Present-day crustal deformation in China constrained by global positioning system measurements

Global Positioning System (GPS) measurements in China indicate that crustal shortening accommodates most of India's penetration into Eurasia. Deformation within the Tibetan Plateau and its margins, the Himalaya, the Altyn Tagh, and the Qilian Shan, absorbs more than 90% of the relative motion between the Indian and Eurasian plates. Internal shortening of the Tibetan plateau itself accounts for more than one-third of the total convergence. However, the Tibetan plateau south of the Kunlun and Ganzi-Mani faults is moving eastward relative to both India and Eurasia. This movement is accommodated through rotation of material around the eastern Syntaxis. The North China and South China blocks, east of the Tibetan Plateau, move coherently east-southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year, respectively, with respect to the stable Eurasia.     

Calcium carbonate production, coral reef growth, and sea level change

Shallow, seaward portions of modern coral reefs produce about 4 kilograms of calcium carbonate per square meter per year, and protected areas produce about 0.8 kilogram per square meter per year. The difference is probably largely a function of water motion. The more rapid rate, equivalent to a maximum vertical accretion of 3 to 5 millimeters per year, places an upper limit on the potential of modern coral reef communities to create a significant vertical structure on a rising sea.     

Earthquake potential along the northern hayward fault, california

The Hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace. Dislocation models of the surface deformation adjacent to the Hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at approximately 7 millimeters per year to the bottom of the seismogenic zone along a approximately 20-kilometer-long northern fault segment. Microearthquakes with the same waveform repeatedly occur at 4- to 10-kilometer depths and indicate deep creep at 5 to 7 millimeters per year. The difference between current creep rates and the long-term slip rate of approximately 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern Hayward fault adjacent to the locked 1868 earthquake rupture, which broke the southern 40 to 50 kilometers of the fault. The potential for a major independent earthquake of the northern Hayward fault might be less than previously thought.     

Slip-rate measurements on the Karakorum Fault may imply secular variations in fault motion

Beryllium-10 surface exposure dating of offset moraines on one branch of the Karakorum Fault west of the Gar basin yields a long-term (140- to 20-thousand-year) right-lateral slip rate of approximately 10.7 +/- 0.7 millimeters per year. This rate is 10 times larger than that inferred from recent InSAR analyses ( approximately 1 +/- 3 millimeters per year) that span approximately 8 years and sample all branches of the fault. The difference in slip-rate determinations suggests that large rate fluctuations may exist over centennial or millennial time scales. Such fluctuations would be consistent with mechanical coupling between the seismogenic, brittle-creep, and ductile shear sections of faults that reach deep into the crust.     

High Rates of Vertical Crustal Movement near Ventura, California

Fission track, radiometric, and paleomagnetic age determinations in marine sedimentary rocks of the Ventura Basin make it possible to estimate the vertical components of displacement rates for the last 2 million years. The basin subsided at rates up to 9.5 +/- 2.5 millimeters per year until about 0.6 million years ago, when subsidence virtually ceased. Since then, the northern margin of the basin has been rising at an average rate of 10 +/- 2 millimeters per year, about the same rate as that based on the geodetic record north and west of Ventura since 1960 but considerably lower than the rate along the San Andreas fault at Palmdale since 1960.     

The Emperor Seamounts: southward motion of the Hawaiian hotspot plume in Earth's mantle

The Hawaiian-Emperor hotspot track has a prominent bend, which has served as the basis for the theory that the Hawaiian hotspot, fixed in the deep mantle, traced a change in plate motion. However, paleomagnetic and radiometric age data from samples recovered by ocean drilling define an age-progressive paleolatitude history, indicating that the Emperor Seamount trend was principally formed by the rapid motion (over 40 millimeters per year) of the Hawaiian hotspot plume during Late Cretaceous to early-Tertiary times (81 to 47 million years ago). Evidence for motion of the Hawaiian plume affects models of mantle convection and plate tectonics, changing our understanding of terrestrial dynamics.     

Changes in surface water supply across Africa with predicted climate change

Across Africa, perennial drainage density as a function of mean annual rainfall defines three regimes separated by threshold values of precipitation. This nonlinear response of drainage to rainfall will most seriously affect regions in the intermediate, unstable regime. A 10% decrease in precipitation in regions on the upper regime boundary (1000 millimeters per year) would reduce drainage by 17%, whereas in regions receiving 500 millimeters per year, such a drop would cut 50% of surface drainage. By using predicted precipitation changes, we calculate that a decrease in perennial drainage will significantly affect present surface water access across 25% of Africa by the end of this century.     

