G34A-01
Indian Plate motion, deformation, and plate boundary interactions
We use 1867 GPS-measured velocities to geodetically constrain Indian plate motion and intraplate strain, and we examine plate boundary deformation and plate interactions around the Indian plate. Our solution includes 15 GPS velocities from continuously recording stations from within the stable Indian plate interior that are used to estimate the rotational parameters of the Indian plate with respect to its neighbors. We test a two-plate Indian system found to be significant only to 89%. We treat the Indian plate as a single plate although intra-plate deformation could be as large as 4 mm/yr in eastern Indian along the Narmada-Son Lineament. We define the Shillong plateau as an independently block rotating counterclockwise. The predicted slip rate along the Dauki fault is 6 mm/yr, less than previous geodetic estimates, but twice the rate suggested by geologic studies. Dense station coverage along the Himalayan range front from NW India to Bhutan allows us to rigorously test boundary other parameterizations and develop a preferred kinematic plate boundary locking model. In our preferred model the Himalayan thrust system accommodates ~50% of the India-Eurasia convergence with as much as 25 mm/yr of slip accumulation along some segments. We estimate interseismic convergence vectors for the plate pairs along the Sumatra subduction zone and constrain the micro-plate motion of the Burma fore-arc sliver. Relative motions of the Australian, Somalian, and Arabian plates are also estimated and compared with slip vectors from earthquakes along the boundaries. We find that geodetic estimates of plate motion are consistent with slip vector orientations along the oceanic boundaries and provide constraints on potential triple junction locations not elucidated in the submarine bathymetry.
G34A-02
New Estimates of Present-day Arabia Plate Motion and Deformation From a Dense GPS Network in Saudi Arabia
Previous investigations of present-day kinematics of Arabia using GPS measurements were primarily obtained from sites located on surrounding plates, with few sites actually located on the Arabian plate itself. Due to the inhomogeneous distribution of these GPS sites and the fact that some of these were actually located in the plate boundary zone, the motion of Arabia was only sensed in a few locations of the rigid plate interior. We provide new estimates for present-day kinematics of Arabia from a dense network of 32 episodic GPS sites in Saudi Arabia and five IGS sites on the Arabian plate, between them covering nearly two thirds of the entire plate. Our new relative motion models for Arabia-Eurasia (28.17° N,18.92° E and 0.431°/Ma) and Nubia-Arabia (31.42° N, 24.47° E and 0.407°/Ma) agree within their uncertainties with those from recently published studies. Our new Arabia absolute motion model (50.420° N, 4.089° W and 0.533 °/Ma) is significantly different from those obtained in previous studies as a result of the number of sites used and their distribution. First results from a strain analysis indicate, in general, little or no deformation within the plate. However, these results also suggest south-western Saudi Arabia to undergo present-day deformation due to its proximity to the Red Sea spreading ridge and potentially also some relative motion between the Arabian shield and Arabian platform.
G34A-03
Low rigidity of the Tibetan plateau and the geodetic slip rate of the Altyn-Tagh fault
The estimates of the Altyn Tagh fault (ATF) slip rate north of the Tibetan Plateau lays at the heart of the ongoing debate about the mechanical behavior of the continental lithosphere in response to continental collision: does Tibet deform as a system of quasi-rigid blocks with deformation confined to block boundaries (high fault slip rate), or does it deform in a more distributed fashion? Although new results suggest that the Tibet could deform as rigid blocks with elastic strain together with a low ATF slip rate, discrepancies still remain between geodetic and geologic estimates. Here we argue that the thick elastic plate model used to estimate fault slip rates from GPS measurements is not appropriate to describe interseismic strain accumulating across the ATF. We propose a conceptual model, consistent with a wide variety of geophysical observations, where the rigidity of the Tibetan plateau is much smaller than the Tarim block. In this model, the interseismic strain due to the locked ATF largely spreads inside the Tibetan plateau. Making the assumption that no other significantly active fault exist south of the ATF, the deduced geodetic ATF slip rate might reach higher values than previously proposed by block model analysis. This implies that geodetic measurements need to be done across the central part of the ATF and in western-central Tibet before one can determine the geodetic fault slip rate of the ATF.
