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'Slow earthquakes' on San Andreas Fault increase risk of large quakes, say ASU geophysicists

June 18, 2018

Detailed study of well-known California fault finds new kind of movement not accounted for in earthquake forecasting

Geologists have long thought that the central section of California's famed San Andreas Fault — from San Juan Bautista southward to Parkfield, a distance of about 90 miles — has a steady creeping movement that provides a safe release of energy.

Creep on the central San Andreas during the past several decades, so the thinking goes, has reduced the chance of a big quake that ruptures the entire fault from north to south.

New research by two Arizona State University geophysicists, however, shows that the earth movements along this central section have not been smooth and steady, as previously thought.

Instead, the activity has been a sequence of small stick-and-slip movements — sometimes called "slow earthquakes" — that release energy over a period of months. Although these slow earthquakes pass unnoticed by people, the researchers say they can trigger large destructive quakes in their surroundings. One such quake was the magnitude 6 event that shook Parkfield in 2004.

"What looked like steady, continuous creep was actually made of episodes of acceleration and deceleration along the fault," said Mostafa Khoshmanesh, a graduate research assistant in ASU's School of Earth and Space Exploration (SESE). He is the lead author of a Nature Geoscience paper reporting on the research.

"We found that movement on the fault began every one to two years and lasted for several months before stopping," said Manoochehr Shirzaei, assistant professor in SESE and co-author of the paper.

"These episodic slow earthquakes lead to increased stress on the locked segments of the fault to the north and south of the central section," Shirzaei said. He points out that these flanking sections experienced two magnitude 7.9 earthquakes, in 1857 (Fort Tejon) and 1906 (San Francisco).

The scientists also suggest a mechanism that might cause the stop-and-go movements.

"Fault rocks contain a fluid phase that's trapped in gaps between particles, called pore spaces," Khoshmanesh said. "Periodic compacting of fault materials causes a brief rise in fluid pressure, which unclamps the fault and eases the movement."

Looking underground from Earth orbit

The two scientists used synthetic aperture radar data from orbit for the years 2003 to 2010. This data let them map month-to-month changes in the ground along the central part of the San Andreas. They combined the detailed ground-movement observations with seismic records into a mathematical model. The model let them explore the driving mechanism of slow earthquakes and their link to big nearby quakes.

"We found that this part of the fault has an average movement of about three centimeters a year, a little more than an inch," Khoshmanesh said. "But at times the movement stops entirely, and at other times it has moved as much as 10 centimeters a year, or about four inches."

The picture of the central San Andreas Fault emerging from their work suggests that its stick-and-slip motion resembles on a small timescale how the other parts of the San Andreas Fault move. 

They note that the new observation is significant because it uncovers a new type of fault motion and earthquake-triggering mechanism, which is not accounted for in current models of earthquake hazards used for California.

As Shirzaei explained, "Based on our observations, we believe that seismic hazard in California is something that varies over time and is probably higher than what people have thought up to now." He added that accurate estimates of this varying hazard are essential to include in operational earthquake-forecasting systems. 

As Khoshmanesh said, "Based on current time-independent models, there's a 75 percent chance for an earthquake of magnitude 7 or larger in both northern and southern California within next 30 years."

Top photo: The southern San Andreas Fault slices across the Carrizo Plain in California. Both the northern and southern sections of the San Andreas have seen large destructive earthquakes, while the central section between north and south has remained largely quiet. New work by ASU geophysicists suggests the central section moves in a new way that makes big quakes more likely. Photo by U.S. Geological Survey

Robert Burnham

Science writer , School of Earth and Space Exploration


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Juneteenth: America’s second Independence Day

June 18, 2018

'Juneteenth emerged in part to reclaim the central importance of Black history — to put African-Americans back into the story of American history,' says ASU professor

Juneteenth, a portmanteau of the words “June” and “nineteenth,” was born out of what was once referred to as the “peculiar institution” A euphemism for slavery used by white southerners in the 19th century.of the United States.

It references the day 153 years ago this year when a quarter of a million people — still held captive in the years after the Emancipation ProclamationIssued on Jan. 1, 1863, the Emancipation Proclamation declared free all people living in slavery in Confederate States that were still in rebellion. — walked away from the fields in which they were forced to toil; out of the houses in which they served under duress; and onto the roads they constructed to begin a new but uncertain future as free men, women and children.

