circum-Pacific Subduction Zones Research Articles

Small-scale heterogeneities in the convective upper mantle beneath circum-Pacific subduction zones: Evidence for fragments of recycled basaltic crust

We show evidence for the presence of compositional heterogeneities at 10 km scale in the convective upper mantle below the circum-Pacific subduction zones. By array-processing we examine seismograms of intermediate-depth earthquakes recorded by large-aperture seismic networks at teleseismic distances. In the P coda until about 25 s after direct P waves, we observe anomalous and significant signals arriving from the directions close to the P waves. The amplitude and duration of the signals decrease with focal depth within the range from about 180 to 300 km. We show that the signals arise mainly from S-to-P scattering in the mantle below the foci. Using the scattering theory for randomly heterogeneous media we also show that the observed features of the signals are matched by 5 to 10 km-scale density and rigidity anomalies that diminish below about 400 km depth. The rigidity anomaly is of the same sign and nearly twice as large as the density anomaly. These properties of the scattering objects could be consistent with basalt embedded in depleted mantle. The results indicate that the upper mantle transition zone beneath subducting slabs is not well mixed even by vigorous convection.

Subduction thermal regime, petrological metamorphism and seismicity under the Mariana arc

Because of the steep subduction of a highly concave slab, researchers have characterized megathrusts under the Marianas as among the coldest and curviest plate coupling interfaces in various circum-Pacific subduction zones. Seismic tomography indicates that the heterogeneous underlying plate varies markedly in its subduction angle, velocity, and flexure along the strike and dip, while their effects on the thermal structure and intraslab earthquake occurrence remain enigmatic. By incorporating the 3-D MORVEL velocity and state-of-the-art slab geometry into thermomechanical modeling, we estimated the 3-D subduction thermal state and hydrothermal regime below the Marianas. We find that (1) the concave slab geometry and the complexity of the intraslab velocity variation in the Marianas are associated with a heterogeneous along-strike thermal regime and a cold mantle wedge beneath the central Marianas; (2) amphibolitization and eclogitization of subducted oceanic crust cause variations in fluid pressure and fluid release from the subduction interface, which may influence the distribution of interface seismicity in the Mariana system; (3) the concentration of active hydrothermal vents in the trench > 8 km deep is accompanied by a large temperature gradient and subsequent remarkable slab dehydration in the southern Marianas; and (4) slab dehydration (> 0.02 wt%/km) from 30 to 80 km indicates notable fluid release and potential fluid migration in subduction channels, which may correspond to the large water flux at depth beneath the Marianas.

Risk Assessment of Tsunamis Along the Chinese Coast Due to Earthquakes

China’s coastal areas are densely populated, economically developed, and located in close proximity to several potential tsunami sources; therefore, tsunami risk cannot be ignored. This study assessed tsunami risk in coastal areas of China by developing a framework for tsunami risk assessment from the perspectives of hazards, vulnerability, and exposure. First, a probabilistic tsunami hazard assessment (PTHA) model was applied to estimate the potential tsunami sources in both local crustal faults and circum-Pacific subduction zones based on numerical simulations. The output of the PTHA includes tsunami wave height distributions along the coast. Then, an indicator system reflecting exposure and vulnerability to tsunamis in the coastal areas of China was established by using the entropy method and analytic hierarchy process. The PTHA findings show that the tsunami wave height is close to 3 m on the southern coast of the Bohai Sea, the Pearl River Estuary, and the Yangtze River Delta and exceeds 2 m near the Taiwan Strait for the 2000-year return period. The results of the tsunami risk assessment show that the cities at the highest risk level (level I) include Tangshan, Yantai, and Hong Kong, while cities at the high risk level (level II) include Fuzhou, Xiamen, and Quanzhou near the Taiwan Strait and many cities on the Yangtze River Delta, the Pearl River Estuary, and the southern coast of the Bohai Sea. Our findings can provide an understanding of differences in tsunami risk between Chinese coastal cities that may be affected by tsunamis in the future.

