tectonic strain concentration. Moreover, the lower fcs of the small events (3.4 ≤ Mw ≤ 3.8) in this sequence are similar to those of off-main-fault events in the 2004 Mid Niigata prefecture earthquake sequence. The characteristics in the scaling relation may be interpreted in the context of the event locations, which are either on- or off-main faults, and may have implications for the seismogenic conditions. Key words: The 2007 Off Mid Niigata earthquake, seismic scaling, seismogenic conditions.