The eruption cycle of re-awakening of Mt. St. Helens volcano, that included the paroxysmal explosive eruption of 18 May 1980, began on 16 March 1980 with a preliminary swarm of volcano-tectonic earthquakes (Figure 2). The first phreatic eruption from its summit crater was observed on March 27, and was followed then by a sequence of small phreatic explosive eruptions up to the climactic eruption on the morning of 18 May (Mullineaux and Crandell, 1981).

According to a series of photos taken during the first minutes of the eruption (Voight, 1981), the eruption began with a large-scale collapse of the volcanic edifice which was followed by a directed blast eruption. The northward-directed lateral blast of the bulging north flank of the volcano erupted about 2 km³ of cold and juvenile material and devastated an area of nearly 600 km². These events triggered a 9-hr magmatic eruption that drove a vertical Plinian column more than 20 km high. A new crater, with an area of 1.5 × 3.0 km² and of depth of about 500 m, formed in the northern flank and summit of the volcano. The height of the volcano was reduced by about 400 m (Christiansen and Peterson, 1981).

The local effects of the 18 May eruption were observed within the 600-km² devastated area. Fifty-seven people were killed and missing. Autopsies showed that most of the people killed in the eruption likely died from asphyxiation after inhaling hot ash (Bernstein et al., 1986). Civil infrastructures such as highways, railways, airports bridges, buildings, etc., immediately north of the volcano were almost completely destroyed by the lateral blast and debris avalanche (Schuster, 1981).

The 18 May eruption sent large amounts of dust and volcanic ash into the atmosphere. The plume spread quickly to the east, bringing darkness, and reduced visibility for the northern states of the USA (Robock, 1982). The dust cloud produced cooling of surface air temperatures during the daytime and warming at night. However, the amount of sulfur injected into the stratosphere was low, and thus had no impact on climate (Robock, 1981).

Figure 2 shows temporal variations in daily number of volcano-tectonic seismic events, recorded by the regional Pacific Northwest Seismic Network (PNSN, https://pnsn.org/pnsn-data-products/earthquake-catalogs), and their maximum magnitudes during two stages of activity: pre-eruptive and before the great 18 May PEE. The 1980 swarm of volcano-tectonic shallow earthquakes began on 16 March with small events; the first significant M_L 4.2 event was recorded beneath Mount St. Helens on 20 March. The number and magnitude of earthquakes sharply increased starting 25 March with peak number on 27 March, when the first phreatic explosion occurred. After the beginning of the eruption, the number of earthquakes gradually decreased during one month; then their number was more or less stable up to 18 May. The magnitudes of volcano-tectonic earthquakes of this period were rather high; seven April earthquakes were in the range of M_L between 5.0 and 5.2. During period from 1 to 16 May, the maximum magnitudes of earthquakes decreased, six of them were in the range of M_L between 4.3 and 4.7.

The pre-eruptive 93 earthquakes, occurring before the 27 March explosions, were located within the area of about 10 × 20 km² along the NW-SE structure, including MSH in its NW part (Figure 3A). These seismic events formed two clusters: a shallow one, at the depth range of 0 to 5 km, and a deep one, at the depth range from 18 to 25 km (Figure 3B). During the active period, from 27 March and until the PEE of 18 May, totally 627 located earthquakes occurred around the volcanic edifice, mainly within the depth range from 0 to 10 km; the deeper pre-eruption cluster disappeared.

The largest earthquake with M_L 5.7 occurred on 18 May at 08:32:11.4¹ on the background of lower-level seismic activity, just before the great directed blast of the volcano that initiated at 08:32:21, about 10 s after the earthquake-trigger (Voight, 1981). This seismic event was located at the depth of 2.8 km beneath the NW flank of MSH (Figure 3). Study of its P-wave focal mechanism (Zobin, 1992) and its comparison with the CMT solution (Dziewonski et al., 1988) showed that this event represented initial left-lateral strike-slip along the NE-SW oriented fault that in a few seconds was transformed in a normal fault triggering a sector failure of volcanic edifice.

As mentioned by Christiansen and Peterson (1981), the Plinian phase of paroxysmal eruption continued for about 9 h. On the background of decreasing intensity of the Plinian phase, a 4-h sequence of 21 magnitude 4.0 and 4.5 events was recorded (Figure 3). The earthquake foci repeated mainly the distribution of the foci recorded during the stage before the 18 May PEE.

