High-resolution structures of the Landers fault zone inferred from aftershock waveform data

High-frequency body waves recorded by a temporary seismic array across the surface rupture trace of the 1992 Landers, California, earthquake were used to determine fault-zone structures down to the seismogenic depth. We first developed a technique to use generalized ray theory to compute synthetic seismograms for arbitrarily oriented tabular low-velocity fault-zone models. We then generated synthetic waveform record sections of a linear array across a vertical fault zone. They show that both arrival times and waveforms of P and S waves vary systematically across the fault due to transmissions and reflections from boundaries of the low-velocity fault zone. The waveform characteristics and arrival-time patterns in the record sections allow us to locate the boundaries of the fault zone and to determine its P - and S -wave velocities independently as well as its depth extent. Therefore, the trade-off between the fault-zone width and velocities can be avoided. Applying the method to the Landers waveform data reveals a low-velocity zone with a width of 270–360 m and a 35–60 per cent reduction in P and S velocities relative to the host rock. The analysis suggests that the low-velocity zone extends to a depth of ∼7 km. The western boundary of the low-velocity zone coincides with the observed main surface rupture trace.     

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

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 V_p / V_s ratio and velocity-depth models. H −κ stacking reveals crustal thicknesses between 25 and 36 km and V_p / V_s 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.     

Evidence for underthrusting beneath the Queen Charlotte Margin, British Columbia, from teleseismic receiver function analysis

The Queen Charlotte Fault zone is the transpressive boundary between the North America and Pacific Plates along the northwestern margin of British Columbia. Two models have been suggested for the accommodation of the ∼20 mm yr⁻¹ of convergence along the fault boundary: (1) underthrusting; (2) internal crustal deformation. Strong evidence supporting an underthrusting model is provided by a detailed teleseismic receiver function analysis that defines the underthrusting slab. Forward and inverse modelling techniques were applied to receiver function data calculated at two permanent and four temporary seismic stations within the Queen Charlotte Islands. The modelling reveals a ∼10 km thick low-velocity zone dipping eastward at 28° interpreted to be underthrusting oceanic crust. The oceanic crust is located beneath a thin (28 km) eastward thickening (10°) continental crust.     

Lithospheric structure of the Arabian and Eurasian collision zone in eastern Turkey from S-wave receiver functions

Crustal and upper-mantle seismic discontinuities beneath eastern Turkey are imaged using teleseismic S -to- P converted phases. Three crustal phases are observed: the Moho with depth ranging between 30 and 55 km, indicating variable tectonic regimes within this continental collision zone; an upper-crustal discontinuity at approximately 10 km depth; and various crustal low-velocity zones, possibly associated with recent Quaternary volcanism. Imaging of the upper mantle is complicated by the 3-D geometry of the region, in particular due to the Bitlis–Zagros suture zone. However, several upper-mantle S -to- P converted phase are identified as being the signature of the lithosphere–asthenosphere boundary (LAB). The inferred LAB for the Eastern Anatolian Accretionary Complex indicates that eastern Turkey has an anomalously thin (between ∼60 and 80 km) lithosphere which is consistent with an oceanic slab detachment model. The observed LAB phases for the Arabian shield and Iranian plateau indicate that lithospheric thickness for these stable regions is on the order of 100 to 125 km thick, which is typical of continental margins.     

A global study of S-to-P and P-to-S conversions from the upper mantle transition zone

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Long-period data of the Global Digital Seismograph Network (GDSN) recorded over the three-year period from 1984 to 1986 were studied for the occurrence of S-P and P-S conversions from the upper mantle transition zone that appear as precursors to teleseismic S arrivals. Conversions of this type were identified on a large number of single-station records. Simple stacking of many records enhanced the appearance of converted phases and demonstrated that no major lateral variations in the nature of the transition zone exist between various tectonic regions. S-P and P-S conversions from the 400 km discontinuity were best observed at distances between 70 and 85 while conversions from the 670 km discontinuity showed up best at distances beyond 87. The analysis of published source mechanisms and comparison with synthetic seismograms suggests that the appearance of converted phases is primarily governed by the earthquake radiation pattern. Phases that have undergone S-P conversions beneath the receiver are best observed from dip-slip events that radiate strong SV - and weak P -waves towards the station. P-S conversions beneath the source area, on the other hand, are frequently observed from events that radiate strong P and little SV energy towards the station, and also from some strike-slip events. Comparison of observed with synthetic seismograms suggests that the PREM model of Dziewonski & Anderson (1981) explains most of the observations. Observed S-P and P-S conversions from the 670 km discontinuity, however, often have larger amplitudes than in the synthetics. Constructive interference of converted waves with the P -wave coda, source radiation effects and a velocity contrast across the 670 km discontinuity which is higher than in PREM may all contribute to the discrepancy.     

