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1.
Ground-motion predictions in Shillong region, northeast India   总被引：1，自引：0，他引：1  
We deliver ground-motion prediction equations for Shillong region, northeast India, based on a database generated by finite-fault stochastic simulations. An examination of the regional seismic source characteristics is carried out beforehand. Micro/minor earthquakes (M W?<?5.0) nucleating at hypocentral depth <21?km in the region recorded at broadband seismic stations are observed to have Brune stress-drop ranging between 2.8 and 99.9?bars. Likewise, macroseismic intensity data for the 1897 Shillong Earthquake that nucleated at a hypocentral depth of ~35?km places the associated stress-drop at 100?200?bars. The apparent variation of the stress-drop parameter with depth is considered with two source zones namely lower-crust and upper-crust. Equations for the lower-crust predict higher ground-motion levels and exhibit affinity to those developed for stable continental region of Eastern North America. The ground-motion levels predicted by the equations for the upper-crust are relatively lower but are still higher compared with those predicted for tectonically active regions, viz., the Himalayas and Western North America.  相似文献   

2.
Generalized inversion of the S-wave amplitude spectra from the strong-motion network data in the East-Central Iran has been used to estimate simultaneously source parameters, site response and the S-wave attenuation (Qs). In this regard, 190 three-component records were used corresponded to 40 earthquakes with the magnitudes M3.5–M7.3. These earthquakes were recorded at 42 stations in the hypocentral distance range from 9 to 200 km. The inverse problem was solved in 20 logarithmically equally spaced points in the frequency band from 0.4 to 15 Hz. The frequency-dependent site amplification was imposed, as a constraint, on two reference site responses in order to remove the undetermined degree of freedom in the inversion and obtain a unique inverse solution. Also, a geometrical spreading factor was assumed for removing the trade-off between geometrical spreading and anelastic attenuation. Different source parameters, such as seismic moment (M0), seismic energy (Es), corner frequency (fc) and Brune stress drop (Δσ), were estimated for each event by fitting an ω2 model to the spectra obtained from the inversion. The stress drop values of earthquakes, obtained in this research, are in good agreement with those of other studies. Also average site response values were correlated to the average shear wave velocities in the uppermost 30 m, in high and low frequency bands. The peak frequencies of site amplifications, estimated by the generalized inversion method, where in good agreement with those of horizontal to vertical (H/V) spectral ratios for the S-wave portion of records. However, no perfect matching in amplitude was obtained due to the deficiencies of the H/V ratio technique. By supposing a free shape for Q factor, a frequency dependent function was found, the logarithm of which could be approximated by a linear function, Q(f)=151f0.75. The uncertainties of model parameters have been evaluated by covariance matrix of least-square fit. The residuals were also analyzed in order to assess the validity of the model. The analysis of residuals with respect to magnitude and distance indicates that they are distributed normally with approximately zero mean. The robustness of the results has been studied concerning their sensitivities to the omission of different datasets, selected randomly from original database. The results obtained here can be used in predicting ground-motion parameters applying stochastic methods.  相似文献   

3.
We useP andS times listed in the International Seismological Summary to relocate 23 historical earthquakes (1927–1963) reported as occurring at or below 670 km. In all cases, our relocated hypocenters are shallower than the starting depths; furthermore, all events converge to 691 km or less, with a precision estimated at ±10 km. This study upholds the results of Stark and Frohlich, who had usedpP–P times of post-WWSSN earthquakes to constrain reliable hypocentral depths to no greater than 684 km. In particular, we reject Rothé's claim that a 1963 event in the vicinity of New Guinea occurred at a depth of more than 780 km.  相似文献   

4.
The Vrancea seismogenic zone in Romania represents a peculiar source of seismic hazard, which is a major concern in Europe, especially to neighboring regions of Bulgaria, Serbia and Republic of Moldavia. Earthquakes in the Carpathian–Pannonian region are confined to the crust, except the Vrancea zone, where earthquakes with focal depth down to 200 km occur. One of the cities most affected by earthquakes in Europe is Bucharest. Situated at 140–170 km distance from Vrancea epicenter zone, Bucharest encountered many damages due to high energy Vrancea intermediate-depth earthquakes; the March 4, 1977 event (Mw=7.2) produced the collapse of 36 buildings with 8–12 levels, while more than 150 old buildings were seriously damaged. A dedicated set of applications and a method to rapidly estimate magnitude in 4–5 s from detection of P wave in the epicenter were developed. They were tested on all recorded data. The magnitude error for 77.9% of total considered events is in the interval [−0.3, +0.3] magnitude units. This is acceptable taking into account that the magnitude is computed from only 3 stations in a 5 s time interval (1 s delay is caused by data packing). The ability to rapidly estimate the earthquake magnitude combined with powerful real-time software, as parts of an early warning system, allows us to send earthquake warning to Bucharest in real time, in about 5 s after detection in the epicenter. This allows 20–27 s warning time to automatically issue preventive actions at the warned facility.  相似文献   

