Explosive volcanic eruptions — V. Observations of plume dynamics during the 1979 Soufrière eruption, St Vincent

Summary During the 1979 eruption of the Soufrière of St Vincent, West Indies, a 14 min period of explosive activity occurred on April 22. The first three minutes of this activity were filmed. Five individual explosions formed distinct plumes which fed an eruption column which eventually ascended to a height of over 18 km. The ascent velocities of the fronts of these plumes ranged from 8.5 to 61.7 ms^-1. The half-angle of spreading of the plume fronts ranged from 21.5 to 24°. One of the plumes was observed to 8 km height and decelerated steadily from 51.5 to 23 ms^-1. The main column fed by individual explosions was, however, observed to accelerate from 8.5 to 58 m s^-1 in the same height interval.
A theoretical treatment of volcanic plume motion is presented. Measurements of dimensions, velocities and spreading rates from the film are used to estimate plume parameters such as temperature, particle content and volume discharge rate of magma from the theoretical relationships. These calculations show that the individual plumes became hotter and richer in juvenile ash with time. The acceleration of the main eruption column was the result of being fed by increasingly hotter and more ash-rich explosions. An average volume discharge rate of 12600m³ s^-1 is estimated from the analysis of the plume motions. This value agrees closely with an estimate of discharge rate based on the heat flux required to form an 18 km high column. This agreement suggests that theories of convective motions in plumes can be successfully adapted to the volcanic case, as long as the effect of hot entrained ash particles is considered.     

Explosive volcanic eruptions—IX. The transition between Hawaiian-style lava fountaining and Strombolian explosive activity

The commonest eruption styles of basaltic volcanoes involve Hawaiian lava fountaining or intermittent Strombolian explosions. We investigate the ways in which magma rise speed at depth, magma volatile content and magma viscosity control which of these eruption styles takes place. We develop a model of the degree of coalescence between gas bubbles in the magma which allows us to simulate the transition between the two extreme styles of activity. We find that magma rise speed is the most important factor causing the transition, with gas content and viscosity also influencing the rise speed at which the transition occurs. Counter to intuitive expectations, a decrease in gas content does not cause a transition from Hawaiian to Strombolian activity, but instead causes a transition to passive effusion of vesicular lava. Rather, a change from Hawaiian to Strombolian style requires a significant reduction in magma rise speed.     

Discrimination Analysis of Earthquakes and Man-Made Events Using ARMA Coefficients Determination by Artificial Neural Networks

A Quadratic Neural Networks (QNNs) model has been developed for identifying seismic source classification problem at regional distances using ARMA coefficients determination by Artificial Neural Networks (ANNs). We have devised a supervised neural system to discriminate between earthquakes and chemical explosions with filter coefficients obtained by windowed P-wave phase spectra (15 s). First, we preprocess the recording's signals to cancel out instrumental and attenuation site effects and obtain a compact representation of seismic records. Second, we use a QNNs system to obtain ARMA coefficients for feature extraction in the discrimination problem. The derived coefficients are then applied to the neural system to train and classification. In this study, we explore the possibility of using single station three-component (3C) covariance matrix traces from a priori-known explosion sites (learning) for automatically recognizing subsequent explosions from the same site. The results have shown that this feature extraction gives the best classifier for seismic signals and performs significantly better than other classification methods. The events have been tested, which include 36 chemical explosions at the Semipalatinsk test site in Kazakhstan and 61 earthquakes (mb = 5.0–6.5) recorded by the Iranian National Seismic Network (INSN). The 100% correct decisions were obtained between site explosions and some of non-site events. The above approach to event discrimination is very flexible as we can combine several 3C stations.     

