Fine S wave velocity structure beneath Iwate volcano, northeastern Japan, as derived from receiver functions and travel times

A genetic algorithm inversion of receiver functions derived from a dense seismic network around Iwate volcano, northeastern Japan, provides the fine S wave velocity structure of the crust and uppermost mantle. Since receiver functions are insensitive to an absolute velocity, travel times of P and S waves propagating vertically from earthquakes in the subducting slab beneath the volcano are involved in the inversion. The distribution of velocity perturbations in relation to the hypocenters of the low-frequency (LF) earthquakes helps our understanding of deep magmatism beneath Iwate volcano. A high-velocity region (dV_S/V_S=10%) exists around the volcano at depths of 2–15 km, with the bottom depth decreasing to 11 km beneath the volcano’s summit. Just beneath the thinning high-velocity region, a low-velocity region (dV_S/V_S=−10%) exists at depths of 11–20 km. Intermediate-depth LF (ILF) events are distributed vertically in the high-velocity region down to the top of the low-velocity region. This distribution suggests that a magma reservoir situated in the low-velocity region supplies magma to a narrow conduit that is detectable by the hypocenters of LF earthquakes. Another broad low-velocity region (dV_S/V_S=−5 to −10%) occurs at depths of 17–35 km. Additional clusters of deep LF (DLF) events exist at depths of 32–37 km in the broad low-velocity zone. The DLF and ILF events are the manifestations of magma movement near the Moho discontinuity and in the conduit just beneath the volcano, respectively.     

西南日本地区地壳P、S波速度结构与地震活动的关系

选用密集的高精度地震台网Hi-net 2002～2004年记录的836个地震事件的23,895条P波和21,969条S波的到时资料,计算得到了日本西南地区水平分辨率约33 km,垂直分辨率4～15 km的三维地壳P波和S波速度结构,并进一步获得泊松比分布.研究表明,在大山火山下存在显著的低速异常,表明该火山可能是一个潜在的活火山.速度异常图像清楚地显示了菲律宾板块俯冲至西南日本地区下,其上方存在的低速高泊松异常表明可能有流体的存在,分析得出可能来自板块俯冲过程中的脱水作用.并进一步探讨了流体在地震孕育及触发的过程中可能起的重要作用.     

Seismicity and magma supply rate of the 1998 failed eruption at Iwate volcano,Japan

Iwate volcano, Japan, showed significant volcanic activity including earthquake swarms and volcano inflation from the beginning of 1998. A large earthquake of magnitude 6.1 hit the south-west of the volcano on September 3. Although a 1 km² fumarole field formed, blighting plants on the ridge in the western part of the volcano in the spring of 1999, no magmatic eruptions occurred. We reconcile the spatio-temporal distributions of volcanic pressure sources determined by previously reported studies in which GPS, strain and tilt data from dense geodetic station networks are analyzed (Miura et al. Earth Planet Space 52:1003–1008, 2000; Sato and Hamaguchi J Volcanol Geotherm Res 155:244–262, 2006). We calculate the magma supply rates from their results and compare them with the occurrence rates of volcanic earthquakes. The results show that the magma supply rates are almost constant or even decrease with time while the earthquake occurrence rate increases with time. This contrast in their temporal changes is interpreted to result from stress accumulation in the volcanic edifice caused by constant magma supply without effusion of magma to the surface. We further show that data showing slight acceleration in strain can be best explained by magma ascent at a constant velocity, and that there is no evidence for increased magma buoyancy resulting from gas bubble growth. This consideration supports the interpretation that the magma stayed at 2 km depth and horizontally migrated. These findings relating magma supply rate and seismicity to magma ascent process are clues to understanding why no magmatic eruption occurred at Iwate volcano in 1998.     

Structural controls on the 1998 volcanic unrest at Iwate volcano: Relationship between a shallow, electrically resistive body and the possible ascent route of magmatic fluid

