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USGS2016

Introduction to “Global tsunami science: Past and future, Volume I”

Twenty-five papers on the study of tsunamis are included in Volume I of the PAGEOPH topical issue “Global Tsunami Science: Past and Future”. Six papers examine various aspects of tsunami probability and uncertainty analysis related to hazard assessment. Three papers relate to deterministic hazard and risk assessment. Five more papers present new methods for tsunami warning and detection. Six papers describe new methods for modeling tsunami hydrodynamics. Two papers investigate tsunamis generated by non-seismic sources: landslides and meteorological disturbances. The final three papers describe important case studies of recent and historical events. Collectively, this volume highlights contemporary trends in global tsunami research, both fundamental and applied toward hazard assessment and mitigation.

Pure and Applied Geophysics PAGEOPH↗
USGS1994

Ductile creep and compaction: A mechanism for transiently increasing fluid pressure in mostly sealed fault zones

A simple cyclic process is proposed to explain why major strike-slip fault zones, including the San Andreas, are weak. Field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock. Ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure. The increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic. The frictional slip associated with earthquakes creates porosity in the fault zone. The cycle adjusts so that no net porosity is created (if the fault zone remains constant width). The fluid pressure within the fault zone reaches long-term dynamic equilibrium with the (hydrostatic) pressure in the country rock. One-dimensional models of this process lead to repeatable and predictable earthquake cycles. However, even modest complexity, such as two parallel fault splays with different pressure histories, will lead to complicated earthquake cycles. Two-dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes. A possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip, stabilizing the fault, and then decreases with further slip, destabilizing it. ?? 1994 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1994

Direct observation of frictional contacts: New insights for state-dependent properties

Rocks and many other materials display a rather complicated, but characteristic, dependence of friction on sliding history. These effects are well-described by empirical rate- and state-dependent constitutive formulations which have been utilized for analysis of fault slip and earthquake processes. We present a procedure for direct quantitative microscopic observation of frictional contacts during slip. The observations reveal that frictional state dependence represents an increase of contact area with contact age. Transient changes of sliding resistance correlate with changes in contact area and arise from shifts of contact population age. Displacement-dependent replacement of contact populations is shown to cause the diagnostic evolution of friction over a characteristic sliding distance that occurs whenever slip begins or sliding conditions change. ?? 1994 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1994

Earthquake mechanism and predictability shown by a laboratory fault

Slip events generated in a laboratory fault model consisting of a circulinear chain of eight spring-connected blocks of approximately equal weight elastically driven to slide on a frictional surface are studied. It is found that most of the input strain energy is released by a relatively few large events, which are approximately time predictable. A large event tends to roughen stress distribution along the fault, whereas the subsequent smaller events tend to smooth the stress distribution and prepare a condition of simultaneous criticality for the occurrence of the next large event. The frequency-size distribution resembles the Gutenberg-Richter relation for earthquakes, except for a falloff for the largest events due to the finite energy-storage capacity of the fault system. Slip distributions, in different events are commonly dissimilar. Stress drop, slip velocity, and rupture velocity all tend to increase with event size. Rupture-initiation locations are usually not close to the maximum-slip locations. ?? 1994 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1990

Effect of anelastic and scattering structures of the lithosphere on the shape of local earthquake coda

A simple model of single acoustic scattering is used to study the dependence of the shape of local earthquake coda on the anelastic and scattering structures of the lithosphere. The model is applied to the coda of earthquakes located near Stone Canyon, central California, and provides an explanation for the features observed in the data, which include an interesting temporal variation in the coda shape. A surficial layer with a Q of 50 and thickness of 10 or 25 km underlain by a zone with a Q of 1000 extending to the bottom of the lithosphere, together with a scattering scale length, a, that varies with depth z according to the relation a=0.3 exp[-(z/45)2] are found to constitute the simplest structure of the medium compatible with the coda data and with body and surface wave attenuation data. The profile of heterogeneity sizes implies that the scattering strength increases strongly with depth, a constraint required by the necessity to boost the energy of the later coda without forcing the intrinsic Q to be excessively high in the uppermost mantle. This constraint is viewed as an artifact of the single scattering model which overstimates the scattering coefficient due to the neglect of multiple scattering. The observed temporal variation of the signal is difficult to explain by a simple change of the intrinsic Q at some depth. Rather, it is suggested that the scattering properties at depth changed with time through a variation of the fractional rms velocity fluctuation on the order of one percent. ?? 1990 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1987