Metasediment-derived Melts in Subduction-zone Magmas and their Influence on Crustal Evolution

AbstractSubduction is a major process cycling material through Earth’s geochemical reservoirs. Although trends in chemical composition of arc magmas imply assimilation of metasediment, the degree of such assimilation and the loci of that metasediment contamination (whether via subducted sediment or country rock assimilation) are poorly understood. To address these issues, we explore compositional data of oceanic and continental arc systems from circum-Pacific subduction zones. We find that high-silica continental arc rocks of the circum-Pacific are associated with higher aluminium saturation indices interpreted to reflect higher degrees of metasediment assimilation, with Sr/Y suggestive of shallow emplacement levels within the crust. In contrast, high-silica oceanic rocks of the circum-Pacific display lower aluminosity and equilibrated at deeper levels within the crust. Continental arc basalts are often assumed to be the source of high-silica continental arc rocks. However, phase equilibrium modelling of partial melting and crystal fractionation of continental arc basalts yield results that question this assumption. Furthermore, continental arc rock compositions show that the assimilated metasediments have protoliths that are most probably felsic greywacke and pelite rather than mafic greywacke. These findings are consistent with the hypothesis that high-silica rocks in continental arcs are directly influenced by anatexis of metasediment at shallow crustal levels (<20 km). Based upon a new method of discriminating the contribution of metasediment-derived melt, approximately one-third of felsic rocks in continental arcs have a demonstrable and unambiguous metasedimentary component. The degree of metasedimentary reworking in continental arc magmas plays an important role in the evolution of the continental crust and highlights the importance of using sediment-sensitive geochemical proxies and a magma’s petrological history when deconvolving the histories of magmatic arcs. This study also underlines the caveats associated with the calculation of depleted mantle model ages, where traditional techniques may lead to discrepancies of the order of 0·5 billion years.

Reconciling lithospheric rheology between laboratory experiments, field observations and different tectonic settings

SUMMARYRecent modelling studies have shown that laboratory-derived rheology is too strong to reproduce observations of flexure at the Hawaiian Islands, while the same rheology appears consistent with outer rise—trench flexure at circum-Pacific subduction zones. Collectively, these results indicate that the rheology of an oceanic plate boundary is stronger than that of its interior, which, if correct, presents a challenge to understanding the formation of trenches and subduction initiation. To understand this dilemma, we first investigate laboratory-derived rheology using fully dynamic viscoelastic loading models and find that it is too strong to reproduce the observationally inferred elastic thickness, Te, at most plate interior settings. The Te can, however, be explained if the yield stress of low-temperature plasticity is significantly reduced, for example, by reducing the activation energy from 320 kJ mol−1, as in Mei et al., to 190 kJ mol−1 as was required by previous studies of the Hawaiian Islands, implying that the lithosphere beneath Hawaii is not anomalous. Second, we test the accuracy of the modelling methods used to constrain the rheology of subducting lithosphere, including the yield stress envelope (YSE) method, and the broken elastic plate model (BEPM). We show the YSE method accurately reproduces the model Te to within ∼10 per cent error with only modest sensitivity to the assumed strain rate and curvature. Finally, we show that the response of a continuous plate is significantly enhanced when a free edge is introduced at or near an edge load, as in the BEPM, and is sensitive to the degree of viscous coupling at the free edge. Since subducting lithosphere is continuous and generally mechanically coupled to a sinking slab, the BEPM may falsely introduce a weakness and hence overestimate Te at a trench because of trade-off. This could explain the results of recent modelling studies that suggest the rheology of subducting oceanic plate is stronger than that of its interior. However, further studies using more advanced thermal and mechanical models will be required in the future in order to quantify this.

Investigating the signatures of long-range persistence in seismic sequences along Circum-Pacific subduction zones