5.
沉积盆地中的中等强度浅源地震起始深度不仅是地震成灾研究的重要参数,还是深地下工程开采诱发地震成因研究的关键信息.然而在台网稀疏、沉积层结构复杂的盆地地区,仅利用直达波到时,常常难以获得较为准确的地震起始深度.基于参考事件的Pn/Pg相对定位方法加入Pn震相,可以有效提高震源深度的测定精度.本文针对松辽盆地中2013年前郭MS5.8地震序列以及Oklahoma州2016年Fairview MW5.1地震序列,利用Pn/Pg方法测定了两个地震序列中较大地震的起始破裂深度.结果显示:在2013年前郭地震序列中,两个较大地震起始深度和质心深度均在3~5 km左右,震源机制中含有较大的非双力偶源成分.而2016年Fairview MW5.1地震,主震起始深度约7 km,质心深度约8 km,5个较大的前震则深度分布在5~9 km.根据上述测定结果,推测2013年11月前郭MS5.8地震序列可能与震源附近的注水活动有关,而2016年Fairview MW5.1地震则可能由较远处注水活动触发.  相似文献   

6.
This study analyzes the S-wave envelope broadening characteristics of 290 earthquakes recorded by 14 stations of the Spanish National Seismograph Network in the Canary Islands region. The S-wave peak delay time (t p ) and envelope duration (t q ) parameters are evaluated phenomenologically to infer the strength of velocity inhomogeneities of the medium along each seismic ray path. Crustal (0?≤?h?≤?18 km) and upper mantle (18?<?h?≤?80 km) events are analyzed separately. Results in the frequency range 1 to 12 Hz for hypocentral distances from 30 to 600 km show that both t p and t q increase according to a power of hypocentral distance and they are independent of frequency. The spatial distribution of the peak delay time reveals weak strength of heterogeneity in most of the region at shallow depths. Relatively strong inhomogeneous zones are generated under the island of Tenerife and Gran Canaria at depths of 11–22 km.  相似文献   

7.
Strong-motion data from eight significant well-documented earthquakes in Iran have been simulated using a stochastic modeling technique for finite faults proposed by Beresnev and Atkinson [Bull Seismol Soc Am 87 (1997) 67–84; Seism Res Lett 69 (1998) 27–32]. The database consists of 61 three-component records from eight earthquakes of magnitude ranging from M 6.3 to M 7.4, recorded at hypocentral distances up to 200 km. The model predictions are in good agreement with available Iranian strong-motion data as evidenced by near-zero average of differences between logarithms of the observed and predicted values for all frequencies. The strength factor, sfact, a quantity that controls the high-frequency radiation from the source is determined, on an event-by-event basis, by fitting simulated to observed response spectra.  相似文献   

8.
Recent improvements in the seismological networks on the Ibero-Maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at South Spain, Alboran Sea and northern Morocco of deep and intermediate depth, with magnitudes between 3.5 and 4.5. Intermediate depth shocks, range from 60 to 100 km, with greater concentration located between Granada and Málaga. Fault-plane solutions of 5 intermediate shocks have been determined; they present a vertical plane in NE-SW or E-W direction. Seismic moments of about 1015 Nm and dimensions of about 1 km have been determined from digital records of Spanish stations.P-wave forms are complex. This may be explained by the crustal structure near the station, discontinuities in the upper mantle and inhomogeneities near the source. Deep activity at about 650 km has only 3 shocks since 1954 (1954, 1973, 1990). Shocks are located at a very small region. Fault-plane solutions show a consistent direction of the pressure axis dipping 45° in E direction. For the 1990 shock seismic moment is 1016 Nm and dimensions 2.6 km. TheP-waves are of simpler form with a single pulse. The intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km. The intermediate shocks may be explained in terms of a recent subduction from Africa under Iberia in SE direction. The very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes.  相似文献   