Crustal and upper mantle structure of stable continental regions in North America and northern Europe

From an analysis of many seismic profiles across the stable continental regions of North America and northern Europe, the crustal and upper mantle velocity structure is determined. Analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques. The P wave velocity structure beneath the Canadian Shield is virtually identical to that beneath the Baltic Shield to a depth of at least 800 km. Two major layers with a total thickness of about 42 km characterize the crust of these shield regions. Features of the upper mantle of these region include velocity discontinuities at depths of about 74 km, 330 km, 430 km and 700 km. A 13 km thick P wave low velocity channel beginning at a depth of about 94 km is also present. A number of problems associated with record section interpretation are identified and a generalized approach to seismic profile analysis using many record sections is described. The S wave velocity structure beneath the Canadian Shield is derived from constrained surface wave data. The thickness of the lithosphere beneath the Canadian and Baltic Shields is determined to be 95-100 km. The continental plate thickness may be the same as the lithospheric thickness, although available data do not exclude the possibility of the continental plate being thicker than the lithosphere. ?? 1987 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1989

Analysis of exceptionally large tremors in two gold mining districts of South Africa

An investigation of ground motion, recorded using broad-band, wide dynamic-range digital seismographs, of large mine tremors from two South African mining districts with different geologic settings, reveals some essential differences in both seismic source and ground motion parameters. In the Klerksdorp district where the strata are offset by major throughgoing normal faults, the largest tremors, with magnitudes ranging as high as 5.2, tend to be associated with slip on these pre-existing faults. Moreover, the seismic source and ground motion parameters are quite similar to those of natural crustal earthquakes. In the Carletonville district, by contrast, where substantial faults do not exist, the large-magnitude tremors appear to result from the failure of relatively intact rock and cause seismic stress drops and ground motion parameters higher than normally observed for natural shocks. Additionally, there appears to be an upper magnitude limit of about 4 in the Carletonville district. Detailed analyses of an exceptionally large event recorded locally from each of these districts serve to highlight these contrasts. ?? 1989 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1989

The Alabama, U.S.A., seismic event and strata collapse of May 7, 1986

On May 7, 1986, the residents of Tuscaloosa, Alabama, felt a seismic event of local magnitude 3.6 that occurred at the same time as a rock burst and roof collapse in an active longwall coal mine. Visual inspection of the seismograms reveals a deficiency in energy at frequencies above 20 Hz compared to tectonic earthquakes or surface blasts. The predominance of energy below 5 Hz may explain reports of body wave magnitudes (mb) greater than 4.2. Also, 1.0 Hz surface waves were more strongly excited than body waves and may explain local felt effects more typically associated with greater epicentral distances. All recorded first motions were dilatational. The concentration of stations in the northern hemisphere allows reverse motion on an east-trending near-vertical plane or strike-slip motion on northwest or southeast trending planes. The reverse focal mechanism is preferred, because the area of roof collapse and the area of active longwall mining are located between two east-striking loose vertical fracture zones. The characteristics of the seismic event suggest that it might have been sudden shear failure resulting from accumulated strain energy in overlying strata behind an active longwall. Although an alternate interpretation of the focal mechanism as an implosion or shear failure in the strata above previously mined out areas is also allowed by the first motion data, this alternate intepretation is not supported by geological data. ?? 1989 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1985

In-situ fluid-pressure measurements for earthquake prediction: An example from a deep well at Hi Vista, California

Short-term earthquake prediction requires sensitive instruments for measuring the small anomalous changes in stress and strain that precede earthquakes. Instruments installed at or near the surface have proven too noisy for measuring anomalies of the size expected to occur, and it is now recognized that even to have the possibility of a reliable earthquake-prediction system will require instruments installed in drill holes at depths sufficient to reduce the background noise to a level below that of the expected premonitory signals. We are conducting experiments to determine the maximum signal-to-noise improvement that can be obtained in drill holes. In a 592 m well in the Mojave Desert near Hi Vista, California, we measured water-level changes with amplitudes greater than 10 cm, induced by earth tides. By removing the effects of barometric pressure and the stress related to earth tides, we have achieved a sensitivity to volumetric strain rates of 10-9 to 10-10 per day. Further improvement may be possible, and it appears that a successful earthquake-prediction capability may be achieved with an array of instruments installed in drill holes at depths of about 1 km, assuming that the premonitory strain signals are, in fact, present. ?? 1985 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1985

