Showing papers in "Bulletin of the Seismological Society of America in 1963"

The behavior of inverted pendulum structures during earthquakes

Abstract: During the Chilean earthquakes of May, 1960, a number of tall, slender structures survived the ground shaking whereas more stable appearing structures were severely damaged. An analysis is made of the rocking motion of structures of inverted pendulum type. It is shown that there is a scale effect which makes tall slender structures more stable against overturning than might have been expected, and, therefore, the survival of such structures during earthquakes is not surprising.

The dynamic behavior of water tanks

Abstract: During the Chilean earthquakes of May, 1960, a number of large elevated water tanks were severely damaged whereas others survived without damage. An analysis of the dynamic behavior of such tanks must take into account the motion of the water relative to the tank as well as the motion of the tank relative to the ground. Some simple expressions are given for the pertinent dynamic properties of tanks with free water surface. A simplified dynamic analysis is indicated for the response of elevated water tanks to earthquake ground motion.

On uniqueness in dynamic poroelasticity

Abstract: Conditions are derived sufficient for uniqueness of solution of the field equations of Biot's theory of liquid-filled porous media, particular attention being paid to continuity requirements at an interface between two such dissimilar materials. It is found that at an interface two distinct sets of conditions will satisfy the demands of the mathematical uniqueness theorem, one of them being discarded on physical grounds. The permissible set is then discussed in relation to a number of possible models of the structure of a pair of elements in contact. The special cases of an impermeable elastic solid or a liquid medium in contact with a saturated porous solid are also examined.

Seismic waves and earth structure in the Canadian shield

Abstract: Careful measurements of phase velocities in the Canadian shield have been made in the period range 3 to 90 sec for Rayleigh waves and 12 to 60 sec for Love waves by phase correlation of wave trains. The continental Love wave phase velocity data are the first to be reported in the literature. The phase and group velocities are higher than yet found in any other continental area, indicating relatively higher shear velocities in the crust and upper mantle. For paths in the Canadian shield a prominent Lg arrival with a velocity of about 3.65 km/sec is observed and an Sn arrival is recorded clearly to distances of about 4000 km with a velocity of about 4.72 km/sec. A theoretical layered model of the crust and upper mantle consistent with the various types of data has been derived by an inversion method employing least-squares curve-fitting of phase velocity data. This model has a three-layered crust 35.2 km thick with shear velocity increasing to about 3.85 km/sec in the lower crust. The upper mantle has a high speed layer with shear velocity 4.72 km/sec down to about 115 km below which the low velocity channel has a shear velocity of about 4.5 km/sec down to a depth of about 315 km. At greater depths the shear velocity closely follows the Gutenberg model. Higher mode phase velocity dispersion curves computed from the theoretical model are used for computing theoretical seismograms for the Canadian shield. These theoretical seismograms possess many of the features of the observed Lg arrivals, showing that Lg can be explained by normal mode wave propagation in a simple, layered crust. This paper shows that new methods of measuring and interpreting surface wave dispersion, combined with travel time data, can provide detailed and reliable information on shear wave velocity distribution down to depths of a few hundred kilometers in regions of horizontally uniform structure.

The stress changes that accompany strike-slip faulting

Abstract: In a previous paper (Chinnery, 1961) an investigation of the deformation of the ground around a strike-slip fault was described. This work is now extended to a calculation of the change in stress distribution that is brought about by such a fault. Contour maps of the components of the stress change are included, both for the general rectangular fault, and for the limiting case of a long shallow fault. These maps have a number of interesting features, and show clearly the complexity of the stress change in the neighborhood of the edges of the fault plane. The assumptions involved in the calculation are discussed at some length.

Comparative spectra of microseisms and swell

Abstract: Spectra of seismic and ocean wave recordings near San Diego, California, show closely related features. The wave spectra consist of a sharp peak whose frequency, f(t) , increases linearly with time and consistent with the expected dispersive behaviour from a source at 6150 nautical miles (presumably a storm in the Ross Sea). The seismic spectra show peaks at f(t) and at 2 f(t) ; the double frequency peak contains 100 times the energy of the peak at the primary frequency. A comparison between the peak frequencies and band widths of the seismic and ocean wave spectra, and an estimate of the direction and beam width of the seismic radiation, leads to the following conclusions: that the microseismic generation area is predominantly local , being confined to a distance of 100 miles up or down the coast. For the primary frequencies the generative strip is presumably confined to shallow water; for the double frequencies it extends 200 miles seaward.