Surface-wave propagation across the Mexican Volcanic Belt and the origin of the long-period seismic-wave amplification in the Valley of Mexico

A network of nine broad-band seismographs was operated from March to May 1994 to study the propagation of seismic waves across the Mexican Volcanic Belt (MVB) in the region of the Valley of Mexico. Analysis of the data from the network reveals an amplification of seismic waves in a wide period band al the stations situated in the southern part of the MVB.
The group velocities of the fundamental mode of the Rayleigh wave in the period range 2–13 s are found to be lower in the southern part of the MVB than in its northern part and in the region south of the MVB. The inversion of dispersion curves shows that the difference in group velocities is due to the presence of a superficial low-velocity layer (with an average S -wave velocity of 1.7 km s^-1 and an average thickness of 2 km) beneath the southern part of the MVB. This low-velocity zone is associated with the region of active volcanism.
Numerical simulations show that this superficial low-velocity layer causes a regional amplification of 8–10 s period signals, which is of the same order as the amplification measured from the data. This layer also increases the signal duration significantly because of the dispersion of the surface waves. These results confirm the hypothesis of Singh et al. (1995), who suggested that the regional amplification observed in the Valley of Mexico is due to the anomalously low shear-wave velocity of the shallow volcanic rocks in the southern MVB     

Amplification of ground motion and waveform complexity in fault zones: examples from the San Andreas and Calaveras Faults

Summary. P -wave seismograms at ranges less than 10 km are synthesized by asymptotic ray theory and by summation of Gaussian beams for point sources located in a low-velocity wedge surrounding a fault. The computations are performed using models of the wedge inferred from the analysis of reflection and refraction experiments across the San Andreas and Hayward-Calaveras faults. Calculations in these models show that the 10–20Hz vertical displacements of earthquakes located at 3–10km depth are amplified by up to an order of magnitude in a 1–2km wide region centred on the fault trace compared to displacements predicted by laterally homogeneous models of the crust. This amplification is not cancelled by high attentuation in the fault zone and compensates for the reduction in amplitudes directly above the source predicted from the radiation pattern of a strike-slip earthquake. Depending on the source depth of the earthquake and the structure and velocity contrast of the wedge, multiple triplications in the travel-time curve of direct P - and S -waves will occur at stations in the fault zone. A wedge model successfully predicts the triplications observed in the P waveforms of aftershocks of the Coyote Lake earthquake recorded in the fault zone, showing that body waves from microearthquakes can be used to determine the three-dimensional velocity structure of the fault zone. The amplification, waveform complexity, and distortion of ray paths introduced by the low- velocity wedge suggest that its effects should be included in the interpretation of strong ground motions and travel times observed in the fault zone. For realistic models of the wedge, asymptotically approximate methods of calculating the body waveforms are strictly valid for frequencies greater than 20Hz. Numerical methods may be necessary to calculate accurately the wavefield at lower frequencies.     

Oceanic crustal structure—Mid-Atlantic Ridge at 45° N

Summary. We present a velocity—depth model for the crust beneath the Mid-Atlantic Ridge at 45° N which is derived from a comparison of waveforms corresponding to observed and synthetic seismograms. The model which best fits the observations includes a high-velocity layer at the base of the crust (layer 3B) and a velocity gradient in the upper mantle. These results are in agreement with other recent seismic studies on the Mid-Atlantic Ridge and indicate that the velocity structure is more complex than that obtained from travel-time analysis. There is no evidence for a low-velocity zone at the base of the crust.     

Forward modelling receiver functions for crustal structure beneath station TBZ (Trabzon, Turkey)

We use teleseismic three-component digital data from the Trabzon, Turkey broadband seismic station TBZ to model the crustal structure by the receiver function method. The station is located at a structural transition from continental northeastern Anatolia to the oceanic Black Sea basin. Rocks in the region are of volcanic origin covered by young sediments. By forward modelling the radial receiver functions, we construct 1-D crustal shear velocity models that include a lower crustal low-velocity zone, indicating a partial melt mechanism which may be the source of surfacing magmatic rocks and regional volcanism. Within the top 5 km, velocities increase sharply from about 1.5 to 3.5 km s⁻¹. Such near-surface low velocities are caused by sedimentation, extending from the Black Sea basin. Velocities at around 20 km depth have mantle-like values (about 4.25 km s⁻¹ ), which easily correlate to magmatic rocks cropping out on the surface. At 25 km depth there is a thin low-velocity layer of about 4.0 km s⁻¹. The average Moho velocity is about 4.6 km s⁻¹, and its depth changes from 32 to 40 km. Arrivals on the tangential components indicate that the Moho discontinuity dips approximately southwards, in agreement with the crustal thickening to the south. We searched for the solution of receiver functions around the regional surface wave group velocity inversion results, which helped alleviate the multiple solution problem frequently encountered in receiver function modelling.
Station TBZ is a recently deployed broadband seismic station, and the aim of this study is to report on the analysis of new receiver function data. The analysis of new data in such a structurally complex region provides constraining starting models for future structural studies in the region.     

