VOLCANO WATCH: Hawai`i Has Seen Oso-Style Landslide in Past
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by Hawaiian Volcano Observatory
Landslides are hazards in areas where slopes are steep. The degree of the hazard depends on the type of rocks that make up the slope.
Large landslides, like other natural hazards, tend to recur in the same locations where they have occurred in the past.
The Oso landslide of March 22, 2014, also known as the SR530 or Steelhead landslide, surprised everyone, even though it occurred in a river valley with frequent landslides.
In minutes, the landslide swept across the valley (0.7 miles), destroying about 40 structures and taking 41 lives (with 2 still missing at last count) in the unincorporated neighborhood of Steelhead Haven, Wash.
It was the deadliest single landslide in the history of the lower 48 states.
In the aftermath of the slide, its cause was variously attributed to a recent earthquake (later ruled out), unusually high rainfall, and/or logging in the area, but its trigger remains unknown. Earlier geologic studies identified several landslide deposits composed of glacial sediments running along the north fork of the Stillaguamish River valley, some as recent as 2006.
Could such a disaster happen in Hawai'i?
The answer may surprise you—it already has.
On April 2, 1868, after a week of constant earthquake activity, the strongest earthquake documented in the Hawaiian Islands struck the Island of Hawaiʻi. Its magnitude is now estimated at 7.9 or stronger, and its effects were most intense in the southeastern portion of the island, with total destruction of all buildings in the Kaʻū District.
The earthquake generated a tsunami that killed 46 Hawaiians and destroyed coastal settlements from Cape Kumukahi near Kapoho to Kalae (South Point).
The earthquake also dislodged part of a valley wall in the Wood Valley area of Kaʻū. In minutes, the landslide covered an area four times the size of the more recent SR530 slide, destroying 10 structures, and killing 31 Hawaiian farmers.
Kaʻū residents who observed the mud landslide from a distance thought it was a lava flow. That was understandable, since Wood Valley is on the southeast flank of the active Mauna Loa volcano and since the mudslide appeared to be red in color and was preceded by many earthquakes.
But witnesses at the scene found that the "lava flow" was cold mud, with streams of water draining down each side. The red color came from the abundant volcanic ash soil in the area. With more time and scrutiny, it became clear that this was a landslide.
Ground vibration caused by the earthquake clearly triggered the 1868 landslide.
But earthquakes occur frequently in this area without landslides. What was different in 1868?
The Kaʻū area experienced heavy rainfall just prior to the earthquake. Perhaps the rainfall saturated the ash layers in the valley walls, weakening or liquefying the ash to the point of failure during strong ground shaking.
The geology of the Kaʻū area is also unique, with one or more thick ash layers interspersed between Mauna Loa lava flows. The ash layers are relatively impermeable, compared with the permeable lava flows.
This means that the ash tends to be a barrier to water percolating down through the ground, resulting in water being concentrated in the lava flows.
Before the sugar industry, Kaʻū had many natural springs, with water gushing out of the lava flows and over ash layers that were exposed in cliffs. Now, an extensive set of tunnels cut into the ash layers extract water more efficiently.
Modern mapping and studies suggest that the 1868 landslide itself was composed of lava-flow blocks and ashy gravels. The debris was probably the result of lava blocks and ash sliding from the hillside.
When saturated with water (rain) and shaken by a strong earthquake, the thick layer of volcanic ash liquefied and flowed like water, removing support for the overlying lava layers.
Liquefaction is a dangerous consequence of strong earthquake shaking and, in the case of the 1868 Ka`ū landslide, can result in life-threatening landslides.
Liquefaction during earthquakes can also be a problem on gentler slopes.
For example, failure of bridge supports in the Hāmākua District and in landfill areas that made up Kawaihae harbor occurred during the October 15, 2006, earthquake.
Liquefaction also occurred in the Oso landslide, causing the hillside to behave like a liquid.
Now that the response to the crisis is over, healing and understanding can begin.
Kīlauea activity update
A lava lake within Halema'uma'u produced nighttime glow that was visible via HVO's Webcam during the past week. Abrupt summit deflation occurred on May 10 and was followed by more gradual deflation over the past week, associated with a drop in lava level. As of Thursday, May 15, the lake level was about 57 m (190 ft) below the rim of the Overlook crater.
