Variation of P-Wave Velocity before the Bear Valley, California, Earthquake of 24 February 1972

Residuals for P-wave traveltimes at a seismnograph station near Bear Valley, California, for small, precisely located local earthquakes at distances of 20 to 70 kilometers show a sharp increase of nearly 0.3 second about 2 months before a magnitude 5.0 earthquake that occurred within a few kilometers of the station. This indicates that velocity changes observed elsewhere premonitory to earthquakes, possibly related to dilatancy, occur along the central section of the San Andreas fault system.     

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.     

Venus tectonics: initial analysis from magellan

Radar imaging and altimetry data from the Magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the Venus surface. The plains record a superposition of different episodes of deformation and volcanism; strain is both areally distributed and concentrated into zones of extension and shortening. The common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes. Ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening; the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression. Venus displays two geometrical patterns of concentrated lithospheric extension: quasi-circular coronae and broad rises with linear rift zones; both are sites of significant volcanism. No long, large-offset strike-slip faults have been observed, although limited local horizontal shear is accommodated across many zones of crustal shortening. In general, tectonic features on Venus are unlike those in Earth's oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide, and separated by stronger and less deformed blocks hundreds of kilometers in width, as in actively deforming continental regions on Earth.     

The 2010 Mw 8.8 Maule megathrust earthquake of Central Chile, monitored by GPS

Large earthquakes produce crustal deformation that can be quantified by geodetic measurements, allowing for the determination of the slip distribution on the fault. We used data from Global Positioning System (GPS) networks in Central Chile to infer the static deformation and the kinematics of the 2010 moment magnitude (M(w)) 8.8 Maule megathrust earthquake. From elastic modeling, we found a total rupture length of ~500 kilometers where slip (up to 15 meters) concentrated on two main asperities situated on both sides of the epicenter. We found that rupture reached shallow depths, probably extending up to the trench. Resolvable afterslip occurred in regions of low coseismic slip. The low-frequency hypocenter is relocated 40 kilometers southwest of initial estimates. Rupture propagated bilaterally at about 3.1 kilometers per second, with possible but not fully resolved velocity variations.     

Water wells as possible indicators of tectonic strain

Coseismic water level changes associated with the Izu-Hanto-oki earthquake of 9 May 1974 were recorded in 59 among 95 observation wells located in the districts of Tokai and Kanto, Japan. The spatial distribution of wells in which the groundwater level rose or fell is rather systematic. The areas in which these wells are located closely coincide with the areas of contraction and dilatation expected by the faulting. This strongly suggests a possible correlation between the observed changes in groundwater level and the tectonic strain. The results may indicate that the water level of wells is able to monitor at least acute coseismic strain changes.     

Physical conditions at the base of a fast moving antarctic ice stream

Boreholes drilled to the bottom of ice stream B in the West Antarctic Ice Sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1.6 bars of the ice overburden pressure. These conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick. The mechanics of ice streaming plays a role in the response of the ice sheet to climatic change.     

High magma storage rates before the 1983 eruption of kilauea, hawaii

After a magnitude 7.2 earthquake in 1975 and before the start of the ongoing eruption in 1983, deformation of Kilauea volcano was the most rapid ever recorded. Three-dimensional numerical modeling shows that this deformation is consistent with the dilation of a dike within Kilauea's rift zones coupled with creep over a narrow area of a low-angle fault beneath the south flank. Magma supply is estimated to be 0.18 cubic kilometers per year, twice that of previous estimates. The 1983 eruption may be a direct consequence of the high rates of magma storage within the rift zone that followed the 1975 earthquake.     

Changes in the west antarctic ice sheet since 1963 from declassified satellite photography

Comparison of declassified satellite photography taken in 1963 with more recent satellite imagery reveals that large changes have occurred in the region where an active ice stream enters the Ross Ice Shelf. Ice stream B has widened by 4 kilometers, at a rate much faster than suggested by models, and has decreased in speed by 50 percent. The ice ridge between ice streams B and C has eroded 14 kilometers. These changes, along with changes in the crevassing around Crary Ice Rise, imply that this region's velocity field shifted during this century.     

