Crustal Velocity Structure Research Articles

Slip rate along the Lijiang-Ninglang fault zone estimated from repeating microearthquakes

Seismicity and slip rates along the Lijiang-Ninglang fault zone between 1999 and 2006 were investigated with the waveform data recorded by the Yunnan digital Seismic Network. The relocated seismicity by the double difference method clearly exhibits different features between the northern and southern segments. More than 76% earthquakes occurred in the southern segment of the fault near the Lijiang area. The relocated seismicity appears to reflect the crustal velocity structure of the study area. Using cross-correlation analysis, we identified a total of 92 doublets and 70 multiplets that show high waveform similarity. Most of these sequences are aperiodic with recurrence intervals varying from a few minutes to hundreds of days. Using two sequences that occurred regularly over the study period, we obtained a fault slip rate of approximately 5 mm/a at ∼23 km, in good agreement with geologic and surface GPS measurement.

Visco-elastic stress triggering model of Tangshan earthquake sequence

We calculated the Coulomb failure stress change generated by the 1976 Tangshan earthquake that is projected onto the fault planes and slip directions of large subsequent aftershocks. Results of previous studies on the seismic failure distribution, crustal velocity and viscosity structures of the Tangshan earthquake are used as model constraints. Effects of the local pore fluid pressure and impact of soft medium near the fault are also considered. Our result shows that the subsequent Luanxian and Ninghe earthquakes occurred in the regions with a positive Coulomb failure stress produced by the Tangshan earthquake. To study the triggering effect of the Tangshan, Luanxian, and Ninghe earthquakes on the follow-up small earthquakes, we first evaluate the possible focal mechanisms of small earthquakes according to the regional stress field and co-seismic slip distributions derived from previous studies, assuming the amplitude of regional tectonic stress as 10 MPa. By projecting the stress changes generated by the above three earthquakes onto the possible fault planes and slip directions of small earthquakes, we find that the “butterfly” distribution pattern of increased Coulomb failure stress is consistent with the spatial distribution of follow-up earthquakes, and 95% of the aftershocks occurred in regions where Coulomb failure stresses increase, indicating that the former large earthquakes modulated occurrences of follow-up earthquakes in the Tangshan earthquake sequence. This result has some significance in rapid assessment of aftershock hazard after a large earthquake. If detailed failure distribution, seismogenic fault in the focal area and their slip features can be rapidly determined after a large earthquake, our algorithm can be used to predict the locations of large aftershocks.

East Greenland Ridge in the North Atlantic Ocean: An integrated geophysical study of a continental sliver in a boundary transform fault setting

The combined Greenland‐Senja Fracture Zones (GSFZ) represent a first‐order plate tectonic feature in the North Atlantic Ocean. The GSFZ defines an abrupt change in the character of magnetic anomalies with well‐defined seafloor spreading anomalies in the Greenland and Norwegian basins to the south but ambiguous and weak magnetic anomalies in the Boreas Basin to the north. Substantial uncertainty exists concerning the plate tectonic evolution of the latter area, including the role of the East Greenland Ridge, which is situated along the Greenland Fracture Zone. In 2002, a combined ocean‐bottom seismometer and multichannel seismic (MCS) survey acquired two intersecting wide‐angle reflection and coincident MCS profiles across and along the East Greenland Ridge. We present the results of integrated reflection seismic interpretation, first‐arrival tomography, 2D kinematic raytracing, full‐wave amplitude modeling, and gravity modeling of the intersecting profiles. The results show that (1) the Greenland Basin is characterized by a normal oceanic crustal velocity structure, (2) the velocity structure of the East Greenland Ridge is of overall continental type, and (3) a major faulted basin province above highly extended continental crust exists to the NE of the ridge. The results further suggest that a zone of extremely thin and faulted continental crust above partially serpentinized mantle peridotite defines the NW edge of the East Greenland Ridge and the transition to the NE Greenland margin.

Crustal Structure and Its Features in Northwest Margin of Bohai Bay and Its Adjacent Areas

AbstractIn this paper wide‐angle reflection and refraction data is used along Tanggu‐Yutian profile and other five profiles intersecting with this profile, and the seismic sounding data was processed and interpreted comprehensively along Tanggu‐Yutian profile. The purpose is to find out the basic features of 2‐D velocity structure, spacial distribution of faults, geological structure condition of shallow and deep crust in the northwest margin of Bohai bay and its adjacent area. The result shows that obvious layered structure appears along the profile, the crustal thickness increases gradually from south to north, and the crustal velocity structures in different regions have obvious inhomogeneity in the lateral and vertical directions. The Moho is a slope between Tanggu and Ninghe near Bohai bay. The crustal depth deepens gradually towards north from 28 km in Tanggu to 31 km in Ninghe.