On Kīlauea's East Rift Zone, the Kahauale'a 2 flow remains active, with its front 8.8 km (5.5 miles) northeast of the vent on Pu'u 'Ō'ō, based on satellite imagery from May 14. Small flows that originated from a spatter cone in the Pu'u 'Ō'ō crater and spilled out of the crater over a week ago appear to be inactive.
There were no earthquakes in the past week reported felt on the Island of Hawai'i.
http://bigislandnow.com/2014/05/16/volcano-watch-hawaii-has-seen-oso-style-landslide-in-past/
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Let's reminisce: Eyjafjallajökull volcano
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First Vesuvius risk map plotted
'Step towards new evacuation plans'
(ANSA) - Pisa, May 15 - Italian geologists have plotted the first complete map of the volcanic risk from Mt Vesuvius near Naples, the only European volcano to have erupted in the last 100 years.
The 650-square-km map, charted by researchers from Pisa and Bari universities, "permits a first major preliminary evaluation of the areas potentially at risk," Pisa University said in a statement.
The map, which was drawn up with the help of the Pisan section of the Italian Institute of Geophysics and Volcanology (INGV), was based on the analysis of 500 years of land movements around the famously active volcano that buried Pompeii in 79 AD.
"It's a first step towards drawing up new detailed evacuation plans," said Pisa University's Giovanni Zanchetta. Naples occasionally goes through scares about its famous volcano.
The last flap was caused by a sonic boom in October 2008.
Citizens of Naples rushed to the phones after hearing a mighty bang over the city that lies in the shadow of Vesuvius.
Switchboards were jammed at the city's eruption hotline until it said the sleeping giant had nothing to do with the noise.
The bang, which was heard across the Naples area and out to sea, was caused by two Italian fighter jets racing to intercept an unidentified intruder.
The sonic boom came as the F16s broke the sound barrier to draw level with the plane and check its credentials.
As the city drew a huge breath of relief, the now-cleared Austrian plane continued its flight home from an aid mission in Chad.
A little over a year before the boom, a previous big fright came in August 2007 after US magazine National Geographic claimed that current evacuation plans wouldn't get people out in time if "the world's most dangerous volcano" blew its stack like it did in 79 AD.
Entitled Vesuvius, Asleep for Now, the report claimed that evacuation plans were not sufficiently up-to-date.
The city's anxiety levels fell after Vesuvius watchers issued a comprehensive denial.
In recent years, Naples officials have repeatedly played down reports that Vesuvius might be set to blow.
Top vulcanologist Franco Barberi recently said that even in the worst-case scenario, Naples' evacuation plan would enable the threatened populace to be smoothly evacuated.
Italy has created simulations of all possible kinds of eruptions, Barberi said.
Recent eruption forecasts have varied, saying the dormant volcano could slumber on for decades or centuries.
Around a million people currently live and work around Vesuvius and at the current rate of expansion this could swell by a further 200,000 by 2016.
In 2003 authorities in Naples started offering people living on the volcano's slopes hefty cash incentives to move away.
So far there have been few takers.
Vesuvius has erupted about three dozen times since it buried the Ancient Roman cities of Pompeii and Herculaneum, killing about 2,000 people.
The most serious blast killed some 4,000 people in 1631.
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Ecuador's Tungurahua Volcano Shoots Ash and Lava
Ecuador's Geophysics Institute is reporting that the Tungurahua volcano has erupted three times, spewing ash and lava in what it called an important increase in activity.
The institute said that two moderate explosions on Saturday were followed by a third of greater size, and pyroclastic…
Ecuador's Geophysics Institute is reporting that the Tungurahua volcano has erupted three times, spewing ash and lava in what it called an important increase in activity.
The institute said that two moderate explosions on Saturday were followed by a third of greater size, and pyroclastic flow stretched 500 meters down its flank. The blasts created a 5-mile (8-kilometer) column of ash above the volcano, which is located 84 miles (135 kilometers) southeast of the capital, Quito.
Authorities said the lava reached the lower part of a ravine called Achupashal, blocking a route to a tourist site called Banos.
Tungurahua is 16,480 feet (5,023 meters) high and has been active since 1999. An outburst in 2006 left four dead and two missing.
http://earthchangesmedia.com/ecuadors-tungurahua-volcano-shoots-ash-and-lava
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