Analysis Of Seismograms Research Articles

Modelling low-frequency volcanic earthquakes in a viscoelastic medium with topography

Magma properties are fundamental to explain the volcanic eruption style as well as the generation and propagation of seismic waves. This study focusses on magma properties and rheology and their impact on low-frequency volcanic earthquakes. We investigate the effects of anelasticity and topography on the amplitudes and spectra of synthetic low-frequency earthquakes. Using a 2-D finite-difference scheme, we model the propagation of seismic energy initiated in a fluid-filled conduit embedded in a homogeneous viscoelastic medium with topography. We model intrinsic attenuation by linear viscoelastic theory and we show that volcanic media can be approximated by a standard linear solid (SLS) for seismic frequencies above 2 Hz. Results demonstrate that attenuation modifies both amplitudes and dispersive characteristics of low-frequency earthquakes. Low frequency volcanic earthquakes are dispersive by nature; however, if attenuation is introduced, their dispersion characteristics will be altered. The topography modifies the amplitudes, depending on the position of the seismographs at the surface. This study shows that we need to take into account attenuation and topography to interpret correctly observed low-frequency volcanic earthquakes. It also suggests that the rheological properties of magmas may be constrained by the analysis of low-frequency seismograms.

Single‐chamber silicic magma system inferred from shear wave discontinuities of the crust and uppermost mantle, Coso geothermal area, California

Analysis of seismograms from teleseismic rays traversing the Coso geothermal area near Ridgecrest, California, suggests the geothermal system lies over a single shallow magma reservoir (∼5 km below the surface) that also plays a crucial role in the local change in deformation style from areas to the north and west. The character of the magma reservoir and the absence of a lower crustal magma reservoir is inferred from three crustal P‐to‐S conversions observed using receiver function analysis: (1) A high‐amplitude, shallow, negative arrival, Ps‐P time of 0.7–0.9 s (3–5 km below sea level (bsl)), (2) a moderate amplitude, positive conversion, Ps‐P time of 2.1–2.5 s (14–17 km bsl), and (3) the Moho conversion, Ps‐P time of 4.0–4.2 s (30–32 km bsl). Observations of Moho converted arrivals indicate that the interface is mostly flat and uncomplicated throughout the study area, while the midcrustal conversion is laterally variable in amplitude and depth. The absence of the large negative amplitude conversion on waveforms recorded at stations outside the geothermal area strongly suggests that the feature lies only underneath the modern geothermal area. In addition, rays sampling the shallow converter also contain later arrivals with retrograde moveout consistent with an origin as reverberations above the conversion. Receiver functions calculated from synthetic data using a single isotropic layer over a half‐space indicates that the shear velocity decreases by 30% across the interface (VS1 = 2.6 km/s; VS2 = 1.8 km/s; layer one thickness 4.9 km), further supporting the presence of shallow magma.

Surface expression of thrust faulting in eastern Iran: source parameters and surface deformation of the 1978 Tabas and 1968 Ferdows earthquake sequences

SUMMARY Previously unrecognised thrust faults in eastern Iran were responsible for a destructive earthquake at Tabas (1978, September 16), which produced over 80 km of distributed and discontinuous surface ruptures above a series of low anticlinal hills to the west of a major range-front. Analysis of long-period body-wave seismograms shows a simple rupture on a gently dipping (∼16 ◦ ) thrust, with a slight right-lateral component. This is compatible with the locally recorded aftershock distribution. Body wave analysis of two later, smaller events show similar source orientations. Several indicators of long-term active folding at Tabas can be recognised in the geomorphology, and surface ruptures from 1978 are consistent with co-seismic fold growth. Drainage incision also indicates uplift at depth on thrust faults dipping eastwards beneath the folds. Body-wave seismograms for two earthquakes near Ferdows, 150 km east of Tabas, in September 1968 also show thrust faulting at depths of ∼10 km. Again, the surface geomorphology indicates a region of folding above an eastward-dipping thrust fault, which is ∼10 km west of a fault bounded range-front. In both Tabas and Ferdows, the active faulting appears to show Quaternary migration away from the range-front, possibly in response to stresses produced by the elevated topography. The identification of zones of active fault-related folding is important for earthquake hazard assessment and also for an understanding of the local tectonics. We conclude that the structures which gave rise to the 1968 and 1978 earthquakes could have been recognised beforehand from the clear signals in the geomorphology, had we known what to look for at that time.

