Double Benioff Zone Research Articles

Evidence for Rupture Through a Double Benioff Zone During the 2017 Mw 8.2 Chiapas, Mexico Earthquake

AbstractWe combine multi‐array relative back‐projection of high‐frequency P waves and finite‐fault modeling of low‐frequency P waves from the 8 September 2017 Mw 8.2 Chiapas, Mexico earthquake to image unilateral rupture on a southeast‐northwest striking subvertical fault plane over depths of 10–70 km. Improved multi‐array relative back‐projection estimates of rupture speed and fault geometry provide key refinements to the slip model from finite‐fault modeling. Compilation of prior seismicity and aftershocks reveals the earthquake initiated in the lower plane of a double Benioff zone (DBZ) and ruptured with large slip across the entire DBZ. The previously aseismic interior does not appear to be due to lack of stress as both planes show downdip extension stresses. Instead, tomographic imaging of DBZs find high Poisson's ratio with fast velocities in the DBZ interior, so we propose that highly hydrated harzburgite generates velocity‐strengthening behavior that inhibits earthquake nucleation, yet it can slip seismically during dynamic weakening from a large throughgoing rupture.

Illumination of deformation by bending stresses and slab pull within the Southern Hikurangi Double Benioff Zone

ABSTRACTDouble Benioff Zones (DBZ) are ubiquitous in subduction systems worldwide, but the stress systems that give rise to them are not well known. We characterise stress orientations in the upper and lower bands of a DBZ in the dipping subducted Pacific plate beneath the southern Hikurangi margin, New Zealand. Stress orientations were calculated from focal mechanisms with 10 or more polarity picks, using a Bayesian stress inversion technique. The stress orientations in the upper band of seismicity are consistent with a down-dip extensional stress regime, whereas stress orientations within the lower band of seismicity are consistent with a down-dip compressional stress regime. This demonstrates that bending stresses are dominant within the Pacific plate beneath the southern North Island, possibly due to subduction beneath thickened mantle lithosphere of the Australian plate.

Controls of faulting and reaction kinetics on serpentinization and double Benioff zones

The subduction of partially serpentinized oceanic mantle may potentially be the key geologic process leading to the regassing of Earth's mantle and also has important consequences for subduction zone processes such as element cycling, slab deformation, and intermediate‐depth seismicity. However, little is known about the quantity of water that is retained in the slab during mantle serpentinization and the pattern of serpentinization that may occur during bending‐related faulting; an initial state that is essential for quantifying subsequent dehydration processes. We present a 2‐D reactive‐flow model simulating hydration processes in the presence of faulting at the trench outer‐rise. We find that the temperature dependence of the serpentinization rate in conjunction with outer‐rise faulting results in plate age and speed dependent patterns of hydration. Serpentinization also results in a reduction in surface heat flux toward the trench caused by advective downflow of seawater into the reaction region. Observed heat flow reductions are larger than the reduction due to the minimum‐water downflow needed for partial serpentinization, predicting that active hydrothermal vents and chemosynthetic communities should also be associated with bend‐fault serpentinization. Our model results agree with previous studies that the lower plane of double Benioff zones can be generated due to dehydration of serpentinized mantle at depth. More importantly, the depth‐dependent pattern of serpentinization including reaction kinetics predicts a separation between the two Benioff planes consistent with seismic observations.

Global Prevalence of Double Benioff Zones

Double Benioff zones provide opportunities for insight into seismogenesis because the underlying mechanism must explain two layers of deep earthquakes and the separation between them. We characterize layer separation inside subducting plates with a coordinate rotation to calculate the slab-normal distribution of earthquakes. Benchmark tests on well-established examples confirm that layer separation is accurately quantified with global seismicity catalogs alone. Global analysis reveals double Benioff zones in 30 segments, including all 16 subduction zones investigated, with varying subducting plate ages and stress orientations, which implies that they are inherent in subducting plates. Layer separation increases with age and is more consistent with dehydration of antigorite than chlorite.

