Earthquake epicentroids in the Beijing-Tianjin-Tangshan-Zhangjiakou region inversed by gravity variation data

Abstract

Gravity variation data observed in the process of seismogenesis and occurrences of earthquakes show that the location with the greatest gravity changes does not necessarily coincide with the epicenter. To explain this we defined the center of effective mass of stress volume as "hypocentroid", and the vertical projection of which on the earth's surface as "epicentroid". Here we adopt three rotating models, including spheroid, ellipsoid and cylinder, to represent the region of an impending earthquake. Based on the models of gravity variations induced by uniform dilatancy, epicentroids associated with sixteen earthquakes with M>4.0 occurred in 1981-2000 in the Beijing-Tianjin-Tangshan-Zhangjiakou region are determined by means of a proposed least squares iterative inversion method. The results indicate that cylinder model is preferable to the other two, and epicentroids obtained by the cylinder model separate from the epicenters by a range of 0-40 kin. Epicentroids are inevitably located within intact tectonic blocks, and usually cluster in groups; while the epicenters are generally located at the terminations of faults or at the intersections of faults. It seems that there exist earthquake-hatching areas in the block among faults. Earthquakes hatch in these areas, but occur around these areas, meanwhile the existence of faults may play an important role in controlling the processes.

Earthquake-related gravity field changes at Beijing-Tangshan gravimetric network during 1987–1998

“…4−7 and Table 1 in case of A, C, F and H earthquakes), but there are also some exception with certain distance from the epicenter (see the pictures of B and G earthquakes) which might be explained by the study of Professor Kuo: in BTTZ region, epicenter usually locates at the fault end or its intersection while the epicentroid, which is the earth surface projection of the beneath mass center of emigrant mass, usually locates in a whole block (Kuo et al, 1999). From the study of a sample of 16 earthquakes (M > 4.0), the difference between the two is mostly in the range of 0−40 km (Zheng et al, 2003). The differences shown in the case of earthquake B and G are now somewhat larger than 40 km.…”

The Types of Thermal Spring and its Geological Distributive Characteristics in Yanshan Orogenic Belt, China

“…The explanation of lithospheric thinning in Yanshan area is that the mantle and thermal column is uplifting stronglly with a mushrooms shape, making a great slippage to the thermal thinning of the earth's crust when detaching from outside in North China [5]. Besides, proofs of crust thinning in North China(including Yanshan Orogenic belt) are provided by artificial earthquake measure and explanation:Tengjiwen(1979) used 8 times Industrial blast combining with natural earthquakes to study crustal structure of Beijing-Tianjin-Tangshan-Zhangjiakou and drew a conclusion: The earth's crust in Beijing and its east area is 35±1.5km, the north area 37±1.0km and the west area 39±1.5km [18].…”