Kristine M. Larson

Kinematics of the India‐Eurasia collision zone from GPS measurements

We use geodetic techniques to study the India-Eurasia collision zone. Six years of GPS data constrain maximum surface contraction rates across the Nepal Himalaya to 18 ± 2 mm/yr at 12°N ±13° (1σ). These surface rates across the 150-km-wide deforming zone are well fitted with a dislocation model of a buried north dipping detachment fault striking 105°, which aseismically slips at a rate of 20 ± 1 mm/yr, our preferred estimate for the India-to-southern-Tibet convergence rate. This is in good agreement with various geologic predictions of 18 ± 7 mm/yr for the Himalaya. A better fit can be achieved with a two-fault model, where the western and eastern faults strike 112° and 101°, respectively, in approximate parallelism with the Himalayan arc and a seismicity lineament. We find eastward directed extension of 11 ± 3 mm/yr between northwestern Nepal Lhasa, also in good agreement with geologic and seismic studies across the southern Tibetan plateau. Continuous GPS sites are used to further constrain the style and rates of deformation throughout the collision zone. Sites in India, Uzbekistan, and Russia agree within error with plate model prediction.

Space geodetic measurement of crustal deformation in central and southern California, 1984-1992

A laboratory type of analyzer for quantitatively determining the percent third element content of a hydrocarbon sample. A unique rhodium/americium radioactive source is disclosed.

Crustal displacements due to continental water loading

The effects of long-wavelength (> 100 km), seasonal variability in continental water storage on vertical crustal motions are assessed. The modeled vertical displace- ments (ARM) have root-mean-square (RMS) values for 1994- 1998 as large as 8 mm, with ranges up to 30 mm, and are predominantly annual in character. Regional strains are on the order of 20 nanostrain for tilt and 5 nanostrain for hori- zontal deformation. We compare ArM with observed Global Positioning System (GPS) heights (Aro) (which include ad- justments to remove estimated effects of atmospheric pres- sure and annual tidal and non-tidal ocean loading) for 147 globally distributed sites. When the Aro time series are ad- justed by ArM, their variances are reduced, on average, by an amount equal to the variance of the ArM. Of the Aro time series exhibiting a strong annual signal, more than half

The motion and active deformation of India

Measurements of surface displacements using GPS constrain the motion and deformation of India and India-Eurasia plate boundary deformation along the Himalaya. The GPS velocities of plate-interior sites constrain the pole of the angular velocity vector of India with respect to Eurasia to lie at 25.6±1.0°N 11.1±9.0°E, approximately 6° west of the NUVEL-1A pole of <3 Ma plate motion. The angular rotation rate of 0.44 ±0.03°Myr−1 is 14% slower than the long-term rate of 0.51° Myr−1. Insignificant velocities between plate interior sites indicate that the exposed Indian plate is stable to within 7 · 10−9 yr−1. The observed contraction vector across the Himalaya (≤20 mm/yr) veers from ∼N20°E in the northwest Himalaya to ∼N25°W in east Nepal, consistent with east-west extension of southern Tibet.

Global plate velocities from the Global Positioning System

We have analyzed 204 days of Global Positioning System (GPS) data from the global GPS network spanning January 1991 through March 1996. On the basis of these GPS coordinate solutions, we have estimated velocities for 38 sites, mostly located on the interiors of the Africa, Antarctica, Australia, Eurasia, Nazca, North America, Pacific, and South America plates. The uncertainties of the horizontal velocity components range from 1.2 to 5.0 mm/yr. With the exception of sites on the Pacific and Nazca plates, the GPS velocities agree with absolute plate model predictions within 95% confidence. For most of the sites in North America, Antarctica, and Eurasia, the agreement is better than 2 mm/yr. We find no persuasive evidence for significant vertical motions (<3 standard deviations), except at four sites. Three of these four were sites constrained to geodetic reference frame velocities. The GPS velocities were then used to estimate angular velocities for eight tectonic plates. Absolute angular velocities derived from the GPS data agree with the no net rotation (NNR) NUVEL-1A model within 95% confidence except for the Pacific plate. Our pole of rotation for the Pacific plate lies 11.5° west of the NNR NUVEL-1A pole, with an angular speed 10% faster. Our relative angular velocities agree with NUVEL-1A except for some involving the Pacific plate. While our Pacific-North America angular velocity differs significantly from NUVEL-1A, our model and NUVEL-1A predict very small differences in relative motion along the Pacific-North America plate boundary itself. Our Pacific-Australia and Pacific-Eurasia angular velocities are significantly faster than NUVEL-1A, predicting more rapid convergence at these two plate boundaries. Along the East Pacific Rise, our Pacific-Nazca angular velocity agrees in both rate and azimuth with NUVEL-1A.

