Isoseismal map of the 1755 Lisbon earthquake obtained from Spanish data

The Royal Academy of History (RAH) of Spain collected very detailed information about the effects on the Spanish territory of the Lisbon, 1755 earthquake, reporting such information within a year after the shock. The data provided by the RAH has been thoroughly analyzed, deriving an M S K intensity value for each one of the localities (more than 1000) included in the report, and an isoseismal map has been obtained from such values. Observations reported, such as tsunami, surface effects, duration of shaking and effects on the population are also discussed. Finally, a comparison is made of the data with the results of recent investigations on the tectonics of the region.     

Deep structure, recent deformation and analog modeling of the Gulf of Cadiz accretionary wedge: Implications for the 1755 Lisbon earthquake

The Gulf of Cadiz spans the plate boundary between Africa and Eurasia west of the Betic-Rif mountain belt. A narrow east dipping subduction zone descends beneath the Gulf of Cadiz and the straits of Gibraltar. The deep crustal structure of the Gulf and the adjacent SW Iberian and Moroccan margins is constrained by numerous multi-channel seismic reflection and wide-angle seismic surveys. A compilation of these existing studies is presented in the form of depth to basement, sediment thickness, depth to Moho and crustal thickness maps. These structural maps image an E-W trending trough, with thin (< 10 km) crust beneath the Gulf of Cadiz. This trough is filled by an eastward thickening wedge of sediments, reaching a thickness of 10-15 km in the eastern Gulf. These sediments are tectonically deformed, primarily along a series of westward-vergent thrust faults and represent a 200-250 km wide accretionary wedge. The northern and especially the southern limits of the accretionary wedge are marked by sharp morphological lineaments showing evidence of recent deformation. These tectonic limits are situated in an internal position with respect to the Miocene deformation front (external Betic and Rif allocthons), which has been abandoned. At the western boundary of the accretionary wedge, near the adjacent Seine and Horseshoe abyssal plains, an E-W trending basement high (Coral Patch Ridge) can be seen indenting the deformation front in an asymmetric manner. Analog modeling is performed using granular materials accreted against a semicircular backstop (representing the basement of the Rif and Betic mountain belts). The modeling initially produces a symmetric, arcuate accretionary wedge. The ensuing collision of an oblique rigid indenter retards accretion on one side, resulting in an embayment and a locally steeper deformation front. The deformation pattern observed in morphology and high-resolution seismic profiles suggests the accretionary wedge and underlying subduction system is still active. The implications of active subduction for the source region of the 1755 Lisbon earthquake and the regional seismic hazard assessment are discussed.     

The Gibraltar Arc seismogenic zone (part 2): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by tsunami modeling and seismic intensity

The Great Lisbon earthquake has the largest documented felt area of any shallow earthquake and an estimated magnitude of 8.5–9.0. The associated tsunami ravaged the coast of SW Portugal and the Gulf of Cadiz, with run-up heights reported to have reached 5–15 m. While several source regions offshore SW Portugal have been proposed (e.g.— Gorringe Bank, Marquis de Pombal fault), no single source appears to be able to account for the great seismic moment as well as all the historical tsunami amplitude and travel time observations. A shallow east dipping fault plane beneath the Gulf of Cadiz associated with active subduction beneath Gibraltar, represents a candidate source for the Lisbon earthquake of 1755.Here we consider the fault parameters implied by this hypothesis, with respect to total slip, seismic moment, and recurrence interval to test the viability of this source. The geometry of the seismogenic zone is obtained from deep crustal studies and can be represented by an east dipping fault plane with mean dimensions of 180 km (N–S) × 210 km (E–W). For 10 m of co-seismic slip an Mw 8.64 event results and for 20 m of slip an Mw 8.8 earthquake is generated. Thus, for convergence rates of about 1 cm/yr, an event of this magnitude could occur every 1000–2000 years. Available kinematic and sedimentological data are in general agreement with such a recurrence interval. Tsunami wave form modeling indicates a subduction source in the Gulf of Cadiz can partly satisfy the historical observations with respect to wave amplitudes and arrival times, though discrepancies remain for some stations. A macroseismic analysis is performed using site effect functions calculated from isoseismals observed during instrumentally recorded strong earthquakes in the region (M7.9 1969 and M6.8 1964). The resulting synthetic isoseismals for the 1755 event suggest a subduction source, possibly in combination with an additional source at the NW corner of the Gulf of Cadiz can satisfactorily explain the historically observed seismic intensities. Further studies are needed to sample the turbidites in the adjacent abyssal plains to better document the source region and more precisely calibrate the chronology of great earthquakes in this region.     

