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We employ P to S converted waveforms to investigate effects of the hot mantle plume on seismic discontinuities of the crust and upper mantle. We observe the Moho at depths between 13 and 17 km, regionally covered by a strong shallow intracrustal converted phase. Coherent phases on the transverse component indicate either dipping interfaces, 3- D heterogeneities or lower crustal anisotropy. We find anomalies related to discontinuities in the upper mantle down to the transition zone evidently related to the hot mantle plume. Lithospheric thinning is confirmed in greater detail than previously reported by Li et al., and we determine the dimensions of the low-velocity zone within the asthenosphere with greater accuracy. Our study mainly focuses on the temperature-pressure dependent discontinuities of the upper mantle transition zone. Effects of the hot diapir on the depths of mineral phase transitions are verified at both major interfaces at 410 and 660 km. We determine a plume radius of about 200 km at the 660 km discontinuity with a core zone of about 120 km radius. The plume conduit is located southwest of Big Island. A conduit tilted in the northeast direction is required in the upper mantle to explain the observations. The determined positions of deflections of the discontinuities support the hypothesis of decoupled upper and lower mantle convection
Receiver functions (RF) are used to investigate the upper mantle structure beneath the Eifel, the youngest volcanic area of Central Europe. Data from 96 teleseismic events recorded by 242 seismological stations from permanent and a temporary network has been analysed. The temporary network operated from 1997 November to 1998 June and covered an area of approximately 400 x 250 km(2) centred on the Eifel volcanic fields. The average Moho depth in the Eifel is approximately 30 km, thinning to ca. 28 km under the Eifel volcanic fields. RF images suggest the existence of a low velocity zone at about 60-90 km depth under the West Eifel. This observation is supported by P- and S-wave tomographic results and absorption (but the array aperture limits the resolution of the tomographic methods to the upper 400 km). There are also indications for a zone of elevated velocities at around 200 km depth, again in agreement with S-wave and absorption tomographic results. This anomaly is not visible in P-wave tomography and could be due to S-wave anisotropy. The RF anomalies at the Moho, at 60-90 km, and near 200 km depth have a lateral extent of about 100 km. The 410 km discontinuity under the Eifel is depressed by 15-25 km, which could be explained by a maximum temperature increase of +200 degrees C to +300 degrees C. In the 3-D RF image of the Eifel Plume we also notice two additional currently unexplained conversions between 410 and 550 km depth. They could represent remnants of previous subduction or anomalies due to delayed phase changes. The lateral extent of these conversions and the depression of the 410 km discontinuity is about 200 km. The 660 km discontinuity does not show any depth deviation from its expected value. Our observations are consistent with interpretation in terms of an upper mantle plume but they do not rule out connections to processes at larger depth
An analysis of the shear (S) waves recorded during the wide-angle reflection/refraction (WRR) experiment as part of the DESERT project crossing the Dead Sea Transform (DST) reveals average crustal S-wave velocities of 3.3-3.5 km s(-1) beneath the WRR profile. Together with average crustal P-wave velocities of 5.8-6.1 km s(-1) from an already published study this provides average crustal Poisson's ratios of 0.26-0.27 (V-p/V-s = 1.76-1.78) below the profile. The top two layers consisting predominantly of sedimentary rocks have S- wave velocities of 1.8-2.7kms(-1) and Poisson's ratios of 0.25-0.31 (V-p/V-s = 1.73-1.91). Beneath these two layers the seismic basement has average S- wave velocities of around 3.6 km s(-1) east of the DST and about 3.7 km s(-1) west of the DST and Poisson's ratios of 0.24-0.25 (V-p/V-s = 1.71-1.73). The lower crust has an average S-wave velocity of about 3.75 km s(-1) and an average Poisson's ratio of around 0.27 (V-p/V-s = 1.78). No Sn phase refracted through the uppermost mantle was observed. The results provide for the first time information from controlled source data on the crustal S-wave velocity structure for the region west of the DST in Israel and Palestine and agree with earlier results for the region east of the DST in the Jordanian highlands. A shear wave splitting study using SKS waves has found evidence for crustal anisotropy beneath the WRR profile while a receiver function study has found evidence for a lower crustal, high S-wave velocity layer east of the DST below the profile. Although no evidence was found in the S-wave data for either feature, the S-wave data are not incompatible with crustal anisotropy being present as the WRR profile only lies 30 degrees off the proposed symmetry axis of the anisotropy where the difference in the two S-wave velocities is still very small. In the case of the lower crustal, high S-wave velocity layer, if the velocity change at the top of this layer comprises a small first-order discontinuity underlain by a 2 km thick transition zone, instead of just a large first-order discontinuity, then both the receiver function data and the WRR data presented here can be satisfied. Finally, the S-wave velocities and Poisson's ratios which have been derived in this study are typical of continental crust and do not require extensional processes to explain them