Geodetic evidence for a low slip rate in the Altyn Tagh fault system

The collision between India and Asia has been simulated with a variety of computational models that describe or predict the motions of the main faults of east Asia. Geological slip-rate estimates of 20–30 mm yr -1 suggest that the largest of these faults, the 2,000-km-long Altyn Tagh fault system on the northern edge of the Tibetan plateau, absorbs as much of the Indo-Asian convergence signal as do the Himalayas—partly by oblique slip and partly by contraction and mountain growth. However, the predictions of dynamic models for Asian deformation and the lower bounds of some geological slip-rates estimates (3–9 mm yr -1; refs 7, 8) suggest that the Altyn Tagh system is less active. Here, we report geodetic data from 89–91° E that indicate left-lateral shear of 9 ± 5 mm yr-1 and contraction of 3 ± 1 mm yr-1 across the Altyn Tagh system. This result—combined with our finding that, at 90° E, Tibet contracts north–south at 9 ± 1 mm yr-1—supports the predictions of dynamic models of Asian deformation.

Transient fault slip in Guerrero, southern Mexico

The Guerrero region of southern Mexico has ac- cumulated more than 5 m of relative plate motion since the last major earthquake. In early 1998, a continuous GPS site in Guerrero recorded a transient displacement. Modeling indicates that anomalous fault slip propagated from east to west along-strike of the subduction megathrust. Campaign GPS and leveling data corroborate the model. The moment release was equivalent to an Mw≥6.5 earthquake. No M> 5 earthquakes accompanied the event, indicating the frictional regime is velocity-strengthening at the location of slip.

GPS Multipath and Its Relation to Near-Surface Soil Moisture Content

Measurements of soil moisture at various spatial and temporal scales are needed to study the water and carbon cycles. While satellite missions have been planned to measure soil moisture at global scales, these missions also need ground-based soil moisture data to validate their observations and retrieval algorithms. Here, we demonstrate that signals routinely recorded by Global Positioning System (GPS) receivers installed to measure crustal deformation for geophysical studies could be used to provide a global network of soil moisture sensors. The sensitivity to soil moisture is seen in reflected GPS signals, which are quantified by using the GPS signal to noise ratio data. We show that these data are sensitive to soil moisture variations for areas of 1000 m2 horizontally and 1-6 cm vertically. It is demonstrated that GPS signals penetrate deeper when the soil is dry than when it is wet. This change in penetration or ¿reflector¿ depth, along with the change in dielectric constant, causes the GPS signal strength to change its frequency and amplitude. Comparisons with conventional water content reflectometer sensors show good agreement (r2=0.9 to 0.76) with the variation in frequencies of the reflected GPS signals over a period of 7 months, with most of the disagreement occurring when soil moisture content is less than 0.1 cm3/cm3.

Earthquake nucleation by transient deformations caused by the M = 7.9 Denali, Alaska, earthquake

The permanent and dynamic (transient) stress changes inferred to trigger earthquakes are usually orders of magnitude smaller than the stresses relaxed by the earthquakes themselves, implying that triggering occurs on critically stressed faults. Triggered seismicity rate increases may therefore be most likely to occur in areas where loading rates are highest and elevated pore pressures, perhaps facilitated by high-temperature fluids, reduce frictional stresses and promote failure. Here we show that the 2002 magnitude M = 7.9 Denali, Alaska, earthquake triggered widespread seismicity rate increases throughout British Columbia and into the western United States. Dynamic triggering by seismic waves should be enhanced in directions where rupture directivity focuses radiated energy, and we verify this using seismic and new high-sample GPS recordings of the Denali mainshock. These observations are comparable in scale only to the triggering caused by the 1992 M = 7.4 Landers, California, earthquake, and demonstrate that Landers triggering did not reflect some peculiarity of the region or the earthquake. However, the rate increases triggered by the Denali earthquake occurred in areas not obviously tectonically active, implying that even in areas of low ambient stressing rates, faults may still be critically stressed and that dynamic triggering may be ubiquitous and unpredictable.

Secular and tidal strain across the Main Ethiopian Rift

Using a combination of laser ranging and GPS data acquired between 1969 and 1997 we derive a separation velocity for the Somali and Nubian plates in Ethiopia (4.5±1 mm/yr at N108±10E). This vector is orthogonal to the NNE-trending neotectonic axis (Wonji fault belt) of the Ethiopian rift axis. Current rifting is concentrated within a 33-km-wide zone that includes a 7-km-wide belt of late Quaternary faulting where maximum surface strain rates are comparable to those at active plate boundaries (0.1 μstrain/yr). The strain-field suggests that thin (<5 km) elastic crust separates thick continental lithosphere, a geometry quite different from oceanic rifting, and a mechanical configuration that favors the amplification of regional strain. Semidiurnal strain tides, however, as measured by kinematic GPS methods are not amplified along or across the rift, indicating that the rift zones low rigidity applies only at periods of years.