B. Gutenberg

Changes in sea level, postglacial uplift, and mobility of the earth's interior

Record of tide gauges indicate that sea level generally is rising at an average rate of about 10 cm per century. The uplift in Fennoscandia and North America is investigated, and maps showing the rate of uplift are given. A discussion of the new material and historic evidence leave no doubt that the uplift is a consequence of isostatic readjustment of the equilibrium disturbed by the postglacial melting of the ice. The remaining uplift is about 200 meters in Fennoscandia and possibly more in North America, where the present rate of uplift has its maximum of about 2 meters per century in the region of Hudson Bay. Originally, the time needed to reduce the defect in mass to one half under the regions of uplift was less than 10,000 years, but it has been increasing with time and now exceeds 20,000 years. Theoretical investigations on the plastic flow in the interior of the earth connected with the uplift are critically discussed and extended. The movements affect the whole interior of the earth below the regions of uplift; their amplitudes decrease slowly in the upper 1000 km. If one assumes a strong lithosphere with a thickness of about 70 km and below the asthenosphere with a viscosity of the order of 10^(22) poises, but little or no strength to prohibit plastic flow, there is no disagreement with observations related to isostasy or deep-focus earthquakes. Tectonic processes connected with isostatic anomalies larger than those in the regions of postglacial uplift must be connected with plastic flow at least down to the core. The importance of the effects of small forces acting during long periods is pointed out.

The energy of earthquakes

Summary Calculation of the energy released during earthquakes, including all great shocks from 1904 to 1954, indicates that the average annual release of energy in earthquakes is roughly 1025 ergs. Since this is only about 0–1 per cent of the energy produced by disintegration of radioactive matter in the earth, processes maintained by the generation of heat could furnish the earthquake energy. In each of the three major depth ranges, (a) shallow shocks, depth h ≤ 60 km., (b) intermediate shocks, 60 < h≤300 km., (c) deep shocks, h > 300 km., the frequency of earthquakes increases about exponentially with decreasing earthquake magnitude m down to at least m = 2, and in each the average energy release between 1904 and 1913 was greater than that in later decades. The largest energy calculated for a single shock during the 51 years, about 2 X 1025 ergs, was found for two shallow shocks. With increasing focal depth h the maximum energy of a single shock decreases to about 6 X 1023 ergs at h = 650 ± km. and to about 4 X 1022 at 720 ± km. No deeper earthquakes are known. The rapid decrease in energy release near 700 km. could be caused by flow processes if the apparent coefficient of viscosity decreases to the order of 1020 poises at a depth of about 700 km.

Depth and geographical distribution of deep-focus earthquakes

Early in the history of seismology it was occasionally suggested that, in addition to earthquakes with foci comparatively near the surface of the earth, shocks might also originate at depths of the order of several hundred kilometers. However, down to a comparatively recent date, all such conclusions were either purely speculative or were based on inadequate or misinterpreted data.

CRUSTAL LAYERS OF THE CONTINENTS AND OCEANS

It has been assumed heretofore that the relatively large phase called [P bar] in records of near-by earthquakes in the continents is the direct longitudinal wave, and that the corresponding velocity of about 5.6 km/sec is characteristic of the “granitic layer.” This leads to contradiction with the calculated origin-time of transverse waves in earthquakes and cannot be reconciled with observations of the longitudinal waves from blasts. It seems more likely that the velocity of longitudinal waves below the sediments is about 6 km/sec, increases to 6 1/2 km/sec or more at a depth of roughly 10 km, and possibly decreases below a depth of between 10 and 15 km. Such a decrease in velocity is to be expected in rocks with an appreciable content of quartz, since in laboratory experiments a decrease of elastic constants of quartz with increasing temperature has been found approaching the temperature at which transformation from alpha- to beta-quartz occurs (corresponding to a depth of 25 km or more). At the bottom of a deeper layer with higher velocity (usually 7-7 1/2 km/sec) the Mohorovicic discontinuity at a depth of between 30 and 40 km, but deeper under some mountain chains, forms the boundary between the simatic crustal layers and the ultra-basic material below (wave velocity 8.2 km/sec). Most earthquake foci seem to be in the hypothetical low-velocity layer. Geophysical and geological evidence indicates a greater difference between the structure of the Pacific basin and the surrounding continental areas than between the bottom of the Atlantic or Indian oceans and the surrounding shelves or continents. In the Pacific, the surface layers seem to consist of sediments, erupted and perhaps some simatic material. Below them is probably ultra-basic material with a boundary (Mohorovicic discontinuity) at a depth of only a few kilometers. In the Atlantic Ocean (and probably similarly the Indian Ocean) the transition from the continents to the basins seems to be more gradual, and the Mohorovicic discontinuity seems to be at greater depth than in the Pacific but much shallower than in the continents. While at present there is no indication of extensive sialic material in the bottom of the Pacific, there may be limited areas with sialic material at least in the eastern part of the Atlantic Ocean while relatively basic simatic (but not ultra-basic) material seems to be close to the surface at least in parts of the western Atlantic basin.