Modifications to the soil pattern of South-Central Chile resulting from seismic and associated phenomenona during the period May to August 1960

Abstract: Genesis and evolution of soils in the earthquake region is discussed and then reviewed in the light of changes induced by the earthquakes. Those effects of the earthquake that could possibly influence soils are reported on in detail. These include many field observations on the mass movement, the tsunamis, flooding, changes in land elevation and regime of rivers, volcanic eruptions, etc. A catalogue of changes, including land lost to production completes the study. (PStA)

General characteristics of active andean volcanoes and a summary of their activities during recent centuries

Abstract: An oceanic deep lies off the Chilean coast, bordered on the east by a coastal mountain range, a discontinuous central valley, and the high cordillera of the Andes. The Chilean volcanoes are found on lineaments that in general coincide with, or are sub parallel to, axis of the Andes. In north Chile they lie along en echelon fractures and, in some cases, along transverse fractures. In the south, the alignment of the volcanoes lies west of the axis of the Andes. Where the Central Valley is not well developed, active volcanoes are scarce. Recent lavas range from basalt in the south to rhyolite in the north. Volcanic activity appears to be decreasing. Details are given of individual volcanoes, and a list of active Andean volcanoes south of Volcan Misti is presented with a historical account of their activity.

The Chilean earthquakes of May, 1960: A structural engineering viewpoint

Abstract: The series of major earthquakes which occurred in south-central Chile during May of 1960 were of large Richter magnitude and were accompanied by major geologic changes. Epicenters were distributed throughout populated regions; the earthquakes violently shook many substantial buildings which had been specifically designed to resist earthquake forces. In general, damage to earthquake resistive structures of reinforced concrete, structural steel, and wood frame was slight and the overall performance was quite satisfactory. When major damage did occur to structures with earthquake bracing, construction practices were almost always recognizably poor. The minor damage to earthquake resistive structures was instructive and a number of case histories are discussed. Of particular interest are the instances of shear wall rotation on compressible soils, construction and design errors, relative rigidity problems, and the performance of inverted pendulum structures.

The Buyin-Zara (Iran) earthquake of September, 1962 a field report

Abstract: The Buyin-Zara earthquake of 1 September 1962 originated on the Ipak fault and it was felt over most of Northern Iran. This newly named fault and its branches run for 64 miles, from Ipak, about W-05°-N, across the mountains south of Buyin, to Takhrijin, 4 miles west of Ab-i-Garm. During the earthquake of September 1, 1962 and its aftershocks, at least 64 miles of surface rupture occurred progressively along this line, and at leas two stages of progressive faulting were involved. Most probably, one from Rudak from where the fracture progressed east to Ipak, and a deeper fracture, with very weak surface evidence from Indrejin progressing east to Tofak. The earthquake killed 12,225 and injured 2,776 persons. It damaged beyond repair 21,310 houses and killed 35% of the livestock in the area. The maximum intensity of the shock did not exceed IX MM. Small landslides and rock falls occurred on steep slopes and temporary springs were formed.

Source-mechanism from spectra of long-period seismic surface waves.3. The Alaska earthquake of July 10, 1958

Abstract: Source-mechanism is derived from amplitude and phase spectra of mantle Love and Rayleigh waves of the Alaska earthquake of July 10, 1958. The signals R_2, R_3, G_2, G_4, G_5 recorded on the Gilman 80–90 and the Press-Ewing 30–90 seismograph systems at Pasadena, California, are separated, digitized, filtered and Fourier-analyzed. An agreement between theory and observations is obtained for a unilateral fault of 300–350 km, which ruptured with a speed of 3-3.5 km/sec in the direction N40°W. Fault length is in good agreement with the extent of aftershock distribution in the month of July, 1958, and the time of rupture checks with the duration of an impressive T-phase recorded at Hawaii. The phases of the signals are corrected for propagation, instrumental shift and the source finiteness. Initial phases thus obtained agree on a mechanism of a right double-couple with a unit step-function in time.