Recognizing explosion sites without seismogram readings: neural network analysis of envelope-transformed multistation SP recordings 3–6 Hz

Seismic-waveform similarities for closely spaced earthquakes and explosions in particular are well established observationally. In many industrialized countries of low seismicity more than 90 per cent of seismic event recordings stem from chemical explosions and thus contribute significantly to the daily analyst workload. In this study we explore the possibility of using envelope waveforms from a priori known explosion sites (learning) for recognizing subsequent explosions from the same site excluding any analyst interference. To ensure high signal correlation while retaining good SNRs we used envelope-transformed waveforms, including both the P and Lg arrivals. To ensure good spatial resolution we used multistation (network) recordings. The interpolation and approximation neural network (IANN) of Winston (1993) was used for teaching the computer to recognize new explosion recordings from a specific site using detector output event files of waveforms only. The IANN output is a single number between 0 and 1, and on this scale an acceptance threshold of 0.4 proved appropriate. We obtained 100 per cent correct decisions between two sets of 'site explosions' and hundreds of 'non-site' explosions/earthquakes using data files from the Norwegian Seismograph Network.     

A near-regional explosion source model for tuff

Summary. A variety of near-regional (300 km) data, including spectral amplitudes of Pg , surface-wave forms, and close-in (5–10 km) accelerograms have been used to build an elastic seismic source model for a 1-Mton explosion in tuff at near-regional distances. The model consists of: (1) a pressure pulse which injects 3 × 10¹² cm³ of volume into the medium, (2) a vertical, upward force impulse that imparts 10¹⁸ dyn-s of momentum to the medium, each source component having a time duration of 0.6 s and a depth of 1.3 km. The force impulse appears to be required by two considerations: (a) the striking similarity, apart from sign, of explosion surface waves with those of their cavity collapses, (b) the observation of considerable SV energy leaving the source of the 1-Mton explosions JORUM and HANDLEY . Scaling curves have been constructed which fit the proposed source model. These scaling curves employ: very slow decrease, as (yield)^−0.10 of the primary corner frequency; decay as (frequency)⁴ or (frequency)³ to high frequency. While these scaling curves are unconventional, they appear to be the only ones which can satisfy the near-regional data. The slow scaling with yield of the spectral carner frequency suggests that it is caused by something other than the equivalent elastic radius, e.g. the time duration of motion at the source. The results, at odds with similar studies at teleseismic distances, suggest that significantly different equivalent elastic sources are required at near-regional (as compared with teleseismic) distances; therefore, the effect of the upward impulse might not be seen at teleseismic distances. Consequently, these results probably do not pertain to the seismic discrimination problem at teleseismic distances.     

Discrimination of small earthquakes and artificial explosions in the Korean Peninsula using Pg/Lg ratios

The purpose of this study is to develop a technique to discriminate artificial explosions from local small earthquakes ( M ≤ 4.0) in the time–frequency domain. In order to obtain spectral features of artificial explosions and earthquakes, 3-D spectrograms (frequency, time and amplitude) have been used. They represent a useful tool for studying the frequency content of entire seismic waveforms observed at local and regional distances (Kim, Simpson & Richards 1994). P and S(L g ) waves from quarry blasts show that the frequency content associated with the dominant amplitude appears above 10  Hz and Rg phases are observed at close distances. P and S(L g ) waves from the Tongosan earthquake have strong amplitudes below 10  Hz. For the Munkyong earthquake, however, a broader frequency content up to 20  Hz is found.
  For discrimination between small earthquakes and explosions, Pg/L g spectral ratios are used below 10  Hz, and through spectrogram analysis we can see different frequency contents of explosions and earthquakes. Unfortunately, because explosion data recorded at KSRS array are digitized at 20  sps, we cannot avoid analysing below 10  Hz because of the Nyquist frequency. In order to select time windows, the group velocity was computed using multiple-filter analysis (MFA), and free-surface effects have been removed from all three-component data in order to improve data quality. Using FFT, a log-average spectral amplitude is calculated over seven frequency bands: 0.5 to 3, 2 to 4, 3 to 5, 4 to 6, 5 to 7, 6 to 8 and 8 to 10  Hz. The best separation between explosions and earthquakes is observed from 6 to 8  Hz. In this frequency band we can separate explosions with log ( Pg/L g ) above −0.5, except EXP1 recorded at SIHY1-1, and earthquakes below −0.5, except the Munkyong earthquake record at station KMH.     

