Los paleoterremotos: estudiando el pasado para entender el futuro

Las fallas activas suelen funcionar de forma cíclica, acumulando esfuerzo durante centenares o miles de años (periodo intersísmico) y liberándolo bruscamente (periodo cosísmico, que equivale a un terremoto importante). Los estudios de peligrosidad sísmica se han basado, hasta hace poco, en datos sísmicos instrumentales e históricos que, a menudo, abarcan un periodo demasiado corto en la actividad de una falla. En ocasiones, estos estudios han infravalorado la peligrosidad sísmica de una región al utilizar sólo una parte (registro histórico e instrumental) de la sismicidad, sin tener en cuenta la ocurrida en el periodo prehistórico. La tectónica activa y la paleosismología, que estudian respectivamente la tectónica más reciente y la secuencia de terremotos prehistóricos generada por una falla, constituyen herramientas complementarias que permiten estudiar periodos de tiempo mucho más largos que abarcan uno o varios ciclos sísmicos. Su aportación es especialmente interesante donde se tiene poca información histórica o donde las fallas deslizan lentamente (con ciclos sísmicos largos) y pueden haber permanecido silenciosas durante el periodo histórico. El análisis paleosísmico requiere de un estudio de tectónica activa previo, basado sobre todo en un análisis gemorfológico que permita detectar las fallas más activas. La Paleosismología se centra en los efectos sobre el terreno del terremoto en la zona de falla (estructuras primarias), pero también en otros efectos producidos fuera de la zona de falla como son las estructuras de licuefacción. El análisis de la zona de falla se centra en el estudio de las modificaciones de la superficie del terreno asociadas al movimiento de la misma y en el estudio en trincheras de los procesos de erosión y sedimentación activados por pulsos tectónicos. Estos estudios, permiten obtener algunos parámetros sísmicos como son: magnitud máxima esperable, período de recurrencia, salto por evento, tiempo desde el último terremoto, geometría de la falla sismogénica, y velocidad de deslizamiento. En la Península Ibérica, la Paleosismología es una disciplina emergente que está aportando en los últimos años datos muy valiosos sobre sus fallas más activas. Los primeros resultados paleosísmicos han permitido reconocer en algunas fallas capacidad para producir terremotos de una magnitud máxima entre 6.7 (por ejemplo la falla de El Camp en Tarragona) y 7.6 (como por ejemplo la falla de Carboneras, en Almería), y con periodos de recurrencia de varios miles de años

Archaeoseismology in the Inka Sacred Valley and in the Cuzco region, an interdisciplinary approach for past seismic impacts characterization on Cultural Heritage as a new marker for paleoevents?

Too often, the seismic hazard evaluation in the Andes is limited to the subduction zone. While it is true that the most powerful earthquakes that affected the Pacific fringe (Lima, 1746; Arica, 1868) had little impact on the Altiplano, this area exhibits a combination of strong seismic hazard and high vulnerability through the presence of active fault segments in densely populated areas (Benavente et al., 2013). Nonetheless, unlike the coastal region where resilience is taken increasingly in account, as a result, in particular, of the violent 2007 Pisco earthquake (D’Ercole et al., 2007), the seismic risk remains largely overlooked in the Highlands. In a such iconic city like Cuzco, the erratic population growth and its consequences on the organisation of the urban landscape represents a further challenge that enhance the risk exposure. The incomplete knowledge of the Quaternary geological settings of the Cuzco region as well as the low recurrence of devastating earthquakes on crustal faults in general lead to a progressive loss of the “risk culture” achieved by the Inkas and their megalithic architecture. To face such difficulties, archaeoseismological approaches demonstrated that disturbed architectural remains may be used as valuable markers (Rodríguez-Pascua et al., 2011) to extend the catalog of palaeoseismological studies (Rosell Guevara, 2018). Moreover, the huge consequences of the damaging events that struck Cuzco in 1650 and 1950 might suggest a similar impact of earthquakes during pre-Hispanic times. Registering and mapping the past seismic effects in Inka citadels like Machu Picchu and Choquequirao as well as sudden variations in constructive techniques of monumental heritage is therefore an interesting tool to provide additional data (recurrence, social impacts) to properly assess the seismic risk and detect “prehistoric” events. Based on an interdisciplinary program, our results will complement other evidences of deformation, issued from fault trenching and proximal lake coring (PATA project). The overall purpose is to extend the knowledge and time window for the local crustal fault activity and emphasize the importance of the seismic risk in the area. Raising awareness will be the first step towards developing risk management policies and implementing mitigation measures to preserve the local Cultural Heritage. Within the framework of this meeting, we aim to present the preliminary results obtained during the two field campaigns in 2019, which are confirming the relevance of Inka sites as good “seismoscopes”

Reappraisal of the 1863 huércal‐overa earthquake (Betic cordillera, se spain) by the analysis of esi‐07 environmental effects and building oriented damage

