The November 2002 eruption at Piton de la Fournaise volcano,La Réunion Island: ground deformation,seismicity, and pit crater collapse

An eruption on the eastern flank of Piton de la Fournaise volcano started on 16 November, 2002 after 10 months of quiescence. After a relatively constant level of activity during the first 13 days of the eruption, lava discharge, volcanic tremor and seismicity increased from 29 November to 3 December. Lava effusion suddenly ceased on 3 December while shallow earthquakes beneath the Dolomieu summit crater were still recorded at a rate of about one per minute. This unusual activity continued and increased in intensity over the next three weeks, ending with the formation of a pit crater within Dolomieu. Based on ground deformation, measured by rapid-static and continuous GPS and an extensometer, seismic data, and lava effusion patterns, the eruptive period is divided into five stages: 1) slow summit inflation and sporadic seismicity; 2) rapid summit inflation and a short seismic crisis; 3) rapid flank inflation, onset of summit deflation, sporadic seismicity, accompanied by stable effusion; 4) flank inflation, coupled with summit deflation, intense seismicity, and increased lava effusion; and finally 5) little deflation, intense shallow seismicity, and the end of lava effusion. We propose a model in which the pre-intrusive inflation of Stage 1 in the months preceding the eruption was caused by a magma body located near sea level. The magma reservoir was the source of an intrusion rising under the summit during Stage 2. In Stage 3, the magma ponded at a shallow level in the edifice while the lateral injection of a radial dike reached the surface on the eastern flank of the basaltic volcano, causing lava effusion. Pressure decrease in the magmatic plumbing system followed, resulting in upward migration of a collapse front, forming a subterranean column of debris by faulting and stoping. This caused intense shallow seismicity, increase in discharge of lava and volcanic tremor at the lateral vent in Stage 4 and, eventually the formation of a pit crater in Stage 5.     

THE CHARACTERISTICS OF VOLCANIC ROCKS STRUCTURE AND LITHOFACIES OF DALIUCHONG VOLCANO IN THE TENGCHONG VOLCANIC FIELD,YUNNAN PROVINCE

A set of grey-purple layered volcanic rocks are found widely distributed from the mountain flank to the main peak of Daliuchong volcano, but it's difficult to identify whether they are volcaniclastic rock or lava rock just by field investigation and the crystal structure observation under microscope. The study of matrix microstructure of the volcanic rocks can help to identify the volcanic facies. We recognize the eruptive facies rocks through observation of the matrix microstructure and pore shape with comparison to those of the volcanic vent facies, extrusive facies and effusive facies rocks under microscope, thus the mentioned layered volcanic rocks could be named as dacitic crystal fragment tuff. Combining the joint work of field investigation, systematic sampling, chemical analyzing and microscopic observation, we summary the Daliuchong volcanic facies as follows:1. The effusive facies lava constitutes the base of Daliuchong volcano and was produced by early eruption.2. The explosive facies is composed of dacite crystal fragment welded tuff and volcanic breccia and mainly distributes on the W, S and NE flank of the volcanic cone.3. The volcanic conduit with its diameter more than one hundred meters is located about 100 meters south of the main peak of the Daliuchong volcano.4. The extrusive facies rock is only exposed near the peak of Daliuchong volcano.Therefore, the volcanism of Daliuchong volcano can be speculated as:Large-scale lava overflowing occurred in the early eruption period; then explosive eruptions happened; at last, the volcanisms ceased marked with magma extrusion as lava dome and plug.     

Stress Field Estimations for Colima Volcano, Mexico, Based on Seismic Data

 For first time, during 1991, seismic activity was recorded during an eruption at Colima volcano. We analyze these data to obtain a stress pattern using a composite focal mechanism technique. From the analysis of regional seismicity, the Tamazula Fault and the Armeria River appear as active features and the dip of the slab east of the Jalisco Block is approximately 12°. Southwest of Colima volcano a vertical alignment of seismic events was observed. We estimate five different composite focal mechanism solutions from our data set, which indicate a change of the stress field at the volcano after the 1991 eruption. These solutions suggest that the stress field in the volcanic edifice was controlled by stresses related to the emplacement of magma superimposed on the regional stress field. No evidence of active local faults in the volcanic edifice was found. We propose a model for the eruptive process that involves tilting of the volcanic edifice. Received: 15 October 1995 / Accepted: 26 October 1998     