The 1933 Ms= 7.3 Baffin Bay earthquake: strike-slip faulting along the northeastern Canadian passive margin

The 1933 November 20 (Ms= 7.3) Baffin Bay earthquake is one of the largest instrumentally recorded passive margin earthquakes. Analysis of seismograms of this earthquake shows strong evidence for strike-slip faulting, which contrasts with the generally accepted belief that Baffin Bay is dominated by thrust faulting. The best-fitting solution consists of a large strike-slip subevent (strike 172°, dip 82°, rake 6°) followed by two smaller oblique-thrust subevents (strike 190°, dip 30°, rake 62°). All subevents occur at a depth of about 10 km. An instrumental moment magnitude of 7.4 was determined. Preliminary analysis of subsequent large (magnitude ≥ 6.0) earthquakes in Baffin Bay finds additional evidence for strike-slip faulting in the region. The results for Baffin Bay, together with those for other passive margin earthquakes, suggest strike-slip faulting may be more prevalent in these regions than was previously believed.

Determination of Site Response in Anchorage, Alaska, on the Basis of Spectral Ratio Methods

This paper deals with the site response (SR) in the Anchorage basin in south-central Alaska. The investigation is based on the analysis of seismograms of 114 earthquakes recorded by 22 weak-motion stations and 46 earthquakes recorded by 19 strong-motion stations in the study area. We have computed SR for 41 sites, using standard spectral ratio and horizontal-to-vertical spectral ratio methods in the frequency range from 0.5 to 11 Hz. Based on these results, we have calculated band-average site response values in two frequency ranges: low frequency (from 0.5 to 2.5 Hz) and high frequency (from 3 to 7 Hz). There is a good correlation between SR values and surficial geology of the Anchorage area in the low-frequency range. SR values increase by a factor of three from the foothills of the Chugach Mountains in the east to the west towards the deeper part of the basin. The highest site response values (SR>2.5) in the same frequency range are observed in the west-central part of the city, which is underlain by cohesive facies of the Bootlegger Cove formation. The SR has a good correlation with the uppermost 30-m time-average shear-wave velocity with a correlation coefficient of 0.82. Moreover, the low-frequency SR values are close to the NEHRP site coefficients for 1 sec. However, high-frequency SR values lack correlation with 30-m average shear-wave velocity and short-period NEHRP site coefficients.

The structure of the upper mantle beneath southern Africa

Abstract A large number of velocity models derived from a variety of seismic data and using different seismic techniques have been published for the Archaean and Proterozoic shields. Here, we focus on the structure beneath southern Africa, where velocity models derived from most regional seismic data find the thickness of the seismic lithosphere to be less than 200 km. In contrast, velocity models derived from teleseismic body-wave and long-period surface-wave data determine the seismic lithosphere to be as much as 400 km thick. We believe that this disagreement is due to the ways in which the various datasets average the velocity structure. Our analysis of regional seismograms from propagation paths largely confined to the stable region shows that the average thickness of the seismic lithosphere beneath southern Africa does not exceed 160 km. We compare the vertical S-wave travel time of our velocity model derived from regional seismic data and those models derived from teleseismic data and find no significant difference. We determine the in situ velocities and densities from nodules from beneath southern Africa using a recently derived geobarometer and geothermometer; these are in excellent agreement with the velocity found in the high-velocity lid from the analysis of the regional seismic data. The lithospheric model that best fits the nodule data has a mechanical boundary layer thickness of 156 km and a lithosphere thickness of 176 km. However, the shear-wave velocity decrease at the base of the lid does not correspond to a change in mineralogy. Recent experimental studies of the shearwave velocity in olivine as a function of temperature and period of oscillation demonstrate that this decrease can result from grain boundary relaxation at high temperatures at the period of seismic waves. This decrease in velocity occurs where the mantle temperature is closest to the melting temperature.

