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The French MOBAL'03 Experiment

The MOBAL'03 Transect


PICS 2001-2003: Project - Results 2002 in French


MoBaL is the acronym for "Mongolian-Baikal Lithosphere seismological Transect". This work was led in the framework of the French CNRS Research Program called PICS ("Programme International de Coopération Scientifique") entitled "Déformations actives et structures lithosphériques en domaine intra continental : étude intégrée du système Mongolie-Baïkal". It implies a collaboration between CNRS in France, Academy of Sciences in Russia (Siberian Branch of the RAS) and Academy of Sciences in Mongolia.

Year 1 (2001) was dedicated to paleoseismology under the direction by Jeff RITZ.
Year 2 (2002) was dedicated to GPS geodesy under the direction by Eric CALAIS.
Year 3 (2003) was dedicated to the MOBAL experiment under the direction by Anne DESCHAMPS.
General coordination was made by Jacques DEVERCHERE.

The 2003 MOBAL Experiment aimed at imaging the crustal and upper mantle structure from the Siberian platform in the north to the Gobi-Altay range in the south, in order to provide geodynamic constraints to deep lithosphere and asthenosphere velocities and deformations, and their relations to the Siberian craton, the Baikal rift, the Hangay dome and the large scale strike-slip faults of Sayan, Bolnay and Bogd. Several observations suggest a hot upper mantle beneath central Mongolia likely associated to a lithosphere thinning, that may also explain the anomalous elevation associated to the Hangay dome.

The MOBAL experiment was divided into 3 phases:
- Prospecting for the definition and location of the seismic network : work done in summers 2001 and 2002.
- Installation and operation of 18 seismic stations (French instrumentation) in 2003 (April to October).
- Distribution of data, processing and interpretation (from November 2003 until now).

The main participants were : Anne Deschamps : Géosciences Azur, CNRS, Nice, France. Jacques Déverchère, Julie Perrot : Domaines Océaniques, UBO/CNRS, Brest, France. Valentina Mordvinova, Alexandre Artemiev, Institute of the Earth Crust, Irkustk, Siberia. T. Dugarmaa, M. Ulziibat, Urtnassan, RCAG, Ulan Bator, Mongolia.

The period of observation was chosen from 25 April to 15 October 2003 to have the longer observation duration and avoid winter problems at the stations.

Field experiment

Sites have been recognized and prepared in 2002 (see report). The main instrumentation involved is the following:

A. Siberia:

- Sensors : 1 CMG3ESP, 1 STS2, 1 CMG3T (all with a similar response at 120 sec) 3 CMG40 (response at 40 sec)
- Digitisers : 6 titan3XT/minititan3XT, with titan GPS for time, which will be installed and on continuous power.
- Spare digitizer 14 hard disks for titan3XT or minititan3XT
- 7 battery chargers 2 laptop for maintenance (dos)
- 1 PC for data storage (+CDROM writer) (windows)
- 1 PC for data processing

B. Mongolia:

- Sensors : 4 STS2, 1 CMG3T (all with a similar response at 120 sec) 7 CMG40 (response at 40 sec)
- Digitisers : 4 titan6NT (see photos) 4 titan3XT (see photos) 4 minititan3XT (as titan3XT), all with GPS time
- 13 hard disks for titan6NT
- 17 hard disks for titan3XT or minititan3XT
- Solar panels (2 per station)
- 2 laptop for maintenance (dos)
- 1 PC for data storage (+CDROM writer) (windows)
- 1 PC for data processing

The location of the network was defined and sites have been prospected in summer 2002: 6 sites are in Siberia, 12 sites in Mongolia.

2 visits have been done to the instruments during the recording period. This work was taken in charge by Mongolian and Siberian colleagues. A visit consists mainly to test how the recorder is working, change the disk and write on the visit page all information necessary for the understanding of the data recorded.

The data set is on the way to be available on the data delivery Center FOSFORE. They will be soon available through the IRIS data Center.




Velocity structure of the lithosphere on the 2003 Mongolian-Baikal transect from SV waves
Izvestiya Phys. Solid Earth, 43 (2), 119-129, ISSN 1069-3513, 2007.
The S wave velocity distribution in the Earth's crust and the first two hundred kilometers of the upper mantle is inferred from data of a seismological linear network including 18 broadband stations installed in the framework of the international teleseismic experiment carried out in 2003 in the south of Siberia and in Mongolia. Models were constructed by using P-lo-S received function inversion beneath each station. Vertical cross sections of S wave velocities from the surface to depths of 65 and 270 km covering the entire 1000-km profile are constructed by the linear spline interpolation of individual velocity models. The vertical sections are also represented as anomalies relative to the standard velocity model. The most intense low velocity anomalies (from -3 to -6%) in the crust and upper mantle are identified beneath the Sayan, Khamar-Daban, and Khangai highlands and the Djida fold zone and agree both with other geophysical data and with the distribution of Late Cenozoic volcanic fields. The results of this work suggest that the activation of Mongolian-Siberian highlands is largely connected with uplift of the asthenosphere to the base of the crust.