Upper mantle structure beneath the Mid-Atlantic Ridge north of the Azores based on observations of compressional wave

Summary. Four seismic refraction profiles have been interpreted which serve to indicate the structure of the lithosphere near the Mid-Atlantic Ridge close to the Azores. An east–west profile which crosses the ridge axis yields a crustal structure. Although energy is propagated across the ridge axis within the crust the axial region marks a clear barrier to propagation within the mantle. A profile parallel to the axis (4 my isochron) shows, below a 7.6 km/s layer, a low-velocity zone underlain by an 8.3 km/s refractor 9 km below the sea bed. On profies normal to the ridge axis higher velocities, which are observed on lines shot towards the ridge, can be attributed to this refractor if it has a dip of several degrees away from the ridge. On another profile parallel to the axis (9 my isochron) a velocity of about 8.3 km/s is only found to exist much deeper at about 30 km depth. These observations are interpreted in the light of seismic refraction results recently obtained by Lewis & Snydsman and of quantitative petrological models, such as that of Bottinga & Allègre. A velocity model based on Bottinga & Allègre's model allows us to understand our results qualitatively. In particular the two 8.3 km/s refractors at 9 and 30 km depth correspond to two different residual peridotite layers. The upper layer contains 1.5–2 wt per cent water and as the lithosphere moves away from the ridge axis the temperature in this layer becomes low enough to start hydration reactions. These cause the low-velocity zone observed at 4 my and the total disappearance of the shallow level refractor before 9 my.     

Crustal structure from an OBS survey of the Nootka fault zone off western Canada

Summary. The Nootka fault zone is the boundary between the small Explorer and Juan de Fuca plates which are situated between the America and Pacific plates off western Canada. To investigate the crustal structure in the region, three explosive/large airgun refraction lines were shot into three ocean bottom seismometers (OBSs) with three-component geophone assemblies. In this phase of the study, P -wave velocity—depth models are interpreted by comparison of the travel time and amplitude characteristics of the observed data with theoretical seismograms computed using a WKBJ algorithm. The interpretation gives relatively consistent results for the upper crust. However, the structure of the lower crust is significantly different among the various profiles. Upper mantle velocities range from 7.5 to 8.3 kms⁻¹ and the sub-bottom crustal thickness vanes from 6.4 to 11 km. Nevertheless, these seismic models are consistent in general terms with oceanic crustal models represented by ophiolite complexes. Some aspects of the differences among profiles can be explained by consideration of a recent tectonic model for the development of the fault zone. This requires, within a 1 Myr time interval, variations in the process of crustal formation at the ridge, crustal 'maturing', or both. The abnormally thick crust near a spreading centre may result in part from the complex interaction of the Juan de Fuca and Explorer plates with the larger and older America and Pacific plates. Upper mantle velocity variations are consistent with the concept of velocity anisotropy. The different record sections show that seismic energy is attenuated for ray paths traversing the Nootka fault zone.     

Massif Central (France): new constraints on the geodynamical evolution from teleseismic tomography

The Massif Central, the most significant geomorphological unit of the Hercynian belt in France, is characterized by graben structures which are part of the European Cenozoic Rift System (ECRIS) and also by distinct volcanic episodes, the most recent dated at 20 Ma to 4000 years BP. In order to study the lithosphere-asthenosphere system beneath this volcanic area, we performed a teleseismic field experiment.
During a six-month period, a joint French-German team operated a network of 79 mobile short-period seismic stations in addition to the 14 permanent stations. Inversion of P -wave traveltime residuals of teleseismic events recorded by this dense array yielded a detailed image of the 3-D velocity structure beneath the Massif Central down to 180 km depth. The upper 60 km of the lithosphere displays strong lateral heterogeneities and shows a remarkable correlation between the volcanic provinces and the negative velocity perturbations. The 3-D model reveals two channels of low velocities, interpreted as the remaining thermal signature of magma ascent following large lithospheric fractures inherited from Hercynian time and reactivated during Oligocene times. The teleseismic inversion model yields no indication of a low-velocity zone in the mantle associated with the graben structures proper. The observation of smaller velocity perturbations and a change in the shape of the velocity pattern in the 60–100 km depth range indicates a smooth transition from the lithosphere to the asthenosphere, thus giving an idea of the lithosphere thickness. A broad volume of low velocities having a diameter of about 200 km from 100 km depth to the bottom of the model is present beneath the Massif Central. This body is likely to be the source responsible for the volcanism. It could be interpreted as the top of a plume-type structure which is now in its cooling phase.     

