Chilean Convergent Margin Research Articles

Technology for Location, Analysis and Interpretation of Magnetic Alterations as Precursors of Seismic Phenomena Update

To date, several studies have shown that the Earth's magnetic field suffers alterations at the local geographical location before an earthquake occur. Its study demonstrates that the Earth’s magnetic alterations at specific local geographical zone, is a local seismic precursor alerting a proximity of an earthquake with a margin of error of approximately 10%. The electromagnetic noise from background is very confusing, but that reason was necessary to identify these electromagnetic signal precursors by filtering a large amount of noise. To isolate the electromagnetic noise, was implemented a Magnetic North deflection detection in Smart Phones Magnetometers. Using it technology, was developed a mathematical algorithm that work in combination with the Smart Phones magnetometers. This research was based using in reference the study carried out by the Department of Physics of the Faculty of Physical Sciences and Mathematics (FCFM) of the University of Chile directed by Cordado, 2018[1], in the paper called “Latitudinal variation rate of geomagnetic cutoff rigidity in the active Chilean convergent margin”.

Deep Structure of the Continental Plate in the South‐Central Chilean Margin: Metamorphic Wedge and Implications for Megathrust Earthquakes

AbstractWe study the deep structure of the continental wedge along the south‐central Chilean convergent margin based on a joint interpretation of wide‐angle reflection and refraction seismic phases acquired between the southern part of the Maule 2010 (36°S) and the northern part of the Valdivia 1960 (41°S) earthquake rupture areas. When combined with results of previous seismic studies, our results provide new insights into the deep structure (to 15–20 km) of the overriding, continental South American plate. We observe a latitudinal variation of P wave velocity, interpreted in terms of the distribution of Permo‐Triassic metamorphic complex near the coast, and deep reflectors at the base of these metamorphic units. A change in the observed deep structural style near 38°S suggests a segmentation in the interplate frictional properties between the Maule and Valdivia earthquakes rupture zones. This regional change in the continental wedge structure indicates a spatial correlation between the coseismic ruptures of large earthquakes (e.g., the 1960 and 2010 megathrust earthquakes) and the rheology/lithology along the interplate boundary.

Latitudinal variation rate of geomagnetic cutoff rigidity in the active Chilean convergent margin

Abstract. We present a different view of secular variation of the Earth's magnetic field, through the variations in the threshold rigidity known as the variation rate of geomagnetic cutoff rigidity (VRc). As the geomagnetic cutoff rigidity (Rc) lets us differentiate between charged particle trajectories arriving at the Earth and the Earth's magnetic field, we used the VRc to look for internal variations in the latter, close to the 70° south meridian. Due to the fact that the empirical data of total magnetic field BF and vertical magnetic field Bz obtained at Putre (OP) and Los Cerrillos (OLC) stations are consistent with the displacement of the South Atlantic magnetic anomaly (SAMA), we detected that the VRc does not fully correlate to SAMA in central Chile. Besides, the lower section of VRc seems to correlate perfectly with important geological features, like the flat slab in the active Chilean convergent margin. Based on this, we next focused our attention on the empirical variations of the vertical component of the magnetic field Bz, recorded in OP prior to the Maule earthquake in 2010, which occurred in the middle of the Chilean flat slab. We found a jump in Bz values and main frequencies from 3.510 to 5.860 µHz, in the second derivative of Bz, which corresponds to similar magnetic behavior found by other research groups, but at lower frequency ranges. Then, we extended this analysis to other relevant subduction seismic events, like Sumatra in 2004 and Tohoku in 2011, using data from the Guam station. Similar records and the main frequencies before each event were found. Thus, these results seem to show that magnetic anomalies recorded on different timescales, as VRc (decades) and Bz (days), may correlate with some geological events, as the lithosphere–atmosphere–ionosphere coupling (LAIC).

Estado de esfuerzos estáticos en la interface de placas Coco-Caribe en Costa Rica: Interpretación mecánica para el origen de asperezas en la zona sismogénica

