Dipping Moho Research Articles

Use of decision tree ensembles for crustal structure imaging from receiver functions

SUMMARY Mode conversion of P waves at the boundary between Earth's crust and upper mantle, when analysed using receiver functions (RFs), allows characterization of Earth structure where seismic station density is high and earthquake sources are favourably distributed. We applied two ensemble decision tree algorithms—Random Forest (RanFor) and eXtreme Gradient Boost (XGBoost)—to synthetic and real RF data to assess these machine learning techniques' potential for crustal imaging when available data are sparse. The synthetic RFs, entailing both sharp increases in seismic velocity across the Moho and gradational Moho structures, calculated with and without added random noise, correspond to idealized crustal structures: a dipping Moho, Moho offset by crustal-scale faults, anti- and synform Moho structures and combinations of these. The RanFor/XGBoost algorithm recovers input structures well regardless of event-station distributions. Useful crustal and upper mantle seismic velocities can also be determined using RanFor and XGBoost, making it possible to image crustal thickness and P- and S-wave velocities simultaneously from RFs alone. We applied the trained RanFor/XGBoost to RFs determined from real seismic data recorded in the contiguous United States, producing a map of the Moho and P- and S-wave velocities of the lowermost crust and uppermost mantle. Use of XGBoost, which evaluates residuals between input RFs and ground-truth to update the decision tree using the gradient of a penalty function, improves the crustal thickness estimates.

Receiver Function Adjoint Tomography for Three‐Dimensional High‐Resolution Seismic Array Imaging: Methodology and Applications in Southeastern Tibet

AbstractA new technique for P‐wave receiver function (PRF) inversion, within the framework of wave equation‐based adjoint tomography and referred to as receiver function adjoint tomography (RFAT), has been developed to obtain models of Vp, Vs, and density. This innovative technique fits the synthetic PRFs with observed PRFs and can better image the lateral variations of Vs from the crust to the uppermost mantle than traditional 1‐D PRF inversion. We utilized RFAT to perform high‐resolution imaging beneath a dense seismic array in Southeastern Tibet, revealing low‐velocity zones extending from the uppermost mantle to the crust, as well as an eastward dipping Moho under the Red River Fault (RRF). Our inversion results provide direct evidence for the existence of a distinct asthenospheric upwelling channel beneath the RRF, and further highlight the effectiveness of RFAT for accurately imaging subsurface structures.

Seismic characteristics of the underthrusting Indian lithosphere beneath the eastern Kumaun Himalaya using regional earthquake waveform analysis

• Pn & Sn seismic velocities are different for earthquakes from north and south directions. • Different Pn & Sn seismic velocities show that the Indian Moho dips to the north. • Moho dips at low-angle beneath the Lesser Himalaya. • Moho dips steeply beneath South Chiplakot Thrust and further north. We investigate seismic nature of uppermost mantle and geometry of underthrusting Indian Moho beneath the eastern Kumaun Himalaya by calculating the uppermost mantle seismic ( Pn and Sn ) velocities using 12 regional earthquakes recorded by 33 digital broadband seismological stations from Gangetic plain to Tethys Himalaya along a ∼ 160 km profile. We use two-way travel time and interstation velocity methods to calculate the Pn and Sn velocities. The results show that the apparent Pn and Sn velocities are different for the earthquakes from north (updip) and south (downdip) directions. The significant difference between the uppermost mantle velocities from the updip and downdip directions clearly shows that the Moho dips in the north direction with an overall dip angle of 3.43°−4.2°. We calculate the true Pn and Sn velocities as 8.06 ± 0.01 km/s and 4.43 ± 0.01 km/s, respectively, throughout the profile. The calculated Pn interstation velocities clearly indicate a very low angle dipping (almost flat) Moho to the south of the South Chiplakot Thrust (SCT) in the Lesser Himalaya. From the SCT, the Moho dips steeply (dip angle 8.65° ± 0.95°), where topography also increases drastically; and continues further north with this dip angle till the Tethys Himalaya.