Earthquakes and faults

Abstract: The hypothesis that earthquakes are caused by faulting has been prominent in seismological theory for half a century, but continues to present many difficulties. Although the chief support comes from studies of large shallow earthquakes that have been accompanied by surface faulting, the evidence given by these infrequent events can be interpreted otherwise. No satisfactory explanation of deep-focus earthquakes has emerged; sudden faulting may be essentially a surface phenomenon. Nor does the hypothesis aid the understanding of such phenomena as sudden regional uplift, or slow fault creep. There is much to encourage the view that fracture of the ground is but a gross form of earthquake damage. On the other hand, the similarity between natural earthquakes and underground nuclear explosions, as radiators of seismic waves, suggests that sudden local phase transitions may provide a source mechanism for earthquakes at all depths.

Velocities of mantle Love and Rayleigh waves over multiple paths

Abstract: Phase velocities of Love waves from five major earthquakes are measured over six great circle paths in the period range of 50 to 400 seconds. For two of the great circle paths the phase velocities of Rayleigh waves are also obtained. The digitized seismograph traces are Fourier analyzed, and the phase spectra are used in determining the phase velocities. Where the great circle paths are close, the phase velocities over these paths are found to be in very good agreement with each other indicating that the measured velocities are accurate and reliable. Phase velocities of Love waves over paths that lie far from each other are different, and this difference is consistent and much greater than the experimental error. From this it is concluded that there are lateral variations in the structure of the earth's mantle. One interpretation of this variation is that the mantle under the continents is different from that under the oceans, since the path with the highest phase velocities is almost completely oceanic. This interpretation, however, is not unique and variations under the oceans and continents are also possible. Group velocities are computed from the phase velocities and are also directly measured from the seismograms. The group-velocity curve of Love waves has a plateau between periods of 100 and 300 seconds with a shallow minimum at about 290 seconds. The sources of error in both Fourier analysis and direct time domain methods of phase velocity measurement are discussed.

Source wave forms of three earthquakes

Abstract: Strain seismograms of the Montana shallow earthquake of August 17, 1959 recorded at Isabella, California have the wave pattern predicted in 1904 by Lamb for a surface pressure pulse. This is equivalent to a dipole source such as given by the vertical bilateral fault slip which was observed at the surface. Similar recordings made at Nana of the deep Peruvian earthquakes of August 19 and 30, 1961, (Δ = 600 ± km, h = 600 ± km) have the pattern calculated by Pekeris for a buried vertical downward force in the form of a step in time. It thus appears that these shocks were generated by a sudden volume contraction at the focus which could be the result of a sudden change of state. Failure of conventional seismographs to record the patterns calculated by Lamb and by Pekeris is due to their low sensitivity to the very long period ground movement components involved, and their relatively high sensitivity to the short period components which are rendered oscillatory by the departure of the crustal characteristics from the homogeneous half space assumed in the theoretical computations.

The seismic motion of the deep ocean floor

Abstract: Results of the first measurements of ambient seismic noise of the deep ocean floor below 2000 fathoms are presented. The measurements were made with vertical component seismometers at nine locations. At five stations in the Atlantic Ocean, amplitudes observed at 1 cps were 100 millimicrons at one site, 35 millimicrons at two sites, and less than 1 millimicron at the other two sites. At a station in the Gulf of Mexico, amplitudes of 35 millimicrons at 1 cps were observed. Amplitudes of about 1 millimicron at 1 cps were observed at three sites in the Arctic Ocean. Amplitudes and periods remained essentially constant during the two days that data were recorded in the Gulf of Mexico and showed no correlation with nearby local meteorological storm activity. During the three days that data were recorded at the noisiest Atlantic site, rising and falling winds and seas over a broad area were weakly reflected as small changes in the amplitude of the ocean bottom noise. Similar small amplitude variations were recorded at Bermuda, 150 nautical miles to the northeast, but the absolute Bermuda amplitudes were an order of magnitude less than those at the noisiest Atlantic site. Measurements in samples of up to one hour in duration were made in the Arctic Ocean at three stations. The measurements were made at intervals during a 16-day period, and a good correlation was found in both time and amplitude between seismic noise on the ice surface and on the ocean bottom in the band from 1 to 10 cps. A P -wave train from a distant earthquake was recorded at the noisiest Atlantic site with a signal-to-noise ratio of about 3:1; this was the same signal-to-noise ratio seen nearby at Bermuda with a seismograph of similar response. In the Arctic, P waves from an Alaskan shock and a possible PKP phase from a New Zealand shock were recorded. The maximum P -wave signal-to-noise ratio recorded in the Arctic for the Alaskan shock was 250:1. After a correction is made for the difference in epicentral distance, the signal-to-noise ratio at Ogdensburg, New Jersey, for this shock is only 20:1. Ogdensburg is quieter than an average station.