La región del antearco a lo largo de la zona de subducción centroamericana muestra una serie de anomalías positivas de gravedad paralelas a la trinchera con anomalías negativas correspondientes, a lo largo de la fosa y hacia la costa. Estas anomalías se extienden desde Guatemala hasta el norte de Nicaragua. Sin embargo, el segmento costarricense del antearco no sigue este patrón debido a la segmentación de la anomalía negativa de la fosa, la ausencia de la anomalía negativa costera y la presencia de masa continental emergida en el lugar de la anomalía positiva del antearco en la Península de Nicoya. Estudios geodésicos y sismológicos a lo largo de la zona de subducción costarricense sugieren la presencia de áreas de acoplamiento en la zona sismogénica bajo la península de Nicoya previo al sismo de magnitud 7,6 Mw del 5 de setiembre de 2012. Estas áreas han sido asociadas anteriormente con asperezas. Publicaciones previas han propuesto un modelo mecánico para el origen de asperezas a lo largo del margen convergente chileno, basado en la estructura de la placa cabalgante. Basado en la estructura de la placa cabalgante, publicaciones previas han propuesto un modelo mecánico para el origen de asperezas a lo largo del margen convergente chileno, en el cual el material ígneo denso en el antearco de la placa cabalgante sobre la zona sismogénica, modifica el estado de esfuerzos estáticos en la interface de placas e influye en los procesos sismogénicos. En Costa Rica, la geología de superficie y los datos de gravedad indican la presencia de corteza densa compuesta por basaltos y gabros que sobreyace la zona sismogénica en donde el acoplamiento está presente. Los valores de anomalía de Bouguer en esta región alcanzan los 120×10-5 m/s2 y son los más altos medidos para Costa Rica. En este trabajo, el estado de esfuerzos estáticos en la interface de placas Cocos-Caribe se calcula basado en la geometría y en la distribución de densidad de un modelo tridimensional de la litósfera de la zona de subducción interpretado a partir de datos de gravedad de modelos geopotenciales combinados. Los resultados muestran una correlación espacial entre las áreas acopladas en la Península de Nicoya y la presencia de anomalías en el estado de esfuerzos estáticos en la interface de placas. Las anomalías de esfuerzo se calcularon para el componente normal del esfuerzo vertical en la zona sismogénica y se interpretaron como originadas por el material denso que compone el antearco en el área. El material denso de las rocas máficas del Complejo de Nicoya y la carga topográfica de la península sobre la zona sismogénica influyen en la distribución de zonas acopladas y el comportamiento sísmico de la región. Esto se debe a que las anomalías en el esfuerzo normal a la interface de placas incrementan el umbral de esfuerzo cortante para generar la ruptura.

Seismic structure of the north‐central Chilean convergent margin: Subduction erosion of a paleomagmatic arc

AbstractWe study the erosive convergent margin of north‐central Chile (at ~31°S) by using high‐resolution bathymetric, wide‐angle refraction, and multichannel seismic reflection data to derive a detailed tomographic 2‐D velocity‐depth model. In the overriding plate, our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6.0–6.5 km/s, which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the Coastal Cordillera. Beneath the lower and middle continental slope, however, the presence of a zone of reduced velocities (3.5–5.0 km/s) is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion. At the landward edge of the outermost fore arc, the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp (~1 km offset), which overlies a strong lateral velocity contrast from ~5.0 to ~6.0 km/s. This pronounced velocity contrast propagates deep into the continental crust, and it resembles a major normal listric fault. We interpret this seismic discontinuity as the volcanic‐continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc. Subduction erosion has, most likely, caused large‐scale crustal thinning and long‐term subsidence of the outermost fore arc.

Material transfer and its influence on the formation of slope basins along the South Central Chilean convergent margin: Insights from scaled sandbox experiments

Material transfer along and across a subduction zone determines not only the morphology of an accretionary prism, but also the architecture and tectono-sedimentary development of their forearc basins. However, the nature of these processes is not fully resolved yet. We investigated, through analog simulations, the relationship among parameters influencing material transfer along the South Central Chilean margin, and the marine basin evolution. Seismic lines revealed half-graben basins, controlled by subvertical faults, governing differential uplift and subsidence through the slope. The identified sedimentary pattern, evidenced three main seismic sequences which have been linked to major climatic fluctuations and episodes of accretion, non-accretion or tectonic erosion which operate since the Pliocene. Our experimental set-up was motivated by the coupled tectonic–climatic history of the Chile–Peru trench fill i.e. climatic episodes represent variations in sediment supply and also in frictional properties of the materials entering in a convergent margin. By using three key sandbox experiments we tested two main parameters: (i) variable trench sediment thickness and (ii) basal frictional properties. We highlight that by changing frictional conditions of sediments along the plate interface and the length of adjacent weak and strong segments, we modified the evolution of the wedge from frontal to basal accretion. In addition, we obtained sub-vertical faults with potential reactivation, such as the observed geometry in seismic lines. The change from a long segment of low basal friction to one of high basal friction offered an efficient mechanism to transport a long sand layer, similar to a subduction channel. This controlled the activation of thrust faults towards the backstop, driving shortening and differential uplift, with small individual extensional basins forming along the top of the wedge. During further underplating the total volume of accommodation space in slope basins decreased, whereas the volume of shelf basins tended to increase.