Volcanic Tremor at Mt. Etna,Italy, Preceding and Accompanying the Eruption of July – August, 2001

The July 17 – August 9, 2001 flank eruption of Mt. Etna was preceded and accompanied by remarkable changes in volcanic tremor. Based on the records of stations belonging to the permanent seismic network deployed on the volcano, we analyze amplitude and frequency content of the seismic signal. We find considerable changes in the volcanic tremor which mark the transition to different styles of eruptive activity, e.g., lava fountains, phreatomagmatic activity, Strombolian explosions. In particular, the frequency content of the signal decreases from 5 Hz to 3 Hz at our reference station ETF during episodes of lava fountains, and further decreases at about 2 Hz throughout phases of intense lava emission. The frequency content and the ratios of the signal amplitude allow us to distinguish three seismic sources, i.e., the peripheral dike which fed the eruption, the reservoir which fed the lava fountains, and the central conduit. Based on the analysis of the amplitude decay of the signal, we highlight the migration of the dike from a depth of ca. 5 km to about 1 km between July 10 and 12. After the onset of the effusive phase, the distribution of the amplitude decay at our stations can be interpreted as the overall result of sources located within the first half kilometer from the surface. Although on a qualitative basis, our findings shed some light on the complex feeding system of Mt. Etna, and integrate other volcanological and geophysical studies which tackle the problem of magma replenishment for the July–August, 2001 flank eruption. We conclude that volcanic tremor is fundamental in monitoring Mt. Etna, not only as a marker of the different sources which act within the volcano edifice, but also of the diverse styles of eruptive activity. An erratum to this article is available at .     

Overview of the 1990–1995 eruption at Unzen Volcano

Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1×10⁸ m³ (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230–540 m high). The eruption finished with extrusion of a spine at the endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO₂ flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.     

Precursory seismicity of the 1994 eruption of Popocatépetl Volcano,Central Mexico

Popocatépetl Volcano is located in the central Mexican Volcanic Belt, within a densely populated region inhabited by over 20 million people. The eruptive history of this volcano indicates that it is capable of producing a wide range of eruptions, including Plinian events. After nearly 70 years of quiescence, Popocatépetl reawakened in December 21, 1994. The eruptive activity has continued up until the date of this submission and has been characterized by a succession of lava dome growth-and-destruction episodes, similar to events that have apparently been typical for Popocatépetl since the fourteenth century. In this regime, the episodes of effusive and moderately explosive activity alternate with long periods of almost total quiescence. In this paper we analyze five years of volcano-tectonic seismicity preceding the initial eruption of the current episode. The evolution of the V-T seismicity shows four distinct stages, which we interpret in terms of the internal processes which precede an eruption after a long period of quiescence. The thermal effects of a magma intrusion at depth, the fracturing related to the slow development of magma-related fluid pathways, the concentration of stress causing a protracted acceleration of this process, and a final relaxation or redistribution of the stress shortly before the initial eruption are reflected in the rates of V-T seismic energy release. A hindsight analysis of this activity shows that the acceleration of the seismicity in the third stage asymptotically forecast the time of the eruption. The total seismic energy release needed to produce an eruption after a long period of quiescence is related to the volume of rock that must be fractured so imposing a characteristic threshold limit for polygenetic volcanoes, limit that was reached by Popocatépetl before the eruption.     

Volcanic tremor at Mt. Etna: Inferences on magma dynamics during effusive and explosive activity

During 1999, the volcanic activity at Mt. Etna was both explosive and effusive at the summit craters: Strombolian activity, lava fountains and lava flows affected different areas of the volcano, involving three of the four summit craters. Results from analysis of the 1999 volcanic tremor features are shown at two different time scales. First, the long-term time variation of the features of the volcanic tremor (including spectral and polarization parameters), during the entire year, was compared with the evolution of the eruptive activity. This approach demonstrated the good agreement between tremor data and observed eruptive activity; the activation of different tremor sources was suggested. Then, a more refined analysis of the volcanic tremor, recorded during 14 lava fountain eruptions, was performed. In particular, a shift of the dominant frequencies towards lower values was noted which corresponds with increasing explosive activity. Similar behaviour in the frequency content has already been observed in other explosive eruptions at Mt. Etna as well as on other volcanoes. This behaviour has been explained in terms of either an increase in the tremor source dimension or a decrease in the sound speed in the magma within the conduit. These results confirm that the volcanic tremor is a powerful tool for better understanding the physical processes controlling explosive eruptions at Mt. Etna volcano.     

