Masaki Kanao

S‐velocity model and inferred Moho topography beneath the Antarctic Plate from Rayleigh waves

Since 2007/2008, seismographs were deployed in many new locations across much of Antarctica. Using the records from 122 broadband seismic stations, over 10,000 Rayleigh wave fundamental-mode dispersion curves have been retrieved from earthquake waveforms and from ambient noise. Using the processed data set, a 3-D S-velocity model for the Antarctic lithosphere was constructed using a single-step surface wave tomographic method, and a Moho depth map was estimated from the model. Using the derived crustal thicknesses, the average ratio of lithospheric mantle and crustal densities of Antarctica was calculated. The calculated density ratio indicates that the average crustal density for Antarctica is much higher than the average values for continental crust or the average density of lithospheric mantle is so low as to be equal to low-density bound of Archean lithosphere. The latter implies that the lithospheric mantle in much of Antarctica should be old and of Archean age. The East Antarctic Mountain Ranges (EAMOR) represent a thick crustal belt, with the thickest crust (~60 km) located close to Dome A. Very high velocities can be found at depths greater than 200 km beneath parts of East Antarctica, demonstrating that the continental lithosphere extends deeper than 200 km. The very thick crust beneath the EAMOR may represent the collision suture of East Gondwana with Indo-Antarctica and West Gondwana during the Pan-African orogeny.

Thick and anisotropic D″ layer beneath Antarctic Ocean

[1] Waveform modeling and travel times analyses of S, ScS and SKS phases recorded at the broad-band permanent station SYO in the Antarctic are used to determine the shear wave velocity structure and transverse isotropy in the D layer beneath the Antarctic Ocean. The SH wave structure has a discontinuity with the velocity increase of 2.0% at 2550 km. The SV structure is similar to PREM model. The magnitude of the anisotropy is highest at the top of D layer and lowest at the core-mantle boundary. The D layer beneath the Antarctic Ocean is significantly thicker than those beneath Alaska and the Caribbean Sea. We attribute this anisotropic D layer to paleo-slab materials. The subduction in and around the Antarctic Ocean has started ∼180 Ma and is the one of the oldest in the world. It has provided a large amount of the slab materials in the lowermost mantle. Citation: Usui, Y., Y. Hiramatsu, M. Furumoto, and M. Kanao (2005), Thick and anisotropic D layer beneath Antarctic Ocean.

Temperature, lithosphere‐asthenosphere boundary, and heat flux beneath the Antarctic Plate inferred from seismic velocities

We estimate the upper-mantle temperature of the Antarctic Plate based on the thermoelastic properties of mantle minerals and S velocities using a new 3-D shear velocity model, AN1-S [An et al., 2015, JGR]. Crustal temperatures and surface heat fluxes are then calculated from the upper-mantle temperature assuming steady-state thermal conduction. The temperature at the top of the asthenosphere beneath the oceanic region and West Antarctica is higher than the dry mantle solidus, indicating the presence of melt. From the temperature values, we generate depth maps of the lithosphere–asthenosphere boundary and the Curie-temperature isotherm. The maps show that East Antarctica has a thick lithosphere similar to that of other stable cratons, with the thickest lithosphere (~250 km) between Domes A and C. The thin crust and lithosphere beneath West Antarctica are similar to those of modern subduction-related rift systems in East Asia. A cold region beneath the Antarctic Peninsula is similar in spatial extent to that of a flat-subducted slab beneath the southern Andes, indicating a possible remnant of the Phoenix Plate, which was subducted prior to 10 Ma. The oceanic lithosphere generally thickens with increasing age, and the age–thickness correlation depends on the spreading rate of the ridge that formed the lithosphere. Significant flattening of the age–thickness curves is not observed for the mature oceanic lithosphere of the Antarctic Plate.