Array Analysis of Seismograms from Explosive Sources: Evidence for Surface Waves Scattered at the Main Topographical Features

In this article, we analyze the seismic wavefield produced by blasts fired at Mt. Vesuvius for a deep seismic sounding experiment and recorded by a dense short-period three-component seismic array. Seismograms from five explosions occurring at the bottom of wells located at different distances and azimuth with respect to the array site have been analyzed with the zero lag cross-correlation technique applied to both vertical and horizontal components in the frequency range 3–7 Hz. The aim is to analyze the portion of the seismograms long after the first P onset in order to obtain the slowness and backazimuth distribution of correlated secondary phases. Polarization analysis in time domain, carried out using the array covariance matrix technique, has also been applied to the well-correlated phases to determine their wave composition. Results show that the correlated phases detected in the coda of the seismogram shots are composed almost entirely of surface waves. Location of the scatterers shows a high concentration of the scatterers in and around the volcanic edifice of Mt. Vesuvius, confirming that topographical irregularities play an important role in the generation of the high-frequency scattering phenomena.

Phase identification and attribute analysis of broadband seismograms at far-regional distances

A trained analyst can frequently provide a rapid assessment of a seismic record and provide identification for many seismic phases. For digital data a challenge is to find methods (or combinations of methods) which can provide equivalent levels of phase identification and attribute analysis. Until now, there have been few discussions on phase attribute analysis for broadband records, even though the character of the major phases has been recognised several decades ago. We introduce a combination of four simple methods into the analysis of broadband seismograms so as to provide a means of improving phase recognition and the full use of broadband information for far-regional distances where the seismograms are particularly complex (because of the influence of the upper mantle discontinuities). For arrival detection we can employ the energy ratios of the short term behaviour to the long-term trend, using the vertical component and horizontal components of unrotated seismic records. We also use auto-regressive analysis to endeavour to separate broadband records into three parts: the seismic signal, microseismic noise and white noise. The higher order auto- and cross-correlation coefficient (representing the similarity of waveform) can be used to identify the presence of seismic phases, by avoiding the influence of the relatively low order correlation of microseismic noise. For each broadband 3-component record a set of complex traces are constructed and then a variety of definitions of instantaneous phase and frequency can be exploited to separate the behaviour of signal and noise. The complex traces can also be used for polarisation analysis. The changes in the character of the eigenvectors are particularly helpful for recognising the phases of broadband records in the far-regional range. The individual methods are quite powerful but when used in combination can provide a very effective means of phase characterisation.

