The Splitting 660 km Discontinuity Associated with Lithospheric Delamination in the Northern Part of the North-South Seismic Zone, China

The north-south seismic zone (NSSZ) is a destructive zone of large-scale earthquakes in China, and the earthquake mechanism associated with deep structures remains unclear. Previous studies have indicated that lithospheric delamination or absence of lithospheres in the western part of the NSSZ may facilitate the eastern extrusion of the Tibetan Plateau and lead to stress accumulation and release. However, the deep process of lithospheric delamination needs to be further clarified. In this study, I collect abundant high-quality teleseismic data recorded by permanent seismic stations and perform common conversion point (CCP) stacking of receiver functions in the north part of the NSSZ. The results show that lithospheric delamination might result in the splitting 660 km discontinuity and a thickening region of the mantle transition zone (MTZ).

Comment on ‘Evidence for a large strike-slip component during the 1960 Chilean earthquake’ by H. Kanamori, L. Rivera, and S. Lambotte

Summary Based on numerous studies of the relevant geodetic data, a low-angle thrusting mechanism has been assigned to the 1960 Chile earthquake. Kanamori, Rivera, and Lambotte recently suggested that a component of dextral slip comparable to the thrusting be included in the mechanism to satisfy long-period, teleseismic observations. The absence of geodetic evidence for that huge strike-slip component is the subject of this comment. The geodetic data are largely measurements of coseismic uplift associated with the earthquake but include 8 measurements of the coseismic change in shear strain. Because strike slip produces relatively little uplift except near the end points of the rupture, identification of that strike-slip component in the geodetic data depends upon the measured, shear-strain change. I consider elastic, half-space models of oblique slip on the plate interface possibly supplemented by simultaneous dextral slip on the nearby, intra-arc Liquiñe-Ofqui Fault Zone. Slip is assumed to be uniform along strike. The best fits to the geodetic data for these models furnish little evidence for strike slip on those structures. To satisfy the long-period, teleseismic data, Kanamori et al. proposed 6 examples, each of which requires a large amount of dextral slip. Because the long-period, teleseismic data do not define the slip distributions, I have used the best fits of those examples to the geodetic data to define those distributions. The large thrusting near the deformation front required by those slip distributions implies large uplift there, contrary to the uplift inferred from the inversion of tsunami data. However, an acceptable fit to the geodetic data and the tsunami data for the 6 examples suggested by Kanamori et al. can be obtained if the seismic moments specified by them are reduced by a factor ∼1.8, a factor within the uncertainties in estimating seismic moments of the 1960 Chile earthquake. The presence of strike slip in those reduced-moment examples despite the lack of geodetic evidence for strike slip is due to a remarkable coincidence that requires careful balancing of contributions from the shallower (depths < 70 km) coseismic sources against those from the deeper coseismic sources to nullify the geodetic evidence for strike slip. Such balancing is possible, but it is remarkable that the balancing is so nearly perfect that it nullifies the geodetic evidence for strike slip and thereby confounds the interpretation of the geodetic data.

Improving depth estimations of African earthquakes using teleseismic data, and influence for the East-African rift seismic hazard characterization

SUMMARY Well-constrained earthquake depth estimations are important for seismic hazard determination. As local networks of the East-African Rift are usually too sparse for reliable depth estimations, we used detections of pP and sP phase arrivals (the so-called depth phases) at teleseismic distance to constrain earthquake depths in this region. We rely on a fully automatic Cepstral analysis approach, first validated at the global scale using the ISC-EHB catalogue, then applied on the East-African seismicity. We investigated 9575 earthquakes from magnitude 2 since 2005 which allows us to constrain the depth estimation of 584 events with magnitude mainly above 3.5, complemented by 139 reliable depth estimations from previous studies based on teleseismic data as well. To ensure a final catalogue as complete as possible, we also identified from regional catalogues 113 earthquakes assumed to be well constrained, based on network geometry empirical criteria. Thanks to this study, we finally propose new earthquake depth distributions for the seismic source zonation defined by Poggi et al., in order to estimate the seismic hazard of the East African Rift region. Including those new distributions in the source models leads to significant changes of seismic hazard assessments results.