Magnetotelluric (MT) measurements were conducted at Iwate volcano, across the entirety of the mountain, in 1997, 1999, 2003, 2006, and 2007. The survey line was 18 km in length and oriented E–W, comprising 38 measurements sites. Following 2D inversion, we obtained the resistivity structure to a depth of 4 km. The surface resistive layer (~ several hundreds of meters thick) is underlain by extensive highly conductive zones. Based on drilling data, the bottom of the highly conductive zone is interpreted to represent the 200 °C isotherm, below which (i.e., at higher temperatures) conductive clay minerals (smectite) are rare. The high conductivity is therefore mainly attributed to the presence of hydrothermally altered clay. The focus of this study is a resistive body beneath the Onigajo (West-Iwate) caldera at depths of 0.5–3 km. This body appears to have impeded magmatic fluid ascent during the 1998 volcanic unrest, as inferred from geodetic data. Both tectonic and low-frequency earthquakes are sparsely distributed throughout this resistive body. We interpret this resistive body as a zone of old, solidified intrusive magma with temperatures in excess of 200 °C. Given that a similar relationship between a resistive body and subsurface volcanic activity has been suggested for Asama volcano, structural controls on subsurface magmatic fluid movement may be a common phenomenon at shallow levels beneath volcanoes.     

Conduit process in vulcanian eruptions at Sakurajima volcano, Japan: Inference from comparison of volcanic ash with pressure wave and seismic data

To elucidate the conduit processes controlling the amplitude of air pressure waves (A _pw) from vulcanian eruptions at the Sakurajima volcano, Japan, we examine ash particles emitted by eruptions preceded by swarms of low-frequency B-type earthquakes (BL-swarms). We measure the water content of glassy ash, an indicator of shallow magma storage pressure, and vesicle textures, such as vesicle number density (VND). These data allow us to reconstruct the shallow conduit by comparing vesicularity with inferred pressure, and therefore depth, of magma storage. The results show that VND increases with depth, implying formation of a dense, outgassed magma cap underlain by more-vesicular, less-outgassed, magma. The VND and water content in the glassy ash positively correlate with the duration of BL-swarms, suggesting that such seismic signals reflect upward migration of deep gas- and vesicle-rich magma. Finally, it is determined that A _pw positively correlates with VND, suggesting that the amplitude of the air pressure waves is controlled by the amount of accumulated gas- and bubble-rich magma below the dense magma cap.     

Tomographic imaging of seismic velocity structure in and around the Onikobe volcanic area, northeastern Japan: implications for fluid distribution

We estimated three-dimensional P- (V_p) and S-wave velocity (V_s) and V_p/V_s structures in and around the Onikobe volcanic area, northeastern Japan, by local travel time tomography. We used travel time data from source and receiver pairs located within and outside the study area, which plays an important role in obtaining the optimum ray coverage and in elucidating the deeper structure more accurately. Detailed information on deeper structures is essential for imaging the complete volcanic system from the magmatic source zone through areas of shallow hydrothermal circulation. More than 50 000 travel time data for the P-waves and 35 000 for the S-waves were used to image the velocity structure. Our results show the following dominant features: (1) two conduits in the upper crust with low V_p and low V_s indicative of H₂O-rich fluid pathways: one lying beneath Naruko volcano, the other beneath the focal area of the 1962 Northern Miyagi earthquake (M6.5); (2) an underlying broad region in the lower crust with low V_p, low V_s and high V_p/V_s, suggestive of a zone of partial melt, from which the fluids in (1) are derived; and (3) low V_p/V_s areas near the surface of the Sanzugawa and Onikobe calderas, suggesting a diffuse vapor-saturated cap.     

Post-eruptive volcanic dome evolution as revealed by deformation and microgravity observations at Usu volcano (Hokkaido, Japan)

Usu volcano (Hokkaido, Japan) is a dacitic volcano, known for its high production rate of lava domes and crypto-domes. It is thus a good target to study processes of volcanic dome evolution (upheaval and/or relaxation). We carried out repeated GPS and microgravity surveys on the three most recent domes of Mt. Usu (1910: Meiji Shinzan; 1943–1945: Showa-Shinzan and 1977–1982: Usu-Shinzan). The repeat period was 1 to 2 months and extended from October 1996 to June 1997. We also compare new data with results from former studies. More than 20 years after the start of Usu-Shinzan dome growth, there is still subsidence at a maximum rate of about 7 to 8 cm/year. The reasons for this subsidence are discussed. Repeated gravity surveys revealed an increase of gravity on the domes (about 60±10 microgal/year for Usu-Shinzan, about 15 microgal at Showa-Shinzan and 10 to 20 microgal for Meiji-shinzan); this gravity increase exceeds that expected due to subsidence. We discuss and interpret the excess gravity change in terms of a density increase in the edifice, caused by a combination of processes (contraction of the edifice, water level change, devesiculisation, cooling and magma intrusion). Quantification of these processes at Usu volcano may help to understand the processes of evolution at domes on other volcanoes such as Merapi (Indonesia), Unzen (Japan) or Montserrat (West Indies).     