Aiming to Predict the Tokai Earthquake

Extensive studies utilizing data from a high-density observation network deployed in the Tokai region have brought about remarkable progress in understanding the subducting process of the Philippine Sea plate. Configuration of the Philippine Sea slab has been modeled using precisely determined hypocentral data (Yamazaki and Oida, 1985; Noguchi, 1996; Harada et al., 1998), and the locked region between the Philippine Sea and the Eurasian plates has been surmised on the basis of spatial variation of the density and focal mechanisms of microerathquakes (Matsumura, 1997). The coupling region has been also estimated by inversion analysis using GPS data (Sagiya, 1998, 1999). Although the result that suggests a large back-slip in the sea region to the south off Omaezaki does not seem to be consistent with the estimation of the locked region that is surmised to exist beneath the western coast of Suruga Bay, we think it is a great achievement to be able to discuss the coupling state on the basis of the observational data. Besides, simulation studies on subduction process modeling the Tokai region have developed considerably in recent years, indicating that there will appear intermediate-term and short-term precursors that should be detectable before a great interplate earthquake (Kato and Hirasawa, 1999). Three-dimensional simulations supply more realistic information about the phenomena that occur in the preparatory process of a great earthquake (Kuroki et al., 2001). Recent detailed analyses on the fault motion of the 1944 Tonankai and the 1946 Nankai earthquakes as well as acoustic sounding investigations on the upper crustal structure in their focal regions have afforded us useful information to assess the focal area of the coming Tokai earthquake. In order to utilize those scientific attainments obtained since the proclamation of the Large-Scale Earthquake Countermeasures Act in 1978, the Central Disaster Prevention Council has commenced to re-consider the focal model of the Tokai earthquake and to propose a new one.In spite of the remarkable progress in studies on tectonic processes in the Tokai region, the probability of a successful prediction of the Tokai earthquake seems not to have much increased in the last 20 years. At present we are not very optimistic concerning the possibility of the detection of immediate precursors compared to when the Earthquake Assessment Committee for the Areas under Intensified Measures against Earthquake Disasters (EAC) had been established. Experiments on rock fracture, analyses of seismograms on the commencement of fault motion, computer simulations on the repeating occurrence of an interplate large earthquake, and other relevant studies all suggest that immediate precursors would not be as large as we expected. Standing on such a recent viewpoint about the magnitude of precursors, the Japan Meteorological Agency has revised criteria to convene EAC in 1998. The new criteriasa was prescribed based upon the detection level of volumetric strainmeters. This revision is considered to express the intension that JMA will do its best to catch the immediate precursors.Some researchers have been presenting the ideas that recently observed seismic quiescence and crustal deformation in the Tokai region exhibit warnings that the Tokai earthquake is actually impending. The data and their interpretations, however, seem insfficient for the majority of the researchers to have a common opinion about the forecasting. The divergence of opinions concerning various kinds of changes in observational data obtained by the densely deployed network itself shows the difficulty of the prediction of the Tokai earthquake. We have not yet achieved a unified method to interpret the data or a scenario to determine how precursory phenomena will appear in sequence. It is very important to acquire more knowledge and know-how concerning these problems.

Characteristics of coda Q-value in the midnorthern part of Ningxia

On the basis of previous studies, this paper, in studying the coda Q-value of near shocks, has proposed using the sampling depth to describe the effect of lapse time on the Q-value, and has investigated the dependence of coda wave of earthquakes on frequency and sampling depth. Analysis of digital seismograms recorded by the Yinchuan Telemetric Seismic Network, Ningxia, shows that not only the coda wave Q-value of shocks is strongly dependent on frequency but also its dependence on sampling depth cannot be neglected. In the commonly used formula that describes the dependence characteristics of coda Q-value of earthquakes, Q = Q0f″, the parameter Q0 rises obviously while n drops when the sampling depth increases, and their changes can be fitted by linear relations. This paper has explained such a characteristic. Whether this characteristic exists universally needs to be verified by more study results because the genetic mechanism of coda wave of earthquakes is more complex.

Source properties of the 1997-98 Central Italy earthquake sequence from inversion of long-period and broad-band seismograms

