Estimación de la tasa de desplazamiento y el máximo desplazamiento acumulado a lo largo de la falla Tambomachay, ciudad del Cusco

La ciudad del Cusco es caracterizada por tener importante actividad sísmica asociada a fallas activas adyacentes a la ciudad. Los sismos son de magnitudes moderadas a elevadas (>5.5 Mw) (Silgado, 1978); sin embargo, la información histórica e instrumental aún es limitada para entender y realizar una coherente evaluación sismotectónica. La falla Tambomachay es una de las fallas más importantes, se ubica a 4km al norte de la ciudad del Cusco (Figura 1a), y se extiende a lo largo de 20 km con una tendencia NNWSSE. Recientes investigaciones (Sébrier et al., 1985; Cabrera, 1988; Benavente et al., 2010; Benavente et al., 2013) muestran evidencias actividad reciente, principalmente morrenas holocenas desplazadas de manera vertical en su terminación oeste, y conos aluviales desplazados en su terminación este (Cabrera, 1988). Su actividad a largo plazo dio como resultado el frente montañoso (Figura 1b) formado a lo largo de toda su longitud, éste exhibe indicadores morfológicos que han retenido la información de los procesos tectónicos y climáticos; la cuantificación de estas geoformas permite calcular tasas de desplazamiento (Tsimi & Ganas, 2015; Whittaker, Attal, & Cowie, 2008) mediante el uso de sensores remotos de alta resolución

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”

Evidence for a great Mw>7 Pre-Hispanic (AD 1300-1400) Crustal Earthquake in the Forearc of Peru

Seismic hazard in South Peru is thought to be dominated by earthquakes on the subduction interface (e.g. Villegaz-Lanza et al., 2016). Little is known about other possible sources of major earthquakes, such as the ~300-km-long Incapuquio Fault System (IFS) outcropping in forearc, active during Cenozoic times (e.g. Jacay et al., 2002; Audin et al., 2006). It is seismically active, but no study deals with its actual potential activity and its seismic hazard. From fieldwork and high-resolution DEMs, we evidenced that the IFS is active with an inverse motion associated to a left-lateral component: the surface is displaced (up to 4.5 m vertical cumulative offset) with outcrops of free faces over ~100 km distance, recent fluvial terraces are overthrusted by Paleozoic rocks, rivers beds are bent, etc. Despite the hyper-arid environment, we found charcoals of roots killed by the earthquake located in the fault plane. Their 14C dating gives a cluster of ages around AD 1325. We interpret it as the last earthquake that occurred along this fault segment with an Mw>7 magnitude (0.7 m vertical, 1.2 m total). The timing of the Mw>7 earthquake coincides with the end of the Chiribayas civilization in Moquegua valley, which has previously been attributed to the “mega-Niño” Miraflores climatic catastrophe (1300-1350) that may have induced the collapse of the irrigation and thus agricultural system (e.g. Satterlee et al., 2000; Goldstein & Magilligan, 2011). This last hypothesis is discussed because in some places, no evidences of mega floods that may have destroyed the canals have been found (Clement & Moseley, 1991), and also because that it has been shown that these human civilization living in this hyper arid area adapted their agricultural system and subsistence to the El Niño climatic fluctuations (Zaro et al, 2013). We thus propose that this collapse could also be due to the ~AD 1325 Mw>7 earthquake on the IFS, or to the sum of the earthquake and the Miraflores climatic Catastrophe

Structural geormophology and paleoseismology in the Altiplano of Peru: First geological evidence of the 1950 earthquake