We report on a receiver function study of the crust and upper mantle within DESERT, a multidisciplinary geophysical project to study the lithosphere across the Dead Sea Transform (DST). A temporary seismic network was operated on both sides of the DST between 2000 April and 2001 June. The depth of the Moho increases smoothly from about 30 to 34-38 km towards the east across the DST, with significant north-south variations east of the DST. These Moho depth estimates from receiver functions are consistent with results from steep-and wide-angle controlled-source techniques. Steep-angle reflections and receiver functions reveal an additional discontinuity in the lower crust, but only east of the DST. This leads to the conclusion that the internal crustal structure east and west of the DST is different. The P to S converted phases from both discontinuities at 410 and 660 km are delayed by 2 s with respect to the IASP91 global reference model. This would indicate that the transition zone is consistent with the global average, but the upper mantle above 410 km is 3-4 per cent slower than the standard earth model
Seismic tomography, imaging of seismic scatterers, and magnetotelluric soundings reveal a sharp lithologic contrast along a similar to 10 km long segment of the Arava Fault (AF), a prominent fault of the southern Dead Sea Transform (DST) in the Middle East. Low seismic velocities and resistivities occur on its western side and higher values east of it, and the boundary between the two units coincides partly with a seismic scattering image. At 1 - 4 km depth the boundary is offset to the east of the AF surface trace, suggesting that at least two fault strands exist, and that slip occurred on multiple strands throughout the margin's history. A westward fault jump, possibly associated with straightening of a fault bend, explains both our observations and the narrow fault zone observed by others
With controlled seismic sources and specifically designed receiver arrays, we image a subvertical boundary between two lithological blocks at the Arava Fault (AF) in the Middle East. The AF is the main strike-slip fault of the Dead Sea Transform (DST) in the segment between the Dead Sea and the Red Sea. Our imaging (migration) method is based on array beamforming and coherence analysis of P to P scattered seismic phases. We use a 1-D background velocity model and the direct P arrival as a reference phase. Careful resolution testing is necessary, because the target volume is irregularly sampled by rays. A spread function describing energy dispersion at localized point scatterers and synthetic calculations for large planar structures provides estimates of the resolution of the images. We resolve a 7 km long steeply dipping reflector offset roughly 1 km from the surface trace of the AF. The reflector can be imaged from about 1 km down to 4 km depth. Previous and ongoing studies in this region have shown a strong contrast across the fault: low seismic velocities and electrical resistivities to the west and high velocities and resistivities to the east of it. We therefore suggest that the imaged reflector marks the contrast between young sedimentary fill in the west and Precambrian rocks in the east. If correct, the boundary between the two blocks is offset about 1 km east of the current surface trace of the AF
Robotic telescopes & Doppler imaging : measuring differential rotation on long-period active stars
(2004)
The sun shows a wide variety of magnetic-activity related phenomena. The magnetic field responsible for this is generated by a dynamo process which is believed to operate in the tachocline, which is located at the bottom of the convection zone. This dynamo is driven in part by differential rotation and in part by magnetic turbulences in the convection zone. The surface differential rotation, one key ingredient of dynamo theory, can be measured by tracing sunspot positions.To extend the parameter space for dynamo theories, one can extend these measurements to other stars than the sun. The primary obstacle in this endeavor is the lack of resolved surface images on other stars. This can be overcome by the Doppler imaging technique, which uses the rotation-induced Doppler-broadening of spectral lines to compute the surface distribution of a physical parameter like temperature. To obtain the surface image of a star, high-resolution spectroscopic observations, evenly distributed over one stellar rotation period are needed. This turns out to be quite complicated for long period stars. The upcoming robotic observatory STELLA addresses this problem with a dedicated scheduling routine, which is tailored for Doppler imaging targets. This will make observations for Doppler imaging not only easier, but also more efficient.As a preview of what can be done with STELLA, we present results of a Doppler imaging study of seven stars, all of which show evidence for differential rotation, but unfortunately the errors are of the same order of magnitude as the measurements due to unsatisfactory data quality, something that will not happen on STELLA. Both, cross-correlation analysis and the sheared image technique where used to double check the results if possible. For four of these stars, weak anti-solar differential rotation was found in a sense that the pole rotates faster than the equator, for the other three stars weak differential rotation in the same direction as on the sun was found.Finally, these new measurements along with other published measurements of differential rotation using Doppler imaging, were analyzed for correlations with stellar evolution, binarity, and rotation period. The total sample of stars show a significant correlation with rotation period, but if separated into antisolar and solar type behavior, only the subsample showing anti-solar differential rotation shows this correlation. Additionally, there is evidence for binary stars showing less differential rotation as single stars, as is suggested by theory. All other parameter combinations fail to deliver any results due to the still small sample of stars available.