Further observations of geologic and geomorphic changes resulting from the catastrophic earthquake of May 1960, in Chile

Abstract: A geomorphic study of the region in which the earthquake occurred, made after the earthquakes is compared with a similar study made before the earthquakes. Evidence is presented to show changes in land level. Discussion of the effects of the tsunami as it affected geomorphic and geologic processes is presented. Landslides are listed and analysed. Distribution of intensity is discussed.

The seismic sea wave of 22 May 1960 along the Chilean coast

Abstract: This paper details the efforts of the Chilean Navy in warning of the tidal waves that followed the earthquakes of May 1960 and the efforts made to collect information concerning the tsunami. Details of time of arrival, damages, wave height and run up are presented for almost all ports in the affected area beginning at Puerto Aysen and working northward. Changes in water depth, caused by erosion, deposition, and changes in land elevation are reported. The fates of several ships caught in the great waves are described in detail. Marigrams of the tsunami are given for all available Chilean ports. The times of high and low tides in the Gulf of Chiloe changed following the earthquakes.

Landslides at Lago Riñihue, Chile

Abstract: Strong earthquakes of 22 May 1960 produced three large landslides which blocked the outlet of Lago Rinihue, 65 kilometers east of the city of Valdivia, Chile. The lake level rose 26.5 meters before water began discharging through artificially constructed canals during the last week in June, 1960. The largest landslide involved about 30 million cubic meters of unconsolidated sediments, the intermediate landslide about 6 million cubic meters, and the smallest landslide about 2 million cubic meters. The surface of rupture of the largest landslide is within an 80-meter sequence of Pleistocene lake clays. The clays are underlain by till and overlain by outwash sands and gravels. Movement of the landslide is interpreted as having been principally block gliding and lateral spreading. Secondary landslides within the larger landslide were produced by rotational slumping, debris falls, and earth flows. Several ancient landslides exist in the vicinity of Lago Rinihue, the largest involving more than 100 million cubic meters of unconsolidated sediments. Many of the older landslides also were probably triggered by earthquakes.

Free spheroidal vibrations of the earth at very long periods, Part I— Calculation of periods for several earth models

Abstract: Periods of free vibrations of the spheroidal type have been calculated numerically on an IBM 7090 for the fundamental and first two shear modes for periods greater than about two hundred seconds. Calculations were made for four different earth models. Phase and group velocities were also computed and are tabulated herein for the first two shear modes. The behavior of particle motions for different modes is discussed. In particular, particle motions for the two shear modes indicate that they behave in some period ranges like Stoneley waves tied to the core-mantle interface. Calculations have been made also for a model which presumes a solid inner core and will be discussed in Part II. The two computer programs which were made for these calculations are described briefly.

Response of soils, foundations, and earth structures to the Chilean earthquakes of 1960

Abstract: Observations and interpretations of damage associated with soil failure in the Chilean earthquakes are presented. Effects of the earthquake are discussed relative to foundations, earth fills, harbor works, and landslides. Damage was extreme in several areas. Of particular interest were the frequent instances of apparent soil liquefaction and the many correlations between foundation conditions and degree of damage to structures.

