Low-velocity Structure Research Articles

Upper crustal velocity and seismogenic environment of the <em>M</em>7.0 Jiuzhaigou earthquake region in Sichuan, China

On August 8, 2017, a magnitude 7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province, China. The deep seismogenic environment and potential seismic risk in the eastern margin of Tibetan Plateau have once again attracted the close attention of seismologists and scholars at home and abroad. The post-earthquake scientific investigation could not identify noticeable surface rupture zones in the affected area; the complex tectonic background and the reason(s) for the frequent seismicity in the Jiuzhaigou earthquake region are unclear. In order to reveal the characteristics of the deep medium and the seismogenic environment of the <i>M</i>7.0 Jiuzhaigou earthquake region, and to interpret the tectonic background and genesis of the seismicity comprehensively, in this paper, we have reviewed all available observation data recorded by the regional digital seismic networks and large-scale, dense mobile seismic array (China Array) for the northern section of the North–South Seismic Belt around Jiuzhaigou earthquake region. Using double-difference seismic tomography method to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Jiuzhaigou earthquake region, we have analyzed and discussed such scientific questions as the relationship between the velocity structure characteristics and seismicity in the Jiuzhaigou earthquake region, its deep tectonic environment, and the ongoing seismic risk in this region. We report that: the P-wave velocity structure of the upper crust around the Jiuzhaigoug earthquake region exhibits obvious lateral inhomogeneity; the distribution characteristics of the shallow P-wave velocity structure are closely related to surface geological structure and formation lithology; the <i>M</i>7.0 Jiuzhaigou earthquake sequence is closely related to the velocity structure of the upper crust; the mainshock of the <i>M</i>7.0 earthquake occurred in the upper crust; the inhomogeneous variation of the velocity structure of the Jiuzhaigou earthquake area and its surrounding medium appears to be the deep structural factor controlling the spatial distribution of the mainshock and its sequence. The 3D P-wave velocity structure also suggests that the crustal low-velocity layer of northeastern SGB (Songpan–Garzê Block) stretches into MSM (Minshan Mountain), and migrates to the northeast, but the tendency to emerge as a shallow layer is impeded by the high-velocity zone of Nanping Nappe tectonics and the Bikou Block. Our results reveal an uneven distribution of high- and low-velocity structures around the Tazang segment of the East Kunlun fault zone. Given that the rupture caused by the Jiuzhaigou earthquake has enhanced the stress fields at both ends of the seismogenic fault, it is very important to stay vigilant to possible seismic hazards in the large seismic gap at the Maqu–Maqên segment of the East Kunlun fault zone.

Morphology and possible origins of the Perm anomaly in the lowermost mantle of Earth

We have constrained a small-scale, dome-shaped low-velocity structure near the core-mantle boundary (CMB) of Earth beneath Perm (the Perm anomaly) using travel-time analysis and three-dimensional (3-D) forward waveform modeling of seismic data sampling of the mantle. The best-fitting dome-shaped model centers at 60.0°E, 50.5°N, and has a height of 400 km and a radius that increases from 200 km at the top to 450 km at the CMB. Its velocity reduction varies from 0% at the top to –3.0% at 240km above the CMB to –3.5% at the CMB. A surrounding 240-km-thick high-velocity D'' structure has also been detected. The Perm anomaly may represent a stable small-scale chemical pile in the lowermost mantle, although the hypothesis of a developing mantle plume cannot be ruled out.

