Component Seismometers Research Articles

Extensions of the Galperin Transformation Matrices for Triaxial Seismometers.

Since its invention in 1955, the Galperin symmetric triaxial seismometer has been widely used for seismic detection on Earth, and most recently on the planet Mars. In this paper, we present detailed physics of such seismometers, which has not yet been published in open literature. We extended Galperin's original work, which is based on idealized geometry and assumptions, to include more practical cases, including (1) non-idealized tilt angles of its component seismometers; (2) component seismometers that are not exactly oriented 120° apart; (3) distributed mass on the boom; and (4) the case of operations at lower frequencies.

Reflection imaging with earthquake sources and dense arrays

AbstractThe Mw 5.8 earthquake that occurred in Louisa County, Virginia, on 23 August 2011 provided an opportunity to record with several “high density” seismic arrays, in addition to traditional, sparse temporary seismic networks. Traditional aftershock networks consist of a few dozen stations spread over tens of kilometers. As a result, the recorded seismic waveforms suffer from spatial aliasing that is so severe that many types of waveform processing are not applicable. Here we report the results of recording with a large number of oil industry‐type instruments deployed at a spacing that is an order of magnitude closer than in traditional deployments. The objective was to image subsurface structure with array methods, using the aftershocks as sources. The dense array recorded continuously for 12 days and consisted of 172 vertical component seismometers that were placed at 200–400 m and a 60 km long three‐component regional profile with stations every 2 km. We demonstrate how processing techniques from Vertical Seismic Profiling can produce high‐resolution 3‐D reflection images of structure beneath the array. These images display reflectivity that correlates with that observed on a nearby deep reflection survey collected by the U.S. Geological Survey. Of particular interest is a strong reflector imaged across multiple profiles. Our analysis demonstrates how a surface array of seismometers can provide 3‐D images of structure using microearthquake sources when wavefields are sampled sufficiently densely.

A first look at the Gargano (southern Italy) seismicity as seen by the local scale OTRIONS seismic network

<p>On April 2013, a local scale seismic network, named OTRIONS, composed of twelve short period (1 Hz) three component seismometers, has been located in the northern part of the Apulia (southern Italy). In the first two months of data acquisition, the network recorded about one hundred very small (M<span><sub>L</sub></span><2) magnitude earthquakes. A three-layer 1D V<span><sub>P</sub></span> velocity model was preliminarily computed, using the recordings of earthquakes occurred in the area in the period 2006-2012 and recorded by the national seismic network of INGV (Istituto Nazionale di Geofisica e Vulcanologia). This model was calibrated by means of a multi-scale approach, based on a global search of the minimum misfit between observed and theoretical travel times. At each step of the inversion, a grid-search technique was implemented to infer the elastic properties of the layers, by using HYPO71 to compute the forward models. In a further step, we used P and S travel times of both INGV and OTRIONS events to infer a minimum 1D V<span><sub>P</sub></span> velocity model, using a classical linearized inversion approach. Owing to the relatively small number of data and poor coverage of the area, in the inversion procedure, the V<span><sub>P</sub></span>/V<span><sub>S</sub></span> ratio was fixed to 1.82, as inferred from a modified Wadati diagram. The final 1D velocity model was obtained by averaging the inversion results arising from nine different initial velocity models. The inferred V<span><sub>P</sub></span> velocity model shows a gradual increase of P wave velocity with increasing the depth. The model is well constrained by data until to a depth of about 25-30 km.</p>

Investigating the dynamics of an Alpine glacier using probabilistic icequake locations: Triftgletscher, Switzerland

[1] In order to improve our understanding of the dynamics of potentially unstable steep glacier tongues, we monitored during summer 2010 the micro seismicity of Triftgletscher, Switzerland. Our system, comprising 8 three‒component seismometers coupled with the ice surface, was installed upstream of the glacier's tongue, which is likely to evolve toward an unstable regime. Complementary surface motion and proglacial runoff measurements allowed the icequake activity to be interpreted in terms of glacier dynamics and hydraulics. The strong contrast in seismic wave velocities due to the underlying bedrock was taken into account using a three‒dimensional (3‒D) velocity model, implemented in a nonlinear probabilistic location procedure allowing to accurately define the hypocenter uncertainty. We located 120 icequakes, with a focal depth accuracy that allowed distinguishing between shallow events (87 events) and near‒bedrock icequakes (33 events). The first motions of most of the deep events argue against pure shear sources expected in case of stick‒slip motion, and our suggested source mechanism is a superimposed tensile crack and shear dislocation. The analysis of surface strain showed that near‒surface events represent the opening and transverse extension of surface crevasses in a confluent flow regime.