Lithospheric structure beneath the Kenya Dome

Summary. The existence of an anomalously low-velocity, low-density zone within the upper mantle beneath the Kenya Dome has been deduced on the basis of previous gravity and seismic studies. This paper describes an experiment to measure teleseismic delay times across the Gregory Rift near the equator and along a SE radius of the Kenya Dome. The delay times have been determined with good relative accuracy and provide further independent evidence for the existence of the anomalous zone. The pattern of delay times along the two profiles and at other stations indicates that the zone thins rapidly to the SE away from the rift axis, mirroring the attenuation observed, from Kaptagat, for the same zone to the NW. The trend is for the thinning to become very much less rapid with distance, but there is also clear evidence for localized thickening of the zone under the Kilimanjaro–Chyulu volcanic area.
Significantly smaller delay times are measured at the centre of the rift than at the edges. This is shown to indicate that the anomalous zone penetrates the crust to form an intrusion of relatively high-velocity material along the rift axis. The clear correlation of the delay time low with the axial Bouguer high indicates that they are both manifestations of the same underlying structure. Thus the delay time results provide independent confirmation of the existence of the axial intrusion previously inferred from gravity data. The width of this intrusion at the normal base of the crust is well defined by the data as 30 km.     

Waveform modelling of teleseismic S, Sp, SsPmP, and shear-coupled PL waves for crust- and upper-mantle velocity structure beneath Africa

We describe a waveform modelling technique and demonstrate its application to determine the crust- and upper-mantle velocity structure beneath Africa. Our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a Very Fast Simulated Annealing (VFSA). We match the S , Sp, SsPmP and shear-coupled PL phases in seismograms of deep (200–800 km), moderate-to-large magnitude (5.5–7.0) earthquakes recorded teleseismically at permanent broad-band seismic stations in Africa. Using our technique we produce P - and S -wave velocity models of crust and upper mantle beneath Africa. Additionally, our use of the shear-coupled PL phase, wherever observed, improves the constraints for lower crust- and upper-mantle velocity structure beneath the corresponding seismic stations. Our technique retains the advantages of receiver function methods, uses a different part of the seismogram, is sensitive to both P - and S -wave velocities directly, and obtains helpful constraints in model parameters in the vicinity of the Moho. The resulting range of crustal thicknesses beneath Africa (21–46 km) indicates that the crust is thicker in south Africa, thinner in east Africa and intermediate in north and west Africa. Crustal P - (4.7–8 km s⁻¹) and S -wave velocities (2.5–4.7  km s⁻¹) obtained in this study show that in some parts of the models, these are slower in east Africa and faster in north, west and south Africa. Anomalous crustal low-velocity zones are also observed in the models for seismic stations in the cratonic regions of north, west and south Africa. Overall, the results of our study are consistent with earlier models and regional tectonics of Africa.     

Crustal structure beneath exposed accreted terranes of Southern Alaska

Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8–9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.     

Joint inversion of active and passive seismic data in Central Java

Seismic and volcanic activities in Central Java, Indonesia, the area of interest of this study, are directly or indirectly related to the subduction of the Indo-Australian plate. In the framework of the MERapi AMphibious EXperiments (MERAMEX), a network consisting of about 130 seismographic stations was installed onshore and offshore in Central Java and operated for more than 150 days. In addition, 3-D active seismic experiments were carried out offshore. In this paper, we present the results of processing combined active and passive seismic data, which contain traveltimes from 292 local earthquakes and additional airgun shots along three offshore profiles. The inversion was performed using the updated LOTOS-06 code that allows processing for active and passive source data. The joint inversion of the active and passive data set considerably improves the resolution of the upper crust, especially in the offshore area in comparison to only passive data. The inversion results are verified using a series of synthetic tests. The resulting images show an exceptionally strong low-velocity anomaly (−30 per cent) in the backarc crust northward of the active volcanoes. In the upper mantle beneath the volcanoes, we observe a low-velocity anomaly inclined towards the slab, which probably reflects the paths of fluids and partially melted materials in the mantle wedge. The crust in the forearc appears to be strongly heterogeneous. The onshore part consists of two high-velocity blocks separated by a narrow low-velocity anomaly, which can be interpreted as a weakened contact zone between two rigid crustal bodies. The recent Java M _w= 6.3 earthquake (2006/05/26-UTC) occurred at the lower edge of this zone. Its focal strike slip mechanism is consistent with the orientation of this contact.     

