# Volcano Watch — The Legacy of the 1906 San Francisco Earthquake

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This week marks the 100-year anniversary of the 1906 San Francisco earthquake, the most deadly earthquake to have hit the United States in recorded history.

Enechalon fractures from The Great 1906 San Francisco earthquake.

(Public domain.)

An estimated 3,000 lives were lost. San Francisco, the commercial, cultural, and financial center of the West, was reduced to smoldering ruins in minutes. Survivors became homeless as those buildings which did not collapse in the earthquake were engulfed in raging fires. While it was not the largest earthquake ever, it was certainly the most sensational.

Four years earlier, another city, on the Caribbean island of Martinique, was also devastated by an unexpected natural disaster-this time a volcanic eruption. The town of St. Pierre, on the lower flanks of Mt. Pelee, was razed to the ground by pyroclastic flows in the space of a few minutes, the first one killing around 23,000 people. By the end of the terrible summer of 1902, pyroclastic flows and surges had killed at least 29,000.

From an early 20th century perspective, these events would have appeared to come out of the blue. Before the 1902 eruption of Mt. Pelee, pyroclastic flows were unknown to science-the people of St. Pierre had little understanding of the phenomenon and, consequently, little chance of mitigating the disaster. Devastating earthquakes had not occurred in the United States in recorded history; the 1906 earthquake caught northern California by surprise, and the city of San Francisco was unprepared for it.

The pace of progress in science is forced by such events. The 1906 earthquake caused scientists to investigate the links between surface fracturing and processes occurring at depth. A scientist named Henry Fielding Reid worked out the mechanism of why earthquakes happen. He deduced that the Earth's surface is not rigid-it bends. Land on either side of a fault may move in opposite directions, but the fault itself may remain locked, under increasing stress. Eventually the fault ruptures, causing the land to rebound violently, releasing accumulated strain very rapidly, resulting in an earthquake.

A century later, with the establishment of the plate tectonic theory, we have a broad explanation of why earthquakes and volcanic eruptions occur in certain parts of the world. The Earth's upper layer, called the crust, sits on top of a ductile mantle. The mantle is capable of slow, convective overturn that drives crustal plate motion. The crust is broken into around 15 plates that move independently of one another to a certain extent. Where the plates interact, life gets very interesting.

Some plates slide against one another, some are in collision, and some are being pulled apart. At the boundaries, the plates are broken and ruptured, forming fault zones, which distribute plate movement over a large area. Movement on the faults takes place as either creep or rupture, or both. Creep is a continuous slow movement; rupture is an earthquake.

Volcanic eruptions occur where plates are pulling apart, causing the ductile mantle rocks to ascend and melt, or where one plate slides beneath another, causing melting. In the case of Hawaii, a mantle hot spot generates melt in the interior of an oceanic crustal plate-a special case of volcanism.

The all-inclusive theory of plate tectonics cannot, however, predict exactly when earthquakes and volcanic eruptions will occur. While the utopia of earthquake prediction is still elusive, the last century brought tremendous advances in monitoring and understanding of earthquake hazards. Now, hazardous faults, such as the San Andreas, are accurately delineated and peppered with seismometers, GPS receivers, and strain meters, allowing scientists to capture their movement. Consequently, we are in a much better position to minimize earthquake hazards, principally by engineering buildings and other structures to survive severe shaking, as well as understanding how the rocks on which they are built respond in an earthquake.

Large, violent events, such as the Asian tsunami in 2004, force scientists and engineers to keep up the pace of research into the causes, mechanisms, hazards, and risks associated with natural disasters. Since large tsunami-spawning earthquakes are so infrequent, there was little effort to understand and plan for one in the Indian Ocean prior to 2004. Where will we stand 100 years from now in terms of how well we mitigate natural hazards? Will we have achieved the long-held dream of being able to predict earthquakes?

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### Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have returned to background values. The number of earthquakes located in the summit area is low (usually less than 10 per day). Inflation of the summit caldera appears to have slowed or paused over the past week. It is premature to say whether this change is significant or not because inflation can pause for days to weeks before resuming.

Eruptive activity at Puu Oo continues. On clear nights, glow is visible from several vents within the crater. Lava continues to flow through the PKK lava tube from its source on the flank of Puu Oo to the ocean, with occasional surface flows breaking out of the tube. In the past week, surface flows were active on the coastal plain in about the same location as the last several weeks: 0.4 km (0.3 mi) inland of the coast at Kamoamoa, about 5.5 km (3.4 mi) from the end of Chain of Craters Road.

As of April 13, lava is entering the ocean at East Laeapuki, in Hawaii Volcanoes National Park. The active lava bench continues to grow following the major collapse of November 28 and is now approximately 1,000 m (3,300 ft) long by 315 m (1,000 ft) wide, with a total area of 17 ha (42 acres). Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were three earthquakes beneath Hawaii Island reported felt within the past week. A magnitude-3.4 earthquake occurred on Thursday, April 6, at 5:11 p.m. and was located 15 km (10 miles) northwest of Kailua at a depth of 16 km (10 miles). A magnitude-2.9 earthquake occurred on Saturday, April 8, at 3:40 p.m. and was located 15 km (10 miles) southwest of Loihi seamount at a depth of 36 km (22 miles). A magnitude-3.1 earthquake occurred on Monday, April 10, at 8:16 a.m. and was located 19 km (12 miles) northwest of Naalehu at a depth of 1 km (1 mile).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (one earthquake was located). Inflation continues to slow down. skip past bottom navigational bar