Electrical resistivity mapping of landscape modifications at the Talgua site,Olancho, Honduras

Geophysical reconnaissance in 1995 provided information useful in developing a successful strategy for the 1996 field season in resistivity mapping of subsurface cultural features at Talgua Village, an archaeological site in eastern Honduras. Ground truth excavations confirmed that high-resistivity anomalies detected by modified dipole–dipole apparent resistivity pseudosections correlate with a layer of small cobbles imported to fill low spots of this prehistoric settlement. Resistivity measurements reveal that mounds on this site were erected on normal subsoil, while at least one plaza was originally a topographic low that has been filled. The volume of imported stones is at least 500 m³, which represents a significant public improvement effort. Similar imported fill under the rest of Talgua Village could be mapped by similar means, and other prehistoric sites of the region could be geophysically tested for similar features. Resistivity profiles provide archaeologists with a quick, inexpensive, accurate, and noninvasive method of determining the extent of landscape modification at Talgua Village. © 1999 John Wiley & Sons, Inc.     

Theory of earthquakes — IV. General implications for earthquake prediction

The scale invariant inclusion theory of failure is applied to the general problem of precursors that precede failure. A precursor is defined to be an effect produced within a physical system which indicates that the process leading to failure of the system has begun. Precursors are grouped into three classes.Class I precursors refer to long-term indicators of impending failure. These may includev _p/v_s, long-term tilt, and crustal uplift anomalies observed to precede some major shallow earthquakes by afew years. Class II precursors refer to short-term indicators of failure and include: S-bend tilt, electromagnetic radiation, radon emanations, and seismicity changes that have been reported to precede major earthquakes by afew hours. Class III precursors refer tovery short-termphenomena such as long-period (strain) waves,rapid changes in surface ground tilts, and seismicity increase in the hypocentral region that are predicted by the inclusion theory to precede major shallow earthquakes by afew seconds.The physical processes that occur within the inclusion zone of an impending failure that indirectly produce the class II precursors are used with the scale invariant properties of failure to show that their time duration is a direct measure of the average length of the cracks that comprise the inclusion zone. This result is used to derive the precursor time-fault length relationship that has been observed to hold for class I precursors of shallow earthquakes, mine failures, and laboratory size failures of rock. The physical model proposed for producing class I, class II, and indirectly, the class III precursors leads to six results when both the Utsu relationship between aftershock area and earthquake magnitude and the Gutenberg-Richter energy-magnitude relationship are satisfied. (1) The seismic efficiency factor for failures satisfying the constraints of the inclusion theory is approximately 0.40%. (2) The energy radiated by aftershocks will be at least 1.0% of the energy radiated bythe mainshock. (3) An upper limiting magnitude of any aftershock in the aftershock sequence isM–1.6, whereM is the mainshock magnitude. (4) The time durations of all three precursor classes are shown to be shortened (or lengthened) by a factor inversely proportional to the rate of increase (or decrease) of the far-field stresses during the time duration of the precursor. Changes in far-field stresses, such as might occur to tidal effects, are shown to be of particular importance in initiating class II precursors, and it is shown that tidal stresses provide a mechanism for triggering large earthquakes (M6.0) in regions that are at the point of incipient failure. Thus, class II precursors may give the appearance of being independent of magnitude for large earthquakes. (5) When fluids are present in the focal volume of the mainshock, the predicted magnitude, calculated by class I precursors, will always be larger than the observed magnitude. (6) Seismic events that produce the inclusion zone of the impending mainshock will not be followed by aftershocks. These events are predicted to be characterized by anomalously long rupture lengths.The inclusion theory is shown to provide a physical basis for criteria required to predict failure. The implications of the inclusion theory to the problem of earthquake prediction are discussed. The theory is applied to existing earthquake-prone regions.     