Sequence of faulting and deformation during the 2000 eruptions of the Usu Volcano, Hokkaido, Japan—interpretation and image analyses of aerial photographs

Significant faulting and deformation of the ground surface has been rarely known during volcanic eruptions. Usu Volcano, Hokkaido, Japan, is a unique example of deformation due to felsic magma intrusion. Usu Volcano has a history of such types of eruptions as phreatic, pumice eruption (Plinian type), pyroclastic flowing and lava doming since 1663. On March 31, 2000, phreatomagmatic to phreatic eruptions took place after 23 years of dormancy in the western piedmont, followed by explosions on the western flank of Usu Volcano. They were associated with significant deformation including faulting and uplift. The eruptions and deformation were continuing up to the end of May 2000.We identified the faulting using total nine sets of aerial photographs taken from before the eruption (March 31, 2000) to more than 1 year (April 27, 2001) after the end of the activity, and traced deformation processes through image processing using aerial photographs. We found that some of the new faults and the associated phreatic eruptions were related to old faults formed during the 1977–1981 eruptive episode.The image processing has revealed that the surface deformation is coincident with the area of faulting forming small grabens and the phreatic explosion vents. However, the faulting and main explosive eruptions did not take place in the highest uplift area, but along the margin. This suggests that the faulting and explosive activities were affected by small feeder channels diverging from the main magma body which caused the highest uplift.     

Model of a strong volcanic blast and a method of estimating the mass ejected

A hydrodynamic model of a strong volcanic blast associated with large (caldera-forming) ash–gas-flow eruptions is presented. We show that the mass erupted from a vent influences considerably the damping of the shock wave. If the ejected mass flux is constant, the shock amplitude as a function of distance will decrease slower (in accordance with the inverse 4/3 power law) than that of an atomic explosion (the inverse cube power law). From the modelling, we extract a relation connecting the effective energy of a blast with the mass ejected during the blast phase. The mass released during the largest explosion of the 1883 eruption of Krakatau, and the lateral blast energy at Mount St. Helens on 1980 May 18 are estimated on the basis of the equation obtained.     

The utility of regional Chinese seismograms for source and path studies in central Asia

Summary. Broadband seismograms from the National Seismic Network of the People's Republic of China (PRC) have recently become available through a data exchange programme between NOAA and the State Seismological Bureau of the PRC. In this study, regional surface waves recorded at the Urumchi station located about 700 km north of the Tibetan Plateau in the Sinkiang Province are used to study East Kazakh explosions and wave propagation in central Asia. The data consist of broadband (flat to displacement between 0.1 and 10 Hz), photographic records from an SK Kirnos galvanometric system. Simultaneous inversion of Rayleigh wave phase and group velocities for the path from East Kazakh through the Dzhungarian Basin yields a crustal model dominated by the presence of very low velocities and a strong positive velocity gradient above 15 km depth. Velocities below 15 km depth are not significantly different from other continental structures underlain by Palaeozoic or Precambrian basement. Seismic moments were estimated for seven East Kazakh explosions using models of explosion sources with associated tectonic strain release. The largest explosion studied occurred on 1980 September 14 and had an m_b of 6.2 and a seismic moment of 2.7 × 10²³ dyn cm. The observed amplitude spectra of Rayleigh waves are richer in high frequencies than spectra calculated from our models. This could be caused by a path effect involving seismic wave focusing by the Dzhungarian Basin, although source medium effects cannot be ruled out.     

Stable inversions for complete moment tensors

The seismic moment tensors for certain types of sources, such as volcanic earthquakes and nuclear explosions are expected to contain an isotropic component. Some earlier efforts to calculate the isotropic component of these sources are flawed due to an error in the method of Jost & Herrmann. We corrected the method after Herrmann & Hutchensen and found great improvement in the recovery of non-double-couple moment tensors that include an isotropic component. Tests with synthetic data demonstrate the stability of the corrected linear inversion method, and we recalculate the moment tensor solutions reported in Dreger et al. for Long Valley caldera events and Dreger & Woods for Nevada Test Site nuclear explosions. We confirm the findings of Dreger et al. that the Long Valley volcanic sources contain large statistically significant isotropic components. The nuclear explosions have strikingly anomalous source mechanisms, which contain very large isotropic components, making it evident that these events are not tectonic in origin. This indicates that moment tensor inversions could be an important tool for nuclear monitoring.     