Seismic activity accompanying the 1974 eruption of Mt. Etna

On January 30, 1974, an explosive eruption began on the western side of Etna. The activity evolved into two eruptive periods (January 30–February 17 and March 11–29). Two spatter cones (Mount De Fiore I and Mount De Fiore II) were formed at a height of about 1650 m a.s.l. and a distance of 6 km from the summit area. The effusive activity was very irregular with viscous lava flows of modest length.A seismic network of four stations was established around the upper part of the volcano on February 3. Moreover additional mobile stations were set up at several different sites in order to obtain more detailed informations on epicenter locations and spectral content of volcanic tremor.The volcanic activity is discussed in relation to the distribution of epicenters and the time-space distribution of the spectral characteristics of volcanic earthquakes and tremor. The characteristics of the seismic activity suggest that the flank eruption of Mount Etna was probably feed by a lateral branch of the main conduit yielding the activity at the Central Crater.     

Seismicity and crustal deformation preceding the January 1996 eruptions at Karymsky Volcanic Center,Kamchatka

Two explosive eruptions occurred on 2 January 1996 at Karymsky Volcanic Center (KVC) in Kamchatka, Russia: the first, dacitic, from the central vent of Karymsky volcano, and the second, several hours later, from Karymskoye lake in the caldera of Akademia Nauk volcano. The main significance of the 1996 volcanic events in KVC was the phreatomagmatic eruption in Karymskoye lake, which was the first eruption in this lake in historical time, and was a basaltic eruption at the acidic volcanic center. The volcanic events were associated with the 1 January Ms 6.7 (Mw 7.1) earthquake that occurred at a distance of about 9–17 km southeast from the volcanoes just before the eruptions. We study the long-term (1972–1995) and short-term (1–2 January 1996) characteristics of crustal deformations and seismicity before the double eruptive event in KVC. The 1972–1995 crustal deformation was homogeneous and characterized by a gradual extension with a steady velocity. The seismic activity in 1972–1995 developed at the depth interval from 0 to 20 km below the Akademia Nauk volcano and spread to the southeast along a regional fault. The seismic activity in January 1996 began with a short sequence of very shallow microearthquakes (M ~0) beneath Karymsky volcano. Then seismic events sharply increased in magnitude (up to mb 4.9) and moved along the regional fault to the southeast, culminating in the Ms 6.7 earthquake. Its aftershocks were located to the southeast and northwest from the main shock, filling the space between the two active volcanoes and the ancient basaltic volcano of Zhupanovsky Vostryaki. The eruption in Karymskoye lake began during the aftershock sequence. We consider that the Ms 6.7 earthquake opened the passageway for basic magma located below Zhupanovsky Vostryaki volcano that fed the eruption in Karymskoye lake.     

Eruptive and earthquake activities related to the 2000 eruption of Mount Cameroon volcano (West Africa)

Mount Cameroon is an active volcano located in the Gulf of Guinea, west of Central Africa. After the March–April 1999 eruption on the SW flank, another eruption of the volcano occurred in 2000. It took place from three sites on the southwest flank and near the summit. The first eruptive site was located 500 m to the southwest of the summit, at 3900 m altitude. Activity on this site was mainly explosive with no lava flow. The second site was located between 3220 and 3470 m altitude. Lava was emitted along NNE–SSE fissures from this site and flew towards Buea, the main city of the area, stopping ~ 4 km from the first houses. The last site was located in the south western flank at 2750 m altitude. The lava ejected from an old cone near the first 1999 eruptive site was divided into two branches, for a total length of around 1 km. The location of active volcanic cones in 1999 and 2000 seems to be linked to the local tectonics. The pre-eruptive period was characterized by a seismic swarm which may be a precursor recorded in March 2000 by an analogue seismic station. The main shock was a magnitude 3.2 event, and was felt by the population in Ekona town located on the eastern flank. It had a Modified Mercalli intensity of III–IV. When the eruption started, a temporary network of short period 3-component seismic stations was set up around the volcano to improve the monitoring of seismic activity. The co-eruptive period from late May to September was characterized by sequences of earthquake swarms, volcanic tremor and a family of earthquakes having similar waveform and appearing regularly in August and early September. Some of the earthquakes were felt by the population in Buea and its environments. The largest seismic event recorded had a magnitude of 4. During the post-eruptive period from mid-September to December, seismicity returned to its background level of 1–3 earthquakes per 3 days. Hypocenter locations reveal a linear narrow structure under the summit zone which could represent the magmatic conduit of the volcano. The frequency/magnitude relationship revealed a b-value of 1.43 higher than those previously determined, but more representative of volcanic media. Seismic energy release was gradual after the 2000 eruption started.     

