Geophysics Section

Apia, Samoa

Geophysics Section

Apia, Samoa
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Eibl E.P.S.,University College Dublin | Bean C.J.,Geophysics Section | Jonsdottir I.,University of Iceland | Hoskuldsson A.,University of Iceland | And 4 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2017

We analyze eruptive tremor during one of the largest effusive eruptions in historical times in Iceland (2014/2015 Holuhraun eruption). Seismic array recordings are compared with effusion rates deduced from Moderate Resolution Imaging Spectroradiometer recordings and ground video monitoring data and lead to the identification of three coexisting eruptive tremor sources. This contrasts other tremor studies that generally link eruptive tremor to only one source usually associated with the vent. The three sources are (i) a source that is stable in back azimuth and shows bursts with ramp-like decrease in amplitude at the beginning of the eruption: we link it to a process below the open vents where the bursts correlate with the opening of new vents and temporary increases in the lava fountaining height; (ii) a source moving by a few degrees per month while the tremor amplitude suddenly increases and decreases: back azimuth and slowness correlate with the growing margins of the lava flow field, whilst new contact with a river led to fast increases of the tremor amplitude; and (iii) a source moving by up to 25° southward in 4 days that cannot be related to any observed surface activity and might be linked to intrusions. We therefore suggest that eruptive tremor amplitudes/energies are used with caution when estimating eruptive volumes, effusion rates, or the eruption explosivity as multiple sources can coexist during the eruption phase. Our results suggest that arrays can monitor both the growth of a lava flow field and the activity in the vents. © 2017. American Geophysical Union.

Williams S.,University of Canterbury | Goff J.,University of New South Wales | Kau J.A.,Geophysics Section | Sale F.,Geophysics Section | And 3 more authors.
Science of Tsunami Hazards | Year: 2013

The September 29, 2009 Samoa Tsunami provided the opportunity to sample the sediments deposited in the Samoan Islands landscape by the tsunami. Analysing the characteristics of the sediment deposits using an established suite of diagnostic criteria, and assessing how they differ from cyclone deposits enables the identification and dating of similar events in the geologic record. This helps to better understand the long-term frequency and likely magnitude of these events. Here we report on a pilot palaeotsunami field-sampling investigation carried out in 2010 at selected sites on Upolu and Savaii Islands in the Independent State of Samoa, and on Ta'u Island in American Samoa. We present empirical stratigraphic data for the investigated sites, and we demonstrate the existence of high energy marine inundation deposits at some of these sites which were laid down by past tsunamis and/or cyclones. We review and discuss the analytical outcomes, as well as summarise the overarching directions of this research. We propose that there is a need for this study to continue and for such studies to be carried out in other islands in the Pacific. By doing this, we can build on the sparse palaeotsunami database in the region, thereby helping to improve our understanding of the long-term frequency, impact distribution, and likely magnitude of these events. Further, we can start assessing their likely sources and the long-term risk these hazards pose to coastal cities and communities in the Pacific.

Chiarabba C.,Italian National Institute of Geophysics and Volcanology | Agostinetti N.P.,Geophysics Section | Bianchi I.,University of Vienna
Geophysical Research Letters | Year: 2016

A persistent seismic gap is hypothesized in the Pollino area (southern Italy), at the boundary between the Apennines and the Calabrian arc. Presently, seismic swarms are active in the gap area, creating concerns for possible future large earthquakes. In this study, we model the deep Earth structure across the Pollino range to give new insights on the kinematics and tectonics of this enigmatic area. Migrated receiver function profiles show a subvertical lithospheric discontinuity, delineated by an abrupt change in Moho depth and mantle fabrics across the range. The lithospheric-scale discontinuity bounds the area of earthquake swarm activity and likely decouples the delamination-related extension of the Apennines from the extensional collapse of the Calabrian fore arc. This large-scale discontinuity implies that the normal faults are segmented across the range, limiting the lateral extent of faults where future earthquakes might occur. ©2016. American Geophysical Union. All Rights Reserved.

Rosenbaum G.,University of Queensland | Piana Agostinetti N.,Geophysics Section
Tectonics | Year: 2015

Lithospheric tear faults are expected to develop in response to along-strike variations in the rates of slab rollback. However, the exact geometry of such structures and their crustal and upper mantle expressions are still debated. We present an analysis of seismic, structural, and morphological features that possibly represent the expression of lithospheric segmentation in the northern Apennines. Geophysical observations show evidence for the existence of a discontinuity in the lithospheric structure beneath the northern Apennines, characterized by a change in the spatial distribution of intermediate-depth seismicity, along-strike variations in the pattern of crustal seismicity, and a bend in the Moho topography. The near-surface expression of this discontinuity is associated with an abrupt change in the morphology and exhumation history of the northern Apennines in the proximity of the Livorno-Sillaro Lineament. We interpret these features as evidence for incipient tearing of the lithospheric slab beneath the northern Apennines, marking the boundary between domains that underwent contrasting styles of lithospheric deformation, which are either associated with different rates of slab rollback or a transition from underplating to retreat. We suggest that similar types of structures may play a crucial role in the evolution of convergent plate boundaries, allowing segmentation of orogenic belts and facilitating the development of orogenic curvatures. Ultimately, further tearing along such structures could potentially lead to the occurrence of tear-related magmatism and the formation of slab windows. ©2015. American Geophysical Union.