Mass wasting at the base of the south central Chilean continental margin: the Reloca Slide

Abstract. Offshore south central Chile (35° S–42° S), the morphology of the lowermost continental slope and trench floor witnesses a voluminous submarine mass-wasting event. The blocky slide body deposited in the Chile Trench at 73°46´ W 35°35´ S was targeted for study during RRS JAMES COOK Cruise JC23 and termed Reloca Slide. Its size of about 24 km3, its steep and high headscarp, the spatial distribution of slide deposits and the cohesive nature of major slide blocks make it interesting to address the issue of tsunami generation. We have obtained seismic reflection data that partly reveal the internal structure of the slide body. Gravity core samples were retrieved that will allow the slide to be dated and linked to the history of sedimentation and slope stability along this particular segment of the Chilean convergent margin. At present we assume a Holocene age for the sliding event.

Asperity generating upper crustal sources revealed by b value and isostatic residual anomaly grids in the area of Antofagasta, Chile

In our study we show that the locations of largest coseismic slip (asperities) on the fault plane of the Mw = 8.0 1995 Antofagasta earthquake in Northern Chile can be mapped by the spatial distribution of the seismic b value obtained from the aftershock sequence of the megathrust earthquake. These areas of high seismic moment release and concurrent high‐b values are congruent with anomalies of the gravity isostatic residual (IR) field in the Antofagasta region. They are superimposed on the seismogenic part of the north Chilean subduction zone where the strongest coupling of the upper and lower plate is expected. The IR anomalies are interpreted to be caused by large Jurassic–Early Cretaceous batholiths which intruded into the upper crust. The observed positive correlations between high seismic b values, IR anomalies, and geologic structures enable us to propose a mechanical model for the generation of the asperities in the Antofagasta region. We suggest that the batholiths in conjunction with buoyant forces acting on the subducted slab of the Nazca plate are responsible for locking the interface where the asperities are located. This implies long‐term conditions for the existence of the asperity generating tectonic situation. Concequently, the asperities around Antofagasta could be stationary features, at least for several seismic cycles. Hence we propose that the IR anomalies along the north Chilean convergent margin can be used as an indicator for high moment release and slip in future large earthquakes.

Evidence for fluid circulation, overpressure and tectonic style along the Southern Chilean margin

The southern Chilean convergent margin, between 50° and 57° S, is shaped by the interaction of the three main plates: Antarctic, South America and Scotia. North of 53° S, the convergence between Antarctic and South America plates is close to orthogonal to the continental margin strike. Here, the deformational style of the accretionary prism is mainly characterized by seaward-verging thrusts and locally by normal faults and fractures, a very limited lateral extension of prism, a very shallow dip (∼ 6°) décollement, and subduction of a thick and relatively undeformed trench sedimentary sequence. South of 53° S, convergence is oblique to the margin, locally, the trench sediments are proto-deformed by double vergence thrusting and the front of the prism grows through landward-verging thrusting. The décollement is sub-horizontal and deep, involving most of the sediment over the oceanic crust in the accretionary process, building a comparatively wide and thicker prism. A Bottom Simulating Reflector is present across the whole prism to the abyssal plan, suggesting the presence of gas in the sediments. The analysis of P- and S-wave velocity reflectivity sections, derived by amplitude versus offset technique (AVO), detailed velocity information and the velocity-derived sediment porosity have been integrated with the structural analysis of the accretionary prism of two selected pre-stack depth migrated seismic lines, aiming to explain the relation between fluid circulation and tectonics. Accretion along double vergence thrust faults may be associated with the presence of overpressured fluid, which decreases the effective shear stress coefficient along the main décollement and within the sediments, and modify the rheolgical properties of rocks. The presence of an adequate drainage network, represented by interconnected faults and fractures affecting the entire sedimentary sequence, can favour the escape of pore fluid toward the sea bottom, while, less permeable and not faulted sediments can favour fluid accumulations. Gravitational and tectonic dewatering, and stratigraphy could control the consolidation and the pore overpressure of sediments involved in subduction along the trench. The results of our analysis suggest the existence of a feedback between tectonic style and fluid circulation.

Crustal structure of the southernmost Chilean margin from seismic and gravity data

