Arrival Of Seismic Waves Research Articles

Coseismic and aseismic strain offsets recorded by the Gran Sasso strainmeter

An underground low-noise high-sensitivity laser strainmeter, crossing a fault in the Gran Sasso massif (central Italy), is discontinuously operating since late May 1994, after a few years of mechanical stabilization of end monuments. A swift extensional strain transient reaching a peak of at least 1 μϵ occurred in about one month from the beginning of the operating time, followed by a four-month-long coarsely stable period, and a slower decay. On June 2nd 1994, in coincidence with two local earthquakes, permanent extensional offsets of about 10 ne have been recorded. A further aseismic slip of the same order of magnitude and sign has been observed about thirty minutes after the last preceding local event, and ten minutes before the next following one. At the same time, the arrival of seismic waves from a distant earthquake has been recorded by the interferometer. No similar signal behaviour has been observed any more since then.

"Near" Realtime Seismology

How often have we heard the term “realtime” applied to some sort of seismic analysis or process? “Realtime epicenter determination,” “realtime moment tensor solution,” and “realtime earthquake notification” are terms being routinely used to describe current seismic processing. Of course, this is usually a misapplication of the term “realtime.” According to Webster, realtime means “the actual time during which something takes place” and according to the Oxford English Dictionary, it means “the actual time during which a process or event occurs, especially, one analyzed by a computer.” In the strictest sense, for earthquake locations and mechanisms “realtime” would mean during the actual faulting process. A more liberal interpretation might be analysis during the arrival of seismic waves. Currently seismologists seem to use the term “realtime” to mean “automatic” or “pretty soon after.” A 1991 report of the National Research Council entitled “Realtime Earthquake Monitoring” included discussions of early warning and...

Electromagnetic Radiation Phenomena before and after the 1995 Hyogo-ken Nanbu Earthquake.

Before and after the 1995 Hyogo-ken Nanbu earthquake on January 17, three kinds of phenomena of electromagnetic radiations were recorded at Uji Station, Kyoto Prefecture, Japan. They were: 1) A series of increases in number of electromagnetic noises starting from 12 days before the main shock to 2 days after the main shock, 2) A clear increase of electromagnetic noises from 5 o'clock on January 17, and 3) A co-seismic change of electric field intensity in the VLF range observed at the time of arrival of seismic waves with large amplitude, then the area around Uji Station was strongly shaken. An investigation by questionnaire about the anomalous electromagnetic phenomena before and after the earthquake was also executed after the earthquake by mailing to amateur radio operators around the focal region, i.e. those in Takarazuka, Itami and Sumoto Cities. The daily variations of the numbers of anomalous electromagnetic phenomena reached a peak just before and just after the earthquake.

Digital Filtering Techniques for Interpreting Seismic Cone Data

Seismic cone penetration testing (SCPT), a relatively new in situ technique, records the arrival of seismic waves generated at the surface using velocity or acceleration transducers installed in an electric piezocone. To distinguish the different seismic arrivals, instruments with fast response times are required, i.e., accelerometers. The fast response time of an accelerometer results in a very sensitive instrument with corresponding noisy time‐domain characteristics. One way to separate events is to apply digital filters, which characterize the signal frequencies and then isolate frequency spectrums of interest. The digital filter introduced in this paper is based on frequency‐domain filtering using cross‐correlation algorithms. The cross‐correlation filter concepts are synthesized by a computer program referred to as PS CrossCor, whose performance was evaluated by interpreting both synthetic and real data. Data from three sites is presented to compare the PS CrossCor and the established crossover method. The results from the analysis indicate that PS CrossCor is an accurate and reliable tool that greatly assists in determining seismic velocities, and has potential for determining other seismic parameters.