9.
Bucharest is one of the cities most affected by earthquakes in Europe. Situated at 150–170 km distance from Vrancea epicentral zone, Bucharest had suffered many damages due to high energy Vrancea intermediate-depth earthquakes. For example, the 4 March 1977 event produced the collapse of 32 buildings with 8–12 levels, while more than 150 old buildings with 6–9 levels were seriously damaged. The studies done after this earthquake had shown the importance of the surface geological structure upon ground motion parameters. New seismic measurements are performed in Bucharest area aiming at defining better elastic and dynamic properties of the shallow sedimentary rocks. Down-hole seismic measurements were performed in a number of 10 cased boreholes drilled in the Bucharest City area. Processing and interpretation of the data lead to the conclusion that shallow sedimentary rocks can be considered weak in the area, down to 150–200 m depth. Seismic wave velocity values and bulk density values presented in the paper associated with local geology are useful primary data in the seismic microzonation of Bucharest City. They are used as 1D models to derive transfer functions and response spectra for the stack of sedimentary rocks in several parts of Bucharest area, leading to a better knowledge of the local site amplification and associated frequency spectra. In a recent study the H/V spectral ratio using Nakamura’s method was applied on the seismic noise measurements in 22 sites in Bucharest City in order to derive the fundamental period associated with these sites. The values confirm the previous results, showing a dominant resonance in the period range of 1.25–1.75 s. The fundamental periods obtained with Nakamura’s method are in good agreement with those computed on the basis of geological and geotechnical data in boreholes, which show an increase of the fundamental period in the Bucharest area from south to north, in the same direction as the increase of the thickness of the Quaternary deposits above the Fratesti layer which is considered the bedrock in the area.  相似文献   

10.
Tarzana station is located in the foothills of the Santa Monica Mountains in California near the crest of a low (<20 m) natural hill with gentle slopes. The hill is about 500 m in length by 130 m in width and is formed of extremely weathered shale at the surface to fresh at depth. Average S-wave is about 250 m/s in the top 17–18 m, and S- and P-wave velocities significantly increase below this depth. According to the NEHRP classification based on VS30∼300 m/s it is a site class D. Strong-motion instrumentation at Tarzana consisted of an accelerograph at the top of the hill, a downhole instrument at 60 m depth, and an accelerograph at the base of the hill. More than 20 earthquakes were recorded by at least three instruments at Tarzana from 1998 till 2003. Comparisons of recordings and Fourier spectra indicate strong directional resonance in a direction perpendicular to the strike of the hill. The dominant peaks in ground motion amplification on the top of the hill relative to the base are at frequencies ∼3.6 and 8–9 Hz for the horizontal components. Our hypothesis is that the hill acts like a wave trap. This results in an amplification at predominant frequencies f=V/4 h (h is layer's thickness) at f∼3.6 Hz for S-waves (using average VS17=246 m/s and h=17 m) and f∼7.9 Hz for P-waves (using average VP17=535 m/s and h=17 m). As was shown by Bouchon and Barker [Seismic response of a hill: the example of Tarzana, California. Bull Seism Soc Am 1996;86(1A):66–72], topography of this hill amplifies and polarizes ground motion in the frequency range of 3–5 Hz. Hill acts as a magnifying polarizing glass: It polarizes ground motion in the direction perpendicular to the strike of the hill and also amplifies ground motions that had been also amplified by a low-velocity layer.  相似文献   

