Shallow Volcano-tectonic Earthquakes Research Articles

The August 2019 Piton de la Fournaise (La Réunion Island) Eruption: Analysis of the Multi-Source Deformation Pattern Detected through Sentinel-1 DInSAR Measurements

Piton de la Fournaise is one of the most active worldwide volcanoes, located on the southeastern part of La Réunion Island. In this work, we focus on the eruption that occurred on the southeastern flank of this volcano, inside the Enclos Fouqué caldera, from 11 to 15 August 2019. This distal event was characterized by the opening of two eruptive fissures and accompanied by shallow volcano–tectonic earthquakes. We exploit the ground displacements using Sentinel-1 Differential Interferometric Synthetic Aperture Radar (DInSAR) measurements, which include the ground deformations generated during both the pre- and co-eruptive phases. To investigate the sources responsible for the detected ground displacements, we perform an analytical modeling of the retrieved DInSAR measurements. Our results reveal the presence of five volcanic sources (i.e., one sill-like source and four dikes), whose concomitant action during the pre- and co-eruptive phases generated the complex detected deformation pattern. The retrieved volcanic sources correlate well with the location of the opened fissures, the spatial distribution and the temporal evolution of the recorded seismicity, and other geophysical evidence already known in the literature.

Magma intrusion and effusion at Sinabung volcano, Indonesia, from 2013 to 2016, as revealed by continuous GPS observation

We analyzed continuous Global Positioning System (GPS) data from Sinabung to capture and model the migration of magma from the pre-eruptive and syn-eruptive time period between June 2013 and January 2016. We divided this time into four periods of significant deformation: two extensional stages followed by two contractional stages. Using a grid search method, we determined the location and volume change of a Mogi source for each deformation stage. Cumulative volume changes during the contraction periods were approximated by an exponentially decaying function with time. Period 1 began in June 2013 with slight extension, for which an inflation source was modeled at a depth of 3–8km below sea level (bsl) and a volume change of 0.3–1.8Mm3. Seismicity in period 1 was marked by a notable increase in deep high frequency volcano tectonic earthquakes (VTs) beginning in July 2013 and shallow VTs in September 2013. Period 2 began in late October 2013 with accelerated extension, with at least 1cm extension in the baseline length. During period 2 the modeled inflation source ascended to a shallower depth of 0.9 (0.4–2.1) km below sea level (bsl) beneath Sinabung with a change in volume of +0.39 (+0.18–+0.60) Mm3, and with accelerated rates of volume increase during the time period when the magma migrated to the surface. Seismicity during period 2 was marked first by an increase in the incidence of shallow volcano-tectonic (VT) earthquakes and later by repetitive self-similar hybrid events as the magma migrated to the surface. Period 3 began in January 2014, after the appearance of the lava dome, and was marked by rapid steady contraction of ~3cm through March 2014. The modeled source located at 8.4 (7.4–9.9) km bsl beneath the eastern flank of Sinabung with a volume change of −20.51 (−26.89 to −14.12) Mm3. Period 4 began in April 2014 with decelerating contraction, and the modeled deformation center shifted to the northeast, reaching a depth of 12.2 (10.1–14.8) km bsl between Sinabung and Sibayak volcanoes and a change in volume of −88.26 (−123.87 to −52.66) Mm3. Approximately 2/3 of the total volume change related to contraction occurred between January 2014 and May 2016, and the current activity of Sinabung is expected to decrease gradually and almost terminate in the early 2020s, assuming no new intrusion or deformation rate changes. Both of the eruptions at Sinabung in 2010 and Unzen in 1991–1995 show characteristics of ground inflation and subsequent deflation, indicating magma migration and effusion processes similar to the current Sinabung activity. The inflation before the 2010 Sinabung eruptions likely started before 2007 and is an indication of magma intrusion before the 2010 and 2013 eruptions.

Ground deformation source model at Kuchinoerabu-jima volcano during 2006\u20132014 as revealed by campaign GPS observation

