Magnitude 8.0 earthquake in Samoa, largest of 2009

Here is today's record on the Franklin and Marshall seismograph:


The U.S. Geological Survey lists this as the largest quake so far of 2009.

The tsunami warning has been cancelled.

Here is the basic info:

Magnitude	8.0
Date-Time	Tuesday, September 29, 2009 at 17:48:11 UTC Tuesday, September 29, 2009 at 06:48:11 AM at epicenter Time of Earthquake in other Time Zones
Location	15.558°S, 172.073°W
Depth	18 km (11.2 miles) set by location program
Region	SAMOA ISLANDS REGION

North Korea nuclear test seismogram - crustal phases

Figure from the Nevada Seismological Laboratory

Back on January 17, I had an entry on crustal phases. We should do a short review to understand the nomenclature shown in the diagram in the last blog for the seismic phases seen on China seismograms after the Korea nuclear test.

You can find a listing of seismic phases here.

Note in particular the phases shown in the previous quake observed for the NK nuclear test:

• Pg - at short distances, an upgoing P wave from a source in the upper crust; otherwise, a wave refracted from a mid-crustal discontinuity
  
• Pn - a P wave refracted along the crust-mantle boundary
  
• Sg - at short distances, an upgoing S wave from a source in the upper crust
  
• Sn - an S wave refracted along the crust-mantle boundary

Rayleigh and L waves are surface waves.

North Korea nuclear test seismograms

Story and image from LDEO:

Seismologists who have intensively studied North Korea’s nuclear testing efforts say Monday’s blast was certainly a nuclear bomb, roughly five times larger than the country’s first test in 2006. The scientists, at Columbia University’s Lamont-Doherty Earth Observatory, say signals picked up by seismic stations including one in China close by, showed clear characteristics of a shallow underground nuclear explosion, with a force of probably several kilotons.

“The second test appears to have a significantly higher yield,” said seismologist Paul G. Richards, a longtime expert on seismic detection of nuclear tests. He and his colleague Won-Young Kim, also at Lamont, are coauthors of articles on the 2006 North Korean test in Nature Physics and Eos Transactions, and have a piece on current advances in nuclear-test detection in the March 2009 Scientific American.

Reports so far have given varying estimate of the explosion's size, and many suggest that only eventual leakage of radioactive particles into the air will confirm that the explosion was nuclear, not chemical. The assessment by Richards and Kim is in line with an anonymous Obama administration official who told The New York Times shortly afterward that it was a nuclear blast of “several kilotons.”

The seismic pattern produced by the latest blast was nearly identical to the Oct. 9, 2006, North Korean test, which later leaked radioactive particles. “The seismogram is almost a carbon copy of the previous image,” said Kim. Seismograms show two sudden, sharp jolts, followed by an angry buzzing of the earth. The dominant seismic waves produced were fast-traveling compressional, or P-waves, which alternately compress and dilate the ground outward from the source--the signature of an explosion. Earthquakes, by comparison, usually start out with weaker P-waves, and produce more shear waves, which shake the ground sideways in relation to their direction of travel. Kim said analysis of signals collected by different stations located the explosion in the same mountainous test area where the last test took place, some 40 kilometers northwest of the city of Kilju.

Richards said it was “implausible” that the blast, which produced shaking equivalent to an earthquake of magnitude 4.5 to 4.7, could have been made by surreptitiously planted conventional explosives. The explosion’s size cannot be exactly quantified because seismic measurements are influenced by the depth of the explosion, and the type of material in which it takes place. Geologic maps show that the surrounding area is made of hard granite and other rocks that transmit energy efficiently, but no one except the North Koreans knows the depth of the blast, nor whether it was in direct contact with bedrock. Kim put the blast in the range of 2.2 to 4 kilotons (a kiloton is equal to 1,000 tons of TNT). By comparison, the 2006 test was estimated to be less than a kiloton, and was considered by many to be only a partial success that did not completely detonate.