11.
—Broad band digital three-component data recorded at UNM, a GEOSCOPE station, were used to estimate Lg coda Q for 34 medium size (3.9 ≤m b ≤ 6.3) earthquakes with travel paths laying in different geological provinces of southern Mexico in an effort to establish the possible existence of geological structures acting as wave guides and/or travel paths of low attenuation between the Pacific coast and the Valley of Mexico. The stacked spectral ratio method proposed by XIE and NUTTLI (1988) was chosen for computing the coda Q. The variation range of Q 0 (Q at 1?Hz) and the frequency dependence parameter η estimates averaged on the frequency interval of 0.5 to 2?Hz for the regions and the three components considered are: i) Guerrero region 173 ≤Q¯ 0≤ 182 and 0.6 ≤Q¯ 0 ≤ 0.7, ii) Oaxaca region 183 ≤Q¯ 0 ≤ 198 and 0.6 ≤Q¯ 0 ≤ 0.8, iii) Michoacan-Jalisco region 187 ≤Q¯ 0 ≤ 204 and 0.7 ≤Q¯ 0 ≤ 0.8 and iv) eastern portion of the Transmexican Volcanic Belt (TMVB) 313 ≤Q 0≤ 335 and η = 0.9. ¶The results show a very high coda Q for the TMVB as compared to other regions of southern Mexico. This unexpected result is difficult to reconcile with the geophysical characteristics of the TMVB, e.g., low seismicity, high volcanic activity and high heat flow typical of a highly attenuating (low Q) region. Visual inspection of seismograms indicates that for earthquakes with seismic waves traveling along the TMVB, the amplitude decay of Lg coda is anomalously slow as compared to other earthquakes in southern Mexico. Thus, it seems that the high Q value found does not entirely reflect the attenuation characteristics of the TMVB but it is probably contaminated by a wave-guide effect. This phenomenon produces an enhancement in the time duration of the Lg wave trains travelling along this geological structure. This result is important to establish the role played by the transmission medium in the extremely long duration of ground motion observed during the September 19, 1985 Michoacan earthquake. ¶The overall spatial distribution of coda Q values indicates that events with focus in the Michoacan-Jalisco and Oaxaca regions yield slightly higher values than those from Guerrero. This feature is more pronounced for the horizontal component of coda Q. A slight dependence of average coda Q ?1 on earthquake focal depth is observed in the frequency range of 0.2 to 1.0?Hz approximately on the horizontal component. Deeper (h > 50?km) events yield lower values of Q ?1 than shallower events. For frequencies higher than 1.0?Hz no clear dependence of Q ?1 on focal depth is observed. However, due to the estimates uncertainties this result is not clearly established.  相似文献   

12.
ＧｅｏｇｒａｐｈｉｃａｌｄｉｓｔｒｉｂｕｔｉｏｎｏｆｈｙｐｏｃｅｎｔｒａｌｄｅｐｔｈｓｏｆＣｈｉｎｅｓｅｅａｒｔｈｑｕａｋｅｓＸＩＮＧ－ＢＥＩＤＵＡＮ（段星北）ＩｎｓｔｉｔｕｔｅｏｆＧｅｏｐｈｙｓｉｃａｌ，ＳｔａｔｅＳｅｉｓｍｏｌｏｇｉｃａｌＢｕｒ...  相似文献   

13.
The three-dimensional P-wave velocity structure beneath the Katmai group of volcanoes is determined by inversion of more than 10,000 rays from over 1000 earthquakes recorded on a local 18 station short-period network between September 1996 and May 2001. The inversion is well constrained from sea level to about 6 km below sea level and encompasses all of the Katmai volcanoes; Martin, Mageik, Trident, Griggs, Novarupta, Snowy, and Katmai caldera. The inversion reduced the average RMS travel-time error from 0.22 s for locations from the standard one-dimensional model to 0.13 s for the best three-dimensional model. The final model, from the 6th inversion step, reveals a prominent low velocity zone (3.6–5.0 km/s) centered at Katmai Pass and extending from Mageik to Trident volcanoes. The anomaly has values about 20–25% slower than velocities outboard of the region (5.0–6.5 km/s). Moderately low velocities (4.5–6.0 km/s) are observed along the volcanic axis between Martin and Katmai Caldera. Griggs volcano, located about 10 km behind (northwest of) the volcanic axis, has unremarkable velocities (5.0–5.7 km/s) compared to non-volcanic regions. The highest velocities are observed between Snowy and Griggs volcanoes (5.5–6.5 km/s). Relocated hypocenters for the best 3-D model are shifted significantly relative to the standard model with clusters of seismicity at Martin volcano shifting systematically deeper by about 1 km to depths of 0 to 4 km below sea level. Hypocenters for the Katmai Caldera are more tightly clustered, relocating beneath the 1912 scarp walls. The relocated hypocenters allow us to compare spatial frequency-size distributions (b-values) using one-dimensional and three-dimensional models. We find that the distribution of b is significantly changed for Martin volcano, which was characterized by variable values (0.8 < b < 2.0) with standard locations and more uniform values (0.8 < b < 1.2) after relocation. Other seismic clusters at Mageik (1.2 < b < 2.2), Trident (0.5 < b < 1.5) and Katmai Caldera (0.8 < b < 1.8) had stable b-values indicating the robustness of the observations. The strong high b-value region at Mageik volcano is mainly associated with an earthquake swarm in October, 1996 that possibly indicates a shallow intrusion or influx of gas. The new velocity and spatial b-value results, in conjunction with prior gravity (Bouguer anomalies up to − 40 mgal) and interferometry (several cm uplift) data, provide strong evidence in favor of partially molten rock at shallow depths beneath the Mageik–Katmai–Novarupta region. Moderately low velocities beneath Martin and Katmai suggest that old, mostly solidified intrusions exist beneath these volcanoes. Higher relative velocities beneath the Griggs and Snowy vents suggest that no magma is resident in the shallow crust beneath these volcanoes.  相似文献   