G34A-04
GPS and Geologic Deformation Rates Agree to Within Uncertainties in the Arabia-Africa- Eurasia Zone of Plate Interaction
Geodetically-derived motions for Arabia and Nubia relative to Eurasia agree within 1 standard deviation with plate rates estimated from geologic observations (McQuarrie et al., GRL, 2003) for the past 11 Myr for Nubia and greater than 25 Myr for Arabia. Furthermore, fault slip rates derived from an elastic block model constrained by GPS agree within uncertainties (about +/- 15 percent) with geologically determined, long-term slip rates in this complex area of plate interaction. Detailed geomorphological studies of the central North Anatolian fault (NAF) constrained by quantitative dating (Kozaci et a al., Geology, 2007) indicate slip rates that agree within uncertainties, but appear to be systematically lower than geodetic rates. While real rate changes of a few mm/yr cannot be ruled out at present, we note that geodetic inversions for coseismic fault slip on the NAF, and most other faults well constrained by geodetic observations, indicate larger slip at depth than at the surface. If this difference persists throughout the earthquake deformation cycle, it would account for the small difference in geodetic and geologic rates. Extrapolating present-day geodetic motions for Arabia relative to Nubia and Somalia to the time of initiation of Red Sea and Gulf of Aden extension indicates that Arabia separated from Nubia and Somalia simultaneously along the full extent of both rifts at about 25 Myr BP, consistent with independent geologic estimates for the style, and age of initiation of Red Sea extension (Omar and Steckler, 1995, Science). In addition, structural offsets across the Gulf of Suez (GoS) and Gulf of Aqaba (GoA) are consistent with a transfer of strain form the GoS to the GoA at around 12 Ma BP, roughly consistent with the age on initiation of the Dead Sea fault system. We further show that the apparent discrepancy between geodetic deformation of the Aegean (plate-like motion with low internal deformation), and geologic deformation (extensive crustal thinning during the Late Miocene), can be accounted for by a change in tectonic deformation due to the NAF cutting across the N Aegean and connecting with the Kephelonia fault during the Pliocene. This allows the S Aegean and Pelloponessis to translate SW with little internal deformation as is observed today.
G34A-05 INVITED
Evolution of Strike-Slip Faults: An Example from the Hunter Mountain-Panamint Valley Fault Zone, Eastern California
Frictional resistance to slip at plate boundaries is generally considered to be low. Since new faults and intact rock are geometrically complex and characterized by high friction, resepctively, this implies that faults undergo a process of simplification and friction reduction. We investigate this process by studying the Hunter Mountain – Panamint Valley fault zone, part of the Eastern California shear zone. This fault is relatively young (less then 3 Ma) and has total offset less than 10 km. New GPS and InSAR data indicate a rate (4-6 mm/yr) that is faster than previous geologic estimates (2-4 mm/yr). In addition, the zone of maximum surface velocity gradient (maximum shear strain) in Panamint Valley, presumably indicating the main zone of shear at depth, lies ~10 km west of the surface trace of the Panamint Valley fault, near the Ash Hill fault. The region of high velocity gradient in the valley is northwest striking, and projects into the central section of the Hunter Mountain fault to the north. The location of high velocity gradient is consistent with a simple model of dextral oblique extension at depth, partitioned at the surface into a paired normal fault – strike slip fault system. Geologic rate estimates on just the normal fault component in Panamint Valley would therefore underestimate the total rate of the fault zone. The simple geometry of the high gradient zone contrasts with the complexity of the various fault segments at the surface. The fault zone may be undergoing geometric simplification, with the zone of maximum velocity gradient observed by space geodesy marking the future location of the simplified fault. A simple model for fault evolution involving a monotonic increase in slip rate with time as the fault matures and straightens, and frictional resistance to slip decreases, is consistent with available geologic and geodetic data. In such a model, integration of the rate-time path from zero rate at fault initiation (constrained by geochronologic data) to present-day rate (measured by geodesy) gives the correct total fault offset.