AbstractIn the present paper, we analyze the signatures of long-range persistence in seismic sequences along Circum-Pacific subduction zones, from Chile to Kermadec, extracted from the National Earthquake Information Center (NEIC) catalog. This region, known as the Pacific Ring of Fire, is the world’s most active fault line, containing about 90% of the world’s earthquakes. We used the classical rescaled range (R/S) analysis to estimate the long-term persistence signals derived from a scaling parameter called the Hurst exponent, H. We measured the referred exponent and obtained values of H > 0.5, indicating that a long-term memory effect exists. We found a possible fractal relationship between H and the bs(q)-index, which emerges from the non-extensive Gutenberg-Richter law as a function of the asperity. Therefore, H can be associated with a mechanism that controls the level of seismic activity. Finally, we concluded that the dynamics associated with fragment-asperity interactions can be classified as a self-affine fractal phenomenon.Keywords: Applied geophysics; Fault and Fracture Analysis; Mathematics applied to geohysics; Seismology; Statistics;geostatistics ResumoNo presente artigo, analisamos as assinaturas de persistência long-range nas sequências sísmicas ao longo das zonas de subducção Circum-Pacific, do Chile até Kermadec, extraídas do catálogo do Centro Nacional de Informações sobre Terremotos (NEIC). Esta região, conhecida como Anel de Fogo do Pacífico, é a linha de falhas mais ativa do mundo, contendo cerca de 90% dos terremotos do mundo. Usamos a análise clássica R / S para estimar a assinatura de persistência a longo prazo derivada do parâmetro de escalonamento chamado expoente de Hurst, H. Como principal objeto de estudo}, medimos o referido expoente e obtivemos todos os valores de H> 0,5, indicando que existe um efeito de memória de longo prazo. A principal contribuição do nosso artigo foi encontrar uma possível relação entre H e o índice bs (q) - que emerge da lei de Gutenberg-Richter não-extensiva como uma função da aspereza, isto é, H pode estar associado ao mecanismo que controla o nível de atividade dos terremotos. Finalmente, concluímos que a dinâmica associada às interações fragilidade-aspereza pode ser classificado como um fenômeno fractal auto-afim.Palavras-chaves: Geofisica Aplicada; Analise de falhas e fraturas; Matematica Aplicada a Geofisica; Sismologia; Estatistica;geoestatistica

Magnesium Isotopes as a Tracer of Crustal Materials in Volcanic Arc Magmas in the Northern Cascade Arc

Fifteen North Cascade Arc basalts and andesites were analyzed for Mg isotopes to investigate the extent and manner of crustal contributions to this magmatic system. The δ26Mg of these samples vary from within the range of ocean island basalts (the lightest being -0.33 ± 0.07‰) to heavier compositions (as heavy as -0.15 ± 0.06‰). The observed range in chemical and isotopic composition is similar to that of other volcanic arcs that have been assessed to date in the circum-pacific subduction zones and in the Caribbean. The heavy Mg isotope compositions are best explained by assimilation and fractional crystallization within the deep continental crust with a possible minor contribution from the addition of subducting slab-derived fluids to the primitive magma. The bulk mixing of sediment into the primitive magma or mantle source and the partial melting of garnet-rich peridotite are unlikely to have produced the observed range of Mg isotope compositions. The results show that Mg isotopes may be a useful tracer of crustal input into a magma, supplementing traditional methods such as radiogenic isotopic and trace element data, particularly in cases in which a high fraction of crustal material has been added.

Deep carbon cycles constrained by a large-scale mantle Mg isotope anomaly in eastern China

Abstract Although deep carbon recycling plays an important role in the atmospheric CO2 budget and climate changes through geological time, the precise mechanisms remain poorly understood. Since recycled sedimentary carbonate through plate subduction is the main light-δ26Mg reservoir within deep-Earth, Mg isotope variation in mantle-derived melts provides a novel perspective when investigating deep carbon cycling. Here, we show that the Late Cretaceous and Cenozoic continental basalts from 13 regions covering the whole of eastern China have low δ26Mg isotopic compositions, while the Early Cretaceous basalts from the same area and the island arc basalts from circum-Pacific subduction zones have mantle-like or heavy Mg isotopic characteristics. Thus, a large-scale mantle low δ26Mg anomaly in eastern China has been delineated, suggesting the contribution of sedimentary carbonates recycled into the upper mantle, but limited into the lower mantle. This large-scale spatial and temporal variation of Mg isotopes in the mantle places severe constraints on deep carbon recycling via oceanic subduction.

Nonextensivity at the Circum-Pacific subduction zones—Preliminary studies

Following the fragment–asperity interaction model introduced by Sotolongo-Costa and Posadas (2004) and revised by Silva et al. (2006), we try to explain the nonextensive effect in the context of the asperity model designed by Lay and Kanamori (1981). To address this issue, we used data from the NEIC catalog in the decade between 2001 and 2010, in order to investigate the so-called Circum-Pacific subduction zones. We propose a geophysical explanation to nonextensive parameter q. The results need further investigation however evidence of correlation between the nonextensive parameter and the asperity model is shown, i.e., we show that q-value is higher for areas with larger asperities and stronger coupling.