We study source properties of the main earthquakes of the 1997–98 Umbria-Marche (central Italy) sequence by analysis of regional-distanceand teleseismic long period and broadband seismograms recorded by MedNet and IRIS/GSN stations. We use a modified Harvardcentroid-moment tensor (CMT) algorithm to allow inversion of long period waveforms, primarily Rayleigh and Love waves, for small earthquakes (4.2 ≤ MW ≤ 5.5) at local to regional distances (Δ 5.2) moment tensors derived from local and regional data agree well with those determined using teleseismic waveforms and standard methods of analysis. We also determine moment tensors for a foreshock and 12 other aftershocks, that were too small for global analysis. Focal depth and rupture propagation are analyzed for three largest shocks by inversion of teleseismic broadband body waves. The earthquakes are generally located at shallow depth (5 km or shallower) and are characterized by normal faulting mechanisms, with a NE-SW tension axis. The presumed principal fault plane dips at a shallow angle towards the SW. Only one of the events analyzed has an entirely different faulting geometry, indicating instead right-lateral strike-slip motion on a plane approximately E-W, or left-lateral faulting on a N-S plane. The other significant exception to the regular pattern of mechanisms is represented by the March 26, 1998, event, located at 51 km depth. Its connection with the shallow earthquake sequence is unclear and intriguing. The time evolution of the seismic sequence is unusual,with the mainshock accounting for only approximately 50% of the total moment release. The broadband teleseismic waveforms of the main, September 26, 09:40, earthquake are very complicated for the size of the event and suggest a complex rupture. In our favored source model, rupture initiated at 5 km depth, propagated updip and was followed, 3 seconds later, by a shallower subevent with a slightly rotated mechanism.

Scenario of the January 1997 West Bohemia Earthquake Swarm

In order to learn more about the nature of the dynamic processes taking place in the West Bohemia/Vogtland earthquake swarm region, we investigated the temporal and spatial variations of the source mechanisms of the January 1997 swarm beneath Nový Kostel (NKC). Visual analyses of WEBNET seismograms of over 800 events revealed that a specific feature of this swarm was the occurrence of eight classes of multiplet events. The result of single-source, absolute moment tensor inversion of the P and SH peak amplitudes of a subset of 70 events representing all multiplet classes indicated that eight statistically significant types of mechanisms occurred during the swarm. Two of them, typesAandBin our denotation, comprised all ML≥ 1.3 events and predominated in the swarm. TypeAwere pure strike-slip mechanisms or strike-slip mechanisms containing a small normal component, with a nearly pure double-couple source. For classBevents, oblique-thrust faulting and non-double-couple components significant at a fairly high confidence level were typical. TypeAevents predominated in the southern subcluster of the swarm, whereas most of typeBevents occurred in the subcluster northwards from NKC. This indicates that two major seismogenic planes were active during the swarm. The swarm essentially developed in four phases: in the first, typeAevents prevailed and the southern plane was active; during the second, characterised by the occurrence of both typeAandBevents (the former in the southern, the latter predominantly in the northern subcluster), the activity of the swarm culminated; in the third and fourth, the occurrence of typeBevents in the northern plane predominated, and only weak single events occurred southwards from NKC. Mechanisms of typesAB,C,D,E,FandG, which were typical for ML≤1.2 events, occurred randomly throughout the swarm. TypeABevents were identified in both the southern and northern clusters, typeC,E,FandGmechanisms only southwards from NKC. TypeDevents exhibited a large scatter of hypocentres which fell in neither the southern nor the northern cluster. Focal mechanisms like those reported in this study and with analogous temporal and spatial variations were observed by other authors already fifteen years ago in the 1985/86 earthquake swarm and may, therefore, be typical for the region under study.

