On tectonophysical interpretation of recent vertical crustal movements

Recent crustal movements are components and the direct continuation of some neotectonic movements i.e. the Neogene-Quaternary stage of the earth's development. According to geodetic and oceanographic data these crustal movements are constantly taking place everywhere on the continents and probably on the ocean floor. Their velocity and velocity gradients are certainly different on platforms from that in orogenic regions. Orogenic regions are characterized by higher velocity, varying from several mm to several cm per year, and to an even greater degree by the different character of the movement. The velocity gradient in these regions reaches values of 10^?6–10^?5/year.A peculiar indication of recent movements is their irregularity in time. Periodic movements are typical for platforms; orogenic regions are characterized by sharp increases in velocity by 1–2 orders of magnitude and their gradients by 2–3 orders, close to the time occurrence of earthquakes.The deep processes generating the measured recent crustal movements are manifested also in earthquakes, the state of stress of rocks and anomalies of gravitational and other fields. Thus estimations of recent tectonic activity should be based on the whole complex of quantitative data, such as the velocity and velocity gradients of recent movements, values of stress in mines, and seismicity.To study the deep processes causing the present tectonic activity, it is necessary to determine the types of crustal deformation occurring in different geotectonic regions. The relationships between recent movements and the distribution of earthquakes in the regions with a high degree of crustal activity allow to identify four types of movement: pleistoseismic, hyposeismic, kryptoseismic and teleseismic.     

Transcontinental profile of recent vertical crustal movements

A transcontinental profile of vertical crustal movement from San Diego, California, to Meldrim (Savannah), Georgia, has been assembled from precise leveling data of the National Geodetic Survey. Assuming constant movement during the time interval between leveling and releveling surveys, rates of movement have been plotted. The overall rate of movement of the East Coast relative to the West Coast derived from leveling is opposite in sign to the corresponding rate from tide gauges at San Diego and Savannah. Though this discrepancy is within the acceptable limits of normal random error associated with leveling measurements, it may also result in part from unavoidable uncertainties at the connection points between the various segments. In the absence of deterministic evidence on the source of the discrepancy, a least-squares adjustment was performed to distribute this difference in the overall trend through the profile. Apart from this trend, the western end of the profile is dominated by pronounced subsidence (rates 10 cm/yr) owing to water withdrawal and associated consolidation of alluvial sediments. With the exception of these movements, rates of vertical movement of the eastern and western United States are similar, suggesting that if such measurements represent significant tectonic activity, this activity may not be confined to the western United States. Movements are correlated with topography only in the peninsular ranges of California. Although these apparent elevation changes may reflect actual ground movement, this correlation may also result from systematic leveling errors. Comparison with gravity, depth to basement and depth to Moho shows no correlation.     

Assessment of benchmark credibility in the study of recent vertical crustal movements

Results of repeated precise leveling in several regions indicate that the magnitude of inferred vertical displacements is controlled by the quality of benchmark monumentation. If ignored, such a correlation may lead to an erroneous tectonic interpretation of the computed pattern of motion.     

Gravimetry for monitoring vertical crustal movements: Potential and problems

Through recent developments in absolute and relative techniques, gravimetry has reached an accuracy of a few hundredths to a few tenths of μm s^{−2 *}in small or large-scale networks, respectively. Consequently, gravimetric techniques can now be employed as one efficient tool for detecting vertical crustal movements. Gravity data are necessary for converting the results of geometric levelling to heights defined in the gravity field, and—by repeated surveys—to control time variations of the height-reference surface. More important is the use of repeated gravity surveys for strengthening and partly replacing levelling, being a time-consuming procedure with unfavourable error propagation over larger distances. The successful application of observed gravity variations with time in the detection of vertical crustal movements depends on the reliability of the conversion factor between gravity and height changes, and on the accuracy of the gravity measurements. The conversion factor should be determined through simultaneous levelling and gravimetry, in representative parts of the survey region. Strategies for establishing gravimetric control are changing now with the availability of transportable absolute gravity meters, and the possibility of accurately calibrating relative instruments and observing small gravity differences with feed-back-systems. Consequently, more attention has to be given now to disturbing effects of environmental character, such as microseismics, atmospheric pressure and groundwater table variations, and to periodic effects such as gravimetric earth tides and polar motion.     