Propagation of an SH-torque pulse in a layered solid

Abstract: In this investigation we determine the motion of the surface of a layered elastic half-space produced by a torque-pulse from a point-source situated inside the layer. The axis of the torque is vertical ( SH ) and its time variation is represented by a step-function with rounded shoulders. The displacement due to the source approaches a saw-tooth shape at large distances. The displacement of the surface was evaluated exactly by the ray-theory method for ranges r = 100 H and r = 400 H , H denoting the thickness of the layer. The spectrum of the forerunner ground wave and of the Love wave can be interpreted in terms of the normal mode theory. The form of the displacement-curve is found to be sensitive to the width Δ of the pulse at the source. For small Δ the Love wave starts with a strong high frequency amplitude comparable to that of the Airy phase arriving later. This is due to a peak in the excitation function of the first mode. As Δ grows, this peak disappears and the Love wave builds up in the normal fashion to a maximum amplitude at the time of arrival of the Airy phase. For the pulse shape assumed, the normal mode theory predicts a factor of [sin 2 (ωΔ/2)]/Δ in the amplitude, where ω denotes the frequency. This is verified in the displacement curves obtained.

Concurrent storms of long and ultralong period microseisms

Abstract: Storms of ultralong period (14-18 seconds) microseisms are shown to be concurrent with storms of the more familiar long period (7-9 seconds) microseisms, and to be related to the same meteorological disturbances. Data from Palisades, New York, were analyzed for two cases, one for a meteorological storm over the northeastern Pacific and one for a meteorological storm in the northwestern Atlantic. The conclusions are based on correlation of continuous spectral analyses of the concurrent microseism storms, and on ground particle motion studies. The periods of the ultralong period microseisms at any given time are, within the precision of the measurements, twice the periods of the long period microseisms at that time.

Computer program for a fault-plane solution

Abstract: In its earthquake mechanism studies the Dominion Observatory has been producing solutions graphically, but a program based on a probability function defined by Knopoff has been written for the IBM 1620 which permits the best solution to be obtained by a series of successive approximations from a given first approximation. The program prints out the strike and dip of the two nodal planes, their standard errors, the azimuth and plunge of their line of intersection, and a list of the stations producing inconsistent data. Weights can be assigned to each station; in practice these weights would depend on the past reliablity of the station. The machine time required depends on the number of stations used, the accuracy of the first approximation and other factors; in general 20 to 30 minutes is required for a solution involving 30-40 stations.

An eigenvalue method for the statistical evaluation of fault plane solutions of earthquakes

Abstract: A method for the calculation of the tectonic motion direction in an area from fault plane solutions of earthquakes is presented. This method is similar to an earlier one described in the literature, but with an improved weighting procedure of the input data. The problem then reduces to that of calculating the eigenvalues and eigenvectors of a certain matrix. The new method enables one for the first time to get easily a value for the scattering of the input data. The method is first applied to a test case, and then to a series of earthquakes that occurred in the vicinity of the Marianas Islands.

Long period seismic waves from large, near-surface nuclear explosions

Abstract: Seismic surface waves were well-recorded from the larger explosions of the U.S. test series detonated in the Marshall Islands during the spring and summer of 1958 and of the U. S. S. R. test series detonated at the Novaya Zemlaya test site during October 1958. In addition to waves of the fundamental Rayleigh mode, some unusual or unexpected waves were identified. These include: 1.) Love waves with lengths as great as 90 km at Hong Kong from the U. S. S. R. explosions, 2.) Love waves at Agra, India, from the U. S. S. R. explosions, 3.) waves of the 1st shear mode at Agra and Uppsala, Sweden, from the U. S. S. R. explosions, and 4.) Love, 1st shear and PL waves at Guam from the U. S. explosions. Group velocity data were derived for many paths and, for the U. S. series, these data are very accurate because they are based on exact knowledge of locations and origin times. For the U. S. S. R. explosions, revised locations and origin times based on a limited number of P-wave observations were used to determine group velocities. Using records from Hong Kong and Honolulu for two U. S. tests, one at Eniwetok and one at Bikini, oceanic phase velocities in the period range of 15 to 40 seconds were measured for the path between these islands. Using Brune9s method for initial phase determination with a known phase velocity, an initial phase, φ 0 , lying between + 3 π 2 and + π 2 was determined for the U.S. explosion, Oak. According to Aki (1960), these values of initial phase are associated, respectively, with forcing functions of a downward impulse and an explosive impulse. Seismic magnitudes of 4.7 and 4.8 were assigned to the U. S. Oak and Poplar events on the basis of the surface wave data, while magnitudes of the larger U. S. S. R. tests range from about 4.0 to 4.5. The ratio of seismic energy as computed from the surface wave magnitude to the total explosive energy available (yield) is apparently greater for the U. S. explosions, assuming equal yield for the two shots compared. This suggests a higher altitude of detonation for the Russian events. The Fourier amplitude spectra of the surface wave trains are generally single peaked where a significant portion of the path traversed is oceanic, but for Uppsala, where the path is short and continental, a second peak occurs at periods of about 10 to 13 seconds. The ratios of the predicted amplitudes, based on the data of one station and on a reasonable dissipation factor, to the actual recorded amplitudes vary with azimuth by as much as a factor of 5. These variations may be explained by asymmetry at the source but might also be explained by instrumental and geologic factors. Long period components of P and S and multiples thereof were sometimes recorded from these events.

