Radiated energy from the mid Niigata, Japan, earthquake of October 23, 2004, and its aftershocks

The Mid Niigata earthquake of October 23, 2004, is a shallow reverse-faulting event with M-w = 6.6. We analyze transverse component accelerograms of the main shock and four aftershocks (M-w = 4.0 similar to 5.5) recorded at 29 strong motion observation stations. The epicenters of the four aftershocks are located within 5 km from that of the main shock. The stations are well distributed around the epicenters and their epicentral distances are between 15 and 60 km. We calculate spectral ratio between records of the main shock and each of the aftershocks in order to obtain source spectral ratio by empirically removing the effects of wave propagation path and surface layers at the stations. Thus obtained source spectral ratio is inconsistent with that expected from the self-similar omega-square model. Fitting omega-square source spectral ratio with variable corner frequency to the observed spectral ratio, radiated energy and corner frequency are estimated. Radiated energy from the main shock is 3.2 x 10(14) J. Although energy-to-moment ratios for the main shock and the aftershocks are all distributed within a narrow range from 8 x 10(-6) to 4 x 10(-5), they definitely increase with the event size. This scale dependence is quite similar to that obtained previously for strike-slip events in Japan. The relationship between the seismic moment, M-0, and the corner frequency, f(0), is M-0 proportional to f(0) (-3.3 +/- 0.2). These results suggest a break in self-similarity.ArticleGEOPHYSICAL RESEARCH LETTERS. 32(21): L21313 (2005)journal articl

Nucleosynthesis in High-Entropy Hot-Bubbles of SNe and Abundance Patterns of Extremely Metal-Poor Stars

There have been suggestions that the abundance of Extremely Metal-Poor (EMP) stars can be reproduced by Hypernovae (HNe), not by normal supernovae (SNe). However, recently it was also suggested that if the innermost neutron-rich or proton-rich matter is ejected, the abundance patterns of ejected matter are changed, and normal SNe may also reproduce the observations of EMP stars. In this letter, we calculate explosive nucleosynthesis with various Ye and entropy, and investigate whether normal SNe with this innermost matter, which we call "hot-bubble" component, can reproduce the abundance of EMP stars. We find that neutron-rich (Ye = 0.45-0.49) and proton-rich (Ye = 0.51-0.55) matter can increase Zn/Fe and Co/Fe ratios as observed, but tend to overproduce other Fe-peak elements. In addition to it, we find that if slightly proton-rich matter with 0.50 <= Ye < 0.501 with s/kb ~ 15-40 is ejected as much as ~ 0.06 Msolar, even normal SNe can reproduce the abundance of EMP stars, though it requires fine-tuning of Ye. On the other hand, HNe can more easily reproduce the observations of EMP stars without fine-tuning. Our results imply that HNe are the most possible origin of the abundance pattern of EMP stars.Comment: 10 pages, 3 figures, Accepted to ApJL; modified reference

Fault extent of the largest aftershock of the 1968 Tokachi-Oki, Japan, earthquake and an interpretation of the normal faulting focal mechanism

The focal mechanism of the 1968 Tokachi-Oki earthquake (MJ = 7.9) was thrust faulting but that of the largest aftershock (MJ = 7.5) was normal faulting. The tectonic implication of the reversal of the focal mechanisms of the two events has not been clarified yet. In order to investigate this, we ought to take into account the relative location of the two fault planes. In the present study, the length and the direction of rupture propagation for the largest aftershock is derived from the azimuth dependence of the duration of observed strong ground motion. If we assume that the rupture initiation point is the hypocenter location determined by the Japan Meteorological Agency (JMA), the location of the fault plane is considerably further north of the tsunami source area as estimated previously, and at the northern end of the aftershock area within 24 hours after the main shock. The fault plane of the largest aftershock is deeper than that of the main shock and the two fault planes are nearly parallel to each other. The thrust faulting of the main shock and the normal faulting of the largest aftershock indicate that the part of the Pacific plate between the two fault planes moved relatively further northwestward due to the two events than the deeper part beneath the fault plane of the largest aftershock.ArticleEARTH PLANETS AND SPACE. 63(12):1213-1216 (2011)journal articl

