Browsing by Author "Chlieh, Mohamed"
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Item Restricted A mixed seismic–aseismic stress release episode in the Andean subduction zone(Nature Research, 2016) Villegas Lanza, Juan Carlos; Nocquet, J. M.; Rolandone, F.; Vallée, M.; Tavera, Hernando; Bondoux, Francis; Tran, T.; Martin, X.; Chlieh, MohamedIn subduction zones, stress is released by earthquakes and transient aseismic slip. The latter falls into two categories: slow slip and afterslip. Slow-slip events emerge spontaneously during the interseismic phase, and show a progressive acceleration of slip with a negligible contribution of synchronous tremors or microseismicity to the energy, or moment release. In contrast, afterslip occurs immediately after large and moderate earthquakes, decelerates over time, and releases between 20 and 400% of the moment released by the preceding earthquake. Here we use seismic and GPS data to identify transient aseismic slip that does not fit into either of these categories. We document a seismic–aseismic slip sequence which occurred at shallow depths along a weakly coupled part of the Andean subduction zone19 in northern Peru and lasted seven months. The sequence generated several moderate earthquakes that together account for about 25% of the total moment released during the full sequence, equivalent to magnitude 6.7. Transient slip immediately followed two of the earthquakes, with slip slowing at a logarithmic rate. Considered separately, the moment released by transient slip following the second earthquake was more than 1,000% of the moment released during the earthquake itself, a value incompatible with classical models of afterslip. Synchronous seismic swarms and aseismic slip may therefore define a stress-release process that is distinct from slow-slip events and afterslip.Item Restricted Active tectonics of Peru: heterogeneous interseismic coupling along the Nazca megathrust, rigid motion of the Peruvian Sliver, and Subandean shortening accommodation(American Geophysical Union, 2016-10) Villegas Lanza, Juan Carlos; Chlieh, Mohamed; Cavalié, O.; Tavera, Hernando; Baby, P.; Chire Chira, J.; Nocquet, J.‐M.Over 100 GPS sites measured in 2008–2013 in Peru provide new insights into the present‐day crustal deformation of the 2200 km long Peruvian margin. This margin is squeezed between the eastward subduction of the oceanic Nazca Plate at the South America trench axis and the westward continental subduction of the South American Plate beneath the Eastern Cordillera and Subandean orogenic wedge. Continental active faults and GPS data reveal the rigid motion of a Peruvian Forearc Sliver that extends from the oceanic trench axis to the Western‐Eastern Cordilleras boundary and moves southeastward at 4–5 mm/yr relative to a stable South America reference frame. GPS data indicate that the Subandean shortening increases southward by 2 to 4 mm/yr. In a Peruvian Sliver reference frame, the residual GPS data indicate that the interseismic coupling along the Nazca megathrust is highly heterogeneous. Coupling in northern Peru is shallow and coincides with the site of previous moderate‐sized and shallow tsunami‐earthquakes. Deep coupling occurs in central and southern Peru, where repeated large and great megathrust earthquakes have occurred. The strong correlation between highly coupled areas and large ruptures suggests that seismic asperities are persistent features of the megathrust. Creeping segments appear at the extremities of great ruptures and where oceanic fracture zones and ridges enter the subduction zone, suggesting that these subducting structures play a major role in the seismic segmentation of the Peruvian margin. In central Peru, we estimate a recurrence time of 305 ± 40 years to reproduce the great 1746 Mw~8.8 Lima‐Callao earthquake.Item Restricted Distribution of discrete seismic asperities and aseismic slip along the Ecuadorian megathrust(Elsevier, 2014-08-15) Chlieh, Mohamed; Mothes, P. A.; Nocquet, J.-M.; Jarrin, P.; Charvis, P.; Cisneros, D.; Font, Y.; Collot, J.