共查询到20条相似文献,搜索用时 46 毫秒
1.
Prantik Mandal 《Pure and Applied Geophysics》2008,165(7):1307-1324
Double difference relocations of the 1402 Kachchh events (2001–2006) clearly delineate two fault zones viz. south-dipping
North Wagad fault (NWF) and almost vertical Gedi fault (GF). The relocated focal depths delineate a marked variation of 4
and 7 km in the brittle-ductile transition depths beneath GF and NWF, respectively. The focal mechanism solutions of 464 aftershocks
(using 8–12 first motions) show that the focal mechanisms ranged between pure reverse and pure strike-slip except for a few
pure dip-slip solutions. The stress inversions performed for five rectangular zones across the Kachchh rift reveal both clockwise
and anticlockwise rotation (7–32°) in the σ1 orientation within the rupture zone, favoring a heterogeneous stress regime with an average N-S fault normal compression.
This rotation may be attributed to the presence of crustal mafic intrusives (5–35 km depth) in the rupture zone of the 2001
Bhuj main shock. Results suggest a relatively homogeneous stress regime in the GF zone favoring strike-slip motion, with a
fault normal N-S compression. 相似文献
2.
We measured and interpreted 30 physical magnetotelluric sounding sites using an SGS-E station and 20 km of electrical profiling
observations using SDVR-4M instrumentation. We constructed a map of seismicity, an interpretation map, and four geoelectric
sections, which give an idea of the deep structure for the Kulu earthquake-generating zone. A general geoelectric upper crustal
model was developed for the zone down to depths of 20–22 km. Three nearly vertical conductive volumes were identified (thickness
3–5 km, depth 10–22 km), which provide the positions of seismically active deep-seated faults that pinpoint the Kulu earthquake-generating
zone. The preliminary boundary of the zone was determined. It was found that earth-quake epicenters are confined to lithosphere
volumes with increased concentrations of conductive layers and zones. 相似文献
3.
Results are reported from a detailed study of central Kamchatka seismicity for the period 1962–1997 based on a modification
of the traditional approach. The approach involves (a) a detailed structure of the seismic region that recognizes the Kronotskii
and Shipunskii geoblocks and two further blocks, the continental slope, and the offshore portion, (b) a study of variations
in the rate of M = 3.0–7.2 earthquakes and the amount of seismic energy released at depths of 0–50 and 51–100 km, (c) a study
of seismicity variability, and (d) separate estimates of the recurrence of crust-mantle earthquakes (depths 0–50 km) and mantle
events (51–100 km). As a result, apart from corroborating the fact of a quiescence preceding the December 5, 1997 Kronotskii
earthquake (M 7.9), we also found that a relationship exists between its beginning and the position of the earthquake-generating
region relative to the mainshock epicenter. The quiescence dominates the seismic process during the pre-mainshock period and
is characterized by a decreased rate of earthquakes (the first feature) and a decreased amount of seismic energy release (the
second feature). Based on the first feature, we found that the quiescence started in 1987 throughout the entire depth range
(0–100 km) in both parts of the Kronotskii geoblock close to the rupture zone of the eponymous earthquake. As to the Shipunskii
geoblock, which is farther from the rupture zone, the quiescence began in the mantle of the inner area first (1988) and somewhat
later at depths of 0–50 km within the continental slope (1989). By the second feature, the quiescence began at shallower depths
in the inner area of the Kronotskii geoblock at the same time and later on (a year later) in the mantle (1988). Under the
continental slope of the trench in the Shipunskii geoblock the shallower quiescence also began in 1987, while it was 3 years
late in the inner zone (1990) and involved the earthquake-generating earth volume at depths of 0–100 km. These data are identical
with or sufficiently close to the estimate for the beginning of this quiescence using a circular area of radius 150 km that
combines the Kronotskii and Shipunskii geoblocks by the RTL method (1990). 相似文献
4.