Hydrologic changes associated with the October 28, 1983, Idaho earthquake

Significant hydrologic changes were observed after the magnitude 7.3 earthquake that occurred on October 28, 1983, in central Idaho. Groundwater levels rose by as much as 3 meters near the epicenter. Discharge in many streams and springs increased, in some instances by more than 100%. One warm spring ceased flowing for several days; the flow then resumed and peaked at about nine times its original rate. Available data show no significant changes in water quality following the earthquake. ?? 1985 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1985

Impulsive radon emanation on a creeping segment of the San Andreas fault, California

Radon emanation was continuously monitored for several months at two locations along a creeping segment of the San Andreas fault in central California. The recorded emanations showed several impulsive increases that lasted as much as five hours with amplitudes considerably larger than meteorologically induced diurnal variations. Some of the radon increases were accompanied or followed by earthquakes or fault-creep events. They were possibly the result of some sudden outbursts of relatively radon-rich ground gas, sometimes triggered by crustal deformation or vibration. ?? 1985 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1985

Prediction of central California earthquakes from soil-gas helium fluctuations

The observations of short-term decreases in helium soil-gas concentrations along the San Andreas Fault in central California have been correlated with subsequent earthquake activity. The area of study is elliptical in shape with radii approximately 160??80 km, centered near San Benito, and with the major axis parallel to the Fault. For 83 percent of the M>4 earthquakes in this area a helium decrease preceded seismic activity by 1.5 to 6.5 weeks. There were several earthquakes without a decrease and several decreases without a corresponding earthquake. Owing to complex and unresolved interaction of many geophysical and geochemical parameters, no suitable model is yet developed to explain the observations. ?? 1985 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1985

Anomalous hydrogen emissions from the San Andreas fault observed at the Cienega Winery, central California

We began continuous monitoring of H2 concentration in soil along the San Andreas and Calaveras faults in central California in December 1980, using small H2/O2 fuel-cell sensors. Ten monitoring stations deployed to date have shown that anomalous H2 emissions take place occasionally in addition to diurnal changes. Among the ten sites, the Cienega Winery site has produced data that are characterized by very small diurnal changes, a stable baseline, and remarkably distinct spike-like H2 anomalies since its installation in July 1982. A major peak appeared on 1-10 November 1982, and another on 3 April 1983, and a medium peak on 1 November 1983. The occurrences of these peaks coincided with periods of very low seismicity within a radius of 50 km from the site. In order to methodically assess how these peaks are related to earthquakes, three H2 degassing models were examined. A plausible correlational pattern was obtained by using a model that (1) adopts a hemicircular spreading pattern of H2 along an incipient fracture plane from the hypocenter of an earthquake, (2) relies on the FeO-H2O reaction for H2 generation, and (3) relates the accumulated amount of H2 to the mass of serpentinization of underlying ophiolitic rocks; the mass was tentatively assumed to be proportional to the seismic energy of the earthquake. ?? 1985 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1985

A study of the depth of weathering and its relationship to the mechanical properties of near-surface rocks in the Mojave Desert

Weathered granite extends 70 m deep at Hi Vista in the arid central Mojave Desert of southern California. The low strength of this granite is due to the alteration of biotite and chlorite montmorillonite. Deep weathering probably occurs in most granites, although we cannot rule out some anomalous mechanisms at Hi Vista. Geophysical instruments set in these slightly altered rocks are limited by the unstable behavior of the rocks. Thus, tectonic signals from instruments placed in shallow boreholes give vague results. Geophysical measurements of these weathered rocks resemble measurements of granitic rocks near major faults. The rheology of the rocks in which instruments are placed limits the useful sensitivity of the instruments. ?? 1985 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1985

