Wednesday, July 30, 2008

Earthquake Prone

So yesterday's quake was another reminder that we live in a "seismically active" area. In case you didn't know, Temecula sits ON TOP OF the Elsinore Fault. A relatively "quiet" fault, yes, but still estimated to be capable of a magnitude 7.5 quake--8 times bigger than the Northridge quake. Along with wildfires, black widows, and cosmetic surgery, quakes are just one of those things that lends Southern California living it's enviable je ne sais quois.

I was sitting on my bed talking to a friend when I felt the rolling begin. I said "Oh my god I have to go," dropped the phone, and ran into the living room where the children were playing. In front of three heavy bookshelves and beneath the swinging ceiling fan.

They looked at me quizzically.

"Didn't you feel that?" I asked.

"I did."

So it was one out of three.

I was rattled; the thing with earthquakes--well, one of the many--is that when one starts, you really don't know whether it's going to last mere seconds or stretch on for minutes, whether it's going to be a little let-off-some-steam rumble or a massive seismic "adjustment" on the scale of the current housing market slide. You also don't know whether the quake you just felt was the main attraction or just a "foreshock."

I moved the kids to a safer spot on the floor. And then I logged on to one of my favorite SoCal resources, the Southern California Earthquake Data Center. Within seconds, I knew the magnitude (5.4), epicenter (near Chino Hills) and that the USGS (US Geological Survey--which boasts another great earthquake info site) estimated that was the main shock, with aftershocks predicted to follow.

Given the results of yesterday's quake--not much damage--it's easy (though I never would have imagined it) to become a little blase about our seismic future here. Here are some things to keep in mind:

The perceived "significance" of a given quake has to do with several factors, including
  • magnitude
  • hypocentral depth
  • epicentral location

The deadly magnitude 6.7 Northdridge quake of 1994 was only the 7th largest quake in recent California history, but the urban epicenter meant toppled overpasses, collapsed buildings, and 60 fatalities. In contrast, the 7.3 mgnitude Landers quake two years earlier had over 5 times greater shaking amplitude and released approximately 15 times more energy, but was much less destructive--resulting in only 3 fatalities.

It's also worth remembering that we are the lucky beneficiaries of improved building codes--even yesterday's "moderate" magnitude 5.4 quake would've caused extensive damage to poorly built structures like those in much of the undeveloped world.

Last year the USGS released a special report predicting the likelihood of a large quake (6.5 or greater) throughout California over the next 30 years. In summary: Buckle up.

The Earthquake Country Aliance has developed the ShakeOut Scenario, which focuses on the eventual "gigantic" (approximately 7.8) quake along the Southern San Andreas, and has planned a week of Earth Quake Preparedness activities for this November.

To understand how significant that quake could be, let's look at the 1857 Fort Tejon magnitude 7.9 quake. The following description of that quake comes from the SCEDC:

"As a result of the shaking, the current of the Kern River was turned upstream, and water ran four feet deep over its banks. The waters of Tulare Lake were thrown upon its shores, stranding fish miles from the original lake bed. The waters of the Mokelumne River were thrown upon its banks, reportedly leaving the bed dry in places. The Los Angeles River was reportedly flung out of its bed, too. Cracks appeared in the ground near San Bernadino and in the San Gabriel Valley. Some of the artesian wells in Santa Clara Valley ceased to flow, and others increased in output. New springs were formed near Santa Barbara and San Fernando. Ridges (moletracks) several meters wide and over a meter high were formed in several places. In Ventura, the mission sustained considerable damage, and part of the church tower collapsed. At Fort Tejon, where shaking was greatest, damage was severe. All around southern and central California, the strong shaking caused by the 1857 shock was reported to have lasted for at least one minute, possibly two or three!"

The Earthquake Country Alliance reports,

"In an earthquake of this size, the shaking will last for nearly two minutes. The strongest shaking will occur near the fault (in the projected earthquake, the Coachella Valley, Inland Empire and Antelope Valley). Pockets of strong shaking will form away from the fault where sediments trap the waves (in the projected earthquake, it would occur in the San Gabriel Valley and in East Los Angeles). An earthquake of this size will cause unprecedented damage to Southern California—greatly dwarfing the massive damage that occurred in Northridge’s 6.7-magnitude earthquake in 1994."

The Fort Tejon earthquake caused a surface rupture between 220 and 250 miles long, and had a maximum displacement of at least 30 feet. The SCEDC Notes, "Were the Fort Tejon shock to happen today, the damage would easily run into billions of dollars, and the loss of life would likely be substantial..."

Temeculans, like our neighbors throughout Southern California, can't prevent or avoid or ignore earthquakes. And we also can't afford to be unprepared.

Coming soon: Tips on earthquake preparedness and safety.


dwija said...

It's a good thing you're posting to this blog because I have most certainly developed the "blase" you mention- seven years of nothing-interesting-coming-of-earthquakes combined with the "can't prevent or avoid" factor has made me....not the best person to comment or advise, let's put it that way! Oh, and the excellent building codes, too :) Yes, it's definitely good thing I've got you to be all practical and safety-oriented. Me? Not so much ;)

RobinV said...

Dwija for a minute there I thought YOU had writted all of this! I was like "damn got some time on your hands!" haha..