Seismic discontinuities in the lithospheric mantle at the Dead Sea Transform

SUMMARY The Dead Sea Transform (DST) was formed in the mid-Cenozoic, about 18 Myr ago, as a result of the breakaway of the Arabian plate from the African plate. Higher resolution information about the sub-Moho structure is still sparse in this region. Here, we study seismic discontinuities in the mantle lithosphere in the region of the DST using a modified version of the P- and S-receiver function methods. We use open data from permanent and temporary seismic stations. The results are displayed in a number of depth profiles through the study area. The Moho is observed on both sides of the transform at nearly 40 km depth by S-to-p and in P-to-s converted signals. The lithosphere–asthenosphere boundary (LAB) on the eastern side of the DST is observed near 180–200 km depth, which is according to our knowledge the first LAB observation at that depth in this region. This observation could lead to the conclusion that the thickness of the Arabian lithosphere east of the DST is likely cratonic. In addition, we observe in the entire area a negative velocity gradient at 60–80 km depth, which was previously interpreted as LAB.

New insights into the structural elements of the upper mantle beneath the contiguous United States from S-to-P converted seismic waves

Summary The S-receiver function (SRF) technique is an effective tool to study seismic discontinuities in the upper mantle such as the mid-lithospheric discontinuity (MLD) and the lithosphere-asthenosphere boundary (LAB). This technique uses deconvolution and aligns traces along the maximum of the deconvolved SV signal. Both of these steps lead to acausal signals, which may cause interference with real signals from below the Moho. Here we go back to the origin of the S-receiver function method and process S-to-P converted waves using S-onset times as the reference time and waveform summation without any filter like deconvolution or bandpass. We apply this ‘causal’ SRF (C-SRF) method to data of the USArray and obtain partially different results in comparison with previous studies using the traditional acausal SRF method. The new method does not confirm the existence of an MLD beneath large regions of the cratonic US. The shallow LAB in the western US is, however, confirmed with the new method. The elimination of the MLD signal below much of the cratonic US reveals lower amplitude but highly significant phases that previously had been overwhelmed by the apparent MLD signals. Along the northern part of the area with data coverage we see relics of Archean or younger north-west directed low-angle subduction below the entire Superior Craton. In the cratonic part of the US we see indications of the cratonic LAB near 200 km depth. In the Gulf Coast of the southern US we image relics of southeast directed shallow subduction, likely of mid-Paleozoic age.

Lithospheric structure beneath the central and western North China Craton and adjacent regions from S-receiver function imaging

Summary To elucidate the nature and extent of the lithospheric modification in the central and western North China Craton (NCC) and adjacent regions, we used the wave equation–based migration technique of S-receiver function on teleseismic data collected from 314 broadband stations in this region to image the lithospheric structure. Incorporating data from previous lithospheric structure studies, we obtained unprecedented high-resolution depth maps of the lithosphere–asthenosphere boundary (LAB) and mid-lithospheric discontinuity (MLD) in the NCC. Our results show more detailed variations of the lithospheric thickness in the central and western NCC and adjacent regions, which ranges from 100 to >170 km, in marked contrast to the thinned lithosphere (60–100 km) in the eastern NCC. Despite its generally thick lithosphere (>130 km), the Ordos Block shows a concordant N–S difference from the surface to deep lithosphere with a boundary at the latitude of 37–38°N. The central NCC is laterally heterogeneous in the lithospheric structure, and the thick lithosphere (∼160 km) in the south is interpreted as a remnant cratonic mantle root. The central Qinling Orogenic Belt preserves a thick lithosphere (∼150 km), which may block the asthenospheric flow driven by extrusion of the Tibetan Plateau to the west of the NCC. Moreover, a negative MLD is widely identified at the depth of 80–110 km within the thick lithosphere, which corroborates the global existence of the MLD in continental regions. The consistence in the depth of the MLD and the shallow LAB in the eastern NCC supports the conjecture that the MLD may have played an important role in the lithospheric modification of the eastern NCC.

Data-oriented constraint on the interpretation of S receiver function and its application to observations of seismic discontinuities in the lithosphere–asthenosphere system

SUMMARY This paper aims to improve the robustness of interpretation in the S receiver function (SRF), a technique commonly used to retrieve forward scattering of S-to-P converted waves (Sdp) originated from the lithosphere–asthenosphere system (LAS) beneath the stations. Although the SRF does not suffer interferences from backward scattering waves such as the first multiples from the Moho, one major drawback in the method is that Sdp phases can interfere with P coda waves and it is conceivable that these signal-generated noise may be misinterpreted as Sdp phase from the LAS beneath seismic stations. Through systematic analysis of full-waveform synthetics and SRFs from catalogued source parameters, we find that the strong P coda waves before the S wave in the longitudinal-component waveforms result in unwanted signal-generated noise before the S wave in the synthetic SRFs. If the mean amplitude of SRFs after the S wave is large, dubious signal-generated noise before the S arrival are strong as well. In this study, we honor the level of these unwanted signal-generated noise and devise data-oriented screening criteria to minimize the interference between P coda waves and genuine S-to-P converted waves. The first criterion is LQR, a direct measure of the amplitude ratio between longitudinal P coda waves and radial S wave in the waveform data. The second criterion is AMP, the amplitude of SRFs after the S arrival. We illustrate that these criteria effectively measure the energy level of mantle waves such as the SP wave. With synthetics and real data, we demonstrate the effectiveness of LQR and AMP criteria in minimizing these unwanted signal-generated noise in the stacked SRFs down to 1–2 per cent, improving detection threshold and interpretation of Sdp phases from seismic discontinuities in the LAS.

Thickness of the lithosphere beneath Turkey and surroundings from S-receiver functions

We analyze S-receiver functions to investigate the variations of lithospheric thickness below the entire region of Turkey and surroundings. The teleseismic data used here have been compiled combining all permanent seismic stations which are open to public access. We obtained almost 12 000 S-receiver function traces characterizing the seismic discontinuities between the Moho and the discontinuity at 410 km depth. Common-conversion-points stacks yield well-constrained images of the Moho and of the lithosphere–asthenosphere boundary (LAB). Results from previous studies suggesting shallow LAB depths between 80 and 100 km are confirmed in the entire region outside the subduction zones. We did not observe changes of LAB depths across the North and East Anatolian Faults. To the east of Cyprus, we see indications of the Arabian LAB. The African plate is observed down to about 150 km depth subducting to the north and east between the Aegean and Cyprus with a tear at Cyprus. We also observed the discontinuity at 410 km depth and a negative discontinuity above the 410, which might indicate a zone of partial melt above this discontinuity.

The lithosphere-asthenosphere boundary observed with USArray receiver functions

The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose, which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities, and is free from multiples, unlike P receiver functions. Only two major discontinuities are observed in the entire area down to about 300 km depth. These are the crust-mantle boundary (Moho) and a negative boundary, which we correlate with the lithosphere-asthenosphere boundary (LAB), since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926). Our S receiver function LAB is at a depth of 70–80 km in large parts of westernmost North America. East of the Rocky Mountains, its depth is generally between 90 and 110 km. Regions with LAB depths down to about 140 km occur in a stretch from northern Texas, over the Colorado Plateau to the Columbia basalts. These observations agree well with tomography results in the westernmost USA and on the east coast. However, in the central cratonic part of the USA, the tomography LAB is near 200 km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100 km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) and Lekic and Romanowicz (2011) as a recently discovered mid-lithospheric discontinuity (MLD). A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al., 2007).