Subduction zone earthquakes and stress in slabs

The pattern of seismicity as a function of depth in the world, and the orientation of stress axes of deep and intermediate earthquakes, are explained using viscous fluid models of subducting slabs, with a barrier in the mantle at 670 km. 670 km is the depth of a seismic discontinuity, and also the depth below which earthquakes do not occur. The barrier in the models can be a viscosity increase of an order of magnitude or more, or a chemical discontinuity where vertical velocity is zero. LongN versus depth, whereN is the number of earthquakes, shows (1) a linear decrease to about 250–300 km depth, (2) a minimum near that depth, and (3) an increase thereafter. Stress magnitude in a subducting slab versus depth, for a wide variety of models, shows the same pattern. Since there is some experimental evidence thatN is proportional toeκσ, where κ is a constant and σ is the stress magnitude, the agreement is encouraging. In addition, the models predict down-dip compression in the slab at depths below 400 km. This has been observed in earlier studies of earthquake stress axes, and we have confirmed it via a survey of events occurring since 1977 which have been analysed by moment tensor inversion. At intermediate depths, the models predict an approximate but not precise state of down-dip tension when the slab is dipping. The observations do not show an unambiguous state of down-dip tension at intermediate depths, but in the majority of regions the state of stress is decidedly closer to down-dip tension than it is to down-dip compression. Chemical discontinuities above 670 km, or phase transitions with an elevation of the boundary in the slab, predict, when incorporated into the models, stress peaks which are not mirrored in the profile of seismicity versus depth. Models with an asthenosphere and mesosphere of appropriate viscosity can not only explain the state of stress observed in double Benioff zones, but also yield stress magnitude profiles consistent with observed seismicity. Models where a nonlinear rheology is used are qualitatively consistent with the linear models.

Hypocentral trend surface analysis: Probing the geometry of Benioff Zones

A hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the “middle” of a Benioff Zone. In this paper we take a relatively simple approach to hypocentral trend surface analysis. The hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface. The vertical deviation of a hypocenter from the hypocentral trend surface is called its residual. Consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution. Any analysis of Benioff Zone geometry trades‐off presumed thickness of the zone with its presumed shape. This trade‐off can be investigated by examining suites of candidate hypocentral trend surfaces (together with their associated residual distributions) generated by varying the number of degrees of freedom available to the trend surface. Hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in Honshu (Japan), Cook Inlet (Alaska) and South Peru. Hypocenters beneath Honshu generate a bimodal distribution of residuals about their trend surface. The vertical separation of the upper and lower sheets of this Double Benioff Zone averages about 33 km. No Double Benioff Zone configuration is discovered below Cook Inlet or South Peru. Regional trend surfaces are established for intermediate‐depth teleseismic data from South and Middle America. In South America (0°–40°S) teleseismic data suggest that along‐strike transitions between “flat” and moderately steeply dipping sections of the Benioff Zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear‐bound flaps or separate tongues of lithosphere. Hypocentral trend surface analysis of teleseismic data from South Peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of Hasegawa and Sacks. An eastward shallowing Benioff Zone beneath southwest Mexico is closely associated with the geometry of the trans‐Mexican volcanic arc. The Benioff Zone beneath Central America is more steeply dipping. The flat slab‐steep slab transition near 96°W may relate to the difference in age of oceanic lithosphere on either side of the Tehuantepec Ridge. While the strong flexure of the subducted Nazca plate beneath Ecuador is closely associated with the Carnegie Ridge, the Nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath South Peru. The flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding South American plate.

Complex distribution of large thrust and normal fault earthquakes in the Chilean subduction zone

Summary. We present the results of a systematic study of events with M, > 6 in northern Chile (20-33S), for the period between 1963 and 1971. Medium to large earthquakes near the coast of this region are of three types: (1) Interplate events at the interface between the downgoing slab and the overriding South American plate. These events can be very large reaching magnitudes greater than 8. (2) Intra-plate earthquakes 20-30 km inside the downgoing slab. They have fault mechanisms indicating extension along the dip of the slab and may have magnitudes up to 7.5. (3) Less frequent,M, - 6 events that occur near the top of the downgoing slab and have thrust mechanisms with an almost horizontal E-W compressional axis. This type of mechanism is very different from that of the events of type 1 which are due to shallow dipping reverse faulting. There is a rotation of about 30 of the compressional axis in the vertical plane between events of types (1) and (3). Three groups of events near 32.5, 25.5 and 21s were studied in detail. Depth and mechanisms were redetermined by P- wave modelling and relative locations were obtained by a master event technique. Near 32.5S, only events of types 1 and 2 were found in the time period of this study. At the two other sites, the three types of events were identified. This shows clearly that there are compressive stresses at the top of the slab and extension at the centre, a situation which is usually found in the areas where a double Benioff-zone has been identified in the seismicity.

