Keith Priestley

Earthquake focal depths, effective elastic thickness, and the strength of the continental lithosphere

Almost all earthquakes on the continents are confined within a crustal layer that varies in thickness (Ts) from about 10 to 40 km, and are not in the mantle. Variations in Ts correlate with variations in the effective elastic thickness ( Te), both of them having similar values, although Te is usually the smaller of the two. These observations suggest that the lower crust, at least in some places, is stronger than the mantle beneath the Moho, contrary to most models of continental rheology. Thus the strength of the continental lithosphere is likely to be contained within the seismogenic layer, variations in the thickness of this strong layer determining the heights of the mountain ranges it can support. The aseismic nature of the continental mantle and the lower crustal seismicity beneath some shields are probably related to their water contents.

Global azimuthal seismic anisotropy and the unique plate-motion deformation of Australia

Differences in the thickness of the high-velocity lid underlying continents as imaged by seismic tomography, have fuelled a long debate on the origin of the ‘roots’ of continents. Some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth, with horizontally polarized shear waves travelling faster than vertically polarized ones. This azimuthally averaged anisotropy could arise from present-day deformation at the base of the plate, as has been found for shallower depths beneath ocean basins. Such deformation would also produce significant azimuthal variation, owing to the preferred alignment of highly anisotropic minerals. Here we report global observations of surface-wave azimuthal anisotropy, which indicate that only the continental portion of the Australian plate displays significant azimuthal anisotropy and strong correlation with present-day plate motion in the depth range 175–300 km. Beneath other continents, azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans. We infer that the fast-moving Australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy. Simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents.

Structure of the crust and uppermost mantle of Iceland from a combined seismic and gravity study

Abstract We present a map of the depth to the base of the upper crust and the total crustal thickness across Iceland constrained by seismic refraction results, receiver function analysis and gravity modelling. Upper crustal thicknesses (as defined by O.G. Flovenz, J. Geophys. 47 (1980) 211–220) lie in the range of approximately 2–11 km, with the thinnest upper crust below active and extinct central volcanoes and the thickest upper crust close to the flanks of the rift zones. The thickest crust (40–41 km) lies above the centre of the Iceland mantle plume, where active upwelling and high mantle temperatures enhance melt production. Thick crust (∼35 km) is also found in eastern Iceland, between the current plume centre and the Faroe–Iceland Ridge. Elsewhere, the crust thins away from the plume centre. The thinnest crust (≤20 km) is found in the active rift in the northern part of the Northern Volcanic Zone, where melt production has been affected by a ridge jump, and in the far southwest of Iceland. The uppermost mantle below Iceland is characterised by reduced densities below the rift zones, suggesting higher mantle temperatures and the possible presence of partial melt in these regions.

Mapping the Hawaiian plume conduit with converted seismic waves

The volcanic edifice of the Hawaiian islands and seamounts, as well as the surrounding area of shallow sea floor known as the Hawaiian swell, are believed to result from the passage of the oceanic lithosphere over a mantle hotspot. Although geochemical and gravity observations indicate the existence of a mantle thermal plume beneath Hawaii, no direct seismic evidence for such a plume in the upper mantle has yet been found. Here we present an analysis of compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations on the Hawaiian islands, that indicate a zone of very low shear-wave velocity (< 4 km s -1) starting at 130–140 km depth beneath the central part of the island of Hawaii and extending deeper into the upper mantle. We also find that the upper-mantle transition zone (410–660 km depth) appears to be thinned by up to 40–50 km to the south-southwest of the island of Hawaii. We interpret these observations as localized effects of the Hawaiian plume conduit in the asthenosphere and mantle transition zone with excess temperature of ∼300 °C. Large variations in the transition-zone thickness suggest a lower-mantle origin of the Hawaiian plume similar to the Iceland plume, but our results indicate a 100 °C higher temperature for the Hawaiian plume.

New views on the structure and rheology of the lithosphere

Over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength, structure and evolution that produce dramatic contrasts between the geological histories of oceans, ancient shields and young orogenic belts. Those developments involve a wide range of observations from a great diversity of geological, geophysical and geochemical disciplines that, none the less, provide a mutually consistent and coherent overall picture. This paper summarizes, in one place, the essential stages in the evolution of the relevant ideas and observations that have led to this situation.

Rayleigh wave phase velocity maps of Tibet and the surrounding regions from ambient seismic noise tomography

Ambient noise tomography is applied to the significant data resources now available across Tibet and surrounding regions to produce Rayleigh wave phase speed maps at periods between 6 and 50 s. Data resources include the permanent Federation of Digital Seismographic Networks, five temporary U.S. Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiments in and around Tibet, and Chinese provincial networks surrounding Tibet from 2003 to 2009, totaling ∼600 stations and ∼150,000 interstation paths. With such a heterogeneous data set, data quality control is of utmost importance. We apply conservative data quality control criteria to accept between ∼5000 and ∼45,000 measurements as a function of period, which produce a lateral resolution between 100 and 200 km across most of the Tibetan Plateau and adjacent regions to the east. Misfits to the accepted measurements among PASSCAL stations and among Chinese stations are similar, with a standard deviation of ∼1.7 s, which indicates that the final dispersion measurements from Chinese and PASSCAL stations are of similar quality. Phase velocities across the Tibetan Plateau are lower, on average, than those in the surrounding nonbasin regions. Phase velocities in northern Tibet are lower than those in southern Tibet, perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained. At short periods ( 20 s), very high velocities are imaged in the Tarim Basin, the Ordos Block, and the Sichuan Basin. These phase velocity dispersion maps provide information needed to construct a 3-D shear velocity model of the crust across the Tibetan Plateau and surrounding regions.

