# Volcano Watch — Contrasting the hazards of earthquakes and volcanoes

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On May 22, 1960, the largest earthquake ever recorded struck the coast of western Chile. The magnitude of this quake was so great that it literally went off the Richter scale; seismologists estimate the effective magnitude at about 9.5. The amount of fault slip during this quake and the area over which the slip occurred were both staggering.

The average slip was 21 m (70 ft), with the maximum slip exceeding 30 m (100 ft). The fault area was more than 160,000 square kilometers (62,000 square miles), about the size of Florida. This single earthquake released nearly one quarter of the 20th century's total seismic energy. It killed about 5000 people, including 61 people in Hilo who drowned in the resulting tsunami.

Twenty years later, almost to the day, on May 18, 1980, Mount St. Helens erupted, producing a tremendous lateral blast that killed 57 people. This eruption was devasting, but in absolute terms, rather moderate. Indeed, in terms of ejected volume, the 1912 eruption of Novarupta Volcano in Alaska was more than 20 times larger.

The 1960 Chilean earthquake, though certainly not the largest earthquake to have ever occurred, was still just about as big as earthquakes get. In contrast, eruptions like Mount St. Helens and Novarupta are at the small end of the eruption continuum. The largest eruptions in human history fill out the continuum somewhat, but the truly monstrous eruptions that dot the geologic past have never been seen by human eyes. For this we should be thankful, since a large eruption can disrupt the global climate on a scale that makes humanity's CO2 emissions seem quaint by comparison.

"Magnitude 9" eruptions occur once or twice every million years. About 760,000 years ago, Long Valley Volcano, in eastern California near the resort town of Mammoth Lakes, erupted more than 700 cubic kilometers (170 cubic miles) of ash and pumice in a matter of days. This is around 350 times more material than Kilauea has erupted in the last two decades. When that much material is erupted, it leaves a big hole or, to use the technical name, a "caldera." At Long Valley the caldera is several hundred meters deep and almost the size of Kauai.

The bottom line is that humanity has experienced earthquakes throughout the magnitude spectrum, but that our experience with volcanoes is concentrated at the low end of the scale. How does this affect our ability to assess seismic and volcanic hazards? Paradoxically, the answer is, "not much."

Our extensive experience with earthquakes that range from imperceptible to planet-shaking has led nowhere in terms of short-term prediction. The situation is better in the long term, however. Seismologists can state with very high confidence that an earthquake will occur along the San Andreas fault before the century is out. Yet, this information, though valuable in terms of how soundly to build our homes, falls well short of what is needed to prevent catastrophe. It is impractical to evacuate California until further notice, but based on (very accurate) long-term predictions of seismic activity, this is probably the safest thing to do.

The problems with volcanic predictions are almost exactly reversed. Volcanologists have an excellent track record of correctly calling imminent eruptions. In fact, the biggest hurdle facing a volcanologist with a short-term eruption forecast is often just getting the authorities to accept the reality of the situation. Long-term volcanic prediction, however, remains a difficult business.

To understand the problems of eruption and earthquake prediction, consider this folksy analogy: Aunty Pele drops by unexpectedly every few years. She never bothers to tell you about her vacation plans, but she usually rings you up an hour or so before appearing at your door. Uncle Shakey also drops by unexpectedly, but his job requires a lot of methodical travel, so you have developed a sense for when he is overdue. Unfortunately, Uncle Shakey hates cell phones, so the first announcement of his arrival is the doorbell. The good news is that Aunty Pele rarely catches you unawares, and Uncle Shakey never sees your house in need of a coat of paint. The bad news is that Uncle Shakey can arrive while you're in the bathtub while Aunty Pele may visit over Super Bowl weekend.

### Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Puu Oo vent during the past week. A third ocean entry of the Mother's Day flow started this past week when lava covered the isolated remnants of roadway at Highcastle and entered the ocean. All three entries have intermittent activity at this time. A large aa flow worked its way down the pali and is now forming a fan at the base of Paliuli. Lava viewing is spectacular, and the National Park Service is allowing visitors to hike out and get up close to the active flows.

The eastern Boundary flow emanating from the "rootless" shields is still dark, but weekly geophysical measurements detect lava flowing beneath the crusted surface.

Three earthquakes were reported felt during the week ending on August 15. Residents of Ahualoa and Paauilo felt an earthquake at 5:18 p.m. on August 9. The magnitude-3.2 earthquake was located 4 km (2.4 mi) southeast of Paauilo at a depth of 9.1 km (5.5 mi). A magnitude-3.5 earthquake was felt at 9:37 p.m. on August 13 by residents of Hawi, Waimea, and Kapulena. The earthquake was located 55 km (33 mi) north of Honokaa at a depth of 10 km (6 mi). A rare earthquake northeast of Oahu was felt by residents of Oahu and Molokai at 8:06 a.m. on August 15. The magnitude-3.9 earthquake was located 14 km (8.4 mi) northeast of Kailua at a depth of 7.9 km (4.7 mi).