Seismic velocity structure of the crust beneath the Japan Islands

More than 13,000 arrival times of 562 local shallow earthquakes selected from the Japan University Network Earthquake Catalog are used to investigate the seismic velocity structure of the crust beneath the Japan Islands. We simultaneously determined the hypocenter parameters, P- and S-wave station corrections and depth distributions of the Conrad and Moho discontinuities beneath the whole of the Japan Islands by applying an inversion method. The P- and S-wave station corrections show similar distribution patterns. They are negative (relatively late arrivals of seismic waves) in Hokkaido and positive (relatively early arrivals of seismic waves) in western Japan. In eastern Honshu, they are positive along the coast of the Pacific Ocean and negative in the land area and along the coast of the Japan Sea. The Conrad discontinuity is located at a depth range from 12 to 22 km. In Hokkaido, the Conrad is shallow in the eastern pan and becomes deep toward the west. In NE Honshu the Conrad is deep beneath the land area, and becomes shallow toward the surrounding seas. In western Japan, the Conrad is shallow in the northern part and becomes deep toward the south. The Moho is located in a depth range from 25 to 40 km. In Hokkaido, the Moho has the shape of a circular cone with a maximum depth of 36 km. In most parts of the Japan Islands, the Moho is deep beneath the land area and becomes shallow toward the surrounding seas. The Moho is deepest, up to 40 km, beneath the central part of the Chubu District.

Instrumentation Corrections to Wave Velocity Data

The detection of the arrival of seismic stress waves for a field measurement of wave velocity requires that the incoming signal be recorded and displayed by an instrument such as a seismograph. Filters, introduced into recording circuits to limit noise or to prevent aliasing in digital recording, can have significant effects on the observed arrival times and on the computation of wave velocity. Such filtration is often not taken into account in processing the data, but the example that follows indicates how the effects can be evaluated and the data corrected by simple techniques. The properties of electronic filters are, of course, well known to electrical engineers, and the following comments are concerned with an application in which their importance has not always been appreciated by those trained in other disciplines. Two basic conclusions can be drawn from this work: 1. The characteristics of instrumentation can have important effects on the results of field geophysical tests. In particular, the effects of filtration should not be ignored. 2. Regression analysis of travel time versus distance is effective in determining timing error and correcting for it. There is a widespread belief that, if one utilizes a hammer with reversible polarity, shear wave arrivals in cross-hole seismic tests can be identified unambiguously because the direction of first motion in the shear wave reverses when the direction of the impact is reversed. The direction of first motion in the P-wave is assumed not to reverse. Unfortunately, shear wave sources such as the Bison hammer also produce P-waves of opposite polarity when the direction of impact changes. The reason that P-waves reverse polarity is that a downward excitation of the hammer causes initial compression below the level of the hammer and initial tension above it. An upward excitation reverses this pattern so the P-waves must reverse polarity. A theoretical argument is that in a linearly elastic material, the superposition of equal and opposite upward and downward excitations gives a case of no net excitation. If the P-waves do not reverse polarity, they will not cancel, and there will be energy propagated with no input. Non-reversing P-waves may be caused by several factors, including late arrivals of reflected or refracted waves and secondary dilatational effects near the source. However, this misconception probably arises because in the down-hole test, shear wave energy is often created by striking a horizontal beam or plate on the surface in opposite horizontal direction. This unreversed vertical load causes an unreversing P-wave, which can be observed in field data. (Author)

Structure of the lunar crust at Highland Site Apollo Station 16

The seismic crustal structure of the moon is known in the region of Oceanus Procellarum from the analysis of artificial impact data. To extend this knowledge we have used data from natural lunar seismic events to search for secondary seismic wave arrivals in the form of peg‐leg multiples caused by reflections at crustal interfaces and converted arrivals caused by refractions at crustal interfaces. A polarization filter has been applied to the data to enhance the rectilinear particle motion expected for the onset of these body wave arrivals in the scattered coda of lunar seismograms. The results of this work tentatively indicate that the highland site at station 16 has a 75 km thick crust with an intermediate 20 km interface, compared to the 60 km crust and 20 km interface in Oceanus Procellarum, a mare region. Since the 20 km upper crust appears to exist at both highland and mare sites, it probably does not represent a mare basalt layer but rather a more general feature of the crust. Crustal thickness may partially control elevation by isostasy.