This work reviews the 1863 Huércal‐Overa earthquake (VI‐VII EMS) based on the environmental seismic intensity scale (ESI‐07) and oriented archaeoseismological building damage. The performed analysis identifies 23 environmental effects (EEEs) and 11 archaeoseismological effects (EAEs), completing a total of 34 intensity data‐points within the intensity zone ≥ VI EMS. The new ESI intensity data quintuplicate the previous intensity data‐points ≥ VI EMS (five localities) for this event. Sixteen of the identified EEEs indicate the occurrence of intensity VII‐VIII within the Almanzora valley, south of Huércal‐Overa, over an area of ca. 12–15 km2. Anomalies in water bodies, slope movements, hydrogeological anomalies, ground cracking, and other effects (gas emissions, tree shaking) are the more diagnostic EEEs—with one of them indicating a local maximum intensity of VIII‐IX ESI‐07 (Alboraija lake). Environmental earthquake damage of intensity ≥ VI covers an area of c. 100 km2, compatible with a VIII ESI intensity event. The spatial distribution of EEEs and EAEs indicates that the zone of Almanzora River Gorge, which was depopulated during the earthquake epoch, was the epicentral area, and compatible with seismotectonic data from active shallow blind thrusting beneath the Almagro Range. The use of ESI data in nearly unpopulated areas help to fill gaps between damaged localities (EMS data) multiplying intensity data‐points, providing a better definition of the intensity zones and offering a geological basis to look for suspect seismic sourcesCGL2015-67169-

Impact of a paleo-earthquake and debris flow in Pikillaqta collapse, Cusco-Perú

In Cusco Valley, have been highlighted the Tambomachay, Pachatusan and Cusco active faults (Cabrera, 1988 & Benavente et al., 2013). Cusco has a historical and instrumental seismic record Mw> 5, events occurred in 1650, 1950 and 1986 (Silgado, 1978, Tavera, 2002). In the same way, during pre-Inca and Inca periods, they suffered the occurrence of earthquakes, this is evidenced in myths and chronicles collected by different chroniclers in XVIth century. These records, however, are limited, due to poor instrumental seismic data. Thus, during our work on paleosismology and archaeoseismology studies in Cusco with the aim of complementing the seismic catalog, we’ve visited several archaeological sites. Among the archaeological centers visited, we were more interested in the Pikillaqta Archaeological Park (PAP), a city that was built at Wari Empire time, a culture that developed in southern Peru between 600 and 1000 AD (Bergh, 2012). The interest of studying this site is basically due to its archaeological evidence of unjustified abandonment around 900 AD (McEwan, 2015). Subsequently, our archeoseismology studies, based on the identification of Earthquake Archeological Effects (Rodriguez-Pascua et al., 2011), and post-seismic effects like new architectural elements dated by 14C in 900 AD, allowed us to observe an important seismic event at this age. In PAP we observed alluvial deposits of up to 2m in height inside the rooms and halls, evidencing a debris flow. Drone images, allowed us to observe drainages related to the entrance of a flow at PAP east, generating an alluvial cone in the PAP main square. Works in situ, allowed us find pottery and bones within the mud flow, dated also around 900 AD. On the other hand, results of paleosismology in the Tambomachay fault (Rosell, 2018), show a seismic event around 900 AD. With the previously mentioned, exist a clear relationship between archaeoseismology and paleosismological results. The event occurred at the end of the 9th century, originating its attempt at reconstruction and subsequent abandonment

Evidence of past seisms in Cusco (Peru) and Tzintzuntzan (Mexico): cultural relations

[ENG] (Evidence of ancient seisms in Cusco, Peru and Tzintzuntzan, Mexico and their cultural relations) At first sight the ancient pre-Columbian cultures seem to have had no awareness of seisms. Purhepecha and Andean cultures nevertheless not only show awareness of these, but also similitude between their anti seismic building techniques. This work exposes clear evidence of coseismic ruptures found In Cusco and in Tzintzuntzan-Patzcuaro. More profound research could nevertheless be helpful to reveal sceneries which are for their most part unknown to current generations

Earthquake Archaeological Effects (EAEs) in Machupicchu. Preliminary results

[ENG] : The National Archaeological Park of Machupicchu (Cusco, Peru) is one of the most important archaeological sites in the world. The relevance of this site makes the necessity of the prevention against natural hazards. Peru is affected by earthquakes from the Pacific Trench, but there are important active faults in the Andean Range that could generate destructive earthquakes. In this study we show the preliminary result of the analysis of Earthquake Archaeological Effects (EAEs) and their differentiation from the effects generated by slope movement (creep) in the archaeological site. This type of studies may be useful in the future for the prevention of earthquake effects in the archaeological site

The AD 1755 Lisbon Earthquake-Tsunami: Seismic source modelling from the analysis of ESI-07 environmental data