The 2003 Bam (Iran) earthquake: Rupture of a blind strike-slip fault

A magnitude 6.5 earthquake devastated the town of Bam in southeast Iran on 26 December 2003. Surface displacements and decorrelation effects, mapped using Envisat radar data, reveal that over 2 m of slip occurred at depth on a fault that had not previously been identified. It is common for earthquakes to occur on blind faults which, despite their name, usually produce long-term surface effects by which their existence may be recognised. However, in this case there is a complete absence of morphological features associated with the seismogenic fault that destroyed Bam.

Metastability, mechanical strength, and the support of mountain belts

Exhumed high-pressure rocks from the Caledonian root zone in Norway provide analogues for processes occurring today under southern Tibet, allowing large-scale geophysical observations, from gravity and earthquakes, to be linked with the mechanical properties of metastable rocks under high mountains. Metastability is essential for the survival of thick mountain roots and, hence, of high mountains and is in turn controlled by water. If water is absent, dry granulite, formed from earlier melting episodes, is both stable and strong and likely to survive in the eclogite stability field of deep root zones for hundreds of million years. But if hydrous fluid is introduced, the transformation of granulite to eclogite is relatively rapid and accompanied by a dramatic loss of strength. In Norway, this transformation was initiated by water infiltration along fractures formed by earthquakes. The same process, as marked by deep earthquakes, may be occurring today beneath southern Tibet.

Lithospheric structure of the Aegean obtained from P and S receiver functions

Combined P and S receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the Aegean region are used to map the geometry of the subducted African and the overriding Aegean plates. We image the Moho of the subducting African plate at depths ranging from 40 km beneath southern Crete and the western Peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern Greece. However, the dip of the Moho of the subducting African plate is shallower beneath the Peloponnesus than beneath Crete and Rhodes and flattens out beneath the northern Aegean. Observed P-to-S conversions at stations located in the forearc indicate a reversed velocity contrast at the Moho boundary of the Aegean plate, whereas this boundary is observed as a normal velocity contrast by the S-to-P conversions. Our modeling suggests that the presence of a large amount of serpentinite (more than 30%) in the forearc mantle wedge, which generally occurs in the subduction zones, may be the reason for the reverse sign of the P-to-S conversion coefficient. Moho depths for the Aegean plate show that the southern part of the Aegean (crustal thickness of 20–22 km) has been strongly influenced by extension, while the northern Aegean Sea, which at present undergoes the highest crustal deformation, shows a relatively thicker crust (25–28 km). This may imply a recent initiation of the present kinematics in the Aegean. Western Greece (crustal thickness of 32–40 km) is unaffected by the recent extension but underwent crustal thickening during the Hellenides Mountains building event. The depths of the Aegean Moho beneath the margin of the Peloponnesus and Crete (25–28 and 25–33 km, respectively) show that these areas are also likely to be affected by the Aegean extension, even though the Cyclades (crustal thickness of 26–30 km) were not significantly involved in this episode. The Aegean lithosphere-asthenosphere boundary (LAB) mapped with S receiver functions is about 150 km deep beneath mainland Greece, whereas the LAB of the subducted African plate dips from 100 km beneath Crete and the southern Aegean Sea to about 225 km under the volcanic arc. This implies a thickness of 60–65 km for the subducted African lithosphere, suggesting that the Aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the Cyclades.

Crustal shear velocity structure of the south Indian shield

[1] The south Indian shield is a collage of Precambrian terrains gathered around and in part derived from the Archean-age Dharwar craton. We operated seven broadband seismographs on the shield along a N-S corridor from Nanded (NND) to Bangalore (BGL) and used data from these to determine the seismic characteristics of this part of the shield. Surface wave dispersion and receiver function data from these sites and the Geoscope station at Hyderabad (HYB) give the shear wave velocity structure of the crust along this 600 km long transect. Inversion of Rayleigh wave phase velocity measured along the profile shows that the crust has an average thickness of 35 km and consists of a 3.66 km s � 1 , 12 km thick layer overlying a 3.81 km s � 1 , 23 km thick lower crust. At all sites, the receiver functions are extremely simple, indicating that the crust beneath each site is also simple with no significant intracrustal discontinuities. Joint inversion of the receiver function and surface wave phase velocity data shows the seismic characteristics of this part of the Dharwar crust to be remarkably uniform throughout and that it varies within fairly narrow bounds: crustal thickness (35 ± 2 km), average shear wave speed (3.79 ± 0.09 km s � 1 ), and Vp/Vs ratio (1.746 ± 0.014). There is no evidence for a high velocity basal layer in the receiver function crustal images of the central Dharwar craton, suggesting that there is no seismically distinct layer of mafic cumulates overlying the Moho and implying that the base of the Dharwar crust has remained fairly refractory since its cratonization. INDEX TERMS: 7203 Seismology: Body wave propagation; 7205 Seismology: Continental crust (1242); 7255 Seismology: Surface waves and free oscillations; KEYWORDS: continental crust, Archean crust, receiver function, Indian shield