14.
Blooms of the toxic dinoflagellates, Karenia spp. occur nearly annually in the eastern Gulf of Mexico with cell abundances typically >105 cells L−1. Thermal and ocean color satellite imagery shows sea surface temperature patterns indicative of upwelling events and the concentration of chlorophyll at fronts along the west Florida continental shelf. Daily cell counts of Karenia show greater increases in cell concentrations at fronts than can be explained by Karenia's maximum specific growth rate. This is observed in satellite images as up to a 10-fold greater increase in chlorophyll biomass over 1–2 d periods than can be explained by in situ growth. In this study, we propose a model that explains why surface blooms of Karenia may develop even when nutrients on the west Florida shelf are low. In the summer, northward winds produce a net flow east and southeast bringing water and nutrients from the Mississippi River plume onto the west Florida shelf at depths of 20–50 m. This water mass supplies utilizable inorganic and organic forms of nitrogen that promote the growth of Karenia to pre-bloom concentrations in sub-surface waters in the mid-shelf region. In the fall, a change to upwelling favorable winds produces onshore transport. This transport, coupled with the swimming behavior of Karenia, leads to physical accumulation at frontal regions near the coast, resulting in fall blooms. Strong thermal fronts during the winter provide a mechanism for re-intensification of the blooms, if Karenia cells are located north of the fronts. This conceptual model leads to testable hypotheses on bloom development throughout the Gulf of Mexico.  相似文献   

15.
Kaguyak Caldera lies in a remote corner of Katmai National Park, 375 km SW of Anchorage, Alaska. The 2.5-by-3-km caldera collapsed ~ 5.8 ± 0.2 ka (14C age) during emplacement of a radial apron of poorly pumiceous crystal-rich dacitic pyroclastic flows (61–67% SiO2). Proximal pumice-fall deposits are thin and sparsely preserved, but an oxidized coignimbrite ash is found as far as the Valley of Ten Thousand Smokes, 80 km southwest. Postcaldera events include filling the 150-m-deep caldera lake, emplacement of two intracaldera domes (61.5–64.5% SiO2), and phreatic ejection of lakefloor sediments onto the caldera rim. CO2 and H2S bubble up through the lake, weakly but widely. Geochemical analyses (n = 148), including pre-and post-caldera lavas (53–74% SiO2), define one of the lowest-K arc suites in Alaska. The precaldera edifice was not a stratocone but was, instead, nine contiguous but discrete clusters of lava domes, themselves stacks of rhyolite to basalt exogenous lobes and flows. Four extracaldera clusters are mid-to-late Pleistocene, but the other five are younger than 60 ka, were truncated by the collapse, and now make up the steep inner walls. The climactic ignimbrite was preceded by ~ 200 years by radial emplacement of a 100-m-thick sheet of block-rich glassy lava breccia (62–65.5% SiO2). Filling the notches between the truncated dome clusters, the breccia now makes up three segments of the steep caldera wall, which beheads gullies incised into the breccia deposit prior to caldera formation. They were probably shed by a large lava dome extruding where the lake is today.  相似文献   

