Stress fields in the Antarctic plate inferred from focal mechanisms of intraplate earthquakes

Typical directional features of intraplate stresses are extracted from focal mechanism solutions of earthquakes in the Antarctic plate. Typical directions of stresses are obtained in the following regions, 1) Bellingshausen Sea, 2) south of Juan-Fernandez microplate, 3) Balleny Island region and 4) Kerguelen region. P axes in regions 1) and 2) have been interpreted by ridge push force. However these interpretations are based on one focal mechanism for each event and on crude physical concept of ridge push. It is difficult to explain intraplate stress fields in these regions only by the local ridge push force. The stress direction in region 3) can be interpreted by both deformation near triple junction and deformation due to deglaciation. Earthquakes near region 4) appear to be normal fault event. Because normal fault events appear only in the younger ocean floor, the stress field may be affected by thermal features such as hot spots Quantitative modeling and superposition of various stress factors are required to discriminate among stress origins. It is difficult to discuss stress directions in and around Antarctic continent, because number of the earthquakes is not enough

ジシン ノ メカニズム カラ ミタ ナンキョク プレート ナイ ノ オウリョクバ

地震のメカニズム解を用いて南極プレート内の特徴的な応力場を抽出した。その結果, 1) Bellingshausen海, 2) Juan-Fernandezマイクロプレート南方沖, 3) Balleny島近傍, 4) Kerguelen島地域, で特徴的な結果が得られた。近傍のリッジ押しのみによって4地域の観測事実を説明するのは困難である。このうち1), 2) での応力場はこれまでリッジ押しで説明されてきたが, データ量の増加とリッジ押しのイメージが明確になったことから, 少なくとも近傍のリッジ押しのみでは応力場は説明できない。3) での地震の起震応力場は, トリプルジャンクション近傍の変形や後氷期地殻変動が震央域を押してやることによって説明可能である。4) では正断層の地震が起こり, ホットスポットなどによる熱応力が重要になる。応力源の特定には定量的モデル化が必要である。大陸内, 大陸縁における応力場は地震が少なく明確な議論は現状では困難である。Typical directional features of intraplate stresses are extracted from focal mechanism solutions of earthquakes in the Antarctic plate. Typical directions of stresses are obtained in the following regions, 1) Bellingshausen Sea, 2) south of Juan-Fernandez microplate, 3) Balleny Island region and 4) Kerguelen region. P axes in regions 1) and 2) have been interpreted by ridge push force. However these interpretations are based on one focal mechanism for each event and on crude physical concept of ridge push. It is difficult to explain intraplate stress fields in these regions only by the local ridge push force. The stress direction in region 3) can be interpreted by both deformation near triple junction and deformation due to deglaciation. Earthquakes near region 4) appear to be normal fault event. Because normal fault events appear only in the younger ocean floor, the stress field may be affected by thermal features such as hot spots Quantitative modeling and superposition of various stress factors are required to discriminate among stress origins. It is difficult to discuss stress directions in and around Antarctic continent, because number of the earthquakes is not enough

Estimation of asthenospheric deformation law for the post glacial rebound based on the deformation mechanism map of olivine

Linear deformation laws have been applied to post glacial rebound, while many speculations based on real rock samples support non-linear rheology in the upper mantle. Essential information is required to support the assumed constitutive relation. Based on a comparison between present plate motion and formation of seismic anisotropy in the asthenosphere, the critical strain rate between linear and nonlinear deformation laws has a value of about 1~10×10^s^. This critical strain rate for formation of anisotropy due to plate motion has a larger value than typical strain rate of post glacial rebound (1~10×10^s^). Thus, the effective deformation mechanism in the asthenosphere for the post glacial rebound is linear rheology

Systematic deviations of earthquake slip vectors from NUVEL1 at the Australia-Antarctica and Pacific-Antarctica plate boundaries

Systematic deviations of slip vectors from predictions based on plate motion model (NUVEL1 and GPS observation) are discussed at the circum Antarctic plate boundaries. Earthquakes with seismic moments larger than 10^ dyne-cm are considered in the analysis. Results show clear systematic deviations at the eastern end of the Australia-Antarctica plate boundary and the eastern part of the Pacific-Antarctica plate boundary. Around the eastern end of the Australia-Antarctica plate boundary, slip vectors deviate from the relative plate motions predicted by either NUVEL1 or GPS observations. Slip vector deviations due to 1) unrecognized microplate hypothesis near the plate boundary and 2) relative motion between East and West Antarctica are inconsistent with stress and slip direction of intraplate earthquake in the Antarctic plate. Deformation of the plate boundary related to reorganization near the triple junction of the Pacific-Australia-Antarctic plates is the most plausible explanation for the slip vector deviations. In contrast, the cause of slip vector deviations observed around the eastern part of the Pacific-Antarctica remains unclear suggesting complicated origins

Local earthquake activity around Syowa Station, Antarctica

The Japanese Antarctic Station, Syowa (69°S, 39°E), is located at the edge of the east Antarctic shield. Seismic observations at Syowa Station were started in 1959. Phase readings of earthquakes have been published by National Institute of Polar Research once a year since 1968, as one of the Data Report Series. Nine local earthquakes were detected empirically on short-period seismograms at Syowa Station in 1990-1996 on the basis of long term records of many different types of events. The seismicity during that period was very low compared with that in 1987-89 when local earthquake locations were determined by the tripartite seismic array. Magnitudes of the nine earthquakes ranged from 0.1 to 1.4. Locations of four earthquakes out of nine are determined by a single station method to be in the Lutzow-Holm Bay and its northeastern coast area

Crust-mantle decoupling revealed by seismic velocity anisotropy beneath Syowa Station, Antarctica

We analyzed shear wave splittings in the crust beneath Syowa Station, using Moho converted Ps waves. Three set of receiver functions and stacked receiver functions from Tonga events are analyzed. Results are interpreted by conbining with seismic anisotropy in the mantle, which have been revealed by SKS splitting. The observed fast polarized direction of crustal shear wave splitting shows N50°W, which is nearly perpendicular to that in the mantle (N49°E). Delay times of crustal anisotropy reach 0.5s. Although part of crustal anisotropy can be caused by the deformation due to Gondwana's break up, the most plausible explanation of crust-mantle decoupled anisotropy is related to the collisional deformation 500 Ma. If delamination between the crust and the mantle occurred in a former stage of metamorphism with subsequent crustal extrusion and mantle subduction, then observed seismic anisotropy in both the crust and the mantle can be explained

Report on workshop "Public use and scientific study of broadband seismic data at Syowa Station, Antarctica"

A workshop on "Public Use and Scientific Study of Broadband Seismic Data at Syowa Station, Antarctica" was held on January 25,1996 at the National Institute of Polar Research, with 12 participants. The history and present status of seismic observations were reported, followed by presentations concerning scientific studies using broadband data in Antarctica. The main contributions are (1) studies of the velocity structure and anisotropy in the crust and upper mantle using short-period body waveforms and travel time data and (2) analyses of the heterogeneity and anisotropy of seismic wave velocity in the core and the lowermost mantle by using short period core phases and free oscillations of the Earth. Finally, discussions concerning data archiving, public use and future plans of the Japanese Antarctic Research Expedition (JARE) were conducted in regarding to the above scientific results