The eruption cycle of re-awakening of El Chichón volcano, that included the PEEs of 3–4 April 1982, began on 1 March 1982 with a swarm of volcano-tectonic earthquakes (Havskov et al., 1983). Near mid-night (at 23:32) of 28 March El Chichón exploded abruptly and vigorously; the eruption then continued until ~ 06:00 the next morning. This powerful explosive eruption fed a 20-km high eruption column and produced heavy ashfalls. During next 6 days (29 March−3 April), the eruptive activity continued intermittently at much-reduced level, consisting of occasional small phreatic explosions. Then, in the evening of 3 April at 19:35 and early morning of 4 April at 05:22, the El Chichón's two paroxysmal eruptions (VEI 5) occurred. The Plinian eruption columns of the explosions that occurred during 3 April and 4 April were 24 km high, with volume of 0.8 × 10⁹ m³, and 22 km high, with volume of 0.8 × 10⁹ m³, respectively. While of shorter duration (each lasting < 2 h), both these eruptions were more powerful than the 28 March eruption. In addition to producing voluminous ashfalls, each eruption also generated pyroclastic flows and surges that swept all flanks of the volcano. As a result of this sequence of great explosions, a nearly complete excavation of El Chichón summit dome was performed, and the 1-km-wide crater formed in its place (Carey and Sigurdsson, 1986; Tilling, 2009).

Local effects of this eruption were tragic. Pyroclastic flows and surges obliterated some villages within a roughly 7 km radius of the volcano, killing about 2,000 people, and ashfalls downwind posed socio-economic hardships for many thousands of inhabitants of the states of Chiapas and Tabasco. The unexpected and vigorous eruption of 28 March caused hasty, confused evacuation of most villagers in the area. Eruptive activity greatly diminished during the next 5 days, and then the most powerful and lethal eruptions occurred 3–4 April—tragically, after many evacuees were allowed by authorities to return home (Tilling, 2009).

The stratospheric cloud, created by the eruption of El Chichón, was about 2.5 km wide and reached an altitude of 25 km; over 20 million tons of sulfur gases were emitted during this event. The stratospheric cloud quickly spread out, encircling the globe in about three week's time. A maximum drop of 0.2°C in the global temperature was measured within two months of the eruption (Vignola, 1999). A significant 10% ozone decrease was recorded in the north temperate regions of North America, Europe, and Asia within the atmospheric layers of 8–16 and 16–24 km during four seasons after the eruption (Angell, 1998).

The eruption was preceded by a 28-day seismic swarm of volcano-tectonic earthquakes (Figures 4, 5), which began on 1 March 1982. Seismic activity was fortuitously recorded by the 14-station seismic network of Comisión Federal de Electricidad, installed by UNAM during 1979–1980 around the Chichoasén hydroelectric plant (~ 50 km south of the volcano's summit; Havskov et al., 1983; Jiménez et al., 1999). Totally 270 events with magnitude Mc ≥ 2 of the 1 March - 4 April swarm were located (Figure 5).

The swarm began with a sharp increase in the number of earthquakes during the first 6 days of activity. Two maximum magnitude events (Mc 4) of the swarm were recorded on 3 and 6 March. Then a gradual decrease in the seismic activity in number and magnitudes of events was observed during the next 2 weeks. The activity increased again beginning from 19 March. The majority (~80%) of earthquakes, preceding the first explosion of 28 March, occurred within an area of 10 × 20 km² having the volcano in the center of this area, ranging within depths from 0 to 25 km (Figure 5B).

Just after the explosion of 28 March seismic activity disappeared but 2 days prior to the PEEs of 3–4 April, the number of earthquakes with Mc ≥ 2 began to increase. After the first explosion of 28 March and before and during the two PEE events of 3 and 4 April, the earthquake foci had filled a vertical N-S oriented volume beneath the volcano at the depth interval from 0 to 8 km (Figure 5B).

The PEE of 4 April (05:35) was followed by a sequence of five intermediate-size (Mc 3.7–3.8) earthquakes that occurred beginning from 07:53 during 5 h on the background of decreasing intensity of the Plinian activity of the volcano (Figure 5). The earthquake foci were distributed around and beneath the focal volume filled during the stage before the 3–4 April PEEs.