Feasibility of Estimating Parameters and Enhancing Reflections of Dipping Moho From Teleseismic P Wave Coda Autocorrelation

AbstractTeleseismic P wave coda autocorrelation can retrieve reflections from subsurface structural boundaries, which can be used in lithospheric imaging. However, dipping structures cause difficulty in extraction of reflections and errors in depth estimation. Based on a previously proposed velocity analysis method, we formulated a method for estimating the dipping parameters and correcting the effects of dipping structures. First, we derived an arrival time formula for the PP reflection of the dipping Moho, which includes the effects of the ray parameters, dipping structures, and azimuthal distribution of available events. Second, the formula was used for move out correction and velocity analysis. Finally, by scanning the peak of the Moho energy group in the velocity spectrum, which varies with the azimuth and dip angles, a method was proposed to estimate the dipping parameters of the Moho. We referred to the scanning map as a dipping spectrum. A field data test at station NE44 revealed the prospects of the practical applications of this method. The dip angle of the Moho below the station was 3°, and exhibited an azimuth angle of 25°NE. The stacked waveform of the Moho reflection was significantly enhanced. This method was successfully applied to more stations adjacent to station NE44. Our results provide new insight on the Moho in this area, which are important for imaging and interpretation in areas that contain a dipping Moho such as plate subduction zones.

Crustal Deformation Patterns in the Tibetan Plateau and Its Adjacent Regions as Revealed by Receiver Functions

ABSTRACT In the past decades, several mechanisms (e.g., the crustal flow and thin viscous sheet models) and combinations of them have been proposed to explain the uplift and outgrowth of the Tibetan plateau. These mechanisms, however, cannot account for all the features observed, and the issue is still highly debated. In this study, we computed receiver functions (RFs) at 254 permanent and portable stations in the Tibetan plateau and its adjacent regions to provide large-scale constraints for the crustal deformation. A harmonic analysis, which has been recently proposed to help distinguishing a dipping Moho from crustal anisotropy, was employed here to investigate the accurate crustal anisotropy. We further applied the H-κ stacking technique to determine the crustal composition using the RFs after harmonic correction or moveout correction. Our results showed the following: (1) Harmonic corrections on Ps phases led to average differences of 2.6 km and 0.04 in crustal thickness and in the VP/VS ratio, respectively; notably, the weighting factors of the Ps, PpPs, and PpSs + PsPs phases affected the uncertainties more obviously than the harmonic corrections. (2) The crustal deformation of the central–southern Tibetan plateau was controlled by the faulting and subduction of the Indian plate; meanwhile, faulting played a dominant role in the crustal deformation of the northern and eastern Tibetan plateau. (3) Widespread upper crustal or crustal shortening and thickening should have occurred beneath the entire study region, except for the Chuan–Dian block (which likely underwent tectonic extension). (4) Crustal partial melting might have enhanced the crustal anisotropy, and promoted the accumulation and release of strain, increasing the probability of fracture in the upper crust.

Simultaneous inversion for crustal thickness and anisotropy by multiphase splitting analysis of receiver functions

SUMMARY We present a technique to derive robust estimates for the crustal thickness and elastic properties, including anisotropy, from shear wave splitting of converted phases in receiver functions. We combine stacking procedures with a correction scheme for the splitting effect of the crustal converted Ps-phase and its first reverberation, the PpPs-phase, where we also allow for a predefined dipping Moho. The incorporation of two phases stabilizes the analysis procedure and allows to simultaneously solve for the crustal thickness, the ratio of average P- to S-wave velocities, the percentage of anisotropy and the fast-axis direction. The stacking is based on arrival times and polarizations computed using a ray-based algorithm. Synthetic tests show the robustness of the technique and its applicability to tectonic settings where dip of the Moho is significant. These tests also demonstrate that the effects of a dipping layer boundary may overprint a possible anisotropic signature. To constrain the uncertainty of our results we perform statistical tests based on a bootstrapping approach. We distinguish between different model classes by comparing the coherency of the stacked amplitudes after moveout correction. We apply the new technique to real-data examples from different tectonic regimes and show that coherency of the stacked receiver functions can be improved, when anisotropy and a dipping Moho are included in the analysis. The examples underline the advantages of statistical analyses when dealing with stacking procedures and potentially ambiguous solutions.

