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1.
Lateral migration of magma away from Miyakejima volcanic island, Japan, generated summit subsidence, associated with summit explosions in the summer of 2000. An earthquake swarm beneath Miyakejima began on the evening of 26 June 2000, followed by a submarine eruption the next morning. Strong seismic activity continued under the sea from beneath the coast of Miyakejima to a few tens of kilometers northwest of the island. Summit eruptive event began with subsidence of the summit on 8 July and both explosions and subsidence continued intermittently through July and August. The most intense eruptive event occurred on 18 August and was vulcanian to subplinian in type. Ash lofted into the stratosphere fell over the entire island, and abundant volcanic bombs were erupted at this time. Another large explosion took place on 29 August. This generated a low-temperature pyroclastic surge, which covered a residential area on the northern coast of the island. The total volume of tephra erupted was 9.3×106 m3 (DRE), much smaller than the volume of the resulting caldera (6×108 m3). Migration of magma away from Miyakejima was associated with crustal extension northwest of Miyakejima and coincident shrinkage of Miyakejima Island itself during July–August 2000. This magma migration probably caused stoping of roof rock into the magma reservoir, generating subsurface cavities filled with hydrothermal fluid and/or magmatic foam and formation of a caldera (Oyama Caldera) at the summit. Interaction of hydrothermal fluid with ascending magma drove a series of phreatic to phreatomagmatic eruptions. It is likely that new magma was supplied to the reservoir from the bottom during waning stage of magmas migration, resulting in explosive discharge on 18 August. The 18 August event and phreatic explosions on 29 August produced a conduit system that allowed abundant SO2 emission (as high as 460 kg s–1) after the major eruptive events were over. At the time of writing, inhabitants of the island (about 3,000) have been evacuated from Miyakejima for more than 3 years. 相似文献
2.
Takayuki Kaneko Atsushi Yasuda Taketo Shimano Setsuya Nakada Toshitsugu Fujii Toshihiko Kanazawa Azusa Nishizawa Yoshihiro Matsumoto 《Bulletin of Volcanology》2005,67(3):243-253
During the early part of a seismic swarm preceding eruption and caldera formation at Miyakejima Volcano, discoloured sea surfaces were observed 1.5 km off the western coast of Miyakejima on 27 June 2000. A later survey of the area using a multi-beam side scan sonar and a remotely operated small submarine revealed four craters of 20–30 m diameter aligned east-west in a 100×10–30 m area on the seafloor, with hot water at 140°C being released from one of the centres. Each crater consists of submarine spatter overlain in part by scoria lapilli. Dredged spatter from the craters was fresh, and there was no evidence of activity of marine organisms on the spatter surface, indicating that the discoloured sea surface resulted from magmatic eruption on the seafloor. This eruption occurred when a westward-propagating seismic swarm, initiated beneath Miyakejimas summit, passed through the area. Finding new magma on the seafloor demonstrates that this seismic swarm was associated with intruding magma, moving outward from beneath Miyakejima. Submarine spatter shows flattened shapes with a brittle crust formed by cooling in water, and its composition is aphyric andesite of 54 wt% SiO2. The spatter is similar in whole rock and mineral composition to spatter erupted in 1983. However, the wide range of Cl in melt inclusions in plagioclase of the 27 June submarine spatter shows that it is not simply a remnant of the 1983 magma, which has only high Cl melt inclusions in plagioclase. The mixed character of melt inclusions suggests involvement of a magma with low Cl melt inclusions. The magma erupted explosively on 18 August from Miyakejimas summit, considered as the second juvenile magma in this eruption, contains low Cl melt inclusions in plagioclase. Based on these observations and the eruption sequence, we present the following model: (1) A shallow magma chamber was filled with a remnant of 1983 magma that had evolved to a composition of 54–55 wt% SiO2. (2) Injection of the 18 August magma into this chamber generated a mixed magma having a wide range of Cl in melt inclusions contained plagioclase. The magma mixing might have occurred shortly before the submarine eruption and could have been a trigger for the initiation of the removal of magma from the chamber as an extensive dyke, which eventually led to caldera subsidence.Editorial responsibility: S Nakada, T Druitt 相似文献
3.
