Decades of discoveries later, earthquakes remain mysterious
One hundred years ago when California shook from the Oregon
border to San Juan Bautista, scientists understood little of what
caused the ground to burst into raging convulsions. A century
later, geologists and seismologists still can’t predict when the
next big one will occur, but they understand a lot more about why
earthquakes happen and are continuing to gather information to help
locals and rescue workers prepare.
Decades of discoveries later, earthquakes remain mysterious
One hundred years ago when California shook from the Oregon border to San Juan Bautista, scientists understood little of what caused the ground to burst into raging convulsions. A century later, geologists and seismologists still can’t predict when the next big one will occur, but they understand a lot more about why earthquakes happen and are continuing to gather information to help locals and rescue workers prepare.
“We’ve discovered huge amounts,” said Steve Walter, a geologist for the United States Geological Survey. “Back then some people thought that earthquakes caused the faults rather than the other way around.”
The earth is a mosaic of loosely joined land masses, called tectonic plates. Since 1906, geologists have learned that California sits along two plates – the North American and Pacific.
“There are plates on the surface of the earth that are moving against each other,” Walter said. “We happen to be at one of those big junctions.”
Faults are places where the boundaries between the plates break and they lurch past each other. Hundreds of small earthquakes happen each week on the dozens of faults throughout California and Nevada.
The USGS tracks seismic activity on its Web site. Visitors can see a “shake map” of recent earthquakes of magnitude 3.5 or higher and they can even include their input as to how much they felt an earthquake through an online survey.
The colored map shows how much shaking and potential damage is likely with a colorful scale called the Modified Mercalli Intensity scale, with red, orange, green and blue shades radiating out from the epicenter of a quake.
The information is helping geologists understand the difference between the shaking recorded by seismologists and how residents actually felt the quake, as well as how much damage really happened.
“Its not just for our information, but for the folks who have to respond,” Walter said.
In addition to gathering information from those living in earthquake country, scientists have been doing a lot of research in the last century to understand why earthquakes happen.
They have come up with a theory of elastic rebound to describe the strain that is released within seconds when an earthquake happens, Walter said.
“There is deformation of the crust on both sides,” Walter said. “They’re being stretched like a rubber band. If you keep stretching it, at some point it springs and breaks.”
The Bay Area has eight known faults that are capable of an earthquake of magnitude 6.7 or greater. The San Andreas Fault creeps through San Benito County from the Pinnacles up through Cienega Valley, passing west of Hollister before traversing San Juan Bautista before it heads northwest into Santa Cruz County. The Calaveras branch of the Hayward Fault system passes directly through Hollister. Gilroy and Morgan Hill are sandwiched between the San Andreas Fault on the west and the Calaveras Fault to the east.
The part of the San Andreas that runs through San Benito County is what scientists refer to as a creeping fault, Walter said. Signs of creep can be seen throughout Hollister, where sidewalks don’t quite line up anymore or asphalt patches on streets have drifted.
“If there is any question of where to live along the San Andreas Fault, that’s it,” he said. “It’s the part of the San Andreas that we wish all faults would behave like.”
The section of the San Andreas through San Benito County is weak, Walter explained, and doesn’t accumulate enough strength for more than small or moderate earthquakes.
“There are small, regular earthquakes of less than magnitude five,” he said.
Even if the chances of a major earthquake being epicentered in San Benito or southern Santa Clara County are slim, that doesn’t mean the cities would be left intact by quakes further up the San Andreas Fault or on one of the other faults in the Bay Area.
The San Andreas Fault ruptured in 1906 near San Francisco in the Great Earthquake and again in 1989 in Santa Cruz County during the Loma Prieta Earthquake.
The famous 100-year-old quake caused a nearly 300-mile rupture of the San Andreas Fault, leaving fallen buildings, fires and death in its wake. The 7.8 magnitude quake is the largest one in California’s recorded history, geologists said.
The much weaker Loma Prieta quake released only 3 percent of the built-up energy that the 1906 quake released.
“Prior to the next big one, we will start seeing more moderate and large earthquakes,” Walter said. “Quakes like the 1906 – they happen about every 350 years.”
While scientists believe there is a 62 percent probability of an earthquake of magnitude 6.7 or higher in the Bay Area in the next few decades, predicting when it will happen or where is much harder. Scientists and geologists are still discovering new faults in California, which leaves them with an incomplete picture of the fault system. Some are unknown until a large quake hits such as the Northridge Fault in Southern California, which erupted in 1994.
It’s also hard to predict the behavior of fault lines because of the relatively short period of history that geologists have been able to piece together of earthquakes.
“We don’t have a complete geological record of all the earthquakes for the last 10,000 years,” Walter said. “Historically, we only go back less than 200 years.”
With only two or three earthquakes greater than 5.5 on each fault, its hard to understand how frequently earthquakes occur on each.
“We haven’t seen anything so far that looks like it happens reliably prior to big earthquakes,” Walter said.
In Parkfield, a section of the San Andreas Fault northeast of San Luis Obispo, scientists believed the fault broke loose every 25 years, causing a larger earthquake, give or take a few years. They expected a quake to erupt along the fault in the 1980s, but instead the quake happened in 2004.
“Knowing or believing that a big earthquake would happen,” Walter said, “We put out creep meters, radios of various types hoping to capture any signal from the earth. We recorded absolutely nothing.”