Seismological evidence for the earliest global subduction network at 2 Ga ago.

The earliest evidence for subduction, which could have been localized, does not signify when plate tectonics became a global phenomenon. To test the antiquity of global subduction, we investigated Paleoproterozoic time, for which seismic evidence is available from multiple continents. We used a new high-density seismic array in North China to image the crustal structure that exhibits a dipping Moho bearing close resemblance to that of the modern Himalaya. The relict collisional zone is Paleoproterozoic in age and implies subduction operating at least as early as ~2 billion years (Ga) ago. Seismic evidence of subduction from six continents at this age is interpreted as the oldest evidence of global plate tectonics. The sutures identified can be linked in a plate network that resulted in the assembly of Nuna, likely Earth's first supercontinent. Global subduction by ~2 Ga ago can explain why secular planetary cooling was not appreciable until Proterozoic time.

Crustal Structure and Evolution of the Eastern Himalayan Plate Boundary System, Northeast India

AbstractWe use data from 24 broadband seismographs located south of the Eastern Himalayan plate boundary system to investigate the crustal structure beneath Northeast India. P wave receiver function analysis reveals felsic continental crust beneath the Brahmaputra Valley, Shillong Plateau and Mikir Hills, and mafic thinned passive margin transitional crust (basement layer) beneath the Bengal Basin. Within the continental crust, the central Shillong Plateau and Mikir Hills have the thinnest crust (30 ± 2 km) with similar velocity structure, suggesting a unified origin and uplift history. North of the plateau and Mikir Hills the crustal thickness increases sharply by 8–10 km and is modeled by ∼30∘ north dipping Moho flexure. South of the plateau, across the ∼1 km topographic relief of the Dawki Fault, the crustal thickness increases abruptly by 12–13 km and is modeled by downfaulting of the plateau crust, overlain by 13–14 km thick sedimentary layer/rocks of the Bengal Basin. Farther south, beneath central Bengal Basin, the basement layer is thinner (20–22 km) and has higher Vs (∼4.1 km s−1) indicating a transitional crystalline crust, overlain by the thickest sedimentary layer/rocks (18–20 km). Our models suggest that the uplift of the Shillong Plateau occurred by thrust faulting on the reactivated Dawki Fault, a continent margin paleorift fault, and subsequent back thrusting on the south dipping Oldham Fault, in response to flexural loading of the Eastern Himalaya. Our estimated Dawki Fault offset combined with timing of surface uplift of the plateau reveals a reasonable match between long‐term uplift and convergence rate across the Dawki Fault with present‐day GPS velocities.

Gravity Anomalies and Isostasy Deduced From New Dense Gravimetry Around the Tsangpo Gorge, Tibet

AbstractWe built the first dense gravity network including 107 stations around the Tsangpo Gorge, Tibet, one of the hardest places in the world to reach, and conducted a gravity and hybrid GPS observation campaign in 2016. We computed the Bouguer gravity anomalies (BGAs) and free‐air gravity anomalies (FGAs) and increased the resolution of the FGAs by merging the in situ data with EIGEN‐6C4 gravity model data. The BGAs around the Tsangpo Gorge are in general negative and gradually decrease from south (−360 mGal) to north (−480 mGal). They indicate a uniformly dipping Moho around the Tsangpo Gorge that sinks from south to north at an angle of 12°. We introduced a method to compute the vertical tectonic stress of the lithosphere, a quantitative expression of isostasy, using BGA and terrain data, and applied it to the area around the Tsangpo Gorge. We found that the lithosphere of the upstream of the Tsangpo Gorge is roughly in an isostatic state, but the lithosphere of the downstream exhibits vertical tectonic stress of ~50 MPa, which indicates the loss of a large amount of surface material. This result does not support the deduction of the valley bottom before uplift of the Tsangpo Gorge by Wang et al. (2014).