Akira Takada 《Bulletin of Volcanology》1997,58(7):539-556
Many basaltic and andesitic polygenetic volcanoes have cyclic eruptive activity that alternates between a phase dominated
by flank eruptions and a phase dominated by eruptions from a central vent. This paper proposes the use of time-series diagrams
of eruption sites on each polygenetic volcano and intrusion distances of dikes to evaluate volcano growth, to qualitatively
reconstruct the stress history within the volcano, and to predict the next eruption site. In these diagrams the position of
an eruption site is represented by the distance from the center of the volcano and the clockwise azimuth from north. Time-series
diagrams of Mauna Loa, Kilauea, Kliuchevskoi, Etna, Sakurajima, Fuji, Izu-Oshima, and Hekla volcanoes indicate that fissure
eruption sites of these volcanoes migrated toward the center of the volcano linearly, radially, or spirally with damped oscillation,
occasionally forming a hierarchy in convergence-related features. At Krafla, terminations of dikes also migrated toward the
center of the volcano with time. Eruption sites of Piton de la Fournaise did not converge but oscillated around the center.
After the convergence of eruption sites with time, the central eruption phase is started. The intrusion sequence of dikes
is modeled, applying crack interaction theory. Variation in convergence patterns is governed by the regional stress and the
magma supply. Under the condition that a balance between regional extension and magma supply is maintained, the central vent
convergence time during the flank eruption phase is 1–10 years, whereas the flank vent recurrence time during the central
eruption phase is greater than 100 years owing to an inferred decrease in magma supply. Under the condition that magma supply
prevails over regional extension, the central vent convergence time increases, whereas the flank vent recurrence time decreases
owing to inferred stress relaxation. Earthquakes of M≥6 near a volcano during the flank eruption phase extend the central
vent convergence time. Earthquakes during the central eruption phase promote recurrence of flank eruptions. Asymmetric distribution
of eruption sites around the flanks of a volcano can be caused by local stress sources such as an adjacent volcano.
Received: 18 March 1996 / Accepted: 14 January 1997 相似文献
4.
A major seismic swarm occurred near Parícutin volcano between the end of May and early July 2006. More than 700 earthquakes
with magnitude (M
L
) exceeding 2.4 were located. Parícutin, located in the Michoacán–Guanajuato volcanic field in western Mexico, is well known
as the site of the 1943 eruption in which a new 400 m cinder cone was constructed in what had been farmland. The 2006 swarm
exhibits all of the characteristics typically associated with swarms of volcanic origins. The earthquake rate showed the typical
ramp up and ramp down over the course of several days. Magnitudes were evenly distributed in time with a notably high b-value
of 2.45. The earthquake locations cluster around a northeast-striking trend extending approximately 6 km. Over the first two
weeks, hypocenters migrated steadily a few hundred meters per day, rising from 9 to 5 km depth and moving northeast about
5 km. On approximately June 7, the ascent of hypocenters stalled. For the next three weeks, hypocenters held their depth while
migrating laterally back to the southwest. Focal mechanisms during the first part of the swarm reflected the increased stress
caused by dike inflation. Following June 7, the stress orientation changed and became more consistent with the inflation of
horizontal sill-like structures. Though only limited information is available from the seismic swarm preceding the 1943 eruption,
several features, including the swarm duration and magnitude relationships, were comparable to those of the 2006 episode.
The strong indicators of a magmatic origin to the 2006 swarm suggest that at this location there are few, if any, traditional
seismic discriminants that could be used to distinguish which seismic swarms and dike emplacement events might culminate in
eruption. 相似文献
5.