The eruption cycle of re-awakening of Pinatubo volcano, which included the paroxysmal explosive eruption of 15 June 1991, began in March 1991 (Figure 6A). Citizens of small communities on the lower northwest flank of Mount Pinatubo felt earthquakes beginning from 15 March, 1991. On 2 April, 1991, during or just after phreatic explosions, vigorous steam vents opened on the upper north slope of the volcano.

From 2 April to late May, steam jetted to heights of 300 to 800 meters. Sometimes these jets were accompanied by ash emissions that reached heights of 1,500 to 3,000 meters. By 9 June, continuous ash emission and occasional small explosions produced ash clouds dense enough to flow slowly down the volcano's western slopes. A new lava dome began to grow in the crater from 9 June. Then a series of strong explosive eruptions began on 12 June, 1991. These culminated in the violent climactic Plinian eruption (VEI 6) of 15 June at 13:42 (L.T.) which caused the collapse of the volcano summit and lasted ~9 h. This eruption produced about 10 km³ of porous, pumiceous deposits, and formed a caldera of 2.5 km in diameter (Sabit et al., 1996; Wolfe and Hoblitt, 1996).

Local effects of this paroxysmal eruption were significant. Pyroclastic flows that were emplaced from the Plinian column during the 15 June paroxysmal eruption traveled as much as 12 to 16 km from the vent in all sectors, impacted directly an area of almost 400 km², and profoundly altered the landscape (Scott et al., 1996). The successive forecasting of the eruptive events and their effects enabled Philippine civil leaders to organize massive evacuations, including the evacuation of Clark Air Base (U.S. Air Force), which was located just east of the volcano. Those evacuations saved thousands of lives. Nonetheless, the climactic eruption coincided with a typhoon on June 15 that caused 200 to 300 deaths and extensive property damage, owing to an extraordinarily broad distribution of heavy, wet tephra-fall deposits (Wolfe and Hoblitt, 1996).

From about 13:40 to about 22:30 on 15 June, Mount Pinatubo produced the largest stratospheric SO₂ cloud ever observed by modern instruments, and the major stratospheric aerosol event since Krakatau exploded in 1883. The giant umbrella cloud covered an area of 300,000 km² at 19:40 and was sustained and growing during the whole 9 h climax, eventually reaching >1,100 km in diameter. The altitude of umbrella cloud was estimated as 25 km at the eastern edge and 34 km at its center for the 15:40 image. The stratospheric cloud was observed by the TOMS satellites to encircle the Earth in about 22 days (Self et al., 1996). After the 1991 Pinatubo eruption, the globally averaged stratospheric temperature rose by about 2°C for about 2 years (Robock, 2000). It was noted also that the sulfuric aerosols, released before and during the eruption, circled the Earth within 3 weeks and remained in the atmosphere for 3 years, reflecting enough sunlight to cool the entire planet by half a degree Celsius during that time (Wendel and Kumar, 2016).

As it was noted, the citizens of small communities on the lower northwest flank of Mount Pinatubo felt earthquakes beginning on 15 March, 1991 but the local seismic observations began by the USGS seismologists only on 7 May (Figure 6A). Therefore, the earthquake swarm prior to the 2 April initial re-awakening of the volcano was not instrumentally recorded. At the same time, no events with magnitude ≥ 4.5 were located by the worldwide seismic survey of USGS (https://earthquake.usgs.gov) in the region of the volcano during March-May.

Figures 6A, 7 show the development of the seismic activity beginning from 7 May, when the local network was installed, and until 12 June, 1991, when the seismic network was destroyed by large explosions. Volcano-tectonic seismic events were recorded within two clusters (Figure 7A): at a distance of about 5 km NW of the summit (I) and just beneath the summit (II). The first cluster of seismic events developed mainly within the depth range from 7 to 3 km while the second group of events was shallower, forming a vertical column within the depth range from 4 km b.s.l. to 1 km a.s.l. This shift of earthquake foci from cluster I to cluster II supposedly indicated the migration of seismicity to the surface beneath the volcano along the fault shown with a dashed line in Figure 7A.