Seismic wave arrivals from LONGSHOT, 0° to 27°

abstract Seismic wave arrivals from the LONGSHOT underground nuclear explosion were recorded at 27 locations in Alaska and two in Yukon Territory at distances between 2.6° and 26.6° from ground zero on Amchitka Island. Five of these stations were on the island chain, three on the Alaska Peninsula, two on other islands, and the remainder on the mainland from Anchorage to Point Barrow. The upper mantle travel times to distances of about 11° may be represented by t = Δ(km)/7.85 + 4.9 ± 0.5. The estimated depth to the M discontinity beneath the island chain is from 22 to 26 km depending on the choice of data. In the distance range from 6° to 16°, recognition of the initial P wave may be a function of the signal-to-noise ratio. From 13° to 27°, travel times average from 1.5 to 4.0 seconds earlier than the JB surface-focus travel times with a steep gradient from “early” in central and northern Alaska to an average of 2 seconds “more early” in eastern Alaska including Circle Hot Springs, Anchorage, and Kodiak. PcP arrivals are recognized at 15.5° and beyond, and their travel times are parallel to the JB PcP times but are from 2 to 4 seconds earlier. PcP was not anomalously early at Kodiak as was P. P wave amplitude maxima to the Bering Sea stations of St. Paul Island and Cape Romanzoff were an order less than at like distances on the Peninsula.

Effects of Thunderstorms and Earthquakes on the Hyderabad Magnetographs

Effects of nearby thunderstorms and earthquakes recorded on the Hyderabad magnetograms during the period January 1, 1965, to June 30, 1966, are discussed in the light of available literature.The thunderstorm effect caused by the magnetic field of the lightning discharge currents appears as a sudden change (kick) of 2-10γ in H or D or both and sometimes 1-2γ in Z also which is followed by very rapid oscillations decaying exponentially in 2-5 minutes.Earthquakes of moderate and great intensity with epicentres in India and its neighbourhood produce characteristic effects on the Hyderabad magnetograms (H, D and Z). The oscillations grow to a maximum of about 30γ in the middle of the effect and then die off gradually over 15-20 minutes. Sometimes the arrivals of seismic body waves P and S and the Rayleigh surface waves LR are clearly seen in the effects recorded on the magnetograms, indicating that the response of the variometers to earthquakes is primarily mechanical. However, a small amount of magnetic effect due to piezomagnetic fluctuations in the rocks underneath the observatory at the time of passage of earthquake waves seems to be inseparably mixed with the mechanical effect.

Characteristics of refracted arrivals of seismic waves

A method has been developed for distinguishing a refracted wave that has been returned to the surface by a positive velocity gradient (direct wave) from a refracted wave that is bound to an interface and constantly radiates energy to the surface of the lower-velocity medium (head wave). The analysis is based on the fact that according to theory the particle velocity of the head wave is proportional to the integral of the particle velocity of the direct wave. Two criteria are given for discrimination between the refracted arrivals. One involves the ratio of frequencies and the other the ratio of the amplitudes of the two waveforms. The frequency-ratio criterion appears to be the more reliable. Examples of classification of the wave types and implications of the interpretation are given.

Ground-coupled air waves from the Great Alaskan Earthquake

Micropressure fluctuations occurring at the same time as the arrival of seismic waves were recorded at many localities following the Alaskan earthquake of March 27, 1964. It is shown that at Palisades, New York, Berkeley, California, and Honolulu, Hawaii, the pressure waves were produced by vertical ground motion associated with local Rayleigh waves arriving from the epicenter. Group velocity dispersion curves typical of Rayleigh wave modes for continental and oceanic paths, respectively, are developed from the air waves from the first two localities. Both oceanic and continental Rayleigh modes are indicated for Berkeley. A later train of waves arrived at a time appropriate for acoustic travel through the atmosphere directly from the epicenter. Although reminiscent of acoustic waves from large explosions, their generation over the large region of vertical ground displacement complicates their study.