Upper mantle flow beneath and around the Hangay dome, central Mongolia
Revised at Earth Planet. Sci. Lett.
Mongolia represents the northernmost area affected by the Indian-Asia collision and is actively deformed along transpressive belts closely associated to large-scale strike-slip faults. The active or frozen mantle flow beneath this region is however poorly known. In order to investigate the deep mantle deformation beneath central Mongolia and its relation with the surrounding major structures such as the Siberian craton, the Altay range and the Baikal rift, a NS trending profile of broadband seismic stations has been deployed in summer 2003 from the southern Siberian craton to the Gobi-Altay range, crossing the whole Hangay dome. Mantle flow is deduced from the splitting of teleseismic shear waves such as SKS phases. In eastern Mongolia, the permanent station ULN in Ulaanbaatar reveals the presence of two anisotropic layers, the upper one being oriented NE-SW, close to the trend of lithospheric structures and the lower one NW-SE, close to the trend of plate motion. Along the NS profile in central Mongolia, seismic anisotropy deduced from SKS splitting reveals a homogeneous NW-SE trending structure, fully consistent with the observations performed in the Altay-Sayan in western Mongolia. Since the observed delay times of 1.5 to more than 2.0 s suggest coherent mantle flow over large mantle thicknesses and since the observed fast directions are parallel to the trend of the lithospheric structures but also close to the trend of the plate motion, we propose that both the lithosphere and the asthenosphere may add their anisotropic effects beneath central Mongolia. In order to interpret the clockwise rotation of the fast directions relative to the plate motion vector, we propose that the root of the Siberian craton could deflect the asthenospheric flow around its southwestern side. GPS vectors and SKS splitting depicts a similar trend beneath central Mongolia, suggesting that the block “escaping” (or pull) toward the east moves consistently with the lithospheric and asthenospheric mantle flow. A strikingly different behaviour is observed in western Mongolia: The GPS vectors trend NS whereas the fast SKS directions trend EW, suggesting that a decoupling occurs somewhere between the upper crust moving northwards and the mantle flowing eastwards.
Asthenospheric imprints on the lithosphere in Centra Mongolia and southern Siberia from a joint inversion of gravity and seismology (MOBAL experiment)
Submitted to Geophys. J. Int. March 2008.
ABSTRACT: We present a joint inversion of gravity and teleseismic data to enlighten the crustal and lithospheric structure of the Mongolia-Baikal region. The study uses a 1000 km long N-S trending seismic transect that crosscuts the main tectonic structures (Altaï belt, Hangai dome, Hövsgöl basin and Siberian craton). The main results obtained are: strong velocity responses in the crust for the Hangai Dome and Hövsgöl depression, a high-velocity lithosphere down to about 150 km for the Siberian platform, and a low velocity/low density region (depth ?) corresponding to the Hangai Dome. Taking advantage of the better spatial coverage of gravity data, we are able to define the 3D geometry of this low density / low velocity body. This body seems to be present from 60 to 140 km depth in his larger width, and it slightly thins to not more than 40 km when the seismic rays samples it (pas clair, est-ce que les rais sismiques ne traversent qu’une zone peu épaisse de ce corps ?). A deep low-velocity zone (down 150 km) is observed on the eastern part of the transect with only a weakly negative density constrast.
The comparison between time residuals of this experiment and the Baikal Rift seismic data show an undeniable disproportion for the Hangai anomaly. The residuals are there far more negative, pleading for an asthenospheric signature which is not seen beneath the Baikal rift.

The 2003 Chuya sequence (North Altay range): tectonic context and seismological study
PhD Thesis, Nice University, France, 2006
PDF (24.6 Mo)
The present work is focussed on the seismological study of the September 27th 2003 (M = 7.3) large earthquake in Altay and try to include the results on a tectonical study of the Altay range (Mongolia/Russia).
In the first chapter is presented tectonic and geodynamic aspect of Altay range. A review of existing works on the morphological and geological context is completed by an interpretation of satellite images. We consider four active fault zones from their continuity. On images we interpret several offsets of river streams. The distribution of offsets is similar along the same active zone, but are different from one fault to the other. We propose a partitioning of movements inside the Altay block and discuss the compatibility of the observations with a global rotation of the Altay.
In the second chapter is discussed the seismic catalogue of Mongolia produced by the RCAG and presented different methods to analyse it, in particular the clustering to reduce the impact of the aftershocks. We show distributions of b-values in the different zones and their relation with the abnormal occurrences of large events on the territory of Mongolia. Next, we focus more on Altay region and its observed seismic activity.
The third chapter is focused on the geological context of the Chuya earthquake. We discuss the available field observations and compared the measured displacement values to the results obtained with a SPOT image correlation made on the southern part of the ruptured zone. Surfaces rupture observed on the field and deduced from spot correlation are geometrically very similar, but amplitude of the horizontal displacement are different.
In the fourth chapter we analyze in detail location of the seismic sequence. Relocation of the first period (main shock and stronger aftershocks) is done with regional data. Next, we discuss the location procedure and show the aftershocks distribution obtained for the period covered by a temporary network. Main activity of aftershock occurred on southern margin of Chuya and Kurai depressions. The cluster is 90 km long in a NW-SE direction, superposed to the surface breaks in its central part, and extend to a depth of 20km.
In the fifth chapter is presented a model of the rupture for the mainshock and two large aftershocks obtained using body wave inversion. The 3 analysed events are modeled as complex events with combination of two or more subevents with different focal mechanisms and different rupture velocities. The source duration, and rupture velocities allow to reconstruct from the epicentral locations a rupture which is in agreement with the surface observation but the history of the sequence can be related to the segmentation of the fault and the presence of different tectonic units