The Thickness of Seismogenic Layer Under the Areas Surrounding the Ordos Block, Northern China

A seismic approach was used to calculate the thickness distribution of the seismogenic layer under the block Ordos in northern China. This block was chosen because of its stability, the complicated tectonics around its boundaries and the completeness of the data set. Several strong earthquakes occurred in this area in the 20th century, with the largest one (Haiyuan, M ¼ 8.4, 1920) in the southwest of the Ordos. Most of the large faults around the Ordos are strike-slip ones. The breaking point (i.e., the saturation of earthquake magnitudesMc) of the self-similarity from small to large events based on the Gutenberg-Richter relation is calculated. Under the assumption of the L model which expresses that the offset is proportional to the length of the fault (SCHOLZ, 1982, 1990, 1994), the thickness (Wc) of the seismogenic layer is directly related to Mc. To display smoothly the change of the thickness of the seismogenic layer and to ensure the reliability of the results, a moving cell of 150 events, with a moving step of 0.1 degree, was adopted. To take advantage of the existing data set, the ‘‘mean value method,’’ which is based on the latest complete data information to make the existing data set a ‘complete’ one, was used to extrapolate it so that the data covers the whole time period. The results show that the average thickness of the seismogenic layer in the southwest, in the northwest and in the east of the Ordos is around 30, 9 and 17 km, respectively. The thickness generally decreases from the southwest to the northeast. This kind of spatial variation is in agreement with the maximum depth distribution of large amounts of microearthquakes, with the correlation coefficient to be about 0.88, and the Moho depth.