Ki Young Kim Jung Mo Lee Wooil Moon Chang-Eob Baag Heeok Jung Myung Ho Hong 《Pure and Applied Geophysics》2007,164(1):97-113
In order to investigate the velocity structure of the southern part of the Korean peninsula, seismic refraction profiles were
obtained along a 294-km WNW-ESE line and a 335-km NNW-SSE line in 2002 and 2004, respectively. Seismic waves were generated
by detonating 500–1000 kg explosives in drill holes at depths of 80–150 m. The seismic signals were recorded by portable seismometers
at nominal intervals of 1.5–1.7 km. Separate velocity tomograms were derived from first arrival times using a series expansion
method of travel-time inversion. The raypaths indicate several mid-crust interfaces including those at approximate depths
of 2–3, 15–17, and 22 km. The Moho discontinuity with refraction velocity of 7.8 to 8.4 km/s has a maximum depth of 37–39
km under the southern central portion of the peninsula. The Moho becomes shallower as the Yellow Sea and the East Sea are
approached on the west and east coasts of the peninsula, respectively. The depth of the 7.6 km/s velocity contour varies from
29.4 km to 36.5 km. The discrepancy in depth between the seismological Moho and the interpreted critically refracting interface
may result from the presence of a gradual transition between the crust and mantle. The velocity tomograms show particular
crustal structures including (1) the existence of an over 70-km wide low-velocity zone centered at 6–7 km depth under the
Okchon fold belt and Ryeongnam massif, (2) existence of high-velocity materials under the Gyeongsang basin, and (3) the downward
extension of the Yeongdong fault to depths greater than 10 km. 相似文献
5.
We report results from a detailed study of seismicity in central Kamchatka for the period from 1960 to 1997 using a modified
traditional approach. The basic elements of this approach include (a) segmentation of the seismic region concerned (the Kronotskii
and Shipunskii geoblocks, the continental slope and offshore blocks), (b) studying the variation in the rate of M = 4.5–7.0 earthquakes and in the amount of seismic energy release over time, (c) studying the seismicity variations, (d)
separate estimates of earthquake recurrence for depths of 0–50 and 50–100 km. As a result, besides corroborating the fact
that a quiescence occurred before the December 5, 1997, M = 7.9 Kronotskii earthquake, we also found a relationship between the start of the quiescence and the position of the seismic
zone with respect to the rupture initiation. The earliest date of the quiescence (decreasing seismicity rate and seismic energy
release) was due to the M = 4.5–7.0 earthquakes at depths of 0–100 km in the Kronotskii geoblock (8–9 years prior to the earthquake). The intermediate
start of the quiescence was due to distant seismic zones of the Shipunskii geoblock and the circular zone using the RTL method, combining the Shipunskii and Kronotskii geoblocks (6 years). Based on the low magnitude seismicity (M≥2.6) at depths of 0–70 km in the southwestern part of the epicentral zone (50–100 km from the mainshock epicenter), the quiescence
was inferred to have occurred a little over 3 years (40 months) before the mainshock time and a little over 2 years (25 months)
in the immediate vicinity of the epicenter (0–50 km). These results enable a more reliable identification of other types of
geophysical precursors during seismic quiescences before disastrous earthquakes. 相似文献
6.
A. A. Cheremisin P. V. Novikov I. S. Shnipov V. V. Bychkov B. M. Shevtsov 《Geomagnetism and Aeronomy》2012,52(5):653-663
Lidar observations during 2007–2008 in Kamchatka revealed aerosol layers in the upper stratosphere at heights of 35–50 km and in the mesosphere at heights of 60–75 km. It is well known that forces of gas-kinetic nature, i.e., photophoretic forces, act on aerosol particles that absorb solar radiation and terrestrial IR radiation; these forces can counteract the gravitational force and even lead to the levitation of these particles at particular heights. The accumulation of particles at these heights may lead to the formation of aerosol layers. We calculated these forces for the conditions of lidar observations in Kamchatka. Aerosol layers were observed at heights where particle levitation can occur. Thus, the stratospheric and mesospheric aerosol layers, detected at heights of 30–50 and 60–75 km, respectively, may be due to the effect of the photophoretic force on aerosol particles. 相似文献
7.
