Jean-Mathieu Nocquet

Seismic and aseismic slip on the Central Peru megathrust

Slip 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 strain suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods. 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 = 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 rate-strengthening 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 = 8.0 in the Pisco area is estimated to be 250 years.

Interseismic coupling and seismic potential along the Central Andes subduction zone

We 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 M_w = 7.7 Nazca, the 2001 M_w = 8.4 Arequipa, the 2007 M_w = 8.0 Pisco and the M_w = 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 M_w∼8.6–8.8 earthquake occurred in 1746. This area ruptured again in a sequence of four M_w∼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 M_w > 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 M_w∼8.8 earthquake of 1868 is equivalent to a magnitude M_w∼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 M_w∼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.

A detailed source model for the Mw9.0 Tohoku‐Oki earthquake reconciling geodesy, seismology, and tsunami records

The 11 March 2011 Mw9.0 Tohoku-Oki earthquake was recorded by an exceptionally large amount of diverse data offering a unique opportunity to investigate the details of this major megathrust rupture. Many studies have taken advantage of the very dense Japanese onland strong motion, broadband, and continuous GPS networks in this sense. But resolution tests and the variability in the proposed solutions have highlighted the difficulty to uniquely resolve the slip distribution from these networks, relatively distant from the source region, and with limited azimuthal coverage. In this context, we present a finite fault slip joint inversion including an extended amount of complementary data (teleseismic, strong motion, high-rate GPS, static GPS, seafloor geodesy, and tsunami records) in an attempt to reconcile them into a single better resolved model. The inversion reveals a patchy slip distribution with large slip (up to 64 m) mostly located updip of the hypocenter and near the trench. We observe that most slip is imaged in a region where almost no earthquake was recorded before the main shock and around which intense interplate seismicity is observed afterward. At a smaller scale, the largest slip pattern is imaged just updip of an important normal fault coseismically activated. This normal fault has been shown to be the mark of very low dynamic friction allowing extremely large slip to propagate up to the free surface. The spatial relationship between this normal fault and our slip distribution strengthens its key role in the rupture process of the Tohoku-Oki earthquake.

Partitioning of oblique convergence in the Northern Andes subduction zone: Migration history and the present-day boundary of the North Andean Sliver in Ecuador

Along the Ecuadorian margin, oblique subduction induces deformation of the overriding continental plate. For the last 15 Ma, both exhumation and tectonic history of Ecuador suggest that the northeastward motion of the North Andean Sliver (NAS) was accompanied by an eastward migration of its eastern boundary and successive progressively narrowing restraining bends. Here we present geologic data, earthquake epicenters, focal mechanisms, GPS results, and a revised active fault map consistent with this new kinematic model. All data sets concur to demonstrate that active continental deformation is presently localized along a single major fault system, connecting fault segments from the Gulf of Guayaquil to the eastern Andean Cordillera. Although secondary faults are recognized within the Cordillera, they accommodate a negligible fraction of relative motion compared to the main fault system. The eastern limit is then concentrated rather than distributed as first proposed, marking a sharp boundary between the NAS, the Inca sliver, and the Subandean domain overthrusting the South American craton. The NAS limit follows a northeast striking right-lateral transpressional strike-slip system from the Gulf of Guayaquil (Isla Puna) to the Andean Cordillera and with the north-south striking transpressive faults along the eastern Andes. Eastward migration of the restraining belt since the Pliocene, abandonment of the sutures and reactivation of north-south striking ancient fault zones lead to the final development of a major tectonic boundary south and east of the NAS, favoring its extrusion as a continental sliver, accommodating the oblique convergence of the Nazca oceanic plate toward South America.

Active tectonics in Quito, Ecuador, assessed by geomorphological studies, GPS data, and crustal seismicity

The Quito Fault System (QFS) extends over 60 km along the Interandean Depression in northern Ecuador. Multidisciplinary studies support an interpretation in which two major contemporaneous fault systems affect Quaternary volcanoclastic deposits. Hanging paleovalleys and disruption of drainage networks attest to ongoing crustal deformation and uplift in this region, further confirmed by 15 years of GPS measurements and seismicity. The resulting new kinematic model emphasizes the role of the N-S segmented, en echelon eastward migrating Quito Fault System (QFS). Northeast of this major tectonic feature, the strike-slip Guayllabamba Fault System (GFS) aids the eastward transfer of the regional strain toward Colombia. These two tectonic fault systems are active, and the local focal mechanisms are consistent with the direction of relative GPS velocities and the regional stress tensor. Among active features, inherited N-S direction sutures appear to play a role in confining the active deformation in the Interandean Depression. The most frontal of the Quito faults formed at the tip of a blind thrust, dipping 40°W, is most probably connected at depth to inactive suture to the west. A new GPS data set indicates active shortening rates for Quito blind thrust of up to 4 mm/yr, which decreases northward along the fold system as it connects to the strike-slip Guayllabamba Fault System. The proximity of these structures to the densely populated Quito region highlights the need for additional tectonic studies in these regions of Ecuador to generate further hazard assessments.

Active tectonics of Peru : heterogeneous interseismic coupling along the Nazca megathrust, rigid motion of the Peruvian Sliver, and Subandean shortening accommodation

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.

Probabilistic Seismic‐Hazard Assessment in Quito, Estimates and Uncertainties

The present study is focused on estimating the probabilistic seismic hazard for the capital city of Ecuador, Quito, the population of which currently exceeds 2 million inhabitants at present. Quito is located at 2800 meters above sea level within the Interandean Depression, bounded by the equatorial line to the north, in an earthquake‐prone environment (Chatelain et al., 1999; Fig. 1). The city and its suburbs have developed in a piggy‐back basin on the hanging wall of a reverse fault system (Fig. 2) that has been recognized as seismically active in historical, geomorphologic, geologic, and geodetic studies (Soulas et al., 1991; Ego and Sebrier, 1996; Hibsch et al., 1997; Egred, 2009; Champenois et al., 2013; Alvarado et al., 2014).