Crustal structure of a Proterozoic craton boundary: East Albany-Fraser Orogen, Western Australia, imaged with passive seismic and gravity anomaly data

We use passive seismic and gravity data to characterize the crustal structure and the crust-mantle boundary of the east Albany-Fraser Orogen in Western Australia, a Proterozoic orogen that reworked the southern and southeastern margin of the Archean Yilgarn Craton. The crustal thickness pattern retrieved from receiver functions shows a belt of substantially thickened crust that follows the trend of the orogen, but narrows to the southwest. Common conversion point profiles show a clear transition from a wide, symmetric Moho trough in the northeast to a one-sided, northwestern Moho dip in the southwest, where the Moho appears to underthrust the craton towards its interior. This change appears to coincide with the inferred trace of the Ida Fault, a major terrane boundary within the Yilgarn Craton.Forward modelling of gravity anomaly data using the retrieved Moho geometry as a geometric constraint shows that a conspicuous, elongated gravity low on the northwestern side of the east Albany-Fraser Orogen is almost certainly caused by thickened Archean crust. To obtain a model that resembles the regional gravity pattern the following assumptions are necessary: high-density rocks occur in the upper crust of the Fraser Zone, at depth inside the Moho trough and in parts of the eastern Nornalup Zone east of the Moho trough. Although our gravity models do not constrain at which crustal level these high-density rocks occur, active deep seismic surveys suggest that large extents of the east Albany-Fraser Orogen’s lower crust include a Mesoproterozoic magmatic underplate known as the Gunnadorrah Seismic Province.The simplest interpretation of the imaged crustal structure is that the Gunnadorrah Seismic Province is underthrust beneath the Yilgarn Craton, likely as a consequence of crustal shortening during accretion further east. The imaged geometry overall appears to show a wedge of Archean lower crust that was driven between the exhumed Fraser Zone and the Gunnadorrah Seismic Province, effectively splitting the Paleo- to Mesoproterozoic crust of the east Albany-Fraser Orogen. The vertical splitting of Proterozoic crust by a cratonic crustal wedge, comparable to what we image in this study, may be a process that contributed to forming many craton margins around the world.

Seismic Character of Moho Beneath the NW Himalaya and Ladakh Inferred from Regional Earthquakes Travel Time Data

We study the uppermost mantle velocities and dip of Indian Moho beneath the NW Himalaya and Ladakh using 42 regional waveform data recorded on 15 seismographs along a ~600 km-long profile. We use the two-way travel time and interstation velocity methods. The apparent Pn and Sn velocities beneath the NW Himalaya are 8.08 ± 0.04 and 4.64 ± 0.07 km/s for earthquakes occurring south of the profile (downdip, western Indian shield) and 8.70 ± 0.13 and 4.76 ± 0.12 km/s for earthquakes from north (updip, western Tibet). Similarly, these velocities beneath Ladakh are 7.18 ± 0.07 and 4.32 ± 0.05 km/s for earthquakes due south (downdip, north Indian shield) and 8.50 ± 0.10 and 4.39 ± 0.12 km/s for earthquakes due north (updip, western Tibet). These velocity variations constrain the Moho dip at ~2.4 ± 0.14o beneath the NW Himalaya and ~6.6 ± 0.54o beneath Ladakh. Considering the varying dips along the profile, we observe that the true Pn (8.37 ± 0.07 km/s) and Sn (4.70 ± 0.1 km/s) velocities are higher for the NW Himalaya than for Ladakh (7.73 ± 0.08 and 4.33 ± 0.09 km/s). The large variation in interstation Pn velocity is observed between the station pairs near the Indus Zangpo Suture zone due to steep dipping (~7.1o to 6.26o) of the Indian Moho. In the Himalaya region, the interstation and average values of the velocities and Moho dip are comparable, whereas a variation is observed in different segments of the Ladakh region. The results show that the Indian Moho is underthrusting at a shallow angle (~2.5o) beneath the Himalaya, steepens abruptly (~6.6o) further north of the Southern Tibetan Detachment and continues at a shallow angle (~3.8o) beneath Ladakh.