Adetunji A.Q.,University of Manitoba | Ferguson I.J.,University of Manitoba | Jones A.G.,Geophysics Section
Journal of Geophysical Research B: Solid Earth | Year: 2015

Magnetotelluric (MT) responses at the Proterozoic Grenville Front in Canada have been interpreted as being caused by lithospheric electrical anisotropy, and the area is often noted as a classic example of lithospheric anisotropy. This study reevaluates evidence for the electrical anisotropy using 56 MT stations. The spatially uniform MT responses noted at the Grenville Front extend to ~200km southeast and for at least 400km along strike and are associated with rocks at less than 150km depth. Examination of induction arrows at longer periods shows arrows at high angle to the MT conductive direction consistent with the presence of macroscopic resistivity structures. New 2-D anisotropic inversions show that electrical anisotropy is not required to fit the MT data. The results indicate that in the resistive mantle lithosphere beneath the Grenville Front, and in conductive lithosphere in adjacent areas, the maximum horizontal resistivity anisotropy is <10%, much less than the factor of 15 determined in earlier 1-D studies. The results suggest that the upper lithospheric mantle in the area is devoid of significant electrical anisotropy and that the observed MT response directionality is due to large-scale resistivity structure. We interpret the spatially consistent MT responses observed at the Grenville Front as being associated with the resistive Archean lithosphere extending southeast beneath the Grenville Front. The obliquity between seismic and MT responses arises because the Superior fabric is oblique to the seismic fast direction. If dextral shearing occurred, it appears to have not caused any significant shape preferred electrical anisotropy. ©2015. American Geophysical Union.

Pham N.D.,Geophysics Section | Pham N.D.,Vietnam Academy of Science and Technology | Igel H.,Geophysics Section | De la Puente J.,Vietnam Academy of Science and Technology | And 2 more authors.
Geophysics | Year: 2010

Rotational motions in homogeneous anisotropic elastic media are studied under the assumption of plane wave propagation. The main goal is to investigate the influences of anisotropy in the behavior of the rotational wavefield. The focus is on P-waves that theoretically do not generate rotational motion in isotropic media. By using the Kelvin-Christoffel equation, expressions are obtained of the rotational motions of body waves as a function of the propagation direction and the coefficients of the elastic modulus matrix. As a result, the amplitudes of the rotation rates and their radiation patterns are quantified and it is concluded that (1) for strong local earthquakes and typical reservoir situations quasi P-rotation rates induced by anisotropy are significant, recordable, and can be used for inverse problems; and (2) for teleseismic wavefields, anisotropic effects are unlikely to be responsible for the observed rotational energy in the P coda. © 2010 Society of Exploration Geophysicists.

Muluneh A.A.,Addis Ababa Institute of Technology | Muluneh A.A.,University of Rome La Sapienza | Kidane T.,Addis Ababa Institute of Technology | Rowland J.,University of Auckland | Bachtadse V.,Geophysics Section
Tectonophysics | Year: 2013

Thirty-four cooling units from the Pleistocene extrusive volcanic rocks exposed in regions bordered by the active Red Sea magmatic segments, in the northwestern central Afar Depression, were sampled for paleomagnetic study. Six to 12 samples were collected from each paleomagnetic site. Samples were demagnetized using Alternating Field (AF) and Thermal (TH) demagnetization techniques. The samples were then measured using the JR-6A spinner magnetometer available at the department of Earth Sciences of Addis Ababa University. The Natural Remanent Magnetization (NRM) direction reveals mostly one or two simple and straightforward components of magnetizations; the first, low-stability component is isolated by heating to 100°C-300°C or by AF of 10-30mT. The magnetization directions after these steps generally define straight lines that are directed towards the origin, and interpreted as primary NRM or ChRM (Characteristic Remanent Magnetization) and the direction of magnetization was determined. Results of the magnetization decay curve plots and rock magnetic analyses using a Variable Field Translation Balance (VFTB) indicate that magnetic mineralogy is dominated by Ti-poor titano-magnetite with magnetic grain sizes in the pseudo-single domain (PSD) range, with minor presence of goethite and maghemite in a few cases. An overall mean direction calculated for the 26 sites located within the overlap zone is obtained (D=354.4°, I=13.2°, N=26, K=43.5, α95=4.3°), and compared to the expected Geomagnetic Axial Dipole (GAD) field, based on the Apparent Polar Wander Path (APWP) Curve of the African plate (Besse and Courtillot, 1991, 2003), a difference in declination δD=-6.5°±4.0° is obtained. This declination difference is interpreted as counterclockwise rotation about a vertical axis, in agreement with rift propagation and right stepping overlap geometry of the Alayta-Dabbahu magmatic segments. The difference in inclination δI=7.8°±3.9° is believed to be related to the long-standing non-dipole field in Afar (Courtillot et al., 1984; Kidane et al., 2003). © 2013 Elsevier B.V.