Determination of Site Factor in and Around Rhine Graben, France in the 1–11 Hz Frequency Band by Non-Reference Network Average Technique

The greatest challenge in estimating site response from instrumental recordings of seismic events is removing the source and path effects. Several methods have been suggested in the past in that direction which may be divided into two main categories, depending on whether or not they need a reference site, with respect to which the particular effects at other sites can be estimated. An alternative approach is to determine the site response relative to the network average. In the present paper we propose one such technique for estimating the site factor in the Rhine graben and its surrounding provinces. Judging the higher signal thresh-hold level (≥3 times the background) only sixteen earthquake events with local magnitude varying between 3 and 4.5 are used from the catalogue of 8 years during 1988–1995 recorded by the Fosse Rhenan network using short period single component seismometers belonging to the Ecole et Observatoire des Sciences de la Terre, Strasbourg, Prance and Survey Baden-Wurttenberg, Freiburg, Germany. Sixteen stations in and around Rhine graben are considered for the site factor computation in the 1–11 Hz frequency band. The contour maps prepared from the spatial distribution of non-reference network average site factor estimates at 1, 3, 5, 7, 9 and 11 Hz exhibit high values along the major faults and the minor tectonic features to the western (Vosges) and eastern (Foret-Noire) provinces. Irrespective of the magnitude of the earthquake, the Rhine graben is comparatively stable with low site factors at all the frequencies. As is expected from Coda site amplification studies, the network average contour patterns remained unchanged showing minor or no variation at all with frequencies.

Microseismic feasibility study: detection of small magnitude events (Ml<0.0) for mapping active faults in the Betic Cordillera (Spain)

We present the results of the first application of the newly developed concept «Nanoseismic Monitoring» on active<br />faults in the region close to Murcia, Spain. The aim of this microseismic feasibility study is to test if it is possible<br />to record small magnitude events (ML<0.0) within a short period of time with surface installations and to investigate<br />if these events are related to the regional catalog in terms of amount of events. The seismic monitoring<br />was performed with one small array called the Seismic Navigating System. It consists of one central three component<br />and three one component seismometers arranged tripartitely around the central station. In the measurement<br />period of two nights at two different sites we were able to detect 19 microearthquakes down to ML = -2.6. The results<br />correlate well with the frequency-magnitude distribution of the regional bulletin. This in turn will allow for<br />estimation of monitoring rates before actual field measurements just from bulletin data. Given an activity rate of<br />5 to 10 events per night one may map active fault zones within just a few weeks of field campaign.

Composition and variation of noise recorded at the Yellowknife Seismic Array, 1991–2007

We analyze seismic noise recorded on the 18 short‐period, vertical component seismometers of the Yellowknife Seismic Array (YKA). YKA has an aperture of 23 km and is sited on cratonic lithosphere in an area with low cultural noise. These properties make it ideal for studying natural seismic noise at periods of 1–3 s. We calculated frequency‐wave number spectra in this band for over 6,000 time windows that were extracted once per day for 17 years (1991–2007). Slowness analysis reveals a rich variety of seismic phases originating from distinct source regions: Rg waves from the Great Slave Lake; Lg waves from the Atlantic, Pacific, and Arctic Oceans; and teleseismic P waves from the north Pacific and equatorial mid‐Atlantic regions. The surface wave energy is generated along coastlines, while the body wave energy is generated at least in part in deep‐water, pelagic regions. Surface waves tend to dominate at the longer periods and, just as in earthquake seismograms, Lg is the most prominent arrival. Although the periods we study are slightly shorter than the classic double‐frequency microseismic band of 4–10 s, the noise at YKA has clear seasonal behavior that is consistent with the ocean wave climate in the Northern Hemisphere. The temporal variation of most of the noise sources can be well fit using just two Fourier components: yearly and biyearly terms that combine to give a fast rise in microseismic power from mid‐June through mid‐October, followed by a gradual decline. The exception is the Rg energy from the Great Slave Lake, which shows a sharp drop in noise power over a 2‐week period in November as the lake freezes. The Lg noise from the east has a small but statistically significant positive slope, perhaps implying increased ocean wave activity in the North Atlantic over the last 17 years.