Nonvolcanic tremors deep beneath the San Andreas Fault

We have discovered nonvolcanic tremor activity (i.e., long-duration seismic signals with no clear P or S waves) within a transform plate boundary zone along the San Andreas Fault near Cholame, California, the inferred epicentral region of the 1857 Fort Tejon earthquake (moment magnitude approximately 7.8). The tremors occur between 20 to 40 kilometers' depth, below the seismogenic zone (the upper approximately 15 kilometers of Earth's crust where earthquakes occur), and their activity rates may correlate with variations in local earthquake activity.     

Local selection and latitudinal variation in a marine predator-prey interaction

Although pairs of species often interact over broad geographic ranges, few studies have explored how interactions vary across these large spatial scales. Surveys along 1500 kilometers of the Pacific coast of North America documented marked variation in the frequency of predation by the snail Nucella canaliculata on the intertidal mussel Mytilus californianus. Laboratory rearing experiments suggest that regional differences in drilling behavior have a genetic basis, and mitochondrial sequence variation confirms that gene flow is low among these snail populations. Marine communities separated by hundreds of kilometers may have intrinsically different dynamics, with interactions shaped by restricted gene flow and spatially varying selection.     

Earthquake source fault beneath Tokyo

Devastating earthquakes occur on a megathrust fault that underlies the Tokyo metropolitan region. We identify this fault with use of deep seismic reflection profiling to be the upper surface of the Philippine Sea plate. The depth to the top of this plate, 4 to 26 kilometers, is much shallower than previous estimates based on the distribution of seismicity. This shallower plate geometry changes the location of maximum finite slip of the 1923 Kanto earthquake and will affect estimations of strong ground motion for seismic hazards analysis within the Tokyo region.     

Precursory chemical changes in ground water: kobe earthquake, Japan

Chloride (Cl(-)) and sulfate (SO(4)(2-)) ion concentrations of ground water issuing from two wells located near the epicenter of the Kobe earthquake in Japan fluctuated before the disastrous magnitude 7.2 event on 17 January 1995. The samples measured were pumped ground water packed in bottles and distributed in the domestic market as drinking water from 1993 to April 1995. Analytical results demonstrate that Cl(-)and SO(4)(2-) concentrations increased steadily from August 1994 to just before the earthquake. Water sampled after the earthquake showed much higher Cl(-) and SO(4)(2-) concentrations. The precursory changes in chemical composition may reflect the preparation stage of a large earthquake.     

Trajectory shifts in the Arctic and subarctic freshwater cycle

Manifold changes in the freshwater cycle of high-latitude lands and oceans have been reported in the past few years. A synthesis of these changes in freshwater sources and in ocean freshwater storage illustrates the complementary and synoptic temporal pattern and magnitude of these changes over the past 50 years. Increasing river discharge anomalies and excess net precipitation on the ocean contributed approximately 20,000 cubic kilometers of fresh water to the Arctic and high-latitude North Atlantic oceans from lows in the 1960s to highs in the 1990s. Sea ice attrition provided another approximately 15,000 cubic kilometers, and glacial melt added approximately 2000 cubic kilometers. The sum of anomalous inputs from these freshwater sources matched the amount and rate at which fresh water accumulated in the North Atlantic during much of the period from 1965 through 1995. The changes in freshwater inputs and ocean storage occurred in conjunction with the amplifying North Atlantic Oscillation and rising air temperatures. Fresh water may now be accumulating in the Arctic Ocean and will likely be exported southward if and when the North Atlantic Oscillation enters into a new high phase.     

Field data and satellite imagery of tsunami effects in Banda Aceh

After the 26 December 2004 earthquake and tsunami, field data on the extent of the inundation in Banda Aceh, Sumatra, were combined with satellite imagery to quantify the tsunami effects. Flow depths along the shores of Banda Aceh exceeded 9 meters, with inundation reaching 3 to 4 kilometers inland. To the southwest, at Lhoknga, flow depths were more than 15 meters at the shoreline and runup exceeded 25 meters. Erosion and subsidence moved the shoreline of Banda Aceh inland up to 1.5 kilometers, and 65 square kilometers of land between Banda Aceh and Lhoknga were flooded.     