The March 25, 1998 Antarctic Earthquake: Great earthquake caused by postglacial rebound

A large Mw = 8.1 earthquake occurred off southeast coast of Antarctica near the Balleny Island region on March 25, 1998. We inverted teleseismic body-wave records to determine the rupture pattern using an iterative deconvolution method. The source parameters obtained are: the centroid depth=20km, (strike, dip, rake)= (282, 83, −1), the seismic moment M0 = 1.6 × 1021 Nm (Mw = 8.1), the length L = 200 km, and the average slip D = 4.4 m. This earthquake occurred in the mid-plate but there has been no reports of such large earthquakes in this region. Furthermore, the source mechanism cannot be related to the plate motion inferred from the nearby transform faults. Therefore this earthquake is not a usual tectonic event. Here we show that the compressional axis of our source mechanism coincides with the horizontal crustal motion predicted by the Earth’s response to present-day and past ice mass changes in Antarctica. Our result suggests that the 1998 Antarctica earthquake is caused by the postglacial rebound in the Antarctica.

Loading and Gravitational Effects of the 2004 Indian Ocean Tsunami at Syowa Station, Antarctica

The 2004 Indian Ocean tsunami reached Syowa Station, Antarctica, approximately 12.5 hr after the December Sumatra–Andaman earthquake. We have analyzed the tsunami signals recorded on ocean-bottom pressure gauges, broadband seismometers (sts-1), and a superconducting gravimeter (sg). We calculated the sea level variation, tilt, and gravity changes induced by the tsunami and compared these results to observations. From this comparison we confirmed the loading and gravity effects of the tsunamis on the sts-1 (horizontal components) and the sg records at Syowa Station. The magnitudes of these effects given as root mean square amplitudes are as follows: for the tilt effects obtained from 20-hr-long sts-1 records at frequencies in the range 0.3–0.6 mHz, 5 and 8 μ Gal (10 −8 m/sec 2 ) in the east–west and north–south directions, respectively; and for the gravity effect obtained from the sg records for the same time period of 20 hr at frequencies in the range 0.1–0.2 mHz, 0.2 μ Gal. By using detailed bathymetry around Syowa Station, the synthetic amplitudes similar to the observed were obtained, although the waveforms of synthetic and observation are not always consistent.

Structure and Evolution of the East Antarctic Lithosphere: Tectonic Implications for the Development and Dispersal of Gondwana

Abstract Lithospheric evolution of the Antarctic shield is one of the keystones for understanding continental growth during the Earths evolution. Architecture of the East Antarctic craton is characterized by comparison with deep structures of the other Precambrian terrains. In this paper, we review the subsurface structure of the Lower Paleozoic metamorphic complex around the Lutzow-Holm area (LHC), East Antarctica, where high-grade metamorphism occurred during the Pan-African orogenic event. LHC is considered to be one of the collision zones in the last stage of the formation of Gondwana. A geoscience program named ‘Structure and Evolution of the East Antarctic Lithosphere (SEAL)’ was carried out since 1996-1997 austral summer season as part of the Japanese Antarctic Research Expedition (JARE). Several geological and geophysical surveys were conducted including a deep seismic refraction/wide-angle reflection survey in the LHC. The main target of the SEAL seismic transect was to obtain lithospheric structure over several geological terrains from the western adjacent Achaean Napier Complex to the eastern Lower Paleozoic Yamato-Belgica Complex. The SEAL program is part of a larger deep seismic profile, LEGENDS (Lithospheric Evolution of Gondwana East iNterdisciplinary Deep Surveys) that will extend across the Pan-African belt in neighboring fragments of Gondwana.

Variations in the crustal structure of the Lützow–Holm Bay region, East Antarctica using shear wave velocity

Abstract Crustal structure in the Lutzow–Holm Bay region (LHB), East Antarctica, is studied by analyzing seismic waveforms derived from records in the vicinity of Syowa Station (SYO; 69.0°S, 39.6°E). One-dimensional crustal models for shear waves, in particular, are obtained in order to investigate crustal heterogeneity. Radial receiver functions developed from teleseismic P-waveforms of broadband seismographs at SYO in 1990–1993 are used in the time domain inversion. As the starting model in the receiver function inversion, a crustal attenuation model (Qs) for shear waves from local earthquakes of telemetry networks in 1987–1989 is derived by shifting the lapse time of the coda part of S-waves. Lateral heterogeneity is investigated by analyzing several backazimuth groups; namely the 50°–100°, 120°–160°, 210°–250° and 300°–360° sectors are investigated. Velocity changes are recognized as a sharp Moho at 36–38 km depth in the continental areas in the 50°–100° and the 120°–160° sectors; in contrast smooth variation of crustal velocities is revealed together with transitional Moho in the bay backazimuths in the 210°–250° and the 300°–360° sectors. In the continental backazimuths, a difference in crustal velocities is recognized between metamorphic terrain of granulite facies and the granulite–amphibolite transitional zone. Shear velocity models are compared with crustal reflectivity derived from the analysis of refraction data on the Mizuho Plateau. The reflective layers are found at depths of 24–45 km and appear to be related to the Bouguer gravity anomalies. By correlating the seismic models of crustal structure with other geophysical and geological evidence, the crustal evolution in LHB can be explained in relation to the regional metamorphism at 500 Ma and the last break-up of Gondwana at 150 Ma.

