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1.
Iwate volcano, Japan, showed significant volcanic activity including earthquake swarms and volcano inflation from the beginning
of 1998. A large earthquake of magnitude 6.1 hit the south-west of the volcano on September 3. Although a 1 km2 fumarole field formed, blighting plants on the ridge in the western part of the volcano in the spring of 1999, no magmatic
eruptions occurred. We reconcile the spatio-temporal distributions of volcanic pressure sources determined by previously reported
studies in which GPS, strain and tilt data from dense geodetic station networks are analyzed (Miura et al. Earth Planet Space
52:1003–1008, 2000; Sato and Hamaguchi J Volcanol Geotherm Res 155:244–262, 2006). We calculate the magma supply rates from their results and compare them with the occurrence rates of volcanic earthquakes.
The results show that the magma supply rates are almost constant or even decrease with time while the earthquake occurrence
rate increases with time. This contrast in their temporal changes is interpreted to result from stress accumulation in the
volcanic edifice caused by constant magma supply without effusion of magma to the surface. We further show that data showing
slight acceleration in strain can be best explained by magma ascent at a constant velocity, and that there is no evidence
for increased magma buoyancy resulting from gas bubble growth. This consideration supports the interpretation that the magma
stayed at 2 km depth and horizontally migrated. These findings relating magma supply rate and seismicity to magma ascent process
are clues to understanding why no magmatic eruption occurred at Iwate volcano in 1998. 相似文献
2.
Decadal evolution of a degassing magma reservoir unravelled from fire fountains produced at Etna volcano (Italy) between 1989 and 2001 总被引:1,自引:1,他引:0
Between 1989 and 2001, five eruptions at Etna displayed a regular alternation between repose periods and episodes rich in
gas, termed quasi-fire fountains and consisting of a series of Strombolian explosions sometimes leading to a fire fountain.
This behaviour results from the coalescence of a foam layer trapped at the top of the reservoir which was periodically rebuilt
prior to each episode (Vergniolle and Jaupart, J Geophys Res 95:2793–2809, 1990). Visual observations of fire fountains are combined with the foam dynamics to estimate the five degassing parameters characteristic
of the degassing reservoir, i.e. the number of bubbles, gas volume fraction, bubble diameter, reservoir thickness and reservoir
volume. The study of decadal cycles of eruptive patterns (Allard et al., Earth Sci Rev 78:85–114, 2006) suggests that the first eruption with fire fountains occurred in 1995 while the last one happened in 2001. The number of
bubbles and the gas volume fraction increase smoothly from the beginning of the cycle (1995) to its end (2001). The increasing
number of bubbles per cubic metre, from 0.61–20×105 to 0.1–3.4×109, results from cooling of the magma within the reservoir. The simultaneously decreasing bubble diameter, from 0.67–0.43 to
0.30–0.19 mm, is related to the decreasing amount of dissolved volatiles. Meanwhile, the thickness and the volume of the degassing
reservoir diminish, from values typical of the magma reservoir to values characteristic of a very thin bubbly layer, marking
the quasi-exhaustion of volatiles. The magma reservoir has a slender vertical shape, with a maximum thickness of 3,300–8,200 m
and a radius of 240 m (Vergniolle 2008), making its detection from seismic studies difficult. Its volume, at most 0.58–1.4 km3, is in agreement with geochemical studies (0.5 km3) (Le Cloarec and Pennisi, J Volcanol Geotherm Res 108:141–155, 2001). The time evolution of both the total gas volume expelled per eruption, and the inter-eruptive gas flux results from the
competition between the increasing number of bubbles and the decreasing bubble diameter. The smooth temporal evolution of
the five degassing parameters also points towards bubbles being produced by a self-induced mechanism within the magma reservoir
rather than by a magmatic reinjection prior to each eruption. The decadal cycles are therefore initiated by a magmatic reinjection,
in agreement with a typical return time of 14–80 years (Albarède 1993). Hence, the 1995 eruption results from a fresh magma being newly emplaced while the magma from the following eruptions is
progressively depleted in volatiles species until reaching a state of quasi-exhaustion in 2001. A magmatic reinjection of
0.13–0.6 km3 every few decades is sufficient to explain the expelled gas volume, including SO2. A scenario is also proposed for the alternation between gas-rich summit eruptions and gas-poor flank eruptions which are
observed during decadal cycles. The scenario proposed for Etna could also be at work at Piton de la Fournaise and Erta ’Ale
volcanoes. 相似文献
3.