A high-resolution aftershock seismicity image of the 2002 Sultandaği-Çay earthquake (Mw = 6.2), Turkey

A moderate-size earthquake (Mw = 6.2) occurred on 3 February 2002 (07:11:28 GMT) in the Sultandagi-Cay region of southwest Turkey. The mainshock was followed by a strong aftershock of Mw = 6.0 just 2 h after the mainshock, at 09:26:49 GMT. A temporary seismic network of 27 vertical component seismometers was installed to monitor aftershock activity. One thousand sixty nine aftershocks (0.2 < ML < 3.3) were recorded during the period from 5 to 10 February 2002. We analyzed the P and S arrival times and P wave first motion data to obtain high-quality hypocenters and focal mechanisms, which revealed fine details of the fault zone. We infer that the mainshock has ruptured a segment of the Sultandag Fault Zone that is approximately 37 km long and 7 km wide at depth. The average slip over the rupture plane during the mainshock is estimated to be 32 cm. The linear distribution of the aftershocks and the location of the mainshock epicenter suggest that rupture has initiated in the eastern bending of the fault and propagated unilaterally to the west. The majority of fault plane solutions indicate E–W to ESE–WNW striking oblique–normal faulting mechanisms with an average dip angle of 62° N ± 10° . The high-resolution aftershock seismicity image also shows that faulting involved a complex array of synthetic and possibly antithetic structures during the evolution of the aftershock sequence. The steady increase of the b value towards the west implies that the highest moment release of the mainshock occurred to the west of the epicenter. The study clearly shows the activation of the WNW–ESE-trending Sultandag Fault Zone along the southern margin of the Aksehir-Afyon Graben (AAG). The westernmost end of the aftershock activity corresponds to a structurally complex zone distinct from the main rupture. It is characterized by both ENE–WSW- and NNE–SSW-trending oblique-slip normal faulting mechanisms, the latter being associated with the NNE–SSW-trending Karamik Graben. The intersection of these two grabens, AAG and Karamik Graben, provides abundant faults available for failure in this region. The occurrence pattern of large events in recent years indicates a possible migration of earthquakes from east to west. Thus, we conclude that this has an important implication for earthquake hazard for the city of Afyon, which lies along the same fault line and only 20 km west of the termination point of the aftershock zone.

Topography of the “410” and “660” km seismic discontinuities in the Izu‐Bonin Subduction Zone

We study the effects of a subducting lithospheric slab on the “410” and “660” km mantle seismic discontinuities in the Izu‐Bonin region using data collected from networks of vertical component seismometers in the United Kingdom and northwestern United States. The consequences of a phase change origin for these discontinuities is tested by examining the effect of laterally changing temperature in and near the subducting slab. We analyse the data collected from 21 earthquakes with depths between 40 and 548 km. Based on arrival time and slowness we identify reflections and mode conversions in slant stacks with the aim of mapping the “410” and “660” km discontinuities. The combination of UK and US array data permits interrogation of the discontinuity properties on both sides of and also inside the slab. Results indicate a “410” minimum depth of 350 km in the interior of the slab and a “660” maximum depth of 700 km. A phase change origin for the discontinuities is consistent with the effects of lateral temperature changes on discontinuities with Clapeyron slopes of opposite signs. Here we see more topography on the “410” than the “660” consistent with thermodynamic Clapeyron slope estimates. “410” elevation suggests temperatures in the slab of 600±75°C at a depth of 350 km. We see weak evidence for a broader 410 near the slab interior, consistent with expected thermochemical effects.

Observation of low-order toroidal modes from the 1989 Macquarie Rise event

SUMMARY At frequencies below 1 mHz toroidal modes are more difficult to observe than spheroidal modes for two reasons. First, the comparatively rapid decay of the signal (low Q factor) of toroidal modes and the consequently short records which have to be analysed result in low resolution in frequency. Secondly, high noise on horizontal component seismometers due to their sensitivity to tilts induced by atmospheric disturbances makes it difficult to observe long-period motion. Here we present observations of the lowest order toroidal modes based largely on digital recordings of the 1989 Macquarie Rise event. We present what we believe to be the first unambiguous observation of the gravest toroidal mode oT2. We estimate multiplet degenerate frequencies and Q for the fundamental modes up to ,TI,, and the overtone lG. The frequencies of individual singlets are estimated for the modes o&-o&, and for the three multiplets o&-oco we have sufficient signal to estimate also the degree 2 aspherical structure coefficients.