-Y.; Villegas Lanza, Juan Carlos; Rolandone, F.; Vallée, M.; Régnier, M.; Segovia, M.; Martín, X.; Yepes, H.A dense GPS network deployed in Ecuador reveals a highly heterogeneous pattern of interseismic coupling confined in the first 35 km depth of the contact between the subducting oceanic Nazca plate and the North Andean Sliver. Interseismic models indicate that the coupling is weak and very shallow (0–15 km) in south Ecuador and increases northward, with maximum found in the rupture areas of large (Mw>7.0) megathrust earthquakes that occurred during the 20th century. Since the great 1906 Mw=8.8 Colombia–Ecuador earthquake may have involved the simultaneous rupture of three to six asperities, only one or two asperities were reactivated during the large seismic sequence of 1942 (Mw=7.8), 1958 (Mw=7.7), 1979 (Mw=8.2) and 1998 (Mw=7.1). The axis of the Carnegie Ridge, which is entering the subduction zone south of the Equator, coincides well with the location of a 50 km wide creeping corridor that may have acted as persistent barrier to large seismic ruptures. South of this creeping region, a highly locked asperity is found right below La Plata Island. While this asperity may have the potential to generate an Mw ~7.0-7.5 earthquake and a local tsunami, until now it is unknown to have produced any similar events. That region is characterized by the presence of slow slip events that may contribute significantly to reduce the long-term moment deficit accumulated there and postpone the failure of that asperity. At the actual accumulation rate, a characteristic recurrence time for events such as those in 1942, 1958 and 1979 is 140±30 yr, 90±20 yr, 153±80 yr, respectively. For the great 1906 event, we find a recurrence time of at least 575 ± 100 yr, making the great 1906 earthquake a rare super cycle event.Item Restricted Estimation of slip scenarios for megathrust earthquakes: a case study for Peru(University of California Santa Barbara, 2011-08-13) Pulido, Nelson; Tavera, Hernando; Perfettini, Hugo; Chlieh, Mohamed; Aguilar, Zenón; Aoi, Shin; Nakai, Shoichi; Yamazaki, FumioThe recent 2011 Tohoku-oki earthquake occurred in a region where giant megathrust earthquakes were not expected. This earthquake proved the difficulty to assess seismic hazard mainly based on information from historical earthquakes. In this study we propose a methodology to estimate the slip distribution of megathrust earthquakes based on a model of interseismic coupling (ISC) distribution in subduction margins as well as information of historical earthquakes, and apply the method to the Central Andes region in Peru. The slip model obtained from geodetic data represents the large scale features of asperities within the megathrust, which is appropriate for simulation of long period waves and tsunami modelling. For the simulation of a broadband strong ground motion it becomes necessary to introduce small scale complexities to the source slip to be able to simulate high frequency ground motions. To achieve this purpose we propose a “broadband” source model in which large scale features of the model are constructed from our geodetic scenario slip, and the small scale heterogeneities are obtained from a spatially correlated random slip model. The good agreement between the power spectral density (PSD) of our geodetic slip model, and the PSD of a slip model of the 2010 Maule earthquake, suggests that our methodology can be appropriate to typify megathrust earthquakes.Item Restricted Interseismic coupling and seismic potential along the Central Andes subduction zone(American Geophysical Union, 2011-12-17) Chlieh, Mohamed; Perfettini, Hugo; Tavera, Hernando; Avouac, Jean-Philippe; Remy, Dominique; Nocquet, Jean-Mathieu; Rolandone, Frédérique; Bondoux, Francis; Gabalda, Germinal; Bonvalot, SylvainWe use about two decades of geodetic measurements to characterize interseismic strain build up along the Central Andes subduction zone from Lima, Peru, to Antofagasta, Chile. These measurements are modeled assuming a 3-plate model (Nazca, Andean sliver and South America Craton) and spatially varying interseismic coupling (ISC) on the Nazca megathrust interface. We also determine slip models of the 1996 Mw = 7.7 Nazca, the 2001 Mw = 8.4 Arequipa, the 2007 Mw = 8.0 Pisco