Quake activity along the San Andreas fault is picking up

Tectonic Summary

GREENBRIER, AR EARTHQUAKE SWARM

This earthquake is part of a swarm of earthquakes that began on Tuesday February 15, 2011 and is continuing. This area is slightly south of and most likely related to similar activity (known as the Guy earthquake swarm) of hundreds of small earthquakes near Guy, Arkansas from August 2010 to present. Central Arkansas has a history of earthquake activity with a swarm of thousands of earthquakes smaller than magnitude 4.5 to 4.7 in the early 1980s and another swarm in 2001 (known as the Enola earthquake swarms). The Center for Earthquake Research and Information (CERI) at the University of Memphis and the Arkansas Geological Survey (AGS) have deployed a local seismic array in the Greenbrier-Enola, Arkansas, area to augment regional seismic stations to carefully monitor this situation. USGS scientists have been working with their AGS and CERI colleagues. The CERI and AGS array and personnel are the best source of the most current information about the new earthquake swarm. The AGS and CERI are investigating whether the earthquakes occur naturally or are related to human activities.

Earthquake swarms are common east of the Rocky Mountains; although none of the others have involved so many small earthquakes as the central Arkansas swarms. Scientists don't know why swarms start, why they stop, or how long to expect them to last. The possibility of a larger earthquake cannot be discounted but none of the other swarms have given us any reason to expect an earthquake large enough to cause significant damage in central Arkansas in the near future. Most of North America east of the Rocky Mountains has infrequent earthquakes that can strike anywhere at irregular intervals. The causes of earthquakes are not understood well enough for us to predict earthquakes reliably.

Earthquakes occur on faults. Most earthquakes occur miles deep. At well-studied plate boundaries like the San Andreas Fault System in California, often seismologists can determine the specific fault on which an earthquake occurred. East of the Rockies, far from plate boundaries, that is rarely the case. Most of the known faults are deep, and probably there are other faults that have not been discovered. It is hard to link an individual earthquake to an individual fault. In most areas, the best guide to earthquake hazards is the earthquakes themselves.

Earthquakes east of the Rocky Mountains, although less frequent than in the West, are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than if the earthquake had occurred on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) and more from where it occurred, and it can cause slight damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes it causes damage as far away as 40 km (25 mi).

http://latimesblogs.latimes.com/lanow/2011/02/quake-experts-surprised-by-level-of-destruction-in-new-zealand-see-parallels-california-dangers.html

New Zealand earthquake surprises experts with its level of destruction; California parallels seen

February 22, 2011 | 1:52 pm



When the New Zealand earthquake struck Tuesday, Jason Ingham was preparing for a seminar on earthquake building standards in a Christchurch hotel, which began to shake.

He and his colleagues from the University of Auckland had studied damaged structures after the last Christchurch quake, which struck in September of last year. But he said they could feel immediately that Tuesday’s quake was more intense.

Ingham and the others quickly evacuated their hotel. Across Cashel Street, they saw people climbing out of the top floors of a separate hotel, which leaned to one side.

Throughout the city center, there was chaos. From the initial devastation alone, Ingham said he knew there would be serious injuries. As his group left the city, they saw many victims who were bloodied and bandaged.

“There was a mass exit of people in every direction,” Ingham said. “The first mode was just to survive.”

His group safely evacuated to a motel about a mile outside the city center. Based on initial observations and reports, Ingham said he believed this quake was very different from the previous one he studied. Although it had a lower magnitude, it occurred closer to the city, and ground acceleration was much higher. And though most of the damage last time occurred to unreinforced-masonry buildings, many modern buildings were damaged Tuesday, Ingham said.

“Our instinct is that this exceeded the loads that even the modern buildings were designed for. We are almost certain,” said Ingham, who is an associate professor of civil engineering. “The assumption is that an earthquake of this size would have caused damage in any modern city anywhere.”

He and the other researchers planned to travel back into Christchurch Tuesday morning. He said there was a chance they would be turned away due to the ongoing search-and-rescue efforts within the city.

Whereas after the last quake he researched unreinforced-masonry buildings almost exclusively, he said this time the biggest question would be how newer buildings fared. In the last 30 or so years, New Zealand engineers have followed the same guidelines used in other quake-vulnerable regions such as California and Japan, Ingham said.

“The thing that will attract a very large amount of attention from people all over the world is the performance of the modern buildings that have been designed to current standards that still received damage,” Ingham said. “At least in the city center, almost every building suffered some sort of damage.”

Ingham noted in previous research that, compared with other cities and towns in New Zealand, Christchurch had taken a more passive approach to updating its building codes. He said Tuesday that about half of all the buildings in Christchurch were unreinforced masonry.

Many of the older buildings were completely destroyed Tuesday, he said. He believes a greater portion of the modern buildings will be recoverable.

“With some modern buildings ... there will be cosmetic damage," he said, "but the structural integrity is still OK.”

Thomas Jordan, director of the Southern California Earthquake Center at USC, said Tuesday’s quake also provided a reminder that the most destructive quake can be an aftershock to the main quake. Aftershocks since the September main quake, which was centered in rural farmland, have been moving eastward in recent months, closer to Christchurch.

“Earthquakes don’t happen individually, but in sequences. We have to be prepared that when we get large earthquakes in California that we recognize that that means the seismic hazard has gone up, not down. One earthquake does not mean the end of story,” Jordan said.

In California, the southern San Andreas fault, which is ripe for a large earthquake, could buckle in a series of quakes, Jordan said.

Christchurch was devastated in large part because of the shallowness of the quake and the fact that the strongest shaking occurred precisely in the downtown area of one of New Zealand’s oldest cities. “It’s really the fact that you had a lot of buildings very close to the very strongest hypocenter," Jordan said, "which means the strongest shaking was right there in town."