Dynamics of slow-moving landslides from permanent scatterer analysis

High-resolution interferometric synthetic aperture radar (InSAR) permanent scatterer data allow us to resolve the rates and variations in the rates of slow-moving landslides. Satellite-to-ground distances (range changes) on landslides increase at rates of 5 to 7 millimeters per year, indicating average downslope sliding velocities from 27 to 38 millimeters per year. Time-series analysis shows that displacement occurs mainly during the high-precipitation season; during the 1997-1998 El Ni?o event, rates of range change increased to as much as 11 millimeters per year. The observed nonlinear relationship of creep and precipitation rates suggests that increased pore fluid pressures within the shallow subsurface may initiate and accelerate these features. Changes in the slope of a hill resulting from increases in the pore pressure and lithostatic stress gradients may then lead to landslides.     

Critical depth for the survival of coral islands: effects on the hawaiian archipelago

Coral islands drown when sea level rise exceeds the maximum potential of coral reefs to grow upward (about 10 millimeters per year). During the Holocene transgression (18,000 years ago to present) sea levels rose at rates of up to 10 to 20 millimeters per year, and most coral island reefs situated deeper than a critical depth of30 to 40 meters below present day sea level drowned. Coral islands that did not drown during the Holocene transgression apparently all developed on antecedent foundations shallower than critical depth. During low stands in sea level during the Pleistocene, these islands were elevated and subject to subaerial erosion. Today, in the Hawaiian Archipelago, the depth of drowned banks is inversely related to summit area; smaller banks are progressively deeper, evidently because of erosional truncation during low sea level stands. Bank summit area may therefore be an important factor determining the failure or success of coral islands.     

Evidence for deep magma injection beneath Lake Tahoe, Nevada-California

A deep earthquake swarm in late 2003 at Lake Tahoe, California (Richter magnitude < 2.2; depth of 29 to 33 kilometers), was coeval with a transient displacement of 6 millimeters horizontally outward from the swarm and 8 millimeters upward measured at global positioning system station Slide Mountain (SLID) 18 kilometers to the northeast. During the first 23 days of the swarm, hypocentral depths migrated at a rate of 2.4 millimeters per second up-dip along a 40-square-kilometer structure striking north 30 degrees west and dipping 50 degrees to the northeast. SLID's transient velocity of 20 millimeters per year implies a lower bound of 200 nanostrains per year (parts per billion per year) on local strain rates, an order of magnitude greater than the 1996 to 2003 regional rate. The geodetic displacement is too large to be explained by the elastic strain from the cumulative seismic moment of the sequence, suggesting an aseismic forcing mechanism. Aspects of the swarm and SLID displacements are consistent with lower-crustal magma injection under Lake Tahoe.     

Rates of surficial rock creep on hillslopes in Western colorado

The average rate of downslope movement of rock fragments on shale hillslopes is directly proportional to the sine of the slope angle or that component of the gravitational force which acts parallel to the hillslope. The rates of surficial rock creep range from a few millimeters per year on a 3degree slope to almost 70 millimeters per year on a 40-degree slope, but these rates vary with natural variations in soil characteristics and microclimate, as well as with accidental disturbances.     

Impact of artificial reservoir water impoundment on global sea level

By reconstructing the history of water impoundment in the world's artificial reservoirs, we show that a total of approximately 10,800 cubic kilometers of water has been impounded on land to date, reducing the magnitude of global sea level (GSL) rise by -30.0 millimeters, at an average rate of -0.55 millimeters per year during the past half century. This demands a considerably larger contribution to GSL rise from other (natural and anthropogenic) causes than otherwise required. The reconstructed GSL history, accounting for the impact of reservoirs by adding back the impounded water volume, shows an essentially constant rate of rise at +2.46 millimeters per year over at least the past 80 years. This value is contrary to the conventional view of apparently variable GSL rise, which is based on face values of observation.     

Global Mean Sea Level Variations from TOPEX/POSEIDON Altimeter Data

The TOPEX/POSEIDON satellite altimeter mission has measured global mean sea level every 10 days over the last 2 years with a precision of 4 millimeters, which approaches the requirements for climate change research. The estimated rate of sea level change is +3.9 +/- 0.8 millimeters per year. A substantial portion of this trend may represent a short-term variation unrelated to the long-term signal expected from global warming. For this reason, and because the long-term measurement accuracy requires additional monitoring, a longer time series is necessary before climate change signals can be unequivocally detected.     

Crustal Deformation from 1992 to 1995 at the Mid-Atlantic Ridge, Southwest Iceland, Mapped by Satellite Radar Interferometry

Satellite radar interferometry observations of the Reykjanes Peninsula oblique rift in southwest Iceland show that the Reykjanes central volcano subsided at an average rate of up to 13 millimeters per year from 1992 to 1995 in response to use of its geothermal field. Interferograms spanning up to 3.12 years also include signatures of plate spreading and indicate that the plate boundary is locked at a depth of about 5 kilometers. Below that depth, the plate movements are accommodated by continuous ductile deformation, which is not fully balanced by inflow of magma from depth, causing subsidence of the plate boundary of about 6.5 millimeters per year.     