The Tibesti massif, one of the most prominent features of the Sahara desert, covers an area of some 100,000 km2. Though largely absent from scientific inquiry for several decades, it is one of the world’s major volcanic provinces, and
a key example of continental hot spot volcanism. The intense activity of the TVP began as early as the Oligocene, though the
major products that mark its surface date from Lower Miocene to Quaternary (Furon (Geology of Africa. Oliver & Boyd, Edinburgh
(trans 1963, orig French 1960), pp 1–377, 1963)); Gourgaud and Vincent (J Volcanol Geotherm Res 129:261–290, 2004). We present here a new and consistent analysis of each of the main components of the Tibesti Volcanic Province (TVP), based
on examination of multispectral imagery and digital elevation data acquired from the Advanced Spaceborne Thermal Emission
and Reflection Radiometer (ASTER). Our synthesis of these individual surveys shows that the TVP is made up of several shield
volcanoes (up to 80 km diameter) with large-scale calderas, extensive lava plateaux and flow fields, widespread tephra deposits,
and a highly varied structural relief. We compare morphometric characteristics of the major TVP structures with other hot
spot volcanoes (the Hawaiian Islands, the Galápagos Islands, the Canary and Cape Verdes archipelagos, Jebel Marra (western
Sudan), and Martian volcanoes), and consider the implications of differing tectonic setting (continental versus oceanic),
the thickness and velocity of the lithosphere, the relative sizes of main volcanic features (e.g. summit calderas, steep slopes
at summit regions), and the extent and diversity of volcanic features. These comparisons reveal morphologic similarities between
volcanism in the Tibesti, the Galápagos, and Western Sudan but also some distinct features of the TVP. Additionally, we find
that a relatively haphazard spatial development of the TVP has occurred, with volcanism initially appearing in the Central
TVP and subsequently migrating to both the Eastern and Western TVP regions.
Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. 相似文献
4.
Giuseppe Pasquale Raffaella De Matteis Annalisa Romeo Rosalba Maresca 《Journal of Seismology》2009,13(1):107-124
The goal of this study was to estimate the stress field acting in the Irpinia Region, an area of southern Italy that has been
struck in the past by destructive earthquakes and that is now characterized by low to moderate seismicity. The dataset are
records of 2,352 aftershocks following the last strong event: the 23 November 1980 earthquake (M 6.9). The earthquakes were
recorded at seven seismic stations, on average, and have been located using a three-dimensional (3D) P-wave velocity model
and a probabilistic, non-linear, global search technique. The use of a 3D velocity model yielded a more stable estimation
of take-off angles, a crucial parameter for focal mechanism computation. The earthquake focal mechanisms were computed from
the P-wave first-motion polarity data using the FPFIT algorithm. Fault plane solutions show mostly normal component faulting
(pure normal fault and normal fault with a strike-slip component). Only some fault plane solutions show strike-slip and reverse
faulting. The stress field is estimated using the method proposed by Michael (J Geophys Res 92:357–368, 1987a) by inverting selected focal mechanisms, and the results show that the Irpinia Region is subjected to a NE–SW extension with
horizontal σ
3 (plunge 0°, trend 230°) and subvertical σ
1 (plunge 80°, trend 320°), in agreement with the results derived from other stress indicators. 相似文献
5.