Frequency-time analysis was applied to records of Rayleigh surface waves due to teleseismic earthquakes (M ≥ 6.0) to obtain
dispersion curves of fundamental mode phase velocities for paths between 51 pairs of digital seismic stations in the Asian
IRIS networks; the range of periods was 10–200 s. For each of these pairs we derived 1D-shear-velocity sections that fit the
corresponding dispersion curves and image the integral earth structure down to depths of ∼650–700 km beneath each of the interstation
seismic paths, which traverse the area of study in different directions. These structures were used by Yanovskaya (2001) to
derive a 3D-shear-velocity model by 2D tomography, imaging the distribution of the larger horizontal inhomogeneities in the
Central Asia mantle for depths of 50 to 600 km, and to construct 2D-velocity sections for five lines passing through the major
tectonic features of Central Asia. 相似文献
8.
In this receiver function study, we investigate the structure of the crust beneath six seismic broadband stations close to
the Sunda Arc formed by subduction of the Indo-Australian under the Sunda plate. We apply three different methods to analyse
receiver functions at single stations. A recently developed algorithm determines absolute shear-wave velocities from observed
frequency-dependent apparent incidence angles of P waves. Using waveform inversion of receiver functions and a modified Zhu
and Kanamori algorithm, properties of discontinuities such as depth, velocity contrast, and sharpness are determined. The
combination of the methods leads to robust results. The approach is validated by synthetic tests. Stations located on Malaysia
show high-shear-wave velocities (V
S) near the surface in the range of 3.4–3.6 km s − 1 attributed to crystalline rocks and 3.6–4.0 km s − 1 in the lower crust. Upper and lower crust are clearly separated, the Moho is found at normal depths of 30–34 km where it
forms a sharp discontinuity at station KUM or a gradient at stations IPM and KOM. For stations close to the subduction zone
(BSI, GSI and PSI) complexity within the crust is high. Near the surface low V
S of 2.6–2.9 km s − 1 indicate sediment layers. High V
S of 4.2 km s − 1 are found at depth greater than 6 and 2 km at BSI and PSI, respectively. There, the Moho is located at 37 and 40 km depth.
At station GSI, situated closest to the trench, the subducting slab is imaged as a north-east dipping structure separated
from the sediment layer by a 10 km wide gradient in V
S between 10 and 20 km depth. Within the subducting slab V
S ≈ 4.7 km s − 1. At station BSI, the subducting slab is found at depth between 90 and 110 km dipping 20° ± 8° in approximately N 60° E. A
velocity increase in similar depth is indicated at station PSI, however no evidence for a dipping layer is found. 相似文献
9.
A detailed dispersion analysis of Rayleigh waves generated by local earthquakes and occasionally by blasts that occurred in
southern Spain, was undertaken to obtain the shear-wave velocity structure of the region at shallow depth. Our database includes
seismograms generated by 35 seismic events that were recorded by 15 single-component short-period stations from 1990 to 1995.
All these events have focal depths less than 10 km and body-wave magnitudes between 3.0 and 4.0, and they were all recorded
at distances between 40 and 300 km from the epicentre. We analysed a total of 90 source-station Rayleigh-wave paths. The collected
data were processed by standard digital filtering techniques to obtain Rayleigh-wave group-velocity dispersion measurements.
The path-averaged group velocities vary from 1.12 to 2.25 km/s within the 1.0-6.0 s period interval. Then, using a stochastic
inversion approach we obtained 1-D shear-wave velocity–depth models across the study area, which were resolved to a depth
of circa 5 km. The inverted shear-wave velocities range approximately between 1.0 and 3.8 km/s with a standard deviation range
of 0.05–0.16 km/s, and show significant variations from region to region. These results were combined to produce 3-D images
via volumetric modelling and data visualization. We present images that show different shear velocity patterns for the Betic
Cordillera. Looking at the velocity distribution at various depths and at vertical sections, we discuss of the study area
in terms of subsurface structure and S-wave velocity distribution (low velocity channels, basement depth, etc.) at very shallow
depths (0–5 km). Our results characterize the region sufficiently and lead to a correlation of shear-wave velocity with the
different geological units features. 相似文献
10.