The events from the distant zone I dominated from 21 May to 5 June. Then seismicity of zone II beneath the volcano began to increase its activity, and the intensive swarm of low magnitude (from 0 to 1; largest magnitude 1.5) events was recorded there during 6 to 7 June (Figure 6A). This swarm culminated in the 7 June lava dome extrusion. Just after the beginning of the lava dome growth, the number of earthquakes sharply decreased, and the strong 12 June explosion occurred on the background of low-level seismicity for both seismic zones.

During the culminated stage of great explosions, which occurred from June 12 to 15, the local seismic network was out of service, and the only information about seismic activity was obtained from the worldwide seismic survey of USGS (https://earthquake.usgs.gov) at the level of magnitudes ≥ 4.5. This suggests that the 9-h climactic sequence of explosions of 15 June that began at 13:42 was accompanied by a swarm of earthquakes “triggered” by the explosion (Figure 6B).

During the 9-h climactic sequence of explosions, totally 25 earthquakes of magnitude range from 4.5 to 5.7 were recorded. They were located within Luzon island to the north and north-east of the volcano (note that during March−14 June no seismic events of this magnitude range were recorded there). Epicenters of five of these earthquakes, located within the area of the seismic swarms occurring 7 May−12 June, are shown in Figure 7A. The precision of the depths estimations of these events is very low, and for all of them the USGS survey took the depths simply as 10 km that means “shallower than 33 km” (https://earthquake.usgs.gov/earthquakes/eventpage/terms.php). Therefore, they are not shown in the distribution of hypocenters of Figure 7B.

The comparative 9-h development of the eruption plume and of the seismic activity is shown in Figure 6B. The first earthquakes of this sequence, with magnitude mb 5.1, occurred at 15:39, about 2 h after the beginning of the explosion sequence (Figure 6B). The number of events and magnitudes of the swarm reached a maximum on the background of decrease in the plume height 5–6 h after the beginning of the explosion. Earthquake with magnitude Mw 5.7 was recorded at 18:41; earthquake with magnitude Mw 5.6 was recorded at 19:15.

The eruption cycle of Usu volcano, which included the paroxysmal explosive eruption of 7 August 1977, began on 6 August 1977 after 32 years of quiescence (Table 2). A major pumice eruption of hypersthene dacite occurred from the summit after 32-h preceding earthquake swarm. Large eruptions lasted for a week from August 7 to 14, opening craters 1–4, each about 100 m across.

The first explosion in this sequence was paroxysmal. An ash-laden gray cloud rose from the southeastern base of Ko-Usu beginning from 09:12 of 7 August without notable explosion sounds. The eruption column grew rapidly, and at 10:30 rose up to a maximum height of 12 km in elevation. The eruption lasted about 2.5 h and ceased at 11:40. A new crater (Crater I), about 100 m across, opened at the southeastern base of the Ko-Usu lava dome (Figure 8B; Katsui et al., 1978, 1985).

Large volume of pumice and ash was ejected and transported to the southeast of the volcano by the westerly wind. The pumice and ash showers caused destruction of cultivated fields and forests around the volcano. The alarm was issued by the Japan Meteorological Agency before the beginning of eruption (Yokoyama et al., 1981), and no one was killed or injured seriously by the eruption activity, though some damage was caused not only to cultivated lands and forests by the ejecta, but also to some buildings, underground constructions and others by the subsequent crustal movements (Katsui et al., 1978).

The first earthquake of the premonitory swarm of volcano-tectonic earthquakes occurred at 01:09 on 6 August (Figure 8A). Beginning from 03:30, the earthquakes were strong enough to be felt in the whole area around the Usu Volcano, up to about 8 km from the volcano, but no damage was caused. Rapid increase in frequency of earthquakes occurred about 01:00 on 7 August and continued to the time of beginning of PEE eruption (Figure 8A; Suzuki and Kasahara, 1979; Katsui et al., 1985).

The earthquake swarm consisted of about 3,000 small (magnitude 1-2) volcano-tectonic earthquakes that were located beneath the summit crater at a depth from 0 to 2 km (Figure 8B). The new eruptive Crater 1 (shown as a cross in Figure 8B) was situated within the epicentral zone of earthquakes. The maximum magnitude M 3.7 earthquake was observed half an hour before the beginning of eruption (Yokohama and Seino, 2000).

