G31D-01 INVITED 08:00h
Contemporary Plate Kinematics and Crustal Deformation in Iran Constrained by Geodetic Measurements
Several GPS networks have been installed in Iran and Northern Oman to measure the active tectonic deformation of this part of the Alpine-Himalayan mountain belt. These studies confirm the rigidity of the Northern part of the Arabian plate and a northward motion relative to Eurasia slower than the NUVEL-1A estimates. This northward shortening is accommodated differently in the western and eastern part of Iran. West of 58$\deg$E, the deformation is distributed in several orogens (Zagros, Talesh, Alborz and Caucasus) surrounding rigid blocks (Central Iranian block and South Caspian block). The CIB/Sanadaj-Sirjan zone is characterized by a very low internal deformation. The relative motion of the Arabia relative to the CIB allows to define long term N-S shortening of the Zagros. At the longitude of Tehran, the Zagros and the Alborz mountain ranges accommodate 6.5 $\pm$ 2 mm/yr and 5 $\pm$ 2 mm/yr respectively. $\sim$3 mm/yr seems to be accommodated somewhere in the Dasht-e-Kavir (i.e., between the CIB and the Alborz Range). The right lateral displacement along the Main Recent Fault in the NW Zagros is about 3 $\pm$ 2 mm/yr, smaller than what was generally expected. By contrast, large right lateral displacement takes place in the NW Iran on the Tabriz fault. The motion of the SCB cannot be precisely inferred by the GPS measurement but it seems that the direction is roughly NW with a rate lesser than $\sim$6 mm/yr. East of 58$\deg$E, most of the deformation is accommodated by the Makran subduction at a rate of 19.5 $\pm$ 2 mm/yr and the remaining deformation is accommodated north of the Lut block in the Kopet-Dag. The differential between eastern and western Iran is accommodated by strike slip motions along Minab fault zone and the NS faults bordering the Lut block.
G31D-02 INVITED 08:20h
Surface displacements and source parameters of the 2003 Bam (Iran) earthquake from Envisat ASAR imagery
The city of Bam in eastern Iran was struck by a deadly Mw 6.6 earthquake on 26th December 2003. This event was one of the first earthquakes for which Envisat ASAR data were available. Using interferograms and azimuth offsets from ascending and descending tracks, we construct a 3D displacement field of the deformation due to the earthquake. Elastic dislocation modelling shows that this observed deformation pattern cannot be explained by models involving slip on a single planar fault, which significantly underestimate eastward and upward motions SE of Bam. We find that the deformation pattern observed can be best explained by slip on two subparallel faults. Approximately 85% of moment release from the earthquake occurred on a previously unknown strike-slip fault running into the centre of Bam, with peak slip of 2.7 m occurring at a depth of 5 km. The location of this fault is consistent with the largest observed surface ruptures. The remainder of the deformation occurred as a combination of strike-slip and thrusting motion on a parallel fault -- a southward extension of the previously-mapped Bam Fault approximately 5 km to the east of the main fault. The geometry and proximity of the two faults suggest that they must interact at depth; similarly complex geometries of faulting have been inferred for recent earthquakes in the Gowk fault zone around 100 km to the NW.
G31D-03 08:40h
Creep Along the North Anatolian Fault at Ismetpasa (Western Turkey) Deduced From InSAR
Although creeping along the North Anatolian Fault (NAF) at Ismetpasa (Turkey) was discovered some thirty years ago, about a decade after the first observation of the phenomenon along the San Andreas Fault in California, little is known about its extent and rate. In order to reveal its three dimensional nature and rupture characteristics, we use Synthetic Aperture Radar Interferometry (InSAR) and dislocations on rectangular faults in elastic half space. Interferograms with temporal baselines ranging between 1.25 and 5 years show that creeping starts at the western termination of the 1943 (Mw=7.6) earthquake rupture and continues about 70-km to the west overlapping with the eastern part of the 185-km-long rupture of the 1944 (Mw=7.3) earthquake. Maximum creep rate is 10 mm/year approximately in the mid point of the creeping part of the rupture segment diminishing gradually towards the edges. Near Ismetpasa, InSAR data yield 7.7 mm/year of creep rate, consistent with those deduced from instrumental (triangulation and creepmeters) measurements (i.e. 9 mm/year). Modeling of the InSAR and GPS data suggests that the fault creep occurs most probably at a shallow depth (0-7 km). InSAR data do not support the previous claims of creep events triggered by the 1999 Izmit earthquake.