16.
Massachusetts Bay, a semi-enclosed embayment (50×100 km) in the Northwest Atlantic, is the focus of a monitoring program designed to measure the effects of relocating the Boston Harbor sewage outfall to a site 15 km offshore in Massachusetts Bay. The Massachusetts Water Resources Authority (MWRA) in situ monitoring program samples selected stations up to 17 times per year to observe seasonal changes in phytoplankton biomass and other water quality variables. We investigated the feasibility of augmenting the monitoring data with satellite ocean color data to increase the spatial and temporal resolution of quantitative phytoplankton measurements. In coastal regions such as Massachusetts Bay, ocean color remote sensing can be complicated by in-water constituents whose concentrations vary independently of phytoplankton and by inaccurate modeling of absorbing aerosols that tend to be concentrated near the coast. An evaluation of in situ and sea-viewing wide field-of-view sensor (SeaWiFS) measurements from 1998 to 2005 demonstrated that SeaWiFS overestimated chlorophyll a mainly due to atmospheric correction errors that were amplified by absorption from elevated concentrations of chlorophyll a and colored dissolved organic matter. Negative water-leaving radiances in the 412 nm band, an obvious artifact of inadequate atmospheric correction, were recorded in approximately 60–80% of the cloud-free images along the coast, while the remaining portions of the Bay only experience negative radiances 35–55% of the time with a clear nearshore to offshore decrease in frequency. Seasonally, the greatest occurrences of negative 412 nm radiances were in November and December and the lowest were recorded during the summer months. Concentrations of suspended solids in Massachusetts Bay were low compared with other coastal regions and did not have a significant impact on SeaWiFS chlorophyll a measurements. A regional empirical algorithm was developed to correct the SeaWiFS data to agree with in situ observations. Monthly SeaWiFS composites illustrated the spatial extent of a bimodal seasonal pattern, including prominent spring and fall phytoplankton blooms; and the approximate 115 cloud-free scenes per year revealed interannual variations in the timing, magnitude and duration of phytoplankton blooms. Despite known artifacts of SeaWiFS in coastal regions, this study provided a viable chlorophyll a product in Massachusetts Bay that significantly increased the spatial and temporal synoptic coverage of phytoplankton biomass, which can be used to gain a comprehensive ecosystem-wide understanding of phytoplankton dynamics at event, seasonal and interannual timescales.  相似文献   

17.
As part of the ECOHAB: Florida Program, we studied three large blooms of the harmful bloom forming dinoflagellate Karenia brevis. These blooms formed on the West Florida Shelf during Fall of 2000 off Panama City, and during Fall 2001 and Fall 2002 off the coastline between Tampa Bay and Charlotte Harbor. We suggest that these blooms represent two different stages of development, with the 2000 and 2001 blooms in an active growth or maintenance phase and the 2002 bloom in the early bloom initiation phase. Each bloom was highly productive with vertically integrated primary production values of 0.47–0.61, 0.39–1.33 and 0.65 g C m−2 d−1 for the 2000, 2001 and 2002 K. brevis blooms, respectively. Carbon specific growth rates were low during each of these blooms with values remaining fairly uniform with depth corresponding to generation times of 3–5 days. Nitrogen assimilation by K. brevis was highest during 2001 with values ranging from 0.15 to 2.14 μmol N L−1 d−1 and lower generally for 2000 and 2002 (0.01–0.64 and 0.66–0.76 μmol N L−1 d−1 for 2000 and 2002, respectively). The highest K. brevis cell densities occurred during the 2001 bloom and ranged from 400 to 800 cells mL−1. Cell densities were lower for each of the 2000 and 2002 blooms relative to those for 2001 with densities ranging from 100 to 500 cells mL−1. The 2000 and 2001 blooms were dominated by K. brevis in terms of its contribution to the total chlorophyll a (chl a) pool with K. brevis accounting generally for >70% of the observed chl a. For those populations that were dominated by K. brevis (e.g. 2000 and 2001), phytoplankton C biomass (Cp,0) constituted <30% of the total particulate organic carbon (POC). However, in 2002 when diatoms and K. brevis each contributed about the same to the total chl a, Cp,0 was >72% of the POC. The fraction of the total chl a that could be attributed to K. brevis was most highly correlated with POC, chl a and salinity. Nitrogen assimilation rate and primary production were highly correlated with a greater correlation coefficient than all other comparisons.  相似文献   

18.
The 2013 Aigion earthquake swarm that took place in the west part of Corinth Gulf is investigated for revealing faulting and seismicity properties of the activated area. The activity started on May 21 and was appreciably intense in the next 3 months. The recordings of the Hellenic Unified Seismological Network (HUSN), which is adequately dense around the affected area, were used to accurately locate 1501 events. The double difference (hypoDD) technique was employed for the manually picked P and S phases along with differential times derived from waveform cross-correlation for improving location accuracy. The activated area with dimensions 6?×?2 km is located approximately 5 km SE of Aigion. Focal mechanisms of 77 events with M?≥?2.0 were determined from P wave first motions and used for the geometry identification of the ruptured segments. Spatio-temporal distribution of earthquakes revealed an eastward and westward hypocentral migration from the starting point suggesting the division of the seismic swarm into four major clusters. The hypocentral migration was corroborated by the Coulomb stress change calculation, indicating that four fault segments involved in the rupture process successively failed by stress change encouragement. Examination of fluid flow brought out that it cannot be unambiguously considered as the driving mechanism for the successive failures.  相似文献   