Broadband Seismic Deployments for Imaging the Upper Mantle Structure in the Lützow-Holm Bay Region, East Antarctica

Broadband seismic deployments have been carried out in the Lutzow-Holm Bay region (LHB), Dronning Maud Land, East Antarctica. The recorded teleseismic and local events are of sufficient quality to image the structure and dynamics of the crust and mantle of the terrain. Passive seismic studies by receiver functions and shear wave splitting suggest a heterogeneous upper mantle. Depth variations in topography for upper mantle discontinuities were derived from long period receiver function, indicating a shallow depth discontinuity at 660 km beneath the continental area of LHB. These results provide evidence of paleo upwelling of the mantle plume associated with Gondwana break-up. SKS splitting analysis anticipated a relationship between “fossil” anisotropy in lithospheric mantle and past tectonics. Moreover, active source surveys (DSSs) imaged lithospheric mantle reflections involving regional tectonic stress during Pan-African and succeeding extension regime at the break-up. By combining the active and passive source studies of the mantle structure, we propose an evolution model of LHB for constructing the present mantle structure.

Origins of the lower crustal reflectivity in the Lützow-Holm Complex, Enderby Land, East Antarctica

The combination of rock velocities with a seismic signature has been used as a clue to understanding the structure and evolution of the continental lithosphere. The lower crustal reflectivity beneath part of the Pan-African orogeny, the Lützow-Holm Complex (LHC), Western Enderby Land, East Antarctica, has been imaged on single-fold seismic reflection profiles using active seismic studies on the continental ice sheet. The set of velocity layers at middle to lower crustal depths were obtained by modeling the later phases of teleseismic receiver functions observed at Syowa Station (39°E, 69°S), in the LHC. The later phases around 10–16 s from P onset in radial components are explained by assuming a layered lower crustal model with velocity changes of 0.3 km/s in shear waves for 0.5–1.0 km thick layers. The origin of lower crustal reflectivity is discussed in terms of high-pressure laboratory measurements on metamorphic rocks from Western Enderby Land. Lower crustal velocities of 6.9 km/s derived by seismic refraction surveys can be explained by a major composition of mafic pyroxene granulite, as occurs in the Archean Napier Complex. A tectonic model involving collision between the paleo East and West Gondwana blocks during the last stage of Pan-African orogeny is presented to explain the high velocities and reflectivity in the lower crust underlying the LHC. The Napier Complex is considered to have descended eastward under part of the Pan-African belt (LHC), generating a higher-pressure mafic granulite composition. The reflectivity of the lower crust of the LHC may have been enhanced subsequently by extensional stress during the breakup process.

Seismic Activity Associated with Surface Environmental Changes of the Earth System, around Syowa Station, East Antarctica

The Japanese Antarctic Station, Syowa (69° S, 39° E; SYO), is located on Lutzow-Holm Bay of western Enderby Land, East Antarctica. Seismic observations at SYO started in 1959, and the arrivaltimes of the major phases for teleseismic events have been reported from the National Institute of Polar Research every year since 1968. Here, we summarize records from local earthquakes around SYO in the last three decades. In particular, the fifteen years since 1987 divided into three periods are examined in detail, with respect to the location of epicenters and estimation of magnitudes. A three-station seismic array was deployed around SYO in 1987–1989. By using these data, epicenters of local earthquakes were determined for the first time. Many different types of earthquakes, such as a mainshock-aftershock sequence, twin earthquake, and earthquake swarms were detected and clearly identified. The seismic activity during this period was higher than that of the following decade. Earthquake location was concentrated along the coast and central Lutzow-Holm Bay.