The tsunami of 22 May 1960 in French Polynesia

Abstract: Except for the Marquesas Islands, the islands of French Oceania are relatively safe from tsunamis because of well-developed reefs, inner lagoons, and steep offshore slopes, hence the Chilean tsunami of 22-23 May 1960 was not well developed. Detailed data are given for Tahiti. The greatest runup height was 3.4 meters above low water, the average was 1.7 meters. Runup of 1 meter was observed in the Leeward Islands. In the Austral Islands the highest wave reached 1.8 meters. The Tuamotu Archipelago suffered water level changes of about 1 meter. In the Marquesas Islands runups of about 3 meters were observed. No human casualties were reported from French Polynesia and damage was slight.

The tsunami of 23 May 1960 in the Hawaiian Islands

Abstract: The tsunami of 23 May 1960 was detected in Hilo Bay at 0007 hours HST and recorded on the Honolulu tide gauge at 0033 HST. Runup heights ranged from 2 to 17 feet except in Hilo where the wave formed a bore and reached 35 feet. Sixty-one people were killed and 282 injured in spite of elaborate warning 5 hours in advance of the arrival of the tsunami. Damage and effects are discussed for all the Hawaiian Islands and recommendations are made to prevent future catastrophes. A brief history of Hawaiian tsunamis is given.

S-wave studies of earthquakes of the North Pacific, Part II: Aleutian Islands

Abstract: The polarization of the S wave at stations distributed azimuthally about the source is examined for each of twenty-five Aleutian Island earthquakes. A combination of data from the first motion of P and from the polarization of S is then used to study the focal mechanisms of the earthquakes. This combination of P and S wave data is found to make possible a good determination of the focal mechanism in cases where data from the first motion of P alone do not suffice. The earthquakes are divided into three groups according to three basic patterns of S wave polarization. The first group (fourteen earthquakes) corresponds to a double couple. The second group (five earthquakes) and the third group (six earthquakes) are conformable to conjugate shears and may therefore be explained by single couple sources of opposite moment, respectively. It is shown that a uniform principal stress system predominates in the region and that the axis of greatest compressive stress is normal to the trend of the island arc.

Rayleigh waves in a two-layer heterogeneous medium

Abstract: The possibility of propagation of Rayleigh waves in an incompressible crust of constant density and rigidity varying exponentially with depth lying on (i) a semi-infinite incompressible homogeneous medium and (ii) a semi-infinite compressible homogeneous medium is studied in this paper. The variation of rigidity is assumed as μ ρ = b e β Z , where b and β are constants. To fix ideas we suppose that the crust has the thickness 37.5 km. in which the rigidity increases exponentially from 2.747 × 10 11 to 4.53 × 10 11 dynes/cm 2 over the ultrabasic material of infinite depth in which the rigidity is constant and equal to 6.47 × 10 11 dynes/cm 2 . Frequency equations and their numerical solutions are obtained in both the cases. The results thus obtained are compared with the results derived from those given by Newlands.