An experimental determination of the true uniaxial stress-strain behavior of brittle rock

SummaryAn Experimental Determination of the True Uniaxial Stress-Strain Behavior of Brittle Rock Results are presented of an experimental study of the behavior of six rock types deformed under uniaxial compression into their respective post-failure regions. Based on the observation that a rock sample in a post-failure state can be considered to be composed of broken and unbroken rock and assuming that the reduction in load-bearing capability of rock in the post-failure region is due to a reduction of the effective cross-sectional area of the specimen resulting from the growth of large cracks within the rock sample, we show that there is a maximum true stress that the unfractured solid rock can sustain without inelastic deformation. This stress is constant and is defined to be the true failure strength of the rock. The value of this stress is calculated by dividing the force on the rock sample at any point along the post-failure curve by the true load-bearing cross-sectional area of the rock sample at that point. Theoretical and experimental techniques are developed which allow an estimate of the true load-bearing area of the rock sample at any point along the post-failure curve of the sample. For the rock types used in the study, which were deformed to preselected positions along their respective post-failure curves and with the assumption that the fractured rock carried none of the applied load, the two techniques of measuring the effective load-bearing area give results which are equivalent.

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ZusammenfassungExperimentelle Bestimmung des wahren Spannungs-Dehnungs-Verhaltens von sprödem Gestein Es wird über Ergebnisse experimenteller Studien über das Verhalten von sechs verschiedenen Gesteinen berichtet, welche unter einachsigem Druck im Nach-Bruch-Bereich deformiert wurden. Aufgrund der Beobachtung, daß eine Gesteinsprobe im Nach-Bruch-Bereich als aus brüchigem und gebrochenem Gestein zusammengesetzt betrachtet werden kann und unter der Annahme, daß die Abminderung der Belastbarkeit des Gesteins im Nach-Bruch-Bereich durch die Verringerung der wirksamen Querschnittsfläche der Probe infolge des Wachsens von großen Rissen in der Gesteinsprobe verursacht ist, wird gezeigt, daß es eine maximale wahre Spannung gibt, welche das ungebrochene, feste Gestein ohne inelastische Deformation ertragen kann. Diese Spannung ist konstant und wird als wahre Bruchfestigkeit des Gesteins angesprochen. Die Größe dieser Spannung errechnet sich durch Division der auf die Gesteinsprobe an irgendeinem Punkt der Nach-Bruch-Kurve ausgeübten Kraft durch die wahre, Belastung tragende Querschnittsfläche der Gesteinsprobe an diesem Punkt. Es wurden theoretische und experimentelle Techniken entworfen, welche eine Schätzung der Verminderung der Belastungsfläche an irgendeiner Stelle längs der Nach-Bruch-Kurve der Probe erlauben. Für Gesteinstypen, welche in diesen Untersuchungen benützt wurden und welche an im Voraus gewählten Stellen längs der Nach-Bruch-Kurve deformiert wurden, gaben unter der Annahme, daß das gebrochene Gestein nichts von der aufgebrachten Belastung trug, die beiden Techniken zur Messung der tatsächlich lasttragenden, wirksamen Fläche gleiche Resultate.

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Résumé On présente les résultats d'une étude expérimentale du comportement de six types de roches chargées en compression simple dans leur domaine respectif de déformation après la rupture. Si l'on remarque qu'un échantillon de roche après la rupture peut être considéré comme composé de roche cassée et de roche non cassée, et en supposant que la réduction de résistance dans le domaine après la rupture est due à la réduction de la section droite intacte de l'échantillon, réduction résultant de la croissance de grandes fissures dans l'éprouvette, on montre qu'il existe une contrainte vraie maximale que la roche peut supporter sans déformation permanente. Cette contrainte est constante et est réputée être la résistance vraie à la rupture. La valeur de cette contrainte est calculée en divisant la force sur l'échantillon en chaque point de la courbe effort-déformation après la rupture, par la section droite supportant alors véritablement la charge. Des techniques théorique et expérimentale sont développées, qui permettent d'estimer la surface portante vraie de l'échantillon en tout point de la courbe après la rupture. Pour les types de roches utilisées dans cette étude, qui furent déformées à des états pré-déterminés du domaine après la rupture, et avec l'hypothèse que la roche fracturée ne supportait plus aucune contrainte, les deux méthodes de mesures de la section portante ont donné des résultats équivalents.

     

Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project

Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the V_S,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical information on the acquisition and analysis of surface wave data by giving some basic principles and specific suggestions related to the most common situations. They are primarily targeted to non-expert users approaching surface wave testing, but can be useful to specialists in the field as a general reference. The guidelines are based on the experience gained within the InterPACIFIC project and on the expertise of the participants in acquisition and analysis of surface wave data.     