Nonlinear response of surface layers at KiK-net stations in Japan

13th World Conference on Earthquake Engineering, August 1-6, 2004, Vancouver, B.C., CanadaArticleProceedings of the 13th World Conference on Earthquake Engineering. : 2690 (2004)conference pape

Scale-dependence of seismic energy-to-moment ratio for strike-slip earthquakes in Japan

We analyzed four pairs of a large (M_w ≈ 6) and a small (M_w ≈ 3.5 to 4) shallow strike-slip earthquakes to investigate the scale-dependence of the seismic energy-to-moment ratio, an important macroscopic parameter which reflects the basic physical process of seismic slip. These earthquakes occurred in the south-western part of Japan, and high-quality close-in records (epicentral distance < 50 km) are available for both the small and large earthquakes. The paired events have almost the same focal mechanism and hypocenter location. We used the spectral ratio of the paired events in order to remove the effects of attenuation along the wave propagation path and the station site response. We then estimated the seismic energy from the source spectra estimated from the spectral ratio. The energy-to-moment ratio increases with the earthquake size. This scale-dependence is very similar to that found earlier for earthquakes in Southern California

Source Characteristics of the Largest Three Aftershocks of the 1983 Japan Sea Earthquake

Intermediate-period P-wave records at far-field stations are analyzed to derive the focal depth and the source characteristics of the largest three aftershocks of the Japan Sea earthquake of May 26, 1983. Event A (June 9, 21 : 49, MJMA=6 .1) and Event B (June 9, 22 : 04, MJMA=6 . 0) occurred near the south edge of the source region of the main shock. Their focal depths are determined as 13 km and 12 km, respectively. Event C (June 21, 15 : 25, MJMA=A7 . 1) took place near the north edge of the source region. Its focal depth is estimated at about 8 km. The essential part of moment rate function for Event A is found to be expressed by a single pulse with a width of about 1 sec, while that for Event B is composed of two pulses separated in time. The moment rate function obtaine d for Event C consists of two pulses with nearly the same width of about 3.5 sec, and the time lag between them is estimated at 2.5 sec.ArticleTohoku Geophysical Journal. 33(1):83-95 (1990)departmental bulletin pape

Flexible band versus rigid ring annuloplasty for functional tricuspid regurgitation

We review and compare our experience with tricuspid ring annuloplasty between usage of the Cosgrove-Edwards flexible band and the MC3 rigid ring for repair of functional tricuspid regurgitation to determine the efficacy and mid-term durability of tricuspid annuloplasty. 117 patients with functional tricuspid regurgitation undergoing open heart surgery and tricuspid valve repair from May 2005 to December 2007 were reviewed. The flexible bands were used in thirty five patients before October 2006. Since then, the rigid rings were used in the next consecutive eighty two cases. Echocardiographic evaluation of tricuspid regurgitation was performed preoperatively and postoperatively in follow-up schedule. The degree of tricuspid regurgitation was reduced from 2.80±0.67 to 0.71±1.0 (regurgitation severity grade: 0 to 4) in the patients with flexible bands at discharge. It was from 2.68±0.70 to 0.22±0.60 in the patients with rigid rings. At thirty six months postoperative period, tricuspid regurgitation grades in patients with flexible bands and rigid rings were 0.80±0.95 and 0.36±0.77, respectively. Freedom from recurrent tricuspid regurgitation (grade 2 or 3) in patients with flexible bands and rigid rings were 68.6% and 87.8%, respectively. Recurrent tricuspid regurgitation was significantly lower in the patients with rigid rings. Although both flexible band and rigid ring annuloplasty provide low rate of recurrent tricuspid regurgitation, rigid ring annuloplasty might be more effective than flexible band annuloplasty for decreasing functional tricuspid regurgitation in immediate and mid-term postoperative periods