Tsunami Risk Management in Pacific Island Countries and Territories (PICTs): Some Issues, Challenges and Ways Forward

The Pacific is well known for producing tsunamis, and events such as the 2011 Tōhoku-oki, Japan disaster demonstrate the vulnerability of coastal communities. We review what is known about the current state of tsunami risk management for Pacific Island countries and territories (PICTs), identify the issues and challenges associated with affecting meaningful tsunami disaster risk reduction (DRR) efforts and outline strategies and possible ways forward. Small island states are scattered across the vast Pacific region and these states have to varying degrees been affected by not only large tsunamis originating in circum-Pacific subduction zones, but also more regionally devastating events. Having outlined and described what is meant by the risk management process, the various problems associated with our current understanding of this process are examined. The poorly understood hazard related to local, regional and distant sources is investigated and the dominant focus on seismic events at the expense of other tsunami source types is noted. We reflect on the challenges of undertaking numerical modelling from generation to inundation and specifically detail the problems as they relate to PICTs. This is followed by an exploration of the challenges associated with mapping exposure and estimating vulnerability in low-lying coastal areas. The latter part of the paper is devoted to exploring what mitigation of the tsunami risk can look like and draw upon good practice cases as exemplars of the actions that can be taken from the local to regional level. Importantly, given the diversity of PICTs, no one approach will suit all places. The paper closes by making a series of recommendations to assist PICTs and the wider tsunami research community in thinking through improvements to their tsunami risk management processes and the research that can underpin these efforts.

Non-volcanic tremor associated with the March 2010 Gisborne slow slip event at the Hikurangi subduction margin, New Zealand

[1] Non-volcanic tremor accompanies slow slip events at most circum-Pacific subduction zones. Previously, the Hikurangi subduction margin, New Zealand, and the Boso Peninsula of Japan have been noted as exceptions where slow slip, which occurs at shallow depth compared to other subduction zones, does not generate seismic tremor. The lack of tremor observations raises questions whether tremor and slow slip are manifestations of the same process and whether different temperature and pressure conditions influence the physical mechanism of tremor production. Analysis of seismic data during the March 2010 Gisborne slow slip event, New Zealand, reveals that this slow slip event at the northern Hikurangi margin is accompanied by tremor with distinctive characteristics compared to local microearthquakes. Both the Gisborne area of New Zealand and the Boso Peninsula of Japan are underlain by thick accumulations of sedimentary rocks that likely inhibit the observation of low amplitude tremor due to high attenuation. Discovery of tremor in New Zealand suggests that tremor may always accompany slow slip and that its apparent absence at the Boso Peninsula, the Hikurangi margin prior to this study, and any future regions discovered to experience slow slip without concurrent tremor, may be the effect of highly-attenuating sediments.

Earthquake swarms in circum-Pacific subduction zones

We systematically and manually search through clusters of earthquakes along circum-Pacific subduction zones to identify potential earthquake swarms. In total, we find 266 potential earthquake swarms: 180 we classify as megathrust and 68 we classify as volcanic due to their proximity to the megathrust or to volcanoes. We focus on the megathrust swarms and demonstrate that: (1) the number of events in a swarm is not a function of the largest earthquake in the swarm, (2) swarms exhibit an approximately constant rate of seismicity that lasts until after the mean timing of events in the swarm, (3) the timing of the largest earthquake in the sequence is no different than the timing of any other earthquake in the sequence, (4) our catalogs of earthquakes comprising swarms (~ 9000 events) have high b-values (1.5 to 2), and (5) when earthquake swarms are considered as single events using total duration and cumulative moment, they appear to be consistent with the slow earthquake magnitude-duration scaling law presented by Ide et al. (2007). The first three observations, along with the observation that swarms can span very large areas compared to their cumulative seismic moment, argue against static stress triggering as a driving mechanism for earthquake swarms. Along strike propagation velocities are observed for several swarms, showing epicentral propagation of ~ 10 km/day, similar to other documented slow slip events. Together, this evidence implies that aseismic slip along the megathrust is likely an important mechanism for the generation of megathrust earthquake swarms in circum-Pacific subduction zones. We then conduct a comparison of swarms and large megathrust earthquakes, finding evidence that the two are broadly anti-correlated: megathrust segments with large earthquake swarm gaps are more likely to experience large (Mw > 8) megathrust events. We characterize the ubiquity of megathrust swarms at different margins, and suggest that fault properties along Marianas-type margins may allow for earthquake swarms to occur regularly, but other margins may rely on other variables, such as the subduction of a ridge or seamount, to facilitate the generation of megathrust earthquake swarms.

