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

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

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

Active tectonics around the Cusco city, Peru: record of earthquakes in the last 14,000 years, from paleosismological data

The seismic activity of Peru has its origin in the convergent margin, where the Nazca plate subducts under the South American plate, a process that generates friction and accumulation of stress, reflected in deformation of the crust and superficial earthquakes (<30km of depth). Cusco was affected by earthquakes in 1950, 1650 and recently in 1986, the first two, major earthquakes of magnitudes greater than 7 MM (Silgado,1978). The Tambomachay active geological fault is located four kilometers to the north of the city of Cusco, and belongs to a large and wide deformation zone, where the structures have NWSE and E-W trends, known as the Zurite-Cusco- Urcos-Sicuani Fault System (Benavente et al., 2013). Recent studies show clear morphological and structural evidence of Quaternary activity on these structures (Sébrier et al., 1985; Mercier et al., 1992; Cabrera, 1988; Benavente et al, 2013), so they should be classified as an important seismogenic source; however, there has not been complete characterization to assess the seismic hazard to the city of Cusco, which currently houses over ~500,000 inhabitants. In this research investigation, we present a paleoseismological study of the western sector of the Tambomachay Fault. We present displacement rates, recurrence intervals and ages of recent seismic event

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

Surface rupture associated with a moderate intraplate earthquake: the Mw 6.2 Parina event (December 1st, 2016) in the Peruvian Altiplano