Source and strong-motion characteristics of two M\u2009>\u20096 buried earthquakes in southwest Taiwan

We used near-field strong-motion data to investigate the complex combination of source effect and site response for two recent disastrous earthquakes in southwest Taiwan. We estimated strong-motion generation areas (SMGAs) of 2.8 km × 2.8 km and 6.0 km × 4.2 km in a frequency band of 0.4–10 Hz for the 2010 Jiashian and 2016 Meinong earthquakes, respectively. The high-stress drops of 26.2 and 17.0 MPa for these two buried events were potentially related to the small dimension and deep rupture. Our results revealed that both earthquakes exhibited westward rupture directivity, whereas the 2016 Meinong event exhibited a stronger directivity effect because of the consistency between the propagation and slip directions. The localized high peak ground velocity (PGV) patch and the nonlinear site response could be attributed to the soft sediment with high pore fluid pressure and low-velocity structure beneath this region. However, the greater seismic moment and closer faulting location to the thick-mudstone-layer region for the 2016 Meinong event reinforced the strong ground shaking and serious damage over the broad area. This implies that this thick-mudstone-layer region in southern Taiwan plays a crucial role in earthquake response, and an investigation of characteristic site effects should be conducted for seismic hazard mitigation.Graphic abstract

Response of Yunnan crustal structure to eastward growth of the Tibet Plateau and subduction of the India plate in Cenozoic

Most previous studies advocate that crustal weakening in the Yunnan area is predominantly the result of eastward growth of the Tibetan Plateau. Here we observe regional variations in the Yunnan crustal structure from the waveform characteristic variations of receiver functions at 342 seismic stations. Furthermore, receiver function imaging along three seismic sections reveals the response of crustal structure to different tectonic evolution. Our results show that northwestern Yunnan is characterized by 49–64 km-thick crust with multi-layer low-velocity materials and local lower crustal “doublet”. These low-velocity structures may reflect partial melting caused by crustal thickening associated with the eastward growth of the plateau. However, in southern and eastern parts of Yunnan, the topography of the Moho varies relatively gently with crustal thicknesses of 26–42 km, and widespread low-velocity zones are located above the crust-mantle boundary. We speculate that these features might favor the action of partial melting of crust triggered by asthenospheric upwelling related to the subduction of the India plate.

3-D travel-time tomography of southernmost Baja California Peninsula

A three-dimensional P and S-wave travel-time tomography, as well as Vp/Vs ratio images, were obtained for the southernmost tip of the Baja California Peninsula in Mexico. The area analyzed include Los Cabos block and part of the Gulf coast. The tomography obtained reveals in detail the heterogeneities of the crustal seismic velocity structure of the Los Cabos block. Results show an overall low velocity structure for both P- and S-waves under the Baja California peninsula down to a depth of ~25 km around the peninsular axis, particularly under Sierra La Laguna and near the Gulf of California (GoC) coast. Low P- and S-wave velocities are also present under the Gulf near the coast whereas fast velocities are observed under the Gulf away from the shoreline. Low and high Vp/Vs ratios are observed to alternate throughout the study area. The low seismic velocities may be explained because the crust in Los Cabos block is highly fractured and fluid-saturated, and has a high heat flux which is manifested as geothermal activity (e.g. hot springs observed all along the block). These low body wave velocities coincide with the existence of an asthenospheric mantle sandwiched by the continental crust of the Los Cabos block and a stalled subducted slab reported by Di Luccio et al. (2014). This mantle strip may be chemically depleted or dry inhibiting the development of active post-subduction volcanism.

Upper crustal velocity structure and geological significance of southwest Shandong Province, China: insights from double-difference seismic tomography

As an important part of the North China Craton extension system, the southwest of Shandong Province comprises several uplift and fault depression structures. However, the tectonic activity there is controversial, and more studies are required for the upper crustal velocity structure. In the present study, the three-dimensional upper crustal P wave velocity structure in southwestern Shandong Province was obtained using double-difference seismic tomography. The findings show that southwestern Shandong Province has many vertical high-velocity structures, indicating that mantle-magma upwelling was strong there and formed the uplift structures. It is reasoned that Taishan Mountain is still undergoing uplift caused by magma upwelling that is concentrated mainly below a depth of 20 km. The shallow–middle low-velocity structures in southwestern Shandong Province correspond strongly to the uplift depression structures. The tectonic activity in southwestern Shandong Province can be divided mainly into (i) brittle-ductile deformation of uplift depression structures at depths of 0–10 km and (ii) mantle-magma upwelling under the uplift structures. The formation of uplift structures promoted the formation of the fault depression structures, and the fault depression and uplift structures are closely related in genesis.

