M5.1 La Habra Earthquake

TJ Earthquake Damage

Typical Damage at Trader Joes

Friday night’s earthquake in La Habra was interesting for a couple of reasons.

It was small, by California standards, at only Magnitude 5.1.  In fact, I have my automatic notification program set to M6.0 or larger because there are so many small earthquakes like this and generally they don’t cause any damage. I found out about this one through Facebook. So I guess yes, there’s at least one good use for Facebook.

Secondly, the shaking was surprisingly strong for an earthquake of this size.  The rupture was shallow, only 1.7 km deep, and the lack of attenuation up to the surface resulted in  peak ground accelerations as high as 0.25g.  I pasted the USGS shake map below with a handy key to the Mercalli Intensity Scale system of classifying earthquakes.

No serious structural damage occurred, though La Habra Utilities is reporting broken water and gas pipes. The lack of damage and injury speaks more to LA’s excellent building codes and earthquake preparedness than it does to the modest earthquake size. In other parts of the world, places that lack the resources, knowledge, and political will of metropolitan Los Angeles, these types of earthquakes cause fatalities. Intensity VII shaking in California knocks over a stack of wine bottles in Trader Joes. The same level of shaking in rural Afghanistan dislodges roofing stones from their timber supports and buries whole populations in their huts.

Small, generally consequence-free events like these are an opportunity for all of us to reconsider our knee-jerk reactions to troublesome regulations and building codes, a chance for us to give genuine thanks to the smart engineers who led the transition from unreinforced masonry buildings like San Francisco had in 1906 to the well built structures that now protect us from preventable misfortune. Here’s my list of earthquake heroes:

Harry Seed – Pioneer geotechncial engineer and shining light of inspiration

John Lysmer – Smartest good-humored numerical modeler

Vitelmo Bertero – Structural engineer, deep thinker, curmudgeon

CB Crouse – Towering intellect, practical seismologist, mentor

Steve Dickenson – Good friend, collaborator, and insightful earthquake engineer

Yumei Wang – Classmate and dedicated earthquake safety advocate

Jason Brown – Cheerful building code enforcer

Because of their work and the work of countless others, your home and office, your kid’s schools, the bridges that you cross, the port that handles your cargo are all safe from damage in these types of events.

Here’s the link to the USGS event summary, for those of you with interest:


Be safe, everyone.

La Habra Shake Map