Rates Of Crustal Movement Research Articles

地殻変動の一様継続性と将来予測

地殻変動の一様継続性と将来予測

Contemporary tectonic deformation of the Basin and Range province, western United States: 10 years of observation with the Global Positioning System

We have estimated patterns and rates of crustal movement across 800 km of the Basin and Range at ∼39° north latitude with Global Positioning System surveys in 1992, 1996, 1998, and 2002. The total rate of motion tangent to the small circle around the Pacific‐North America pole of rotation is 10.4 ± 1.0 mm/yr, and motion normal to this small circle is 3.9 ± 0.9 mm/yr compared to the east end of our network. On the Colorado Plateau the east end of our network moves by ∼1–2 mm/yr westerly with respect to North America. Transitions in strain rates delimit six major tectonic domains within the province. These deformation zones coincide with areas of modern seismicity and are, from east to west, (1) east‐west extension in the Wasatch Fault zone, (2) low rate east‐west extension centered near the Nevada‐Utah border, (3) low rate east‐west contraction between 114.7°W and 117.9°W, (4) extension normal to and strike‐slip motion across the N10°E striking Central Nevada Seismic Zone, (5) right lateral simple shear oriented N13°W inside the Walker Lane Belt, and (6) shear plus extension near the Sierra Nevada frontal faults. Concentration of shear and dilatational deformation across the three westernmost zones suggests that the Walker Lane Belt lithosphere is rheologically weak. However, we show that linear gradients in viscosity and gravitational potential energy can also effectively concentrate deformation. In the Basin and Range, gradients in gravitational potential are spatially anticorrelated with dilatational strain rates, consistent with the presence of horizontal variations in viscosity of the lithosphere.

POSSIBILITY OF A GREAT EARTHQUAKE IN THE TOKAI DISTRICT, CENTRAL JAPAN

The Tokai district has been considered to be one of the probable sites of great interplate earthquakes in the near future. This idea is based on the time series of historical great earthquakes and the fairly high rate of crustal movement observed in this region for more than 80 years. The last great earthquake was the Ansei earthquake of 1854 (M=8.4). The western half of the source region of this earthquake ruptured again in the 1944 Tonankai earthquake, but the eastern half, i.e., the Tokai district, remain unruptured for 122 years. The recurrence time interval of great earthquakes in this region estimated from historical earthquakes, topographical and geological evidences, observed strain rates, the rate of plate convergence, etc. varies from 120 years to more than 600 years. There are several observations which may suggest that this region has already been in the long-term precursory stage. However, it is questionable that these observations represent real precursors to the future Tokai earthquake. It seems possible that the next great earthquake will occur south of the Izu Peninsula and will release the strain in the Tokai district. In any case, the Tokai district should be watched carefully for every conceivable precursors in order to make a successful prediction.

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.

CRUSTAL MOVEMENT OF THE GREAT LAKES—TIME SERIES ANALYSIS

ABSTRACT Time series techniques were employed to determine rates of vertical crustal movement within the Great Lakes region of North America. Observations of water level elevations as recorded at gauges around the lakes, and differences in elevations between pairs of gauges were analysed for linear trends, periodicities and stochastic components. It was found that the variance of time series of elevations consisted mainly of first-order linear trends and small periodic components. Relative rates of crustal movement were computed from a linear trends analysis of elevation differences. These rates were converted to absolute rates of movement using the Nipissing zero isobase as a datum. This study shows that, in general, the northeastern area of the Great Lakes region is rising at a rate of about 1·00 ft per 100 years relative to the southwest of the region.

Stochastic analysis of Lake Superior elevations for computation of relative crustal movement

Using Lake Superior mean monthly elevations as recorded at five gauges around the lake, time series of elevations and differences in elevations between pairs of gauges were analysed for trends, periodicities and autoregressive components. It was found that the variance of the time series of elevations consisted of 4–12% linear trends, 35–44% periodicities in the mean, 0.23–0.66% periodicities in the variance, a 43–54% autoregressive component and a 5% random component. On the other hand, the time series of differences in lake elevations were found to consist of 30–52% linear trends, 5–35% periodicities in the mean and variance, up to a 30% autoregressive component and a random component of 21–31%. Rates of crustal movement were computed from the trends in gauge differences.