G34A-06 INVITED
Geodetic Constraints on Strain Transfer Between the Colorado Plateau and the Basin and Range Province, Southwestern United States
Geodetic measurements of crustal deformation in the western United States have provided unprecedented constraints on the kinematics of deformation of the Pacific-North America plate boundary zone. In particular, it is now well established that deformation in the northern Basin and Range Province is mostly taking place along its western and eastern margins with very little internal strain in the middle part of the Province. Westward motion of the bulk of the Province of 2-3 mm/yr relative to North America/Colorado Plateau is being accommodated along a roughly 100-km wide deformation zone centered near the Wasatch fault. We present a new model of how these 2-3 mm/yr of relative motion are being accommodated further south using velocities of continuous GPS sites in the area, mainly those belonging to the BARGEN and EarthScope's Plate Boundary Observatory networks. The velocity field allows for an interpretation in which the relatively narrow deformation zone across the Wasatch broadens southward to about 700-km wide in the southern Basin Range at the latitude of southern Arizona. If true, the idea of an autonomous, rigid, and independently moving Colorado Plateau needs to be reassessed. Alternatively we find that subsets of the GPS velocities on the Plateau can be interpreted as having rigid-body rotation, but the Euler pole of the rotation is highly dependent on which subset of stations is being considered. Consequently, the inferred motion along the Plateau's margins vary considerably between models and can only be tested by acquiring more geodetic data or by considering independent kinematic indicators such as earthquake slip vectors. Relative to sites on the southwestern Plateau we observe 1.7 and 1.2 mm/yr of westward motion of sites on the Nevada Test Site and Spring Mountains (southern Nevada), respectively, while we observe no extension directly across the Hurricane-Toroweap fault system (the Plateau's western physiographic boundary at this latitude). These results suggest a westward jump of the eastern perimeter of active deformation of the northern Basin and Range. We also observe 2 mm/yr of sinistral motion across the eastern part of the ENE-SSW striking Pahranagat shear zone. This shear zone is seismically very active, with consistent strike-slip earthquake mechanisms, and exhibits evidence of long-term sinistral offset. This result suggests that the Pahranagat shear zone acts as key component in the transfer of extension from the Wasatch in the east to the Eastern California Shear Zone in the west.
G34A-07
Geodetic strain rates in the New Madrid Seismic Zone, 2000-2008: Converging Toward Zero
The occurrence of large earthquakes in plate interiors is taken as
evidence that significant amounts of elastic strain accumulate along
intraplate faults. As in the case of plate boundary faults, hazard
assessment strategies ususally assume that characteristic earthquakes
regularly unload the accumulated strain. However, although geodetic
methods have long been able to quantify strain accumulation on plate
boundary faults, there is still no unchallenged detection of similar
deformation within plate interiors.
The New Madrid Seismic Zone (NMSZ) has been the focal point of this
issue because of the M7+ events that struck the area in 1811-1812
and raised the issue of earthquake hazard to high priority in this
now populated area of the american midcontinent. Geodetic strain
rates remain debated, with recent estimates ranging from "comparable
in magnitude to those across active plate boundaries" (Smalley et
al., Nature, 2005) to "no statistically significant site motions
or strains" (Calais et al., Nature, 2005).
Here, we present a new geodetic solution for the NMSZ. We find
that velocity uncertainties have decreased by at least a factor of
2 at all sites compared to the 2005 solution, thanks to longer time
series and improved GPS processing techniques. Residual velocities
(w.r.t. stable North America) have decreased as well, with an overall
RMS of 0.2 mm/yr (compared to 0.7 mm/yr in the 2005 solution). Sites
with the worse quality position time series (large number of outliers,
large amount of non-white noise, large seasonal signals) such as
RLAP also have the largest velocity residuals. We use simulated
data to show that none of the observations require residual velocities
different from zero.
http://web.ics.purdue.edu/~ecalais/projects/noam/
G34A-08
Long-term CGPS Measurements (1995-2008) in the Hellenic Deformation Zone Between the Eurasian and African Plates
The Eastern Mediterranean forms the seismically most active region of the Alpine-Mediterranean plate boundary. It is characterized by the collision between the Eurasian and African plates. The collision is closely related to continental subduction and formation of the pronounced Hellenic trench system. In addition to the relatively slow CCW rotation of the African plate, rapid motion of the Anatolian-Aegean region is encountered, directed towards west-southwest, reaching velocities of up to 4 cm/yr along the Hellenic arc, relative to Eurasia. A long-term record of continuous GPS (CGPS) time series (1995-2008) and campaign- type GPS measurements (1991-2008) will be presented. The data has been analyzed to derive rates of plate and microplate motion and to study the strain rate field in the deforming zone between the Eurasian and African plates. This includes the deformation belt extending from the Ionian islands to the North Aegean Sea, Greece. While the Ionian islands are characterized by the Kephalonia fault zone which terminates the subduction of the Hellenic arc the most important feature in the North Aegean sea is the North Aegean trough which is considered to form the western continuation of the North Anatolian Fault Zone. Most recent GPS results will be presented for both regions and discussed in terms of ongoing deformation processes including dextral faulting and transtension, encountered in the northern Hellenic boundary region between the Eurasian and African plates.