and the Mw = 7.7 Tocopilla earthquakes. We find that the data require a highly heterogeneous ISC pattern and that, overall, areas with large seismic slip coincide with areas which remain locked in the interseismic period (with high ISC). Offshore Lima where the ISC is high, a Mw=8.6–8.8 earthquake occurred in 1746. This area ruptured again in a sequence of four Mw=8.0 earthquakes in 1940, 1966, 1974 and 2007 but these events released only a small fraction of the elastic strain which has built up since 1746 so that enough elastic strain might be available there to generate a Mw > 8.5 earthquake. The region where the Nazca ridge subducts appears to be mostly creeping aseismically in the interseismic period (low ISC) and seems to act as a permanent barrier as no large earthquake ruptured through it in the last 500 years. In southern Peru, ISC is relatively high and the deficit of moment accumulated since the Mw=8.8 earthquake of 1868 is equivalent to a magnitude Mw=8.4 earthquake. Two asperities separated by a subtle aseismic creeping patch are revealed there. This aseismic patch may arrest some rupture as happened during the 2001 Arequipa earthquake, but the larger earthquakes of 1604 and 1868 were able to rupture through it. In northern Chile, ISC is very high and the rupture of the 2007 Tocopilla earthquake has released only 4% of the elastic strain that has accumulated since 1877. The deficit of moment which has accumulated there is equivalent to a magnitude Mw=8.7 earthquake. This study thus provides elements to assess the location, size and magnitude of future large megathurst earthquakes in the Central Andes subduction zone. Caveats of this study are that interseismic strain of the forearc is assumed time invariant and entirely elastic. Also a major source of uncertainty is due to fact that the available data place very little constraints on interseismic coupling at shallow depth near the trench, except offshore Lima where sea bottom geodetic measurements have been collected suggesting strong coupling.Item Restricted Postseismic relocking of the subduction megathrust following the 2007 Pisco, Peru, earthquake(American Geophysical Union, 2016-05-03) Remy, D.; Perfettini, Hugo; Cotte, N.; Avouac, J. P.; Chlieh, Mohamed; Bondoux, Francis; Sladen, A.; Tavera, Hernando; Socquet, A.Characterizing the time evolution of slip over different phases of the seismic cycle is crucial to a better understanding of the factors controlling the occurrence of large earthquakes. In this study, we take advantage of interferometric synthetic aperture radar data and 3.5 years of continuous Global Positioning System (GPS) measurements to determine interseismic, coseismic, and postseismic slip distributions in the region of the 2007, Mw 8.0 Pisco, earthquake, Peru, using the same fault geometry and inversion method. Our interseismic model, based on pre‐2007 campaign GPS data, suggests that the 2007 Pisco seismic slip occurred in a region strongly coupled before the earthquake while afterslip occurred in low coupled regions. Large afterslip occurred in the peripheral area of coseismic rupture in agreement with the notion that afterslip is mainly induced by coseismic stress changes. The temporal evolution of the region of maximum afterslip, characterized by a relaxation time of about 2.3 years, is located in the region where the Nazca ridge is subducting, consistent with rate‐strengthening friction promoting aseismic slip. We estimate a return period for the Pisco earthquake of about 230 years with an estimated aseismic slip that might account for about 50% of the slip budget in this region over the 0–50 km seismogenic depth range. A major result of this study is that the main asperity that ruptured during the 2007 Pisco earthquake relocked soon after this event.Item Restricted Scenario source models and strong ground motion for future mega-earthquakes: application to Lima, Central Peru(Seismological Society of America, 2015-01) Pulido, Nelson; Aguilar, Zenón; Tavera, Hernando; Chlieh, Mohamed; Calderón, Diana; Sekiguchi, Toru; Nakai, Shoichi; Yamazaki, FumioThe 2011 moment magnitude (Mw) 9.0 Tohoku-Oki Japan earthquake occurred in a region