Use of P coda for determination of yield on nuclear explosions

Summary. Spectral analysis of P and P coda from NTS explosions recorded at NORSAR shows that magnitudes based on spectral integration of P coda provide a significantly more precise measure of yield than those based on the initial P . Coda amplitudes and spectra are considerably less sensitive to near-source variations than the direct P . A plot of P -coda magnitude, after correcting for the effects of attenuation in the upper mantle and the source spectrum based on knowledge of approximate yield or m_b , versus log yield appears to have a slope of unity. This implies that P coda from several stations (with different but known t * values) can be combined to improve the yield determinations further. Single station P coda from Soviet explosions can also be used to determine relative yields that appear to be at least as precise as those from Lg based on a network of stations.     

Frequency–size distribution of global seismicity seen frombroad-band radiated energy

The frequency–energy distribution of global seismicity is studied using broad-band radiated energy of shallow earthquakes from January 1987 to December 1994 estimated by NEIC. Rank-ordering statistics are applied to enhance the resolution in retrieving the power-law distribution with undersampled data, namely a few tens of events. Seen in the perspective of broad-band radiated energy with higher resolution, a single (Gutenberg–Richter-type) power-law distribution can fit the data. For earthquakes with energy larger than 10¹⁴ J, the number N of events with energy E depends on E via N∝E ^−B , with the scaling constant B = 0.64 ± 0.04, corresponding to b = 0.95 ± 0.06. This relation is different from that of scalar seismic moment, which shows a transition of power-law distributions between small and large earthquakes. To demonstrate such a difference we use the same set of earthquakes with both broad-band energy estimation and CMT estimation. It is found that for the same data set, the energy distribution and the moment distribution show different patterns. The moment distribution has a clear kink between small and large earthquakes, while the energy distribution shows a single power law with no convincing kink between small and large earthquakes. To investigate the effect of different focal mechanisms and different seismic regions, events with strike-slip mechanisms and events within the Japan–Kuril region are considered. For these subsets of events, a similar pattern exists, in which the moment distribution shows a kink between small and large earthquakes, while the energy distribution shows a single power law.     

Kinematic multistation discriminator between local quarry blasts and earthquakes

It has long been known that S waves on seismograms of local explosions are often accompanied by strong low-frequency, low-velocity, Rg surface wave trains, often significantly diminished for earthquakes. We utilize this fact to construct a new formal discriminator between earthquakes and explosions by measuring the S -surface-wave group velocity. The method is based on analysing the velogram; that is, the display of the envelope of ground motion versus group velocity V = R/T , where R is the epicentral distance and T  the traveltime. We examine the distribution of seismic energy in time and space using envelopes of records from the Israel Seismic Network (ISN), from which we compute the velograms and observe differences in the velograms of quarry blasts and earthquakes. The data include 143 seismic events occurring in three areas (Galilee, Dead Sea, and Gilad) monitored by the ISN; the magnitude range is M _L = 1.0–2.8 at distances of 15–310 km. From the velograms we measure the group velocity, V _{m s} , within the 1–4 km s⁻¹ range at which the velogram reaches its maximum for each available station. The resulting V _{m s} (R) function is closely fitted by the empirical relationship a + b  ln  R , with a and b coefficients varying from event to event. A simple linear function c = b + 0.33a at a threshold C = 0.69 completely separates ( a,b ) pairs for the 67 Galilee events, and, for the 76 remaining events, one earthquake and four explosions are wrongly classified. After data validation and application of the Fisher linear discriminator, adapted to the events from Galilee, only two misidentified events remain for the whole data set.     