During the PEE, volcano-tectonic earthquakes disappeared but volcanic tremor accompanied the total process of the volcanic column growth. Figure 9 shows the temporal variations in the height of the eruption column together with the variation in the amplitudes of tremor observed at the seismic station SSU (2.5 km from Crater 1). There is a good correlation between these parameters. The amplitude of tremors reached maximum values when the eruption cloud reached its maximum height (Suzuki and Kasahara, 1979).

The eruption cycle of Soufriere Hills volcano (SHV), which included the paroxysmal explosive eruption of 17 September 1996, began with phreatic explosion on 18 July 1995 after 450 years of dormancy without any preliminary seismic activity beneath the volcano (http://volcano.si.edu/volcano.cfm?vn=360050). Several steam vents formed a NNW line across the Castle Peak Dome. Phreatic explosions with associated steam and ash columns rose to 3 km in height. The largest explosion occurred on 21 August. These explosions continued for four months and gave a way to continuous eruption of juvenile andesitic magma in the form of lava dome and lava flows that began around 15 November 1995 and continued until the PEE of 17 September 1996 (Young et al., 1998).

On 17 September 1996, the SHV started a 9-h period of dome collapse. This collapse began at 11:30 and was followed by pyroclastic flows generation involving totally 11.7 × 10⁶m³ (Dense Rock Equivalent, DRE) in which 40% of the dome collapsed. After 2.5 h of quiescence, a paroxysmal explosive eruption occurred at 23:42. Explosive activity continued for about 50 min. A large crater-like depression 210 m deep formed in the dome open to the east. The height of plume reached 11-15 km, the volume of ejecta was estimated as 3.2 × 10⁶ m³ (DRE). Pumice and lithic lapilli fell widely across southern Montserrat and houses were destroyed in the nearest village (Robertson et al., 1998).

The image of 4-band Advanced Very High Resolution Radiometer (AVHRR) multispectral data, provided from the NOAA polar-orbiting satellite series, shows that the cloud, elongating from Montserrat about 1 h after the 17 September PEE, was 175 km long E-W and 75 km wide N-S. The image, taken later in about 5 h, shows that with a distance from the island the cloud quickly became very diffuse (http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/images).

We use the catalog of volcano-tectonic earthquakes prepared by (Aspinall et al., 1998; Miller et al., 1998). This catalog included about 600 volcano-tectonic earthquakes with magnitudes from −0.5 to 2.4 that were located from 28 July 1995 (10 days after beginning of the eruption on 18 July) to the end of February 1997. Gardner and White (2002) wrote that the 18 July vent opened without notable accompanying seismicity.

The pre-PEE July 1995-September 1996 volcano-tectonic earthquakes occurred within three epicentral zones (Figure 10A): beneath St. George Hill (SGH), that is situated about 5 km to the NW from the volcano (I), along the structure extending to the NE of the volcano (II), and beneath Soufriére Hills volcano (III).

As noted before, the eruption developed into two main stages: phreatic and magmatic, separated by a long (about 1 year) stage of the construction of lava dome and emplacement of associated lava flows. We describe here the variations in seismic activity associated with the stage of phreatic explosions that reached its peak on 21 August 1995 and the stage of occurrence of magmatic PEE that occurred on 17 September 1996.

Increase in seismic activity before the phreatic explosion of 21 August began with a 2-day swarm of earthquakes along the NE extent of the SHV (Zone II) from 5 to 6 August, then a new two-day swarm was recorded beneath SGH (Zone I) that continued from 12 to 13 August (Figures 10A, 11A,C). These seismic events occurred at the depth range from 6 to 3 km b.s.l. Beginning from 20 August the seismic foci migrated upward to the surface filling the volume III beneath the volcano. During 2 days, 20 and 21 August, about 140 earthquakes with magnitudes between −0.5 and 0.4 were recorded. The seismic events recorded beneath the volcano clustered within a vertical column shape volume extending from 3 km b.s.l. to the summit of the volcano.

The explosion occurred at 08:03 of 21 August (B. Voight, personal communication, 2002) during the maximum level of seismic activity within zone III beneath the volcano. Just after the explosion, the number of seismic events sharply decreased.

