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1.3: Across-axis cross-section from northwest to the southeast perpendicular to rift valley (from 27.47 @BULLET S, 65.73 @BULLET E to 27.80 @BULLET S, 65.82 @BULLET E). OBS positions are represented by red triangles, thick dashed black line marks the reference niveau of earthquake depths and is at 5.461 km below sea level (station RUM41). Earthquake depths with errors ≤6 km are coloured dark-green (90), locations with fixed depths are shown in grey (3). Solid grey line at 7 km brl marks the Moho depth according to the velocity model. The background colours illustrate the layers of the local velicty model (Tab. 3.5.1). There is no clear evidence for a pronounced detachment fault.
Source publication
The mid-ocean ridge system is the longest mountain range in the world, running across all the worlds oceans. At these places, lithospheric plates drift apart and ∼18 cubic km of new oceanic crust is formed every year. The ultraslow spreading Southwest Indian ridge (SWIR) represents a global end-member of the mid-ocean ridge system, with a full spre...
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Citations
... Further details . At SWIR Segment-8, eight densely spaced OBSs were deployed to investigate this ultraslow spreading ridge (Scholz 2014;Schlindwein & Schmid 2016). For OBS deployment depths and positions, see Table S1 in the Supporting Information or Stähler et al. (2016). ...
We present two independent, automated methods for estimating the absolute horizontal misorientation of seismic sensors. We apply both methods to 44 free-fall ocean-bottom seismometers (OBSs) of the RHUM-RUMexperiment (http://www.rhum-rum.net/). The techniques measure the 3-D directions of particle motion of (1) P-waves and (2) Rayleigh waves of earthquake recordings. For P-waves, we used a principal component analysis to determine the directions of particle motions (polarizations) in multiple frequency passbands. We correct for polarization deviations due to seismic anisotropy and dipping discontinuities using a simple fit equation, which yields significantly more accurate OBS orientations. For Rayleigh waves, we evaluated the degree of elliptical polarization in the vertical plane in the time and frequency domain. The results obtained for the RHUM-RUM OBS stations differed, on average, by 3.1° and 3.7° between the methods, using circular mean and median statistics, which is within the methods' estimate uncertainties. Using P-waves, we obtained orientation estimates for 31 ocean-bottom seismometers with an average uncertainty (95 per cent confidence interval) of 11° per station. For 7 of these OBS, data coverage was sufficient to correct polarization measurements for underlying seismic anisotropy and dipping discontinuities, improving their average orientation uncertainty from 11° to 6° per station. Using Rayleigh waves, we obtained misorientation estimates for 40 OBS, with an average uncertainty of 16° per station. The good agreement of results obtained using the two methods indicates that they should also be useful for detecting misorientations of terrestrial seismic stations. © The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.
... Assuming that the skew accumulated linearly over the deployment period, the clock error can be corrected for any moment in time. Previous studies (Hannemann et al., 2014;Scholz, 2014) show that linearity is a good first order approximation for the clocks used in the DEPAS instruments. For the LCPO2000 instruments used in the INSU pool, Gouedard et al. (2014) found that drift rates can vary over the course of days. ...
... Using cross-correlation of ambient noise, Sens-Schönfelder (2008) presented a method to determine the relative clock error between two seismometers a posteriori, which Hannemann et al. (2014) successfully applied to OBS data. Likewise, Scholz (2014) Figure 7. Data availability and quality for all RHUM-RHUM ocean-bottom stations. Green indicates availability of good data. ...
RHUM-RUM is a German-French seismological experiment based on the sea floor surrounding the island of La Réunion, western Indian Ocean (Barruol and Sigloch, 2013). Its primary objective is to clarify the presence or absence of a mantle plume beneath the Reunion volcanic hotspot. RHUM-RUM's central component is a 13-month deployment (October 2012 to November 2013) of 57 broadband ocean bottom seismometers (OBS) and hydrophones over an area of 2000 × 2000 km2 surrounding the hotspot. The array contained 48 wideband OBS from the German DEPAS pool and 9 broadband OBS from the French INSU pool. It is the largest deployment of DEPAS and INSU OBS so far, and the first joint experiment.
This article reviews network performance and data quality: of the 57 stations, 46 and 53 yielded good seismometer and hydrophone recordings, respectively. The 19 751 total deployment days yielded 18 735 days of hydrophone recordings and 15 941 days of seismometer recordings, which are 94 and 80 % of the theoretically possible yields.
The INSU seismic sensors stand away from their OBS frames, whereas the DEPAS sensors are integrated into their frames. At long periods (> 10 s), the DEPAS seismometers are affected by significantly stronger noise than the INSU seismometers. On the horizontal components, this can be explained by tilting of the frame and buoy assemblage, e.g. through the action of ocean-bottom currents, but in addition the DEPAS intruments are affected by significant self-noise at long periods, including on the vertical channels. By comparison, the INSU instruments are much quieter at periods > 30 s and hence better suited for long-period signals studies.
The trade-off of the instrument design is that the integrated DEPAS setup is easier to deploy and recover, especially when large numbers of stations are involved. Additionally, the wideband sensor has only half the power consumption of the broadband INSU seismometers. For the first time, this article publishes response information of the DEPAS instruments, which is necessary for any project where true ground displacement is of interest. The data will become publicly available at the end of 2017.