The Gibraltar Arc seismogenic zone (part 1): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by seismic data, gravity and thermal modeling

The Great Lisbon earthquake of 1755 with an estimated magnitude of 8.5–9.0 is the most destructive earthquake in European history, yet the source region remains enigmatic. Recent geophysical data provide compelling evidence for an active east dipping subduction zone beneath the nearby Gibraltar Arc. Marine seismic data in the Gulf of Cadiz image active thrust faults in an accretionary wedge, above an east dipping decollement and an eastward dipping basement. Tomographic and other data support subduction and rollback of a narrow slab of oceanic lithosphere beneath the westward advancing Gibraltar block.Although, no instrumentally recorded seismicity has been documented for the subduction interface, we propose the hypothesis that this shallow east dipping fault plane is locked and capable of generating great earthquakes (like the Nankai or Cascadia seismogenic zones). We further propose this east dipping fault plane to be a candidate source for the Great Lisbon earthquake of 1755. In this paper we use all available geophysical data on the deep structure of the Gulf of Cadiz–Gibraltar region for the purpose of constraining the 3-D geometry of this potentially seismogenic fault plane. To this end, we use new depth processed seismic data, have interpreted all available published and unpublished time sections, examine the distribution of hypocenters and perform 2-D gravity modeling. Finally, a finite-element model of the forearc thermal structure is constructed to determine the temperature distribution along the fault interface and thus the thermally predicted updip and downdip limits of the seismogenic zone.     

LISBON RECALLED: ALL SAINTS DAY, 1 NOVEMBER 1755

In this, the eighth in an occasional series of earth phenomena, the author recounts his experience of by far the largest earthquake to hit Europe in the last few centuries.     

Rebel with a cause? Iconography and public memory in the Southern United States

Recent years have witnessed debates in the American South between traditional white Southerners and African American Southerners over whether and how symbols from the region's two defining historical events – the Civil War and the Civil Rights movement – are displayed on the region's landscape. This paper examines the most contentious of these debates, the conflict over government sanction for flying the various flags of the Confederate States of America. This article first discusses the concepts of iconography and public memory, and then the role of Confederate flags in traditional white Southern iconography. We then examine four recent attempts in the states of Georgia, Alabama and South Carolina, and within the chambers of the U.S. Senate to remove government sanction for flying Confederate flags. We conclude from these debates that while icons can act as centripetal forces binding a people together, they can also emerge as centrifugal forces, further splitting apart a region's population along major cultural and racial divisions.     

Governing Metropolitan Lisbon: A tale of fragmented urban governance

This article is concerned with the nature of the present governance framework in the Lisbon metropolitan area and with the conditions responsible for the lack of any consistent intergovernmental and inter-tier co-ordination. It shows how Lisbon, where a form of metropolitan government was introduced only in 1991, is different from other European cities, but at the same time similar with regard to the limits associated with the associative or (polycentric) model adopted. The article concludes by discussing alternative scenarios for the reform of metropolitan government as well as the institutional and policy challenges facing the Lisbon metropolitan area. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Influence of the North Atlantic Oscillation on Rainfall Triggering of Landslides near Lisbon

