Earthquakes have baffled civilizations for thousands of years.
What causes them? Can we predict them? What can we do to lessen their impact?
Modern science and engineering have helped us to gain a better appreciation and understanding of our world’s mighty tectonic forces. We have come a long way in the study of seismology, but there’s still so much more to learn.
In ancient times, many civilizations believed earthquakes were the result of giant animals, angry gods, and giants.
In Japan, people believed a giant catfish living at the bottom of the ocean would shake the Earth when he moved (see photo of an ancient Japanese wood -block print showing the fish, Namazu).
Folklore in India explained that the Earth is held up by four elephants that stand on the back of a turtle who is balanced on top of a cobra. When any of these animals moves, an earthquake occurs.
Norse mythology attributed earthquakes to the Fenris Wolf, a giant animal that was tricked by the gods and chained by the roots of the mountains underground. His roars shake the mountains, crack the Earth and send rocks tumbling down the mountainside.
The Greeks blamed Poseidon, the god of the sea.
In New Zealand, a popular superstition explained that Mother Earth carried a child in her womb, the young god Ru, who shook the ground whenever he stretched and kicked.
Mexican folklore explained quakes as an act of the devil, El Diablo, himself. Legend has it he made huge crevices in the earth from which he and his helpers could emerge to stir up trouble on Earth.
What We’ve Learned
Today, we know that the rigid, outermost shell of the Earth – called the lithosphere – is comprised of several large tectonic plates that are constantly in motion but get stuck at times due to friction. When the stress overcomes the friction there is a jolt of energy released in waves that travel through the earth’s crust and cause the shaking we call an earthquake.
There are two tectonic plates in California:
- The Pacific Plate, consisting of most of the Pacific Ocean floor and the California coastline. (It is said to creep in a northwesterly direction at a rate of about 2-4 inches per year.)
- The North American Plate, making up most of the continent and parts of the Atlantic Ocean floor. (It moves to the west-southwest at a rate of about nine-tenths of an inch per year.)
The main boundary between these two plates is the infamous San Andreas Fault: 650 miles long and at least 10 miles in depth. Other smaller faults branch off from there, making up the San Andreas Fault Zone.
The convergence of these two plates is what helped to shape the spectacular Sierra Nevada mountain range. It’s also what gives California its special disposition to earthquakes.
Lessons from the Past
At 5:04 p.m. on Tuesday, Oct. 17, 1989 a magnitude 6.9 earthquake, centered near the Santa Cruz mountains and named after the nearby Loma Prieta peak, jolted the San Francisco and Monterey Bay region area for 15 to 20 seconds, crumpling freeways, toppling storefronts and flattening buildings, many of which were “soft-story” structures: wood-framed apartments or commercial space built over a ground-floor parking area.
All total, 63 people were killed, and 350 were hospitalized. The quake destroyed a freeway viaduct in Oakland, collapsed a portion of the Bay Bridge, flattened historic buildings and soft-story buildings, severed communications and caused as much as $10 billion in damage.
It was the largest earthquake to strike the Bay Area since the Great San Francisco Quake of 1906, which was a magnitude 7.9.
The Loma Prieta earthquake exposed California to our vulnerability of future quakes, some of which will inevitably be more powerful even than that one. It also prompted many organizations, universities and government bodies to work together to study the impacts of the quake and apply this knowledge to reduce future losses.
New Building Codes
In 2013 San Francisco passed legislation that requires the evaluation and retrofit of “multi-unit soft-story buildings,” defined as: wood-frame structures, containing five or more residential units, having two or more stories over a “soft” or “weak” story, and permitted for construction prior to January 1, 1978. The City of Berkeley requires owners of owners of soft, weak or open front (SWOF) buildings with five or more dwelling units to retrofit their buildings. The law took effect January 4, 2014 and applies to wood frame buildings constructed prior to 1978. Other Bay Area jurisdictions adopting such codes include Santa Clara County and the City of San Jose, and the cities of Alameda, Richmond and Fremont.
We have seen the same problems here in Los Angeles, particularly as the result of the 1994 Northridge quake, a 6.7 magnitude jolt that killed more than 60, injured more than 9,000 and caused as much as $25 billion in widespread damage.
Both disasters – just five years apart from each other – prompted a sweeping call for stricter building codes throughout California: a mandate for seismic retrofitting of non-ductile concrete frame buildings and wood structures with soft-story conditions.
Both disasters have given us the ability to learn from the past and develop spectacular methods for preparing a structure for the next big quake with minimal cost when compared to the risk of loss when nothing is done.
California today is able to withstand the impact of major quakes better than most other parts of the entire globe.
And because there are so many earthquakes here – thanks to the Pacific and North American tectonic plates – it seems only fitting that we be ahead of the game in learning to prepare for them.