Formation of the North–South Seismic Zone and Emeishan Large Igneous Province in Central China: Insights from P-Wave Teleseismic Tomography

ABSTRACT The geodynamic features of the north–south seismic zone (NSSZ) and the formation of the Emeishan large igneous province (ELIP) in China remain controversial. In this study, we conducted detailed P-wave teleseismic tomography studies in the NSSZ and adjacent regions. The results revealed large high-velocity anomalies beneath the Songpan–Ganzi Block and the South China Block, possibly representing large-scale lithospheric delamination. We further identified low-velocity structures at 50–200 km depths in the western and southern parts of the NSSZ, suggesting an upwelling asthenosphere induced by delamination and the absence of a rigid lithosphere. Two high-velocity structures beneath the Sichuan basin and the Alashan block were also revealed, which may represent the lithospheric roots of these structures. These rigid lithospheric roots may have obstructed the eastward extrusion of the Tibetan plateau and led to stress accumulation and release (triggering earthquakes) in the Longmenshan Orogenic Belt and the northern part of the NSSZ. Because of this obstruction, the eastward extrusion was redirected southeastward to Yunnan in the southern part of the NSSZ, which led to stress accumulation and release causing earthquakes along the Honghe and Xiaojiang faults. The results from this study reveal a high-velocity structure with a subducted slab-like appearance that may represent vestiges of the Paleo-Tethys oceanic lithosphere, which subducted beneath the ELIP and initiating large-scale mantle return flow or mantle upwelling, contributing to the formation of the ELIP.

Investigation on the Dynamic Rupture of the 1970 Ms 7.7 Tonghai, Yunnan, China, Earthquake on the Qujiang Fault

ABSTRACTRegional stress states and fault geometries play important roles in earthquake rupture dynamics. Using the curved grid finite-difference method, we conducted 3D spontaneous rupture simulations of the nonplanar Qujiang fault (QF) to investigate the rupture processes of the 1970 Tonghai earthquake and potential future earthquakes. A nonplanar fault model including topography was adopted and embedded in heterogeneous media. Regional stress orientations with an interval of 5° were tested, and various fault geometry models with different fault surface traces and fault dips were discussed. We also provided explanations for the unbroken northwestern segment of the QF and the seismic intensity anomaly in the Tonghai basin during the 1970 Tonghai event. Finally, we presented several future potential earthquake scenarios occurring on the QF at three nucleation locations. Our simulation results suggested that the maximum principal stress azimuth around the Tonghai area is N25°W and that the QF is most likely a complex dipping fault—the southeastern segment dips to the northeast, whereas the northwestern segment dips to the southwest. Our simulations also revealed that multiple explanations, including a regional stress rotation and an increase in the cohesion force, could account for the unbroken northwestern segment of the QF. Furthermore, the seismic intensity anomaly in the Tonghai basin can be explained by a low-velocity structure. Future earthquake scenarios demonstrated that potential earthquakes nucleating at Eshan and Wujie in a complex dipping fault model could rupture the entire QF, thereby posing severe seismic risks to nearby regions. In contrast, when the nucleation point was located at Quxi, the rupture was constrained to the initial fault segment of the QF; however, caution should still be exercised in the Quxi area because this scenario produces a maximum intensity of VIII.