We analyzed campaign Global Positioning System observation data in Kuchinoerabu-jima during 2006–2014. Most benchmarks located around Shin-dake crater showed crater-centered radial horizontal displacements. Horizontal displacements at western rim of the Shin-dake crater were tended to be larger compared to those at eastern rim. In addition, benchmark KUC14 which locates near the cliff at Furu-dake showed westward horizontal displacement rather than crater-centered radial (southward) one. Meanwhile, small displacements were detected at the benchmarks located at the foot of Kuchinoerabu-jima. We modeled the observed displacements applying a finite element method. We set entire FE domain as 100 × 100 × 50 km3. We set top of the domain as a free surface, and sides and bottom to be fixed boundaries. Topography was introduced in the area within Kuchinoerabu-jima using digital elevation model data provided by Kagoshima prefecture and elevation information from Google earth, and elevation of the outside area was assumed to be sea level. We assumed a stratified structure based on a one-dimensional P-wave velocity structure. We applied a vertical spheroid source model and searched optimal values of horizontal location, depth, equatorial and polar radiuses, and internal pressure change of the source using the forward modeling method. A spherical source with a radius of 50 m was obtained beneath the Shin-dake crater at a depth of 400 m above sea level. The internal pressure increase of 361 MPa yields its volume increase of 31,700 m3. Taking effects of topography and heterogeneity of ground into account allowed reproduction of overall deformation in Kuchinoerabu-jima. The location of deformation source coincides with hypocenters of shallow volcano-tectonic (VT) earthquakes and the aquifer estimated from a two-dimensional resistivity model by audio-frequency magnetotellurics method. The obtained deformation source may be corresponding to the pressurized aquifer, and shallow VT earthquakes and demagnetization may be caused by pressure and strain accumulation in the rocks around the aquifer. Applying the obtained spherical source to the tilt change before August 3, 2014 eruption, we found that 520 m3 of volcanic materials were supplied toward shallower in 1.5 h before the eruption. The depth and volume change of deformation source before May 2015 eruption detected by precise leveling surveys is deeper and two orders of magnitude greater compared to that before August 2014 eruption.

Seismic observations of Redoubt Volcano, Alaska — 1989–2010 and a conceptual model of the Redoubt magmatic system

Seismic activity at Redoubt Volcano, Alaska, has been closely monitored since 1989 by a network of five to ten seismometers within 22km of the volcano's summit. Major eruptions occurred in 1989–1990 and 2009 and were characterized by large volcanic explosions, episodes of lava dome growth and failure, pyroclastic flows, and lahars.Seismic features of the 1989–1990 eruption were 1) weak precursory tremor and a short, 23-hour-long, intense swarm of repetitive shallow long-period (LP) events centered 1.4km below the crater floor, 2) shallow volcano-tectonic (VT) and hybrid earthquakes that separated early episodes of dome growth, 3) 13 additional swarms of LP events at shallow depths precursory to many of the 25 explosions that occurred over the more than 128day duration of eruptive activity, and 4) a persistent cluster of VT earthquakes at 6 to 9km depth.In contrast the 2009 eruption was preceded by a pronounced increase in deep-LP (DLP) events at lower crustal depths (25 to 38km) that began in mid-December 2008, two months of discontinuous shallow volcanic tremor that started on January 23, 2009, a strong phreatic explosion on March 15, and a 58-hour-long swarm of repetitive shallow LP events. The 2009 eruption consisted of at least 23 major explosions between March 23 and April 5, again accompanied by shallow VT earthquakes, several episodes of shallow repetitive LP events and dome growth continuing until mid July. Increased VT earthquakes at 4 to 9km depth began slowly in early April, possibly defining a mid-crustal magma source zone.Magmatic processes associated with the 2009 eruption seismically activated the same portions of the Redoubt magmatic system as the 1989–1990 eruption, although the time scales and intensity vary considerably among the two eruptions. The occurrence of precursory DLP events suggests that the 2009 eruption may have involved the rise of magma from lower crustal depths. Based on the evolution of seismicity during the 1989–1990 and 2009 eruptions the Redoubt magmatic system is envisioned to consist of a shallow system of cracks extending 1 to 2km below the crater floor, a magma storage or source region at roughly 3 to 9km depth, and a diffuse magma source region at 25 to 38km depth. Close tracking of seismic activity allowed the Alaska Volcano Observatory to successfully issue warnings prior to many of the hazardous explosive events that occurred in 2009.

La erupción estromboliana violenta y freatomagmática de 1723 en el volcán Irazú, Costa Rica