The attenuation of coda waves is analysed for nine seismic stations in the area of convergent motion of the Adriatic microplate
and the Dinarides. The frequency dependent coda quality factor of the form Qc = Q0 fn is estimated for up to seven central frequencies (1.5, 3, 6, 9, 12, 18 and 24 Hz) and for 21 successive 30 s long time windows.
Q0 was found to increase from 68–353 for short lapse times of 20–50 s, to 158–373 for lapse times of 90–100 s. Parameter n is
observed to vary between 0.46 and 0.89, with a pronounced tendency to decrease with increasing Q0, and vice versa. Both Q0 and n seem to stabilize for lapse times larger than 50–80 s, indicating that a change in rock properties controlling coda
attenuation occurs at depths of about 100–160 km. The spatial distribution of observed Q0 is well correlated with observed seismicity and inferred tectonic activity. In particular, we observe significant negative
correlation of Q0 with the peak ground acceleration (PGA) estimate for the return period of 475 years. The degree of frequency dependence n,
is the smallest for stations on the islands, where the crust is the thinnest. The largest n is observed over the thickest
crust in the region, where the Moho lies at depths of over 55 km. 相似文献
11.
Corrado Di Martino Maria Luce Frezzotti Federico Lucchi Angelo Peccerillo Claudio A. Tranne Larryn W. Diamond 《Bulletin of Volcanology》2010,72(9):1061-1076
Fluid inclusion studies together with volcanological and petrochemical data allow reconstruction of the magma feeding system
of basaltic-andesitic to andesitic activity during the oldest and intermediate stages of development of Lipari Island (223–81 ka).
A major magma storage zone is active during the overall investigated time span at depths of 22 km, close to the crust-mantle
Moho transition, at which mantle-derived mafic magmas tend to accumulate due to neutral buoyancy conditions. Beneath central-type
volcanoes (M. Mazzacaruso, M. S.Angelo, M. Chirica-Costa d’Agosto), a shallower magma reservoir is located within the upper
crust at 5.5–3.5 km, associated with a major lithological discontinuity. For fissural-type volcanoes (Timpone Ospedale, Monterosa,
M. Chirica), tectonic structures are suggested to influence further magma ascent and storage at mid-crustal depths (∼14 km),
with no ponding at shallower levels. Partial crustal melting processes at the roofs of the deep magma reservoirs (∼17 km)
are invoked to explain the origin of cordierite-bearing lavas beneath M. S.Angelo and M. Chirica-Costa d’Agosto volcanoes,
which were active during the intermediate stages of development of Lipari (105–81 ka). The generation of felsic anatectic
melts in the lower crust could have created density and rheologic barriers to impede the passage of mafic melts and promote
their ponding, with influence on the subsequent evolution of Lipari volcano. 相似文献
12.
S. A. Kovachev I. P. Kuzin L. I. Lobkovskii 《Izvestiya Physics of the Solid Earth》2009,45(9):777-793
The results of detailed seismological observations with bottom seismographs in the Central Kurile segment in August-September,
2006 are discussed. The system of six bottom seismographs was placed on the island slope of the Kurile deep-sea trench southeast
of Urup Island and southwest of the Bussol Strait. Over 230 earthquakes with M
LH = 0.5–5.5 were registered in the area with a radius of 150 km around the center of the observation system at depths up to
300 km during 16 days. Records of 80 earthquakes with hypocenters in the earth crust (h = 0–30 km) beneath the island slope of the Kurile deep-sea trench were first obtained by bottom seismographs. These data
are inconsistent with previous concepts of aseismicity of this zone. The discovery of the unique morphological structure of
the Benioff zone beneath the central Kurile Arc represents the most important result of detailed seismological observations.