Crustal structure of Iraq from receiver functions and surface wave dispersion: implications for understanding the deformation history of the Arabian–Eurasian collision

We report the crustal structure for two locations in Iraq estimated by joint inversion of P -wave receiver functions (RFs) and surface (Rayleigh) wave group velocity dispersion. RFs were computed from teleseismic recordings at two temporary broad-band seismic stations located in Mosul (MSL) in the Zagros Fold Belt and Baghdad (BHD) in the Mesopotamian Foredeep. Group velocity dispersion curves at the sites were derived from continental-scale tomography. The inversion results show that the crustal thicknesses are 39 km at MSL and 43 km at BHD. We observe a strong Ps _Moho at BHD consistent with a sharp Moho discontinuity. However, at MSL we observe a weak Ps _Moho suggesting a transitional Moho where crustal thickening is likely to be occurring in the deep crust. Both sites reveal low velocity surface layers consistent with sedimentary thickness of about 3 km at station MSL and 7 km at BHD and agreeing well with the previous reports. Ignoring the sediments, the crystalline crustal velocities and thicknesses are remarkably similar at both stations. The similarity of crustal structure suggests that the crust of the northeastern proto-Arabian Platform was uniform before subsidence and deposition of the sediments in the Cenozoic. If crystalline crustal structure is uniform across the northern Arabian Platform then crustal thickness variations in the Zagros Fold Belt and Thrust Zone should reveal the history of deformation and crustal shortening in the Arabian–Eurasian collision zone and not reflect pre-existing crustal thickness variations in the Arabian Plate.     

Seismic Delays in the Eastern Caribbean

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The intervals between the arrivals of the same body wave phases from distant earthquakes at a close network of stations are compared with those expected from the known surface speeds of the phases. The arrival at the station on the island of Barbados is shown to be
2.94 = 0.34 s
later than expected relative to the other stations.
This is believed to be due to differences in crustal structure associated with the belt of negative gravity anomalies east of the West Indian arc.
A crustal section consistent with the seismic delays, gravity anomalies and seismic refraction profiles is presented.     

Crustal thicknesses in Fennoscandia

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As a supplement to seismic profiling surveys, crustal thicknesses have been estimated for 11 Fennoscandian seismograph stations equipped with three-component long period instruments, using the so-called spectral ratio technique of Phinney. The largest Moho depths, of the order of 45 km, were found for stations located in the north-east areas of Norway and Sweden and in Finland, with a local maximum in the Bothnian Bay. The coastal area of south-east Norway and Zealand, Denmark exhibit crustal thicknesses in the range 28–33 km. The agreement between our results and those obtained by conventional refraction profiling is good, when this comparison is restricted to profiles of lengths 300 km or more, and when the associated crustal thickness estimate is averaged over the central parts of the profiles in question. Also, a comparison between our results and other available geophysical information gives that the oldest tectonic provinces of the Baltic Shield also are characterized by relatively modest heat flow, and exhibit the greatest crustal thicknesses. Post-glacial uplift data and large wavelength free air gravity data appear to be uncorrelated with crustal thickness. The same partly applies to Bouguer gravity anomalies, thus implying that the isostatic compensation mechanism in Fennoscandia is of both Airy and Pratt type.     

Slab low-velocity layer in the eastern Aleutian subduction zone

Local earthquakes in the vicinity of the Alaskan Peninsula's Shumagin Islands often produce arrivals between the main P and S arrivals not predicted by standard traveltime tables. Based on traveltime and polarization, these anomalous arrivals appear to be from P -to- S conversions at the surface of the subducted Pacific Plate beneath the recording stations. The P -to- S conversion occurs at the top of a low-velocity layer which extends to at least 150 km depth and is 8 ˜ 2 per cent slower than the overlying mantle. The slab is ˜ 7 per cent faster than the mantle. The low-velocity layer contains the foci of the earthquakes in the upper plane of the double seismic zone and confines PS ray paths to lie within it. These observations indicate that layered structures persist to positions well past the surface location of the volcanic front. Reactions forming high-pressure minerals do not yield slab-like velocities until beyond the point that subduction zone magma genesis occurs. If the subducted oceanic crust forms the layer, it is subducted essentially intact.