Crustal Structure and Its Tectonic Responses During Mesozoic and Cenozoic in Beijing‐Tianjin‐Tangshan and Its Adjacent Region

AbstractTaking the North China block as the demonstration zone of “Images of the Earth's Crust & Upper Mantle (IGCP Project 474)”, we calculated the average velocity and thickness of the upper, middle, and lower crust in Beijing‐Tianjin‐Tangshan (BTT) and adjacent areas by integrating crustal velocity structure from eight wideangle reflection seismic profiles in this region. Using Kriging interpolating technique, the authors have constructed many significant images such as average velocity, thickness and spatial variation of the bottom boundary depths of upper crust, mid‐crust and lower crust. These images show that uplift and depression spreading along nearly EW and NE directions dominate in the study region. Faulted‐swells and faulted‐ depressions controlled by NE faults are present too. The formation of geological structure is closely related to tectonic movement that occurred since the Mesozoic in the study region. The authors inferred that there were at least three stages of intense tectonic movements in the Mesozoic and Cenozoic in the study region. Combining with regional tectonic researches, the authors conclude that the nearly EW uplifts and depressions formed in the Triassic and the NE‐trending uplifts and depressions formed in the Jurassic. The faulted‐uplifts and faulted depressions controlled by NE faults emerged in the Cretaceous. Because the middle crust is a low‐velocity layer and its deformation is mainly ductile, which is completely different from the upper and lower crust, it is thickened and has low velocity in uplifted areas, whereas its thickness decreased and has high velocity in depressions. The overall strength of the crust is reduced in places of thick middle crust, where faulting and deformation are easy to occur in later tectonic movements, thus affecting the occurrence of present earthquakes in the study region.

Seismic images under the Beijing region inferred from P and PmP data

In this study a new tomographic method is applied to over 1500 high-quality PmP (Moho reflected wave) travel-time data as well as over 38,500 high-quality first P-wave arrivals to determine a detailed 3D crustal velocity structure under Beijing and adjacent areas. Results of detailed resolution analyses show that the PmP data can significantly improve the resolution of the model in the middle and lower crust. After the PmP data are included in the tomographic inversion, our new model not only displays the tectonic feature appeared in the previous studies, but also reveals some new features. The Zhangjiakou-Bohai Sea fault zone (Zhang-Bo zone) is imaged as prominent and continuous low-velocity (low-V) anomalies in the shallower crust, while in the middle and lower crust it shows intermittent low-V anomalies extending down to the uppermost mantle. Furthermore, the pattern of low-V anomalies is different along the Zhang-Bo zone from the southeast to the northwest, indicating that there exist large differences in the dynamic evolution of Taihangshan and Yanshan uplifts and North China depression basin. Prominent low-V anomalies are visible under the source area of the 4 July 2006 Wen-An earthquake (M 5.1), suggesting that the occurrence of the Wen-An earthquake is possibly related to the effect of the crustal fluids probably caused by the upwelling of the hot and wet asthenospheric materials due to the deep dehydration of the stagnant Pacific slab in the mantle transition zone. The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the Wen-An earthquake. This is somewhat similar to the cause of the 1695 Sanhe-Pinggu earthquake and the 1976 Tangshan earthquake in the region, as well as the 1995 Kobe earthquake in Japan and the 2001 Bhuj earthquake in India.

1D velocity structure beneath broadband seismic stations in the Cretaceous Gyeongsang Basin of Korea by receiver function analyses