G31D-04 08:55h
GPS Constraints on Continental Deformation in the Eastern Mediterranean and Caucasus Region
(Presented on behalf of the E. Med/Caucasus GPS Consortium). We use GPS observations during the period 1988 to 2004 to constrain an elastic block model for deformation within the zone of interaction of the Eurasian, African, and Arabian plates. We constrain present-day motions of the African (Nubian), Arabian, and Eurasian plates, regional deformation within the inter-plate zone, and slip rates for major faults. Kinematically, we interpret the deformation field in terms of a block-like response of the continental lithosphere including the westward motion of Anatolia and the N-E motion of the eastern Turkey/Lesser Caucasus region away from the Arabia-Eurasia collision zone. The eastern Turkey and Lesser Caucasus region is undergoing counterclockwise rotation that results in increasing rates of convergence from west to east along the Main Caucasus Thrust. We observe an apparent change in orientation across the Sevan fault in the Lesser Caucasus consistent with right-lateral strike slip (probable source of the 1988, M=6.9 Spitak Earthquake), but relative motions are constrained to be less than 1 to 2 mm/yr, about an order of magnitude less than the motion between the Lesser and Greater Caucasus blocks. We observe deformation within the Greater Caucasus block at about latitude 47.5°E that may be associated with a change in the strike of the Greater Caucasus thrust fault from ESE to approximately N-S where it may transition into a right-lateral fault along the west side of the Caspian Sea. Although poorly constrained, our investigations suggest shallow locking depths (about 5 km) for the Main Caucasus Thrust and the Hellenic subduction zone, and normal locking depths for other faults (15 to 20 km). This may help account for the low level of historic seismic strain release observed in the Caucasus and along the Hellenic Arc. A particularly interesting result of our model is that the East Anatolian Fault (boundary between the Anatolian and Arabian plates) is transtensional. If this result persists with improved GPS control on Arabia plate motion, it would imply that the westward motion of Anatolia is currently being driven completely by buoyancy forces (i.e., not "extrusion"), including foundering of the down-going African plate along the Hellenic trench.
G31D-05 09:10h
Constraints on Glacial Isostatic Adjustment in Europe and Tectonics in the Western Mediterranean from Permanent GPS array solutions
We present a new velocity field for Europe and the Mediterranean spanning 8 years of data between 1996.0-2004.7 period, derived from a rigorous combination of (1) a selection of sites from the ITRF2000 solution, (2) a subset of sites from the European Permanent GPS Network solution, (3) a solution of the French national geodetic permanent GPS network (RGP), and (4) a solution of a permanent GPS network in the western Alps (REGAL). The resulting velocity field describes horizontal crustal motion at $\sim$150 sites with an accuracy of the order of 1 mm/yr or better. We first investigate the Glacial Isostatic Adjustment deformation in Europe south of Fennoscandia. Uplift in Fennoscandia is surrounded by a subsidence trough reaching as far south as the Alps, with a maximum subsiding rate of 1.5~mm/yr located at latitude 50.5-53$^\circ$N. The vertical GPS velocities agree with the model prediction of Milne et al. (2001) at 0.7 mm/yr (wrms). Looking at the horizontal velocities, shortening occurs between Fennoscandia and northern central Europe with magnitude of strain rate of the order 10$^{-9}$ $yr^{-1}$. The principal direction of shortening changes longitudinally and systematically points toward the Gulf of Botnia. No deformation can be detected south of latitude 52$^\circ$N. In particular, A stability analysis shows that no motion can be resolved across the Rhine Graben as well as across the Pyrenees and the islands of western Mediterranean. As a consequence, the totality of the shortening must be accommodated in Northern Africa and in the Betics in Spain. In Italy, although a counter-clockwise rotation of the Adriatic plate explains the general pattern of velocities, detailed study indicates internal deformation of the microplate and NE-SW extensional strain rates as high as 40 ± 10 .10$^{-9}$ yr$^{-1}$ occurs in the Apennines. Finally, we find that the Dinarides accommodates $\sim$3.5 mm/yr of NE-SW shortening between Dubrovnik and Sarajevo.
