The 23 October 2004 Chuetsu Earthquakes and Related Disasters, Niigata, Japan I Posters
Presiding: R Sidle, Disaster Prevention Research Institute, Geohazards Division, Kyoto University; Y Iio, Disaster Prevention Research Institute, Research Center for Earthquake Prediction, Kyoto University
S51A-01 0830h
Crustal Heterogeneity in the Source Region of the 2004 Mid Niigata Prefecture Earthquake :Analyses of Coda Envelopes and Fault-zone Trapped Waves
The 2004 Mid Niigata Prefecture Earthquake (MJMA6.8) and its aftershock sequences generated complicated, i.e., several conjugate fault planes in their source region. In order to understand the occurrence process of these earthquakes, we estimated the 3D distribution of small-scale heterogeneities in the source region by inversion of coda envelopes of aftershocks, and also analyzed fault-zone trapped waves using seismograms recorded on the Obiro fault, where coseismic flexure deformation was detected. In the coda analysis, the fluctuation of coda envelopes from average decay curve was measured as the observational data and inverted to estimate a 3D distribution of relative scattering coefficient. We analyzed 138 seismograms from 30 events, recorded at 7 telemetry stations. The result shows strong scattering in the source region at depths from 0 to ~20 km, corresponding to the complicated distribution of aftershocks. We will discuss the detailed heterogeneity related to the earthquake generation by adding more data to increase the resolution. In the analysis of fault-zone trapped waves, we used the event-array method. This method has an advantage of reducing the site effect and we can detect fault-zone trapped waves by rotating the linear alignment of the event array, parallel and perpendicular to the strike of the fault. Possible trapped waves are detected for aftershocks in several limited source areas. Preliminary analysis suggests that the Obiro fault has a low velocity fault-zone connecting to both of the two parallel fault planes of the mainshock and the largest aftershock (MJMA6.5).
S51A-02 0830h
High-resolution Image of Aftershock Activity of the 2004 Niigata Chuetsu Earthquake (M6.8)
The 23 October 2004 Chuetsu earthquake (M6.8) was followed by intense aftershock activity, which included the occurrence of four events with M ≥ 6.0. In order to analyse in detail the spatial and temporal characteristics of this earthquake sequence, we first determined accurate hypocenter locations using the double-difference algorithm (Waldhauser and Ellsworth, 2000). The relocated events show an increased spatial clustering. One can recognise at least three main fault-like structures, which are clearly defined by the aftershock distribution. About three days after the mainshock the seismic activity extended to the NW and SE. One of the large aftershocks (M6.1), which occurred to the SE, was preceded by few small events. The seismic activity of each cluster of aftershocks is analysed for its frequency-magnitude distribution and decay rate of aftershocks. To have a more accurate estimate of the aftershock decay immediately after the mainshock (minutes-hours), we analyse the continuous Hi-Net waveform data at several stations located closely to the aftershock distribution and try to detect as many early aftershocks as possible.
S51A-03 0830h
Earthquake Induced Landslides In The Watershed Of The Imo River
The 2004 Niigata Chuetsu earthquake induced >1600 landslides in various types and dimensions. It has been general understanding until now that because of strong ground shaking during earthquakes a lot of slope failures occur on the steep slopes but only few reactivated landslides occur on gentle slopes. However, also many reactivated landslides occurred in the neighborhood of the epicenter area especially in the Yamakoshi Village. Landslides induced by the earthquake can be classified manly into the following categories: (1) Shallow slope failures on steep slopes near ridges, (2) Shallow slope failures on steep slopes along river channels, (3) Reactivated landslides on gentle hillslopes, (4) Landslide dams were formed by the displaced soils mass by the previous three categories. For the mitigation of subsequent disaster, it was a matter of great urgency to implement emergency countermeasures against overtopping and failure of the landslide dams. The Yamakoshi Village locates on the Higashiyama Hills trending a NNE-SSW direction with the altitude between 400m and 700m. The Imo River is flowing through eastern part of the village. The geology around the village belongs to Pliocene and Pleistocene formations that consist mainly of mudstone, sandstone and their alternation. The geological structure is controlled by the anticlines and synclines almost parallel to the direction of the hills. Landslides occurred especially densely in the watershed of the Imo River along the Kajigane Syncline and slipped along dip direction of geologic formations. Originally, heavily landslide prone areas are widely distributed in the Tertiary mudstone areas in Niigata Prefecture. Reactivated