Guang-hui Cai Hang Wu 《Stochastic Environmental Research and Risk Assessment (SERRA)》2006,20(1-2):1-5
Let {Y, Y
i
, −∞ < i < ∞} be a doubly infinite sequence of identically distributed and asymptotically linear negative quadrant dependence random
variables, {a
i
, −∞ < i < ∞} an absolutely summable sequence of real numbers. We are inspired by Wang et al. (Econometric Theory 18:119–139, 2002) and Salvadori (Stoch Environ Res Risk Assess 17:116–140, 2003). And Salvadori (Stoch Environ Res Risk Assess 17:116–140, 2003) have obtained Linear combinations of order statistics to estimate the quantiles of generalized pareto and extreme values
distributions. In this paper, we prove the complete convergence of under some suitable conditions. The results obtained improve and generalize the results of Li et al. (1992) and Zhang (1996). The results obtained extend those for negative associated sequences and ρ*-mixing sequences.
CIC Number O211, AMS (2000) Subject Classification 60F15, 60G50
Research supported by National Natural Science Foundation of China 相似文献
6.
Comprehensive evaluation of regional flood frequency analysis by L- and LH-moments. I. A re-visit to regional homogeneity 总被引:1,自引:0,他引:1
Ali Meshgi Davar Khalili 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(1):119-135
As part I of a sequence of two papers, previously developed L-moments by Hosking (J R Stat Soc Ser B Methodol 52(2):105–124,
1990), and the LH-moments by Wang (Water Resour Res 33(12):2841–2848, 1997) are re-visited. New relationships are developed for regional homogeneity analysis by the LH-moments, and further establishment
of regional homogeneity is investigated. Previous works of Hosking (J R Stat Soc Ser B Methodol 52(2):105–124, 1990) and Wang (Water Resour Res 33(12):2841–2848, 1997) on L-moments and LH-moments for the generalized extreme value (GEV) distribution are extended to the generalized Pareto
(GPA) and the generalized logistic (GLO) distributions. The Karkhe watershed, located in western Iran is used as a case study
area. Regional homogeneity was investigated by first assuming the entire study area as one regional cluster. Then the entire
study area was designated “homogeneous” by the L-moments (L); and was designated “heterogeneous” by all four levels of the
LH-moments (L1 to L4). The k-means method was used to investigate the case of two regional clusters. All levels of the L- and LH-moments designated the
upper watershed (region A), “homogeneous”, and the lower watershed (region B) “possibly-homogeneous”. The L3 level of the GPA and the L4 level of the GLO were selected for regions A and B, respectively. Wang (Water Resour Res 33(12):2841–2848, 1997) identified a reversing trend in improved performance of the GEV distribution at the LH-moments level of L3 (during the goodness-of-fit test). Similar results were also obtained in this research for the GEV distribution. However,
for the case of the GPA distribution the reversing trend started at L4 for region A; and at L2 for region B. As for the case of the GLO, an improved performance was observed for all levels (moving from L to L4); for both regions. 相似文献
7.
8.
I. Alberico L. Lirer P. Petrosino R. Scandone 《Journal of Volcanology and Geothermal Research》2002,116(1-2)
The volcanological history of Campi Flegrei suggests that the most frequent eruptions are characterized by the emplacement of pyroclastic flow and surge deposits erupted from different vents scattered over a 150-km2 caldera. The evaluation of volcanic risk in volcanic fields is complex because of the lack of a central vent. To approach this problem, we subdivided the entire area of Campi Flegrei into a regular grid and evaluated the relative spatial probability of opening of vents based on geological, geophysical and geochemical data. We evaluated the volcanic risk caused by pyroclastic flows based on the formula proposed by UNESCO (1972), R=H×V×Va, where H is the hazard, V is the vulnerability and Va is the value of the elements at risk. The product H×V was obtained by performing simulations of type eruptions centered in each cell of the grid. The simulation is based on the energy cone scheme proposed by Sheridan and Malin [J. Volcanol. Geotherm. Res. 17 (1983) 187–202], hypothesizing a column collapse height of 100 m for eruptions of VEI=3 and 300 m for eruptions of VEI=4 with a slope angle of 6°. Each simulation has been given the relative probability value associated with the corresponding cell. We made use of the GIS software ArcView 3.2 to evaluate the intersection between the energy cone and the topography. The superposition of the areas invaded by pyroclastic flows (124 simulations for VEI=3 and 37 for VEI=4) was used to obtain the relative hazard map of the area. The relative volcanic risk map is obtained by superimposing the urbanization maps. 相似文献
9.