The zone consists of an inner seismoactive subzone, which is located beneath the island slope of the arc at depths of 15–210
km, being characterized by an angle of incline of 50° under the latter and crosses the ocean bottom approximately 80 km away
from the trench axis, and outer low-activity subzone. The latter is traceable beyond the trench almost parallel to the inner
zone beginning from a depth of 50 km below the sea bottom up to a depth of approximately 300 km. Due to the slightly lower
incline (∼45°) of the outer subzone, both subzones gradually converge downward. The integral thickness of the Benioff zone
varies from 150 km in its upper part to 125 km at depths of 210–260 km. The medium sandwiched between these subzones is practically
aseismic. The reality of this defined structure is confirmed by the distribution of aftershocks of the earthquake that occurred
on November 15, 2006 (M = 8.3). These seismic events served as foreshocks for the subsequent strong earthquake of January 13, 2007 (M = 8.1) with the hypocenter located beyond the trench under the ocean bottom. Such a structure of this zone within the central
Kurile Arc segment is unique, having no analogues either in the flanks of the Kurile-Kamchatka Arc or other arcs. The results
of detailed seismological observations obtained two months before the first of the catastrophic Central Kurile earthquakes
appeared to be typical for the period of foreshocks (the lower seismic activity of the Simushir block, which hosted the hypocenter
of the earthquake that occurred on November 15, 2006, particularly at depths of 0–50 km, the gentler incline of the recurrence
plot, and other features). 相似文献
13.
Previous studies of the stress regime in the northern Apennines report compression in the outer sector and extension in the
inner sector. In this study we focus on the relationships between the two regimes and, by making use of 54 focal mechanism
solutions, we try to shed some more light on a tectonic setting that appears more complex than the previous models. The focal
mechanisms, computed using the first onset technique, are inverted for stress parameters. The method applied requires subdividing
the volume to be investigated into homogeneous sectors: to comply with the complexity of the area under study, we included
depth as a parameter for sub-zoning. Our results show that the shallow regime (0–10 km of depth) is transtensive even in the
sector previously reported to be compressive. In fact, at a shallow depth, very few thrust focal solutions lie in a spatially
limited sector in the eastern part of the area under study. Just below 10 km in depth, the stress regime converts to transpressional:
stress axes are approximately inverted. Deeper than 45 km, thrust solutions are found. They are not numerous enough to perform
an inversion but indicate the existence of a compressional regime at depth. It is worth noting that a gap of seismicity is
observed in the layer 30–45 km. 相似文献
14.
FU Mingxi HU Shengbiao & WANG Jiyang Institute of Geology & Geophysics Chinese Academy of Sciences Beijing China Correspondence should be addressed to Hu Shengbiao 《中国科学D辑(英文版)》2005,48(6):840-848
Many evidences published in recent years reveal that the thickness,chemical composition and thermal state of lithosphere in eastern North China have ex-perienced dramatic transition during the Phanero-zoic.Comparative study[1—8]of early Paleozoic dia-mond-and xenoliths-bearing kimberlites to Cenozoic mantle peridotite xenoliths-bearing alkali basalts indi-cates that the early Phanerozoic lithosphere is thick and stable to depths within the diamond stability field(180–200km)with depleted ma… 相似文献
15.
A set of two hundred shear-wave velocity models of the crust and uppermost mantle in southeast Europe is determined by application
of a sequence of methods for surface-waves analysis. Group velocities for about 350 paths have been obtained after analysis
of more than 600 broadband waveform records. Two-dimensional surface-wave tomography is applied to the group-velocity measurements
at selected periods and after regionalisation, two sets of local dispersion curves (for Rayleigh and Love waves) are constructed
in the period range 8–40 s. The shear-wave velocity models are derived by applying non-linear iterative inversion of local
dispersion curves for grid cells predetermined by the resolving power of data. The period range of observations limits the
velocity models to depths of 70 km in accordance to the penetration of the surface waves with a maximum period of 40 s. Maps
of the Moho boundary depth, velocity distribution above and below Moho boundary, as well as velocity distribution at different
depths are constructed. Well-known geomorphologic units (e.g. the Pannonian basin, southeastern Carpathians, Dinarides, Hellenides,
Rodophean massif, Aegean Sea, western Turkey) are delineated in the obtained models. Specific patterns in the velocity models
characterise the southeast Carpathians and adjacent areas, coast of Albania, Adriatic coast of southern Italy and the southern
coast of the Black Sea. The models obtained in this study for the western Black Sea basin shows the presence of layers with
shear-wave velocities of 3.5 km/s–3.7 km/s in the crust and thus do not support the hypothesis of existence of oceanic structure
in this region. 相似文献
16.