The crustal velocity structures beneath four broadband seismic stations (GKP, GSU, HDB, and BUS) in the Cretaceous Gyeongsang Basin, southeastern Korea, are estimated by using receiver function analyses employing teleseismic waveforms. The genetic algorithm is adopted to avoid the inherent non-uniqueness problem of the inversion. The inversion results are constrained by surface-wave dispersions to complement the shortcoming of the receiver function. The selected records of earthquakes distributed in three quadrants from each seismic station in the years from 2002 to 2005 are analyzed. Although limited quantity of data is used due to short operation periods, the resolution and confidence level are expected to be better than those using less data. The Moho depth under GKP is estimated to be 30 km showing no distinctive indication of dipping. This depth is well coincident with those in nearby velocity cross-sections obtained from crustal-scale seismic profiles compared to the 36-km deep Moho previously reported by other independent work. At GSU the Moho appears at the 32-km depth and a distinct low-velocity anomaly is detected at the depth of about 10 km. They agree with those in the velocity tomogram of the nearby survey line. An adakitic intrusion which results from the partial melting of a young and hot subducted oceanic crust in the basin during the Cretaceous is suggested as a possible geologic interpretation of the low-velocity zone. The Moho beneath HDB is 28-km deep, and agrees with those in nearby velocity cross-sections obtained from crustal-scale seismic profiles. This Moho depth at HDB is rather shallower than those of other stations. The significant P S phase amplitude and arrival time differences in the radial receiver functions conclude the Moho to dip southwestwardly. This is supported by polarities of direct P and P S in transverse receiver functions. Beneath BUS, P wave velocity increases gradually from the 32-km depth and it reaches 7.6 km/s at the 36-km depth. The Moho discontinuity beneath BUS is thought to be at an around 35-km depth.

Dalrymple Trough: An active oblique-slip ocean–continent boundary in the northwest Indian Ocean

The Dalrymple Trough marks part of the transform plate boundary between India and Arabia in the northern Arabian Sea. Oblique extension is presently active across this portion of the boundary at a rate of a few millimetres per year, and seismic reflection profiles across the trough confirm that it is an extensional structure. We present new swath bathymetric and wide-angle seismic data from the trough. The bathymetric data show that the trough is bounded by a single, steep, 3-km-high scarp to the southeast and a series of smaller, en-echelon scarps to the northwest. Wide-angle seismic data show that a typical oceanic crustal velocity structure is present to the northwest, with a crustal thickness of ~ 6 km. There is an abrupt change in crustal thickness and velocity structure at the northwestern edge of the trough, and the trough itself is underlain by 12-km-thick crust interpreted as thinned continental crust. Therefore we infer that Dalrymple Trough is an unusual obliquely extending plate boundary at which continental crust and oceanic crust are juxtaposed. The extensional deformation is focused on a single major fault in the continental lithosphere, but distributed over a region ~ 60 km wide in the oceanic lithosphere.

Basin and crustal velocity structure models for the simulation of strong ground motions in the Kinki area, Japan

We constructed a prototype of the basin and crustal structure model for the Kinki area, southwest of Japan, for the simulation of strong ground motions of hypothetical crustal and subduction earthquakes. We collected results of the deep seismic velocity profiles obtained by the reflection experiments and seismic imaging results, which were conducted in the Kinki area. The obtained profiles give underground velocity structures of the crust, from the surface to the subducting slab. We also gather the basin velocity structure information of the Osaka, Kyoto, Nara, and Ohmi basins. To examine the applicability of the constructed velocity structure model to the ground motion simulation, we simulated waveforms of an intermediate size event occurred near the source area of the hypothetical subduction earthquakes. Simulated ground motions using the basin and crustal velocity structure model are fairly well reproducing the observations at most of stations, and the constructed basin and crustal velocity structure model is applicable for the long-period ground motion simulations.

Crustal Velocity Structure in Italy from Analysis of Regional Seismic Waveforms

Abstract In this article, we use regional seismic waveforms recorded by the recently installed Istituto Nazionale di Geofisica e Vulcanologia (INGV) national network and the Mediterranean Very Broadband Seismographic Network (MedNet) stations to develop 1D crustal velocity models for the Italian peninsula. About 55,000 P -wave and 35,000 S -wave arrival times from 4727 events are used to derive average seismic parameters in the crust and uppermost mantle. We define four regions, according to geological constraints and recent travel-time tomography results. Based on the average seismic parameters, we combine broadband seismic waveforms and travel times of regional phases to model crustal structures for the four regions by applying the genetic algorithm. Our results indicate smooth velocity gradients with a depth beneath the Apennines and a deep Moho beneath the central Alps. Green’s functions from the regionalized 1D velocity models are used to determine source depths and focal mechanisms for 37 events with a magnitude larger than 3.5 by a grid search technique. Our results show that normal and strike-slip faulting source mechanisms dominate the Apenninic belt and that most thrust faulting events occur in the Adriatic Sea and the outer margin of the northern Apennines.