The model for the 2000 dike intrusion event between Kozushima and Miyakejima volcano, Japan, was reinvestigated. After the sudden earthquake swarm in Miyakejima volcano, a dike intrusion of large volume was detected by the nationwide GPS network (Geonet). The displacements detected with GPS stations over an area with a radius of about 200 km shows a distribution that is consistent with the dike source being located near Miyakejima volcano.The dike was intruded northwestwards between Miyakejima and the neighboring Kozushima volcano. We searched for the parameters in the models that reproduce the regional displacements due to dike intrusion between Miyakejima and Kozushiima islands. We tested three models, (1) the model with a single dike, (2) the model with a dike and a point dislocation source which represents a creep dislocation source and (3) the model with a dike and a deflation source which represents a magma reservoir. Though all three models can match the horizontal displacements near the source area, model 1 fails to reproduce the regional displacements in the central part of Japan. Both models 2 and 3 can reproduce the regional displacement for horizontal components. Model 3 produces slightly better results than model 2 for vertical components. The balance in the volume budget for models 2 and 3 is also consistent with the observations. These results show that we cannot distinguish between the two models using only GPS observation. As there is no direct evidence for such a large creep or ductile source (corresponds to M7 or more) as proposed in model 2 and the active seismic region migrated back and forth within the linear swarm region, the model with a dike and a deep magma source is preferable. For the deflation point source, we obtained a deflation volume of 1.5 km3 at the depth of 20 km below the dike. An additional ~0.95 km3 of volume loss through caldera collapse and edifice deflation took place at Miyakejima. We conclude that the magma that intruded the dike came in part from below Miyakejima and in part from below the sea floor between Miyakejima and Kozushima, perhaps from reservoirs at the Moho.Editorial responsibility: S Nakada, T Druitt 相似文献
6.
2002和2003年夏季流动地震观测揭示，天池火山口附近存在大量的微震活动和一系列震群活动.地震定位结果表明地震主要发生在火山口附近，以震群形式发生的地震全部集中在天池火山口西南部，东北部地震密集区没有观测到震群活动.在夏季以外的其他季节，天池火山区只有一个固定地震台站（CBS）用于地震监测.利用CBS台不同时间的观测纪录，通过波形相关分析发现其他季节的主要震群活动仍然集中在天池西南部.震群的高精度相对定位揭示震源位置沿北西－南东向分布，倾向西南，倾角约80°. 2003年7月13日的震群发生期间，地震震源位置出现从深到浅的迁移现象，同时震源深度较大的地震在不同台站的地震波初动方向几乎全部向上，表明震源具有明显的膨胀分量.考虑到长白山天池火山2002年以来出现明显的地表形变、地球化学异常和谐频地震等现象，我们认为震群活动可能与5 km深度附近存在岩浆热液活动和岩浆增压有关. 相似文献
7.
During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO2), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO2). The submarine spatter contains two types of crystal-clots, A-type and A-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A-type clots contain Ca-poor plagioclase (An < 70). We consider that the A-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.Editorial responsibility: S Nakada, T Druitt 相似文献
8.