Small scale heterogeneity in the mid-lower mantle beneath the circum-Pacific area

Data from a western United States short-period seismic network are investigated in order to detect anomalous later P-wave arrivals within a time window from 50s to 170s after direct P waves for deep and intermediate-depth earthquakes at circum-Pacific subduction zones. We interpret the anomalous arrivals as S-to-P scattered waves from heterogeneities in the mid-lower mantle. The searched regions (depths from ∱/4800km to 2200km) cover approximately 2% of the entire lower mantle. The heterogeneous scattering objects are located by array processing of the seismic waves using semblance, with careful investigations of arrival time, slowness and azimuth of the later phases relative to direct P waves. We usually observe two or three later phases which are separated in arrival time by 5•20s and have similar arrival azimuth and slowness. This observation suggests that a few scatterers often form a cluster with a typical spacing of 100•200km vertically. More than ten clusters or groups of S-to-P scattering objects are detected in the mid-lower mantle beneath the circum-Pacific regions; northern Kuril, Izu-Bonin, Mariana, northern Fiji, and west coast of South America. The S-to-P scattering objects are found in a depth range from 1100 to 1800km, and the most prominent objects of each region tend to be located around the middle of the depth range. The amplitudes of the scattered waves range from 1% to 10% of the direct P waves, with exceptionally large values as large as 40%. The associated magnitudes of the changes in elasticity is not tightly constrained but probably are nearly 1% and may occasionally exceed 4% of shear velocity. Considering their high efficiency at converting short-period waves (wavelengths of 5•10km), the changes in elastic properties at the heterogeneous objects appear to occur within several kilometers. Seismic tomography studies have not detected prominent seismic wave speed anomalies except for those beneath South America which appear to be located in the former Farallon plate and those at north of Fiji which appear to be located around the Pacific superplume. The mid-lower mantle heterogeneity should represent sharp chemical variations in major element composition of the lower mantle rocks. The most plausible tectonic explanation of the variations is remnant of subducted and folded former oceanic crust which preceded the current subduction of the Pacific plate. The extreme estimates of low shear velocity (>4%) may indicate the prominent drops in the shear modulus due to ferro-elastic phase transition in stishovite (SiO2). Our observations imply that heterogeneity of kilometer-scale persists for billions of years despite of stirring due to convection and therefore is ubiquitous in the lower mantle. It is qualitatively consistent with some recent convection models.

Seismic scatterers at the shallowest lower mantle beneath subducted slabs

Data from a western United States short-period seismic network are analyzed in order to investigate anomalous later phases within a time window from about 10 s to 120 s after direct P waves for deep and intermediate-depth earthquakes in circum-Pacific subduction zones. The anomalous phases are best interpreted as S-to-P scattered waves (wavelengths of ~ 10 km) from heterogeneities in the shallow lower mantle (depths ≤ 950 km). Several S-to-P scatterers where elastic properties of the rocks must substantially change within several kilometers are detected in the shallowest 300 km of the lower mantle beneath four circum-Pacific regions: Kuril, Bonin, Fiji, and Peru. Around most of the observed scatterers the seismic tomography models have delineated high seismic velocity anomalies which are associated with recently subducted Pacific slab or Nazca slab. The most likely origin of the scatterers would be basalt which used to form the oceanic crust. Interestingly enough the majority of the scatterers are located near the bottom boundaries of the slabs. Given rather moderate degrees of folding and/or buckling of the Pacific and Nazca slabs in the shallowest lower mantle at the study areas, the existence of the oceanic crust associated with the most recent slab subduction at the scattering sites is unlikely. Alternative and more likely explanations include: the presence of ancient basaltic rocks, localized dehydration of hydrous minerals, or a sharp boundary between different rock fabrics such as isotropic and anisotropic lower mantle rocks associated with mantle flow dragged by the slab subduction.