Seismotectonic deformation of the volcanic edifice prior to the 1998 lava eruption of Volcán de Colima, México

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.     

Magma discharge variations during the 2011 eruptions of Shinmoe-dake volcano, Japan, revealed by geodetic and satellite observations

We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption.     

Earthquake activity related to the 1991 eruption of the Hekla volcano,Iceland

The 1991 eruption of the Hekla volcano started unexpectedly on 17 January. No long-term precursory seismicity was observed. The first related activity was a swarm of small earthquakes that began approximately half an hour before the eruption. Intensive seismicity, both earthquakes and volcanic tremor, accompanied the violent onset of the eruption. Almost 400 events up to M_L magnitude 2.5 were recorded during the first few hours. During the later phases of the eruption, the earthquake activity was modest and the main volcano-related seismic signal was the persistent volcanic tremor. The tremor died away, together with the eruption on 11 March, and Hekla was seismically quiet until the beginning of June 1991, when a sudden swarm of numerous small shallow earthquakes occurred. This activity is atypical for Hekla and is interpreted to be a failed attempt to resume the eruption.     

The occurrence of Mt Barca flank eruption in the evolution of the NW periphery of Etna volcano (Italy)

Geological surveys, tephrostratigraphic study, and ⁴⁰Ar/³⁹Ar age determinations have allowed us to chronologically constrain the geological evolution of the lower NW flank of Etna volcano and to reconstruct the eruptive style of the Mt Barca flank eruption. This peripheral sector of the Mt Etna edifice, corresponding to the upper Simeto valley, was invaded by the Ellittico volcano lava flows between 41 and 29 ka ago when the Mt Barca eruption occurred. The vent of this flank eruption is located at about 15 km away from the summit craters, close to the town of Bronte. The Mt Barca eruption was characterized by a vigorous explosive activity that produced pyroclastic deposits dispersed eastward and minor effusive activity with the emission of a 1.1-km-long lava flow. Explosive activity was characterized by a phreatomagmatic phase followed by a magmatic one. The geological setting of this peripheral sector of the volcano favors the interaction between the rising magma and the shallow groundwater hosted in the volcanic pile resting on the impermeable sedimentary basement. This process produced phreatomagmatic activity in the first phase of the eruption, forming a pyroclastic fall deposit made of high-density, poorly vesicular scoria lapilli and lithic clasts. Conversely, during the second phase, a typical strombolian fall deposit formed. In terms of hazard assessment, the possible occurrence of this type of highly explosive flank eruption, at lower elevation in the densely inhabited areas, increases the volcanic risk in the Etnean region and widens the already known hazard scenario.     

Evolution of the seismic activity from the 1982 eruption of El Chichon Volcano, Chiapas, Mexico

 We analyzed more than 1700 earthquakes related to the 1982 eruption of El Chichon volcano in southern Mexico. The data were recorded at specific periods throughout the whole eruptive interval of March to April 1982, by three different networks. The seismic activity began several months before the first eruption on 28 March. During this period the seismicity consisted of hybrid and long-period shallow earthquakes most likely related to processes of faulting, fracturing, and fluid movement underneath the volcano. The foci of events occurring before the eruption circumscribe an aseismic zone from approximately 7 to 13 km below the volcano. After the eruption, the seismic activity consisted of tectonic-type earthquakes that peaked at 1200 events/h. This later activity occurred over a wide range of depths, mostly between 5 and 20 km, that includes the former aseismic zone and is roughly limited by the major tectonic faults in the area. Received: 19 May 1998 / Accepted: 13 June 1999     

