The number and magnitudes of earthquakes under Sierra Negra volcano in Galapagos, Ecuador are steadily growing since mid-2016. A significant uptick occurred on May 25, 2018, with 104 earthquakes within 24 hours. This, along with ‘very important deformation’ suggests a new eruption is possible in weeks and months ahead.
Sierra Negra is one of the largest volcanoes in the Galapagos Islands and has an oval caldera, which measures 9 km (5.6 miles) in the east-west direction and 7 km (4.3 miles) in the north-south direction. This volcano has produced at least 10 eruptions in the historical epoch, with an average resting period of 15 years between each eruptive period. Its last two eruptions occurred in the years 1979 and 2005.
Earthquakes detected under the volcano are predominantly volcano-tectonic, related to internal rock fracturing. However, a significant number are tremors reflecting fluid oscillations and movements inside the volcano.
In addition to the number of earthquakes, magnitudes are also increasing. Since January 2018, there have been 11 earthquakes with magnitudes greater than or equal to 4.0.
The persistence of these high levels of seismicity suggests that magma is moving within the Sierra Negra volcano, which could produce an eruption, IGEPN said in a special bulletin released June 8, 2018.
Number of earthquakes recorded per day in the Sierra Negra volcano. The colors of the bars correspond to volcano-tectonic (VT), long-period (LP) and hybrid (HB) earthquakes. Credit: IGEPN
The quakes are located mainly on the edges of the caldera, except its NE edge, the source of eruptions in 1979 and 2005.
Given the characteristics of the unrest, an eruption is possible in the days, weeks or months ahead. At the moment, IGEPN thinks the most likely scenario is a moderate-sized eruption.
The broad shield volcano of Sierra Negra at the southern end of Isabela Island contains a shallow 7 x 10.5 km (4.3 x 6.5 miles) caldera that is the largest in the Galápagos Islands. Flank vents abound, including cinder cones and spatter cones concentrated along an ENE-trending rift system and tuff cones along the coast and forming offshore islands.
The 1124-m-high (3 690 feet) volcano is elongated in a NE direction. Although it is the largest of the five major Isabela volcanoes, it has the flattest slopes, averaging less than 5 degrees and diminishing to 2 degrees near the coast. A sinuous 14-km-long (8.7 miles), N-S-trending ridge occupies the west part of the caldera floor, which lies only about 100 m (328 feet) below its rim. Volcán de Azufre, the largest fumarolic area in the Galápagos Islands, lies within a graben between this ridge and the west caldera wall.
Lava flows from a major eruption in 1979 extend all the way to the north coast from circumferential fissure vents on the upper northern flank. Sierra Negra, along with Cerro Azul and Volcán Wolf, is one of the most active of Isabela Island volcanoes. (GVP)
Sierra Negra volcano began erupting on 22 October 2005, after a repose of 26 years. A plume of ash and steam more than 13 km high accompanied the initial phase of the eruption and was quickly followed by a ~2-km-long curtain of lava fountains.
The eruptive fissure opened inside the north rim of the caldera, on the opposite side of the caldera from an active fault system that experienced an mb 4.6 earthquake and ~84 cm of uplift on 16 April 2005.
The main products of the eruption were an `a`a flow that ponded in the caldera and clastigenic lavas that flowed down the north flank. The `a`a flow grew in an unusual way. Once it had established most of its aerial extent, the interior of the flow was fed via a perched lava pond, causing inflation of the `a`a. This pressurized fluid interior then fed pahoehoe breakouts along the margins of the flow, many of which were subsequently overridden by `a`a, as the crust slowly spread from the center of the pond and tumbled over the pahoehoe.
The curtain of lava fountains coalesced with time, and by day 4, only one vent was erupting. The effusion rate slowed from day 7 until the eruption’s end two days later on 30 October.
Although the caldera floor had inflated by ~5 m since 1992, and the rate of inflation had accelerated since 2003, there was no transient deformation in the hours or days before the eruption.
During the 8 days of the eruption, GPS and InSAR data show that the caldera floor deflated ~5 m, and the volcano contracted horizontally ~6 m.
The total eruptive volume is estimated as being ~150×106 m3. The opening-phase tephra is more evolved than the eruptive products that followed. The compositional variation of tephra and lava sampled over the course of the eruption is attributed to eruption from a zoned sill that lies 2.1 km beneath the caldera floor.