John E. Hathaway Richard O. Gilbert John E. Wilson Brent A. Pulsipher 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(2):253-261
After site clean-up teams have removed all of what they believe to be UXO within a specific impact area, statistical compliance
sampling is a possible method for verifying with a specified probability that this area has been cleaned to specifications.
Schilling [J Qual Technol 10(2):47–51, 1978, Acceptance sampling in quality control. Marcel Dekker, Inc., New York, 1982] developed a compliance sampling methodology based on the hypergeometric distribution. Bowen and Bennett (1987) also use compliance sampling where they provide an approximation for estimating the number of samples (n) required to state with desired probability that the entire population of sample units (N, where n < N) are in compliance with cleanup goals. This article describes two methods (anomaly and transect) for applying the Schilling
[J Qual Technol 10(2):47–51, 1978, Acceptance sampling in quality control. Marcel Dekker, Inc., New York, 1982] compliance sampling method to military training sites. After describing these methods, a simulation study is presented which
demonstrates the performance of transect compliance sampling calculations based on varied degrees of clustered UXO within
a specific impact area and different types of sampling routines. 相似文献
10.
Hiromasa Ishikawa Tsukasa Ohba Hirokazu Fujimaki 《Journal of Volcanology and Geothermal Research》2007
The ratio of 87Sr/86Sr was measured from different water samples of thermal/mineral (hot spring as well as crater lake) and meteoric origins, in order to specify the location and to verify the detailed model of a volcano-hydrothermal system beneath Zao volcano. The ratio showed a trimodal distribution for the case of thermal/mineral water: 0.7052–0.7053 (Type A, Zao hot spring), 0.7039–0.7043 (Type B, Okama crater lake and Shin-funkiko hot spring), and 0.7070–0.7073 (Type C, Gaga, Aone, and Togatta hot springs), respectively. However, in comparison, the ratio was found to be higher for meteoric waters (0.7077–0.7079). The water from the central volcanic edifice (Type B) was found to be similar to that of nearby volcanic rocks in their Sr isotopic ratio. This indicates that the Sr in water was derived from shallow volcanic rocks. The 87Sr/86Sr ratio for water from the Zao hot spring (Type A) was intermediate between those of the pre-Tertiary granitic and the Quaternary volcanic rocks, thus suggesting that the water had reacted with both volcanic and granitic rocks. The location of the vapor–liquid separation was determined as the boundary of the pre-Tertiary granitic and the Quaternary volcanic rocks by comparing the results of this strontium isotopic study with those of Kiyosu and Kurahashi [Kiyosu, Y., Kurahashi, M., 1984. Isotopic geochemistry of acid thermal waters and volcanic gases from Zao volcano in Japan. J. Volcanol. Geotherm. Res. 21, 313–331.]. 相似文献
11.
Large data sets covering large areas and time spans and composed of many different independent sources raise the question
of the obtained degree of harmonization. The present study is an analysis of the harmonization with respect to the moment
magnitude M
w within the earthquake catalogue for central, northern, and northwestern Europe (CENEC). The CENEC earthquake catalogue (Grünthal
et al., J Seismol, 2009) contains parameters for over 8,000 events in the time period 1000–2004 with magnitude M
w ≥ 3.5. Only about 2% of the data used for CENEC have original M
w magnitudes derived directly from digital data. Some of the local catalogues and data files providing data give M
w, but calculated by the respective agency from other magnitude measures or intensity. About 60% of the local data give strength
measures other than M
w, and these have to be transformed by us using available formulae or new regressions based on original M
w data. Although all events are thus unified to M
w magnitude, inhomogeneity in the M
w obtained from over 40 local catalogues and data files and 50 special studies is inevitable. Two different approaches have
been followed to investigate the compatibility of the different M
w sets throughout CENEC. The first harmonization check is performed using M
w from moment tensor solutions from SMTS and Pondrelli et al. (Phys Earth Planet Inter 130:71–101, 2002; Phys Earth Planet Inter 164:90–112, 2007). The method to derive the SMTS is described, e.g., by Braunmiller et al. (Tectonophysics 356:5–22, 2002) and Bernardi et al. (Geophys J Int 157:703–716, 2004), and the data are available in greater extent since 1997. One check is made against the M
w given in national catalogues and another against the M
w derived by applying different empirical relations developed for CENEC. The second harmonization check concerns the vast majority
of data in CENEC related to earthquakes prior to 1997 or where no moment tensor based M
w exists. In this case, an empirical relation for the M
w dependence on epicentral intensity (I
0) and focal depth (h) was derived for 41 master events, i.e., earthquakes, located all over central Europe, with high-quality data. To include
also the data lacking h, the corresponding depth-independent relation for these 41 events was also derived. These equations are compared with the
different sets of data from which CENEC has been composed, and the goodness of fit is demonstrated for each set. The vast
majority of the events are very well or reasonably consistent with the respective relation so that the data can be said to
be harmonized with respect to M
w, but there are exceptions, which are discussed in detail. 相似文献
12.