The largest of the recorded historic eruptions at Irazu volcano began on February 16, 1723 and lasted until at least December 11. We here critically examine deposits of this eruption exposed on the summit of Irazu. Our reconstruction of the eruption is based on the unique chronicle of the Spanish governor Diego de la Haya. The eruption began with a < 10 cm thick surge deposit of phreatic origin showing block sag structures. The deposit is overlain by 6 m-thick coarse-grained basaltic andesitic non-graded juvenile fallout tephra consisting of highly vesicular (22-59 vol.%) bombs and lapilli with minor hydrothermally altered lapilli (1-7 vol.%) and rare light colored andesitic vesicular lapilli (< 1%). These fallout deposits are interpreted as strombolian, possible generated during a short-lived scoria cone at the end of February 1723, dominate volumetrically in the proximal facies. Overlying <1.2 m thick phreatomagmatic deposits of finely laminated lapilli-bearing gray ash (fallout and surge deposits) some with contorted bedding and sag structures, are in turn overlain by a 1.2 m thick bed of ash matrix-rich bomb/block deposit. The 1723 eruption was accompanied by shallow volcano-tectonic earthquakes (Modified Mercalli scale Intensity MMI VI-VII, magnitude ML ~5.5) that possibly facilitated magma/water interaction. Phenocrysts in the basaltic andesite (~53-55 wt.% SiO2) bombs comprise plagioclase (6.1-21.6 vol.%, An52-35), clinopyroxene (2.5-10 vol.%), orthopyroxene (0.7-2 vol.%), olivine (0.1-2.2 vol.%; Fo76-88) and Fe/Ti-oxides (0.1-1%), in a groundmass (66.5-90.3 vol. %), dominated by plagioclase (An69-54), clinopyroxene and opaques in brown and black glass with the same range of chemical composition (SiO2= 57-64 wt.%). Rare white pumiceous lapilli in the scoria deposits are high-K, hornblende andesite (SiO2: 58-60 wt.%), geochemically unrelated to the scoria deposits. Thus, two different magmas co-existing in the magma chamber were mingled shortly before, and during, the eruption, suggesting that the eruption was triggered by magma mingling between hornblende andesite and basaltic andesite magma.

Temporal changes in stress preceding the 2004–2008 eruption of Mount St. Helens, Washington

The 2004–2008 eruption of Mount St. Helens (MSH), Washington, was preceded by a swarm of shallow volcano-tectonic earthquakes (VTs) that began on September 23, 2004. We calculated locations and fault-plane solutions (FPS) for shallow VTs recorded during a background period (January 1999 to July 2004) and during the early vent-clearing phase (September 23 to 29, 2004) of the 2004–2008 eruption. FPS show normal and strike-slip faulting during the background period and on September 23; strike-slip and reverse faulting on September 24; and a mixture of strike-slip, reverse, and normal faulting on September 25–29. The orientation of σ 1 beneath MSH, as estimated from stress tensor inversions, was found to be sub-horizontal for all periods and oriented NE–SW during the background period, NW–SE on September 24, and NE–SW on September 25–29. We suggest that the ephemeral ~ 90° change in σ 1 orientation was due to intrusion and inflation of a NE–SW-oriented dike in the shallow crust prior to the eruption onset.

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.

Modelling of seismic energy transport at volcanoes with real topography and complex propagation medium

In stratovolcanoes multiple scattering of seismic waves is caused by a strongly heterogeneous propagation medium. Additionally, these waves are scattered by the rough topography of the volcano. We devised an algorithm to model seismic energy transport in volcanoes based on a Monte Carlo simulation of the equation of radiative transfer. This algorithm takes into account both the real topography of volcanoes, which is described by a digital elevation model and the complex subsurface structure of the volcanic edifice, which is treated as a random medium. Theoretical energy densities of two different models were compared to observed energy densities derived from seismograms of shallow volcano-tectonic earthquakes that occurred at Merapi volcano, Indonesia. A model consisting of a heterogeneous volcano-shaped medium over a homogeneous half space was found to be suitable to explain the observed data. For Merapi volcano, our modelling results in a mean free path of 250 m and a thickness of 1.7 km of the scattering volcanic edifice. The systematic decrease of the coda energy with source distance of the observed data, referred to as coda localization, can be successfully explained by our model.