《Earth and Planetary Science Letters》2006,241(3-4):789-803
We present a combined method, using sP depth-phase data and double-difference arrival times, to determine the precise hypocenter locations of earthquakes that occur under the Pacific Ocean outside of the area covered by the land-based seismic network. We assess the effectiveness of the combined method using a data set of P- and S-wave arrival times and sP depth phase from suboceanic earthquakes recorded by both land-based seismic stations and offshore seismic stations (OFS). The hypocenters of the offshore earthquakes relocated using the combined method are consistent with those determined using the standard location method and OFS data. The differences in the hypocenters relocated by the two methods are less than 4 km. We applied the method to the subduction region that underlies the Kanto district, central Japan, and located a large number of earthquakes that occurred beneath the Pacific Ocean. We then determined the detailed 3D seismic velocity structure by inverting a large number of arrival times of P- and S-waves and sP depth phase from the relocated earthquakes in the study region. High-velocity anomalies related to the cold subducting Pacific slab and low-velocity anomalies related to the hot mantle wedge are clearly imaged. Beneath active volcanoes, low-velocity zones are visible from the surface to a depth of 100 km, reflecting fluids released by dehydration of the subducting Pacific slab. Strong lateral heterogeneities are revealed on the upper boundary of the Pacific slab beneath the forearc region. The low-velocity areas under the offshore region are associated with low seismicity and weak interplate coupling. A low-velocity layer is imaged along the upper boundary of the Philippine Sea slab in the northern part of Kanto district, which may reflect dehydration of the slab. Our tomographic images indicate that the overlaying Philippine Sea plate has effects on the spatial distribution of active volcanoes related to the subducting Pacific slab in the study region. 相似文献
9.
本文使用双差定位法对2014年9月12日至12月30日浙江珊溪水库发生的4184次地震进行重定位,并采用CAP方法对11次ML≥3.0地震事件的震源机制解进行反演,讨论了震群序列的活动特征及其与断裂之间的关系,分析了水库水位与地震之间的关系.重新定位的结果显示,在空间分布上,2014年震群序列发生在2006年震群序列NW向延伸的方向上,两者形成一条线性条带,该条带平行分布于双溪—焦溪垟断裂南侧.重定位得到的震源主要在0.7—6 km深度范围内分层分布,垂直于地震条带走向的震源剖面刻画出的结构面以高角度倾向SW.震源机制解结果显示多数地震为走滑型,均存在一个与地震条带走向一致的NW向节面,呈右旋走滑错动性质.考虑到断裂的定位误差,线性分布的震群活动极有可能沿双溪—焦溪垟断裂的破裂面活动,精定位的震源位置和震源机制刻画出了该断裂的几何结构和活动性质.但由于多数地震的震源深度在6 km以上,因此震群活动不能归为双溪—焦溪垟断裂活动的结果,即双溪—焦溪垟断裂不是这两次震群的发震构造,而且仍然属于水库诱发地震,而水库地震存在激发该断裂发生构造地震的可能.水库水位上升或者下降与震群活动关系不大,震群活动有随时间进一步增强的趋势, 可能是库水沿库底断层破裂面长期渗透和扩散增加了孔隙压所致. 相似文献
10.
Hm. Okada H. Watanabe H. Yamashita I. Yokoyama 《Journal of Volcanology and Geothermal Research》1981,9(4):311-334
An earthquake swarm, and the major pumice eruptions in August 1977 which followed, marked the start of the dacitic doming activity of Usu volcano in southwestern Hokkaido, Japan. The sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data. The distribution of the abundant hypocenters shows clearly an earthquake-free zone beneath the summit crater. The hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations, considered due to magma intrusion into this earthquake-free zone. The earthquake mechanism solutions are mostly of dip-slip type and are interpreted in terms of the doming deformations. The existence of earthquake families (earthquakes with similar waveforms) is the main cause of the peculiar occurrence of earthquakes in space, time and magnitude. The concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm, and the observed episodic deformations. 相似文献
11.
日本所在的西太平洋地区是世界上中深源地震发生最为频繁的地区. 早期研究已表明, 日本东北地区下方的中深源地震呈双层分布. 为进一步分析该双震带的空间分布特征, 本文通过方法测试证明了采用球坐标系下的三维射线追踪法改进后的双差定位法进行地震重定位的精确性和有效性, 对使用该方法进行重定位前、 后各方向上的误差进行了分析, 并确定了最佳的定位参数. 在此基础上, 对日本东北地区的中深源地震进行了高精度重定位, 并对重定位得到的震源位置进行了空间拟合, 其结果表明地震呈明显的双层分布, 且与西太平洋俯冲板块几近平行. 本文研究结果对揭示双震带中地震的发震机理以及俯冲板块内的精细结构均具有重要意义. 相似文献
12.