The majority of landsliding episodes in the area north of Lisbon are associated with rainfall events of short (less than 5 days) medium (5–20 days) or long duration (more than 20 days). The precipitation regime in Portugal is highly irregular, with large differences between wet and dry years. We have assessed the impact of the North Atlantic Oscillation (NAO) on both the winter precipitation and the timing and magnitude of associated landslide events. Results show that the large inter-annual variability of winter precipitation is largely modulated by the NAO mode. The precipitation composite corresponding to high NAO index presents a considerable lower median value (47 mm/month) than the corresponding low NAO index class (134 mm/month). The entire precipitation distribution associated with the low NAO index composite encompasses a wider range of values than the corresponding high NAO index composite. This non-linear behavior is reflected in the probability of occurrence of a very wet month (precipitation above the 90% percentile) that is just 1% for the positive NAO class and 23% for low NAO index months. Results for the low NAO class are crucial because these months are more likely associated with long-lasting rainfall episodes responsible for large landslide events. This is confirmed by the application of a 3-month moving average to both NAO index and precipitation time series. This procedure allowed the identification of many months with landslide activity as being characterized by negative average values of the NAO index and high values of average precipitation (above 100 mm/month). Finally, using daily data we have computed the return periods associated with the entire set of landslide episodes and, based on these results, obtained a strong linear relationship between critical cumulative rainfall and the corresponding critical rainfall event duration.     

Gated communities, commodification and aestheticization: The case of the Lisbon metropolitan area

This paper focuses on the commodification processes and the aestheticization practices that shape gated communities and on the way these are connected to their segregational dimension, which is illustrated through the case of the Lisbon Metropolitan Area (LMA). First, I discuss the (increasing) aestheticization of commodities, emphasising some of the more problematic theoretical issues it involves. Following this discussion I present and analyse the ‘Lisbon case’. I provide a brief overview of gated communities in the LMA and then present and analyse the results obtained through a content analysis of LMA gated community advertisements and observation of their site semantics as conveyed through general built form, including physical location, planning, architecture, landscaping, and toponymy. I relate these findings to the LMA gated community market characteristics and to the broader prevailing socio-spatial context regarding their emergence.     

遥感图像处理在“5．12”汶川地震抗震救灾工作中的应用

遥感技术在本次“5．12”汶川特大地震抗震救灾工作中发挥了前所未有的重要作用,为应急抗震救灾、应急次生地质灾害排查和灾后规划重建工作提供了重要的第一手资料。本文介绍了我们通过参与震后大量应急图像处理工作,在实际工作中总结出一套特殊的应急图像处理手段、方法和取得的圆满成果。     

Landslides in the North of Lisbon Region (Portugal): Conditioning and triggering factors

Detailed geomorphological mapping carried out in 5 sample areas in the North of Lisbon Region allowed us to collect a set of geological and geomorphological data and to correlate them with the spatial occurrence of landslide. A total of 597 slope movements were identified in a total area of 61.7 km², which represents about 10 landslides per km².The main landslide conditioning factors are: lithology and geological structure, slope angle and slope morphology, land use, presence of old landslides, and human activity.The highest landslide density occurs in Cretaceous marls and marly limestones, but the largest movements are in Jurassic clays, marls and limestones.The landslide density is higher on slopes with gradients above 20 °, but the largest unstable area is found on slopes of 10 ° to 15 °, thus reflecting the presence of the biggest slope movements. There is a correlation between landslides and topographical concavities, a fact that can be interpreted as reflecting the significance of the hydrological regime in slope instability.Concerning land use, the highest density of landslides is found on slopes covered with shrub and undergrowth vegetation.About 26% of the total number of landslides are reactivation events. The presence of old landslides is particularly important in the occurrence of translational slides and complex and composite slope movements.20% of the landslide events were conditioned by anthropomorphic activity. Human's intervention manifests itself in ill-consolidated fills, cuts in potentially unstable slopes and, in a few cases, in the changing of river channels.Most slope movements in the study area exhibit a clear climatic signal. The analysis of rainfall distribution in periods of recognised slope instability allows the distinction of three situations: 1) moderate intensity rainfall episodes, responsible for minor slope movements on the bank of rivers and shallow translational slides, particularly in artificial trenches; 2) high intensity rainfall episodes, originating flash floods and most landslides triggered by bank erosion; 3) long-lasting rainfall periods, responsible for the rise of the groundwater table and triggering of landslides with deeper slip surfaces.     