Comparison of recent crustal movements with quaternary movements in japan and its plate-tectonic implication

Recent crustal movements may be classified into two categories: one being associated with major earthquakes and the other being creep deformations without a direct association with any major earthquake. The spatial distribution of the rate of creep deformation during the last 70 years as detected by the precise re-leveling, shows a similar configuration on a map to the distribution of amount of Quaternary vertical movements, throughout the Japanese Islands with the exception of Hokkaido.Comparison of the rate of recent movement with the total amount of Quaternary movement suggests the following two interpretations, which have the advantage of simplicity compared with other possibilities:

1. (1) The rate of the recent movements is four times larger than the average rate for the Quaternary.

2. (2) If the rate of crustal movements is almost uniform as most of creep deformations can be shown to be, then the Quaternary movements must have started about 0.5 million years ago.

Nevertheless, the author suggested in 1967, that the active period of Quaternary tectonic movements might have begun about one million years ago. This suggestion was based upon the measurement of the total amount of movement compared with the rate of movement as detected from the deformation of late Quaternary terraces. These tectonic movements, naturally include the crustal movements associated with major earthquakes.

It is highly probable that the movements associated with major earthquakes and the creep deformations not directly associated with major earthquakes became active at the same time. If so, a third interpretation could be advanced:

3. (3) The rate of the recent creep deformations is about twice the average, and the crustal movements in the Japanese Islands commenced their active period about one million years ago.

On the basis of a bend in the hot-spot trace along the Hawaiian volcanic chain, the Pacific plate seems to have changed the position of its rotation axis and its angular velocity about one million years ago. The agreement of both of these dates with the increasing rates of activity suggests that the Quaternary tectonic movement in Japan was activated by the change in the pole of rotation of the Pacific plate which took place at about one million years ago and in doing so caused the bend in the eastern end of the Hawaiian island chain.     

Statistical analysis of geodetic measurements for the investigation of crustal movements

Geodetic networks are designed to obtain data that can be used to monitor crustal movements. The relative position on the earth's surface is determined from these networks by means of coordinates. The coordinates of stations and its variance—covariance matrix are based on the computational model. In spatial networks at least three points, the base points, should be chosen to define the coordinate system “fixed” to the earth. In monitoring crustal movements these base points are considered to be stationary over the time span of the motion involved. A procedure for testing the stability of the base points, together with other stable points, is described.The coordinate differences between two time epochs, t₀ and t₁ are considered to investigate crustal movements. A statistical test is introduced to determine whether crustal movements have actually occurred.The reliability, i.e., the influence, of nondetected errors in the observations or computations, should be considered. Two types of decisions can be made which may lead to incorrect conclusions. These conclusions are as follows:

1. (1) That no movement has taken place, although a nondetected error leads to the opposite conclusion.

2. (2) That a movement has occurred, although a nondetected error in the observations leads to the opposite conclusion.

The chance of arriving at these conclusions can be computed. Boundary values for assumed crustal motion in specified latitudinal and longitudinal directions give a better insight into the desired specifications for geodetic networks.The testing procedure and the above-mentioned method of computing boundary values can be used for all types of networks e.g., those obtained by conventional triangulation or by a satellite-borne ranging system.     