Eikonal Equation-Based Seismic Tomography of the Source Areas of the 2008 Mw 7.9 Wenchuan Earthquake and the 2013 Mw 6.6 Lushan Earthquake

ABSTRACT We use the eikonal equation-based seismic travel-time tomography method to image the source areas of the 2008 Wenchuan earthquake and the 2013 Lushan earthquake in the Longmenshan fault zone. High-resolution VP and VS models are obtained by inverting 75,686 P-wave and 74,552 S-wave travel times of local earthquakes during the period from 2009 to 2018. The tomographic models reveal strong crustal velocity heterogeneities in the study area. A significant velocity contrast exists across the Longmenshan fault zone: The western Songpan–Ganzi block is a high-velocity body, whereas the eastern Sichuan basin is a low-velocity anomaly. The hypocenter of the 2008 Wenchuan earthquake is between a high-velocity and a low-velocity anomaly. Beneath the Wenchuan mainshock, there is a significant low-velocity structure in the lower crust. The 2013 Lushan earthquake occurred in rocks associated with a high-velocity anomaly. A distinct low-velocity zone with low seismicity is imaged between the 2008 Wenchuan earthquake and the 2013 Lushan earthquake, where the crustal ductile deformation is likely to occur. The Baoxing complex to the northwest of the Lushan hypocenter exhibits as a high-velocity anomaly, which may be a carrier of stress accumulation and more prone to seismic activities in the future.

Transfer functions between teleseismic data components

SUMMARYDifferent data components of teleseismic waveforms are related by transfer functions that depend only on receiver-side structure. This is the common basis of a number of teleseismic techniques, including receiver functions and shear wave splitting analysis. Common trace misfits used in these analysis techniques are shown to be equivalent to band-limited comparisons of real and synthetic transfer functions. The data deconvolution used in receiver function analysis leads to reduced structural resolution compared to direct trace-based misfits such as cross-convolution, with direct transfer function modelling of a data trace having the particular advantage of a physically meaningful misfit. Having established that the intertrace transfer function contains all available structural information, the sensitivity of transfer functions to structure is examined for a series of teleseismic scenarios. Transfer functions for the teleseismic P coda show a strong sensitivity to shallow low-velocity structures such as sedimentary basins; the Sp precursors used in S receiver functions are less affected. Examination of transfer functions for shear wave splitting shows that response complexities occur at frequencies too high to be observable in teleseismic studies, and that the dominant control on the response is the splitting intensity.

Comparing performance of multi-frequency bands Occam’s receiver function inversion to standard linearized receiver function inversion

Receiver function (RF) is a characterized waveform that sensitive to the sub-surface velocity structure near the seismic station. In the geophysical problem, an inversion scheme has been used for determining the structure model that corresponded to the observed data. Because RF inversion is a non-linear problem, the standard method such as linearized inversion suffered in the non-uniqueness of the results. Thus, using a stochastic optimization algorithm (e.g. Monte Carlo or Generic Algorithm) is the most interesting trend for solving the problem. However, their computation cost is high and the efficiency is strongly depended on user setting. In this work, Occam’s inversion algorithm, which is popular in the electromagnetic method, and multi-frequency bands receiver function have been integrated to improve the efficiency of receiver function inversion. The propagation method has been applied for the calculation of seismogram in this work. The synthetic cases, which are thin low velocity zone model (TLVZ), broad low velocity zone model (BLVZ), thin high velocity zone model (THVZ) and broad high velocity zone model (BHVZ), were used to compare the performance of this newly implemented algorithm and the standard linearized inversion. Single frequency band (SFO) and multi-frequency bands Occam’s inversion (MFO) have been tested for each case. For, TLVZ, THVZ and BHVZ, the MFO inversion provided better fitting results than SFO in most cases. Comparing to linearized inversion of CPS330, the MFO results still provide better performance. For the smooth low velocity structure in BLVZ model, both linearized and MFO can recover a significant structure of the true model. More concisely, the linearized inversion can recover the absolute velocity of the true model better than MFO. By the way, the MFO provides the best data fitting for the BLVZ. In summary, the MFO receiver function inversion that implemented in this study provides a new improved tool for a seismologist to investigating the subsurface structure.