This work presents a macroseismic analysis of the AD 1755 Lisbon Earthquake-Tsunami event by means of the combination of intensity data derived from the EMS-98 scale and the ESI-07 scale (Environmental damage). About 600 records of secondary earthquake environmental effects (EEEs) for the whole Spain have been used to define intensities, focused on the SW portion of the Iberian Peninsula. The Spanish data have been complemented with 308 EEEs records from Portugal. The analyses indicate maximum intensities of X EMS-ESI along the Atlantic margin of the Iberian Peninsula with 76 records of Tsunami environmental effects (TEEs). An important amplification (VIII – VII) occurred all along the Guadalquivir basin and the adjacent Betic front at epicentral distances of 300–700 km. In these zones 55 records of ground effects (ground cracks, Liquefactions and slope movements) are catalogued. In the rest of the territory of the Peninsula the most widespread effects were hydrogeological changes with 505 records in Spain and 196 in Portugal (total 701 records) covering all the intensity levels. Increase of flow discharges in springs and elevation of water level in wells was the common groundwater response to seismic shacking, especially in SW Iberia. In this zone water elevation in wells was between 5 and 3 m and persistent increases of discharges long-lasting (several days to two months). Persistent discharges on springs were linked in 143 cases to important SW-NE crustal faults (e.g., Alentejo-Plasencia Fault). From the Intensity distribution the historic seismic scenarios are explored by means of the development of empirical ShakeMaps. These consider the three classical seismic sources proposed for this earthquake: Gorringe Bank (G); Marques de Pombal Fault (M) and Atlantic delamination beneath the Gulf of Cadiz (C). However, individually these seismic sources are too small and unable to generate the resulting seismic scenario depicted by the intensity map developed in this work, with onshore seismic accelerations (PGA) up to 0.82 g. These acceleration values and the great amplification experienced throughout the Guadalquivir basin (0.34–0.52 g) are only possible considering a combination of the three seismic sources (GMC Source) probably related to shallow subduction or lithospheric delamination beneath SW Iberia and the Gulf of Cadiz. This will suggest an NNE-SSW offshore rupture length of 350–360 km with an overall rupture area of c. 84,500 km2 resulting in an event magnitude 8.6 Mw calculated from empirical relationships. The results demonstrate the efficacy of these kind of approaches for better identifying and modelling seismic sources for historical eventsThis work was supported by the Spanish Research Project MINECOFEDER CGL2015-67169-P (QTECSPAIN - USAL). It is a contribution of the Earthquake Geology Group (TPPT) of the INQUA TERPRO Commission. Authors are grateful to the constructive comments of Joao Fonseca and an anonymous reviewer who significantly improved the content of this pape

Did earthquakes strike Machu Picchu?

The Historic Sanctuary of Machu Picchu (Cusco, Peru) is one of the most important archaeological monuments in Peru and worldwide. Machu Picchu is classified as a UNESCO World Heritage site and at risk from climatic change. However, the seismic centennial history of Peru reports large earthquakes generated both along the subduction zone (Mw8) and on active crustal faults along the Andean Cordillera (Mw7). It is therefore important to know if Machu Picchu is located in an area of seismic hazard and then to take measures to mitigate potential seismic hazards. Due to the short historical earthquake catalogue (< 500 years) and the absence of significant recent instrumental seismicity in the site’s vicinity (radius of < 30 km), our knowledge about the seismic hazard in Machu Picchu is limited. The earthquakes of 1650 and 1950 affected Cusco city and surrounding areas, but without damage descriptions inMachu Picchu (80 km away) (Silgado Ferro 1978). In this study, we make the first attempt to use the analysis of earthquake archaeological effects (EAEs) and their differentiation fromthe effects generated by slope movements (creep) to investigate the past occurrence of strong earthquakes at the site. The application of geological structural analysis to the deformations observed in Machu Picchu shows two directions of the mean ground movement: N020° E and N110° E. Two earthquakes that affected Machu Picchu during its construction generated these directions. This kind of data should be used in the future to protect this important archaeological site

Archaeoseismological Evidence of Seismic Damage at Medina Azahara (Córdoba, Spain) from the Early 11th Century

The “Caliphal City of Medina Azahara” was built in 936–937 CE or 940–941 CE (depending on the source) by the first Caliph of al-Andalus Abd al-Rahman III, being recently inscribed (2018) on the UNESCO World Heritage List. The abandonment and destruction of the city have been traditionally related to the civil war (“fitna”) that started between 1009 and 1010 CE. However, we cannot rule out other causes for the rapid depopulation and plundering of the city just a few decades after its foundation. The archaeoseismological study provides the first clues on the possible role played by an earthquake in the sudden abandonment and ruin of the city. Eleven different types of Earthquake Archaeological Effects (EAEs) have been identified, such as dropped key stones in arches, tilted walls, conjugated fractures in brick-made walls, conjugated fractures and folds in regular pavements and dipping broken corners in columns, among others. Besides that, 163 structural measures on EAEs were surveyed resulting in a mean ground movement direction of N140°–160° E. This geological structural analysis clearly indicates a building-oriented damage, which can be reasonably attributed to an earthquake that devastated Medina Azahara during the 11st or 12th centuries CE. If this were the case, two strong earthquakes (≥VIII MSK/EMS) occurred in 1024–1025 CE and 1169–1170 CE could be the suspected causative events of the damage and destruction of the cit