The 1934 Pahiatua earthquake sequence

Descriptive accounts and analysis of local seismograms establish that the epicentre of the 1934 March 5 Ms7.6 earthquake, known as the Pahiatua earthquake, was nearer to Pongaroa than to Pahiatua. Conspicuous and severe damage (MM8) in the business centre of Pahiatua in the northern Wairarapa led early seismologists to name the earthquake after the town, but it has now been found that the highest intensities (MM9) occurred about 40 km to the east and southeast of Pahiatua, between Pongaroa and Bideford. Uncertainties in the location of the epicentre that have existed for sixty years are now resolved with the epicentre determined in this study lying midway between those calculated in the 1930's by Hayes and Bullen.
 Damage and intensity summaries and a new isoseismal map, derived from extensive newspaper reports and from 1934 Dominion Observatory "felt reports", replace previous descriptions and isoseismal maps. A stable solution for the epicentre of the mainshock has been obtained by analysing phase arrivals read from surviving seismograms of the rather small and poorly equipped 1934 New Zealand network of twelve stations (two privately owned). The addition of some teleseismic P arrivals to this solution shifts the location of the epicentre by less than 10 km. It lies within, and to the northern end of, the MM9 isoseismal zone. Using local instrumental data larger aftershocks and other moderate magnitude earthquakes that occurred within 10 days and 50 km of the mainshock have also been located. Approximate locations of other associated moderate magnitude earthquakes until October 1934 have been identified by their maximum intensity and S-P intervals read from the Wellington Wood-Anderson seismograph records. The distribution of S-P intervals of aftershocks (magnitudes M > 3.5) within 24 hours of the mainshock is used to delineate the probable mainshock rupture zone.
 Neither contemporary sources nor recent inquiries directed to old residents yield historical evidence of a surface fault rupture. Nevertheless, the strike-slip mechanism at 20 km depth determined by preliminary teleseismic body wave modelling of Doser and Webb suggests that rupture could have extended to the surface. Recent investigation of two of the freshest-looking, active faults that lie within the MM9 isoseismal by Schermer and others indicates that one of them could have ruptured in the 1934 Pahiatua earthquake.

The 1997 May 10 Zirkuh (Qa'enat) earthquake (Mw 7.2): faulting along the Sistan suture zone of eastern Iran

Summary The destructive Zirkuh-e-Qa’enat earthquake of 1997 May 10 (Mw 7.2, Ms 7.3, mb 6.3) produced 125 km of NNW–SSE right-lateral strike-slip surface faulting on the Abiz fault in the Sistan suture zone of eastern Iran: the longest known surface rupture associated with an Iranian earthquake. Analysis of the body-wave seismograms from the main shock shows that rupture occurred in four main subevents, propagating in a sequence from north to south. Although predominantly strike-slip, the orientation of the faulting in each subevent varies, with appreciable reverse components in the north-central part and at the southern end of the Abiz fault. This change in fault style along the Abiz fault inferred from the seismograms is also seen in the coseismic surface ruptures and the geomorphology. Average coseismic surface displacements were approximately 2 m, implying a static stress drop of only 5 bar (0.5 MPa). The 1997 surface ruptures followed clear traces of late Quaternary slip on the Abiz fault, and for its northern 50 km re-ruptured fault segments that had slipped in previous earthquakes of Ms 6.0–6.6 in 1936 and 1979. The 1997 earthquake ruptured the northern end of the N–S right-lateral strike-slip system of the Sistan suture zone, ending where it abuts a system of E–W left-lateral strike-slip faults which have also slipped in large earthquakes during the last 30 years. The earthquakes on this conjugate system of strike-slip faults form a sequence that may have been triggered by the enhancement of stress on one fault as a result of slip on a neighbouring fault. Together, these faults achieve N–S right-lateral shear of deforming Iran against stable western Afghanistan by N–S slip on the right-lateral faults and clockwise rotation of the E–W left-lateral faults.

Hydrate distribution off Vancouver Island from multifrequency single‐channel seismic reflection data

On the northern Cascadia margin, single channel seismic data clearly image the bottom‐simulating reflector (BSR) that marks the base of the hydrate stability zone. Airguns with three distinct source frequencies (30, 75, and 150 Hz) were recorded along several coincident lines, and at all frequencies the BSR appears as a single pulse. BSR reflection coefficients are slightly larger for the 30‐Hz data than for the 75‐ or 150‐Hz data. Multi‐frequency analysis indicates that the BSR is due to a single sharp interface at the base of the hydrate zone, whereas the velocity structure away from the interface must be gradationally‐varying so as to produce no separate reflection and no tuning effects at the 75‐ and 150‐Hz frequencies. In contrast, the seafloor reflection in some areas has significantly larger amplitudes for data recorded with the 75‐Hz source, relative to the 30‐Hz source. Synthetic seismogram analysis of the two data sets shows that the high amplitudes can be produced by a 2‐m‐thick high‐velocity layer at the seafloor, with velocities and densities corresponding to those of carbonate. Using the 75‐Hz source, seafloor and BSR amplitudes and reflection coefficients were measured over a tight grid of lines, nominally spaced at 200 m. Maximum BSR reflection coefficients of 0.15–0.18 were observed beneath topographic highs, while maximum seafloor reflection coefficients of up to 0.5–0.6 were found on the flanks of the topographic highs. This suggests that topography provides a major control on the flow of methane‐bearing fluids. Amplitude measurements, converted to values of velocity above the BSR, were used to estimate hydrate concentrations, which reaches a maximum of 15–18% of the total sediment volume with mean values near 5–10%. Over the 100 km2 survey area, the volume of methane gas in the hydrate reservoir is estimated as 6.4 × l010 m3, or 2.1 trillion cubic feet.