Seismological evidence for roots of mountains

Theoretical problems concerned with the calculation of the velocities of seismic waves in the various layers of the earths crust, and the determination of the thickness of such layers, are discussed. The results are applied to seismograms obtained from artificial explosions, and sources of error in interpretation are investigated. Seismograms registered in different parts of the earth from epicenters less than 1000 km from stations are used to investigate the layering in these regions and the problem of mountain roots. The bearing of the findings on the theory of isostasy is discussed.

Earthquakes and structure in southern California

During recent years the accuracy in determining the arrival times of earthquake waves in southern and central California has been notably improved, so that more detailed results concerning the structure of the earths crust in California are to be expected from the study of such records. Fifty shocks were selected, and their epicenters, depths of foci, origin times, and magnitudes determined and discussed. All known corrections have been applied; in particular, the effect of the altitude of some of the stations is beyond the probable error of the readings. With this improved material it is possible not only to investigate more accurately than heretofore the wave velocities in the various layers, but also differences in structure, especially local effects near the stations.

SEISMICITY OF THE EARTH (SUPPLEMENTARY PAPER)

This paper supplements Seismicity of the earth (Gutenberg and Richter, 1941). Additional epicenters for shallow and deep earthquakes are reported. Sixty-four great earthquakes are now identified for 1904-1943, and 201 major earthquakes for 1922-1943. New maps are given. Relative seismicity of active regions is discussed quantitatively (shallow shocks only). The Pacific belt has about 80 per cent and the trans-Asiatic zone about 10 per cent of the general activity. Earthquakes, volcanism, and gravity anomalies are discussed in their geographical and dynamical relation to structural arcs of Pacific type. These must be maintained by persistent processes in which subcrustal currents play a part.

SEISMIC EXPLORATIONS ON THE FLOOR OF YOSEMITE VALLEY, CALIFORNIA

The depth and configuration of the bedrock floor beneath Yosemite Valley were determined by seismic surveys in 1935 and 1937. Seismic velocities of roughly 1.7, 2.5, 3.0, and 5.2 km/sec, and good to excellent reflections delineate at least three distinct layers within the valley fill resting on granitic bedrock. The upper layer with a maximum thickness of about 150 m extends from Mirror Lake to the Wisconsin end moraines near Bridalveil Meadow. It is thought to be primarily deltaic lake deposits of Wisconsin age. The intermediate and basal layers have maximum thicknesses of 220 and 300 m respectively, and the intermediate layer lies in a U-shaped trough seemingly gouged out of the basal layer. Both layers are thought to be remnants of earlier lake fillings, and at least the basal layer is pre-Wisconsin. The greatest thickness of fill, about 600 m, is near the head of the valley between Ahwahnee Hotel and Camp Curry. The bedrock floor of Yosemite Valley is an undulating surface with three separate basins and a total bedrock closure of at least 400 m, possibly approaching 500 m. The bedrock floor slopes steeply from the head of the valley to its deepest point, 600 m above sea level, between Ahwahnee Hotel and Camp Curry. Down-valley it rises rapidly about 300 m across a broad sill opposite Rocky Point. The second basin, 800 m above sea level, is opposite Cathedral Spires. From here the floor rises gradually down-valley to at least 1000 m above sea level opposite Artist Creek. It may rise another 100 m before the drop into a small basin more than 100 m deep at the Cascades. The amount of glacial excavation on the bedrock floor, essentially double the 450 m previously estimated, is attributed wholly to pre-Wisconsin glaciation. The greatest depth of excavation is in massive granitic rocks, and it is suggested that thick ice, exfoliation sheeting developed by pressure relief, and compressive flow in the glacier combined at this point to produce exceptionally effective erosion.

Depth and geographical distribution of deep-focus earthquakes (second paper)

This paper is a continuation of a previous publication (1938). The writers distinguish (1) shallow shocks, at depths not exceeding about 50 kilometers; (2) intermediate shocks, at depths from about 50 to 300 kilometers; (3) deep shocks. Separate maps are drawn for intermediate and for deep shocks, and new or revised determinations are presented and discussed. Previous conclusions as to mechanism and origin remain unmodified.

Pacific Science Congress

The Seventh Pacific Science Congress which was held in New Zealand during February 1949 was the first after the war. The Congresses are mainly intended to unite scientists in fields with special interest in the Pacific Ocean. Thus, pure physics is not represented, but there are divisions of geophysics, geology, meteorology, and oceanography, in addition to other sections such as botany, zoology, public health, anthropology, and social science.