Shear wave velocity structure beneath Bandung basin, West Java, Indonesia from ambient noise tomography

Summary We investigated the seismic shear wave velocity structure of the upper crust beneath the Bandung area in West Java, Indonesia, using ambient seismic noise tomography. We installed 60 seismographs to record ambient seismic noise continuously in the city of Bandung and its surrounding area for 8 months. After inter-station cross-correlation of recordings of ambient seismic noise, we obtained empirical Green's functions for Rayleigh waves. Group velocity dispersion curves for Rayleigh waves between periods of 1 s and 8 s were measured on each inter-station path by applying the multiple filter analysis method with phase-matched processing. The spatial variation of group velocities shows a good correlation with the geological structure of the Bandung Basin. The Rayleigh wave dispersion maps were inverted to obtain the 1D shear wave velocity profiles beneath each station, which were interpolated to infer a pseudo-3D structure under the study region. The results show that the Bandung Basin has a thick layer of sediment. Along the northern, eastern and southern mountains surrounding the Bandung Basin there is high-velocity structure, except to the west of the Tangkuban Parahu volcano, where a massive low-velocity structure extending throughout the upper crust might indicate the presence of fluids or partial melts.

Evidence for an upwelling mantle plume beneath the Songliao Basin, Northeast China

A mushroom-like low-velocity body (MLLVB) was defined by tomography beneath Northeast China, which may be linked to a vestige of the Mesozoic upwelling mantle plume. However, multiple lines of evidence indicate that the necessary condition of the upwelling is that the upwelling mantle plume has a temperature sufficiently higher than that of the surrounding mantle. The upwelling mantle plume cannot be defined only based on the low-velocity structure. To investigate this issue, I conducted common conversion point (CCP) stacking of receiver functions in Northeast China and designed six CCP stacking profiles crossing Northeast China. The results show that when the CCP stacking profiles cross the location of the MLLVB, the topography of the 410 and 660 km discontinuities did not exhibit obvious variations; instead, two 520 km discontinuities merged into a single discontinuity, which indicates that the MLLVB had an upwelling temperature condition. The CCP stacking image of the receiver functions also reveals a deepening of the 410 km discontinuity and shallowing of the 660 km discontinuity at the center of the Songliao Basin along the 45°N profiles, which might be associated with the Mesozoic upwelling mantle plume.

3D shear wave velocity model of the crust and uppermost mantle beneath the Tyrrhenian basin and margins

The Tyrrhenian basin serves as a natural laboratory for back-arc basin studies in the Mediterranean region. Yet, little is known about the crust-uppermost mantle structure beneath the basin and its margins. Here, we present a new 3D shear-wave velocity model and Moho topography map for the Tyrrhenian basin and its margins using ambient noise cross-correlations. We apply a self-parameterized Bayesian inversion of Rayleigh group and phase velocity dispersions to estimate the lateral variation of shear velocity and its uncertainty as a function of depth (down to 100 km). Results reveal the presence of a broad low velocity zone between 40 and 80 km depth affecting much of the Tyrrhenian basin’s uppermost mantle structure and its extension mimics the paleogeographic reconstruction of the Calabrian arc in time. We interpret the low-velocity structure as the possible source of Mid-Ocean Ridge Basalts- and Ocean Island Basalts- type magmatic rocks found in the southern Tyrrhenian basin. At crustal depths, our results support an exhumed mantle basement rather than an oceanic basement below the Vavilov basin. The 3D crust-uppermost mantle structure supports a present-day geodynamics with a predominant Africa-Eurasia convergence.

Near‐Surface Environmentally Forced Changes in the Ross Ice Shelf Observed With Ambient Seismic Noise

AbstractContinuous seismic observations across the Ross Ice Shelf reveal ubiquitous ambient resonances at frequencies &gt;5 Hz. These firn‐trapped surface wave signals arise through wind and snow bedform interactions coupled with very low velocity structures. Progressive and long‐term spectral changes are associated with surface snow redistribution by wind and with a January 2016 regional melt event. Modeling demonstrates high spectral sensitivity to near‐surface (top several meters) elastic parameters. We propose that spectral peak changes arise from surface snow redistribution in wind events and to velocity drops reflecting snow lattice weakening near 0°C for the melt event. Percolation‐related refrozen layers and layer thinning may also contribute to long‐term spectral changes after the melt event. Single‐station observations are inverted for elastic structure for multiple stations across the ice shelf. High‐frequency ambient noise seismology presents opportunities for continuous assessment of near‐surface ice shelf or other firn environments.