Crustal structure beneath the Mount Cameroon region derived from recent gravity measurements

In this study, the recent update of the gravity database with new measurements has raised the opportunity of improving the knowledge of the crustal structure beneath the large volcanic system called Mount Cameroon, and its implication in the regional tectonics. The multi-scale wavelet analysis method was applied to highlight the geologic features of the area, and their depths were estimated using the logarithmic power spectrum method. The results reveal a complex crustal structure beneath Mount Cameroon with high variation in the lateral distribution of crustal densities. The upper and lower crusts are intruded by dense materials originating from the mantle with less lateral extension. The trends of Tiko and Ekona faults along the intrusion suggest tectonic activities as deep as 25 km. The difference in mantle composition or temperature between the East and the West of the studied area is clearly seen in detailed wavelet images and agrees with a mantle origin for the Cameroon Volcanic Line.     

Solvability and multiquadric analysis as applied to investigations of vertical crustal movements

A variety of geodetic measurements can be combined, in network fashion, to yield adjusted velocities of elevation change. However, it is not always apparent which network junctions have solvable point velocities. When a velocity surface is desired, it is not always apparent how many coefficients should be used. A solvability algorithm, devised to operate on observation equations, answers these questions, and therefore permits the adjustment process to continue with the assurance that the result will be mathematically justified. Using both the hyperboloid and the reciprocal hyperboloid as quadric forms, multiquadric (MQ) analysis has been applied to leveling and tide gauge data in the vicinity of Puget Sound, to obtain heights corresponding to a selected date, and coefficients which collectively define a velocity surface. The solvability algorithm was used to tell which junctions in the level network had solvable point velocities, and consequently where MQ nodal points should be placed for an optimized solution. Networks of simulated data were also used with the solvability algorithm to help determine data requirements for height—velocity adjustments, and to evaluate the ability of MQ analysis to predict velocities.     

Earthquake hazard assessment from the recent space and surface measurements of the Earth's gravity field

The distinctive nature of the structure of the Earth's gravity field over high seismicity areas, as observed in some limited scale studies, indicates that it is possible to use these associative patterns to outline areas of high seismicity or high earthquake hazard potential from a knowledge of the finer structure of the Earth's gravity field. The global scale investigation of such relationships as a function of the geophysical characteristics of various tectonic provinces, and the parameterisation of such relationships by hypocentral depth and earthquake intensity/energy data, become even more attractive because of the availability of (1) satellite-determined gravity models which provide global information on the long wavelength components of the gravity field, (2) satellite altimetry data which provide oceanwide information on the detailed geoidal structure, (3) surface gravity data which provide information on the short wavelength components in the areas of surface gravity coverage, and (4) bathymetric and topographic data which, though still somewhat limited in spite of their recent extensions of coverage, are available in more and more areas and provide information on the tectonic and morphological environments of an area to enable its gravity data conversion to some standard environments for direct comparisons of underlying structures. Additionally, the rapidly mounting geological and geophysical evidence of considerable intraplate tectonic activity, not so fashionable until recently, makes the study of these correlative patterns even more attractive and productive from a scientific viewpoint. This paper presents the major elements of the theoretical formulation for conducting such investigations.     

Holocene deformation and crustal movements in some type areas of India

Recent crustal movements have been observed and studied in several parts of India including the Himalayan and sub-Himalayan regions, the Precambrian shield of peninsular India and also the coastal tracts. The results of studies of Holocene deformation and crustal movements in two type areas are presented, one in the extreme southeastern part of the peninsula and the other in northeastern India.The Precambrian shield in the extreme southeastern part is characterised by a major NE—SW trending fault zone in the Tirupattur—Mattur areas of Tamil Nadu with some major extended faults, one of which apparently cuts through the entire crust and Moho as indicated by gravity data and which is associated with occurrences of alkaline and basic intrusions and carbonatite complex. Evidence of Recent crustal movements in this zone is afforded by geomorphic features and recent and current seismicity of a mild nature which is apparently to be attributed to slow movements along the fault plane.The Shillong plateau in northeastern India occurs as block-uplifted horst, comprising for the most part Archaean crystalline rocks with plateau basalts and Cretaceous and Tertiary sediments occurring on its southern margin. The plateau is bounded by major faults and is located in a zone of high seismicity lying astride and parallel to the eastern Himalayas intervened by the alluvium of the Brahmaputra Valley. Geomorphic features such as raised terraces, straight-edged scarps, etc., provide evidence for Recent crustal movements with dominant vertical movements along the fault planes which have continued through Tertiary and Recent times. Repeated precision levelling measurements conducted by the Survey of India indicate a rate of uplift of 4–5 cm per 100 years during the period 1910–1975.The gravity data pertaining to this region are also discussed in relation to the crustal movements.     