High-precision relocation of seismic sequences above a dipping Moho: the case of the January–February 2014 seismic sequence on Cephalonia island (Greece)

Abstract. Detailed velocity structure and Moho mapping is of crucial importance for a high precision relocation of seismicity occurring out of, or marginal to, the geometry of seismological networks. Usually the seismographic networks do not cover the boundaries of converging plates such as the Hellenic arc. The crustal thinning from the plate boundary towards the back-arc area creates significant errors in accurately locating the earthquake, especially when distant seismic phases are included in the analysis. The case of the Cephalonia (Ionian Sea, Greece) sequence of January–February 2014 provided an excellent example where the hypocentral precision was greatly affected by the crustal thinning from the plate boundary at the Ionian sea towards the Aegean sea. This effect was examined in detail by testing various velocity models of the region in order to determine an optimal model. Our tests resulted in the adoption of a velocity model that resembles the crustal thinning of the region. Then, a relocation procedure was performed in the Cephalonia sequence for the time period of 26 January to 15 May 2014 by applying probabilistic non-linear location algorithms. The high-precision relocation resulted in an improved spatial distribution of the seismicity with respect to the preliminary locations and provided a reliable basis to examine seismotectonic implications of the Cephalonia sequence.

Analysis of teleseismic P waves with a 5200‐station array in Long Beach, California: Evidence for an abrupt boundary to Inner Borderland rifting

AbstractWe analyze teleseismic P waves from four Mw ≥ 6.5 earthquakes recorded by a petroleum industry survey in Long Beach, California. The survey used a 2‐D array with up to 5200 seismometers, 120 m mean spacing, and 7 – 10 km aperture. At frequencies near 1 Hz, P wave travel times and amplitudes exhibit coherent lateral variations over scales as short as ~400 m, including locally delayed travel times and increased amplitudes at the crest of the Long Beach anticline. Deeper heterogeneity is indicated by P wave phase velocities that deviate from reference model predictions for events from southwestern azimuths. We postulate that a sharp northeastward increase in Moho depth from the Inner Borderland (IB) to mainland southern California causes the anomalous phase velocities. Elastic forward modeling finds the travel times are fit well by a Moho that dips 65° to the northeast and flattens ~10 km southwest of the Newport‐Inglewood fault zone. Constraining the felsic thickness of mainland crust to 28 km requires an 8 km thick layer with a P‐velocity of 7 km/s beneath it, which could result from basal accretion of former Farallon ocean crust or magmatic underplating during Miocene volcanism. Forward models with a 65° Moho dip predict a P‐to‐s conversion with a phase velocity of ~5 km/s. Deconvolution of the array's mean P wave signal isolates a similar later arriving phase. The steep crust thickness transition supports a locally abrupt boundary to IB rifting. Our results highlight the utility of dense short‐period arrays for passive imaging at near surface to uppermost mantle depths.

