Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Medium to large quakes peak every three years on central San Andreas Fault


Medium to large earthquakes occurring along the central San Andreas Fault appear to cluster at regular three-year intervals - a previously unnoticed cycle that provides some hope for forecasting larger quakes along this and other California faults.

The San Andreas Fault as it stretches across California
(USGS graphic)

A study by University of California, Berkeley, seismologists shows a higher probability of moderate to large quakes - magnitude 4, 5 and 6 - just as the frequency of smaller quakes, called microquakes, begins to increase along the northern half of a 110-mile segment of the central San Andreas Fault. The scientists found that the frequency of these repeating microquakes along the fault segment rises and falls over a three-year period, and that moderate to large earthquakes are six to seven times more likely to occur at the upswing of this cycle.

"Larger earthquakes along the northern portion of the central San Andreas occur preferentially when the pulse starts up," said Robert Nadeau, an assistant research geophysicist at the Berkeley Seismological Laboratory. "Quakes greater than magnitude 3.5 tend to happen within one year of the pulse’s initiation or start.

Among the earthquakes that occurred at an upswing in microquakes was the destructive 1989 Loma Prieta quake, a magnitude 7.1 temblor. Assuming the three-year cycle has continued, the next upswing in microquakes should occur in late 2004, Nadeau said.

The study covers 16 years of data, from 1984 until 2000, on quakes along a section of the San Andreas Fault between Loma Prieta in the north, near the city of Santa Cruz, and Parkfield in the south. The San Andreas, which stretches from Los Angeles into the Pacific Ocean off the northern coast of California, broke along its northern half in 1906 to cause the great San Francisco earthquake and fire.

"This phenomenon suggests that something is going on deep under the seismogenic zone, loading this zone and inciting quakes," Nadeau said. The seismogenic zone is the brittle upper crust where earthquakes occur. "This has promise for forecasting larger quakes, though this is our first look and it needs to be refined more."

It’s still unclear whether major quakes such as the 8.0 quake that struck San Francisco in 1906 are associated with quasiperiodic pulses of microquakes, he noted.

Nadeau reports the findings in the Jan. 9 issue of Science in a paper coauthored by his late collaborator, UC Berkeley seismologist Thomas V. McEvilly, a professor of earth and planetary science who died a year ago. Nadeau also will discuss his findings at a two-day workshop, Jan. 13-14, at U.S. Geological Survey headquarters in Menlo Park, as part of the National Earthquake Hazards Reduction Program.

Microquakes are quakes that cannot be felt, typically lower than magnitude 3.5. Once thought to carry little information, McEvilly and Nadeau have turned that thinking around in recent years thanks to studies at Parkfield. UC Berkeley installed 10 borehole seismometers several hundred meters underground at Parkfield more than 15 years ago to collect more precise data on the position of microquakes, and have shown that a large number - perhaps half - actually occur repeatedly in the same spot, popping off every few months to every few years. One has repeated 23 times in the same location.

Nadeau thinks that these "repeaters" represent places where the San Andreas Fault is not lubricated by fluid or mud and thus doesn’t slide as easily as surrounding areas. This may result from an irregularity on one side of the strike/slip fault that repeatedly catches the other side, breaks, and rewelds, only to break again when the strain reaches a certain threshold.

Whatever the cause, these microquakes "are a new way to estimate where zones are locked, how fast the fault creeps, and the tectonic loading in the area," Nadeau said. "Repeating microquakes also let us fill in gaps along the fault where we don’t have reliable measures of creep."

Locked zones, which generate larger quakes when they break, do not produce microquakes. Apart from a small number of locked zones, most of the central San Andreas Fault seems to slip smoothly without large quakes, putting stress on the small stuck patches, which eventually break and cause microquakes.

From their study of microquakes at Parkfield, Nadeau and McEvilly discovered in 1999 that these repeating microquakes could be used to predict the rate at which the fault is slipping deep underground. The faster the slip, the more frequently a given patch generates microquakes.

With this background, they decided to look at the central San Andreas Fault north of Parkfield, stretching all the way to the presumed rupture or breakage of the 1989 Loma Prieta earthquake. The stretch between Parkfield and Loma Prieta creeps fairly evenly at about one inch per year.

When they looked at 2,594 microquakes occurring at 515 repeater spots along the 110-mile segment, with data coming from UC Berkeley’s borehole seismometers and from the Northern California Seismic Network, they found periodic pulses of microquakes every three years on the northern half, indicating a periodic increase in slip rate as deep as 6.3-6.9 miles below ground. When they looked back at 45 quakes greater than 3.5 magnitude that occurred along the same segment during that time, these quakes tended to cluster within one year after the start of microquake activity.