地下水位对近震和远震异常响应的比较——以汶川地震和苏门答腊地震为例

地震引起的深层地下水位异常具有多样性和复杂性的特点。为了进一步探索深层地下水位动态与地震作用过程之间的联系机制,发挥深层地下水位对地震活动的指示作用,以2008年汶川Ms8.0地震和2007年苏门答腊Ms8.5地震为研究背景,对川、滇、陕、甘、渝地区井水位对两次地震的响应特征进行了比较。研究发现:地下水位远震响应形态主要以振荡型和阶变型为主,异常出现的时间较晚;地下水位近震响应形态比较复杂,以阶变型、脉冲型以及振荡型为主,异常出现的时间几乎与地震的发生同步。根据深层地下水位对地壳应力的响应机理,分析了各典型井水位对远震和近震不同响应的原因。结果表明:地下水位对远震的响应主要是由于含水层介质受到地震波应力的作用;对近震异常响应原因比较复杂,主要是含水层介质受到区域构造应力和地震波应力的共同作用的结果,震中距越小,含水层受到震源构造应力场的控制作用越大。     

地震石从何而来——青川东河口地震遗迹地质公园

汶川地震后,一块名为地震石的巨石伫立在青川东河口地震遗迹地质公园里。对其来由,大家众说纷纭。通过实地调研,对其进行了显微鉴定,认为该石(蚀变辉绿玢岩)形成于海西—印支构造运动时期,并经后期的蚀变作用。在此次地震中,该石借王家山高速远程滑坡—碎屑流之力,最终抵达现在所处的位置。     

地震研究进展与趋势——全球地震科学钻探研究综述

近年来大地震频繁发生,造成了巨大的人员伤亡和财产损失,认识地震和地震发生机制已成为地质学家和地球物理学家共同的奋斗目标。科学钻探是获取地下深部物质、了解深部信息的最直接、最有效、最可靠的方法,因此,科学钻探是认识和揭示地震断裂作用的最佳手段。本文介绍了目前世界上主要的地震科学钻探计划,包括位于环太平洋地震带的日本野岛(Nojima)断层科学钻探计划、台湾车笼埔断裂钻探计划(TCDP)、圣安德烈斯断裂深部观测钻探计划(SAFOD)、新西兰深部断层钻探计划(DFDP)、日本南海海槽发震带试验钻探计划(NanTroSEIZE)、日本海沟地震快速钻探计划(JFAST)和大陆内部地震的汶川地震断裂带科学钻探计划(WFSD),简要概括了这些科学钻探计划所取得的有关地震研究的重要进展与贡献,并且通过这些成果探讨了未来地震研究趋势。     

从汶川“5·12”地震看地球物理预测地震

汶川5·12地震,破坏之大,死亡人数之多,让世界震惊,所以人们重新把眼光投向地震预报。地震预报的确是艰难的,中国的海城地震是一个成功的典范,但唐山、松潘和汶川地震依然让人们陷入迷茫。地震发生前有许多先兆(包括前震),通过仪器接收,表现为一些地球物理场异常。引起这些地球物理场的异常有诸多的原因,不可能在短时间内分辨出那些是因为即将地震而引起的。目前地球物理预测地震理论已经日趋完善,可进行多方位的数据分析,并在实践中验证,下逐渐走向相对科学地、精确地对地震进行预测。     

Aftershock of the Wenchuan Earthquake in Sichuan Triggers Large Earthquake in Minxian County, Gansu

Because of the confining compression of the Pacific,Eurasian and Indian plates,the Chinese mainland is frequently stricken with earthquakes,especially in the Qinghai-Tibet Plateau and surrounding areas and along the NS-trending tectonic zone from Yinchuan to Lanzhou,Chengdu,and finally to the Kunming tectonic belt (Fig.1).Historical records show that there are 14 earthquakes of＞Ms 8 occurring in the two regions,eight of which occurred along the latter tectonic belt.The 2008 Wenchuan earthquake located on both the NS tectonic zone and the Longmenshan fault zone along the eastern margin of the Qinghai-Tibet Plateau.