landslides occur frequently in the western part of the village where the mudstone is distributed. This time most of landslides occurred in the western part of the village where the sandstone is distributed. More than 30 landslide dams were formed along the main channel of the river and its tributaries. There are two critical dams because of their dimensions among them, namely Higashitakezawa landslide dam and Terano landslide dam. Both of them have a length of about 350m and a volume of >106 m3. The length of the buried river channel is about ten times of the maximum water depth of the reservoir in both cases. Thus the possibility of the destructive collapse of the dams by water pressure and piping were estimated to be low. However, there remained apparent danger of overtopping and successive collapse of the dams. Therefore, the inhabitants of the downstream area had to evacuate. It was urgently necessary to lower the water table of the reservoir. The reservoir at Higashitakezawa located most downstream has a critical significance. In order to mitigate the danger of overtopping, the following emergency maneuvers were arranged. At first, the water table was tried to lower by pumps and siphons. At the beginning 6 pumps and after then additional 6 pumps were installed. Because of maintenance problems of the pumps, later an alternative diversion pipelines were installed as reserve measure for unexpected alarm cases. Finally, an open channel with a sufficient cross section area also for water discharge including snow melting during early spring was constructed. It was absolutely necessary to keep the stability of the displaced soil mass against the secondary landslide during construction works of the channel. Therefore, cutting operation of the upper part of the soil mass was immediately carried out as an appropriate emergency countermeasure.
S51A-04 0830h
Anomalies Of Groundwater Temperature and Chemistry In Relation To The 2004 Niigata Chuetsu Earthquake
The 2004 Niigata Chuetsu earthquake of Mw6.6 occurred at the 220km north of Tokyo, Japan, on October 23, 2004. It was the special interest that this earthquake and several large aftershocks occurred at the northern segment of Niigata-Kobe Tectonic Zone (NKTZ) proposed by Sagiya (2001). Some of investigators regard the NKTZ as the plate boundary between the Eurasian plate and the North American plate. In the northern segment of the NKTZ, the thick accumulation of the Neogene sediments has been folded to make basins and ranges almost in parallel with the NNE-SSW coastal line of the Japan Sea. There are several active faults and a local tectonic line called "Shibata-Koide Tectonic line (SKTL)" around the epicentral area. Our field investigations indicated no evidence of the reactivation of these faults and SKTL. In addition, we found a spring water discharged at the eastern margin of the epicentral area close to SKTL. The water showed higher temperature and extremely higher electric conductivity than the surrounding shallow groundwaters. Then we focused on the behavior of groundwaters around the epicentral area, and measured the temperature, electric conductivity and chemistry using "the snow-melting wells". The Niigata area has been suffered from the heavy snow-fall more than three meters thick. The groundwaters from >10,000 wells at the depth of 40-100m have been utilized for snow-melting since 1960s. Considering the average aquifer depth of 70m for the wells, the local geothermal gradient of 30°C/km, and the annual mean temperature of 11.5 to 12°C in research area, we decided the temperature of >15°C was anomalous. We found the groundwaters with 15 to 17°C in the western margin of the epicentral area where the Yukyu-zan active fault with a NNE-SSW trend is located at the eastern border of the Nagaoka plain and the Higashi-yama Hill. We also found the waters that the temperature and the electric conductivity were anomalous in the southeastern margin of the epicentral area where SKTL with the NNE-SSW trend is located between the Higashi-yama Hill and the Echigo Mountains. A few groundwaters with 18 to 25°C were found in the downtown area of Ojiya City located in the southwest extension of the Yukyu-zan active fault, although no active fault has been known there. We consider that their anomalies are influenced by deep geothermal systems associated with the natural gas and oil reserviors beneath the folded ranges. Furthermore, the high relief of basin and range structure bordered by major faults creates deep circulation of groundwater, and consequently thermal and mineral springs sometimes discharge along the faults. It is likely that deep geothermal waters have seeped upward through fault fractures, injected into shallow aquifers and mixed with groundwaters. The excess fluid pressure and the friction heat induced by a series of earthquakes might be released through the Yukyu-zan active fault and SKTL to the near surface. Such process might be one of the major causes of groundwaters' anomalies in temperature, electric conductivity and chemistry.