Coherence measurements of deep-ocean seismoacoustic noise and signals

In April 1992, an array of six ocean bottom seismometer/hydrophone instruments was deployed on the seafloor in 3800-m-deep water. The instrument site was approximately 300 km west of San Diego, California and was within a few miles of the location of a smaller aperture array used in an experiment conducted in 1987. Initial calculations of the spatial coherence between vertical component seismometer recordings of ambient noise show remarkable similarities to data collected in the earlier experiment. The ratio of vertical to horizontal particle motion at the seafloor during ambient noise recording periods also is consistent with the earlier observations. The combination of coherence structure and particle motion ratios derived from the earlier experiment is used to infer that the ambient noise field at the site is dominated by seismic interface waves. With the new data set, this hypothesis is tested by examining: (1) the behavior of spatial coherence over a larger aperture (∼400 m) array, (2) the spatial coherence of the near-bottom pressure field and horizontal particle motion field, (3) the ratios of particle motion to near-bottom pressure, and (4) the particle motion ratios and spatial coherence of signals from near sea-surface and on-bottom explosive shots. [Work funded by ONR and ONT.]

An instrument for discriminating between acceleration and ground tilt in horizontal component seismometers

Seismometers measuring horizontal vibrations are seriously affected by ground tilt caused by atmospheric and other effects. An angular accelerometer has been developed, responding only to ground tilt, which enables the seismometer records to be improved. A description of the instrument is given, and an attempt is made to improve a wind-disturbed seismic record. An analysis of the instrumental noise level shows that it can detect a tilt of about 10-8 radians in the seismic band used, and it appears possible to improve this by an order of magnitude.

DESTRUCTIVE EARTHQUAKES AND TSUNAMI WARNING SYSTEM

AbstractThe Portuguese coast has been affected several times in the past by strong earthquakes that generated tsunamis severely damaging the city of Lisbon.The most significant event known was the Lisbon earthquake of 1 November 1755. It is generally assumed that the location of this event was the Gorringe Bank region in the North Atlantic. This ridge, located 200 km south‐west of the Portuguese shore, was also the location of the 28 February 1969 magnitude Ms= 7.9 earthquake (Fukao, 1973), that generated a tsunami of small amplitude clearly recorded at the tidal stations of the Portuguese south and south‐west coasts.The need to reduce the social and economic impact of an event of this type, greatly amplified by the urban concentration of coastal areas, led to the research project ‘Destructive Earthquakes and Tsunami Warning System in SW Portugal’. This project, sponsored by the European Economic Community and the public Portuguese research funding agencies, has been conducted by the Geophysical Centre of the University of Lisbon, since April 1988.The main targets of the project are:•the installation of a pilot warning system against tsunamis, based on two ocean bottom stations, comprising a 3 component seismometer and a bottom mounted pressure sensor, linked by cable to a surface buoy. This buoy is equipped with a data acquisition and data transmission system. Seismic and water level data will be collected on an almost real‐time basis and will be transmitted to Lisbon via satellite;•the refinement of existing geological models, in order to clarify the genesis of the bank and the seismic activity in this area;•the installation of an adequate network of seismic monitoring stations in order to better locate off shore earthquakes (Fig. 1);•the evaluation of seismic and tsunami risk around the Iberian Peninsula.

A study of ground motion attenuation in the Southern Great Basin, Nevada‐California, using several techniques for estimates of Qs, log A0, and coda Q

As a first step in the assessment of the earthquake hazard in the southern Great Basin of Nevada‐California, this study evaluates the attenuation of peak vertical ground motions using a number of different regression models applied to unfiltered and band‐pass‐filtered ground motion data. These data are concentrated in the distance range 10–250 km. The regression models include parameters to account for geometric spreading, anelastic attenuation with a power law frequency dependence, source size, and station site effects. We find that the data are most consistent with an essentially frequency‐independent Q and a geometric spreading coefficient less than 1.0. Regressions are also performed on vertical component peak amplitudes reexpressed as pseudo‐Wood‐Anderson peak amplitude estimates (PWA), permitting comparison with earlier work that used Wood‐Anderson (WA) data from California. Both of these results show that Q values in this region are high relative to California, having values in the range 700–900 over the frequency band 1–10 Hz. Comparison of ML magnitudes from stations BRK and PAS for earthquakes in the southern Great Basin shows that these two stations report magnitudes with differences that are distance dependent. This bias suggests that the Richter log A0 curve appropriate to California is too steep for earthquakes occurring in southern Nevada, a result implicitly supporting our finding that Q values are higher than those in California. The PWA attenuation functions derived from our data also indicate that local magnitudes reported by California observatories for earthquakes in this region may be overestimated by as much as 0.8 magnitude units in some cases. Both of these results will have an effect on the assessment of the earthquake hazard in this region. The robustness of our regression technique to extract the correct geometric spreading coefficient n and anelastic attenuation Q is tested by applying the technique to simulated data computed with given n and Q values. Using a stochastic modeling technique, we generate suites of seismograms for the distance range 10–200 km and for both WA and short‐period vertical component seismometers. Regressions on the peak amplitudes from these records show that our regression model extracts values of n and Q approximately equal to the input values for either low‐Q California attenuation or high‐Q southern Nevada attenuation. Regressions on stochastically modeled WA and PWA amplitudes also provides a method of evaluating differences in magnitudes from WA and PWA amplitudes due to recording instrument response characteristics alone. These results indicate a difference between MLWA and MLPWA equal to 0.15 magnitude units, which we term the residual instrument correction. In contrast to the peak amplitude results, coda Q determinations using the single scatterer theory indicate that Qc values are dependent on source type and are proportional to ƒp, where p = 0.8 to 1.0. This result suggests that a difference exists between attenuation mechanisms for direct waves and backscattered waves in this region.