The Pachatusan Faults System (SFP) is located 5km NW of the city of Cusco. It presents sub-parallel segments that are distributed along 25 km in a width of 2 km. Deposits and geoforms of fluvial-glacial environment are affected, as well as quaternary volcanic rocks (0.5 Ma). Benavente et al. (2013) y Sébrier et al. (1988) they describe moraines, and morphologies associated with glaciers, affected by normal type faults. Peru has a limited historical catalogue of earthquakes, in this sense, with this work we contribute to widen the window of observation in the region of Cusco, region known for earthquakes of great magnitude (1650, 1950, 1986). Studies focused on acquisition of high-resolution images and DEMs (5cm x pixel), which allowed detailed morphostructural analyses to be carried out. In addition, we excavated a paleosismological trench that allowed, together with new radiocarbon ages, to reconstruct holocene deformation associated with SFP. The morphostructural analysis of 201 Swath profiles, in morphologies of the last glacial maximum advance (14 ka), resulted in a vertical displacement of 20 m, resulting in a slip rate of 1.4 mm/year. The paleosismological analysis, from a trench 8 m long and 3 m high, allowed to identify 4 reactivations with superficial rupture in the last 4 ka. Being the last event or reactivation between 1876 - 1948 cal AD. We propose that this event would be associated with the 1950 earthquake, where great damage was recorded in the city of Cusco (Silgado, 1978). According to the length of rupture and vertical displacement, this earthquake was 6.3 M (Wells & Coppersmith, 1994)

Active faulting, paleoseismology and seismic hazard in forearc of southern Peru: First evidence of a crustal earthquake in the 19th century

Subduction earthquakes are considered the main source for seismic hazard in Peru (Villegas-Lanza et al., 2016). However, in recent years, it has been made an effort to generate a database of active faults for southern Peru (Benavente Carlos et al., 2017, Benavente et al., 2018), to propose these as a potential danger. In this way, we carried out morpho-structural and paleoseismological analyzes to get data for use it in empirical relationships and propose the magnitude of a potential earthquake. The Purgatorio Mirave fault (PMF) is located at forearc of southern Peru. Benavente et al. (2017) propose its activity in the last 6 ka, through TCN (Be10) dating over the fault scarps. In order to reconstruct the deformation and seismic history of PMF, we carried out studies of paleoseismology. Before the excavation of paleosismological trench, we acquired high resolution DEM and imagery with LIDAR and drone with the purpose to do a detailed mapping of all segments associated to PMF and to identify favorable areas for morpho-structural and paleosismological analyzes. With these, we determine 200km of superficial ruptures, that are distributed inside an area of 70 km for length and 2 km for width. The high resolution images (5cm x pixel) shows sub-parallel segments, forming sigmoidal structures, in echelons, pop-up and pressure ridges. The trench was excavated at the bottom of a valley (near the town of Mirave-Tacna), where there is a 2-meter-high fault scarp, that is affecting unconsolidated alluvial deposits. The trench (5 m long and 4 m deep) shows a reverse fault that places Oligocene rocks (Fm. Moquegua) over Holocene deposits. We identified two colluvial wedge, which are dated in 15 191-14 690 cal BC and 1668-1787 cal AD by radiocarbon C14. The most recent event generated a vertical rupture of 0.90 m. According to our results (rupture length and vertical displacement of the PMF) and using the scale proposed by Wells and Coppersmith (1994), we determine a magnitude of 7.5Mw for the last event. In this sense, between the end of 18th and the beginning of 19th century, in southern Peru important geological processes were registered: a) A large cortical earthquake associated with the reactivation of the PMF, and b) The explosive eruption of the Tutupaca volcano (Samaniego et al., 2015), located 30 km from the PMF

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

Active tectonics around the Cusco City, Perú: Record of earthquakes in the last 14,000 years, from paleoseismological data