During the Pamir Himalayan project in the year 1975 seismic refraction and wide-angle reflection data were recorded along
a 270 km long Lawrencepur-Astor (Sango Sar) profile in the northwest Himalayas. The profile starts in the Indus plains and
crosses the Main Central Thrust (MCT), the Hazara Syntaxis, the Main Mantle Thrust (MMT) and ends to the east of Nanga Parbat.
The seismic data, as published by Guerra et al. (1983), are reinterpreted using the travel-time ray inversion method of Zelt and Smith (1992) and the results of inversion
are constrained in terms of parameter resolution and uncertainty estimation. The present model shows that the High Himalayan
Crystallines (HHC, velocity 5.4 km s−1) overlie the Indian basement (velocity 5.8–6.0 km s−1). The crust consists of four layers of velocity 5.8–6.0, 6.2, 6.4 and 6.8 km s−1 followed by the upper mantle velocity of 8.2 km s−1 at a depth of about 60 km. 相似文献
17.
Climate-driven shifts in diatom assemblages recorded in annually laminated sediments of Sacrower See (NE Germany) 总被引:1,自引:1,他引:0
E. P. Kirilova O. Heiri P. Bluszcz B. Zolitschka A. F. Lotter 《Aquatic Sciences - Research Across Boundaries》2011,73(2):201-210
Sacrower See is a eutrophic lake with annually laminated sediments extending back to A.D. 1868. Analysis of annual layers
revealed multi-decadal periods of distinct diatom assemblages at A.D. 1868–1875, 1876–1940, 1941–1978, and 1979–2000. Detrended
correspondence analysis performed on individual seasonal sediment layers showed decadal-scale patterns of turnover in the
diatom flora. The spring–summer layers showed higher sample scores until the early 1960s, after which the differences with
the autumn–winter layers became smaller. Rates-of-change analysis revealed that the seasonal variability in diatom assemblages
was higher than the annual changes. Summer diatom rates of change over the period A.D. 1894–1960 was on average higher than
for winter, whereas between the 1960s and 1970s the winter rates of change became higher than the summer ones. Redundancy
Analyses showed that seasonal temperatures and wind strength were significant explanatory variables for diatom assemblages
in both annual and seasonal layers. These results suggest that meteorological changes indirectly affected diatom assemblages
via the mixing regime of the lake. A comparison of the diatom rates of change with the amplitude of inter-annual climate change
shows a statistically significant correlation for the spring-summer layers in the period of A.D. 1963–2000, showing that the
sensitivity of diatom assemblages to meteorological changes has varied over the past century, with a stronger effect on diatoms
registered during the past 40 years. 相似文献
18.
The first P-arrival time data from local earthquakes are inverted for two-dimensional variation of the depths to the Conrad
and Moho discontinuities in the Kyushu district, southwest Japan. At the same time, earthquake hypocenters and station corrections
are determined from the data. The depths to the discontinuities are estimated by minimizing the travel time residuals of first
P-arrival phases for 608 earthquakes observed at 57 seismic stations. In the land area of Kyushu, the Conrad and Moho discontinuities
are located within the depth ranges of 16–18 and 34–40 km, respectively. The Conrad discontinuity is not as largely undulated
as the Moho discontinuity. The depth to the Moho is deep along the east coast of Kyushu, and the deepest Moho is closely related
to markedly low velocity of P wave. We regard the deepest Moho as reflecting the Kyushu–Palau ridge subducting beneath the
Kyushu district, together with the Philippine Sea slab. In western Kyushu, the shallow Moho is spreading in the north–northeast–south–southwest
direction in the Okinawa trough region. Based on the presence of low-velocity anomaly in three-dimensional velocity structure
and seismogenic stress field of shallow crustal earthquakes, the shallow Moho is interpreted as being due to lower crustal
erosion associated with a small-scale mantle upwelling in the Okinawa trough region. The velocity discontinuity undulation
basically has insignificant effect on hypocenter determination of the local earthquakes, but the Moho topography makes changes
in focal depths of some upper mantle earthquakes. The depth variation of the Moho discontinuity has a good correlation with
the Bouguer gravity anomaly map; i.e., the shallow Moho of western Kyushu and the deep Moho of eastern Kyushu closely correlate
with the positive and negative Bouguer gravity anomalies, respectively. 相似文献
19.