Crustal structure of the southern Dead Sea basin derived from project DESIRE wide-angle seismic data
(2009)
As part of the DEad Sea Integrated REsearch project (DESIRE) a 235 km long seismic wide-angle reflection/ refraction (WRR) profile was completed in spring 2006 across the Dead Sea Transform (DST) in the region of the southern Dead Sea basin (DSB). The DST with a total of about 107 km multi-stage left-lateral shear since about 18 Ma ago, accommodates the movement between the Arabian and African plates. It connects the spreading centre in the Red Sea with the Taurus collision zone in Turkey over a length of about 1 100 km. With a sedimentary infill of about 10 km in places, the southern DSB is the largest pull-apart basin along the DST and one of the largest pull-apart basins on Earth. The WRR measurements comprised 11 shots recorded by 200 three-component and 400 one-component instruments spaced 300 m to 1.2 km apart along the whole length of the E-W trending profile. Models of the P-wave velocity structure derived from the WRR data show that the sedimentary infill associated with the formation of the southern DSB is about 8.5 km thick beneath the profile. With around an additional 2 km of older sediments, the depth to the seismic basement beneath the southern DSB is about 11 km below sea level beneath the profile. Seismic refraction data from an earlier experiment suggest that the seismic basement continues to deepen to a maximum depth of about 14 km, about 10 km south of the DESIRE profile. In contrast, the interfaces below about 20 km depth, including the top of the lower crust and the Moho, probably show less than 3 km variation in depth beneath the profile as it crosses the southern DSB. Thus the Dead Sea pull-apart basin may be essentially an upper crustal feature with upper crustal extension associated with the left- lateral motion along the DST. The boundary between the upper and lower crust at about 20 km depth might act as a decoupling zone. Below this boundary the two plates move past each other in what is essentially a shearing motion. Thermo-mechanical modelling of the DSB supports such a scenario. As the DESIRE seismic profile crosses the DST about 100 km north of where the DESERT seismic profile crosses the DST, it has been possible to construct a crustal cross-section of the region before the 107 km left-lateral shear on the DST occurred.
Clear S-to-P converted waves from the crust-mantle boundary (Moho) and lithosphere-asthenosphere boundary (LAB) have been observed on the eastern part of the Dead Sea Basin (DSB), and are used for the determination of the depth of the Moho and the LAB. A temporary network consisting of 18 seismic broad-band stations was operated in the DSB region as part of the DEad Sea Integrated REsearch project for 1.5 years beginning in September 2006. The obtained Moho depth (similar to 35 km) from S-to-P receiver functions agrees well with the results from P-to-S receiver functions and other geophysical data. The thickness of the lithosphere on the eastern part of the DSB is about 75 km. The results obtained here support and confirm previous studies, based on xenolith data, geodynamic modeling, heat flow observations, and S-to-P receiver functions. Therefore, the lithosphere on the eastern part of the DSB and along Wadi Araba has been thinned in the Late Cenozoic, following rifting and spreading of the Red Sea. The thinning of the lithosphere occurred without a concomitant change in the crustal thickness and thus an upwelling of the asthenosphere in the study area is invoked as the cause of the lithosphere thinning.
The geothermal potential in Tarutung is controlled by both the Sumatra Fault system and young arc volcanism. In this study we use the spatial distribution of seismic attenuation, calculated from local earthquake recordings, to image the 3-D seismic attenuation of the area and relate it with the temperature anomalies and the fluid distribution of the subsurface. A temporary seismic network of 42 stations was deployed around Tarutung and Sarulla (south of Tarutung) for a period of 10 months starting in 2011 May. Within this period, the network recorded 2586 local events. A high-quality subset of 229 events recorded by at least 10 stations was used for the attenuation inversion (tomography). Path-average attenuation (t(p)*) was calculated by using a spectral inversion method. The spread function, the contour lines of the model resolution matrix and the recovery test results show that our 3-D attenuation model (Q(p)) has good resolution around the Tarutung Basin and along the Sarulla graben. High attenuation (low Q(p)) related to the geothermal system is found in the northeast of the Tarutung Basin suggesting fluid pathways from below the Sumatra Fault. The upper part of the studied geothermal system in the Tarutung district seems to be mainly controlled by the fault structure rather than by magmatic activities. In the southwest of the Tarutung Basin, the high attenuation zone is associated with the Martimbang volcano. In the Sarulla region, a low-Q(p) anomaly is found along the graben within the vicinity of the Hopong caldera.