Crustal Strains in the Eastern Mediterranean and Middle East as Derived from GPS Observations

Velocity vectors estimated by GPS observations in the Eastern Mediterranean and Middle East region were used to investigate the crustal strain distribution in this region. Nine years of data from 1988 to 1997, estimated by McCLUSKY et al. (2000) were used to derive principal components of strains. We employed the Least-Squares Prediction (LSP) technique to segregate signals and noise in the data. Estimated signals were used to reconstruct the strains; dilatations, maximum shear strains, and principal axes of strains. Results of crustal strains clearly portray active tectonic strains in the study region. There are large convergent strain rates in the Mediterranean Sea, along the Hellenic Arc and south of it, which are directly related to the subduction of the African plate along this arc. Large extension strain rates with orientations varying between NNE-SSW and N-S are found in northern Aegean Sea and northwestern Anatolia. The southern Aegean Sea is characterized by relatively small strain rates. Convergent strain rate is dominant in the northeastern Africa, but occurs at a relatively low rate. Patterns of strain rates located north and south of the Bitlis suture are quite similar, suggesting that most of the motion of Arabia is being transferred directly to Anatolia. East of the Karliova triple junction (KTJ), the compressional axes of strains show a tendency of being more easterly toward the NNE, resulting in shortening normal to the Caucasus thrust front. There is no indication of active deformation (almost strain-free) along the Cyprean Arc, south of Turkey, and near the Gulf of Iskenderum. Strain rates and level of earthquake occurrence are low in the central part of Anatolia, indicating that internal deformation in this region is very small. The principal axes of GPS strain rates show remarkable agreement with seismic data and fault plane solutions. In general, distinct seismic clusters accompany the areas of high geodetic strain rates, whereas the strain-free regions are nearly aseismic

Strain Rate Distribution in Taiwan before the 1999 Chi-Chi, Taiwan Earthquake Derived from GPS Observations

In order to investigate strain distribution in Taiwan before the 1999 Chi-Chi, Taiwan earthquake, displacement rate vectors estimated by GPS observations in Taiwan were used to derive the principal components of strains. Five years of data for the period from 1990 to 1995 published by Yu et al. (1997) were used for this purpose. We employed the Least Squares Prediction technique for the strain analysis. The results suggest that Taiwan might be categorized into four regions of different tectonic backgrounds in terms of strain distributions. I) Central to south of eastern coastal range, where arc-continent collision is predominant and seismicity is very high. II) The southwestern part of the island where strain and seismicity due to subduction of the Eurasia plate is large. III) Northeastern corner of the island where effects from the backarc rifting of the Okinawa trough is eminent and positive dilatation is predominant. And, IV) the Northwestern corner where strain rate is comparatively low. The Chi-Chi earthquake occurred at the central west of the island where these four regions merge.1999年台湾集集地震がどのようなひずみ場の中で発生したかを調べるため,この地震が発生する前にGPS観測によって取得されていた変位速度データを解析した.データは,Yu et al. (1997)によって発表された, 1990年から1995年にかけて台湾内でほぼ毎年繰り返されたGPS観測による観測点の変位速度リストを用いた.このデータに我々が開発してきた最小二乗予測法を適用し,空間的になめらかな変位場を得た後,面積ひずみ,最大ずりひずみ及びひずみの主軸の,それぞれ5年間の平均的なひずみ速度を求めて図化した.その結果,台湾島内がひずみ速度分布から見て以下の4つの地域に分けられることが見て取れた; I)東部沿岸地域の中央から南部にかけてのひずみ速度が非常に大きい地域, II)台湾南西部の,ユーラシアプレートの沈み込みによる影響と見られるひずみ速度の大きな地域, III)台湾北東部の伸張ひずみが卓越する地域,及びIV)台湾北西部のひずみ速度が比較的小さい地域.集集地震はこれら4つの領域のちょうど交わる台湾中西部で発生した