where giant megathrust earthquakes were not expected. This earthquake proved the difficulty in assessing seismic hazard by relying mainly on information from historical and instrumental seismicity. To help improve the seismic-hazard assessment for such rare events, we propose a methodology to estimate the slip distribution of future megathrust earthquakes based on a model of interseismic coupling distribution in subduction margins, as well as information of historical earthquakes, and apply the method to the central Peru region, Lima. The slip model obtained from geodetic data represents the large scale features of asperities within the megathrust, which is appropriate for simulation of long-period waves and tsunami modeling. For the simulation of a broadband strong ground motion, we add small scale heterogeneities to the source slip to be able to simulate high frequencies. To achieve this purpose, we propose broadband source models constructed by adding short-wavelength slip distributions obtained from a Von Karman power spectral density function, to the slip model inferred from interseismic geodetic data. Using these slip models and assuming several hypocenter locations, we calculate a set of strong ground motions for Lima and incorporate site effects obtained from microtremors surveys and geotechnical data. Our simulated average pseudospectral accelerations (period 0.3 s) are above 1:5g for wide areas in Lima, which may be critical in terms of damage of low- to midrise masonry and reinforced concrete buildings, which characterize the majority of buildings in Lima.Item Restricted Seismic and aseismic slip on the Central Peru megathrust(Nature Research, 2010-05-06) Perfettini, Hugo; Avouac, Jean-Philippe; Tavera, Hernando; Kositsky, Andrew; Nocquet, Jean-Mathieu; Bondoux, Francis; Chlieh, Mohamed; Sladen, Anthony; Audin, Laurence; Farber, Daniel L.; Soler, PierreSlip on a subduction megathrust can be seismic or aseismic, with the two modes of slip complementing each other in time and space to accommodate the long-term plate motions. Although slip is almost purely aseismic at depths greater than about 40 km, heterogeneous surface strain1–8 suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods9–11. Thus, active faults seem to comprise areas that slip mostly during earthquakes, and areas that mostly slip aseismically. The size, location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place, and what fraction of the long-term slip rate it accounts for. Here we address this issue by focusing on the central Peru megathrust. We show that the Pisco earthquake, with moment magnitude Mw 5 8.0, ruptured two asperities within a patch that had remained locked in the interseismic period, and triggered aseismic frictional afterslip on two adjacent patches. The most prominent patch of afterslip coincides with the subducting Nazca ridge, an area also characterized by low interseismic coupling, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past. The seismogenic portion of the megathrust thus appears to be composed of interfingering rate-weakening and ratestrengthening patches. The rate-strengthening patches contribute to a high proportion of aseismic slip, and determine the extent and frequency of large interplate earthquakes. Aseismic slip accounts for as much as 50–70% of the slip budget on the seismogenic portion of the megathrust in central Peru, and the return period of earthquakes with Mw 5 8.0 in the Pisco area is estimated to be 250 years.Item Open Access Sismo de Yauca-Acarí del 25 de septiembre del 2013 (7.0 Mw) - Arequipa: aspectos sismológicos(Instituto Geofísico del Perú, 2013-09) Tavera, Hernando; Fernández, Efraín; Guardia Anampa, Patricia Alejandra; Villegas Lanza, Juan Carlos; Chlieh, Mohamed; Yauri Condo, Sheila Alodia; Arredondo García, Luz Mercedes; Flores Guerra, Edden Christian; Martínez Herrera, Julio CésarEl Perú es parte del llamado Cinturón de Fuego del Pacífico y en su borde occidental se desarrolla el proceso de convergencia de la placa de Nazca bajo la Sudamericana a una velocidad promedio del orden de 7-8 cm/año (DeMets et al, 1980; Norabuena et al, 1999); siendo el mismo, responsable