Subaqueous explosive eruption and welding of pyroclastic deposits

Silicic tuffs infilling an ancient submarine caldera, at Mineral King in California, show microscopic fabrics indicative of welding of glass shards and pumice at temperatures >500 degrees C. The occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water, which would cause chilling. Intracaldera progressive aggradation of pumice and ash from a thick, fast-moving pyroclastic flow occurred during a short-lived explosive eruption of approximately 26 cubic kilometers of magma in water >/=150 meters deep. The thickness, high velocity, and abundant fine material of the erupted gas-solids mixture prevented substantial incorporation of ambient water into the flow. Stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general, both above the vent and along any flow path, may be the main process giving rise to buoyant-convective subaqueous eruption columns and attendant fallout deposits.     

Interaction of the san jacinto and san andreas fault zones, southern california: triggered earthquake migration and coupled recurrence intervals

Two lines of evidence suggest that large earthquakes that occur on either the San Jacinto fault zone (SJFZ) or the San Andreas fault zone (SAFZ) may be triggered by large earthquakes that occur on the other. First, the great 1857 Fort Tejon earthquake in the SAFZ seems to have triggered a progressive sequence of earthquakes in the SJFZ. These earthquakes occurred at times and locations that are consistent with triggering by a strain pulse that propagated southeastward at a rate of 1.7 kilometers per year along the SJFZ after the 1857 earthquake. Second, the similarity in average recurrence intervals in the SJFZ (about 150 years) and in the Mojave segment of the SAFZ (132 years) suggests that large earthquakes in the northern SJFZ may stimulate the relatively frequent major earthquakes on the Mojave segment. Analysis of historic earthquake occurrence in the SJFZ suggests little likelihood of extended quiescence between earthquake sequences.     

Correlation of changes in gravity, elevation, and strain in southern california

Measurements made once or twice a year from 1977 through 1982 show large correlated changes in gravity, elevation, and strain in several southern California networks. Precise gravity surveys indicate changes of as much as 25 microgals between surveys 6 months apart. Repeated surveys show that annual elevation changes as large as 100 millimeters occur along baselines 40 to 100 kilometers long. Laser-ranging surveys reveal coherent changes in areal strain of 1 to 2 parts per million occurred over much of southern California during 1978 and 1979. Although the precision of these measuring systems has been questioned, the rather good agreement among them suggests that the observed changes reflect true crustal deformation.     

Strain release mechanism of the 1906 san francisco earthquake

Reexamination of geodetic data has shown that aseismic slip occurred on or near the San Andreas fault in the period of about 20 years after 1906. The inferred displacements are comparable to but at greater depths than the sudden slip that occurred at the time of the earthquake. The postseismic slip is constrained only between late 1906 and 1925, and data are insufficient to determine the movements, if any, below about 20 kilometers on the fault. Two independent observations also indicate stubstantial anomalous crustal deformation away from the fault at least 30 years before the earthquake.     

An experiment in earthquake control at rangely, colorado

An experiment in an oil field at Rangely, Colorado, has demonstrated the feasibility of earthquake control. Variations in seismicity were produced by controlled variations in the fluid pressure in a seismically active zone. Precise earthquake locations revealed that the earthquakes clustered about a fault trending through a zone of high pore pressure produced by secondary recovery operations. Laboratory measurements of the frictional properties of the reservoir rocks and an in situ stress measurement made near the earthquake zone were used to predict the fluid pressure required to trigger earthquakes on preexisting fractures. Fluid pressure was controlled by alternately injecting and recovering water from wells that penetrated the seismic zone. Fluid pressure was monitored in observation wells, and a computer model of the reservoir was used to infer the fluid pressure distributions in the vicinity of the injection wells. The results of this experiment confirm the predicted effect of fluid pressure on earthquake activity and indicate that earthquakes can be controlled wherever we can control the fluid pressure in a fault zone.     

Strain measurements and the potential for a great subduction earthquake off the coast of washington

Geodetic measurements of deformation in northwestern Washington indicate that strain is accumulating at a rate close to that predicted by a model of the Cascadia subduction zone in which the plate interface underlying the continental slope and outer continental shelf is currently locked but the remainder of the interface slips continuously. Presumably this locked segment will eventually rupture in a great thrust earthquake with a down-dip extent greater than 100 kilometers.