The magmatic PEE of this SHV's eruptive cycle occurred 1 year later, on 17 September 1996. The seismic activity that preceded this paroxysmal explosion from 10 August is shown in Figures 10B, 11B,D. The zones I and II, that were active before the 21 August 1995 phreatic explosion, now were calm. The earthquake foci were clustered mainly beneath the SHV (zone III) forming a vertical column rising from the depth of 3 km b.s.l. to the summit of the volcano.

The swarm of earthquakes had no clear beginning (Figure 10B); it occurred on the background of continuous seismic activity due to the process of partial collapsing of the dome that began in July 1996. Nevertheless, we can mark the gradual increase in earthquake numbers beginning from 29 August; therefore, the swarm continued for about 23–24 days. The earthquakes were of low magnitudes between 0 and 1.9; the largest event with magnitude 1.9 occurred on 17 September at 10:01, just before the 9-h crater collapse preceding the PEE. The explosion occurred during the apparent decrease in number of seismic events. After the PEE event, the number of earthquakes sharply decreased.

The eruption cycle of Volcán de Colima, which included the 2004–2005 episode with paroxysmal explosive eruption of 5 June 2005, began on 22 November 1997 with 1-year sequence of volcano-tectonic earthquake swarms. Beginning from 20 November 1998, sequence of four endogenous episodes of lava dome construction and three episodes of lava domes destructed by Vulcanian explosions, was recorded. The most intense Vulcanian explosions (VEI 3) were recorded during the 2004–2005 episode of lava dome construction-destruction. The extrusions of andesitic magma, which occurred during September-November 2004, had formed a new lava dome filling the active crater. The March 2005 explosions destroyed the 2004 lava dome. The repetitive building of new small-size domes was then observed during the April to July, 2005 explosive activity; each of them was destroyed by the following explosions. Finally, a crater ~ 260 m across and ~ 30 m deep was left (Zobin et al., 2010). A photo, taken on 16 June from the southern side of the volcano (Zobin et al., 2010), shows the erosion channel at the summit, which was formed by pyroclastic flows related to the recent explosions. Many small impact craters formed on the floor of the crater.

The explosion, which occurred on 5 June 2005 at 14:20, was the largest in this sequence and may be considered as the paroxysmal event of the 2004–2005 episode as well as the total 1997–2011 cycle. This PEE blasted an ash plume which reached 9 km a.s.l. and had a length of 222 km. The time duration of the PEE, considering the time of explosion column growth and development, was not estimated but it continued during less than half of hour. The eruption column collapse was followed by emplacement of pyroclastic density currents with runout of 5.1 km and volume of 2.3 × 10⁵ m³. Fall of ash was observed in the nearest villages. The local airport canceled all flights for a day. After the explosion of 5 June, evacuation was carried out of the village of Juan Barragan located nearest to the volcano (Zobin et al., 2016; http://volcano.si.edu/volcano.cfm?vn=341040).

The 1998–2013 sequence of the lava dome constructions-destructions was preceded by a swarm of volcano-tectonic earthquakes only before the first 1998–1999 episode. The third episode of the cycle that developed during September 2004 to December 2005 (http://volcano.si.edu/volcano.cfm?vn=341040) was not preceded by any volcano-tectonic seismicity. As it can be seen in Figures 12, 13, the seismic activity before and during the episode was produced by numerous rockfalls, pyroclastic flows, and small explosions. Their daily number sharply increased during the lava dome extrusion occurring from the end of September 2004 to the beginning of November 2004 and stayed at a level of 200–300 rockfall events and 50–60 small explosions. Before the beginning of and during the sequence of the March-April 2005 large explosions, the number of the seismic signals of rockfalls and small explosions was at low level, between 1 to 5 daily events. The number of the seismic events of both types began to increase until 10-20 daily earthquakes in the middle of May. The beginning of June, before the 5 June PEE, was characterized by a sharp decrease of small explosions but also increase in the number of rockfalls (Figure 12). Another type of the seismic activity before the PEE was represented by microearthquakes that sharply increased in their number about 12 h before the PEE (Figure 13B). These microearthquakes, clearly seen on the seismogram recorded at a distance of 1.6 km from the crater (Figure 13A), were identified as micro-explosions and micro-rockfalls (Zobin et al., 2010).