4.7 Quake Shakes Up Los Angeles Basin

USGS Shake Map and Chart 

May 17, 2009 LA Basin Earthquake

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News sources first identified the epicenter as Lennox CA, but have not settled on a name so far.  Identifications of earthquakes often are changed by USGS and news media. 

A 4.7 magnitude quake that, according to the US Geological Survey , appeared consistent with striking on the Newport Inglewood fault that runs near the coast (onshore and offshore) between the two named cities appears to have ruptured in the heart of the LA Basin.  According to the ederal organization's information the noisy shaker hit 3 miles east  of the Los Angeles International Airport in the City of Lennox and at a depth of 8.5 miles. The USGS says that two subsequent aftershocks seem to be west of the N-I fault, and that other movements may clarify the location of the slippage.

The USGS says damage should be light and mostly we are seeing problems with things that have fallen off flat surfaces.  In a store in Norwalk, CA a light smattering of wine bottles were the only casualties.

The Newport-Inglewood fault caused a deadly earthquake in 1933 (called the Long Beach Earthquake) in which 120 people died including 5 young people at the city's schools in a gymnasium building that fell.   A long time resident of the (though not alive in 1933) recalls the fallen school building was the Woodrow Wilson High School pool facility. 

The 1933 quake which struck in the early evening helped spur retrofitting of structures and re-enforcement of brick facades as well as a second round of standards tightening for new buildings, in those design requirements and in the matter of increasing lateral stress capacities.  Schools were especially targeted as the people in the town realized that the lateness of the strike may have been the only factor that kept hundreds of the city's children alive. (Some of the information on the subject above was obtained or confirmed from National Information Service for Earthquake Engineering: Long Beach Earthquake 1933).

More information, pictures on the 1933 LB Quake at Southern California Earthquake Center; Long Beach Earthquake: 70th Anniversary

The USGS reports that the largest quake likely on the Newport-Inglewood fault would be c.7.4 on the Richter scale, not considered a devastating strength according to the US agency.

The last big earthquake in the basin, the 1994 Northridge Quake on the Northridge or Pico thrust fault was magnitude 6.7 on the Richter scale according to the Southern California Earthquake Data Center , but later reports showed that some areas shook as if they were in a much larger earthquakes due to convergence of waves in bowl like structure of the San Fernando Valley, slipping secondary faults, and varying soil structure.   In addition the main '94 quake is considered to have been a blind-strike quake (upward motion) instead of the slip strike kind experienced  along the Newport Inglewood. 

One can also hope that poor construction and design that brought down some structures in the '94 quake are not repeated often in the areas of LA County and Orange County (through which the fault extends), but multi storied building with 'soft' parking structures at the ground or basement level failed spectacularly in one case and caused dangerous and unlivable conditions in many more locations.  Specifically, one three storied apartment building pancaked the bottom floor of the structure into the basement garages killing 16 people, and a large parking structure collapsed on a cleaning vehicle, though inflatable bags were used to rescue the driver.

Looking at the full size example of the shake map for Sunday night's temblor available by clicking on the image at top of report, one can see the strength of the quake was widely spread, possibly due to the extra depth of the quake, with the brightest yellow blotchily appearing over areas to at least 20 miles South and East of the epicenter. 

Liquification where earthquake waves are magnified in some soils is also considered a risk along parts of the Newport Inglewood fault.

tags: earthquake  los angeles  lennox  hawthorne  inglewood  newport inglewood fault  

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