de la actual geodinámica y geomorfología presente sobre todo el territorio peruano. Del mismo modo, este proceso ha dado origen a un gran número de sismos de diversa magnitud y focos ubicados a variadas profundidades, todos asociados a la fricción de placas (oceánica y continental), deformación interna de la placa oceánica por debajo de la cordillera y deformación cortical a niveles superficiales en el interior del continente. En el Perú, la ocurrencia de sismos es continua en el tiempo y cada año el Instituto Geofísico del Perú registra y reporta un promedio de 150 sismos percibidos por la población con intensidades mínimas de II-III (MM) y magnitudes ML4.0. Los sismos con magnitudes mayores son menos frecuentes y en general, tienen su origen en el proceso de fricción de placas produciendo importantes daños en áreas relativamente grandes, tal como sucedió en la región Sur de Perú el 23 de Junio de 2001 (Mw=8.2) y en Pisco, el 15 de Agosto de 2007 (Mw=8.0). Los sismos con origen en los procesos de deformación de la corteza a niveles superficiales son menos frecuentes, pero cuando ocurren, producen daños de consideración en áreas relativamente pequeñas, por ejemplo los sismos del Alto Mayo (San Martín) del 30 de Mayo de 1990 y 5 de Abril de 1991, ambos con magnitudes de 6.0 y 6.5 Mw. Los sismos de foco intermedio son pocas veces percibidos por la población en superficie, pero cuando alcanzan magnitudes ≥7.0 Mw tienen gran radio de percepción, llegando algunas veces a producir daños leves en viviendas; además, de procesos de licuación de suelos y/o deslizamientos de tierra y piedras en zonas de gran pendiente. El análisis de la distribución espacial de la sismicidad en el Perú (Figura 1), permite identificar la ubicación de las principales fuentes sismogénicas, todas descritas ampliamente por Tavera y Buforn (2001) y Bernal y Tavera (2002). Estudios recientes realizados sobre la historia sísmica de Perú (Dorbath et al, 1990; Tavera, 2005) y sobre la presencia, en su borde occidental, de áreas de importante acumulación de energía (Chlieh et al, 2011), muestran que el territorio peruano puede ser afectado en el futuro por sismos de gran magnitud. Por ejemplo, es conocida la existencia de las llamadas “lagunas sísmicas” en el borde occidental de la región central debido a que no ocurre un sismo importante desde el año 1746, otra en la región sur que no es afectada aún por un sismo similar al ocurrido en el año 1868, ambos habrían presentado magnitudes de ~8.5Mw. Del mismo modo, áreas de menor tamaño han sido identificadas frente a la localidad de Yauca, que en el año 1913 dio origen a un sismo de magnitud del orden de 7.7 Ms (Silgado, 1978). Este sismo, produjo daños de consideración en Caraveli, Chuquibamba y Caylloma en donde se desplomaron gran cantidad de viviendas y la iglesia mayor en Chuquibamba. En Arequipa hubo averías en edificios públicos. Según Umlauf (1913) el área afecta es de 30,000 km2 encerrada por una isosista de grado X (MM); mientras que la isosista de grado VII (MM) limita un área de 58,000 km2. Según información de los observatorios de Lima y La Paz, el área epicentral estaría entre las localidades de Chala y Atico (Silgado, 1978). Después de este terremoto, se produjo un tsunami que inundó la zona costera entre las localidades de Lomas y Chala, escenario que obligó a la población de la localidad de Yauca trasladarse y establecerse a mayor altura; es decir, en su actual ubicación. En la zona centro-sur del Perú, el día 25 de Septiembre del 2013, ocurrió un sismo de magnitud moderada (6.9ML, 7.0 Mw) y epicentro ubicado a 66 km al SO de la localidad de Yauca, 75 km al OSO de la localidad de Chala y 86 km, al SSO de la localidad de Acarí (Provincia de Caravelí, Departamento de Arequipa). El sismo ocurrió a una profundidad de 31 km (foco superficial) y en general, presentó un área de percepción con radio del orden de 350 km (Imax=II), siendo la mayor intensidad del orden de VI (MM) evaluada en las localidades de Yauca, Acarí y Chala. Este informe presenta los parámetros hipocentrales del sismo, intensidades evaluadas, réplicas, orientación de la fuente y su respectiva interpretación sismotectónica.