Magnitude corrections for attenuation in the upper mantle

Summary. The m _b: M _s relation for explosions at the Nevada Test Site (NTS) differs from those for explosions in other parts of the world. There is considerable evidence that this results mostly from high body-wave attenuation in the upper mantle beneath the western US. The authors have developed an empirical magnitude correction for body-wave attenuation and applied it to both source and receiver ends of the teleseismic body-wave path. The results imply that m _b values are lower for NTS explosions than for Soviet explosions of comparable yield and seismic coupling. The authors have also developed and applied a source-depth correction to account for _pP-P interference in the P -wave arrival.
The body-wave magnitude resulting from these corrections is designated m_o to distinguish it from other definitions of m _b. Values of m_Q determined for a world-wide set of large explosions show that a single m_Q : yield relation is a fair fit to the data for the explosions with high seismic coupling. However, grouping the explosions under two m_Q :yield relations gives a better fit to the data.
All the studied explosions in salt or granite or below the water table fit a common M _s:yield relation. Explosions from North America, Eurasia and Africa have a common m_Q : M _s relation.     

Seismic hazard in central southern Africa

Seismic hazard maps of central-southern Africa where hazard has been expressed in terms of peak ground acceleration for an annual probability in excess of 10^-1 show relatively high values that distinguish the seismic hazard potential of the Deka fault zone, the mid-Zambezi basin-Luangwa rift and western central Mozambique. In areas such as central-southern Africa where little is known about the geology of the region and the fault systems have not been fully mapped, seismic hazard potential may be estimated from seismicity and broad-scale fault features. For this region, such potential is based on earthquake magnitude M_s ≥ 6. Events of such magnitude have recently occurred in the mid-Zambezi basin, southern Zimbabwe and western-central Mozambique. This paper follows the conventional probabilistic hazard analysis procedure, defining seismic source zones from seismicity based on instrumental records from a cataloque that spans a period of 83 years. Geological and geomorphological features in the region are described on the mesoscale and are correlated with the seismicity as broad fault zones. The scarcity of strong-motion accelerogram data necessitated the formulation of attenuation values based on random vibration theory (RVT).     

On the linear relation between mb and Ms for discrimination between explosions and earthquakes

Summary. The statistical capability of the m _b: M _s discriminant for the discrimination of earthquake and explosion populations is examined by application of discriminant functions to a group of 83 explosions and 72 earthquakes in Eurasia. Equations are derived for the probability that an event is an earthquake or an explosion. The positive sign of DIS in the decision index equation, DIS _i = 34.3383 – 11.9569 mb _t + 7.1161 M _si , indicates that the event i is an earthquake. Its negative sign indicates that event i is an explosion. The probability of correct classification for an event, P _i , is related to its DIS _i value, by P _i = [1-exp (DIS _i )]⁻¹, where a large, positive DIS indicates a high probability that an event is an earthquake and a large, negative DIS indicates a high probability that an event is an explosion. The discrimination line M _s = 1.680 m _b– 4.825, or m _b= 0.595 M _s+ 2.872 very successfully separates the explosion population from the earthquake population. The points on this line have an equal chance of being an earthquake or an explosion; moreover, for any event, the distance parallel to the M _s-axis from the point representing that event in the m _b: M _s plane to this line is a measure of the probability for the correct classification of that event.     

Internal structure and eruptive history of a kilometre-scale mud volcano system, South Caspian Sea

We describe the internal structure of a multi‐kilometre scale mud volcano edifice from the South Caspian Sea using three‐dimensional (3D) seismic reflection data leading to a reconstruction of the volcano system's eruptive history. By adapting elements of classic seismic stratigraphy to the study of this volcano, we have found its edifice to consist of a series of stacked mud cones. This internal architecture is most likely to have formed as a result of repeated episodes of expulsion of a fluid‐mud mix. Underlying the stack of cones is an asymmetric fault‐bounded caldera measuring approximately 2 km in diameter. This caldera shows close structural similarity to the trapdoor type of magmatic caldera. Based on the geometrical relationships of individual mud cones to this caldera, we conclude that caldera‐like collapse of the edifice floor initiated following the deposition of the first mud cone (the pioneer cone). Growth of the caldera continued until the later stages of edifice evolution when it eventually abated. This eruptive history shows strong similarities to recent models for magmatic caldera eruption cycles. The study therefore highlights the potential analogue value of mud volcano systems to the study of igneous volcanism. Furthermore, it identifies 3D seismic data as a potentially useful tool in reconstructing the history of mud volcanic eruption and fluid and sediment expulsion from sedimentary basins.     