Observations of the phase SP at distances less than 40

Summary. A detailed analysis of short-period shear waves using an array of three component seismometers near Tennant Creek in the centre of northern Australia has found only isolated samples of the phase Sp which, for a mode conversion at the crust/mantle boundary precedes the S arrival by about 6 s. Two strong phases commonly observed in the S wavetrain in the distance range 30-40 with a time separation of between 6 and 8 s have been determined to be the phases S and SP respectively. Contrary to the Jeffreys Bullen seismological tables, SP is first generated at a minimum distance not of 40°, but rather in the vicinity of 20.

やや深い構造のS波速度 (III)

Continued to previous works at Iwatsuki and Shimohsa, S wave velocities in deep soil deposits were measured by using the observation well at the Fuchu Deep-Borehole Crustal Activity Observatory of the National Research Center for Disaster Prevention, Japan.Measurements were conducted at 17 different depths to the borehole bottom with the depth of 2, 750m at intervals of 100-250m. S waves were excited by the SH wave generator on the ground surface. The excited waves were observed by a set of three component seismometers assembled into the specially designed capsule. The capsule can fix itself to the borehole wall at any depths.The measurement revealed that the deposit at the Fuchu is fundamentally composed of three layers which covers the basement rocks. S wave velocities of those layers are, from the top to the bottom, 0.54, 0.78, and 1.19km/sec, and that of the substratum is 2.54km/sec. The velocity structure, including the P wave one, is consistent with such data as geologic section, sonic velocity, bulk density, and electric resistivity loggings of the observation well.Through a comparison of thus obtained structures at three separate sites, Iwatsuki, Shimohsa, and Fuchu, one can visualize the underground structure three dimensionally, though vaguely and restricted in the Tokyo metropolitan area. A relation between the seismic wave velocity and the geological condition is also obtained. The relation is useful to estimate the velocity structure at deep-wells which were bored for the purpose of the geological investigation. Finally, it is found that the P wave velocity of the basement obtained from our direct method is rather small compared with the one from the refraction method. The difference is confirmed to be meaningful. It seems to be caused from the basement structure, that is, the basement is not uniform but has a thin layer at its top. The velocity difference between this layer and the lower basement is so small that the upper-most layer is masked in the ordinary refraction exploration.

S-wave velocity measurements in deep soil deposit and bedrock by means of an elaborated down-hole method

abstractDeep S-wave velocity measurements were planned at two separate sites in the Tokyo area from the earthquake engineering point of view, and actually carried out down to 2 to 3 km in depth using geophysical observation wells.S-waves were produced by means of ordinary small explosions and a specially designed SH-wave generator. A set of three component seismometers was installed in a capsule having a device that is clamped to the borehole wall. Measurements to the bottom of the wells were conducted at about 15 different depths at intervals of 100 to 500 m.The S-wave velocities are around 0.8 km/sec in Pleistocene soils, 1.2 to 1.6 km/sec in Miocene soils, and 2.5 to 2.7 km/sec in Cambrian rocks. The corresponding P-wave velocities are 2.0 to 2.3 km/sec, 2.6 to 3.0 km/sec, and 4.7 to 4.9 km/sec, respectively. These data show both S- and P-wave velocities in deep soil deposit increasing with depth. The greatest velocity difference is at the boundary above the pre-Tertiary rocks.The velocity structures completely agree with the known data such as sonic logs, density distributions, and geological sections. A comparison with velocity profiles at two separate sites was also made as the first step to visualize the three-dimensional underground structure in the Tokyo metropolitan area.The seismological and earthquake engineering importance of shear-wave velocity measurements for thick soil deposits was demonstrated by approximate calculations of the amplification of seismic waves between ground surface and bedrock.