Among those large quakes were the 7.1 magnitude 1989 Loma Prieta quake and the 1998 5.4 magnitude San Juan Bautista quake, both of which coincided with a rise in microquakes.

Nadeau said, however, that it is unclear whether quakes as large or larger than the Loma Prieta quake will always correlate with upswings in microquake activity. The slippage that generates these microquakes also puts strain on locked zones like that near Loma Prieta, which eventually will cause them to break and generate major quakes.

Interestingly, the southern portion of the central San Andreas, just north of Parkfield, showed a different periodicity in microquakes, about 1.5 years. Large quakes are not known to occur on this part of the fault, however. The region below Parkfield showed no consistent pattern of microquakes, probably because this zone is locked, which suppresses microquakes.

Fault pulsing where large earthquakes occur has been seen at two other places, in Japan and in the Pacific Northwest, but both of these sites are subduction zones where the earth’s crust is diving beneath adjacent tectonic plates.

"People are still trying to understand what is going on in the phenomenon," Nadeau said.

Borehole seismometers implanted by UC Berkeley at a depth of 200 meters along the Hayward Fault have already shown microquake activity preceding a few magnitude 4 quakes, but it is unclear yet whether larger quakes on that fault are correlated with upticks in microquake activity, he added.

The work is supported by the USGS and the National Science Foundation

Robert Sanders | UC Berkeley
Further information:

More articles from Earth Sciences:

nachricht New technologies and computing power to help strengthen population data
22.03.2018 | University of Southampton

nachricht New interactive map shows climate change everywhere in world
22.03.2018 | University of Cincinnati

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Researchers Discover New Anti-Cancer Protein

An international team of researchers has discovered a new anti-cancer protein. The protein, called LHPP, prevents the uncontrolled proliferation of cancer cells in the liver. The researchers led by Prof. Michael N. Hall from the Biozentrum, University of Basel, report in “Nature” that LHPP can also serve as a biomarker for the diagnosis and prognosis of liver cancer.

The incidence of liver cancer, also known as hepatocellular carcinoma, is steadily increasing. In the last twenty years, the number of cases has almost doubled...

Im Focus: Researchers at Fraunhofer monitor re-entry of Chinese space station Tiangong-1

In just a few weeks from now, the Chinese space station Tiangong-1 will re-enter the Earth's atmosphere where it will to a large extent burn up. It is possible that some debris will reach the Earth's surface. Tiangong-1 is orbiting the Earth uncontrolled at a speed of approx. 29,000 km/h.Currently the prognosis relating to the time of impact currently lies within a window of several days. The scientists at Fraunhofer FHR have already been monitoring Tiangong-1 for a number of weeks with their TIRA system, one of the most powerful space observation radars in the world, with a view to supporting the German Space Situational Awareness Center and the ESA with their re-entry forecasts.

Following the loss of radio contact with Tiangong-1 in 2016 and due to the low orbital height, it is now inevitable that the Chinese space station will...

Im Focus: Alliance „OLED Licht Forum“ – Key partner for OLED lighting solutions

Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, provider of research and development services for OLED lighting solutions, announces the founding of the “OLED Licht Forum” and presents latest OLED design and lighting solutions during light+building, from March 18th – 23rd, 2018 in Frankfurt a.M./Germany, at booth no. F91 in Hall 4.0.

They are united in their passion for OLED (organic light emitting diodes) lighting with all of its unique facets and application possibilities. Thus experts in...

Im Focus: Mars' oceans formed early, possibly aided by massive volcanic eruptions

Oceans formed before Tharsis and evolved together, shaping climate history of Mars

A new scenario seeking to explain how Mars' putative oceans came and went over the last 4 billion years implies that the oceans formed several hundred million...

Im Focus: Tiny implants for cells are functional in vivo

For the first time, an interdisciplinary team from the University of Basel has succeeded in integrating artificial organelles into the cells of live zebrafish embryos. This innovative approach using artificial organelles as cellular implants offers new potential in treating a range of diseases, as the authors report in an article published in Nature Communications.

In the cells of higher organisms, organelles such as the nucleus or mitochondria perform a range of complex functions necessary for life. In the networks of...

All Focus news of the innovation-report >>>



Industry & Economy
Event News

Virtual reality conference comes to Reutlingen

19.03.2018 | Event News

Ultrafast Wireless and Chip Design at the DATE Conference in Dresden

16.03.2018 | Event News

International Tinnitus Conference of the Tinnitus Research Initiative in Regensburg

13.03.2018 | Event News

Latest News

Modular safety concept increases flexibility in plant conversion

22.03.2018 | Trade Fair News

New interactive map shows climate change everywhere in world

22.03.2018 | Earth Sciences

New technologies and computing power to help strengthen population data

22.03.2018 | Earth Sciences

Science & Research
Overview of more VideoLinks >>>