In colloquial expression it has been called America’s “second Independence Day” after the Continental Congress approved the final textAmerica’s independence was formally declared two days earlier on July 2, 1776. of the Declaration of Independence from British rule on July 4, 1776. But for many, particularly those whose ancestry and lineage have been all but scrubbed from the annals of American history due to 250 years of that peculiar institution, Juneteenth, or Freedom Day, has come to represent a true day of independence from slaveholding rule.

Growing in observances and recognition, Juneteenth is recognized as a state holiday or observance in 45 statesNorth Dakota, South Dakota, New Hampshire, Montana and Hawaii do not recognize Juneteenth, according to the National Juneteenth Observance Foundation.. But it has been a slow journey to awareness — not unlike the protracted process of emancipation, said author and Professor Calvin SchermerhornA historian of slavery, capitalism and African-American literature, Calvin Schermerhorn is the author of several books on related topics including the forthcoming “Unrequited Toil: A History of United States Slavery,” due for publication in 2018. of the School of Historical, Philosophical and Religious Studies at Arizona State University.

“Juneteenth celebrations were as much about the unfinished work of freedom as about the accomplished fact of slavery’s end,” Schermerhorn said. “Freedom was already and not yet complete since economic and civil rights were slow to follow. Even the soldiers that proclaimed Black Texans free of slavery encouraged them to keep their shoulders to the plow, stay in their places and to accept peace over justice.”

Schermerhorn recently shared more about the history and significance of Juneteenth in this Q&A with ASU Now:

Question: What is Juneteenth, and what is the tradition behind the observance of the day?

Answer: Juneteenth began in 1865 in Texas when some 250,000 formerly enslaved people were officially freed — two and a half years after President Abraham Lincoln issued the Emancipation Proclamation. On June 19, 1865, Union Gen. Gordon Granger issued General Order No. 3, which read: “The people of Texas are informed that, in accordance with a proclamation from the Executive of the United States, all slaves are free.” The order went on to assert “an absolute equality of personal rights and rights of property between former masters and slaves.” Juneteenth, argues one historianRobert C. Conner in his 2013 book “General Gordon Granger: The Savior of Chickamauga and the Man Behind Juneteenth.”, “was a new word signifying a new world.”

African-Americans in Texas turned the day into a celebration of freedom and an occasion to push for equal rights and opportunities. At first, it was specific to Texas but moved with migrating Black Texans all over the United States. Towns like Covert, Michigan, and Tucson, Arizona, were holding Juneteenth celebrations by the early 20th century. Since African-Americans migrating out of Texas tended to go west, the holiday migrated west with them, though by the late 20th century it was popular all over the U.S.

Calvin Schermerhorn

Q: Why was the Emancipation Proclamation not enforced in Texas?

A: The Emancipation Proclamation was a war measure authorizing the president as commander-in-chief of the armed forces to liberate enslaved people in states and parts of states in active rebellion against the U.S. on Jan. 1, 1863. It signaled that the Union war effort was now a war against slavery, but it was not enforceable in places outside Union control. Only when Texas as a state of the Confederacy fell to Union forces did emancipation arrive in law, despite the fact that many Black Texans had already freed themselves as a practical matter. Gen. Granger arrived in June 1865 to reassert federal — not Confederate — supremacy and issued the emancipation order. Many, if not most white Texans did not accept the freedom verdict of the Civil War, and Texas did not ratify the 13th Amendment abolishing slavery until 1870, some five years after Congress passed it. Reconstruction-era Texas also saw some of the worst racist violence and murders of African-American people by white militants.

Q: What were some of the immediate challenges for the belatedly freed men and women of Texas after they received the news of their emancipation?

A: Freedom came with no back wages, no land transfer and no other compensation to those who were formerly enslaved. They received nothing but freedom and had to return to work under many circumstances for former owners who still owned land and other means of production. Many formerly enslaved people emerged in freedom on a war-torn landscape of sickness and scarcity. Some who had toiled all their lives were ill and unable to toil on. Others were orphans, injured or dislocated. The Civil War devastated large swaths of the American South, and economic recovery was haltingly slow.