Thrust and extensional faulting under the Chilean coast: 1965, 1971 Aconcagua earthquakes

Summary. Earthquakes under coastal northern Chile, from 18° S to 33° S occur at two very different depths. The shallower interplate events have the typical thrust mechanisms associated with subduction of the Nazca Plate under South America. The deeper (close to 70 km) events have normal fault mechanisms with a tension axis approximately parallel to the dip of the downgoing slab. We study in detail a. couple of Ms∼ 7.5 events that took place about 60 km north of Valparaiso, Chile. The 1971 July 9 event was a shallow thrust interplate event at 40 km depth, while the 1965 March 28 earthquake had a normal fault mechanism and a depth of 72 km. We carry out a detailed analysis of long period P-and surface waves observed in the WWSSN network of stations in order to improve fault plane solutions and depth determinations. For the 1971 event we find a seismic moment of 5.6 ∼ 1027 dyne cm from both P-and surface waves. The source area for this event is relatively small compared to that of other underthrust events with similar moments. Assuming a circular fault shape and a rigidity of 7 ∼ 1011 dyne cm−2 we find a stress drop of 38 bar. The 1965 event is more complex; seismic moments of 1.8 and 1.0 ∼ 1027 dyne cm are found from surface and body waves respectively. The stress drop estimated from P-waves and a circular fault model is close to 91 bar. The existence of earthquakes at two different depths near the Chilean coast may be interpreted in terms of a double layered Benioff zone, as has been found in Honshu, Kuriles or central Peru. Unfortunately the hypo-central determinations of events in this region are not precise enough to separate the two layers. An alternative method to demonstrate the existence of a double Benioff zone is to look for compressional events associated with the upper layer of the down-going slab. A search among available fault plane solutions is ambiguous; three shallow coastal events are found in Stauder's catalogue with horizontal pressure axes normal to the trench, but they might be intra-continental plate events.

Lithospheric flexure

During the last four years, major progress has been made in mapping regional variations in the strength of the lithosphere and in understanding the rheological properties of the upper mantle which are responsible for the plate‐like behavior of the lithosphere. It has long been recognized that the apparently rigid translations of the plates and the support of large topographic loads on the earth's surface require a strong lithosphere overlying a weak asthenosphere. Recently, interest in defining the detailed mechanical properties of the lithosphere has greatly increased, perhaps stimulated by the discovery of a double Benioff zone beneath Japan which may be related to the unbending of the subducted oceanic plate. Many investigators seem to be reaching a general consensus on a model of the oceanic lithosphere which is consistent with thermal models of the upper mantle, experimental rock mechanics, gravity and bathymetrie surveys, and patterns of seismicity and focal mechanisms.

OCEANIC CRUST IN THE DYNAMICS OF PLATE MOTION AND BACK-ARC SPREADING

Some of the fundamental features of plate tectonics are interpreted in connection with the behavior of oceanic crust. It is shown to be likely that the oceanic crust which is produced at the mid-ocean ridge by chemical differentiation may be removed from the downgoing slab by melting at the depth of asthenosphere behind the deep-sea trench. The melting of crustal material after the subduction is made possible by an efficient supply of heat through the well-developed asthenosphere with a low-velocity and high-attenuation of seismic waves. The removal of subducted oceanic crust from the slab is consistent with the positive gravity anomaly behind trenches and the double Benioff zone recently discovered. We propose new type of driving forces of plate motion, which arises from the density contrast between the crust and mantle when the oceanic crust is either created or destructed. The proposed driving mechanism is consistent with the non-uniform size and shape of individual plates, the migration of mid-ocean ridges and compressional intraplate stress, while these facts are difficult to understand in the framework of conventional models. A continuous accumulation of basaltic magma beneath the trench-arc system results in a catastrophic overflow of material, which corresponds to back-arc spreading. The picture presented in this paper explains the evolution of marginal basins that is characterized by the presence of remnant arcs, the changes in stress field and the dip angle of the slab, and the anomalous depth-age relationships.