I. Yokoyama H. Yamashita H. Watanabe Hm. Okada 《Journal of Volcanology and Geothermal Research》1981,9(4):335-358
The 1977–1978 eruption of Usu volcano is discussed from the geophysical standpoint as a classic example of dacite volcanism. The activities of dacitic volcanoes are characterized by persistent earthquake swarms and remarkable crustal deformations due to the high viscosity of the magmas; the former include shocks felt near the volcanoes and the latter accompany formation of lava domes or cryptodomes.The hypocenters of the earthquakes occurring beneath Usu volcano have been located precisely. Their distribution defines an earthquake-free zone which underlies the area of doming within the summit crater. This zone is regarded as occupied by viscous magma. The domings within the summit crater forming the cryptodomes have amounted to about 160 m. In addition to uplift they showed thrusting towards the northeast. As a result, the northeastern foot of the volcano has contracted by about 150 m. The relation between crustal deformation and earthquake occurrence is examined, and it is found that the abrupt domings are accompanied by the larger earthquakes (M = 3–4.3). Both the seismic activity and the ground deformation are shown to have a unique and common energy source.The energy of activities of Usu volcano consists of the explosive type, the deformation type and the seismic type; the second and the third are in parallel with each other in discharges, and both energies are complementary to the explosive energy. The explosive energy and the seismic energy have been calculated for an explosion sequence, and it is concluded that the deformation energy is about 10 times greater than the seismic energy. The discharge rate of the seismic energy and the upheaval rates of the cryptodomes have continued to decrease since the outburst of the eruption, except for a small increase at the end of January 1978. Eruptions are governed not only by the supply of the energies but also by the depth of the magma, which has gradually approached the surface. The last eruption occurred in October 1978; however, the crustal deformations and the earthquake swarms are still proceeding as of January 1980, albeit at a lower rate of activity. 相似文献
13.
Paricutin volcano is a monogenetic volcano whose birth and growth were observed by modern volcanological techniques. At the time of its birth in 1943, the seismic activity in central Mexico was mainly recorded by the Wiechert seismographs at the Tacubaya seismic station in Mexico City about 320 km east of the volcano area. In this paper we aim to find any characteristics of precursory earthquakes of the monogenetic eruption. Though there are limits in the available information, such as imprecise location of hypocenters and lack of earthquake data with magnitudes under 3.0.The available data show that the first precursory earthquake occurred on January 7, 1943, with a magnitude of 4.4. Subsequently, 21 earthquakes ranging from 3.2 to 4.5 in magnitude occurred before the outbreak of the eruption on February 20. The (S - P) durations of the precursory earthquakes do not show any systematic changes within the observational errors. The hypocenters were rather shallow and did not migrate.The precursory earthquakes had a characteristic tectonic signature, which was retained through the whole period of activity. However, the spectra of the P-waves of the Paricutin earthquakes show minor differences from those of tectonic earthquakes. This fact helped in the identification of Paricutin earthquakes. Except for the first shock, the maximum earthquake magnitudes show an increasing tendency with time towards the outbreak. The total seismic energy released by the precursory earthquakes amounted to 2 × 1019 ergs. Considering that statistically there is a threshold of cumulative seismic energy release (1017–18ergs) by precursory earthquakes in polygenetic volcanoes erupting after long quiescence, the above cumulative energy is exceptionally large. This suggests that a monogenetic volcano may need much more energy to clear the way of magma passage to the earth surface than a polygenetic one.The magma ascent before the outbreak of Paricutin volcano is interpretable by a model of magma-filled crack formation proposed by Weertman, based on seismic data and other field observations. 相似文献
14.