Water-rock interactions in fault gouge

Measurements were made of the amounts of D,18O, and H2O+ in fault gouge collected over a depth of 400 m in the San Andreas fault of California. The amounts and isotopic compositions of the pore fluids, also analyzed, suggest that formation waters from adjacent Franciscan rocks have migrated into the gouge and mixed with local meteoric water. Thus the gouge is an open system permeable to fluid flow. This permeability has important implications concerning heat flow along the fault zone. Analyses of the fault gouge itself give information on the amounts, timing, and conditions of formation of the clay minerals. Stable-isotope analyses of materials from fault zones are good indicators of water-rock interactions that bear importantly on processes taking place in seismically active regions. ?? 1985 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1989

Estimation of strong ground motions from hypothetical earthquakes on the Cascadia subduction zone, Pacific Northwest

Strong ground motions are estimated for the Pacific Northwest assuming that large shallow earthquakes, similar to those experienced in southern Chile, southwestern Japan, and Colombia, may also occur on the Cascadia subduction zone. Fifty-six strong motion recordings for twenty-five subduction earthquakes of Ms???7.0 are used to estimate the response spectra that may result from earthquakes Mw<81/4. Large variations in observed ground motion levels are noted for a given site distance and earthquake magnitude. When compared with motions that have been observed in the western United States, large subduction zone earthquakes produce relatively large ground motions at surprisingly large distances. An earthquake similar to the 22 May 1960 Chilean earthquake (Mw 9.5) is the largest event that is considered to be plausible for the Cascadia subduction zone. This event has a moment which is two orders of magnitude larger than the largest earthquake for which we have strong motion records. The empirical Green's function technique is used to synthesize strong ground motions for such giant earthquakes. Observed teleseismic P-waveforms from giant earthquakes are also modeled using the empirical Green's function technique in order to constrain model parameters. The teleseismic modeling in the period range of 1.0 to 50 sec strongly suggests that fewer Green's functions should be randomly summed than is required to match the long-period moments of giant earthquakes. It appears that a large portion of the moment associated with giant earthquakes occurs at very long periods that are outside the frequency band of interest for strong ground motions. Nevertheless, the occurrence of a giant earthquake in the Pacific Northwest may produce quite strong shaking over a very large region. ?? 1989 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1978

Induction of auroral zone electric currents within the Alaska pipeline

The Alaskar pipeline is a highly conducting anomaly extending 800 miles (1300 km) from about 62?? to 69?? geomagnetic latitude beneath the most active regions of the ionospheric electrojet current. The spectral behavior of the magnetic field from this current was analyzed using data from standard geomagnetic observatories to establish the predictable patterns of temporal and spatial changes for field pulsation periods between 5 min and 4 hr. Such behavior is presented in a series of tables, graphs and formulae. Using 2- and 3-layer models of the conducting earth, the induced electric fields associated with the geomagnetic changes were established. From the direct relationship of the current to the geomagnetic field variation patterns one can infer counterpart temporal and spatial characteristics of the pipeline current. The relationship of the field amplitudes to geomagnetic activity indices, Ap, and the established occurrence of various levels of Ap over several solar cycles were employed to show that about half of the time the induced currents in the pipe would be under 1 A for the maximum response oscillatory periods near 1 hr. Such currents should be of minimal consequence in corrosion effects for even a section of the pipeline unprotected by sacrificial electrodes. Of greater interest was the result that the extreme surges of current should reach over one-hundred amperes in the pipeline during high activity. ?? 1978 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1986

The effects of sliding velocity on the frictional and physical properties of heated fault gouge

The frictional properties of a crushed granite gouge and of gouges rich in montmorillonite, illite, and serpentine minerals have been investigated at temperatures as high as 600??C, confining pressures as high as 2.5 kbar, a pore pressure of 30 bar, and sliding velocities of 4.8 and 4.8??10-2 ??m/sec. The gouges showed nearly identical strength behaviors at the two sliding velocities; all four gouges, however, showed a greater tendency to stick-slip movement and somewhat higher stress drops in the experiments at 4.8??10-2 ??m/sec. Varying the sliding velocity also had an effect on the mineral assemblages and deformation textures developed in the heated gouges. The principal mineralogical difference was that at 400??C and 1 kbar confining pressure a serpentine breakdown reaction occurred in the experiments at 4.8??10-2 ??m/sec but not in those at 4.8 ??m/sec. The textures developed in the gouge layers were in part functions of the gouge type and the temperature, but changes in the sliding velocity affected, among other features, the degree of mineral deformation and the orientation of some fractures. ?? 1986 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