Contribution of the Patagonia Icefields of South America to sea level rise

Digital elevation models of the Northern and Southern Patagonia Icefields of South America generated from the 2000 Shuttle Radar Topography Mission were compared with earlier cartography to estimate the volume change of the largest 63 glaciers. During the period 1968/1975-2000, these glaciers lost ice at a rate equivalent to a sea level rise of 0.042 +/- 0.002 millimeters per year. In the more recent years 1995-2000, average ice thinning rates have more than doubled to an equivalent sea level rise of 0.105 +/- 0.011 millimeters per year. The glaciers are thinning more quickly than can be explained by warmer air temperatures and decreased precipitation, and their contribution to sea level per unit area is larger than that of Alaska glaciers.     

Global positioning system measurements for crustal deformation: precision and accuracy

Analysis of 27 repeated observations of Global Positioning System (GPS) position-difference vectors, up to 11 kilometers in length, indicates that the standard deviation of the measurements is 4 millimeters for the north component, 6 millimeters for the east component, and 10 to 20 millimeters for the vertical component. The uncertainty grows slowly with increasing vector length. At 225 kilometers, the standard deviation of the measurement is 6, 11, and 40 millimeters for the north, east, and up components, respectively. Measurements with GPS and Geodolite, an electromagnetic distance-measuring system, over distances of 10 to 40 kilometers agree within 0.2 part per million. Measurements with GPS and very long baseline interferometry of the 225-kilometer vector agree within 0.05 part per million.     

Snowfall-driven growth in East Antarctic ice sheet mitigates recent sea-level rise

Satellite radar altimetry measurements indicate that the East Antarctic ice-sheet interior north of 81.6 degrees S increased in mass by 45 +/- 7 billion metric tons per year from 1992 to 2003. Comparisons with contemporaneous meteorological model snowfall estimates suggest that the gain in mass was associated with increased precipitation. A gain of this magnitude is enough to slow sea-level rise by 0.12 +/- 0.02 millimeters per year.     

Recent sea-level contributions of the Antarctic and Greenland ice sheets

After a century of polar exploration, the past decade of satellite measurements has painted an altogether new picture of how Earth's ice sheets are changing. As global temperatures have risen, so have rates of snowfall, ice melting, and glacier flow. Although the balance between these opposing processes has varied considerably on a regional scale, data show that Antarctica and Greenland are each losing mass overall. Our best estimate of their combined imbalance is about 125 gigatons per year of ice, enough to raise sea level by 0.35 millimeters per year. This is only a modest contribution to the present rate of sea-level rise of 3.0 millimeters per year. However, much of the loss from Antarctica and Greenland is the result of the flow of ice to the ocean from ice streams and glaciers, which has accelerated over the past decade. In both continents, there are suspected triggers for the accelerated ice discharge-surface and ocean warming, respectively-and, over the course of the 21st century, these processes could rapidly counteract the snowfall gains predicted by present coupled climate models.     

Advances in helioseismology

Globally coherent oscillation modes were discovered in the sun about a decade ago, providing a unique seismological probe of the solar interior. Current observations detect modes that are phase-coherent for up to 1 year, with surface velocity amplitudes as low as 2 millimeters per second, and thousands of mode frequencies have been measured to accuracies as high as 1 part in 10(5). This article discusses the properties of these oscillation modes and the ways in which they are adding to our understanding of the structure and dynamics of the sun.     

Crust formation and plate motion in the early archean

Mounting evidence for voluminous continental crust formation in the early Archean involving intracrustal melting and selective preservation of granitoid rocks suggests that initial crust formation crust formation and growth were predominantly by magmatic underplating in plumegenerated Iceland-type settings. Collision of these early islands to give rise to larger blocks is suggested by extensive horizontal shortening in both supracrustal and granitoid assemblages. Preservation of early Archean high-grade gneisses that were once at depths of 20 to 30 kilometers implies that these blocks developed thick, subcrustal roots despite high mantle heat flow. Rigid continental plates must have existed since at least 3.5 billion years ago, and greenstone belts (composed of mixed metavolcanic and metasedimentary sequences intruded by granitoid plutons) probably developed on or near these microcontinents. Paleomagnetic data with good age control from at least one ancient craton suggest that plate motion was at normal minimum average velocities of about 17 millimeters per year with respect to the poles during the period 3.5 billion to 2.4 billion years ago. If this is true on a global scale, Archean plate motion was not faster than in later geologic times.