2‐D Crustal Structures Along Wuqing‐Beijing‐Chicheng Deep Seismic Sounding Profile

AbstractIn this paper wide angle reflection and refraction data is used to obtain the 2‐D velocity structure, spatial distribution of faults, and geological structure condition along Wuqing‐Beijing‐Chicheng profile. The results reveal strong vertical and lateral heterogeneities in the crust. The crust has layered features and its thickness gradually increases from 31.0 km in Wuqing to 40.0 km in Yanqing. The strong variation of topography of the crustal interfaces appears around the Beijing area, and the Moho deepens rapidly in the northwest of Beijing area. Five shallow faults in the crust are inferred along this profile, based on the crustal velocity structures and the variations of the crustal interfaces

Crustal structure across the Dabie–Sulu orogenic belt revealed by seismic velocity profiles

The Tan-Lu fault (TLF) separates the Dabie and Sulu orogenic belts, well known for their ultra high pressure (UHP) metamorphic rocks in eastern China. We reinterpret one of the wide-angle seismic profiles traversing the TLF using traveltime tomography methods, and compare the results with the interpretation of three other seismic profiles across the TLF, to enable us to study the relationship of the five tectonic units comprising the North China plate (NCP), the Yangtze plate (YTZP), the TLF, the Dabie–Sulu orogenic belt (DSOB), and the ultra-high pressure metamorphic belt (UHPMB) that is exposed within the DSOB. The results demonstrate that there is strong lateral heterogeneity within the studied area. The TLF's penetrating depth deepens along a S–N direction. In the central section of the fault, the TLF can be traced to the middle crust but in the northern section it penetrates to the Moho. The average P-wave velocity in the UHPMB and DSOB is 0.1–0.4 km s−1 faster than that of the YTZP, NCP and TLF for upper crusts with depths 13 km. The bottom borders of the middle and lower crusts of the UHPMB and DSOB are apparently deeper than the other three tectonic units, and the Moho beneath UHPMB around Dabieshan may be deeper than 40 km. The general similarities of the crustal velocity structures between the Dabie and Sulu UHPMB may suggest a similar exhuming mechanism of UHP metamorphic rocks, before the large-scale TLF strike slip, driven by the subduction of the Yangtze block. The velocity gradient of the crust–mantle transition beneath the Sulu UHPMB implies the intrusion of basaltic melts from the upper mantle.

A crustal seismic velocity model for the UK, Ireland and surrounding seas

SUMMARY A regional model of the 3-D variation in seismic P-wave velocity structure in the crust of NW Europe has been compiled from wide-angle reflection/refraction profiles. Along each 2-D profile a velocity‐depth function has been digitised at 5 km intervals. These 1-D velocity functions were mapped into three dimensions using ordinary kriging with weights determined to minimise the difference between digitised and interpolated values. An analysis of variograms of the digitised data suggested a radial isotropic weighting scheme was most appropriate. Horizontal dimensions of the model cells are optimised at 40 × 40 km and the vertical dimension at 1 km. The resulting model provides a higher resolution image of the 3-D variation in seismic velocity structure of the UK, Ireland and surrounding areas than existing models. The construction of the model through kriging allows the uncertainty in the velocity structure to be assessed. This uncertainty indicates the high density of data required to confidently interpolate the crustal velocity structure, and shows that for this region the velocity is poorly constrained for large areas away from the input data.

Crustal velocity inhomogeneities along the Hirapur–Mandla profile, central India and its tectonic implications

Wide-angle seismic and gravity data across the Narmada-Son lineament (NSL) in central India are analyzed to determine crustal structure, velocity inhomogeneities and hence constrain the tectonics of the lineament. We present the 2-D crustal velocity structure from deep wide-angle reflection data by using a ray-trace inverse approach. The main result of the study is the delineation of fault-bounded horst raised to a subsurface depth (∼1.5 km) and the Moho upwarp beneath the NSL. The crust below the basement consists of three layers with velocities of 6.45–6.7, 6.2–6.5 and 6.7–6.95 km/s and interface depths of about 5.5–8.7, 14–17 and 18–23 km along the profile. The low-velocity (6.2–6.5 km/s) layer goes up to a depth of 5 km and becomes the thickest part (∼13 km), while the overlying high-velocity (6.45–6.7 km/s) layer becomes the thinnest (∼3 km) and upper boundary lies at a depth of ∼1.5 km beneath the NSL. The overall uncertainties of various velocity and boundary nodes are of the order of ±0.12 km/s and ±1.40 km, respectively. The up-lifted crustal block and the up-warping Moho beneath the NSL indicate that the north and south faults bounding the NSL are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust. Gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap. The lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the Narmada south fault during the 1997 Jabalpur earthquake.

