The broadcast media wasted little time adding to public apprehension in the aftermath of the two California earthquakes July 4 and 5. The networks laid out in detail the twin quakes that caused zero deaths and, in the end, limited property damage in Ridgecrest on the edge of the Mojave Desert.
South Carolina residents best remember hearing of the Charleston Earthquake that happened on Aug. 31, 1886. Just after 9 p.m. residents felt extreme shaking and witnessed the destruction of many buildings in their city from the quake estimated at more than 7.0 magnitude. At least 60 lives were lost across the Lowcountry. Few buildings escaped with minimal damage. The quake was felt as far away as Milwaukee, New Orleans, Cuba and Bermuda.
Following the Ridgecrest event, modern-day seismographs detected thousands of aftershocks, more than 70,000 to date. Lost in the discussion is the realization most are minor, not reaching the threshold of human perception. Earthquakes are gauged on a scale from 1 to 10. For example a magnitude 2 is 10 times stronger than a 1, and so on up the scale. A magnitude 4 causes limited damage while a 5 inflicts noticeable structural damage with severity dependent on terrain and architecture. Highly destructive quakes registering 7 or greater are rare and generally restricted to the most active tectonic zones.
Just after the news broke that a 6.4 quake struck Ridgecrest, a woman watching TV coverage far from the scene was overheard saying this is a sure sign California is doomed to fall into the Pacific on the day “The Big One” finally arrives.
Such notions illustrate the lack of understanding how an earthquake unfolds.
A network TV reporter during the coverage was little help, repeating the same question several times to a Cal Tech professor during a news conference in Los Angeles. Dr. Lucy Jones patiently explained that the chance of another strong aftershock decreases statistically as days tick by. Accordingly, an even larger 7.1 quake struck 24 hours later along the same fault line. No more since.
The jolt from the second quake was felt from San Francisco down into Mexico. But in Los Angeles, fans at the Dodgers-Padres game noticed only brief minor shaking while players on the field were unaware a 7.1 quake had just happened 125 miles to the northeast.
The incident recalls the third game of the 1989 World Series at San Francisco’s Candlestick Park. The deadly Loma Prieta earthquake interrupted play and inflicted multiple fatalities (63 deaths) and severe infrastructure damage around the Bay Area. Nothing resembling that level of damage happened at Ridgecrest. Near the epicenter there were minor injuries and a house fire attributed to a ruptured gas line.
Remoteness of the fault from population centers limited injuries and casualty damage from the Ridgecrest sequence. Strong California earthquakes such as the Great San Francisco Earthquake (and fire) of 1906, as frightening as they are, have tended to be less catastrophic in comparison with mega earthquakes in Chile, Indonesia and other active areas around the Pacific Rim.
Geophysicists explain the potential severity of a quake is limited by the length of the generating fault line. They tell us a magnitude 9.5 is the practical upward limit for severity. For a quake to register 10 the fault line would need to stretch halfway around the world.
The geologic process known as “plate tectonics” causes lateral movement of large pieces of the Earth's crust. As two massive plates grind slowly past each other in opposite directions-- a few inches a year--a great amount of strain builds up in adjoining layers of rock lying to either side of the fault. The Charleston quake was an exception. The Carolina faults are part of a preexisting system in the North American continent dating from early in Earth's history. But they are still believed to be active.
Once internal strain builds to a critical point in a fault system, rupture suddenly releases the stored energy, usually from miles deep in the crust, in waves surging upward through rock layers and loose sediment to create the shaking felt at the surface.
The U.S. Geological Survey is working on a system for advanced warning, when and where a major quake is likely to occur. A current experimental system is inadequate to provide timely, reliable warnings and is prone to trigger false alarms.
A satisfactory system awaits further development before Angelinos can sleep more confident that they will be awakened in time to escape to safety in advance of the next inevitable San Andreas temblor. The same may apply to Lowcountry residents with a lesser known, but potentially lethal fault system beneath them.
William D. Balgord has a doctorate in geochemistry and heads Environmental & Resources Technology Inc. in Middleton, Wis.