Environmental geology: our professional public responsibility

 Conflicts between different interest groups for use of natural resources is one area where state geological surveys can provide assistance. A state geological survey working within the scientific constraints of specific issues can remain objective in its presentations and maintain the faith of both the conflicting interest groups and the public. One cannot vary from the objective view or you will quickly be criticized. Criticism can still occur from one side of a natural resource issue as your data might counter their views. However, the final decisions are almost always made in some legislators, or regulators, area of responsibility. The responsibility of the state geological survey is to provide the important data that will assist in making correct decisions. Should one party in the conflict become extreme in their demands, a potential compromise that is beneficial to both sides can be lost. In Kansas, the classical natural resource problem of resource/recreation in a populated area is presented as a case study. The state geological survey presented data on sand resources in the Kansas River and its valley in northeast Kansas. That information was important to both recreation and dredging interests where the political problem is a conflict of sand use as a construction material resource versus use of the alluvial river as an important recreation area, especially for canoeing. However, when a reasonable compromise was near completion in the Kansas Legislature one side, in a bold move to develop an advantage, ruined that potential for compromise. Received: 12 May 1998 · Accepted: 6 July 1998     

Pressure overloads within a subducting plate and the density contrast between the plate and mantle: Some geophysical implications

Summary The density of a subducting plate is shown to become greater than the density of the adjacent mantle rock at a subduction depth on the order of 60 km. The density increase results from high pressures generated within the plate during its motion into mantle rock whose long-term strength is much less than the long-term strength of the plate. This positive density contrast between the plate and mantle is in addition to any positive density contrast that may result from the plate not being in thermal equilibrium with the adjacent mantle rock. The effect of this pressure induced density contrast on the overall down-dip stress distribution in the plate is shown to be insignificant on earthquake formation except, possibly, near the upper, near-surface portions of the plate. Implications of this result in terms of the inclusion collapse theory of deep earthquakes are discussed.     

Integrated climate model-oxygen isotope evidence for a North American monsoon during the Late Cretaceous

During the Late Cretaceous, western North America was characterized by a close geographic association between the Sevier highlands and the Western Interior Seaway. In this paper, an atmospheric general circulation model (AGCM) is used to simulate the impact of this geographic association on surface pressure, wind direction, and precipitation, and it is predicted that seasonal changes in these variables resulted in a strong monsoon along the eastern flank of the Sevier Highlands. Confirmation that these model simulations are accurate is provided by isotopic data from foreland basin sediments. In particular oxygen isotope records from different environments (large rivers, small streams and ponds) and proxies (unionid bivalve shells and paleosol carbonates) indicate that foreland basin streams were recharged by local precipitation with high oxygen isotope ratios while large trunk rivers were recharged by high-elevation precipitation. This hydrologic pattern is observed from Alberta to Utah and is consistent with east to west monsoonal air mass movements and associated seasonal rainfall. Recognition of a highland-driven monsoon has implication in regard to studies of fossil taphonomy, of water vapor transport, and of links between climate and mountain uplift and exhumation in this region.     

Adaptive multiuser detection for underwater acoustical channels

An underwater acoustic local area network (ALAN) provides multipoint-to-point telemetry between many high-rate, ocean-bottom sensors and a central, surface-deployed receiver in the 10-30 kHz vertical acoustical channel. Ocean-bottom modems initiate the transmission process by requesting data channel time slots via a common narrow-band request channel. Request packets overlap in time and frequency in this channel, and the throughput and average transmission delay rely heavily on the successful resolution of the request packet collisions. This paper presents the design, analysis, and experimental demonstration of a request channel receiver capable of resolving collisions between several asynchronous and cochannel packets. The receiver algorithm differs from standard capture schemes (by demodulating the data from both strong and weak transmitters), conventional spread-spectrum receivers (by overcoming the near-far problem), and existing multiple-access demodulation techniques (by adapting to the number of interfering signals, and the unknown phase, Doppler, amplitude, and timing of each signal in the collision). The receiver demodulates the collided packets by decision-directed techniques through a novel method of estimating the interference for each user which minimizes error propagation due to inaccurate tentative decisions. An inwater experiment illustrates that this technique is extremely desirable for collision resolution in underwater acoustic local area networks, and also for underwater autonomous vehicles with both sidescan sonar as well as acoustic telemetry links     

SWprocess: a workflow for developing robust estimates of surface wave dispersion uncertainty

Journal of Seismology - Non-invasive surface wave methods are increasingly being used as the primary technique for estimating a site’s small-strain shear wave velocity (Vs). Yet, in...