Tomographic Determination of the Upper Crustal Structure in Jiashi Strong Earthquake Swarm Region‐Joint Inversion of Explosion and Earthquake Data

AbstractThe Jiashi strong earthquake swarm region is located at the juncture area of three tectonic units, i.e. the Tianshan fold zone, Pamir tectonic arc and Tarim block. Many strong earthquakes occurred recently in this area. In order to obtain the fine crustal velocity structures of this area, the data from a 3‐D seismic transmission experiment and a portable seismograph network acquired in the Jiashi strong earthquake swarm region in 1997 are processed. Joint inversion of explosion and earthquake data is carried out to reconstruct the 3‐D images of velocity perturbations of the upper crust under the seismic array. The temporary seismic array was used to record the seismic waves generated by 8 shots fired at different azimaths, and the additional 20 seismic stations were deployed in this area to record local earthquakes. The results show that the structure of the upper crust in the study area is characterized by obvious inhomogeneities both laterally and vertically. The low P wave velocity zone centered at Saruxi beneath the Jiashi earthquake swarm area is gradually replaced by high P wave velocity with the increase of depth. There is a high P wave velocity anomaly striking NNW at the depth of 12 km under the earthquake swarm area. It extends downwards to the depth of 16 km at least, and is surrounded by relatively low velocities. VP/VS ratios are also characterized by high values in this region. This kind of structural differences may be related to the occurrence of the Jiashi strong earthquake swarm. It can be found from the P wave velocity images at the depth of 16 km that the Jiashi strong earthquake swarm generally occurred in the regions with comparatively high P wave velocity perturbations or in the transition zones between high and low velocity perturbations. The existence of the high P wave velocity anomaly in the crust may play an important role in the seismogenic process.

The Crustal Structure of Western Shandong and the High‐Velocity Body in the Crust

AbstractThe crustal velocity structure of western Shandong is revealed by a 320 km long DSS profile from Liaocheng to Lianyungang. In this area, the upper crust has a two‐layer structure with a total thickness of 18~20 km and a velocity of 5.4~6.2 km/s; the lower crust is also divided into two layers with a total thickness of 13~15 km and a velocity of 6.4~6.7 km/s. The depth of Moho is from 33 km to 35 km. The Pn velocity in this region is 7.9 km/s. The velocity in the crust varies considerably laterally. The average velocity is from 6.2 km/s to 6.5 km/s, which is slightly higher than the normal value. In the crust there are two nearly upright high‐velocity blocks, extending from the lower crust to the upper crust till the bottom of the sedimentary basin. It is possible that a large amount of magma from the upper mantle rose up into the crust, making the local average velocity higher. The high velocity blocks are probably the channels of magma uprising. The study on the crustal structure of western Shandong is of certain significance for investigating the mechanism of disintegration and subsidence of ancient platform.

Strong ground motion simulation of the 1999 Chi‐Chi, Taiwan earthquake from a realistic three‐dimensional source and crustal structure

We simulate the strong ground motion of 1999 Chi‐Chi, Taiwan earthquake (Mw = 7.6) by considering a three‐dimensional source rupture model in a full waveform three‐dimensional wave propagation study. The strong ground motion records during the 1999 Chi‐Chi earthquake show various characteristics at different sites in Taiwan. We adopt a three‐dimensional source model derived from an inversion study with identical path effects as considered in this three‐dimensional forward study. Comparisons between the simulation results and observed waveforms from dense island‐wide strong motion stations demonstrate that the fault geometry, lateral velocity variation, and complex source rupture process greatly influence the distribution of strong ground shaking. The simulation has reproduced the heavy damage area that is mainly concentrated in the hanging wall, especially close to the surface break of the Chelungpu fault. The source directivity effect is also reproduced and shows serious shaking along the northward rupture direction. Low‐velocity material in the shallow part of the Western Plain is found to generate significantly amplified ground motions. In the Central Range, the shaking is relatively weak owing to the energy radiation characteristics of a low‐angle thrust of the Chelungpu faulting system. The wavefield is then amplified by a high‐velocity gradient under the Coastal Range. Our simulation results in the frequency range of 0.01–0.5 Hz give good agreement with the extensive strong motion observations of the Chi‐Chi earthquake. We find that adequate source representation, good three‐dimensional crustal velocity structures, and careful numerical work are necessary to make the ground motion prediction feasible.