G31D-06 INVITED 09:25h
A new strain rate model for the Great Basin and its application to tectonic and geodynamic studies
The Great Basin in the western United States covers a large portion of the diffuse PA-NA plate boundary zone. Yet the seismic potential of its many faults as well as the evolution of, and the driving forces behind, the deformation remain largely unknown or disputed. To advance our understanding it is important to quantify the spatial distribution of the rate, style and direction of the present-day deformation field. GPS velocity measurements are the single most important input to fulfill this objective, and many data are now available from continuous (e.g., BARGEN network) and campaign style measurements (USGS and others). We use the Haines and Holt technique to present a new strain rate model, which is superior in its use of the latest GPS solutions and a denser model grid. Furthermore, the release of the 2003 USGS fault database makes it possible to use geologic data (i.e., slip rate and/or fault geometry) either as an additional constraint in or as a comparison with models based on the interpolation of GPS velocities alone. The ultimate aim of this work is; 1) to compare present-day style and rate of deformation with finite strain markers to place constraints on the Quaternary evolution of deformation, particularly in the northern Walker Lane, 2) to use objective means in distinguishing potential rigid blocks, 3) to identify zones of transient deformation, 4) to further develop the observed relationship between shear strain rate, fault orientation and geothermal output, and 5) to improve geodynamic models by comparing modeled present-day strain rate directions with finite strain orientations in the middle to lower crust as shown in metamorphic complexes and in the lithosphere as inferred from seismic anisotropy. For this presentation we will discuss the data synthesis as well as the resolution and reliability of the model. Furthermore, a few examples will be highlighted to underline the potential of the model in addressing the goals described above. Finally, a brief introduction will be given to the semi-permanent Mobile Array of GPS for NEvada Transtension (MAGNET) network (currently 40-50 stations, and growing) that will greatly improve our spatial resolution and velocity precision in the western Great Basin.
G31D-07 09:45h
Crustal Deformation across the Sierra Nevada-Northern Walker Lane, Basin and Range Transition, Western United States Measured with GPS, 2000-2004
In the northern Walker Lane (WL) of northwest Nevada and northeast California, approximately 20-25% of the contemporary Pacific/North America relative plate motion is accommodated east of the Sierra Nevada in a focused zone of dextral shear and extension. Previous measurements with the Global Positioning System (GPS) have identified a partitioning of shear and extension and a widening of the zone of active deformation north of 39? north latitude. This deformation pattern may indicate that northwest-directed extension in the Basin and Range east of the WL becomes more active northward, making up for a northward decrease in northwest-directed dextral slip on the WL strike slip faults. Alternatively (or additionally), it may be an effect of postseismic relaxation following the 20th century earthquakes in the Central Nevada Seismic Belt (CNSB), east of the WL. We present velocities and strain rate tensors obtained from newly collected GPS data along networks that extend from the Central Great Valley of California, across the Sierra Nevada, Walker Lane and into the central Basin and Range province. Campaign surveys in September 2000 and September 2004 occupied 56 sites that have greater spatial density ($<$20 km), and lie outside the footprint of the planned deployment of the Plate Boundary Observatory Extension/Backbone continuous GPS cluster. These networks fill gaps in previous GPS coverage, and between the sparsely (near 100 km) spaced continuously recording Basin and Range Geodetic Network (BARGEN), whose data we include in our GIPSY/OASIS II processing. Additionally, our solution will include GPS data from 40+ stations of a new network of semi-continuous sites: the Nevada Mobile Array of GPS for Nevada Transtension (MAGNET), which commenced operation the end of January 2004 and currently has $<$1 year of data, but provides a complementary spatial distribution of sites and will, in future, greatly enhance constraint on WLB surface deformation. We will use the combined solution to provide new constraints on postseismic relaxation on the CNSB and on plate boundary kinematics in the northern WL.