Systematic biases in subduction zone hypocenters

[1] Earthquake hypocenters from global datasets commonly have regional biases in their locations due to the use of a one-dimensional velocity model in their location. To analyze this effect, we group intermediate-depth earthquakes into forty-four 500-km-long sections of subduction zone. We relocate earthquakes in each group relative to one another and in a three-dimensional global velocity model. Hypocenters shift up to 25 km regionally when these effects are included; most earthquakes in circum-Pacific subduction zones are pulled toward the Pacific. Location uncertainties from the relative relocations are reduced to one-fifth of those from single-event locations; Wadati-Benioff zone thicknesses are reduced to two-thirds of their original thickness. These biases alter estimates of depth to slab beneath arc volcanoes up to 50 km, and alter measurements such as the volume of mantle wedge available for water storage or melt generation, and the size of the thrust zone on which large earthquakes can occur.

Non-volcanic tremor beneath the Central Range in Taiwan triggered by the 2001Mw7.8 Kunlun earthquake

SUMMARY Non-volcanic tremor (NVT) is an extended-duration and non-impulsive seismic signal observed away from volcanic regions. Although NVT has been found along the circum-Pacific subduction zones and the transform plate boundary in California, it is not clear whether NVT occurs in other tectonic environments. NVT is often associated with slow-slip events, and can be triggered instantaneously during the surface waves of teleseismic events. However, the underlying mechanisms of tremor generation remain mysterious. Here we show clear evidence of NVT beneath the Central Range (CR) in Taiwan triggered by the 2001 M w 7.8 Kunlun earthquake in northern Tibet. Tremor occurs when the Love wave displacement is to the southwest (the lateral motion direction for the CR), suggesting a simple frictional response to the driving force. We hypothesize that tremor occurred on the weak basal detachment fault beneath the CR. Our observations indicate that tremor may exist at a wide range of active tectonic environments.

The Bend: Origin and significance

Research Article| March 01, 2007 The Bend: Origin and significance Rex H. Pilger Rex H. Pilger 1Landmark Graphics, 1805 Shea Center Drive, Suite 400, Highlands Ranch, Colorado 80129, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2007) 119 (3-4): 302–313. https://doi.org/10.1130/B25713.1 Article history received: 29 Aug 2004 rev-recd: 21 Aug 2006 accepted: 26 Aug 2006 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Rex H. Pilger; The Bend: Origin and significance. GSA Bulletin 2007;; 119 (3-4): 302–313. doi: https://doi.org/10.1130/B25713.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The origin of the bend in the Hawaiian-Emperor Island seamount chain is the prototypical problem at the heart of the ongoing hotspot debate: Does the bend represent a change in motion between the Pacific plate and underlying hotspot (plume?), or might it represent a change in the intraplate stress field?Reconstructions of continental plates relative to the Pacific plate produce loci with changes in motion that correspond in age and geometry with the bend, especially the North and South American plates. This implies that the two continental plates moved very slowly relative to the Pacific hotspots, especially between 75 and 30 Ma, and that the Hawaiian-Emperor bend is the result of change of Pacific plate motion relative to the underlying hotspot reference frame (or vice versa). Additional reconstructions imply that the Pacific hotspot frame is distinct from the Atlantic and Indian Ocean hotspots over the same time period.Passage of some hotspot traces from one plate to another suggests that the hotspots are at least partially sublithospheric phenomena and, therefore, cannot simply represent fractures propagated by intraplate stress fields. Contemporary and paleostress fields correspond to hotspot kinematic models for at least three continental plates. If the stress fields represent drag of the continental plates against the underlying asthenosphere and mesosphere, this suggests that the sub-lithospheric hotspots are embedded within the mesosphere (and do not move independent of it); both hotspot traces and stresses record the same motion.The correspondence of North and South American and Hawaiian hotspot motions, combined with the evidence for embedding of the hotspots in the mesosphere, leads to the inference that motion of the two continental plates (along with the Eurasian) constrains the motion of the sub-Pacific mesosphere. Thus, the Pacific and Atlantic-Indian hotspot reference frames are plausibly independent and separated by the deep circum-Pacific subduction zones. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Using Earthquake Source Durations along the Sumatra–Andaman Subduction System to Examine Fault-Zone Variations