Crustal complexity in the Lachlan Orogen revealed from teleseismic receiver functions

There is an ongoing debate about the tectonic evolution of southeast Australia, particularly about the causes and nature of its accretion to a much older Precambrian core to the west. Seismic imaging of the crust can provide useful clues to address this issue. Seismic tomography imaging is a powerful tool often employed to map elastic properties of the Earth's lithosphere, but in most cases does not constrain well the depth of discontinuities such as the Mohorovičić (Moho). In this study, an alternative imaging technique known as receiver function (RF) has been employed for seismic stations near Canberra in the Lachlan Orogen to investigate: (i) the shear-wave-velocity profile in the crust and uppermost mantle, (ii) variations in the Moho depth beneath the Lachlan Orogen, and (iii) the nature of the transition between the crust and mantle. A number of styles of RF analyses were conducted: H-K stacking to obtain the best compressional–shear velocity (V P /V S) ratio and crustal thickness; nonlinear inversion for the shear-wave-velocity structure and inversion of the observed variations in RFs with back-azimuth to investigate potential dipping of the crustal layers and anisotropy. The thick crust (up to 48 km) and the mostly intermediate nature of the crust−mantle transition in the Lachlan Orogen could be due to the presence of underplating at the base of the crust, and possibly to the existing thick piles of Ordovician mafic rocks present in the mid and lower crust. Results from numerical modelling of RFs at three seismic stations (CAN, CNB and YNG) suggest that the observed variations with back-azimuth could be related to a complex structure beneath these stations with the likelihood of both a dipping Moho and crustal anisotropy. Our analysis reveals crustal thickening to the west beneath CAN station which could be due to slab convergence. The crustal thickening may also be related to the broad Macquarie volcanic arc, which is rooted to the Moho. The crustal anisotropy may arise from a strong N–S structural trend in the eastern Lachlan Orogen and to the preferred crystallographic orientation of seismically anisotropic minerals in the lower and middle crust related to the paleo-Pacific plate convergence.

Lithospheric and upper mantle structure of the northeastern Tibetan Plateau

We use receiver functions calculated for data collected by the INDEPTH‐IV seismic array to image the three‐dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern Tibet. Our results indicate an average crustal thickness of 65 to 70 km in northern Tibet. In addition, we observe a 20 km Moho offset beneath the northern margin of the Kunlun Mountains, a 10 km Moho offset across the Jinsha River Suture and gently northward dipping Moho beneath the Qaidam Basin. A region in the central Qiangtang Terrane with higher than normal crustal Vp/Vs ratio of ∼1.83 can be the result of the Eocene magmatic event. In the Qiangtang Terrane, we observe a significant lithospheric mantle discontinuity beneath the Bangong‐Nujiang Suture at 80 km depth which dips ∼10° to the north, reaching ∼120 km depth. We interpret this feature as either a piece of Lhasa Terrane or remnant oceanic slab underthrust below northern Tibet. We detect a ∼20 km depression of the 660‐km discontinuity in the mantle transition zone beneath the northern Lhasa Terrane in central Tibet, which suggests this phase transition has been influenced by a dense and/or cold oceanic slab. A modest ∼10 km depression of the 410‐km discontinuity located beneath the northern Qiangtang Terrane may be the result of localized warm upwelling associated with small‐scale convection induced by the penetration of the sinking Indian continental lithosphere into the transition zone beneath the central Tibetan Plateau.

Crustal structure and deformation in the northeast India-Asia collision zone: constraints from receiver function analysis

SUMMARY Crustal structure across the India–Asia collision zone (Tidding Suture) in the northeast Himalaya bounded by the Eastern Himalayan Syntaxis (EHS) is investigated using the P-wave receiver function (RF) method. The analysed data included three-component waveforms of teleseismic earthquakes recorded by a linear array of 11 broad-band seismic stations. The RFs and inverted shear wave velocity models reveal azimuthally varying crustal structure. The RFs for earthquakes from the northeast back azimuths are conspicuous by absence of P-to-S converted phase at the Moho discontinuity. Inverted velocity model ascribe this to absence of the typical step velocity jump at the Moho in a narrow section of the EHS bordering the indenting Indian Plate and pierced by travelling waves. In contrast, teleseismic waves arriving from southeast back azimuth sample different litho-tectonic blocks of the Himalayan collision zone and inverted models show northeast dipping Moho across the Tidding Suture. Compared to an overall thickness of >70 km in the northwest and central Himalaya, the crust across the Tidding Suture is only about 55 km thick. This is attributed to a slower rate of convergence in this part of the collision zone. The Moho structure beneath Indian Plate and southeastern Tibetan Plateau reveal opposite dip directions with their colliding margin placed just east of the Walong Thrust. The inverted shear wave velocity models show evidence of intracrustal low-velocity layer whose strength varies across the Tidding Suture. The magnitude of velocity reduction beneath Lohit Plutonic Complex (Trans Himalaya) favour partial melt as a possible mechanism whereas south of the Tidding Suture, where velocity reduction is comparatively less, fluids generated by dehydration reactions appear to be the source for velocity reduction.