In Texas as in many other areas, former enslavers took a dim view of people of African descent, offering labor contracts that looked a lot like slavery. Those gave way to sharecropping arrangements that put the croppers or tenants in perpetual debt. There were urban Black populations who became middle class, voted and held public office. But despite the heroic struggles of civil rights leaders, African-American political participation and power was fleeting. The height of Black political participation was in the years between when Congress remilitarized the South in 1867 and the panic of 1873, which sapped the political will of federal authorities to combat a militant white supremacist insurgency that took the form of the Ku Klux Klan and other Democratic Party political organizations.

Q: When and how was the first Juneteenth celebrated?

A: It was celebrated first in Austin in 1867, and by 1872 it was a celebration that drew crowds of thousands in parts of Texas. In subsequent years, some localities let prisoners out of jails for the day to join the celebration. In Galveston and elsewhere, women took the lead in celebrating Juneteenth and making it the pretext for political discussions. In 1919, when the National Association for the Advancement of Colored People (NAACP) opened a branch in Galveston, women activists used Juneteenth as a date to meet at churches and urge more resources for education for African-American children. By then Juneteenth was so important that even white employers gave Black workers the day off.

It was celebrated with barbecues, parades, floats and speeches, many of which were organized by the Women’s Nineteenth of June Committee in Galveston. It was therefore a day of protest and of celebration as an occasion to mark freedom that was only partly accomplished.

As Texas historian Elizabeth Hayes Turner argues, “No other public event instilled African-Americans with such awareness and historic pride, and perhaps no other event so publicly managed to voice the desire for sexual equality that Black women had been quietly demonstrating in their churches, schools and clubs.”

Q:  It feels like we are at a point in time culturally where Americans are taking a deeper look at history and some of the narratives that have been left out of history books. Where does Juneteenth fall in with those “lost” narratives?  

A: Juneteenth as an African-American celebration of emancipation rose in the late 20th century as a way of reviving earlier Freedom Day celebrations and, more generally, as a way of raising awareness of African-Americans’ struggles against slavery and the racist violence of Jim Crow. It became the pretext for the Poor People’s March on Washington, D.C., in 1968. Until the 1960s — and in many history books much later — Black Americans were marginalized. Historians routinely ignored or downplayed slavery, Jim Crow violence and the racist legacies of both. Instead, a grand patriotic narrative held sway and American history was a triumphal story of white generals, inventors and leaders. We see a version of this process in debates and conflicts over Confederate monuments. Monuments to Confederate leaders and generals were put up as assertions of white supremacy cloaked in a language of local struggles against a distant tyrannical government — the government of Abraham LincolnHistorians, including Schermerhorn, say had the Confederacy won the war, millions of African Americans would have remained enslaved.. That and other such stories distorted or ignored troubled parts of American history.

The same is true of Native Americans, women and many immigrant groups. Juneteenth emerged in part to reclaim the central importance of Black history — to put African-Americans back into the story of American history. It also revived a Black holiday of remembrance. In the post-Civil War nation, Freedom Day celebrations varied from state to state among African-descended Americans who had been enslaved or were members of formerly enslaved families. African-Americans in parts of Virginia, for instance, celebrated Surrender Day on April 9, the day when the Confederate Army of Northern Virginia under Robert E. Lee surrendered. Whenever it was celebrated, the day usually coincided with an event signaling Black freedom. But such celebrations fell off in the early 20th century, and they were revived generations later. Juneteenth became an official holiday in Texas in 1980, and proclamations in other states and locales have broadened it to a holiday commemorating emancipation and promoting respect and dignity of all peoples in freedom struggles.

Top Photo: "Emancipation" by Thomas Nast. Image courtesy of the Library of Congress

For more on the cultural significance of Juneteenth: Read our Q&A with Associate Professor Duku Anokye of the School of Humanities, Arts and Cultural Studies at ASU's New College.

Emancipation Proclamation, author Abraham Lincoln. Image courtesy of the Library of Congress, Rare Book and Special Collections Division, Alfred Whital Stern Collection of Lincolniana

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