Initial ensemble generation and validation for ocean data assimilation using HYCOM in the Pacific 总被引:2,自引:0,他引:2
A method to initialize an ensemble, introduced by Evensen (Physica, D 77:108–129, 1994a; J Geophys Res 99(C5):10143–10162, 1994b; Ocean Dynamics 53:343–367, 2003), was applied to the Ocean General Circulation Model (OGCM) HYbrid Coordinate Ocean Model (HYCOM) for the Pacific Ocean.
Taking advantage of the hybrid coordinates, an initial ensemble is created by first perturbing the layer interfaces and then
running the model for a spin-up period of 1 month forced by randomly perturbed atmospheric forcing fields. In addition to
the perturbations of layer interfaces, we implemented perturbations of the mixed layer temperatures. In this paper, we investigate
the quality of the initial ensemble generated by this scheme and the influence of the horizontal decorrelation scale and vertical
correlation on the statistics of the resulting ensemble. We performed six ensemble generation experiments with different combinations
of horizontal decorrelation scales and with/without perturbations in the mixed layer. The resulting six sets of initial ensembles
are then analyzed in terms of sustainability of the ensemble spread and realism of the correlation patterns. The ensemble
spreads are validated against the difference between model and observations after 20 years of free run. The correlation patterns
of six sets of ensemble are compared to each other. This study shows that the ensemble generation scheme can effectively generate
an initial ensemble whose spread is consistent with the observed errors. The correlation pattern of the ensemble also exhibits
realistic features. The addition of mixed layer perturbations improves both the spread and correlation. Some limitations of
the ensemble generation scheme are also discussed. We found that the vertical shift of isopycnal coordinates provokes unrealistically
large deviations in shallow layers near the islands of the West Pacific. A simple correction circumvents the problem.
相似文献
Liying WanEmail: |
13.
Karim Kelfoun Pablo Samaniego Pablo Palacios Diego Barba 《Bulletin of Volcanology》2009,71(9):1057-1075
We use a well-monitored eruption of Tungurahua volcano to test the validity of the frictional behaviour, also called Mohr–Coulomb,
which is generally used in geophysical flow modelling. We show that the frictional law is not appropriate for the simulation
of pyroclastic flows at Tungurahua. With this law, the longitudinal shape of the simulated flows is a thin wedge of material
progressively passing, over several hundreds of metres, from an unrealistic thickness at the front (<<1 mm) to some tens of
centimetres. Simulated deposits form piles which accumulate at the foot of the volcano and are more similar to sand piles
than natural pyroclastic deposits. Finally, flows simulated with a frictional rheology are not channelised by the drainage
system, but affect all the flanks of the volcano. In addition, their velocity can exceed 150 m s−1, allowing pyroclastic flows to cross interfluves at bends in the valley, affecting areas that would not have been affected
in reality and leaving clear downstream areas that would be covered in reality. Instead, a simple empirical law, a constant
retarding stress (i.e. a yield strength), involving only one free parameter, appears to be much better adapted for modelling
pyroclastic flows. A similar conclusion was drawn for the Socompa debris avalanche simulation (Kelfoun and Druitt, J Geophys
Res 110:B12202, 2005). 相似文献
14.
