Seismic refraction and wide-angle reflection exploration by JARE-43 on Mizuho Plateau, East Antarctica

The 43rd Japanese Antarctic Research Expedition (JARE-43) carried out seismic exploration experiments on Mizuho Plateau,East Antarctica,in the austral summer season of ,2001-2002. The exploration was composed of seven large explosions and 161seismic stations distributed along the 151 km-long seismic line.The first arrival time data are analyzed by a refraction method.It is found that the ice sheet is composed of two layers:the upper layer with P wave velocity of 2.7-2.9 km/s has thickness of 35-45 m,and the lower layer with P wave velocity of 3.7-3.9 km/s continues to the bedrock.Lateral velocity variation in the upper-most crust is revealed:P wave velocity for the upper-most crust in the southern and central parts is 6.1-6.2 km/s and that in the northern part is5.9 km/s. This result implies that the geological boundary observed along the coast in the Lu tzow-Holm Bay and Prince Olav Coast regions possibly continues to the inland area.The S wave velocity is also obtained to be roughly 3.5km/s for the whole upper-most crust.Travel time analysis of two distinct reflection phases shows two horizontal reflecting planes located at 19 and 40km depth; the latter corresponds to the Moho discontinuity

トウカガタ ジシンケイ (ペネトレータ) ノ ナンキョク ミズホ コウゲン デノ シケン カンソク ダイ43ジ ナツタイ ホウコク

南極氷床上のクレバス帯等の,地上からは到達困難な地域での人工地震観測を目的とした投下型地震観測装置(南極ペネトレータ)を開発し,第43次日本南極地域観測隊で実施する東南極みずほ高原における人工地震探査で使用するために,22本のペネトレータを昭和基地に持ち込んだ.しかし,開発の遅れに伴う国内試験の不足から種々の不具合が発生し,今回は本観測での使用をあきらめざるを得なかった.当初の目的は果たすことができなかったが,国内では得られない環境でのペネトレータ投下実験を行い,投下姿勢,着地衝撃力,温度変化等の貴重なデータを得るとともに,南極内陸部での実際のヘリコプター運用への知見を得ることができた.これらの成果はペネトレータ型地震計の改良のみならず,今後の各種投下型観測機器の開発・製作に多いに役に立つものと考えられる.We have developed an Antarctic penetrator that is applicable to seismic explosion experiments along a difficult traverse route on a continental ice sheet with crevasses. In the 43rd Japanese Antarctic Research Expedition (JARE-43, 2001-2002) seismic explosion experiments on the Mizuho Plateau, in East Antarctica, we equipped 22 penetrators for seismic observation. However, due to electrical trouble, we could not apply the penetrators to actual observation. Then, we carried out a running test of the penetrators and acquired much valuable data that cannot be obtained in the domestic environment in Japan, such as the declination angle of a body intruding into the Antarctic ice sheet, impact shock, and daily change of temperature of the body in the snow. These data are not only useful for development of the Antarctic penetrator, but also for development of touchdown-type physical observation equipment for use in Antarctica

ヒガシ ナンキョク ミズホコウゲン ニ オケル クッセツホウ オヨビ コウカク ハンシャホウ ジシン タンサ カンソク ガイヨウ (ダイ43ジ ナツタイ ホウコク)

第43次日本南極地域観測隊夏隊(JARE-43)では,東南極みずほ高原において,JARE-41に引き続き,みずほ高原下の地殻の速度構造を明らかにするため,屈折法および広角反射法地震探査を実施した.探査測線は,みずほルート上のH176地点を起点に北北東方向に101 km,南南西方向に50 kmの測線長約151 kmであり,JARE-41の探査測線と斜交している.この測線上に161台の地震観測点を配置し,7カ所で薬量700 kg程度の大発破,1カ所で薬量20 kgの小発破を行った.大発破点からの距離が100 km以下では,明瞭な初動が観測された.また,氷床中を伝播して来た波,分散波および地殻深部(コンラッド面とモホ面)からの反射波と推定される後続波も観測された.発破点近傍の観測で得られた初動走時から,氷床が厚さ36-45 mでみかけ速度2.7-2.9 km/sの上部層とみかけ速度3.7-3.9 km/sの下部層から成り立つことが推定された.測線全体の初動走時からは,氷床直下の基盤層(岩盤)のみかけ速度が,測線の北部では5.9 km/s,中央部～南部では6.1-6.2 km/sとなり,基盤層の速度差が示唆された.本報告は,実験の概要と得られたデータについて述べる.A seismic refraction and wide-angle reflection exploration was successfully conducted along a profile crossing the JARE-41 seismic profile on the Mizuho Plateau, in East Antarctica, in the austral summer season of 2001-2002 (JARE-43). One hundred sixty-one seismic stations were temporarily installed along a profile about 151 km long and seven large shots with about 700 kg of dynamite were fired. In addition, one shot with charge size of 20 kg was also arranged along the profile. The obtained seismic records show the clear onsets of the first arrivals at distances of less than 100 km from each large shot. In particular, seismic waves traveling through the ice sheet and dispersed surface waves were clearly observed. Some later reflection phases were also detected. The obtained first travel time data show that the ice sheet is a two-layered structure consisting of an upper layer with a P wave velocity of 2.7-2.9 km/s and a lower layer of 3.7-3.9 km/s. The thickness of the upper layer is estimated to be about 36-45 m. The apparent velocity in the basement rock just beneath the ice sheet is 6.1-6.2 km/s in the central and southern parts of the profile and almost 5.9 km/s in the northern part. This report describes basic outlines of the exploration and the obtained seismic data

Seismic exploration at Fuji volcano with active sources : The outline of the experiment and the arrival time data

Fuji volcano (altitude 3,776m) is the largest basaltic stratovolcano in Japan. In late August and early September 2003, seismic exploration was conducted around Fuji volcano by the detonation of 500 kg charges of dynamite to investigate the seismic structure of that area. Seismographs with an eigenfrequency of 2 Hz were used for observation, positioned along a WSW-ENE line passing through the summit of the mountain. A total of 469 seismic stations were installed at intervals of 250-500 m. The data were stored in memory on-site using data loggers. The sampling interval was 4 ms. Charges were detonated at 5 points, one at each end of the observation line and 3 along its length. The first arrival times and the later-phase arrival times at each station for each detonation were recorded as data. P-wave velocities in the surface layer were estimated from the travel time curves near the explosion points, with results of 2.5 km/s obtained for the vicinity of Fuji volcano and 4.0 km5/s elsewhere

Reflection profiling and velocity structure beneath Mizuho traverse route, East Antarctica

A seismic survey with a high density network was performed by JARE-41 in the austral summer of 1999 to 2000. The seismic line is spread out over 190 km along the Mizuho traverse route. 160 seismic stations programmed on timer operation were installed. A clear reflection is obtained in some shot records. It was described as a PmP reflection in a previous study. This survey geometry enables us to apply reflection analysis. A single fold profile and corresponding velocity structures for the JARE-41 seismic line were obtained in this study. The dipping Moho reflector and dipping reflectivity layer are revealed beneath the full spread of the seismic line in the profile. The Moho reflector is located at approximately 42 km beneath the shot point S-6 and at approximately 29 km below sea level beneath the coastal end of the profile. There is a lateral change in reflection character in the reflective layer in the neighborhood of the shot point S-4, although the velocity of the upper crust from refraction analysis is constant in this region. The structure obtained might be one case of Seaward Dipping Reflector Sequences in the continental margin area of the past Gondwana super-continent