Rupture Kinematics of the 11 January 2021 Mw 6.7 Hovsgol, Mongolia, Earthquake and Implications in the Western Baikal Rift Zone

The Mongolia plateau is the farthest intracontinental region of the India–Eurasia collision and is a transition zone between north–south convergence to the south in the Tien Shan and northwest–southeast extension to the north in the Baikal rift. Mongolia has experienced four M 8 earthquakes since 1905, but due to limited observations, the mechanism of these strong earthquakes and regional tectonics are poorly understood. The 11 January 2021 Mw 6.7 Hovsgol, Mongolia, earthquake is the largest event that has occurred in the Hovsgol graben, which is noted for being the northernmost convergence region of the India–Eurasia collision and the youngest extension region of the Baikal rift. In this article, the coseismic displacements are retrieved by space geodesy for the first time in this region, providing good constraints for the deformation pattern. We use a finite-fault inversion of InSAR and teleseismic data, and a backprojection analysis to reveal the rupture kinematics of this event. The geometry of the Hovsgol fault is determined as east-dipping with a dip of 45°. The rupture process is characterized by a northwestward propagation with a moderate average rupture velocity of ∼2.0 km/s and a complex slip pattern composed of two major slip patches with dimensions of 40 km×20 km. The oblique slip, illustrated by predominate extension and significant dextral striking, confirms the right-lateral-striking faulting in the Hovsgol rift, which indicates that the eastwardly north–south convergence across the southwest segment of the Baikal rift has decreased.

Moho and uppermost mantle structure in the greater Alpine area from S-to-P converted waves

In the frame of the AlpArray project we analyze teleseismic data from permanent and temporary stations of the greater Alpine region to study seismic discontinuities down to about 140 km depth. We average broadband teleseismic S waveform data to retrieve S-to-P converted signals from below the seismic stations. In order to avoid processing artefacts, no deconvolution or filtering is applied and S arrival times are used as reference. We show a number of north-south and east-west profiles through the greater Alpine area. The Moho signals are always seen very clearly, and also negative velocity gradients below the Moho are visible in a number of profiles. A Moho depression is visible along larger parts of the Alpine chain. It reaches its largest depth of 60 km beneath the Tauern Window. The Moho depression ends however abruptly near about 13° E below the eastern Tauern Window. The Moho depression may represent the mantle trench, where the Eurasian lithosphere is subducted below the Adriatic lithosphere. East of 13° E an important along-strike change occurs; the image of the Moho changes completely. No Moho deepening is found in this easterly region; instead the Moho is updoming along the contact between the European and the Adriatic lithosphere all the way into the Pannonian Basin. An important along strike change was also detected in the upper mantle structure at about 14° E. There, the lateral disappearance of a zone of negative P-wave velocity gradient indicates that the S-dipping European slab laterally terminates east of the Tauern Window in the axial zone of the Alps. The area east of about 13° E is known to have been affected by severe late-stage modifications of the structure of crust and uppermost mantle during the Miocene when the ALCAPA (Alpine, Carpathian, Pannonian) block was subject to E-directed lateral extrusion.

The seismic structure of the lithosphere in the greater Alpine area from S-to-P converted waves

<p>In the frame of the AlpArray project we analyse teleseismic data from permanent and temporary stations of the greater Alpine region to study seismic discontinuities in the entire lithosphere. We use broadband S-to-P converted signals from below the seismic stations. In order to avoid sidelobes, no deconvolution or filtering is applied and S arrival times are used as reference. We show a number of north-south and east-west profiles through the greater Alpine area. The Moho signals are always seen very clearly, and also negative velocity gradients below the Moho are visible in a number of profiles. The subducting European Moho is visible in the Eastern Alps west of 13.5°E (the eastern edge of the Tauern Window) and reaches there about 60km depth at 47°N. East of about 13.5°E, the image of the Moho changes completely. No south dipping European Moho is found anymore, instead the Moho is shallowing towards the Pannonian Basin. This suggests severe post-nappe emplacement modifications east of about 13.5°E, most probably associated with delamination of the mantle lithosphere within the formerly subducting European slab, i.e. mantle that separated from the crustal parts of the Alpine-West Carpathian orogen during the last ca. 20 Ma when the Pannonian basin formed and the ALCAPA block underwent its E-directed lateral extrusion.</p><p>Ratschbacher, L., Frisch, W., Linzer, H.-G. and Merle, O. (1991) Lateral extrusion in the Eastern Alps, Part 2: Structural analysis. Tectonics, vol.10, No.2, 257-271.</p>