Probabilistic aftershock hazard assessment II: application of strong ground motion simulations 总被引:1,自引:1,他引:0
Probabilistic aftershock hazard assessment (PAHA) has been introduced by Wiemer (Geophys Res Lett 27:3405–3408, 2000). The method, in its original form, utilizes attenuation relations in evaluating peak ground velocity (PGV) exceedence probability.
We substitute the attenuation relations together with their uncertainties by strong ground motion simulations for a set of
scenarios. The main advantage of such an approach is that the simulations account for specific details of the aftershock source
effects (faulting style, slip distribution, position of the nucleation point, etc.). Mean PGVs and their standard deviations
are retrieved from the simulation results obtained by the new hybrid k-squared source model, and they are used for the PAHA analysis at a station under study. The model chosen for the testing
purposes is inspired by the Izmit A25 aftershock (M
w
= 5.8) that occurred 26 days after the mainshock. The PAHA maps are compared with (1) those obtained by the use of attenuation
relations and (2) the peak values of ten selected strong-motion recordings written by the aftershock at epicentral distances
<50 km. We conclude that, although the overall hazard decay with increasing fault distance is similar, the PAHA maps obtained
by the use of simulations exhibit remanent radiation pattern effect and prolongation in the strike direction due to the directivity
effect pronounced for some of the scenarios. As regard the comparison with real data, we conclude that the PAHA maps agree
with observed peak values due to appropriate attenuation model adopted in the analysis. 相似文献
15.
Probabilistic aftershock hazard assessment I: numerical testing of methodological features 总被引:1,自引:1,他引:0
Probabilistic aftershock hazard assessment (PAHA, Wiemer, Geoph Res Lett 27:3405–3408, 2000), provided for California within the frame of the STEP project, is based on a methodology, two features of which are addressed
in detail: (1) the parameters of Omori’s law and (2) application of attenuation relations in evaluating peak ground velocity
exceedence probability. Concerning the first point, we perform a simple parametric study. We assume the generalized Omori’s
law by Shcherbakov et al. (Geoph Res Lett 31:L11613, 2004), in which characteristic time c scales with aftershock magnitude. The study shows that, among all the parameters, the hazard is most sensitive to the choice
of m* (controlling the overall aftershock productivity) and least sensitive to the scaling of c. We also conclude that the hazard is mainly due to very early (less than 1 day) aftershocks. As regards the second point,
we employ various attenuation relations from different tectonic areas to study their effect on the hazard analysis. We conclude
that the resulting variations are relatively large, comparable to those obtained for varying m*. 相似文献
16.
Liverpool Bay is a region of freshwater influence which receives significant freshwater loading from a number of major English
and Welsh rivers. Strong tidal current flow interacts with a persistent freshwater-induced horizontal density gradient to
produce strain-induced periodic stratification (SIPS). Recent work (Palmer in Ocean Dyn 60:219–226, 2010; Verspecht et al. in Geophys Res Lett 37:L18602, 2010) has identified significant modification to tidal ellipses in Liverpool Bay during stratification due to an associated reduction
in pycnocline eddy viscosity. Palmer (Ocean Dyn 60:219–226, 2010) identified that this modification results in asymmetry in flow in the upper and lower layers capable of permanently transporting
freshwater away from the Welsh coastline via a SIPS pumping mechanism. Observational data from a new set of observations from
the Irish Sea Observatory site B confirm these results; the measured residual flow is 4.0 cm s−1 to the north in the surface mixed layer and 2.4 cm s−1 to the south in the bottom mixed layer. A realistically forced 3D hydrodynamic ocean model POLCOMS succeeds in reproducing
many of the characteristics of flow and vertical density structure at site B and is used to estimate the transport of water
through a transect WT that runs parallel with the Welsh coast. Model results show that SIPS is the dominant steady state,
occurring for 78.2% of the time whilst enduring stratification exists only 21.0% of the year and enduring mixed periods, <1%.
