华北地区地震震源机制分区特征

利用华北地区（北纬36°—42°；东经111°—125°范围）2010年1月至2014年6月≥M_L2.5的918个地震事件波形资料，采用FOCMEC方法计算震源机制，根据参与计算的清晰初动数量及振幅个数，以及在分别采用5°、10°、15°不同步长搜索震源机制解结果必须为同一组接近解的原则，我们共得到572个可靠的震源机制解.对于震级相对较大且波形低频部分信噪比较好的地震，同时采用TDMT全波形反演方法反演了矩张量，最终得到14个地震的矩张量，并与利用FOCMEC方法得到的震源机制解进行了比较.同时我们还搜集了1937年以来华北地区中强地震的震源机制解结果.根据震源机制类型特征及构造特点，我们从空间上对震源机制结果进行了分区分析.结果表明：研究区内中小地震的震源机制类型相对复杂，但仍能看出中小地震震源机制有显著的分区特征，震源机制主要类型是正断型和走滑型，并且大部分正断型震源机制分布在山西断陷带、唐山老震区、海城老震区内.该现象表明研究区内主要变形以平移和拉张为主；同时通过大于4级以上地震震源机制类型主要为走滑型可得出，走滑型应力在华北地区应力场上占绝对优势，但是局部地区的正断型应力也比较显著，比如山西断陷带、海城老震区、唐山老震区、渤海内（烟台—蓬莱段局部地区）.

Zonational characteristics of earthquake focal mechanism solutions in North China

Using waveform data we applied the FOCMEC method to calculate focal mechanisms of 918 M_L ≥ 2.5 earthquakes occurred during the period of Jan. 2010 to Jun. 2014 in North China (latitude 36°-42°; longitude 111°-125°). Among the computed solutions, 572 were found robust based on their clear first motions and amplitudes in the event waveforms, and the similarities among solutions using different (5°, 10°, and 15°) step sizes. Using the TDMT full waveform inversion method, we further calculated the moment tensors for 14 large events which have higher signal-to-noise ratio at low frequencies, and compared the solutions with that by the FOCMEC method. In addition, we compiled focal mechanism solutions for moderate and large earthquakes since 1937 in North China. Based on the focal mechanism types and their correlation with surface tectonics, we characterized the focal mechanism solutions into several spatial clusters. Our results show the solutions for moderate and small earthquakes are generally complex in the study region, but some intriguing patterns can be recognized. The focal mechanisms of moderate and small earthquake have a clear zoning characteristic, and they are mainly normal and strike-slip fault types, suggesting that crustal deformation is dominated by translation and tension. Most of the normal type focal mechanisms are distributed in the Shanxi rift zone, Tangshan and Haicheng seismic zones. For major earthquakes (M_L ≥ 4.0), we found that the focal mechanism type is mainly of strike-slip. We conclude that the horizontal stress is dominant in North China, while in localized regions such as the Shanxi rift zone, Tangshan and Haicheng seismic zones, and the Bohai sea (the local region of Yantai-Penglai section), the tensile stress may be prevailing.