Abstract The 26 December 2004 great Sumatran–Andaman earthquake in the Indian Ocean caused extensive damage and loss of life from intense shaking and the resulting tsunami. Several studies of this earthquake suggest that portions of the fault ruptured at variable speeds, with both fast and slow rupture velocities observed along the 1200 km long rupture length. Variations in rupture velocity during the earthquake may indicate along-strike variations in megathrust frictional conditions that may influence other earthquakes along the zone. Previous work on global subduction zone systems suggests depth-dependent frictional conditions arising from heterogeneous conditions along the fault. In many of the circum-Pacific subduction zones, shallow earthquakes along the subduction megathrust have longer scaled source durations than deeper earthquakes, possibly resulting from variations in frictional conditions with depth. This study focuses on thrust mechanism earthquakes on the Sumatra– Andaman megathrust, examining aftershocks with Mw >6.0 of the 2004 earthquake, as well as earthquakes that occurred in the region between 1992 and 2004. Source duration, depth, and slip distribution are determined for this set of earthquakes to explore the possibility of both along-strike and depth-dependent variations in source parameters and frictional conditions. There is evidence of depth-dependent source parameters for these events, with longer scaled durations for the shallower earthquakes, consistent with previous global studies. No temporal change is apparent in this relationship, as source parameters for previous large earthquakes are similar to those after the earthquake. Along-strike patterns suggest long-duration character for several earthquakes in the southern portion of the rupture zone but no strong evidence of slow character in the northern portion of the rupture zone. It appears unlikely that any long-term variations in the fault-zone character influenced possible slow rupture of the 2004 earthquake.

Pole-Tide Modulation of Slow Slip Events at Circum-Pacific Subduction Zones

Episodic slow slip (ESS) events have been detected at several circum- Pacific subduction zones, such as Cascadia, Japan, and Mexico. Notably, at least eight ESS events along the northern Cascadia subduction zone recurred with a period of 13-16 months. We study the relationship between pole-tide (associated with Chan- dler wobble with a period of 14 months)-induced stress and the occurrence of the ESS events. Our quantitative analysis shows that 14 of the 20 documented ESS events occurred during the ascension phase, prior to the maximum, of a pole-tide-induced Coulomb failure stress change, and three events occurred at the stress-change peak. The pole tides modulate the stress field at the downdip edge of the transition zone along the plate interface and may trigger ESS events when conditions are favorable. The phase advance of the triggered events with respect to the induced Coulomb failure stress change may reflect that the fault slip is dictated by a rate- and state- dependent friction law inferred from laboratory experiments.

Seismic low-velocity layer at the top of subducting slabs: observations, predictions, and systematics

Layered structure persists at the tops of subducting plates to depths in excess of 150 km. A low-velocity channel several km thick produces guided seismic guided waves, which indicate that the top of the downgoing plate remains coherent and probably has not eclogitized to >150 km depth. The surrounding medium has seismic velocities 3–6% faster than standard earth models, as inferred from direct travel times, presumably because the waveguide lies atop cold descending lithosphere. However, guided waves travel more slowly. Several tests with full waveform synthetics show that the guided waves can be observed only if excited by sources that lie near or within the low-velocity channel. Thus, the frequent observation of dispersed body waves indicates that intermediate-depth earthquakes tend to lie in waveguides. Also, receivers must be favorably placed. Observations for seven circum-Pacific subduction zones show such signals, which can be explained as low-velocity channels 2–8 km thick. Velocity anomalies within low-velocity channels are largest at shallowest depths, up to 14% slower than surroundings at depths less than 150 km, diminishing to <3% at greater depths. This variation in depth indicates that low-velocity channels metamorphose at depths that can exceed 150 km to a rock closely resembling eclogite or peridotite. Metamorphism probably involves dehydration of a layer initially abundant in hydrous minerals, although equilibration of variable fractions of metastable gabbro may explain some observations. The size of the low-velocity anomaly correlates with slab dip, with steeply dipping slabs exhibiting the greatest anomalies, but does not correlate with thermal parameter or convergence velocity. This correlation may reflect a greater ability of steep slabs to funnel fluid up the slab rather than into the overlying wedge, so that slow seismic velocities integrate effects of subduction of hydrous materials and their subsequent up-dip fluid transport. At depths exceeding 150 km, the waveguide velocities place an upper bound on the amount of water that could be delivered to deeper mantle, to 2 wt.% or less within this 4 km thick layer.