The southern Chilean convergent margin south of the Strait of Magellan, between 52 and 57°S, is undergoing a style of subduction where obliquity plays a significant role. Seismic reflection and gravity data collected during 1988 by LDEO (RC2902) across the study area image crustal structures from oceanic crust (seaward of the trench) to the continental shelf. Pre-stack and post-stack depth migration have been applied to MCS profile RC2902-790, from which we have obtained a depth section, with true geometries and an associated accurate velocity model. Additional velocity information from sonobuoys has been used to further constrain the depth-converted section. 2D gravity modelling has supported the deduced crustal structure. Four main domains are imaged, which, from SW to NE, are: (1) the oceanic domain, where the Chilean trench is buried with a clastic sedimentary wedge reaching maximum thickness of 4 km, lying on a 7–8-km-thick crystalline oceanic crust that gently dips landward (3–4°); (2) the accretionary prism domain, about 40 km wide, of highly deformed sediments with a clear Bottom Simulating Reflector (BSR); (3) the forearc basin domain, a 25-km-wide basin (nearly undeformed) with a maximum sediment thickness of 4.5 km, which is bordered along its seaward edge by a zone of outer-arc structural highs and the accretionary prism forming the so-called ‘Fuegian terrace’; and (4) the continental domain which consists of an 8° dip continental slope and the continental shelf. Seismic reflection data along the continental shelf reveal that between 5 and 8 s twt, there is package of reflections that could mark the presence of a high reflective lower crust. Results from the corresponding sonobuoy are not conclusive enough to confirm such a hypothesis, but the brightness of these reflectors suggests the presence of a 10-km-thick reflective lower crust between 14 and 24 km depth. At greater depths (between 11 and 14 s twt), there is a thin package of bright landward-dipping reflectors, which may indicate the top of the subducting slab as deduced from gravity data. Furthermore, gravity modelling shows that the slab subducts at a very low angle of about 7–8°. In spite of the lack of seismicity, our results favour the conclusion that subduction still occurs in this region of the Chilean margin or has recently ceased.

Crustal dynamics and active fault mechanics during subduction erosion. Application of frictional wedge analysis on to the North Chilean Forearc

The forearc region of the non-accreting South American Plate margin in northern Chile is characterised by subduction erosion and regional uplift. Neotectonic deformation structures reflect simultaneous extensional and contractional fault kinematics. In the outer forearc, where the brittle crust directly overlies the subducting Nazca Plate, the stress regime changes from extension in the upper part to compression at the base of the forearc wedge as seen in neotectonic surface structures and seismic data. In the inner forearc, surface structures indicate a compressional stress regime also affecting the western rim of the magmatic arc. This stress regime is limited to a brittle crustal wedge segment which overlies the ductile part of the inner forearc lithosphere. The shape of the two brittle forearc wedges at the leading edge of the South American Plate is controlled by exogenetic surface processes, internal deformation processes, contemporaneous basal tectonic erosion and underplating. Mechanical parameter sets controlling and reflecting the recent tectonic processes and geometrical wedge segmentation within the forearc system are evaluated and applied to general frictional wedge models. The states of stress within the crustal wedges are controlled by spatial variations of the basal mechanical parameters in the down-dip direction of the forearc wedge base. The new models illustrate the fundamental kinematics and dynamic processes of ‘Coulomb-type’ basal tectonic erosion and mass transfer modes along active non-accretive convergent margins. The frictional wedge models explain the dynamics of the simultaneous and contrary deformation processes affecting the forearc crust at the North Chilean convergent margin shaped by active basal tectonic erosion.

Sonobuoy and teleseismic study of Gulf of California transform fault earthquake sequences : REICHLE M. S., G. F. SHARMAN and J. N. BRUNE, 1976. Bull. seism. Soc. Am., 66 (5): 1623–1641

The southern Chilean convergent margin south of the Strait of Magellan, between 52 and 57°S, is undergoing a style of subduction where obliquity plays a significant role. Seismic reflection and gravity data collected during 1988 by LDEO (RC2902) across the study area image crustal structures from oceanic crust (seaward of the trench) to the continental shelf. Pre-stack and post-stack depth migration have been applied to MCS profile RC2902-790, from which we have obtained a depth section, with true geometries and an associated accurate velocity model. Additional velocity information from sonobuoys has been used to further constrain the depth-converted section. 2D gravity modelling has supported the deduced crustal structure. Four main domains are imaged, which, from SW to NE, are: (1) the oceanic domain, where the Chilean trench is buried with a clastic sedimentary wedge reaching maximum thickness of 4 km, lying on a 7–8-km-thick crystalline oceanic crust that gently dips landward (3–4°); (2) the accretionary prism domain, about 40 km wide, of highly deformed sediments with a clear Bottom Simulating Reflector (BSR); (3) the forearc basin domain, a 25-km-wide basin (nearly undeformed) with a maximum sediment thickness of 4.5 km, which is bordered along its seaward edge by a zone of outer-arc structural highs and the accretionary prism forming the so-called ‘Fuegian terrace’; and (4) the continental domain which consists of an 8° dip continental slope and the continental shelf. Seismic reflection data along the continental shelf reveal that between 5 and 8 s twt, there is package of reflections that could mark the presence of a high reflective lower crust. Results from the corresponding sonobuoy are not conclusive enough to confirm such a hypothesis, but the brightness of these reflectors suggests the presence of a 10-km-thick reflective lower crust between 14 and 24 km depth. At greater depths (between 11 and 14 s twt), there is a thin package of bright landward-dipping reflectors, which may indicate the top of the subducting slab as deduced from gravity data. Furthermore, gravity modelling shows that the slab subducts at a very low angle of about 7–8°. In spite of the lack of seismicity, our results favour the conclusion that subduction still occurs in this region of the Chilean margin or has recently ceased.