Shallow/upper crustal shear wave structure of the Tehran region (Central Alborz, Iran) from the inversion of Rayleigh wave dispersion measurements

Surface wave dispersion curves from microearthquakes are used to obtain group velocity dispersion maps. The calculation of the local dispersion curves for each grid point from these maps then produces the input data to retrieve the 3D shear wave velocity model of the Tehran region. The group velocity maps indicate that the tomographic results agree well with the three main tectonic features and the geological units in the study area. The tomographic maps generally possess high-velocity structures across most of the mountain belts (Central Alborz and east-southeast mountains), whereas the Tehran Basin correlates to a low-velocity structure. Increasing the period in the study area highlights four independent low-velocity zones that reflect faults and fault junction systems. The shear wave velocity profiles indicate that the depth to bedrock exhibits southward variation ranging from ~ 300 m to ~ 1500 m. We also focus our analysis on the existence of faults within the shear wave profiles and discuss the low shear wave velocity anomalies deeper than 2 km result from the main fault structures (e.g., North Tehran, North-South Rey and Parchin). Furthermore, we argue that the dip angle of the North Tehran fault varies along fault strike, whereas the North-South Rey fault possesses a constant dip angle. Moreover, initial model uncertainties and checkerboard resolution tests are used to identify reliable and robust anomaly features in the 3D shear wave velocity model and 2D tomographic maps, respectively. Microearthquake analysis provides an effective approach for studying the upper crustal structure heterogeneity, especially the fault structure, of the Tehran region.

Deep Structure of the Zone of Tolbachik Fissure Eruptions (Kamchatka, Klyuchevskoy Volcano Group): Evidence from a Complex of Geological and Geophysical Data

With the use of the method of low-frequency microseismic sounding, the configuration of the magmatic feeding system of the Tolbachinsky Dol—a regional zone of areal basaltic volcanism in the southern part of the Klyuchevskoy volcano group in Kamchatka—is studied. The initial data are obtained by a stepby-step recording of the background microseismic noise in 2010–2015 within a thoroughly marked-out survey area covering the zones of fissure eruptions in 1975–1976 and 2012–2013 and, partly, the edifice of the Ploskii (flat) Tolbachik volcano. The depth sections reflecting the distributions of the relative velocities of seismic waves in the Earth’s crust are constructed. For a more reliable interpretation of the revealed deep anomalies, the results of independent geological and geophysical studies are used. The ascertained low-velocity structures are closely correlated to the manifestations of present-day volcanism. It is shown that the feeding structure of the Tolbachinsky Dol is spatially heterogeneous, incorporating subvertical and lateral pipeshaped magma conduits, closely spaced magma feeding channels, and shallow magma reservoirs. A longlived local transcrustal magma conducting zone is revealed, and regularities in the deep structure of the feeding systems of fissure eruptions are identified. The configuration of the established subvertical magma conduits permits basalts moving to rise to the surface by different paths, which, inter alia, explains the contrasting magma compositions observed during a single eruption. Thus, based on the instrumental data, it is shown that the magmatic feeding structure of the Tolbachinsky Dol has a number of specific peculiarities and is significantly more complicated than has been previously thought about the areal volcanic fields.

Role of mantle dynamics in rebuilding the Tianshan Orogenic Belt in NW China: A seismic tomographic investigation

The Tianshan orogenic belt, Junggar terrane and Altai terrane are located at the southwestern part of the Central Asian Orogenic Belt (CAOB). Here, we investigate the velocity structure beneath the Xinjiang region in NW China, which includes the Tarim terrane, Tianshan orogenic belt, Junggar terrane and Altai terrane with a view to evaluate the mantle dynamics based on teleseismic data recorded by 103 seismic stations. Our tomographic results show both high and low velocity perturbations beneath the Tianshan orogenic belt. We suggest that the high velocity perturbations beneath this orogenic belt might represent the northward subducted lithosphere of the Tarim Basin and the southward subducted lithosphere of the Junggar Basin. The low velocity structure beneath the Tianshan orogenic belt might represent asthenosphere upwelling that triggered the extensive magmatism which contributed to rebuilding of the Tianshan orogenic belt.