SIPS produces a persistent offshore flow of freshened surface water throughout the year. The estimated net flux of water in
the surface mixed layer is 327 km3 year
−1, of which 281 km3 year−1 is attributable to SIPS periods. Whilst the freshwater component of this flux is small, the net flux of freshwater through
WT during SIPS is significant, the model estimates 1.69 km3 year−1 of freshwater to be transported away from the coast attributable to SIPS periods equivalent to 23% of annual average river
flow from the four catchment areas feeding Liverpool Bay. The results show SIPS pumping to be an important process in determining
the fate of freshwater and associated loads entering Liverpool Bay. 相似文献
17.
In this study, we address the issue of short-term to medium-term probabilistic seismic hazard analysis for two volcanic areas,
Campi Flegrei caldera and Mt. Vesuvius in the Campania region of southern Italy. Two different phases of the volcanic activity
are considered. The first, which we term the pre-crisis phase, concerns the present quiescent state of the volcanoes that is characterized by low-to-moderate seismicity. The second
phase, syn-crisis, concerns the unrest phase that can potentially lead to eruption. For the Campi Flegrei case study, we analyzed the pattern
of seismicity during the 1982–1984 ground uplift episode (bradyseism). For Mt. Vesuvius, two different time-evolutionary models
for seismicity were adopted, corresponding to different ways in which the volcano might erupt. We performed a site-specific
analysis, linked with the hazard map, to investigate the effects of input parameters, in terms of source geometry, mean activity
rate, periods of data collection, and return periods, for the syn-crisis phase. The analysis in the present study of the pre-crisis phase allowed a comparison of the results of probabilistic seismic hazard analysis for the two study areas with those provided
in the Italian national hazard map. For the Mt. Vesuvius area in particular, the results show that the hazard can be greater
than that reported in the national hazard map when information at a local scale is used. For the syn-crisis phase, the main result is that the data recorded during the early months of the unrest phase are substantially representative
of the seismic hazard during the whole duration of the crisis. 相似文献
18.
We present a simple and efficient hybrid technique for simulating earthquake strong ground motion. This procedure is the combination
of the techniques of envelope function (Midorikawa
et al. Tectonophysics 218:287–295, 1993) and composite source model (Zeng
et al. Geophys Res Lett 21:725–728, 1994). The first step of the technique is based on the construction of the envelope function of the large earthquake by superposition
of envelope functions for smaller earthquakes. The smaller earthquakes (sub-events) of varying sizes are distributed randomly,
instead of uniform distribution of same size sub-events, on the fault plane. The accelerogram of large event is then obtained
by combining the envelope function with a band-limited white noise. The low-cut frequency of the band-limited white noise
is chosen to correspond to the corner frequency for the target earthquake magnitude and the high-cut to the Boore’s f
max or a desired frequency for the simulation. Below the low-cut frequency, the fall-off slope is 2 in accordance with the ω2 earthquake source model. The technique requires the parameters such as fault area, orientation of the fault, hypocenter,
size of the sub-events, stress drop, rupture velocity, duration, source–site distance and attenuation parameter. The fidelity
of the technique has been demonstrated by successful modeling of the 1991 Uttarkashi, Himalaya earthquake (Ms 7). The acceptable
locations of the sub-events on the fault plane have been determined using a genetic algorithm. The main characteristics of
the simulated accelerograms, comprised of the duration of strong ground shaking, peak ground acceleration and Fourier and
response spectra, are, in general, in good agreement with those observed at most of the sites. At some of the sites the simulated
accelerograms differ from observed ones by a factor of 2–3. The local site geology and topography may cause such a difference,
as these effects have not been considered in the present technique. The advantage of the technique lies in the fact that detailed
parameters such as velocity-Q structures and empirical Green’s functions are not required or the records of the actual time
history from the past earthquakes are not available. This method may find its application in preparing a wide range of scenarios
based on simulation. This provides information that is complementary to the information available in probabilistic hazard
maps. 相似文献
19.