[ESP] Los análisis de desplazamiento de falla y riesgo sísmico emplean relaciones empíricas para predecir la magnitud potencial del terremoto ("relaciones de escala"; por ejemplo, Wells y Coppersmith, 1994), deslizamiento de la superficie), funciones de probabilidad de ruptura de la superficie y cantidad de deslizamiento de la superficie (por ejemplo, "probabilidad condicional de ruptura" "Y" probabilidad de superación ", respectivamente; ver Youngs et al; 2003). Esas relaciones comparten el problema común de que dependen de un número limitado de casos de magnitud moderada a grande (> = 6.5) y anteriores a 2000. Los terremotos del oeste de EE. UU. y Japón están ampliamente representados, y los casos intraplaca son pocos. Aquí, informamos evidencia de fallas en la superficie que ocurrió durante un terremoto moderado que ocurrió en el Altiplano del sur del Perú. Presentamos datos de campo y de alta resolución que mejoran el conocimiento geodinámico de la región y proporcionan pistas para actualizar las herramientas de riesgo sísmico. El terremoto de falla normal Parina 2016 Mw 6.2 ocurrió dentro de los altos Andes del sur del Perú en una región con escasa sismicidad reciente y sin tensión horizontal geodésica observable. Las observaciones de campo y los DEM de alta resolución de las rupturas de la superficie permiten investigar la relación entre el deslizamiento en la falla de Parina, la geomorfología local y la tectónica regional. Mapeamos un segmento principal de tendencia NW-SE y 6 km de largo, con deslizamiento vertical de hasta ~ 27 cm (hacia abajo hacia el SW) y apertura tensional de ~ 25 cm. El deslizamiento de la superficie no se distribuye fuera de la falla principal, con la excepción de un cordón paralelo a 200 m de la falla principal en su extremo norte. Un punto llamativo es un segmento menor de tendencia NW-SE y un segmento roto de 1.5 km de largo con valores de deslizamiento más pequeños (hasta 8 cm) distantes por 5 km al norte, a lo largo de la misma zona de falla. Las dos trazas de ruptura mapeadas coinciden directamente con la proyección ascendente del plano de falla co-sísmica inferido de las mediciones de InSAR y, por lo tanto, pueden representar dos secciones superficiales distintas de la falla sísmica primaria, separadas por un espacio de superficie. Esta brecha ocurre donde la geología superficial está constituida por sedimentos sueltos. Las rupturas coinciden con escarpes de 10-20 m de altura que atraviesan depósitos fluvio-glaciales que son arrojados hacia el SW, y forman la extensión hacia el sudeste del sistema de fallas Lagunillas-Mañazo más grande que tiende el NO-SE en el Altiplano peruano. Una estimación preliminar lleva a inferir un deslizamiento de sentido normal repetido en la falla de Parina desde la última glaciación mayor (~ 10-30 ka), lo que implica una tasa de deslizamiento vertical ~ 1 mm / a. Además de su interés regional en términos de tectónica activa y geodinámica (Wimpenny et al., 2018), la ruptura de la superficie de Parina 1) constituye un nuevo caso para enriquecer la base de datos SURE pendiente con nuevos datos precisos, especialmente para eventos intraplaca, 2) geología de superficie es un parámetro clave que influye en el deslizamiento de la superficie, 3) ilustra una vez más que los terremotos moderados pueden romper la superficie en un patrón complejo, 3) muestra que las técnicas de alta resolución permiten mejorar la caracterización de las rupturas de la superficie (longitud de la ruptura y desplazamiento máximo / medio) y 4) cuestiona potencialmente los parámetros de falla que se infirieron en el pasado cuando dichos enfoques no estaban disponibles. Esos son argumentos que apoyan la idea de la necesidad de una revisión profunda de las relaciones empíricas, basadas en catálogos de terremotos modernos.[ENG] Fault displacement and Seismic hazard analyses employ empirical relationships to predict potential earthquake magnitude ("scaling relationships"; e. g., Wells and Coppersmith, 1994), surface slip), probability functions of surface rupture and surface slip amount (e. g., “conditional probability of rupture” and “probability of exceedance”, respectively; see Youngs et al; 2003). Those relationships share the common issue that they rely on a limited number of moderate-to-large magnitude (>=6.5) and pre-2000 cases. Earthquakes from western US and Japan are largely represented, and intraplate cases are few. Here, we report surface faulting evidence that occurred during a moderate earthquake that occurred in the Altiplano of Southern Peru. We present field and high-resolution data that improve the geodynamic knowledge of the region and provide clues to upgrade seismic hazard tools. The 2016 Mw 6.2 Parina normal-faulting earthquake occurred within the high Andes of southern Peru in a region with sparse recent seismicity and no observable geodetic horizontal strain. Field observations and high-resolution DEMs of the surface ruptures allow investigating the relationship between slip on the Parina Fault, local geomorphology and the regional tectonics. We mapped one major NW-SE-trending and 6-km-long segment, with up to ˜27 cm vertical slip (downthrown to the SW) and ˜25 cm tensional opening. Surface slip is not distributed off the main fault, with the exception of a parallel strand 200-m off the major one at its northern tip. One striking point is a minor NW-SE-trending and 1.5-km-long ruptured segment with smaller slip values (up to 8 cm) distant by 5 km to the north, along the same fault zone. The two mapped rupture traces directly coincides with the up-dip projection of the co-seismic fault plane inferred from InSAR measurements, and they therefore may represent two distinct surface sections of the primary earthquake fault, separated by a surface gap. This gap occurs where surface geology is constituted of loose sediments. The ruptures coincide with 10-20 m high scarps cutting through fluvio-glacial deposits that are downthrown to the SW, and they form the southeastward extension of the larger Lagunillas-Mañazo fault system that trends NW-SE across the Peruvian Altiplano. A preliminary estimation leads to infer a repeated normal-sense slip on the Parina Fault since the last major glaciation (˜10-30 ka), implying a vertical slip rate ˜1 mm/y. Besides its regional interest in terms of active tectonics and geodynamics (Wimpenny et al., 2018), the Parina surface rupture 1) constitutes a new case to enrich the pending SURE database with new accurate data, especially for intraplate events, 2) surface geology is a key parameter influencing the surface slip, 3) illustrates once again that moderate earthquakes can rupture the surface in a complex pattern, 3) shows that high-resolution techniques allows improving the characterization of surface ruptures (rupture length and max/mean displacement) and 4) potentially questions the fault parameters that were inferred in the past when such approaches were not available. Those are arguments that support the idea of the need for a deep revision of empirical relationships, based on catalogues of modern earthquakes