Seismic Tomography of Southern Tyrrhenian by means of teleseismic data

<p>The topic of my work is a seismic tomography which has as object the investigation of Southern Tyrrhenian. This tomography has been obtained by means of inversion of teleseismic data to investigate subduction zones in the Southern Tyrrhenian oceanic back-arc basin. The subducting lithosphere has been mostly consumed along the Tyrrhenian-Apennine system has been consumed with the exception of the Calabrian arc sector. This kind of inversion could provide a good resolution to depth of 500-600 km, whereas previous local tomographies of Southern Tyrrhenian show results to depth of 250-300 km. The adopted database consists of 1929 teleseisms  recorded in period 1990-2012 by 122 southern Italian seismic station directly connected to ISC (International Seismological Centre). The software FMTT was employed for the inversion of these arrival times. I have implemented a grid of 0-500 km in depth, 7°E-20°E in longitude and 35°-48° in latitude, with a grid spacing of 50 km in depth, 0.8 degrees in longitude and 0.4 degrees in latitude. I have made 10 horizontal sections of final model from 50 km of depth to 500 km of depth, with an interval of 50 km of depth from each other. I have made 8 vertical sections, 4 NS vertical sections at fixed longitude and 4 WE vertical sections at fixed latitude. Finally, I have made 3 transversal sections. Summarising, the horizontal sections show an evolution of the high velocity body that represents the Ionian slab. It is visible both at depth of 50 km and at depth of 100 km, beneath the Calabrian arc and extends to northern Sicily beneath the Aeolian arc with a maximum of 0.6-0.8 km/s. At depth of 250 km, the tomography evidences a sort of “transition” due to the absence of the Southern Tyrrhenian HVA and the occurrence of a low velocity region with maximum of -0.5 km/s scattered between the Aeolian Islands and Calabria. In the depth interval from 250 km to 400 km, there are two impressive high velocity areas in northern Sicily and along southern Campania with a value of 0.3 km/s, separated by a low velocity area (LVA) along the Calabrian arc and the Aeolian Islands in the range [0.4 ; 0.6] km/s. Extensions of HVAs and LVAs previously mentioned have been estimated by means of vertical and transversal sections. This evidence could be interpreted as the effect of a three-dimensional circulation of astenospheric flow provoked by slab roll-back. A new evidence from the tomography is the presence of a LVA in the [250 ; 400] km depth interval with an extension of 100-150 km that practically splits the Tyrrhenian slab into two parts, in Neapolitan region and in the southern Calabria-northern Sicily region. The presence of this “window slab” could be interpreted as a tear in which unperturbed mantle insert itself.</p>

A Harmonised Instrumental Earthquake Catalogue for Iceland and the Northern Mid-Atlantic Ridge

A comprehensive catalogue of historical earthquakes, with accurate epicentres and homogenised magnitudes is a crucial resource for seismic hazard mapping. Here we update and combine catalogues from several sources to compile a catalogue of earthquakes in and near Iceland, in the years 1900–2019. In particular the epicentres are based on local information, whereas the magnitudes are based on teleseismic observations, primarily from international on-line catalogues. The most reliable epicentre information comes from the catalogue of the Icelandic Meteorological Office, but this is complemented with information from several technical reports, scientific publications, newspaper articles, and modified by some expert judgement. The catalogue contains 1272 MW ≥ 4 events and the estimated completeness magnitude is W 5.5 in the first years, going down to MW 4.5 for recent years. The largest magnitude is MW 7.01. Such melting of local and teleseismic data has not been done before for Icelandic earthquakes, and the result is an earthquake map with no obviously mislocated events. The catalogue also lists additional 5654 earthquakes on the Mid-Atlantic Ridge, north of 43°, with both epicentres and magnitudes determined teleseismically. When moment magnitudes are not available, proxy MW values are computed with χ2-regression, normally on MS, but exceptionally on mb. All the presented magnitudes have associated uncertainty estimates. The actual combined seismic moment released in the Icelandic earthquakes is found to be consistent with the moment estimated using a simple plate motion model. The catalogue is named ICEL-NMAR and it is available online at http://dx.doi.org/10.17632/7zh6xg22cv.1.