Temporal change of characteristics of shallow volcano-tectonic earthquakes associated with increase in volcanic activity at Kuchinoerabujima volcano, Japan

Shallow volcano-tectonic (VT) earthquakes recorded at the Kuchinoerabujima island volcano in southwest Japan are analyzed in order to clarify the role of hydrothermal activity in the development of volcanic seismicity. From analysis of shallow VT earthquakes in 2006, two specific episodes of elevated seismicity are observed in April and November 2006. The VT earthquakes have hypocenters at depths of 0–0.4 km beneath the summit crater, and normal fault focal mechanisms with WNW–ESE extension consistent with the tensional stress field indicated by the alignment of craters and fissures. Although the hypocenters and focal mechanisms are found to be largely invariant during these episodes, the corner frequencies of the VT earthquakes underwent a pronounced increase and decrease accompanying the changes in seismicity rates. The corner frequencies increased to 20–25 Hz approximately one month prior to the onset of elevated seismicity, and then decreased to 10–15 Hz in the period of peak seismicity. The rupture length also decreased at the onset of seismicity, thereafter increasing as the seismicity continued. The peak seismicity in terms of the daily number of VT events was accompanied by inflation around the crater, suggestive of a pressure increase in the volcanic system. It is inferred that the increase in shallow VT seismicity and rupture length is related to the development of a fractured zone. The pressure increase in the volcanic system is attributed to the intrusion of hydrothermal fluids, which is supported by an observed increase in fumarolic temperature and activity. The preceding monochromatic events are thus considered to be generated by the effect of fluid-filled cracks. The shortening of rupture length is then inferred to be related to the closing of non-fluid-filled cracks in the fracture zone under the increasing pressure field, leading to a transition from monochromatic events to low-frequency and shallow VT seismicity.     

Magnetic and electric field observations during the 2000 activity of Miyake-jima volcano, Central Japan

Magnetic and electric field variations associated with the 2000 eruption of Miyake-jima volcano are summarized. For about 1 week prior to the July 8 phreatic explosion, significant changes in the total intensity were observed at a few stations, which indicated uprising of a demagnetized area from a depth of 2 km towards the summit: this non-magnetic source can be regarded as a vacant space itself. Electric and magnetic field variations were observed simultaneously associated with the tilt-step event, which was the abrupt (∼50 s) inflation at a few km depth within the volcano followed by gradual recovery (∼several hours). The electric field is ascribed to the electrokinetic effect most probably due to forced injection of fluids from the source, while the magnetic field to the piezomagnetic effect due to increased pressure. Large magnetic variations amounting to a few tens of nT were observed at several stations since July 8, and they turned almost flat after the August 18 largest eruption. Magnetic changes are explained mostly by the vanishing of magnetic mass in the summit and additionally by the thermal demagnetization at a rather shallow depth. A large increase in the self-potential by 130 mV was also observed near the summit caldera associated with the August 18 eruption, which suggests that the hydrothermal circulation system sustained within the volcano for the past more than 10 years was destroyed by this eruption.     

Constraints on the source mechanism of harmonic tremors based on seismological, ground deformation, and visual observations at Sakurajima volcano, Japan