Crustal‐scale transcurrent fault development in a weak‐layered crust from an integrated geophysical research: Carboneras Fault Zone, eastern Betic Cordillera, Spain

New magnetotelluric and receiver transfer function studies provide insights from the upper to the lower crust of the eastern Betic Cordillera, which is deformed by large folds, normal faults, and a major transcurrent left‐lateral fault, the Carboneras Fault Zone (CFZ). Receiver function analysis determines a NNW dipping Moho reaching 20° that increases in depth, from 20 km south of the CFZ up to 34 km in the Sierra de Los Filabres. In addition, seismic discontinuities determined in the upper crust are interpreted as major contacts between metamorphic complexes that are detached and folded. The MT inversion model reveals a conductive zone, also representing a crustal seismic discontinuity, associated with the Alpujárride/Nevado‐Filábride contact and fitting the N vergent geometry of the Sierra Alhamilla antiform. A small flexure at Moho coincides with the CFZ, as revealed by the Bouguer anomaly trend, in agreement with the receiver function results. Moreover, the Bahr strike and tipper angle at the stations placed closest to the CFZ clearly reveal the continuity of the CFZ at least down to approximately 15 km in depth, crossing all the detected crustal discontinuities up to the Moho. The lack of a clear Moho offset associated with the Carboneras Fault supports the idea that some large strike‐slip faults tend to accommodate the deformation by a broadening fault zone at lower crustal levels. Its nucleation could occur at the base of a thin crust, where melting processes critically reduced the lithospheric strength during the late Miocene, to then propagate upward, reaching the topographic surface. Northward, the lithosphere comprised moderately larger strength, and the crustal discontinuities favored the development of larger folds with kilometric amplitude instead of strike‐slip faults since the late Miocene.

P-Wave Back-Azimuth and Slowness Anomalies Observed by an IMS Seismic Array LZDM

The seismic array LZDM in China is a primary station of the International Monitoring System (IMS) for verifying compliance with the Comprehensive Nuclear-Test-Ban Treaty. P -wave back-azimuth and slowness anomalies were found at LZDM during research on teleseismic events; that is, the back-azimuth and slowness errors of the P phases were found to be systematic and varied with theoretical back azimuth in a sinusoidal-like and cosinusoidal-like manner, respectively. The maximum values of back-azimuth and slowness errors reached 87.1° and 8.68 sec/°, respectively. Analysis showed that these errors could not be expressed as measurement errors because they were mostly caused by the dipping (Moho) below LZDM. These large systematic errors were well compensated by introducing several dipping Moho models. A velocity contrast of 0.84 km/sec across the Moho was chosen for these models. The best-fit dipping Moho model for all the data was found to have a 115° strike and a 9° dip angle. Moreover, the events in different slowness domains were adapted to specific submodels, which resulted in smaller average errors. The strikes of all the models were all around the point where the slowness errors change polarity from either positive to negative or from negative to positive. The corrected results for 151 teleseismic events were clearly improved by the introduction of the single dipping Moho model. Error analysis after structural correction showed that the average errors of back-azimuth and slowness residuals drop from 31.5° to 11.6° and from 3.2 sec/° to 1.2 sec/°, respectively; that is, the error reductions of back-azimuth and slowness residuals are 63.3% and 62.5%, respectively. To validate the model, we used another group of 166 events and did the structural corrections. Error analysis after structural correction showed that the average errors of back-azimuth and slowness residuals drop from 30.0° to 13.2° and from 3.5 sec/° to 1.3 sec/°, respectively; that is, the error reductions of back-azimuth and slowness residuals are 55.9% and 63.3%, respectively. Several submodels for different slowness domains have been introduced for the first 151 events group and 21 events from southeast with low slowness values, the error reduction ratios were higher (about 10% in back azimuth and slowness, respectively) than the application of the single Moho model. Error analysis after structural correction showed that the average errors of back-azimuth and slowness residuals drop from 30.8° to 8.9° and from 2.8 sec/° to 0.7 sec/°, respectively. The structural correction results of the second group of 166 events by the using of submodels gave a 9% improvement in the back-azimuth and slowness determination than the single Moho model, which implied the complexity that exists below the array. Error analysis after structural correction showed that the average errors of back-azimuth and slowness residuals drop from 30.0° to 10.6° and from 3.5 sec/° to 1.0 sec/°, respectively.

