Lots of my relatives and friends in Southern California are reporting that they are feeling shaken up tonight. The
magnitude 5.1 earthquake (revised from 5.3) took place about 2 miles east of La Habra, which may place it on one of the strands of the Puente Hills blind thrust system, according the the
U.S. Geological Survey. My friends are reporting minor damage, mostly in the form of broken glass and fallen flat-screen televisions and monitors. The fault has been responsible for
a number of historical quakes, most notably the 5.9 Whittier Narrows quake of 1987, which killed 8 people and caused around 350 million dollars of damage. The 2008 Chino Hills quake 5.4 may have occurred on the same system.
The event on the Puente Hills blind thrust was oblique, with reverse (compressional) and right lateral displacement, as shown by the
focal mechanism from the quake (below). It is called a blind thrust because it doesn't have a clear surface expression which makes it difficult to assess the hazard level and the history of the fault. It is thought to be capable of generating a magnitude 7.2-7.5 magnitude event, which would have catastrophic consequences for the urban areas of Los Angeles and Orange counties. Such events are thought to have taken place
around four times in the last 11,000 years.
Earthquakes such as those tonight serve to remind all Californians that their home is earthquake country, and all who live in the region must take this into account. The Puente Hills fault is just one of numerous active faults in Southern California. The San Jacinto, Santa Susana, Cucamonga, Elsinore, Whittier Hills faults, and the granddaddy of them all the San Andreas are all capable of causing great mayhem. Education and preparation are the best defense against tragedy. Here is a good place to start:
http://www.data.scec.org/earthquake/preparedness.html.
9 comments:
Felt it in Fullerton for sure. Very jarring being so close to the epicenter. Makes me appreciate what kind of shaking was needed over how much time to raise our local hills and mountains. The sphere image does little to convey information about the quake to me. Translation?
The symbol is a plot of compression and dilations (the stretching and squeezings) of the earthquake waves which is used to interpret what kind of fault produced the quake. Faults can move laterally (like the San Andreas), or vertically (like the faults that produced the San Gabriel Mountains). The earthquake tonight was oblique (sort of diagonal).
Was'nt the rolling type of earthquake-jarred & shook-Should we expect another quake so large-or did this one release built up pressure & we're done for awhile? Thank you-shaken up in La Puente
You've probably already noticed the 4.1 magnitude aftershock today. There is nothing unusual about aftershocks and they are expected after any moderate to large earthquake. As the US Geological Survey notes, there is about a 5% chance of a larger quake within a day or two, because there are such things as foreshocks, but we can't tell the difference (until after the fact). All quakes release pressure, but given that a 5.1 releases only about a thousandth of the energy of a 7.1, it didn't release much accumulated stress on the fault system.
I grew up in Hacienda Heights so this is all very interesting. I roamed around the Puente Hills as a kid in the late 1960s, our little piece of wilderness, except for the cattle. I never thought the immediate area was prone to earthquakes having been through a number of small earthquakes and the larger San Fernando earthquake in 1971 (M6.6), all comparatively distant from our quiet little enclave tucked in the hills overlooking the San Gabriel Valley. What is really interesting is the recent understanding of these blind thrust fault systems and the recent earthquakes they've spawned (1987 Whittier Narrows earthquake, M5.9; 1994 Northridge earthquake, M6.7).
Lifelong resident of Whittier and now Fullerton. My Dad was visiting from Victorville today. He said Hal Fishman (or some other venerated local newsman) explained to him in his day that a 6 was a thousand times bigger than a 5. I recanted that on the log scale a 6 was 10 times bigger than a 5 but that I didn't know how it related specifically to earthquakes. Maybe there was a fudge factor. Does your post above indicate that? 7.1 a thousand times "bigger" than a 5.1? A quick scan of wikipedia today indicated Richter was related to amplitude and not energy released.
The increase in amplitude of waves is, as you say, by a factor of 10 times, so a magnitude 5 is 10 times bigger than a 4, and 100 times larger than a 3 (10x10). This works up to about magnitude 6.5; after that the waves don't get any larger, the quakes just last longer over a larger area. But the energy required to make those waves increases by a factor of about 31 times, so a 5 has 31 times more energy than a 4 and about a thousand times more energetic than a 3 (31x31). That's where the 1,000 comes in.
Thanks Garry. My Dad will feel vindicated. I am so frazzled from aftershock after aftershock I am happy to be distracted by your reply.
Hang in there! Best wishes, and thanks for the comments!
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