The 1999 Chi-Chi, Taiwan, earthquake (Mw = 7.6) was one of the strongest earthquakes in recent years recorded by a large number of strong-motion devices. Though only
surface records are available, the obtained strong-motion database indicates the variety of ground responses in the near-fault
zones. In this study, accelerograms of the Chi-Chi earthquake were simulated at rock and soil sites, and models of soil behavior
were constructed at seven soil sites (TCU065, TCU072, TCU138, CHY026, CHY104, CHY074, and CHY015), for which parameters of
the soil profiles are known down to depths of at least ~70 m and at 24 other soil sites, for which parameters of the soil
profiles are known down to 30–40 m; all the sites were located within ~50 km from the fault. For reconstructing stresses and
strains in the soil layers, we used a method similar to that developed for the estimation of soil behavior based on vertical
array records. As input for the soil layers, acceleration time histories simulated by stochastic finite-fault modelling with
a prescribed slip distribution over the fault plane were taken. In spite of the largeness of the earthquake’s magnitude and
the proximity of the studied soil sites to the fault plane, the soil behavior at these sites was relatively simple, i.e.,
a fairly good agreement between the spectra of the observed and simulated accelerograms and between their waveforms was obtained
even in cases where a single stress-strain relation was used to describe the behavior of whole soil thickness down to ~70–80
m during strong motion. Obviously, this is due to homogeneity in the characteristics of soil layers in depth. At all the studied
sites, resonant phenomena in soil layers (down to ~40–60 m) and nonlinearity of soil response were the main factors defining
soil behavior. At TCU065, TCU110, TCU115, CHY101, CHY036, and CHY039 liquefaction phenomena occurred in the upper soil layers,
estimated strains achieved ~0.6–0.8%; at other stations, maximum strains in the soil layers were as high as 0.1–0.4%, according
to our estimates. Thus, valuable data on the in situ soil behavior during the Chi-Chi earthquake was obtained. Similarity in the behavior of similar soils during the 1995 Kobe,
2000 Tottori (Japan), and Chi-Chi (Taiwan) earthquakes was found, indicating the possibility of forecasting soil behavior
in future earthquakes. In the near-fault zones of the three earthquakes, “hard-type” soil behavior and resonant phenomena
in the upper surface layers prevail, both leading to high acceleration amplitudes on the surface. 相似文献
20.
Twenty-two peat samples collected at different depths of a core including the layer affected by the 1908 explosion in Tunguska
area of Central Siberia, Russia, and three basalt samples collected near the site, are analyzed by ICP-MS. The concentrations
of Pd, Ni, Co, ΣREE, Ti and Sr in the event layers are 4–35 times higher than the background values in the normal layers.
The variation of Pd is closely related to Ni, Co and ΣREE in the event layers, but not to these elements in the normal layers.
It indicates that these excess elements came from the same source, i.e. the Tunguska explosion body. In addition, the patterns
of Cl-chondrite-normalized REE in the event layers ((La/Yb)N ≈2–3) are much flatter than those in the normal layers ((La/Yb)N ≈7–143), and differ from those in the three basalt samples. The concentrations of REE in the three basalt samples are tens
times higher than those in the event layers. It may be inferred that these excess elements could not be produced by the contamination
of the terrestrial material, but probably by the Tunguska explosion body. Additionally, the ratios of Ti/Ni and Sr/Co in the
event layers are close to those in comet. It implies that the solid part of the explosion body was compositionally similar
to carbonaceous chondrites (Cl) and more probably a small comet. In terms of the Pd excess fluxes in the explosion area, it
can be estimated that the celestial body that exploded over Tunguska in 1908 weighed more than 107 tons, corresponding to a radius of > 126 m. 相似文献