The “Big One”-L.A.’s Next “Great Quake”

You hear it almost every day here in L.A….public officials reminding us to be prepared for the “Big One”, the next “Great Quake” that will hit this area in the near future. 

Seismologists often mention that the area is overdue for the next magnitude 8 earthquake.  They base this, in part, on the fact that the southern San Andreas Fault–the only fault that we know of in this area that can deliver a quake of this size–has not had this large of an earthquake since the last one in 1857, the “Great Fort Tejon Earthquake” (M 7.9).

Fort Tejon, in fact, was not the epicenter of this earthquake.  The fault rupture originated northwest of Parkfield in Monterey County and propagated southeastward for over 360 km (225 miles) along the San Andreas Fault to the Cajon Pass northwest of San Bernardino. 

Technically, Parkfield was the epicenter of this quake, as it was the origin of the rupture, but most scientists would be more concerned with the extent and location of the entire rupture; Fort Tejon was approximately the midway point of the rupture. The earthquake actually acquired its name because Fort Tejon was the only populated locality near the fault, and naturally, the Fort suffered more damage than the rest of sparsely-populated 1857 Southern California.

So, was this really the last “Great Quake” to strike Southern California, considering that Parkfield is nearly 200 miles away from the Los Angeles Basin?  When was the last “Big One” that struck a little closer to home?

The last “Great Quake” to shake this area may have occurred around 1690, before another major earthquake on the southern San Andreas Fault, in 1812, with an epicenter near Wrightwood.  The 1812 earthquake ruptured the San Andreas Fault on the Wrightwood (southern) half  of the Mojave segment, from north of Pallett Creek to south of Cajon Pass.  It is believed that this quake was smaller in magnitude    (M 7.3) than the 1857 earthquake, though it did severely damage the Mission at San Juan Capistrano–over 50 miles away.  Damage was also reported at the Mission in San Gabriel, and in San Diego. 

Another major (M 7.1) earthquake shook the area two weeks later (potentially triggered by the Wrightwood quake) and caused damage to missions in Santa Barbara and Ventura counties.  The epicenter originally was believed to be either offshore or in Santa Barbara or Ventura counties, and there may have been some confusion in the records over which quake caused damage to which missions. 

Recent studies have indicated that the second quake may have also been on the San Andreas Fault, rupturing the northern half of the Mojave segment from north of Pallett Creek to south of Fort Tejon in the Tejon Pass.  The entire Mojave segment that may have ruptured in the two quakes of 1812 ruptured again in the 1857 quake.  This may be the pattern that the San Andreas sometimes follows, with a “Great Quake” being set up decades earlier by 1 or 2 major earthquakes (M 7.0+).

The approximate dates–based on paleoseismic data–of other major (M 7.0+) or “Great” (M 8.0+)earthquakes along the southern San Andreas Fault near Wrightwood before 1812 are:   1690–1480–1346–1100–1048–997–797–737–677. 

The approximate dates–based on paleoseismic data–for other major or “Great” quakes along the southern San Andreas Fault between Fort Tejon and Parkfield (near the Carrizo Plain, which is closer to the epicenter of the 1857 quake) are:   1640–1585–1393–1310–1216–878–455.    

It appears that the same thing may have happened before the “Great San Francisco Quake” of 1906, with the major 1838 San Francisco earthquake (M 7.4) on the northern San Andreas Fault potentially setting up the 1906 shock (M 7.8).  Does this mean that the major 1989 Loma Prieta earthquake (M 7.1) south of San Francisco may be setting up the next “Great Quake” on the northern San Andreas Fault in the Bay area?  The next 40 or 50 years will tell us a lot about whether this pattern sometimes holds true for many of the “Big One’s” on both the northern and southern San Andreas Fault.

If this pattern is applied to the southern San Andreas Fault south of Cajon Pass to the Mexican border, the last major quake (M 7.0+) on this part of the fault was the Imperial Valley earthquake (M 7.1) of 1940.  It was located on the Imperial Valley Fault, part of the southern San Andreas Fault system between the Salton Sea and the Mexican border.  

 

To be continued…              

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