Alaskan Earthquake of 27 March 1964: Remote Seiche Stimulation

After the intense Alaskan earthquake of 27 March 1964, unusual waves up to 6 feet (about 2 meters) in height occurred at about the time of arrival of seismic waves at many localities along the coasts of Louisiana and Texas. The parameters of local channels at the tide gauge in Freeport, Texas (the only instrument to record significant waves) yield seiche periods close to those of the seismic surface waves suggesting that the water waves were generated by and in resonance with the seismic waves.

Crustal structure in Nevada and southern Idaho from nuclear explosions

The time of first arrival of seismic waves generated by four underground nuclear explosions at the Nevada test site and recorded along a line extending north into southern Idaho is expressed as TD = 0.00 + DELTA /3.0 (assumed), T1 = 0.40 + DELTA /6.03, and T2 = 6.15 + DELTA /7.84, where time is in seconds and the shot-detector distance ( DELTA ) is in kilometers. Constant velocities and horizontal layers were assumed, and the crustal thickness in the vicinity of NTS was found to be 28 km. Delays in the travel-time segment T2 , which represents Pn, indicate that the crust may thicken to 32 km in northern Nevada. A third phase, expressed as T3 = 14.48 + DELTA /7.84, was also recognized. It has arrival times appropriate for SPS. Amplitudes of P were determined at seven places from recordings of seismic waves from one underground nuclear explosion.

Crustal structure from the Nevada test site to Kingman, Arizona, from seismic and gravity observations

The time of the first arrival of seismic waves generated by explosions at the Nevada Test Site and recorded along a 300-km line southeastward through Kingman, Arizona, is expressed as T0 = Δ/5.2, T1 = 0.34 + Δ/6.15, and T2 = 5.82 + Δ/7.81, where time is in seconds and the shot-detector distance (Δ) is in kilometers. Assuming constant velocities for the layers, the thicknesses are H0 = 1.7 km, H1 = 26.7 km, and H0 + H1 ≐ 28 km (below a 1-km datum). The average Bouguer anomaly is about −120 milligals, and the average elevation is about 1.1 km. Seismograms were examined for P waves indicating the presence of other discontinuities within and below the H2 layer, but the separation between seismic stations was too great to establish the presence of such discontinuities. A questionable alignment of weak arrivals following the T2 refraction time by less than 1 second may indicate the presence of a discontinuity below 28 km. Fair alignments of strong second arrivals in the range 200 to 400 km might be interpreted as direct P waves or channel waves in the H1 layer.

THE ELECTROSEISMIC EFFECT

The electroseismic effect manifests itself as an electrical potential generated in the subsurface by the passage of seismic waves. It can be detected at the surface of the ground with electrode pairs. The time of occurrence of the voltage clearly distinguishes the effect from either the ionization potential produced at the time of explosion or the seismic electric effect produced coincident with the arrival of seismic waves at the surface of the ground. Evidence is presented which associates the electroseismic effect with the base of the weathered layer and demonstrates its characteristics in several areas and at various distances from the shotpoint. Its immediate utility appears to be that of determining the travel time through the weathered layer at the shotpoint.

Kew Records of the Italian Earthquake

THE accompanying traces show records of the Italian earthquake of December 28, 1908, given by the Milne seismograph and the declination magnetograph respectively at Kew. The former shows a very sudden arrival of seismic waves of some magnitude at 4h. 23.6m. a.m. (G.M.T.). If these represent “preliminary tremors” from an earthquake originating in Italy, about 2000 kilometres away, one would expect the large waves not to commence until some seven minutes later. Ordinarily, the maximum amplitude occurs some little time after the large waves commence. In the present case, however, the maximum is shown at either 4h. 31.Im. or 4h. 32.7m., the lateral traces overlapping at both these times. This means a movement of more than 17 mm. at the end of the boom.