A Late-Night Jolt: The Bay Area’s Latest Reminder

-
At 1:40 AM PDT on April 2nd, the Bay Area received a sudden wake-up call. A 4.6 Mw earthquake centered near Boulder Creek, about 48 miles southeast of San Francisco—shook the region, with reports on the USGS "Did You Feel It?" map spanning from the Golden Gate down to Monterey1.

While a 4.6 isn't exactly a "Big One" on the scale of 1906 or 1989, it’s certainly more than a minor tremor. It originated within the San Andreas Fault system, perhaps the most famous transform boundary in the world, where the Pacific and North American plates relentlessly grind past one another.

Why This One Felt More Personal

For many, this was just another Northern California quirk, but for me, it triggered two specific points of interest:


The San Ramon Connection: It strikes while the well-publicized San Ramon swarm is still active. 2


A Moment of Déjà Vu: A mid-4.0 quake in this specific region was actually my first introduction to seismic life. On the evening of August 6, 1989, I felt my first rattle while staying in a shady hotel just north of Palo Alto. That quake—centered at Lake Elsman3—was located in the same 

general neighborhood as April 2nd’s Boulder Creek event. 


The August 6th 1989 Lake Elsman event was not thought terribly significant until October 17th, 1989, at 5:04 p.m. PDT  when the Loma Prieta 6.7 earthquake interrupted the World Series and more importantly Killed 63 people and injured another  3,7574. It is now seen that my August 6th shaker was a pre-shock, hindsight is wonderful. 


The Boulder Creek event does not necessarily mean another 6.7 is coming,  There have been quite a few >4Mw events since 1898 in the region with no knock on effect. But it quite often a release of stress in one part of a fault system can increase stress in another (see map 1)



Map 1

More Connected Than We Thought?

The geography here is telling. While the San Ramon swarm is typically associated with the Calaveras Fault, and the Boulder Creek/Lake Elsman events hug the San Andreas, the lines between these systems are blurring.


Recent USGS research suggests that many of these "separate" faults may actually be interconnected at much deeper levels. We are seeing evidence that major systems—like the San Andreas and Cascadia5, or the Hayward and Calaveras6—might share deeper structural links.

It’s a fascinating, if slightly unsettling, reminder that beneath the beautiful California landscape, the earth is far more integrated (and active) than it appears on the surface.


Footnote: Moment Magnitude vs. Richter Scale

The Richter scale measures earthquake size using the strength of seismic waves recorded by instruments, but it works best for small to moderate earthquakes and tends to underestimate very large ones because it “saturates.” The Moment Magnitude scale (Mw), now the standard used by scientists, measures earthquakes based on the physical characteristics of the fault rupture, including the area that slipped, the amount of movement, and the rigidity of the rocks, which directly reflects the total energy released. Moment Magnitude is preferred because it remains accurate for both small and extremely large earthquakes, allows consistent global comparison, and provides more reliable information for scientific studies, hazard assessment, and engineering design.

References:

  1. Did you feel it

  2. San Ramon Swarm

  3. LA Times August 1989 Earthquake Report

  4. October 1989 Loma Prieta Report
  5. Cascadia - San Andreas Connection
  6. Hayward - Calaveras Connection

<Enter stunningly witty and clever tagline here>

Bazza

Comments

Post a Comment

Popular posts from this blog

Tibia Trivia

-
Image
The ankle is a joint where the foot meets the leg. It consists of the bones of the leg - the fibula and its larger neighbour the tibia - and the top foot bone part called the talus. The fibula and tibia run between the knee and the ankle. If you were to cause a fracture to the one of these bones anywhere from the knee to about three quarters down, you would be considered as having broken a leg. If the break, in either or both of these fine bones, happens in the general area of the top of the foot then you have broken your ankle. To break either the fibula or tibia is bad, to break both is worse. This is a picture of a clearly broken fibula and, less obviously broken tibia (it is two images of the same foot). The bones in the X-ray belong to my wife and that is why I now know more about the bone structure of the lower extremities than I did last Friday morning. Those same bones are also the reason I did not post a blog last week. We were attending the wedding of some good friends out at...
Read more

During leap years February is a little longer in South Korea

-
Image
As I described in 2 Green Dots and 28 Green Bars my car has a little green clock. Well those cunning South Korean Hyundai engineers also put in a calendar. On the 12th of March my wife and I were driving back from dinner and she happened to notice the date that was being displayed on the calendar. Seems the Hyundai engineers may have missed something … Feb 29 + 12 = 41 so at least it is right! <Enter stunningly witty and clever tagline here> Bazza
Read more

The Soft-dying Day

-
Image
I awoke this morning in my southern New England home to the sound of the wind pushing against the house. Outside the sky was a crisp blue. The multi-colour leaves of autumn were lining up like golden sky-divers; waiting for their cue to leap from the trees and then tumble and dance down the hilly road. The wind breathes excitement and animation outside my window. The heavy foliage of summer stops the hanging green frog from spinning over our deck. But now the fall has come and the cover is thinned, so that you can see patches of sky, the frog spins madly throwing off spirals of reflected light. Speedy, Sparky and their friends that inhabit the grassy bank, are nervous. The sweet smelling autumn wind moves everything and drowns out the normal sounds of safety. The change of air is tangible to them as they stand on their hind legs, twitching and sniffing. The small mammals are obvious now because they are the only stationary things. Ironic that they should be so exposed by the very act ...
Read more