FIR filter effects and nucleation phases

The symmetric impulse response of linear phase Finite Impulse Response (FIR) filters most commonly used in modern seismic recording systems produces precursory signals to impulsive arrivals. These acausal filter-generated artefacts may result in misinterpretations of various onset properties. Prior to any onset interpretation, these effects have to be removed from the seismic record. This can be achieved without loss of bandwidth by post-filtration of the digital seismograms if the filter coefficients and the decimation ratios are known. We have analysed numerous signals from different instruments and sampling rates for precursory phases and found that—in contrast to commonly held beliefs—FIR-filter-related precursory signals are not always easy to recognize visually from their waveform signature. Furthermore, they can exhibit surprisingly similar properties to those reported for nucleation phases, although the majority of nucleation phases reported in the past have been obtained on instruments with a causal response. We demonstrate examples of filter-related precursory signals for events scanning nine orders of moment, from 10¹⁰ N m to 10¹⁹ N m. Surprisingly, the lower bound of the artefact durations as a function of seismic moment scales close to the cube root of the seismic moment. We interpret this as being caused by the fact that above a certain seismic moment, the attenuated source signal acts as a causal lowpass filter of a smaller bandwidth than the FIR filter. Assuming an ω^-2 source model, constant stress drop and an empirical relationship between the maximum artefact duration and the cut-off frequency of the FIR filter, the artefact durations are expected to scale proportional to the 1/2.5 power of the seismic moment, in comparison to 1/3 as proposed for nucleation phases.     

Reply to comment by J. R. Murphy on 'Q for short-period P waves: is it frequency dependent?' by A. Douglas

Using network averaged spectra Murphy (1989) demonstrates that if it is assumed that the source functions of explosions at Pahute Mesa, Nevada Test Site (NTS) are as predicted by the Mueller-Murphy (M-M) source model then the average t * at around 1 Hz for P waves radiated from the test site must be about 0.75 s. With this value of t * Murphy (1989) estimates the best fitting M-M spectrum for each explosion studied, by adjusting A, t _o & c ; A & t _o being the amplitude and delay time of pP relative to P and c the wave speed for the material in which the explosion was fired. The absolute amplitudes of the theoretical spectra are obtained using a calibration factor estimated from the data. Murphy (1993) extends the analysis to explosions in granite at the Nevada, French Sahara and E. Kazakh test sites. For the French Sahara explosion t * is assumed to be 0.75 s (Murphy's estimate for NTS explosions), and for the E. Kazakh explosion a t * of 0.55 s is used. For the French Sahara and E. Kazakh explosions Murphy (1993) shows that by using the same calibration factor as for the NTS it is possible by varying A & t _o to fit the estimated average network spectra using the M-M granite source. Murphy (1993) states that the amplitudes and spectra for the largest NTS explosion in granite (PILE DRIVER) can also be predicted using the M-M model but these results are not shown.     

极地冰芯中记录的火山作用及其对气候的影响

大多数火山喷发以强酸 (主要是硫酸 )的形式在极地冰芯中留下痕迹 ,通过冰芯连续电导率 (ECM)或各个雪冰样品SO42 - 浓度测定能够恢复历史上的火山作用。极地冰芯中已知年代的火山喷发还可用于冰芯定年。极地冰芯记录的火山信号大小依赖于火山喷发的规模和类型 ,火山喷发的地理位置和酸性气体组成 ,大气气溶胶传输 ,以及沉降地点的年积累率和沉积后生过程等。单个火山喷发通常会导致数月至数年的半球乃至全球范围的气候相对变冷 (这种现象在极区变得更明显 ) ,但过去大规模的爆炸性火山喷发对气候的影响及其与气候变化之间的联系仍有待于进一步研究