A Whole Lot of Shakin’ (and Debate) Goin’ On

Like the strike zone in baseball, deciding what is an aftershock tends to be in the eye of the beholder.

In the moments, hours and days after a big earthquake, it is obvious that the incessant rattlings going on in the neighborhood are aftershocks.

But what about three years after a quake? Are suddenly powerful temblors that follow periods of comparative seismic quiet--such as those last weekend centered miles away from the Northridge earthquake--still aftershocks?

And 45 years after the Tehachapi quake of 1952, can occasional rumblings in Kern County be classified as aftershocks?

To Lucy Jones of the U.S. Geological Survey, a leading expert on aftershocks, the answer is yes.

But, Jones is quick to acknowledge, “ ‘aftershock’ is a semantic term. There is not a hard scientific definition.”

Ordinary folks may have quite a different answer to those questions. Occasionally, even scientists will disagree among themselves.

Most scientists were fairly certain from the outset that the 139 quakes that shook an area of the Santa Susana Mountains between the Simi and Santa Clarita valleys last Saturday and Sunday were Northridge aftershocks.

But a few, such as Oregon State University’s Robert Yeats, an expert on the area, were initially skeptical.

Yeats concluded that they were “probably not” aftershocks because they happened at a different depth and on a different plane than the main Northridge fault.

A few days later, Yeats revised that opinion. After exchanging information on the precise locations with Caltech seismologists, he concluded: “They’re in the general zone of aftershocks of Northridge. . . . It’s not unusual to find a 5.0 this late in the series.”

But, he added, “You can’t completely rule out a migration of quakes to the west, to the Ventura Basin, a new series of quakes in a new area that would not be aftershocks.”

According to Jones and another of California’s eminent aftershock experts, Paul Reasenberg of the Geological Survey, the weekend quakes fit the most commonly accepted scientific definition of aftershocks:

First, they were no more distant from the nearest point of the original Northridge rupture than the length of that rupture. The rupture was 10 miles long, and the weekend shocks were within 10 miles of the northern part of it.

Second, they were smaller than the magnitude 6.7 Northridge quake. The Saturday morning quake was a 5.0 and the Sunday morning temblor was a 4.9.

Third, they continued a rate of frequency of earthquakes within the area that was higher than that before the Jan. 17, 1994, quake.

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And fourth, as a clincher, they happened at the precise location as some of the first potent aftershocks of the Northridge quake, including a 5.1 that took place two days after the main event.

In fact, as Caltech seismologist Egill Hauksson points out, the location of last weekend’s quakes was in the center of the second thickest cluster of Northridge aftershocks, even though they were at the northernmost edge of the acknowledged zone.

If there was any quibbling, it was over the depth. The fault that caused the Northridge quake dipped to the south, and at the point where the recent aftershocks occurred the original fault plane was miles above the new quakes.

Never mind, said Reasenberg. These were still Northridge aftershocks.

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“Earthquakes are always interacting with surrounding faults,” he said. They shift the stresses in various regions, and they prime certain other faults to break.

“Being on the same fault is really rather irrelevant,” agreed Jones, although she does acknowledge that at some point the distance of quakes occurring soon after main shocks grows so great that scientists refuse to consider them aftershocks.

In the days after the magnitude 7.3 Landers earthquake of June 28, 1992, so-called “sympathetic” quakes occurred as far away as Yellowstone National Park. But these were not considered aftershocks.

On the other hand, the 6.2 Big Bear quake that occurred three hours after the Landers main shock, about 20 miles to the west of it, was considered an aftershock.

Northridge aftershocks now number nearly 14,000. At 5:51 a.m. Saturday, a 3.3 magnitude aftershock struck seven miles northeast of Simi Valley, the 14th magnitude 3 or greater in the latest series.

Landers aftershocks exceed 66,000. No one knows how many Tehachapi aftershocks have occurred because the seismic network recording the tiny jolts was not in existence when it took place.

Aftershocks are obviously a big presence in Southern California.

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But Jones is concerned that many people are unaware how large and potentially destructive they can be. Residents may be too sanguine in their belief that after the main shock, the worst is over.