19.
《Geofísica Internacional》2013,52(2):173-196
An analysis of local and regional data produced by the shallow, thrust Ometepec-Pinotepa Nacional earthquake (Mw 7.5) of 20 March 2012 shows that it nucleated at 16.254°N 98.531°W, about 5 km offshore at a depth of about 20 km. During the first 4 seconds the slip was relatively small. It was followed by rupture of two patches with large slip, one updip of the hypocenter to the SE and the other downdip to the north. Total rupture area, estimated from inversion of near-source strong-motion recordings, is ~25 km × 60 km. The earthquake was followed by an exceptionally large number of aftershocks. The aftershock area overlaps with that of the 1982 doublet (Mw 7.0, 6.9). However, the seismic moment of the 2012 earthquake is ~3 times the sum of the moments of the doublet, indicating that the gross rupture characteristics of the two earthquake episodes differ. The small-slip area near the hypocenter and large-slip areas of the two patches are characterized by relatively small aftershock activity. A striking, intense, linear NE alignment of the aftershocks is clearly seen. The radiated energy to seismic moment ratios, (Es/M0), of five earthquakes in the region reveal that they are an order of magnitude smaller for near-trench earthquakes than those that occur further downdip (e.g., 2012 and the 1995 Copala earthquakes). The near-trench earthquakes are known to produce low Amax. The available information suggests that the plate interface in the region can be divided in three domains. (1) From the trench to a distance of about 35 km downdip. In this domain M~6 to 7 earthquakes with low values of (Es/M0) occur. These events generate large number of aftershocks. It is not known whether the remaining area on this part of the interface slips aseismically (stable sliding) or is partially locked. (2) From 35 to 100 km from the trench. This domain is seismically coupled where stick-slip sliding occurs, generating large earthquakes. Part of the area is probably conditionally stable. (3) From 100 to 200 km from the trench. In this domain slow slip events (SSE) and nonvolcanic tremors (NVT) have been reported.The earthquake caused severe damage in and near the towns of Ometepec and Pinotepa Nacional. The PGA exceeded 1 g at a soft site in the epicentral region. Observed PGAs on hard sites as a function of distance are in reasonable agreement with the expected ones from ground motion prediction equations derived using data from Mexican interplate earthquakes. The earthquake was strongly felt in Mexico City. PGA at CU, a hard site in the city, was 12 gal. Strong-motion recordings in the city since 1985 demonstrate that PGAs during the 2012 earthquake were not exceptional, and that similar motion occurs about once in three years.  相似文献   

20.
We present analyses of two swarms of long-period (LP) earthquakes at > 30 km depth that accompanied the geodetically observed 2002–2005 Mauna Loa intrusion. The first LP earthquake swarm in 2002 consisted of 31 events that were precursory and preceded the start of Mauna Loa inflation; the second LP swarm of two thousand events occurred from 2004–2005. The rate of LP earthquakes slowed significantly coincident with the occurrence of the December 26, 2004 Mw 9.3 Sumatra earthquake, suggesting that the seismic waves from this great earthquake may have had a dynamic triggering effect on the behavior of Mauna Loa's deep magma system. Using waveform cross correlation and double difference relocation, we find that a large number of earthquakes in each swarm are weakly similar and can be classified into two families. The relocated hypocenters for each family collapse to compact point source regions almost directly beneath the Mauna Loa intrusion. We suggest that the observed waveform characteristics are compatible with each family being associated with the resonance of a single fluid filled vertical crack of fixed geometry, with differences in waveforms between events being produced by slight variations in the trigger mechanism. If these LP earthquakes are part of the primary magma system that fed the 2002–2005 intrusion, as indicated by the spatial and temporal associations between mantle seismicity and surface deformation, then our results raise the possibility that this magma system may be quite focused at these depths as opposed to being a diffuse network. It is likely that only a few locations of Mauna Loa's deep magma system met the geometric and fluid dynamic conditions for generating LP earthquakes that were large enough to be recorded at the surface, and that much of the deep magma transfer associated with the 2002–2005 intrusion occurred aseismically.  相似文献   

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