A Pragmatic Approach to Seismic Parameters in a region with Low Seismicity: The Case of Eastern Iberia

In intra-plate regions with low-seismic activity, seismic cycles last between 10³ and 10⁵ years and, consequently, quiescent faults may be capable of producing catastrophic earthquakes. Paleoseismic studies, which are necessary to define capable faults in these regions and to establish their seismic parameters, are scarce and not always possible. In order to overcome the scarcity of paleoseismic data, this paper proposes an approach to evaluate the capability and seismic parameters of faults. This approach takes into account instrumental, historical, paleoseismological and geological data. Assuming that in a given structural region with definite climatic and geological characteristics similar geomorphic features reflect a similar structural-seismic evolution, the capability of faults is established by comparing the fault related geomorphic features of the few proven seismogenic faults with those of the rest of the faults. The seismic parameters were estimated using some mathematical relationships from geological and geomorphological data, where the slip rate of the faults was determined from geological and geomorphological criteria. The case of eastern Iberia is presented to illustrate this approach. This area is characterized by low seismicity, few historical destructive earthquakes, and only one fault with good paleoseismological data. Of the 249 potential faults (longer than 10 km and adequately oriented with respect to the current stress field), 23 were regarded as capable faults. Some of these were located in seismic quiescent areas, and their main seismic parameters were estimated.     

A new interpretation of Stephanian deformation in the Decazeville basin (Massif Central, France): consequences on late Variscan tectonism

Five stages of faulting were observed in and around the Stephanian Decazeville basin, in the SW French Massif Central, at the southern edge of the Sillon houiller fault. The older stage ends during middle Stephanian time, and corresponds to a strike-slip regime with N–S shortening and E–W extension. Before the end of the middle Stephanian, three other stages were recorded: two strike-slip regimes with NW–SE, then E–W compression and NE–SW, then N–S extension; and finally a NNE–SSW extensional regime during the main subsidence of the basin from the end of the middle Stephanian to late Stephanian. Based on mining documents, a new interpretation of the N–S striking folds of the Decazeville basin is proposed. Folding may not be associated with E–W compression but with diapirism of coal seams along syn-sedimentary normal faults during the extensional phase. A last strike-slip regime with N–S compression and E–W extension may be related to Cainozoic Pyrenean orogeny. At a regional scale, it is suggested that from the end of the middle Stephanian to the late Stephanian, the main faults in the Decazeville basin may represent a horsetail splay structure at the southern termination of the Sillon houiller fault.     

浅谈活断层及其研究方法

介绍了活断层的基础理论知识,包括活断层性质及地震的概念及特性。从理论上探讨了地震与活断层的关系,认为地震的产生往往伴随断层位移且可常见于地表。以已有的科研成果为例,总结了近年专家、学者研究活断层的几种方法及其应用,提出了综合应用活断层研究方法及相互论证是预测地震更为有效的技术。阐述了对地震产生的动力机制的认识,并认为活断层往往是地震孕育场所,但不一定所有地震都在活断层的基础上发生。     

Trenching studies of active faults in Kamchatka, eastern Russia: Palaeoseismic, tectonic and hazard implications

The central part of the Kamchatka Peninsula is characterized by a well defined depression associated with active volcanism, aligned NE–SW. On the east, the depression is bounded by a prominent system of active faults known as the East Kamchatka Fault Zone (EKFZ). In order to improve understanding of the behaviour and kinematic role of this fault zone a fieldwork programme, including study of trenches, was conducted in the north-central part of this system. Aerial photograph analysis, ground-truthed, indicates a westward fault dip with predominantly normal slip, while lateral offsets of river terraces and stream channels demonstrate a combined dextral component. Over 20 excavated pits and natural exposures were examined to confirm a detailed tephra succession extending from the early Holocene to recent historic eruptions. This chronological framework then provided age control on five past faulting events recognised in three trenches. These events took place at about 10.5, 6.0, 4.5 and, in a two-event succession within a short time span, at 3.3–3.2 ka BP. Event clustering may be characteristic and fault length–displacement values suggest earthquakes of M6.5, thus representing a significant new element in regional seismic hazard evaluations; additional to events generated at the subduction interface. The relatively long gap in faulting since the two most recent events may also be significant for hazard scenarios and there is a possible link between the faulting and volcanic activity in the depression. Overall, the EKFZ, together with the Nachiki Transverse Zone farther south, is thought to define a regional-scale block that is extending eastwards independently from the rest of Kamchatka.     

Active rifting in the Japan and Okhotsk seas and the tectonic evolution of the Central Sakhalin Fault zone in the Cenozoic

Large-scale geological maps available for individual areas in the Central Sakhalin Fault zone and geological-geophysical maps of Sakhalin and surrounding sea areas were analyzed to elucidate the tectonic evolution of the fault zone determined by movements of crustal blocks due to the opening of rift basins. Changes in the direction of horizontal compression in the Sakhalin fold system from diagonal (NW-SE) to near-latitudinal resulted in the transformation of near-meridional right-lateral strike-slip faults into reversed faults in the Late Miocene. This allows Sakhalin faults to be interpreted as a zone of recent right-lateral shear between Eurasian and Sea of Okhotsk plates.     