The andesitic stratovolcano Volcán de Colima is one of the most active volcanoes in Mexico. The recent eruption of Volcán de Colima began in November 1998 and was preceded by a 12-month period of seismic activity that included five earthquake swarms. About 600 events with magnitudes from -0.5 to 2.7 were located within a 50-km2 area extending northward from the crater of Volcán de Colima to the Pleistocene volcano Nevado de Colima. The majority of hypocenters within this area did not exceed 5 km depth below sea level. We investigated earthquake focal mechanisms and seismotectonic deformations of the volcanic edifice. Focal mechanisms during four earthquake swarms indicated normal faulting associated with extensional processes, which is in agreement with the general stress regime near the volcano revealed by field measurements of fault slips. Earthquakes in the fifth swarm had focal mechanisms associated with inverse faulting, showing a significant change in the stress situation just before the beginning of the eruption. The calculated deformations varied from 1.3᎒-11 to 2.7᎒-9. The first swarm of November-December 1997 resulted in a N-S horizontal elongation that was two times greater than the E-W horizontal shortening. The volume was also subject to vertical shortening. The second and third swarms, observed in March and May 1998, showed uniform horizontal N-S and E-W elongations accompanied by a vertical elongation of the volume. In June-July 1998, the situation of November-December 1997 was repeated, with N-S horizontal elongation greater than the E-W horizontal shortening of the volume accompanied by intensive vertical shortening. During the last swarm of October-November 1998, slight E-W elongation of the seismic volume was accompanied by strong N-S shortening and very slight vertical shortening. We assume that the seismic activity prior to the 1998 eruption of Volcán de Colima developed along two intersecting tectonic structures, the N-S-trending Colima rift, and the E-W-trending system of faults associated with Tamazula fault. During the first stage (November 1997-July 1998) the passageway for magma was developed along the Tamazula fault system under horizontal extension without any surface manifestation. In October-November 1998, the seismic events began to cluster along the Colima rift structures under predominantly compressional stresses; this condition culminated with the extrusion of andesitic block lava from the summit crater. 相似文献
15.
Abstract The variation of sulphur isotopic composition during the 2000 eruption of the Miyakejima Volcano was examined in order to monitor the temporal change of the volcanic activity. The δ34 S values of water-soluble sulphate leached from volcanic ash effused during intermittent eruptions from July to September 2005 range from +5 to +11‰ with a fluctuation of ca 3‰ within a single eruption. The δ34 S value of sulphuric acid mist collected with 'Cu-metal trap' placed on the flank of the volcano from December 2000 to January 2001 is +6.2‰. These sulphur isotopic compositions of sulphate, which were isotopically equilibrated in the subvolcanic hydrothermal system, indicate that the temperature of the hydrothermal system beneath the caldera increased after the period of intermittent phreatic and phreatomagmatic eruptions. Then, the δ34 S value of sulphuric acid trapped from January to March 2001 was +9.0‰ and the δ34 S value of water-soluble sulphate on volcanic ash emitted with minor eruption in May 2001 was +11.0‰, suggesting a decrease in temperature of the subvolcanic hydrothermal system. 相似文献
16.
利用华北遥测台网和首都圈数字地震台网112个台站记录到的1993—2004年发生在首都圈地区3983次地震的P波绝对到时资料和相对到时资料,采用双差地震层析成像方法联合反演首都圈地区的地震震源参数,给出了2809次地震的重新定位结果.经地震重新定位后,P波绝对走时均方根残差由初始的1.2s降为0.3s,定位精度有了非常显著的提高.重新定位后的地震震中更集中分布在断层带地区,条带状更为清晰.在唐山地区的唐山—大城断裂带,地震主要集中在断裂带内,两侧的地震比较稀少.从重新定位后的震中分布可以看出,研究区域内的地震活动带呈现更明显的北北东向和北西西向的条带状分布,说明这两组方向的断裂最为活跃.用双差地震层析成像方法得到的唐山地区的地震震源位置,沿北东方向剖面在深度上呈现明显的3个小震群的特点,震源最大深度为25km.唐山地区地震重新定位结果的对比性研究表明,双差层析成像方法得到的震源参数的精度高于常规地震层析成像方法和双差法. 相似文献
17.