GPS観測から求めた東地中海および中東地域の地殻歪

Velocity vectors estimated by GPS observations in the Eastern Mediterranean and Middle East region were used to investigate the crustal strain distribution in this region. Nine years of data from 1988 to 1997, estimated by McCLUSKY et al. (2000) were used to derive principal components of strains. We employed the Least-Squares Prediction (LSP) technique to segregate signals and noise in the data. Estimated signals were used to reconstruct the strains; dilatations, maximum shear strains, and principal axes of strains. Results of crustal strains clearly portray active tectonic strains in the study region. There are large convergent strain rates in the Mediterranean Sea, along the Hellenic Arc and south of it, which are directly related to the subduction of the African plate along this arc. Large extension strain rates with orientations varying between NNE-SSW and N-S are found in northern Aegean Sea and northwestern Anatolia. The southern Aegean Sea is characterized by relatively small strain rates. Convergent strain rate is dominant in the northeastern Africa, but occurs at a relatively low rate. Patterns of strain rates located north and south of the Bitlis suture are quite similar, suggesting that most of the motion of Arabia is being transferred directly to Anatolia. East of the Karliova triple junction (KTJ), the compressional axes of strains show a tendency of being more easterly toward the NNE, resulting in shortening normal to the Caucasus thrust front. There is no indication of active deformation (almost strain-free) along the Cyprean Arc, south of Turkey, and near the Gulf of Iskenderum. Strain rates and level of earthquake occurrence are low in the central part of Anatolia, indicating that internal deformation in this region is very small. The principal axes of GPS strain rates show remarkable agreement with seismic data and fault plane solutions. In general, distinct seismic clusters accompany the areas of high geodetic strain rates, whereas the strain-free regions are nearly aseismic

Precise troposphere delay model for Egypt, as derived from radiosonde data

Tropospheric delay computation is necessary to improve GPS measurements accuracy. Precise determination of these propagation delays requires knowledge of the full refraction profile at signal path. In the present research, precise troposphere slant delay model (PTD) is derived based on ten stations of radiosonde data well-distributed over and around Egypt. To derive the PTD, the troposphere is divided into regular small layers. Ray tracing technique of actual signal path traveled in the troposphere is used to estimate tropospheric slant delay. Real GPS data of six stations in 8-day period were used for the assessment of zenith part of PTD model against the available international models. These international models include Saastamoinen, Hopfield, and the local Egyptian dry model proposed by Mousa & El-Fiky. The data were processed using Bernese software version 5.0. The closure error results indicate that the PTD model is the best model in all session, but when the available radiosonde stations are less, the accuracy of PTD model is near to classic models. As radiosonde data for all ten stations are not available every session, it is recommended to use one of the regularization techniques for database to overcome missing data and derive consistent tropospheric delay information

1999年台湾集集地震発生前のGPS観測から得られた台湾のひずみ速度分布

In order to investigate strain distribution in Taiwan before the 1999 Chi-Chi, Taiwan earthquake, displacement rate vectors estimated by GPS observations in Taiwan were used to derive the principal components of strains. Five years of data for the period from 1990 to 1995 published by Yu et al. (1997) were used for this purpose. We employed the Least Squares Prediction technique for the strain analysis. The results suggest that Taiwan might be categorized into four regions of different tectonic backgrounds in terms of strain distributions. I) Central to south of eastern coastal range, where arc-continent collision is predominant and seismicity is very high. II) The southwestern part of the island where strain and seismicity due to subduction of the Eurasia plate is large. III) Northeastern corner of the island where effects from the backarc rifting of the Okinawa trough is eminent and positive dilatation is predominant. And, IV) the Northwestern corner where strain rate is comparatively low. The Chi-Chi earthquake occurred at the central west of the island where these four regions merge.1999年台湾集集地震がどのようなひずみ場の中で発生したかを調べるため,この地震が発生する前にGPS観測によって取得されていた変位速度データを解析した.データは,Yu et al. (1997)によって発表された, 1990年から1995年にかけて台湾内でほぼ毎年繰り返されたGPS観測による観測点の変位速度リストを用いた.このデータに我々が開発してきた最小二乗予測法を適用し,空間的になめらかな変位場を得た後,面積ひずみ,最大ずりひずみ及びひずみの主軸の,それぞれ5年間の平均的なひずみ速度を求めて図化した.その結果,台湾島内がひずみ速度分布から見て以下の4つの地域に分けられることが見て取れた; I)東部沿岸地域の中央から南部にかけてのひずみ速度が非常に大きい地域, II)台湾南西部の,ユーラシアプレートの沈み込みによる影響と見られるひずみ速度の大きな地域, III)台湾北東部の伸張ひずみが卓越する地域,及びIV)台湾北西部のひずみ速度が比較的小さい地域.集集地震はこれら4つの領域のちょうど交わる台湾中西部で発生した