关键词: 震源机制解, 构造应力场, 鄂尔多斯地块

Abstract: This paper discusses mainly the spatial distribution and change of stress orientation at focal depth around the Ordos fault block. The stress orientation are inverted by using the Grid Test Method from more than 3 000 earthquake focal mechanism solutions calculated from earthquakes with magnitude less than 5 recorded by local seismological stations around the Ordos during the years of 1980 to 1999. The region around the Ordos is divided into ten subregions based on neotectonic, geomorphologic and GPS data. The P and T axes being close to horizontal in all subregions implies that the Ordos and its vicinity are dominated by strike-slip motion. The spatial distribution pattern of the principal compressive stress axes around the Ordos Block appears as a fan convex toward the southwest of the Ordos Block. The mean orientation of the P axis is NE-directed in the west, ENE-directed in the north, and nearly E-W-directed in the south of the Ordos Block. In fact, the orientation of P axis in each subregion may vary with time. In 1992 and 1996, the direction of P axis in each subregions was gradually turned toward N75癊 direction. Based on the changes of P axis orientation with time in each subregion, three types of subregions are identified: the first type includes the areas where the orientation of P axis is relatively unchanged, such as the Weihe, Liupanshan and Yingchuan subregions; the second type includes the areas where the change of the orientation of P axis with time is complex, such as the Linfen and Tongxin subregions; the third type includes the areas where the orientation of P axis is sensitive to the earthquakes occurred nearby, such as the Datong and Baotou subregions. In Datong and Baotou subregions, the changes of P axis orientation doesn't simultaneously respond to the occurrence of an earthquake; when the direction of P axis in one subregion turns to NNE direction, then an earthquake will occur in this area. However, during the same period, the orientation of P axis in the other subregions doesn't turn to NNE direction. Another interesting thing is that the inconsistency ratio of the Grid Test Method for Datong subregion decreased before the Datong earthquake in 1989, while the ratio for Baotou subregion decreased before the Baotou earthquake in 1996.

Key words: earthquake focal mechanism, tectonic stress field, Ordos fault block