An overview of the seismicity of Merapi volcano (Java, Indonesia), 1983–1994

From 1982 to 1995, Merapi volcano (Java, Indonesia) experienced two main eruptive cycles, from 1984 to 1986 and from 1992 to 1994. Each cycle had a duration slightly larger than 2 years. Four major eruptive events occurred in 1984, 1986, 1992 and 1994. These eruptions were monitored by a telemetered seismic network. We present an overview of the seismicity observed between 1983 and 1995. From seismological observations, several types of characteristic events are recognised. Volcanotectonic earthquakes are standard microearthquakes, either shallow (<1.5 km below the summit) or deep (between 2.5 and 5 km). Volcanotectonic earthquakes seem to correspond to an injection of magma into the upper part of the volcano and to a high gas pressure regime. The explosive eruptions in 1984 and 1992 were both preceded by deep and shallow volcanotectonic earthquakes but the 1986 and 1994 eruptions were not. Multiphase events are associated with the formation of the dome and with the supply of magma and of volatiles to the dome. Their hypocentres are near the crater, beneath the dome. Multiphase events occur when the magma flow in the chimney is large; sometimes the dome grows without multiphase activity. Low-frequency events and tremor are associated with fluid resonance. Rock falls (guguran in Indonesian) dominate the seismic records by their large number. The number of multiphase events is correlated to the number of rock falls. Both are related to dome growth. An aseismic zone is situated between two seismic zones, at a depth between 1.5 and 2.5 km below the summit: we postulate that it is a small shallow magma reservoir where magma injected from below can be stored temporarily. We try to relate seismic observations during both eruptive cycles to the initial injection of deep magma to the superficial magma system. The first eruption in each case seems to involve magma enriched in gas that extrudes the content of the conduit and explodes the old dome. A regular outflow of magma follows the initial explosion and gradually a dome is building up. Then it stops and a period of repose is observed. Later a second episode of dome growth occurs. The height of the dome reaches a maximum corresponding to a state of lithostatic equilibrium. The time delay of more than 2 years between the beginning and the end of a cycle may result from crystallisation in a volatile rich magma and from the building up of pressure in the superficial magma pocket. Some important events, like the collapse of the dome on 22 November 1994, are not preceded by any seismic precursor. When it reaches a large volume, the dome is not stable and is likely to collapse by gravity, for instance after the sudden influx of matter or gas or after a heavy rain. On Merapi, tremors are observed but do not always precede a dangerous event.

Seismic precursors of the February 10, 1990 eruption of Kelut volcano, Java

Kelut volcano, eastern Java, Indonesia erupted on February 10–12, 1990. Seismic activity and an increase of temperature and acoustic noise in the crater lake appeared three months before the eruption and were the main precursory signs of this eruption. During the pre-eruptive stage, the number of events was eight per day on average and the cumulative seismic energy released was 5.6 × 10 15 erg; these low values are uncommon for an andesitic volcano. Seismic activity mainly consisted of swarms of very shallow volcano-tectonic earthquakes, many of which were part of seismic multiplets. In the last three weeks before the eruption, seismic activity remained at a very low level. This quiescent period was followed, 24 hours prior to the eruption initiation, by an intense swarm of low-energy events which progressively became continuous spasmodic tremor. Most of the data obtained during the pre-eruptive stage are consistent with a process of fracturation of a shallow and low-strength plug blocking the eruptive conduit before the eruption. Several observations suggest that water is an important element in the preeruptive and eruptive phenomena at Kelut volcano. In particular, the monochromatic events observed at Kelut are characterized by one narrow peak in their spectra; the estimations of Q-value and impedance contrast for these events suggest that the fluid which fills the source resonator could be water.

Seismic evolution of the 1989–1990 eruption sequence of Redoubt Volcano, Alaska

Redoubt Volcano in south-central Alaska erupted between December 1989 and June 1990 in a sequence of events characterized by large tephra eruptions, pyroclastic flows, lahars and debris flows, and episodes of dome growth. The eruption was monitored by a network of five to nine seismic stations located 1 to 22 km from the summit crater. Notable features of the eruption seismicity include : (1) small long-period events beginning in September 1989 which increased slowly in number during November and early December; (2) an intense swarm of long-period events which preceded the initial eruptions on December 14 by 23 hours; (3) shallow swarms (0 to 3 km) of volcano-tectonic events following each eruption on December 15; (4) a persistent cluster of deep (6 to 10 km) volcano-tectonic earthquakes initiated by the eruptions on December 15, which continued throughout and beyond the eruption; (5) an intense swarm of long-period events which preceded the eruptions on January 2; and (6) nine additional intervals of increased long-period seismicity each of which preceded a tephra eruption. Hypocenters of volcano-tectonic earthquakes suggest the presence of a magma source region at 6–10 km depth. Earthquakes at these depths were initiated by the tephra eruptions on December 15 and likely represent the readjustment of stresses in the country rock associated with the removal of magma from these depths. The locations and time-history of these earthquakes coupled with the eruptive behavior of the volcano suggest this region was the source of most of the erupted material during the 1989–1990 eruption. This source region appears to be connected to the surface by a narrow pipe-like conduit as inferred from the hypocenters of volcano-tectonic earthquakes. Concentrations of shallow volcano-tectonic earthquakes followed each of the tephra eruptions on December 15; these shocks may represent stress readjustment in the wall rock related to the removal of magma and volatiles at these depths. This shallow zone was the source area of the majority of long-period seismicity through the remainder of the eruption. The long-period seismicity likely reflects the pressurization of the shallow portions of the magmatic system.