Earthquake Watch: California Is Overdue for a ‘Big One’

Earthquake Watch: California Is Overdue for a 'Big One'

California earthquakes are a geologic inevitability. The state straddles the North American and Pacific tectonic plates and is crisscrossed by the San Andreas and other active fault systems. The magnitude 7.9 earthquake that struck off Alaska’s Kodiak Island on Jan. 23, 2018 was just the latest reminder of major seismic activity along the Pacific Rim.

Earthquake a M3 > does not release the stress of the California fault lines. To do that there needs to be a M6-7 or greater. These smaller earthquake is a indication that stress has built along the Faults of California. And there are many faults.

Earthquake Watch: California Is Overdue for a 'Big One'

Most people only know the names of a few of these faults. San Andres being the most famous. There is also the Hayward Fault which becomes the Rogers Creek Fault, the Calaveras, Greenville Fault, The Concord Fault, Green Valley Fault, West Napa Fault, ( It is believed to be the northern extension of the Calaveras Fault in the East Bay region) all in the Bay Area.

The 1906 earthquake was so large is released all the stress in the crust in the Bay Area, which has been rebuilding. There is a 72% chance this area will have another large earthquake by the year 2043. A M6-7 or larger.

Tragic quakes that occurred in 2017 near the Iran-Iraq border and in central Mexico, with magnitudes of 7.3 and 7.1, respectively, are well within the range of earthquake sizes that have a high likelihood of occurring in highly populated parts of California during the next few decades.

The earthquake situation in California is direr than people who aren’t seismologists may realize. Although many Californians can recount experiencing an earthquake, most have never personally experienced a strong one. For major events, with magnitudes of 7 or greater, California is actually in an earthquake drought. Multiple segments of the expansive San Andreas Fault system are now sufficiently stressed to produce large and damaging events.

The last big one

Earthquake Watch: California Is Overdue for a 'Big One'

California occupies a central place in the history of seismology. The April 18, 1906 San Francisco earthquake (magnitude 7.8) was pivotal to both earthquake hazard awareness and the development of earthquake science – including the fundamental insight that earthquakes arise from faults that abruptly rupture and slip. The San Andreas Fault slipped by as much as 20 feet (six meters) in this earthquake.

Earthquake Watch: California Is Overdue for a 'Big One'

Although ground-shaking damage was severe in many places along the nearly 310-mile (500-kilometer) fault rupture, much of San Francisco was actually destroyed by the subsequent fire, due to the large number of ignition points and a breakdown in emergency services. That scenario continues to haunt earthquake response planners. Consider what might happen if a major earthquake were to strike Los Angeles during fire season.

Earthquake Watch: California Is Overdue for a 'Big One'

The quake, estimated at a 7.9 magnitude, caused a fire that burned for three days. By the time the conflagration had been contained, the natural disaster had killed about 3,000 people, left more than 200,000 more homeless and destroyed more than 28,000 buildings and 500 city blocks, according to the Library of Congress.

Although the damage and death toll were less than the 1906 quake, the Loma Prieta—like its predecessor—did the most harm to structures built on soft soil, and buildings with unreinforced masonry.

Emergency Food Supplies

Earthquake Watch: California Is Overdue for a 'Big One'

The October 17, 1989 Loma Prieta earthquake slip during the earthquake occurred on 35 km of fault at depths ranging from 7 to 20 km. Maximum slip was approximately 2.3 m. The earthquake may not have released all of the strain stored in rocks next to the fault and indicates a potential for another damaging earthquake in the Santa Cruz Mountains in the near future may still exist.

Earthquake Watch: California Is Overdue for a 'Big One'

It was the most damaging and costly earthquake to strike California since 1906. However, because of its distance from major population centers and the short duration of shaking, Loma Prieta was not a rigorous test of the performance of the Bay Area’s built environment.

The moderately large (7.1 on the Richter Scale) Loma Prieta earthquake of October 17, 1989, took 64 lives, cost $12 billion, and damaged more than 27,000 structures. It resulted from a slip along a 25-mile segment of the San Andreas fault where it traverses the Santa Cruz Mountains,

Earthquake Watch: California Is Overdue for a 'Big One'

The deaths on the Cypress Viaduct, 35 in total, accounted for more than fifty percent of the 64 lives taken during the earthquake. Fortunately, only about eighty vehicles were on the affected stretch of highway during the earthquake — a fact attributed to a sporting event: the San Francisco Giants were playing in the World Series and people were indoors anticipating the commencement of the game.