A study on seismic source process in short-term and imminent stage before destructive mining shock

Three destructive mining shocks successively occurred nearby the seismic Station No.6 located in a shaft of the Fangshan Coal Mine of Beijing Mining Service in 4 minutes at 19 o’clock of May 15, 1993. The largest shock is of M=2.3 (M 0=1.5×1011 N·m). Analysis of synthetic seismogram provides that the three shocks exhibited predominantly a dip-slip movement mode, which is consistent with the collapse of coal mass from the coal bed at high dip-angle, as observed by those who were present at the site. The near-field records of the main shock and a series of events prior to it made at the Station No.6 are different from normal records, it had not only high-frequency vibration, but also low-frequency vibration. By using elastic wave theory and nucleation theory of seismic fracture during recent years, analysis of the data indicates that the low-frequency vibration maybe long-period wave as the subcritical extension is pushing forward. It is an unrecovered deformation. The high-frequency vibration is just the mining shock event, exhibiting a wave field of brittle fracture radiation. From the dominant frequency of low-frequency vibration in the records of M=2.3 event and the records of foreshocks at 5.4 s before it, it is inferred that the volume of dilatation zone at the termination of shock-generating fracture has rapidly enlarged during the occurrence of the main shock. In 20 or more days before the main shock, the source process was of the following characteristics: the subcritical extension occurred for many times; during this period the volume of dilatation zone at the termination of shock-generating fracture little changed. The subcritical extension not only exits long-period waves, but also induced small events at the same time. The dominant orientation of subcritical extension is basically consistent with the direction of slip movement during main shock.

A fast and simple diagnostic method for identifying tsunamigenic earthquakes

An analysis of regional broadband seismograms of moderate and large subduction‐zone earthquakes in Mexico shows that earthquakes which occur near the trench are abnormally depleted in high‐frequency radiation. This observation leads to a simple and fast method to assess regional tsunami potential from earthquakes which occur along the Pacific coast of Mexico. A significant advantage of the method is that a single broadband seismograph is sufficient for the purpose. The method is based on the ratio of the total energy to the high‐ frequency energy (between 1 and 5 Hz), ER, computed from the seismograms. For earthquakes with the same seismic moment, ER is an order of magnitude greater for a source area near the trench as compared to those near the coast. The same seismograms are used to compute energy magnitude, ME, which is tied to the moment magnitude, MW. A regional tsunami may be expected along the coast if ME ≥ 6.5 and ER corresponds to a source near the trench. If, however, ER corresponds to a near‐coast source, then ME may have to be greater than about 7.3 for a tsunami of similar size to occur along the coast. The method holds promise for a fast regional tsunami warning in many Pacific basin countries which lack an adequate seismic network.