Study on the basement structure in the southeastern North China by blasting seismic

Since the Mesozoic, southeastern North China Craton has experienced intense crustal thinning and lithosphere destruction. Some of the responses of the deep activity in the upper crust crystalline basement have been retained in a series of tectonic evolution. The study of the upper crust velocity structure, especially the properties of the basement interface, is of great significance for studying the tectonic evolution and seismic hazard in the southeastern part of North China. In this study, we selected Pg waves of the blasting seismic data in the southeastern part of North China in recent years, which reflect the west Shandong uplift, offshore sedimentary basins and the Tanlu Fault zone and the Sulu orogenic transition zone, to study the structural and seismological characteristics of basement in North China Craton. The results of this study showed as follows: First, the obvious lag of Pg wave arrival time in Dongying depression and North Jiangsu basin reveals the thick sedimentary, low velocity and unstable basement structure. Second, the advance Pg wave arrival time with high apparent velocity, which reflects the basement structure of the west Shandong uplift, indicates the thin sediments and the shallow basement. Third, combined with many geophysical phenomena, such as electrical structure, density structure and terrestrial heat flow, we hold that the Tanlu tectonic belt and the Sulu orogenic belt have experienced great lithosphere destruction and there is shallow crust and the thinnest lithosphere in the vicinity of the Tanlu fault zone.

Constraining the hydration of the subducting Nazca plate beneath Northern Chile using subduction zone guided waves

Guided wave dispersion is observed from earthquakes at 180–280 km depth recorded at stations in the fore-arc of Northern Chile, where the 44 Ma Nazca plate subducts beneath South America. Characteristic P-wave dispersion is observed at several stations in the Chilean fore-arc with high frequency energy (>5 Hz) arriving up to 3 s after low frequency (<2 Hz) arrivals. This dispersion has been attributed to low velocity structure within the subducting Nazca plate which acts as a waveguide, retaining and delaying high frequency energy. Full waveform modelling shows that the single LVL proposed by previous studies does not produce the first motion dispersion observed at multiple stations, or the extended P-wave coda observed in arrivals from intermediate depth events within the Nazca plate. These signals can however be accurately accounted for if dipping low velocity fault zones are included within the subducting lithospheric mantle. A grid search over possible LVL and faults zone parameters (width, velocity contrast and separation distance) was carried out to constrain the best fitting model parameters. Our results imply that fault zone structures of 0.5–1.0 km thickness, and 5–10 km spacing, consistent with observations at the outer rise are present within the subducted slab at intermediate depths. We propose that these low velocity fault zone structures represent the hydrated structure within the lithospheric mantle. They may be formed initially by normal faults at the outer rise, which act as a pathway for fluids to penetrate the deeper slab due to the bending and unbending stresses within the subducting plate. Our observations suggest that the lithospheric mantle is 5–15% serpentinised, and therefore may transport approximately 13–42 Tg/Myr of water per meter of arc. The guided wave observations also suggest that a thin LVL (∼1 km thick) interpreted as un-eclogitised subducted oceanic crust persists to depths of at least 220 km. Comparison of the inferred seismic velocities with those predicted for various MORB assemblages suggest that this thin LVL may be accounted for by low velocity lawsonite-bearing assemblages, suggesting that some mineral-bound water within the oceanic crust may be transported well beyond the volcanic arc. While older subducting slabs may carry more water per metre of arc, approximately one third of the oceanic material subducted globally is of a similar age to the Nazca plate. This suggests that subducting oceanic lithosphere of this age has a significant role to play in the global water cycle.