But the biggest aftershocks have occasionally caused death and lots of damage on their own.

* On Aug. 22, 1952, 32 days after the Tehachapi quake killed 12 people, an aftershock centered near Bakersfield killed two, injured 35 and caused $10 million in damage.

* A 5.3 aftershock three days after the 5.9 Whittier Narrows earthquake in 1987 killed another person, bringing the death toll to eight.

* More than six months after the March 10, 1933, Long Beach earthquake of 6.3, a 5.2 aftershock near Signal Hill caused substantial additional damage. The renowned quake scientist Charles Richter called the temblor of Oct. 2, 1933, an aftershock, although, technically, it may have extended the original rupture zone.

* In what may have been the largest aftershock in California’s recorded history, an earthquake of about magnitude 7.5 that occurred Dec. 8, 1812, near Wrightwood was followed 13 days later by an approximate 7.0 quake centered in the Santa Barbara Channel.

“It’s a marginal call whether this was an aftershock,” Jones said. “But since the rupture of the Wrightwood quake was 200 miles long, the distance at which another quake could be called an aftershock was very considerable.”

Reasenberg notes that aftershocks have their own ripple effect on other faults, and at some point, one large quake sequence can trigger another.

Some scientists think the Sylmar-San Fernando quake of 1971 may have triggered the Northridge temblor 23 years later.

And some scientists, such as Allan Lindh of the Geological Survey and Lynn Sykes of Columbia University, believe that the Northridge, San Fernando and other magnitude 5 and 6 quakes along the San Gabriel and Santa Susana mountain ranges since 1970 may ultimately trigger a much larger quake, possibly of magnitude 8, on the San Andreas fault, 40 miles northeast of Los Angeles.

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In that case, all the quakes along the mountain ranges since 1970 might be called “preshocks” of the Big One. And if they had happened just a few hours or days before it, they would be called “foreshocks.”

Which gets back to Jones’ point about semantics. A quake, depending on what follows, can be an aftershock and a preshock at the same time.

On April 22, 1992, for instance, a 6.1 earthquake known as the Joshua Tree quake, rippled through the Little San Bernardino Mountains southeast of Yucca Valley.

For the next two months there was a very robust aftershock series, and then, on June 28, came the 7.3 Landers quake a few miles northwest of the Joshua Tree rupture zone.

The Joshua Tree aftershocks could be called preshocks of the Landers quake, but even so they were still aftershocks, Jones said.

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Incessant Rattlings

Determining which earthquakes qualify as aftershocks is sometimes a matter of scientific debate, but, generally speaking, quakes of smaller magnitude are considered aftershocks when they happen within or close to the rupture zone of a main shock and occur at a higher rate of frequency than was taking place in the same area beforehand.

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California Faults

These are the leading surface faults in California. Thrust faults that do not intersect the surface are not shown.

Eurek

San Francisco

San Andreas Fault

Santa Barbara

Los Angeles

San Diego

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Recent Earthquakes in Southern California

No. of Quakes by Quarter

In thousands 1992: Landers

1994: Northridge

1995: Ridgecrest

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3.0 or greater

1992: Landers

1994: Northridge

1995: Ridgecrest

Source: ESRI, Redlands, Calif.; U.S. Geological Survey, Caltech

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Aftershock Facts

The biggest aftershocks, less than one order of magnitude below the main shocks, can cause death and much damage on their own. Two people died and 35 were injured when a 5.8 aftershock of the 7.7 Tehachapi quake struck near Bakersfield.

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Northridge aftershocks now number nearly 14,000. As of March 31 of this year, 403 had been between magnitude 3.0 and 3.9, 53 had been magnitude 4.0 and 4.9 and 10 had been magnitude 5.0 or higher.

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Aftershocks of the 1992 Landers earthquake exceed 66,000. As of March 31, 1997, 1,619 had been between 3.0 and 3.9, 169 between 4.0 and 4.9, and 23 had been 5.0 or higher.

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The Techachapi quake is still producing what many scientists classify as aftershocks 45 years later.

* Sources: ESRI, Redlands, Calif.; U.S. Geological Survey; Caltech.