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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↗

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↗

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↗

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↗

USGS1986

Seismic measurements of the internal properties of fault zones

The internal properties within and adjacent to fault zones are reviewed, principally on the basis of laboratory, borehole, and seismic refraction and reflection data. The deformation of rocks by faulting ranges from intragrain microcracking to severe alteration. Saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity, but, from borehole measurements, densely fractured rocks do show significantly reduced velocities, the amount of reduction generally proportional to the fracture density. Highly fractured rock and thick fault gouge along the creeping portion of the San Andreas fault are evidenced by a pronounced seismic low-velocity zone (LVZ), which is either very thin or absent along locked portions of the fault. Thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone; high pore pressure within the pronounced LVZ may be conductive to fault creep. Deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust. Models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated, seismically anisotropic mylonites.

Pure and Applied Geophysics PAGEOPH↗

USGS1994

Seismicity trends and potential for large earthquakes in the Alaska-Aleutian region

The high likelihood of a gap-filling thrust earthquake in the Alaska subduction zone within this decade is indicated by two independent methods: analysis of historic earthquake recurrence data and time-to-failure analysis applied to recent decades of instrumental data. Recent (May 1993) earthquake activity in the Shumagin Islands gap is consistent with previous projections of increases in seismic release, indicating that this segment, along with the Alaska Peninsula segment, is approaching failure. Based on this pattern of accelerating seismic release, we project the occurrence of one or more M???7.3 earthquakes in the Shumagin-Alaska Peninsula region during 1994-1996. Different segments of the Alaska-Aleutian seismic zone behave differently in the decade or two preceding great earthquakes, some showing acceleration of seismic release (type "A" zones), while others show deceleration (type "D" zones). The largest Alaska-Aleutian earthquakes-in 1957, 1964, and 1965-originated in zones that exhibit type D behavior. Type A zones currently showing accelerating release are the Shumagin, Alaska Peninsula, Delarof, and Kommandorski segments. Time-to-failure analysis suggests that the large earthquakes could occur in these latter zones within the next few years. ?? 1994 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗

USGS1994

Triaxial testing of Lopez Fault gouge at 150 MPa mean effective stress

Triaxial compression experiments were performed on samples of natural granular fault gouge from the Lopez Fault in Southern California. This material consists primarily of quartz and has a self-similar grain size distribution thought to result from natural cataclasis. The experiments were performed at a constant mean effective stress of 150 MPa, to expose the volumetric strains associated with shear failure. The failure strength is parameterized by the coefficient of internal friction ??, based on the Mohr-Coulomb failure criterion. Samples of remoulded Lopez gouge have internal friction ??=0.6??0.02. In experiments where the ends of the sample are constrained to remain axially aligned, suppressing strain localisation, the sample compacts before failure and dilates persistently after failure. In experiments where one end of the sample is free to move laterally, the strain localises to a single oblique fault at around the point of failure; some dilation occurs but does not persist. A comparison of these experiments suggests that dilation is confined to the region of shear localisation in a sample. Overconsolidated samples have slightly larger failure strengths than normally consolidated samples, and smaller axial strains are required to cause failure. A large amount of dilation occurs after failure in heavily overconsolidated samples, suggesting that dilation is occurring throughout the sample. Undisturbed samples of Lopez gouge, cored from the outcrop, have internal friction in the range ??=0.4-0.6; the upper end of this range corresponds to the value established for remoulded Lopez gouge. Some kind of natural heterogeneity within the undisturbed samples is probably responsible for their low, variable strength. In samples of simulated gouge, with a more uniform grain size, active cataclasis during axial loading leads to large amounts of compaction. Larger axial strains are required to cause failure in simulated gouge, but the failure strength is similar to that of natural Lopez gouge. Use of the Mohr-Coulomb failure criterion to interpret the results from this study, and other recent studies on intact rock and granular gouge, leads to values of ?? that depend on the loading configuration and the intact or granular state of the sample. Conceptual models are advanced to account for these descrepancies. The consequences for strain-weakening of natural faults are also discussed. ?? 1994 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗

