华北北部地区地震中短期前兆场的震中区前兆空区模式

华北北部地区３次强震的交兆场研究结果表明,在地震发生前几个至十几（或几十个）个月的时间尺度上,未来震中区表现为前兆平静或空白的特征,同期外转眍中小地震活动十分活跃,地震前兆异常也大量涌现。根据这些现象,提出了地震中短期前兆场的震中区前兆空区模式。     

鲁苏皖交界地区地震活动特点浅析

从历史及现代两个时期对三省(鲁苏皖)交界区域地震活动特点、规律进行了讨论和分析。从历史活动周期及小震空区空间演化特征,以不同的角度讨论分析研究了所属区域的震情发展趋势。指出近期(1998年起)形成的空区为孕震空区,且空区周围应力场背景显示增强的态势,并依据历史地震南北迁移特点,认为34°N以南区域为未来发生5级以上地震的地区,应注意加强监视。     

具有动力学含义的强震动态图像预测方法研究

地震空区方法是利用地震活动图像预报地震效果较好的方法,然而单纯通过空区预报地震具有一定的局限性和不确定性。建立具有动力学含义的动态图像预测方法,把空区的形成和演化放在板块运动和构造特征的基础上考虑。通过对华东和华南地区26次5级以上地震的震例分析,给出主震发生与空区、活动断裂位置以及空区与活动断裂位置和走向的关系,并从两个区域的构造差异上给出空区形成和演化规律差异的动力学解释,从而为使用空区预测地震提供物理基础,减少了不确定性。     

Resistivity structure of a seismic gap along the Atotsugawa Fault, Japan

Seismicity along the Atotsugawa Fault, located in central Japan, shows a clear heterogeneity. The central segment of the fault with low-seismicity is recognized as a seismic gap, although a lot of micro-earthquakes occur along this fault. In order to elucidate the cause of the heterogeneity in seismicity, the electrical resistivity structure was investigated around the Atotsugawa Fault by using the magnetotelluric (MT) method. The regional geoelectrical strikes are approximately parallel to the fault in a low-frequency range. We constructed two-dimensional resistivity models across the fault using TM-mode MT responses to minimize three-dimensional effects on the modeling process. A smooth inversion algorithm was used, and the static-shifts on the apparent resistivity were corrected in the inversion process.A shallow, low resistivity zone along the fault is found from the surface to a depth of 1-2 km in the best-fit model across the high-seismicity segment of the fault. On the other hand, the corresponding low resistivity zone along the low-seismicity segment is limited to a shallower depth less than 1 km. The low resistivity zone along the Atotsugawa Fault is possibly due to fluid in the fracture zone; the segment with higher levels of seismicity may have higher fluid content in the fault zone compared with the lower seismicity segment. On a view of the crustal structure, a lateral resistivity variation in a depth range of 3-12 km is found below the fault trace in the high-seismicity segment, while a resistive layer of wide extent is found at a depth of about 5 km below the fault trace in the low-seismicity segment. The resistive layer is explained by less fluid condition and possibly characterized as high rigidity. Differences in the resistivity structures between low and high-seismicity segments of the fault suggest that the seismic gap in the central part of the Atotsugawa Fault may be interpreted as a locked segment. Thus, MT is an effective method in evaluating a cause and future activity of seismic gaps along active faults.The lower crust appears as a conductive zone beneath the low-seismicity segment, less conductive beneath the high-seismicity segment. Fluid is inferred as a preferable cause of the conductive zone in this study. It is suggested that the conductive lower crust beneath the low-seismicity segment is recognized where fluid is trapped by an impermeable layer in the upper crust. On the other hand, fluid in the lower crust may upwell to the surface along the high-seismicity segment of the fault.     

江苏常熟—太仓5.1级地震前兆异常的识别

本文讨论了當熟—太仓5.1级地震前,邻近地区曾多次出现小震震群,形成一个空区之后,在黄海“老震区”出现相对的波形异常。研究结果表明,这些空区和波形异常的出现,除了与地质条件有关外,还显示了出现主震前兆异常的可能性。     

地震动态空区法与中、强震预报

本文在组合模式和立交模式的基础上提出了预报中、强地震的动态空区法，应用该法对我国大震进行了回顾性检验。结果表明，这一方法比单空区确定地震危险区有重要改进。文中还讨论了由动态空区出现的频次增多、地震共轭条带以及中小地震活动性加剧等判定震源断层的致锁和解锁时间，由此预测未来主震的震级和发震时间。     

苏联斋桑7.3级地震前地震活动的某些特征

1990年6月14日苏联斋桑7.3级强震发生在重力异常梯度带的拐弯处;强震前存在明显的M_s≥3.5级地震围成的空区;空区走向为北西西向,与斋桑强震发震构造走向一致,长轴约170km。1990年2月14日4.0级地震可视为信号震。最后初步讨论了阿勒泰活动区的地震趋势。     

山东地区及其海域近期地震活动特点

本通过对山东地区及其海域的历史地震和近代小震活动的分析，认为该区的历史地震活动，自１６８８年以来存在三个Ｍ≥５级地震活动空段；而１９６０年以来，该区ＭＬ≥４．５级地震具有８年的活跃时间段的特点。     

六盘山断裂带的地震构造特征与强震危险背景

集成活动构造与震源机制解、重新定位小震分布、历史与现今地震、GPS速度场等资料,综合分析了六盘山断裂带的构造动力学条件与变形方式、横剖面构造、历史强震破裂背景、GPS形变以及现代地震活动性,进而探讨了该断裂带的强震危险背景.结果表明：NNW向六盘山断裂带的运动与变形主要缘于青藏地块东北缘的向东水平挤出受到相对稳定的华北地块西缘（鄂尔多斯地块）阻挡而聚集的水平挤压作用;此外,海原和陇县-宝鸡两条NW向走滑断裂带的左旋运动在右阶区的局部会聚作用,也由六盘山断裂带的变形与运动来承受与转换.横剖面上,六盘山断裂带表现为向东推覆的大型逆冲构造带,主滑脱带位于~25 km深处,之下很可能存在分隔青藏与华北地块的超壳-岩石圈型深断裂带.沿六盘山断裂带中-南段以及更靠南东的陇县-宝鸡断裂带存在总长为120~140 km、至少最近~1400年未发生M ≥ 6½强震破裂的地震空区.地震空区内的断裂,GPS形变显示已有显著应变积累,地震活动上出现为小震稀疏或空缺的部位,以及低b值区,反映那里的断面业已闭锁,并已有高应力积累.因此,六盘山断裂带中-南段和陇县-宝鸡断裂带应是未来可能发生强震/大地震的两个危险地段,潜在地震的最大矩震级估值分别为M_W=7.3±和7.2±.

     

Seismotectonic Reactions to the Arc-continent Convergence in Central Philippines

Abstract. High seismic activity in the Philippines originates from tectonic convergence related to surface and subsurface seismotectonic features. Based on earthquake data, the archipelago can be divided into the seismically-active Philippine Mobile Belt and the aseismic North Palawan Block. The latter represents a rifted continental fragment of the Eurasian margin that juxtaposed with the rest of island arc units in central Philippines. Earthquake hypocenter plots on planar and in vertical profiles show that the seismic events are associated with known seismotectonic features. In addition, data suggest that the collision zone between the North Palawan Block and the Philippine Mobile Belt is characterized by a decreased amount of hypocenters at > 100 km depths. Although field evidence favors the presence of a subducted slab or slabs beneath western central Philippines, these are difficult to image using the present seismicity distribution dataset.