Kidane T.,Addis Ababa Institute of Technology | Bachtadse V.,Geophysics Section | Alene M.,Addis Ababa Institute of Technology | Kirscher U.,Geophysics Section
Geophysical Journal International | Year: 2013

One hundred fourteen oriented palaeomagnetic core samples were collected from 13 palaeomagnetic sites on subhorizontal to tilted glacial sediments at five localities of Northern Ethiopia. Combined alternating field (AF) and stepwise thermal demagnetization techniques were successfully applied to resolve the complete directional spectrum. A viscous remagnetization (VRM) and one stable component of magnetization were identified in most of the specimens. The VRM is removed between a temperature range of 120-350 °C and AF of up to 30mT. Further heating until ~650 °C results in smooth decay of the natural remanent magnetization (NRM) intensity to about 50 per cent and the rest of the NRM is efficiently removed by heating to 690 °C, while only 30-50 per cent of NRM is removed by the maximum AF available suggesting haematite as remanence carrier. Results of the magnetization decay curve plots and rock magnetic analyses using the variable field translation balance indicated the presence of magnetite with minor goethite, pyrrhotite as well. The high stability component defining a straightline segment, starting 350 °C and/or 30mT is mostly directed towards the origin and interpreted as the characteristic remanent magnetizations (ChRMs). The direction of magnetization is determined both by best-fitting line using the least-square technique of Kirschvink and remagnetization circles of Halls for few unresolved overlapping components. The site mean directions of the sediments from two sites are normal polarity and are close to present-day field directions at the sample site. The site mean directions from 11 sites, on the other hand, are reversed in polarity with better grouping in the tilt-corrected coordinate and pass the McFadden fold test. This overall site mean direction is Dec = 143.4o, Inc = 58.8° (N = 11, α95 = 9.7°) with a corresponding mean pole position of Lat = 26.0°, Lon = 249.5° (N = 11, A95 = 13.1°). This geomagnetic pole position is later rotated into West Africa coordinates to allow for extensional rifting in the Benue Trough about an Euler pole position, at 19.2°N, 352.6°E through an angle -6.3  (clockwise). The resulting pole position is located at φs = 246.6°E, λs = 31.8°S (N = 11, A95 = 13.1°), this pole with its 95 per cent confidence circle intersects the 270-310 Ma, segment of the APW path for West Africa consistent with ages of between late Carboniferous and early Permian. The result also implies that the Late Carboniferous Dwyka land ice sheet had probably extended more than 1000 km further north to Ethiopia than previously known. © The Authors 201.

Kidane T.,Addis Ababa Institute of Technology | Bachtadse V.,Geophysics Section | Alene M.,Addis Ababa Institute of Technology
Physics of the Earth and Planetary Interiors | Year: 2014

Eighty-one paleomagnetic cores were collected from 10 locations across a black limestone unit within the core of Negash Synclinorium, northern Ethiopia in order to test the proposed Snowball Earth events for the diamictite unit of the Tambien Group. Cores were cut into two standard paleomagnetic specimens and were subjected to stepwise demagnetizations using both Thermal (TH) and alternating field (AF) techniques. Rock magnetic analyses on representative specimens were done and results revealed goethite, pyrrhotite, titano-magnetite, and titano-hematite to be the major magnetic materials carrying the magnetizations with PSD (pseudo single domain) grain size range. In most cases paleomagnetic directions are defined by a single component of magnetization, where a viscous component is present it is usually removed by heating to a temperature of ~200°C or an AF of ~10mT. The high stability component isolated above temperature of 200°C or AF of 15mT, defined straight line trajectories directed towards the origin and considered as the Characteristic Remanent Magnetization Direction (ChRM). The direction of magnetization of the ChRM is determined for samples with stable straight line segments by the best-fit line using the least square technique of Kirschvink (1980). In the cases of overlapping spectra and unblocking temperatures, direction of magnetization is determined by remagnetization circles of Halls (1976, 1978). When site mean ChRM directions are plotted on stereogram, their distribution is relatively clustered in geographic coordinates and the overall mean direction is Decg=358.5°, Incg=16.6° (α95=3.8°, K=162.8, N=10). After a structural restoration to the horizontal is made the directions disperse and fail the fold test of both McElhinny's and McFadden's tests and the mean direction is Decs=353.5°, Incs=8.8° (α95=18.9°, K=7.5, N=10). This is interpreted to result from a later remagnetization of the black limestone. All directions are normal in polarity and have mean unrestored paleomagnetic directions comparable to the Quaternary paleomagnetic directions. Virtual Geomagnetic poles (VGP) in the unrestored position is used to calculate overall mean VGP position resulting long=235.7°E, latg=84.5°N (A95=3.0°, N=10). Comparison of the obtained pole with the apparent polar wander path (APWP) curve for Africa of Besse and Courtillot (1991, 2003) and with the 2Ma reference pole of stable Africa (Kidane et al., 2003) is found to be consistent with remagnetizations during the Quaternary period. Hence supporting evidence for the proposed Snowball Earth event of the Sturtian glaciation in the Negash rocks could not, unfortunately, be obtained from paleomagnetism. © 2014 Elsevier B.V.

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