USGS1979

Seismic gaps and source zones of recent large earthquakes in coastal Peru

The earthquakes of central coastal Peru occur principally in two distinct zones of shallow earthquake activity that are inland of and parallel to the axis of the Peru Trench. The interface-thrust (IT) zone includes the great thrust-fault earthquakes of 17 October 1966 and 3 October 1974. The coastal-plate interior (CPI) zone includes the great earthquake of 31 May 1970, and is located about 50 km inland of and 30 km deeper than the interface thrust zone. The occurrence of a large earthquake in one zone may not relieve elastic strain in the adjoining zone, thus complicating the application of the seismic gap concept to central coastal Peru. However, recognition of two seismic zones may facilitate detection of seismicity precursory to a large earthquake in a given zone; removal of probable CPI-zone earthquakes from plots of seismicity prior to the 1974 main shock dramatically emphasizes the high seismic activity near the rupture zone of that earthquake in the five years preceding the main shock. Other conclusions on the seismicity of coastal Peru that affect the application of the seismic gap concept to this region are: (1) Aftershocks of the great earthquakes of 1966, 1970, and 1974 occurred in spatially separated clusters. Some clusters may represent distinct small source regions triggered by the main shock rather than delimiting the total extent of main-shock rupture. The uncertainty in the interpretation of aftershock clusters results in corresponding uncertainties in estimates of stress drop and estimates of the dimensions of the seismic gap that has been filled by a major earthquake. (2) Aftershocks of the great thrust-fault earthquakes of 1966 and 1974 generally did not extend seaward as far as the Peru Trench. (3) None of the three great earthquakes produced significant teleseismic activity in the following month in the source regions of the other two earthquakes. The earthquake hypocenters that form the basis of this study were relocated using station adjustments computed by the method of joint hypocenter determination. ?? 1979 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗

USGS1979

On Chinese earthquake history - An attempt to model an incomplete data set by point process analysis

Since the 1950s, the Academia Sinica in Peking, People's Republic of China, has carried out extensive research on the Chinese earthquake history. With a historical record dating back some 3000 years, a wealth of information on Chinese earthquakes exists. Despite this monumental undertaking by the Academia Sinica, much work is still necessary to correct the existing earthquake data for historical changes in population, customs, modes of communication, and dynasties. In this paper we report on the status of our investigation of Chinese earthquake history and present some preliminary results. By applying point process analysis of earthquakes in 'Central China', we found suggestions of (1) lower earthquake activity at intervals of about 175 years and 375 years, and (2) higher earthquake activity at an interval of about 300 years.

Pure and Applied Geophysics PAGEOPH↗

USGS1993

Radon anomalies on three kinds of faults in California

Radon emanation is known to be anomalously high along active faults in many parts of the world. We tested this relationship in California during July and early August 1992, using a portable radonmeter to conduct soil-air radon surveys at 5 sites across three kinds of faults: Creeping, locked, and freshly broken. Along a 350-m long survey line across a creeping segment of the San Andreas fault at Nyland Ranch in San Juan Bautista, we found anomalous radon concentrations not in the creep zone itself as determined by a creepmeter, but on the adjacent sides, 10 and 30 meters from the center line of the fault. The anomalous values were 5 times higher than the background values measured farther away from the fault. A similar radon anomaly was observed along a 420-m long survey line across a creeping segment of the Calaveras fault near 7th Street in Hollister. There, the anomalous values were about 6 to 11 times the background values and about 40 and 50 m from the center line of the fault. The double-peaked featire of the anomalies may be indicative of a relatively low gas permeability of the fault-gouge materials in the creeping zones and high permeability of fractured rocks in the adjacent shear zones. Along a 144-m survey line across the currently locked segment of the San Andreas fault at the Earthquake Trail near Olema, the radon concentration was indeed anomalously high in the fault zone, by a factor of two above background values. However, the maximum values (3 to 6 times background) again were recorded about 10 meters from the center line. Three weeks after the magnitude 7.5 Landers earthquake of 28 June 1992, we conducted a survey along a 300-m line across the earthquake fault alongside Encantado Road in the epicenter area. The radon values measured at the two main fault breaks were an order of magnitude higher than the background values. A similar result was found along a 420-m line alongside Reche Road about 1.7 km south of Encantado Road. ?? 1993 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