We consider the origin of rhyolites associated with tholeiitic basalt in bimodal provinces, as exemplified by the Rattlesnake
Tuff of the High Lava Plains of eastern Oregon, in comparison to rhyolites associated with calcalkaline suites in light of
recent models of extraction of rhyolite from crystal mush (Hildreth, J Volcanol Geotherm Res, 136:169–198, 2004; Bachmann and Bergantz, J Petrol, 45:1565–1582, 2004). The High Lava Plains encompass a strongly bimodal, tholeiite-rhyolite suite, spatially and compositionally
related to the Snake River Plain and Yellowstone Plateau. In our assessment we draw the distinction between fractionation
dominated processes to make rhyolites from rhyolites and processes required to make the parental rhyolite melt. New isotopic
data and compositional zoning profiles in phenocrysts confirm that crystal fractionation dominated the generation of progressively
more evolved, discrete rhyolites in the zoned Rattlesnake Tuff and are consistent with an origin of the least evolved high-silica
rhyolites by partial melting of a mafic crust. While the most evolved rhyolites are compositionally virtually indistinguishable
from those of calcalkaline suites, the parental rhyolites from bimodal suites are more Fe-rich than their calcalkaline counterparts.
Oxygen isotope thermometry yields pre-eruptive temperatures of 860°C, in keeping with 800–880°C zircon saturation temperatures.
High magmatic temperatures are common among rhyolites of bimodal suites, distinguishing them from cooler rhyolites of calcalkaline
suites. Extraction of interstitial melt from a granodioritic mush cannot produce compositions of the Rattlesnake Tuff on the
basis of major and trace element arguments (especially Fe, Ba, Sr, and Eu) and on the basis of temperature considerations.
Chemically viable parental crystal mushes are syenite and alkali (A-type) granites for the production of all more evolved
Rattlesnake Tuff rhyolites; ferro-dacitic mush is required for production of the least-evolved, parental Rattlesnake Tuff
rhyolite. Paucity of such ferro-dacitic compositions in tholeiitic bimodal suites, especially compared to the abundance of
dacitic (granodioritic) compositions in calcalkaline suites, argues against the mush extraction model for the parental rhyolite.
Furthermore, rhyolites of bimodal suites lack associated voluminous eruptions of crystal-rich ignimbrite that might represent
a parental mush, as exemplified by the “monotonous intermediate” Fish Canyon Tuff in calcalkaline suites. We conclude that
extensive fractionation is common among rhyolites and may obscure their ancestry. Fe-rich parental rhyolites common in bimodal
tholeiitic suites, as represented by Rattlesnake Tuff, may often be the result of partial melting of mafic to intermediate
crust, in contrast to calcalkaline high-silica rhyolites that are related to voluminous suites of intermediate intrusive rocks
where the pre-plutonic mush-extraction model works better.
This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic
rhyolites. 相似文献
20.
An automatic scheme for baseline correction of strong-motion records in coseismic deformation determination 总被引:2,自引:0,他引:2
Coseismic deformation can be determined from strong-motion records of large earthquakes. Iwan et al. (Bull Seismol Soc Am
75:1225–1246, 1985) showed that baseline corrections are often required to obtain reliable coseismic deformation because baseline offsets lead
to unrealistic permanent displacements. Boore (Bull Seismol Soc Am 91:1199–1211, 2001) demonstrated that different choices of time points for baseline correction can yield realistically looking displacements,
but with variable amplitudes. The baseline correction procedure of Wu and Wu (J Seismol 11:159–170, 2007) improved upon Iwan et al. (Bull Seismol Soc Am 75:1225–1246, 1985) and achieved stable results. However, their time points for baseline correction were chosen by a recursive process with
an artificial criterion. In this study, we follow the procedure of Wu and Wu (J Seismol 11:159–170, 2007) but use the ratio of energy distribution in accelerograms as the criterion to determine the time points of baseline correction
automatically, thus avoiding the manual choice of time points and speeding up the estimation of coseismic deformation. We
use the 1999 Chi-Chi earthquake in central Taiwan and the 2003 Chengkung and 2006 Taitung earthquakes in eastern Taiwan to
illustrate this new approach. Comparison between the results from this and previous studies shows that our new procedure is
suitable for quick and reliable determination of coseismic deformation from strong-motion records. 相似文献