Vulcanian-type eruptive activity has occurred from the summit crater of Sakurajima volcano, Japan, since 1955. Over this period, harmonic tremors have commonly occurred either several hours after swarms of B-type earthquakes (herein termed HTB: Harmonic Tremor following B-type earthquake swarm) or immediately after explosive eruptions (herein termed HTE: Harmonic Tremor after an Eruption). In this study, we analyzed the spectra and particle motions of HTBs and HTEs. Both HTBs and HTEs have spectra with peaks at fundamental frequencies and higher frequencies that are integer multiples of the fundamental frequencies. The peak frequencies of HTBs remained within a certain range, whereas those of HTEs showed a gradual increase. The spectra of an HTB that occurred on 20 July 1990 had stable fundamental frequencies of 1.46–1.66 Hz and at least 9 peaks of higher modes; in contrast, the HTE that occurred 3 minutes after an explosive eruption at 11 h 15 m (JST) on 11 October 2002 showed clear frequency gliding from 0.8 to 3.7 Hz in the fundamental mode. The peak frequencies of higher modes of the HTE also showed an increase corresponding to the shift of the fundamental mode towards a higher frequency. Particle motion analysis mainly identified Rayleigh waves from the prograde elliptical motion at the deepest borehole station (HAR) and retrograde motions at the other shallower stations. Love waves were dominant at the stations north and south of the crater. The distribution patterns of Rayleigh and Love waves of HTBs are similar to those of HTEs. The nature of the dominant surface waves of both HTBs and HTEs suggest that the sources of harmonic tremors are located at a shallow depth, corresponding to a gas pocket in the uppermost part of the volcanic conduit. Differences in the temporal characteristics of the HTB and HTE spectra reflect the internal condition of the gas pocket: HTBs are associated with inflation of the conduit, whereas HTEs occur following an eruption, associated with deflationary ground deformation. HTBs are caused by resonance of the gas pocket embedded beneath the lava dome. Although HTEs occur within the open conduit, the small size of vents enables resonance within the bubbly magma conduit. The positive gliding of dominant peaks toward higher frequencies is interpreted to result from shortening of the bubbly magma conduit due to a rise in the bubble nucleation level; this rise results from the re-pressurization that accompanies the ascent of magma from deep within the reservoir.     

Array analyses of volcanic earthquakes and tremor recorded at Las Cañadas caldera (Tenerife Island, Spain) during the 2004 seismic activation of Teide volcano

We analyze data from three seismic antennas deployed in Las Cañadas caldera (Tenerife) during May–July 2004. The period selected for the analysis (May 12–31, 2004) constitutes one of the most active seismic episodes reported in the area, except for the precursory seismicity accompanying historical eruptions. Most seismic signals recorded by the antennas were volcano-tectonic (VT) earthquakes. They usually exhibited low magnitudes, although some of them were large enough to be felt at nearby villages. A few long-period (LP) events, generally associated with the presence of volcanic fluids in the medium, were also detected. Furthermore, we detected the appearance of a continuous tremor that started on May 18 and lasted for several weeks, at least until the end of the recording period. It is the first time that volcanic tremor has been reported at Teide volcano. This tremor was a small-amplitude, narrow-band signal with central frequency in the range 1–6 Hz. It was detected at the three antennas located in Las Cañadas caldera. We applied the zero-lag cross-correlation (ZLCC) method to estimate the propagation parameters (back-azimuth and apparent slowness) of the recorded signals. For VT earthquakes, we also determined the S–P times and source locations. Our results indicate that at the beginning of the analyzed period most earthquakes clustered in a deep volume below the northwest flank of Teide volcano. The similarity of the propagation parameters obtained for LP events and these early VT earthquakes suggests that LP events might also originate within the source volume of the VT cluster. During the last two weeks of May, VT earthquakes were generally shallower, and spread all over Las Cañadas caldera. Finally, the analysis of the tremor wavefield points to the presence of multiple, low-energy sources acting simultaneously. We propose a model to explain the pattern of seismicity observed at Teide volcano. The process started in early April with a deep magma injection under the northwest flank of Teide volcano, related to a basaltic magma chamber inferred by geological and geophysical studies. The stress changes associated with the injection produced the deep VT cluster. In turn, the occurrence of earthquakes permitted an enhanced supply of fresh magmatic gases toward the surface. This gas flow induced the generation of LP events. The gases permeated the volcanic edifice, producing lubrication of pre-existing fractures and thus favoring the occurrence of VT earthquakes. On May 18, the flow front reached the shallow aquifer located under Las Cañadas caldera. The induced instability constituted the driving mechanism of the observed tremor.     

首都圈地区地壳P波和S波三维速度结构及其与大地震的关系

选用华北地震遥测台网和首都圈数字化地震遥测台网1993～2005年记录的2866个地震事件中的33487条P波和31822条S波的到时资料,计算得到了水平分辨率25km到50km之间的首都圈地区（38.5°N～41°N, 114°E～120°E）地壳三维P波和S波速度结构,并进一步获得泊松比分布.研究表明,首都圈地区P波和S波速度分布表现出强烈的横向不均匀性,浅层速度分布同地表地质结构分布相一致.分析得出震区强震多发生在低速体与高速体之间、低泊松比地区,且震源下部存在低速、高泊松比异常体.经过与其他地区研究结果相比较,对地震触发与流体的关系进行了探讨.流体在地震孕育及触发的过程中可能起着重要作用.     