The Alban Hills, a Quaternary volcanic center lying west of the central Apennines, 15–25 km southeast of Rome, last erupted
19 ka and has produced approximately 290 km3 of eruptive deposits since the inception of volcanism at 580 ka. Earthquakes of moderate intensity have been generated there
at least since the Roman age. Modern observations show that intermittent periods of swarm activity originate primarily beneath
the youngest features, the phreatomagmatic craters on the west side of the volcano. Results from seismic tomography allow
identification of a low-velocity region, perhaps still hot or partially molten, more than 6 km beneath the youngest craters
and a high-velocity region, probably a solidified magma body, beneath the older central volcanic construct. Thirty centimeters
of uplift measured by releveling supports the contention that high levels of seismicity during the 1980s and 1990s resulted
from accumulation of magma beneath these craters. The volume of magma accumulation and the amount of maximum uplift was probably
at least 40×106 m3 and 40 cm, respectively. Comparison of newer levelings with those completed in 1891 and 1927 suggests earlier episodes of
uplift. The magma chamber beneath the western Alban Hills is probably responsible for much of the past 200 ka of eruptive
activity, is still receiving intermittent batches of magma, and is, therefore, continuing to generate modest levels of volcanic
unrest. Bending of overburden is the most likely cause of the persistent earthquakes, which generally have hypocenters above
the 6-km-deep top of the magma reservoir. In this view, the most recent uplift and seismicity are probably characteristic
and not precursors of more intense activity.
Received: 15 April 1997 / Accepted: 9 August 1997 相似文献
18.
A significant number of volcano-tectonic (VT) earthquake swarms, some of which are accompanied by ground deformation and/or
volcanic gas emissions, do not culminate in an eruption. These swarms are often thought to represent stalled intrusions of
magma into the mid- or shallow-level crust. Real-time assessment of the likelihood that a VT swarm will culminate in an eruption
is one of the key challenges of volcano monitoring, and retrospective analysis of non-eruptive swarms provides an important
framework for future assessments. Here we explore models for a non-eruptive VT earthquake swarm located beneath Iliamna Volcano,
Alaska, in May 1996–June 1997 through calculation and inversion of fault-plane solutions for swarm and background periods,
and through Coulomb stress modeling of faulting types and hypocenter locations observed during the swarm. Through a comparison
of models of deep and shallow intrusions to swarm observations, we aim to test the hypothesis that the 1996–97 swarm represented
a shallow intrusion, or “failed” eruption. Observations of the 1996–97 swarm are found to be consistent with several scenarios
including both shallow and deep intrusion, most likely involving a relatively small volume of intruded magma and/or a low
degree of magma pressurization corresponding to a relatively low likelihood of eruption. 相似文献
19.
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
20.
利用云南腾冲火山地区15个固定台站记录到的7923次地震的P波到时资料,采用双差层析成像方法,反演得到腾冲火山及周边地区地壳及上地幔顶部三维P波速度结构和地震重定位结果.研究发现,腾冲火山区域地壳内存在明显的地震波低速区,P波速度低于整个区域地壳速度平均值超过15%,上地幔顶部存在规模较大的低速异常区.推测腾冲火山地区存在较大规模的地幔热物质上涌以及向地壳的侵入,热物质在地壳内以岩浆囊形式存储,并且壳内岩浆囊之间可能存在岩浆通道.通过联合反演获得的地震重定位结果显示,丛集地震位置更加集中,其展布特征与断裂构造具有显著的对应关系,表明研究区域断裂构造比较活跃.获得的高分辨率三维P波层析成像结果,为进一步认识火山地区岩浆存储特征以及地震分布与区域构造之间的关系提供了新的地震学依据. 相似文献