During the Loma Prieta earthquake, the entire Viaduct structure began to vibrate tremendously. Whereas well-graded soils helped to dampen vibrations the soft “bay mud” upon which most of the structure was constructed actually served to increase the amplitude of vibrations by up to five times in comparison to that of the rest of the freeway which was built on rock.

California’s earthquake drought

The 1906 San Francisco earthquake was the last quake greater than magnitude 7 to occur on the San Andreas Fault system. The inexorable motions of plate tectonics mean that every year, strands of the fault system accumulate stresses that correspond to a seismic slip of millimeters to centimeters. Eventually, these stresses will be released suddenly in earthquakes.

But the central-southern stretch of the San Andreas Fault has not slipped since 1857, and the southernmost segment may not have ruptured since 1680. The highly urbanized Hayward Fault in the East Bay region has not generated a major earthquake since 1868.

Reflecting this deficit, the Uniform California Earthquake Rupture Forecast estimates that there is a 93 percent probability of a 7.0 or larger earthquake occurring in the Golden State region by 2045, with the highest probabilities occurring along the San Andreas Fault system.

Parkfield had a M6 earthquake in 2007 and shouldn’t have another on in this area for about 10 years.

Earthquake Watch: California Is Overdue for a 'Big One'

Reflecting this deficit, the Uniform California Earthquake Rupture Forecast estimates that there is a 93 percent probability of a 7.0 or larger earthquake occurring in the Golden State region by 2045, with the highest probabilities occurring along the San Andreas Fault system.

January 28, 2018 rupture of a massive pipeline in Hayward that sent water, rocks and mud running down city streets may have been caused by a “creep” on the Hayward Fault. The fault moved 10 inches in the last 40 years.

A creep is defined as the constant gradual land movement or surface displacement that occurs on an active fault without an earthquake.

The most recent large earthquake on the Hayward fault was in 1868, 140 years ago. Because the past five large earthquakes on the Hayward fault have been about 140 years apart, the Hayward and Rodgers Creek faults are the most likely faults to produce a large earthquake in the Bay Area.


 

Can California do more?

California’s population has grown more than 20-fold since the 1906 earthquake and currently is close to 40 million. Many residents and all state emergency managers are widely engaged in earthquake readiness and planning. These preparations are among the most advanced in the world.

Earthquake Watch: California Is Overdue for a 'Big One'

For the general public, preparations include participating in drills like the Great California Shakeout, held annually since 2008, and preparing for earthquakes and other natural hazards with home and car disaster kits and a family disaster plan.

No California earthquake since the 1933 Long Beach event (6.4) has killed more than 100 people. Quakes in 1971 (San Fernando, 6.7); 1989 (Loma Prieta; 6.9); 1994 (Northridge; 6.7); and 2014 (South Napa; 6.0) each caused more than US$1 billion in property damage, but fatalities in each event were, remarkably, dozens or less. Strong and proactive implementation of seismically informed building codes and other preparations and emergency planning in California saved scores of lives in these medium-sized earthquakes. Any of them could have been disastrous in less-prepared nations.

Earthquake Watch: California Is Overdue for a 'Big One'

Nonetheless, California’s infrastructure, response planning and general preparedness will doubtlessly be tested when the inevitable and long-delayed “big ones” occur along the San Andreas Fault system. Ultimate damage and casualty levels are hard to project, and hinge on the severity of associated hazards such as landslides and fires.

Managing earthquake risk requires a resilient system of social awareness, education and communications, coupled with effective short- and long-term responses and implemented within an optimally safe built environment. As California prepares for large earthquakes after a hiatus of more than a century, the clock is ticking.





Mary Greeley News
www.marygreeley.com
credit: http://abc7news.com/weather/exclusive-creep-on-hayward-fault-may-have-broken-water-main/3013039/
https://earthquake.usgs.gov/learn/topics/shakingsimulations/hayward/M7.2.php

https://www.livescience.com/61601-california-overdue-earthquake.html

https://www.engineering.com/Blogs/tabid/3207/ArticleID/73/categoryId/7/Cypress-Street-Viaducts.as