New insight into the factors leading to the 1998 flank collapse and lahar disaster at Casita volcano,Nicaragua

During unusually high rainfall associated with Hurricane Mitch in 1998, a flank collapse leading to a disastrous lahar occurred at Casita volcano, Nicaragua. The lack of a similar failure during a rainfall event of comparable magnitude in 1982 (Tropical Storm Alleta) suggests that additional factors influenced the 1998 collapse. We have investigated the potential contribution of erosional flank undercutting, seismic activity and anthropogenic land-cover change at the collapse site. Except for seismic events prior to Mitch, which may have increased flank instability, none of these factors appears significant. Instead, for failure to occur, the following conditions were required: (1) highly fractured slope material allowing deep and rapid infiltration of meteoric water, (2) a less permeable underlying layer to obstruct drainage, (3) strong, continuous antecedent rainfall to build up high pore-water pressure, and (4) episodic, high-intensity precipitation during Mitch to generate recurrent pressure waves. Prerequisite (1) was provided by edifice-wide deformation towards the south-east, seismic activity and proximity to a prominent fault, and local subsidence. Condition (2) was met by clay-rich layers resulting from hydrothermal alteration. More than 1,900 mm of rain fell in the 6 months prior to Mitch without significant interruption, while intense episodic precipitation occurred during the hurricane, satisfying conditions (3) and (4), respectively. The main difference with Alleta was that it occurred at the beginning of the rainy season and, therefore, without sufficient antecedent rainfall. Anthropogenic activity, including land-cover change, did not affect slope stability (i.e. the hazard). However, vulnerability was generated when two towns were established in the lowlands south of Casita, on top of previous lahar deposits. It was greatly increased when approximately 4 km of forest between the collapse site and the towns were cleared, paving the way for a largely unobstructed debris flow. Deforestation also facilitated erosion along the flanks to provide about 78% of the material contained in the lahar when it destroyed the towns, killing more than 2,500 people.     

Tidal triggering of seismic and volcanic phenomena during the 1879–1880 eruption of Islas Quemadas volcano in El Salvador, Central America

The Islas Quemadas dome in Lake Ilopango, El Salvador, was extruded during December 1879 to March 1880. The eruption took place in six distinct periods, five of which began near fortnightly minima in the amplitude of the semidiurnal solid earth tide. An earthquake swarm preceded dome extrusion. Felt earthquakes were carefully recorded during a two-day period preceding a destructive main shock that occurred on December 27, 1879. During this foreshock sequence the periods of highest earthquake activity occurred at times of maxima and minima in semidiurnal solid-earth tide. The four largest earthquakes, including the main shock, occurred at maxima in the semidiurnal tide.     

Seismic velocity structure around the Hyuganada region, Southwest Japan, derived from seismic tomography using land and OBS data and its implications for interplate coupling and vertical crustal uplift

The Hyuganada region, a forearc region of Southwest Japan, is characterized by several interesting geological and geophysical features, i.e., significant aseismic crustal uplift of 120 m during the past 120 thousand years at the Miyazaki Plain, negative free-air gravity anomalies with the maximum magnitude of −130 mgal, and relatively less cohesive interplate coupling compared with that for off the Shikoku and Kii Peninsula. In order to examine the causes of these observations, we determined a detailed three-dimensional seismic velocity structure based on the seismic data observed by ocean bottom seismometers (OBS) and land stations. P- and S-wave tomographic velocity structures clearly indicate the subducting slab and also the zones of high Poisson's ratio at 25–35 km depth along the coastline of the northeastern part of the Hyuganada. The region with high Poisson's ratio may correspond to the serpentinized mantle wedge as suggested for other mantle wedges, and appears to be coincident with the zone for observed aseismic slips such as the slow-slip and after-slip events. Also, the detection may be related to a relatively weak interplate coupling in the Hyuganada region. The tomographic structures also indicate low velocity zones with a horizontal scale comparable to the Kyushu-Palau Ridge in and around the subducting slab. If we assume that the low velocity zones correspond to the subducted Kyushu-Palau Ridge, then the predicted gravity anomaly due to the density contrast between the low velocity zones and the surrounding region can explain about 60% of the gravity anomaly in the Hyuganada region. The buoyancy is probably an important factor for the crustal uplift observed in the Miyazaki Plain, the steep bending of the subducting slab and the normal fault-type earthquakes around the Hyuganada region.