Some implications of crustal movement in engineering planning

Movements of the earth's crust causing progressive changes in the levels of large bodies of water relative to their shorelines may influence the formulation of water resource projects and/or their continuing effectiveness with time. In the Great Lakes basin there is evidence of an uplift of the earth's crust, of about 1 ft per 100 y, in the northeasterly part of the basin relative to that in the southwest. This results in a corresponding lowering of water levels along the northeasterly shorelines and a rise in water levels along the southwest shores. In at least two of the lakes, Lake Huron and Lake Ontario, the average depth of water will change with time. In Lake Huron, it will gradually decrease because the bed underlying the lake is rising with respect to the lake outlet. In Lake Ontario, the depth of water will increase since the lake outlet is rising with respect to the remainder of the lake. This paper reviews some of the engineering implications of the relative rates of crustal movement in the Great Lakes region on long-term management of the water levels of the Great Lakes.

Correlation between Heights of Younger and Older Shorelines for Estimating Rates and Regional Differences of Crustal Movements

The study of crustal movements depending upon former shorelines has been one of the intersting fields of the Japanese Quaternary. Herein an attempt was made for estimating the rates and regional differences of Quaternary crustal movements in Japan by using correlation between the heights of older and younger shorelines. The method of approach is as follows:In a coastal region, the heights of former shorelines of two different ages (t1 and t2 years B. P.) are measured. Pairs of older and younger shoreline-heights in close distance are designated as h1 and h2 respectively (fig. 4). When many pairs of h1 and h2 are obtained in a coastal region, the values are compared on a correlation diagram. Six regions studied are shown in fig. 1 and 11 cases of correlation obtained from the six regions are shown in fig. 3. In four regions, Kanto, Muroto Peninsula, Nishitsugaru and Osado, shoreline-heights of three different ages were measured and two correlation diagrams were drawn for each region.From fig. 3, it can be said that the relationship between h1 (in ordinate) and h2 (in abscissa) is linear in most cases. Therefore, the regression line, h1=ah2+b (1)was applicable to many cases, and calculated values of ‘a’ and ‘b’ are shown in each case. Under what conditions is it possible to be applicable to the above equation (1) for variables of h1 and h2?When we use the signs shown in fig. 4, h1 and h2 are expressed as follows:h1=z1+v1(t1-t2)+v2t2 (2)h2=z2+v2t2 (3)where z1 and z2 are sea-level heights of eutstatic change, v1 and v2, mean rates of crustal movements. From (1), (2) and (3), the following equation is led (a≠1):v2=t1-t2/t2(a-1)v1+z1-az2-b/t2(a-1) (4)That is, v2 is expressed by a linear formula of v1, where the gradient and intercept are constants peculiar to the concerning region. Therefore, (4) is also expressed by the following formula, using ‘m’ and ‘n’ which are constants peculiar to the concerning region:v2=mv1+n (5)where ‘m’ means how v1 is succeeded by v2, and is named, therefore, a ‘successional coefficient of the rate of crustal movement’. On the other hand ‘n’ means the rate of crustal movement which is added after t2 years B. P. and is uniform through the whole concerning region. Therefore, ‘n’ is named a ‘regional addend of the rate of crustal movement’. A model of areal distribution of v1 and v2 is shown in fig. 5 (in case of m=3).If (5) is right in one region, (1) is also led, then ‘a’ and ‘b’ are expressed as follows:a=1/m(t1/t2-1)+1 (6)b=(z1-z2)-1/m(t1/t2-1)(nt2+z2) (7)That is, ‘a’ is a function of ‘m’, and ‘b’ is a function of ‘m’ and ‘n’. For the correlated two shorelines of plural regions, if ‘a’ of each region is equal to one another, then ‘m’ is the same. Moreover, if ‘a’ values of many regions, having different geologic structures and different physical properties of rock, are equal, then every ‘m’ value can be estimated as 1, because it is improbable that ‘m’ values of various regions take the same value except 1. (m=1_??_a=t1/t2)