Recent vertical movements and their determination in the Rhenish Massif

The uplift of the Rhenish Massif started in late Tertiary time. The determination of the rates of recent vertical movements helps to interpret the whole, phenomenon. The data obtained by relevellings of high precision will be analysed by two different velocity models based on the concepts of Holdahl: the single-point model and the velocity surface model. Both models yield adjusted heights corresponding to a selected reference time and the velocities. The significance of the model parameters will be increased by introducing free adjustments with generalized inverses.     

Objectives and results of the investigation of recent vertical movements in the Carpatho-Balkan Region

The author, as coordinator of the investigation of recent vertical crustal movements in the Carpatho-Balkan Region, reports on the purpose, conditions and method of recent (starting in 1980) investigations, as well as on the results obtained so far. He presents the geodetic and oceanographic data applied, together with their reliability and the standard error of deduced velocities and absolute height data, in tabular form. Deduced height-value differences between the mean sea levels of the three seas considered (Baltic, Black and Adriatic seas) are also shown. Up to now, the joint adjustment and compilation of national maps of movements have been completed. At present compilation of the overall map at a scale of 1: 1,000,000 is under way, for publication in 1985, and its presentation was scheduled for the Budapest International Symposium in October, 1985.

As far as further investigations are concerned, the derivation of movement velocity gradients and their cartographic presentation are discussed.     

New results on the properties of recent crustal movements in the Bohemian Massif and its boundary with the West Carpathians

Preliminary analysis of the new results of repeated levellings within the territory of the Bohemian Massif and its border with the Carpathians is made, and the correlation between horizontal gradients of vertical movements and main fault zones discussed. The results indicated the continuance of the main movement's tendencies, determined by the foregoing measurements. Also discussed are the vertical and horizontal crustal movements between the Bohemian Massif and the Carpathians. The active zones are connected with the main fault systems, and the horizontal movements indicated the spreading tendencies between both geological structures. These results are in accord with the tendencies of horizontal movements in the Soviet Carpathians.     

Recent vertical crustal movements from precise leveling surveys in the Blue Ridge and Piedmont Provinces, North Carolina and Georgia

Examination of two lines of repeated leveling in North Carolina and Georgia reveals

1. (1) apparent uplift at the Blue Ridge-Piedmont physiographic boundary (the AtlanticGulf drainage divide) relative to the Atlantic Coastal Plain on the east and the Valley and Ridge province to the west; and

2. (2) large tilts over short baselines superimposed upon the regional pattern in the vicinity of the nearby Blue Ridge—Piedmont geologic boundary (the Brevard fault zone). In the North Carolina profile a very pronounced correlation between topography and movement suggests possible systematic leveling error, but the observed movements appear to be larger than those normally attributed to leveling error. Thus, either refraction or rod errors are larger than expected, or the movement is real and strongly correlates with topography along this portion of the leveling line.

Anomalously high stream-gradients over both resistant and nonresistant lithologies are found around the drainage divide in North Carolina, and may be associated with the relative uplift inferred from releveling. The drainage divide in Georgia, also characterized by relative uplift on the movement profile, approximately separates two different types of stream patterns. In both cases evidence presented here suggests that stream morphology may be responding to contemporary deformation as implied by the observed elevation changes. The relative uplift in North Carolina also correlates with a positive Bouguer gravity anomaly of 30–40 mGal in the midst of the regional Blue Ridge gravity low, although the significance of the correlation is unclear.The close spatial correspondence between the zone of maximum uplift and the drainage divide suggests that the vertical movements and geomorphic anomalies may result from the same mechanism, although the nature of such is unclear. One possible mechanism could be displacement at depth along the nearby Brevard zone. However, on the basis of dislocation modeling it appears that the geodetic observations cannot be adequately explained by surface deformation associated with any simple models of slip on the Brevard zone.     