Estimation of the Crustal Vp/Vs Ratio with Dipping Moho from Receiver Functions

AbstractThe Moho discontinuity is often deformed by complex forces in tectonic zones, which may lead to a dipping Moho locally. In order to obtain accurate crustal Vp/Vs ratio for these regions, this paper proposed a H‐κ‐θ stacking method to estimate the crustal Vp/Vs and thickness, which considers the responses of a dipping Moho. Both radial and tangential receiver functions were stacked, so that improves the scanning precision. Results of its application to southeastern Tibet indicate that the crustal thickness beneath southeastern Tibet varied notably. The average crustal thickness is about 60 km, 47 km and 43 km in Songpan‐Garzê fold system, Sichuan basin and Yangtze platform, respectively, with a normal value in Sanjiang fold zone and southeastern Yangtze platform. The crustal Vp/Vs ratio is higher in the region between the Songpan‐Garzê fold system and Yangtze platform, and is abnormally highest to the west of Sichuan basin (> 1.88). We presume that the eastward crustal material flow was resisted by the Sichuan basin with high strength, and accumulated at west of Sichuan, consequently resulting in the unusual high Vp/Vs anomaly.

Seismic anisotropy in northern and central Gulf of California region, Mexico, from teleseismic receiver functions and new evidence of possible plate capture

We present anisotropic models for the shallow lithosphere in the northern and central Gulf of California obtained from receiver function (RF) analysis. This region has undergone intense plate reorganization over the past 25 Ma that led to the transfer of the plate boundary east of the modern Baja California Peninsula posterior to the cease of the subduction of the Farallon plate beneath the North American plate. Our main interest in modeling the seismic anisotropy is to characterize rock fabric inherited from past and recent tectonism and to provide new information for understanding the tectonic processes involved in the evolution of the local plate boundary. We calculate RFs from teleseismic P waves recorded by the Network of Autonomously Recording Seismographs(NARS)‐Baja array, and we stack them in 20° wide back azimuthal bins to enhance coherent information. These RFs yield significant transverse energy whose azimuthal variations are not consistent with that expected for a crust with dipping Moho. Therefore we interpret the conspicuous RFs energy of the transverse component as convincing evidence of anisotropy. We present forward models obtained by fitting the radial and transverse RFs with synthetic seismograms calculated with a reflectivity code. Our analysis suggests substantial variations in the structure of the shallow lithosphere along the Baja California Peninsula and the presence of stalled fragments of the extinct Farallon plate beneath its central part. The results at station NE81 in Sonora are interpreted as frozen crustal anisotropy inherited from Miocene Basin and Range extension, consistent with previous observations of the upper mantle anisotropy beneath this station.