Historical seismicity and rates of crustal deformation along the margins of the Ordos block, north China

摘要

Earthquakes in China show an empirical relation between seismic moment (M_0) and the areal distribution of Modified Mercalli intensities VI and VIII (log M_0 = 16.66 + 0.91 log A^(VIII) and log M_0 = 14.35 + 1.16 log A^(VI), where A and M_0 are measured in squared kilometers and Newton-meter, respectively). The empirical relations may be used to estimate M_0 for historical earthquakes in China to within a factor of three, on average, when sufficient isoseismal data exist. This observation and an extensive collection of isoseismal maps are used to estimate M_0 for large earthquakes that occurred along the margins of the Ordos block during the last 700 yr. Focal parameters of the historical events are inferred from the orientation and displacements across Quaternary faults. Average rates of crustal deformation are then estimated from the 700-yr historical record with formulas that relate the occurrence rate of seismic moment in a region to rates of crustal strain. The Shanxi and northern Ningxia graben systems are situated along the eastern and northwestern edges of the Ordos block, respectively. Normal faults in the two systems trend northeasterly and are characterized by a component of right-lateral slip. The deformation resulting from slip during earthquakes in each of the respective fault systems is estimated at about 0.5 to 1.0 mm/yr of both right-lateral shear and north by northwest extension. The Weihe graben system bounds the southern edge of the Ordos block, strikes easterly and conjugate to the Shanxi and northern Ningxia fault systems, and exhibits left-lateral normal fault displacements. The average rate of deformation across Weihe is described by about 1.0 mm/yr of north by northwest extension and 1.5 mm/yr left-lateral east-west shear. The Hetao fault system delineates the northern edge of the Ordos block and displays Quaternary faults similar in orientation and mechanism to that observed in Weihe. Although mapped faults in Hetao exhibit evidence of Quaternary displacement, crustal deformation rates are not estimated because there exists no historical record of large earthquakes in the area. In southern Ningxia, at the southwest boundary of the Ordos block, deformation occurs by slip on left-lateral strike-slip faults oriented in an easterly azimuth and thrust faults with strikes ranging from southeast to south. The average deformation rate in southern Ningxia is found to be about 4.0 mm/yr of east by northeast contraction and 10.0 mm/yr of left-lateral shear. Deformation of each of the fault systems is consistent with a regional compressive stress that trends northeast and results in an average of about 3.0 mm/yr each of contraction at N70°E and extension of N160°E across the entire region. Inasmuch as uncertainties in estimates of M0 for historical earthquakes are about a factor of three, a similar uncertainty is attached to rates of crustal strain determined in this manner.