The city of Cusco in Peru was hit by several strong earthquakes in historical times (Silgado, 1978), but the seismogenic source of these earthquakes are yet unknown. Cusco is surrounded by geological faults with evidence of Quaternary tectonic activity (Sébrier et al., 1985; Cabrera, 1988; Mercier et al., 1992; Benavente et al., 2013), but there is not enough data to establish a history of fault reactivations and the seismogenic potential of each one these structures. The Tambomachay Fault is the closest to the city of Cusco (~4 km). The NW sector has a well-preserved morphology. In this sector, we observe 14,000-year-old lateral moraines dated from cosmogenic nuclide 10Be, that show normal type fault displacements. This make this sector a prime target for paleoseismology studies. Our paleoseismological results with the ages obtained from dating C14, suggest that the Tambomachay Fault generated at least four seismic events, with surface ruptures, in the last 14,000 years. Using empirical relationships, we conclude that the Tambomachay Fault is capable of generating earthquakes with magnitudes greater than 6.7Mw, putting at high risk the inhabitants of the city and the archaeological remains declared as world heritage by the UNESCO. In addition, the last seismic event, dated between 856-988 cal AD, coincides with the abandonment of the citadel of Pikillacta by the Wari culture (McEwan, 2015), pre-Inca culture located to the south of the city of Cusco and adjacent to Tambomachay Fault

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

Deformación transcurrente y cuaternaria asociada al sistema de falla Pachatusan-Cusco

El sistema de fallas Pachatusan está ubicado a 5 km de la ciudad de Cusco (Figura 1.b). Tiene una longitud aproximada de 25 km y consiste en segmentos sub-paralelos de dirección N130ºE a N140ºE. Los segmentos de fallas que componen todo este sistema afectan depósitos y morfologías de ambientes fluvio-glaciares y rocas volcánicas del Cuaternario. Cabrera (1988) describe desplazamientos verticales de morrenas posiblemente holocenas, además señala desplazamientos de hasta 10m. Benavente et. al. (2013) muestran un cartografiado más detallado y describen desplazamientos verticales mayores a 15m. En la actualidad existen nuevas herramientas que permiten identificar fallas activas con mayor precisión, estas herramientas permiten estimar deformación vertical y lateral. En este sentido, en base a trabajos de sensores remotos y de campo mostramos nuevas evidencias de deformación transcurrente en la zona noroeste del sistema de fallas Pachatusan, denominado Sector Huacoto, por su cercanía al poblado de Huacoto (Figura 1.c). Cabe resaltar que la información generada es importante para la caracterización del sistema de falla Pachatusan como fuente sismogénica, ya que las capacidades sismogénicas de una falla normal pura y una falla transcurrente son diferentes. En este sentido, estos primeros resultados nos permitirán hacer una evaluación sobre el peligro sísmico al que se encuentra expuesta la ciudad del Cusco

Paleoseismic Evidence of an Mw 7 Pre-Hispanic Earthquake in the Peruvian Forearc

We present the results of a paleoseismic survey of the Incapuquio Fault System, a prominent transpressional fault system cutting the forearc of South Perú. High-resolution Digital Elevation Models, optical satellite imagery, radiocarbon dating, and paleoseismic trenching indicate that at least 2–3 m of net slip occurred on the Incapuquio Fault generating a complex, ∼100-km long set of segmented fault scarps in the early 15th century (∼1400–1440 CE). We interpret the consistent along-strike pattern of fault scarp heights, geometries and kinematics to reflect a surface rupture generated by a single Mw 7.4–7.7 earthquake, suggesting that brittle failure of the forearc poses a significant, yet mostly overlooked, seismic hazard to the communities in coastal areas of Perú. The timing of this earthquake coincides with the collapse of the Chiribaya civilization in ∼1360–1400 CE, and we present evidence of damaged buildings along the fault trace that may be of Chiribayas age. Our surface faulting observations, when combined with observations of deformation in the forearc from geodesy and seismology, also demonstrate that the forearc in South Perú experiences a complex, time-varying pattern of permanent strain, with evidence for trench-parallel shortening, trench-parallel extension, and trench-perpendicular shortening all in close proximity but in different periods of the megathrust earthquake cycle. The kinematics of recent slip on the Incapuquio Fault are consistent with the sense of interseismic strain within the forearc measured by GPS, suggesting the fault is loaded toward failure between megathrust earthquakes