Four-dimensional modeling of recent vertical movements in the area of the southern california uplift

This paper describes an analytical technique that utilizes scattered geodetic relevelings and tide-gauge records to portray Recent vertical crustal movements that may have been characterized by spasmodic changes in velocity. The technique is based on the fitting of a time-varying algebraic surface of prescribed degree to the geodetic data treated as tilt elements and to tide-gauge readings treated as point movements. Desired variations in time can be selected as any combination of powers of vertical movement velocity and episodic events. The state of the modeled vertical displacement can be shown for any number of dates for visual display. Statistical confidence limits of the modeled displacements, derived from the density of measurements in both space and time, line length, and accuracy of input data, are also provided. The capabilities of the technique are demonstrated on selected data from the region of the southern California uplift.     

Regional investigations of vertical crustal movements in the U.S., using precise relevelings and mareograph data

In the past two years, the National Geodetic Survey has been creating a data base of vertical crustal movement information. The data elements are relative elevation changes along lines of releveling which are used to produce graphic displays on microfilm. In separate studies, the releveling data, together with information extracted from mareograph records, have been used to create two networks of velocity differences. The two networks, one in the vicinity of Chesapeake Bay and the other covering the Gulf Coast states, have been adjusted in order to prepare maps showing the velocities of elevation change. In the adjustments, the velocities derived from mareograph records were treated as observations. The results indicate annual subsidence ranging between −1.2 mm and −4.0 mm in the Chesapeake Bay area, with significant local variation. In the Gulf Coast region there is generally slight subsi'dence along the coast, ranging between 0.0 and −1.5 mm/year. Stability and slight uplift is indicated to the north where bedrock reaches the terrain surface. Anomalous subsidence of −7.0 mm/year occurs at New Orleans, La., and at Houston, Texas, there has been several decimeters change in the last ten years.     

Recent crustal movements and the Moneron earthquake of 1971

From the results of high-precision levelling conducted in the southern part of Sakhalin Island, the dynamics of the earth's surface have been determined before the Moneron earthquake in 1971, during its numerous aftershocks and in the following periods. Great subsidence, of up to 7–8 cm, preceded the earthquake, then a 3-year period of steady state followed and at the end of this period the earthquake happened. After this event vertical movements show an inherited character related to the very recent geological structures. Horizontal displacements of the triangulation net, 50 km distant from the earthquake epicentre, were also noted; they had a northeast direction and an amplitude of 7 cm.     

Analysis of recent crustal movement in the central and northern Sierra Nevada, California, using repeated geodetic leveling data

A comparative analysis of repeated geodetic leveling data was made along nine subparallel, E—NE-trending leveling lines located in the central to northern Sierra Nevada and the eastern Central Valley. The analysis was made to identify relative changes of elevation and evaluate these changes with respect to the regional geology and tectonics. The analysis used National Geodetic Survey first- and second-order, unadjusted, observed elevations.The relative changes in elevation indicate that crustal deformation is continuing to occur in the Sierra Nevada along pre-existing zones of crustal weakness and that this deformation is localized along some strands of Late Cenozoic faulting within the Mesozoic Foothills fault system. This deformation is characterized by variable and nonunifor westward tilt of the Sierran block west of the Melones fault zone, and relatively consistent eastward tilt of the Sierran block east of the Melones fault zone. Variable elevation changes occur within the Foothills fault system and are often associated with prominent geological or structural contacts. In addition, subsidence in the Central Valley appears to be of small magnitude and localized in extent, indicating nontectonic changes in elevation problably due to compaction of unconsolidated sediments.