Other estimates of the current test have ranged widely. The Russian Defense Ministry said it might have yielded 15 or 20 kilotons—the size of the bombs that leveled Hiroshima and Nagasaki—while European scientists have estimated three to eight kilotons. However, some of these other estimates were made using data from stations far from the source, or which did not have readings from the 2006 event to use as a baseline. Richards and Kim took data only from a handful of stations that monitored the 2006 test, as well as earlier known chemical explosions and natural earthquakes in the same area that could be used for comparison. This included readings from the closest station from which data is available, in Mudanjiang, China, some 370 kilometers north of the test site. Much Chinese seismic data is unavailable to other countries, but Mudanjiang, operated with the U.S. Geological Survey and an international consortium of universities, transmits readings to researchers worldwide almost in real time. Kim and Richards also obtained information on signals from stations in South Korea, Australia, Alaska and Kyrgyzstan. Each showed amplitudes three to seven times higher than in 2006, leading to the conclusion that the new test had a yield roughly five times larger than the previous one.

By any account, the latest test blast was small compared with the bombs now stocked by major countries including the United States, which run upward of 50 megatons. But, said Richards, “Even the 2006 North Korean test would have been a disaster if it had gone off in a metropolitan area. This one was bigger.”

Mount St. Helens seismogram, May 18, 1980

From the USGS Cascades Volcano Observatory

Note the harmonic tremors in the bottom half of the record, which are characteristic signals accompanying magma rising through a volcano.

But the event at 15:32 GMT (8:32 am Pacific time) is the quake that accompanied the main blast.

From the U.S. Geological Survey:

Within 15 to 20 seconds of a magnitude 5.1 earthquake at 8:32 a.m., the volcano's bulge and summit slid away in a huge landslide - the largest on Earth in recorded history. The landslide depressurized the volcano's magma system, triggering powerful explosions that ripped through the sliding debris. Rocks, ash, volcanic gas, and steam were blasted upward and outward to the north. This lateral blast of hot material accelerated to at least 300 miles per hour, then slowed as the rocks and ash fell to the ground and spread away from the volcano; several people escaping the blast on its western edge were able to keep ahead of the advancing cloud by driving 65 to 100 miles an hour! The blast cloud traveled as far as 17 miles northward from the volcano and the landslide traveled about 14 miles west down the North Fork Toutle River.

The lateral blast produced a column of ash and gas (eruption column) that rose more than 15 miles into the atmosphere in only 15 minutes. Less than an hour later, a second eruption column formed as magma erupted explosively from the new crater. Then, beginning just after noon, swift avalanches of hot ash, pumice, and gas (pyroclastic flows) poured out of the crater at 50 to 80 miles per hour and spread as far as 5 miles to the north. Based on the eruption rate of these pyroclastic flows, scientists estimate that the eruption reached its peak between 3:00 and 5:00 p.m. Over the course of the day, prevailing winds blew 520 million tons of ash eastward across the United States and caused complete darkness in Spokane, Washington, 250 miles from the volcano.

During the first few minutes of this eruption, parts of the blast cloud surged over the newly formed crater rim and down the west, south, and east sides of the volcano. The hot rocks and gas quickly melted some of the snow and ice capping the volcano, creating surges of water that eroded and mixed with loose rock debris to form volcanic mudflows (lahars). Several lahars poured down the volcano into river valleys, ripping trees from their roots and destroying roads and bridges.

The largest and most destructive lahar was formed by water seeping from inside the huge landslide deposit through most of the day. This sustained flow of water eroded material from both the landslide deposit and channel of the North Fork Toutle River. The lahar increased in size as it traveled downstream, destroying bridges and homes and eventually flowing into the Cowlitz River. It reached its maximum size at about midnight in the Cowlitz River about 50 miles downstream from the volcano.

Note the sequence of events: thge earthquake first, the landslide next, and then the uncorked volcano blew its stack.

Has it blown yet?

Redoubt Volcano seismograph locations:

Click on above for larger version
from Alaskan Volcanological Observatory

The current seismogram at AVO can be seen here. Notice all the activity, the fairly constant amplitude, and the high frequency signals.

If you live in Anchorage, better buy your goggles and dust mask!

Pakistan earthquake - a teachable moment

(click above for more detail)

For better of for worse, major earthquakes make us wonder about this geophysical phenomenon. Educators can use these events in the news to help their students think about science. So let's try a few entries focused on the Pakistani earthquake.

First, a seismogram (the record) from the Lamont-Doherty seismograph (the instrument) in Palisades, NY.

Where can you get this seismogram? What can you do with it? What can you learn from it? Stay tuned for subsequent posts.