Relations between left-lateral strike-slip faults and right-lateral monoclinal kink bands in granodiorite,Mt. Abbot quadrangle,Sierra Nevada,California

Small left-lateral strike-slip faults and right-lateral monoclinal kink bands with subvertical fold axes may be related to the formation of a very large right-lateral kink band (Bear Creek kink band), about 8 km wide and at least 15 km long, trending N27W along Bear Creek Valley in the Mt. Abbot quadrangle, Sierra Nevada, California.A foliation within Bear Creek Valley is defined by vertical slabs of granodiorite bounded by joints and faults. Small strike-slip faults and larger fault zones have nucleated along preëxisting joints and accommodated shearing between granodiorite slabs. The orientations of small cracks that occur near the tips of faults or connect adjacent fault segments indicate that the direction of maximum compression was about 20° counterclockwise from traces of joints at the time the faults nucleated. In some places where faults are closely spaced there are small, right-lateral kink bands with widths of 1 to 20 m. The slabs of granodiorite are gently curved through the kink bands, and analysis of the orientations of slabs in the limbs of the small kink bands indicates that the direction of maximum compression during kink-band formation was 15° to 20° counterclockwise from the traces of faults outside the kink bands. The orientation of the maximum compression for the formation of the small cracks at tips of many strike-slip faults and for the formation of the small kink bands, relative to the orientation of the maximum compression inferred from the joints on the limb of Bear Creek kink band, suggests that the foliation within the Bear Creek Valley has reoriented a maximum of 40° to 60° clockwise. Although the various orientations of joints, faults, and kink bands could be explained in terms of different regional compression directions at different places and at different times in the Mt. Abbot quadrangle, a much simpler interpretation, based on analysis of large and small structures in the granodiorite in Bear Creek Valley, is that they all formed in response to one maximum regional compression in the direction N25E.     

Brecciation processes in fault zones: Inferences from earthquake rupturing

Surface-rupture patterns and aftershock distributions accompanying moderate to large shallow earthquakes reveal a residual brittle infrastructure for established crustal fault zones, the complexity of which is likely to be largely scale-invariant. In relation to such an infrastructure, continued displacement along a particular master fault may involve three dominant mechanical processes of rock brecciation: (a)attrition brecciation, from progressive frictional wear along principal slip surfaces during both seismic and aseismic sliding, (b)distributed crush brecciation, involving microfracturing over broad regions when slip on the principal slip surfaces is impeded by antidilational jogs or other obstructions, and (c)implosion brecciation, associated with the sudden creation of void space and fluid-pressure differentials at dilational fault jogs during earthquake rupture propagation. These last, high-dilation breccias are particularly favorable sites for hydrothermal mineral deposition, forming transitory low-pressure channels for the rapid passage of hydrothermal fluids. Long-lived fault zones often contain an intermingling of breccias derived from all three processes.     

海底浅层地质灾害的高分辨率地震识别技术

将渤海某油田最新采集的二维高分辨率资料处理解释后,结合区域地球物理及地质概况,利用地震相分析、波阻抗反演、井(孔)震标定等深层油气勘探的成熟技术,系统研究了各类海底浅层地质灾害因素的成因、特征、危害及展布规律,总结了一套完整的利用高分辨率地震识别海底浅层地质灾害的技术方法。结果表明,浅层断裂、浅层气和埋藏古河道是研究区海底浅层发育的主要地质灾害因素,通过刻画不同期次地质灾害因素的类型及其分布范围,为今后该油田海上施工提供了可靠的工程地质调查成果。因此,高分辨率地震技术能够很好地应用于海底浅层地质灾害的识别。     

Interpretation of gravity profiles across the northern Oaxaca terrane,its boundaries and the Tehuacán Valley,southern Mexico

A gravity study was conducted across the northern Oaxaca terrane and its bounding faults: the Caltepec and Oaxaca Faults to the west and east, respectively. These faults juxtapose the Oaxaca terrane against the Mixteca and Juarez terranes, respectively. The Oaxaca Fault also forms the eastern boundary of the Cenozoic Tehuacán depression. On the west, at depth, the Tehuacán valley is limited by the normal buried Tehuacán Fault. This gravity study reveals that the Oaxaca Fault system gives rise to a series of east tilted basamental blocks (Oaxaca Complex). The tectonic depression is filled with Phanerozoic rocks and has a deeper depocenter to the west. The gravity data also indicate that on the west, the Oaxaca Complex, the Caltepec and Santa Lucia faults continue northwestwards beneath Phanerozoic rocks. A major E–W to NE–SW discontinuity is inferred to exist between profiles 1 and 2.     

氡(Rn)射气测量在胜利油田隐伏断裂研究中的应用

土壤氡(Rn)射气测量是探测隐伏断裂的存在,判定断裂的位置、走向、倾向的一种有效手段。通过在胜利油田进行的氡射气探测并结合其它勘探手段,发现该区主要的隐伏潘动断裂有无棣—益都断裂、胜北断裂和埕子口断裂。其中,无棣—益都断裂活动性最强。