Detection of a new summit crater on Bezymianny Volcano lava dome: satellite and field-based thermal data

An explosive eruption occurred at the summit of Bezymianny volcano (Kamchatka Peninsula, Russia) on 11 January 2005 which was initially detected from seismic observations by the Kamchatka Volcanic Eruption Response Team (KVERT). This prompted the acquisition of 17 Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite images of the volcano over the following 10 months. Visible and infrared data from ASTER revealed significant changes to the morphology of the summit lava dome, later seen with field based thermal infrared (TIR) camera surveys in August 2005. The morphology of the summit lava dome was observed to have changed from previous year’s observations and historical accounts. In August 2005 the dome contained a new crater and two small lava lobes. Stepped scarps within the new summit crater suggest a partial collapse mechanism of formation, rather than a purely explosive origin. Hot pyroclastic deposits were also observed to have pooled in the moat between the current lava dome and the 1956 crater wall. The visual and thermal data revealed a complex eruption sequence of explosion(s), viscous lava extrusion, and finally the formation of the collapse crater. Based on this sequence, the conduit could have become blocked/pressurized, which could signify the start of a new behavioural phase for the volcano and lead to the potential of larger eruptions in the future.     

Reconstruction of the eruptive history of Usu volcano,Hokkaido, Japan,inferred from petrological correlation between tephras and dome lavas

Usu volcano has erupted nine times since 1663. Most eruptive events started with an explosive eruption, which was followed by the formation of lava domes. However, the ages of several summit lava domes and craters remain uncertain. The petrological features of tephra deposits erupted from 1663 to 1853 are known to change systematically. In this study, we correlated lavas with tephras under the assumption that lava and tephra samples from the same event would have similar petrological features. Although the initial explosive eruption in 1663 was not accompanied by lava effusion, lava dome or cryptodome formation was associated with subsequent explosive eruptions. We inferred the location of the vent associated with each event from the location of the associated lava dome and the pyroclastic flow deposit distribution and found that the position of the active vent within the summit caldera differed for each eruption from the late 17th through the 19th century. Moreover, we identified a previously unrecognized lava dome produced by a late 17th century eruption; this dome was largely destroyed by an explosive eruption in 1822 and was replaced by a new lava dome during a later stage of the 1822 event at nearly the same place as the destroyed dome. This new interpretation of the sequence of events is consistent with historical sketches and documents. Our results show that petrological correlation, together with geological evidence, is useful not only for reconstructing volcanic eruption sequences but also for gaining insight into future potential disasters.     

Magnetic monitoring at Merapi volcano, Indonesia

Merapi volcano, located 30 km north of the heavily populated city of Yogjakarta, Java, is one of the most active of the 129 volcanoes in Indonesia. About every 2 years a new phase of activity is observed. Depending on the past activity the unrest gives rise either to an endogenous dome which partly collapses in the southwest direction or to pyroclastic flows which travel as far as 15 km. The 1990–1997 period has involved a plume emission on 30 August 1990, an extrusion on 20 January 1992, and a pyroclastic eruption on 22 November 1994. The intensity of the Earth magnetic field has been measured simultaneously and digitally recorded at four stations since 1990. Two Overhauser magnetometers with resolution of 0.01 nT have been installed in the summit area to strengthen the volcano monitoring. Outstanding magnetic changes appear to correlate with volcanic activity. Three types of volcanomagnetic signals can be identified: long-term trends up to 15 nT with period >10 years; medium-term cyclic variations, at most 3 nT in amplitude and with 1–2 years period; and small events, reaching 1.5 nT, lasting a few months, and associated with any remarkable volcanic activity. Merapi volcano began a new cycle of activity in 1995 leading to a dome growth in July 1996, and accompanied by 27 nuées ardentes in August. The comparison between magnetic data, seismicity, and surface phenomena suggests that some long-term trends of decade periods could be of thermomagnetic origin, while mid-term volcanomagnetic variations associated with the cycles of Merapi activity could be of piezomagnetic origin. Short-term variations of a few weeks duration, less than 1.5 nT, are well correlated with the 1995–1996 seismic activity.