Continuous excitation of planetary free oscillations by atmospheric disturbances

Seismology provides a powerful tool for probing planetary interiors1,2, but it has been considered inapplicable to tectonically inactive planets where earthquakes are absent. Here, however, we show that the atmospheres of solid planets are capable of exerting dynamic pressure on their surfaces, thereby exciting free oscillations with amplitudes large enough to be detected by modern broad-band seismographs. Order-of-magnitude estimates of these forces give similar amplitudes of a few nanogals for the Earth, Venus and Mars despite widely varying atmospheric and ambient conditions. The amplitudes are also predicted to have a weak frequency dependence. Our analysis of seismograms, recorded continuously from 1992 to 1993 at 13 globally distributed stations, shows strong evidence for continuously excited fundamental-mode free oscillations on the Earth. This result, together with other recent studies3,4,5, is consistent with our estimate of atmospheric forcing and we therefore propose that it may be possible to detect atmospheric excitation of free oscillations on Venus and Mars as well.

Preliminary results from a passive seismic array over the Chicxulub impact structure in Mexico

Abstract A passive, 20-element, short-period (1 Hz) and broadband seismic array was deployed over the Chicxulub impact structure for c. 100 days in early 1996. The principal objective was to study the shear-wave anisotropy associated with the structure; in particular, to determine the presence (or absence) of radial symmetry which will allow comment on the time variance of that anisotropy. A total of 15 teleseismic, 75 regional, and 100 local events were recorded. Preliminary results from studies of the surface-wave dispersion of the local events, and a receiver function analysis of a single teleseismic event are reported here. Thirty local events have been located, a number of which originated from quarries within the array. Analysis of seismo grams from three of these events demonstrates a bimodal distribution; those whose ray-paths cross the outer part of the impact structure show a strong inverse dispersion, whereas those with ray-paths crossing the centre do not. The pattern may be produced by the sedimentary depositional environment, with deeper water sedimentation in the outer part of the post-impact crater basin and shallower water sedimentation over the upraised peak-ring block at the centre. Receiver functions derived for an event originating in Peru are dominated by an efficient mode conversion, simply modelled as a P-S multiple from the base Tertiary boundary. This shows a strong correlation with distance from the centre of the impact structure and implies it has an S-wave radial symmetry. The multiple also has a variable delay probably related to the depth of the conversion boundary. Unfortunately, the Moho conversion occurs at almost exactly the same time as this surface layer sediment multiple, restricting any modelling of Moho topography and its influence on the receiver functions.

Reconciling physical properties with surface seismic data from a layered mafic intrusion

Compositional layering of intrusive rocks is often cited as a source of seismic reflectivity in the crystalline crust. Direct evidence for this is based primarily on synthetic seismograms calculated from laboratory measurements of rock velocity and density, or estimates of aggregate rock properties from measurements of mineral velocities and densities. Little is known about how in situ effects such as fracturing, chemical alteration, or anisotropy, may change the reflectivity character of the surface seismic data. An ideal dataset for examining the hazards of using physical properties data for estimating crustal reflectivity occurs at the Nellie intrusion, a middle Proterozoic-age layered mafic intrusion that lies beneath the Permian Basin of west Texas. Due to a well that penetrated 4.5 km of the intrusion, we have a dataset that spans the scale from compositional data on the intrusion derived from well cuttings, to log data, to surface reflection data. Sonic and density log data from the well measure velocities and densities 10 to 15% below the values calculated from modal mineralogy. We attribute this difference primarily to pervasive post-emplacement alteration and fracturing of the rock mass in situ. Thus, this unique dataset provides a quantitative estimate of the magnitude of difference between observed and theoretical velocities that can be expected for crystalline rocks in the upper crust and its cause. Despite the difference, analysis of synthetic seismograms shows that primary compositional variation of the intrusion is still responsible for sub-horizontal layered reflectivity in both the log-based and petrology-based synthetic seismograms. Boundaries of intrusive cycles are characterized by large negative reflection coefficients due to the juxtaposition of mafic-rich basal layers against plagioclase-rich layer tops. In detail, only the log-based synthetic provides a good match to the surface seismic reflection data. This suggests that physical properties from modal mineralogy are useful in obtaining a generalized model for reflectivity, but that differences in bulk physical properties of the rock mass preclude the possiblity of obtaining a one-to-one match with surface seismic data.