Automatic analysis of microearthquake network data

The microearthquake data collected by the dense network of seismographs in California may contain a wealth of information about the tectonic process, structure and properties of the earth's crust. However, before this data can be fully exploited for the purposes of earthquake prediction research, a relatively automatic, accurate, and uniform procedure for the analysis of seismic data is needed. A simple algorithm which identifies earthquakes and times first arrivals using data from an array of single component seismometers is being studied. The algorithm is simple enough that it could easily be implemented on a microprocessor monitoring a single station in real time. A unique aspect of the algorithm is that it not only picks arrivals but also objectively determines arrival time accuracy. Earthquakes are located by an iterative method. First, the most accurate arrivals are used to locate the event. Then arrivals which do not agree with this location are repicked using a more sensitive algorithm. This process is repeated until a stable location is obtained. For clear, unambiguous arrivals, the arrival times determined by the algorithm agree to within a few hundredths of a second of arrival times determined by an analyst. For weaker, more ambiguous arrivals, computer picks and hand picks are always within the accuracy prescribed by the algorithm. Using such an algorithm in routine processing of seismic data could require as little as 2 percent of the human effort now required by manual processing, and would provide for the first time, an accurate objective data base for earthquake prediction research. Syntactic structural analysis (a formal method of symbolic description) of seismic wave-forms is being investigated as a tool to improve the descriptive capability of the algorithm and as a formalism in which other picking algorithms can be expressed easily and compared. Using syntactic procedures to automatically provide descriptions of the first-arrival waveform is proposed as way of providing useful information about the earth that is not presently available on a routine basis.

やや長周期の微動観測と地震工学への適用 (4)

In the previous papers it was confirmed that the long-period microtremors change their spectral character very sensitively to deep soil deposits and also the amplification characteristic of earthquake strong motions is, specially in the long-period range, correlated to that of the microtremors. This clarified the engineering importance of observing the long-period microtremors. However, observational and analytical processes in those papers were concentrated to the elucidation of the spectral peaks of the microtremors and their relation to the underground structures, that is the microtremors were treated as if they were standing waves. For further improvement of their applicability in earthquake engineering, considerations from wave-theoretical point of view are better to be introduced. The best research way is to understand their generation, propagation, and amplification due to ground layers as a series of events.In this paper, as the first step of reserch in this way, a chiefly concerned investigation with the propagative characteristics of the long-period microtremors was carried out. To do this, a temporay array observation was prepared putting six sets of three component seismometers in an equal space of 60m, and the sequential observations were performed along three independent lines with an angle of 45° each other.Analysis was begun by obtaining apparent propagation velocities of the microtremors by means of phase correlation method, and by determining the direction of the wave propagation. Then the true phase velocities resulted as a function of frequency. Group velocities of the microtremors were roughly estimated by considering flows of the wave energies figured out on running power spectra at the two observation points far away from each other. Next, these phase and group velocities were compared with those calculated by knowing the underground structure at the site. Amplitudes of the microtremors in horizontal and vertical components were investigated in relation to their particle orbits at the ground surface.The results by these analyses are summarized as follows.1. The long-period microtremors are interpreted as an ensemble of dispersive waves. In the period range as we concern, the M11, and M21 modes of Rayleigh waves are dominant. Adding to these, the fundamental mode of Love waves is also mixed.2. The propagation direction of the long-period microtremors is, though slightly time-variant, decisive and mainly from open sea side (Pacific ocean).Finally, we would like to add that further experimental observations for the second step of the wave-theoretical analysis of the long-period microtremors are on schedule.

The La Coste Suspension--Principles and Practice

Summary For 35 years the La Coste suspension has been used in gravimeters and vertical component seismometers, yet its precise design and best adjustment have been handicapped by limited knowledge of the forces which combine to produce a stable instrument. The present analysis gives quantitative relations which merit consideration in design, and provides a rational step-by-step basis for obtaining the best possible adjustment of any instrument using the La Coste principle. The magnitude and effect of spurious forces such as may be caused by magnetic impurities or faulty construction is discussed briefly. The special case of the inclined boom seismometer is covered in a discussion of the symmetrical triaxial instrument invented by the author.