Evolution of 3D velocity structure before Dayao M=6.2 and M=6.1 earthquakes in 2003

In this paper the crustal velocity structure are imaged at the annual scale for determining the evolution of 3D velocity structure before the Dayao earthquakes in 2003, and the relation between physical variation of medium and cause of earthquake occurrence is discussed. Checkerboard resolution tests show that the spatial resolution at 15 km depth is approximate 0.6 around Dayao. Error analyses show that the error (Approximate 0.02 km/s) at the focus area is far smaller than the magnitude (approximate 0.15 km/s) of velocity variation. Results of the studies show that a NNW-striking juncture zone of high-and low-velocity is formed before Dayao earthquake, whose strike is basically consistent with that of aftershock distribution of Dayao earthquakes and the focal mechanism, and the hypocenters lie on the high-velocity side of juncture zone. Furthermore, there is a low-velocity body under the hypocenters after Dayao earthquakes. The evolution of velocity structure provides restriction for discussing the circumstances of earthquake gestating and occurring.

Three-dimensional structure across the Tintina strike-slip fault, northern Canadian Cordillera, from seismic refraction and reflection tomography

Accepted 2006 June 5. Received 2006 June 3; in original form 2005 September 4 SUMMARY The development of the northern Canadian Cordillera involved major strike-slip displacement of accreted terranes relative to North America along faults such as the Tintina, which has experienced ∼425 km of dextral motion since the Palaeocene. The SNORE seismic refraction/wideangle reflection experiment was carried out in 1997 as one component of Lithoprobe’s SlaveNorthern Cordillera Lithospheric Evolution (SNORCLE) transect. In addition to four 2-D profiles, two sets of broadside recordings were acquired to image the 3-D structure across the Tintina fault (TF) in areas centred at about 59.5 ◦ N and 62 ◦ N. Simultaneous and independent refraction and reflection traveltime tomography were applied to the combined inline and broadside data set for each region to establish the range of lower crustal velocity, Moho depth and upper mantle velocity structure consistent with the data. Our preferred models are the average of the simultaneous and independent models since they represent the robust features required by the data. The preferred 3-D models are generally consistent with the 2-D models obtained from the inline data in previous independent studies. There are along-strike variations across the TF, perhaps due to the change in strike direction or the amount of motion along the fault in the north compared to the south. In the lower crust, the only correlation with the TF that is required by the data is a 0.1 km s −1 drop in velocity to the southwest of the fault in the northern study area. The absence of a strong correlation with the TF in the lower crust is consistent with the interpreted continuity of lower crustal units across the fault in the SNORCLE reflection data. The Moho is relatively flat throughout the study area, 34‐35 km depth, but there is broad crustal thickening of a few kilometres centred ∼50 km southwest of the TF in the northern and southern study areas. This thickening may be the result of a period when there was a component of compression along the TF. There is strong evidence for a 0.3‐0.4 km s −1 drop in upper mantle velocity to the west of the TF in the south, and weak evidence for ∼0.1 km s −1 drop to the southwest in the north. The upper mantle variations in the south indicate that the TF is the boundary between cooler and/or more refractory ancestral North American mantle to the east and warmer and/or more fertile mantle beneath accreted North America to the west. In the north, the mantle appears to be more intermediate in its properties on both sides of the TF.

Analysis of the crustal velocity structure of the British Isles using teleseismic receiver functions

SUMMARY The onshore crustal and upper mantle velocity structure of the British Isles has been investigated by teleseismic receiver function analysis. The results of the study augment the dense offshore and sparse onshore models of the velocity structure beneath the area. In total almost 1500 receiver functions have been analysed, which have been calculated using teleseismic data from 34 broadband and short-period, three-component seismic recording instruments. The crustal structure has primarily been investigated using 1-D grid search and forward modelling techniques, returning crustal thicknesses, bulk crustal Vp/Vs ratio and velocity-depth models. H − κ stacking reveals crustal thicknesses between 25 and 36 km and Vp/Vs ratios between 1.6 and 1.9. The crustal thicknesses correlate with the results of previous seismic reflection and refraction profiles to within ±2 km. The significant exceptions are the stations close to the Iapetus Suture where the receiver function crustal thicknesses are up to 5 km less than the seismic refraction Moho. This mismatch could be linked to the presence of underplated magmatic material at the base of the crust. 1-D forward modelling has revealed subcrustal structures in northern Scotland. These correlate with results from other UK receiver function studies, and correspond with the Flannan and W-reflectors. The structures are truncated or pinch out before they reach the Midland Valley of Scotland. The isolated subcrustal structure at station GIM on the Isle of Man may be related to the closure of the Iapetus Ocean.