Know Your Faults: Puente Hills Fault

(This is an occasional series exploring the many earthquake fault lines in and around Southern California. This series is not intended to be a scholarly, scientific review of earthquake faults throughout Southern California, but hopefully will be a jumping off point for you to understand and further explore the fault lines that cross Southern California. )   

LA HABRA - It has been a weekend of cleaning up shattered glass, having homes assessed and reassessed to see if they are still livable, and dealing with aftershocks in what is turning out to be perhaps the most damaging earthquake in the Los Angeles Basin since the 1994 Northridge Earthquake.

At only magnitude 5.1 it is considered to be a low-end moderate earthquake, but when you get a typical Southern California shallow quake in the magnitude 5 range under an urbanized area that is when you begin to see damage, and that has been painfully proven for residents and businesses in the Fullerton and La Habra area.

The La Habra quake is bringing much needed attention to something rather unpleasant, the Puente Hills Fault, or otherwise called, the Puente Hills Thrust System.  (For the rest of this piece we shall just call it the Puente Hills Fault.)

How Unpleasant?

Just how unpleasant is the thought of this fault to geologists, seismologists and emergency planners? Well, a major quake, "The Big One," on the San Andreas Fault in Southern California is going to be a major life altering event, but a major earthquake on the Puente Hills Fault, with an expected magnitude 7.2-7.5, could take such a disastrous event to a whole new level. In fact, a major event on this fault is expected to be worse than a major event on the Newport-Inglewood Fault.

Officials at the United States Geological Survey (USGS) believe this La Habra earthquake was caused by the Puente Hills Fault. It is worth noting USGS also believes the 1987 Whittier-Narrows Earthquake, which seismologists originally thought was on the then newly discovered Elysian Park Fault, was also caused by this fault.

Why So Dangerous?

Using rough directions, the Puente Hills Fault runs about 25 miles, going east-to-west, from about the hills above Brea, across the lower San Gabriel Valley, going northwest into Downtown L.A., and further northwest ending just about before Griffith Park. Seeing and understanding where this fault runs you can understand why a major quake on this fault is such a dreadful thought to officials. 

Now unlike, say, the Whittier Fault or Newport-Inglewood Fault, which has noticeable scars in the earth, like hills, the Puente Hills Fault is a blind thrust fault with no surface scars. 

A Different Kind of Shaking

One of the many problems with a major earthquake on this fault is, unlike the Newport-Inglewood Fault or even the San Andreas Fault where those faults are vertical faults resulting in intense shaking near where the fault reaches the surface, the Puente Hills Fault is a horizontal fault with intense shaking felt over a much larger area.

 
A shake-map scenario of a M7.1 Puente Hills Fault rupture created by the Southern California Seismic Network at the California Institute of Technology (Caltech) in partnership with USGS. No copyright infringement intended; Shown for educational purposes only.

Just How Bad?

The Southern California Earthquake Center (SCEC) along with the University of Southern California conducted a study in 2003 that showed a major earthquake on the Puente Hills Fault could cause "fatalities ranging between 3,000 and 18,000," along with "displaced households ranging from 142,000 to 735,000, with an average of 274,000."

With a fault running from the lower San Gabriel Valley, into Downtown L.A. up to Griffith Park you would have a major earthquake occurring in Southern California's oldest neighborhoods. Seismologists along with emergency planners believe such a quake could result in severe, catastrophic damage to Downtown L.A.'s older, historic buildings along Broadway, Main Street and Grand Avenue. 

What about the tall, modern skyscrapers that gives L.A. its modern, worldly skyline?

Thomas H. Jordan, director of the SCEC, told the L.A. Downtown News in March 2011 that, “A 7.5 at Puente Hills would pretty much be a worst-case scenario for Downtown,” with shaking lasting more than a minute. Mr. Jordan says, “It’s conceivable that some of the high-rise buildings would collapse [...] A lot of the modern structures in Downtown are very well constructed, so it would take a very extreme event like [a 7.5] to really cause damage to those very well-constructed buildings.”

A major quake could also have a catastrophic affect on the older industrial neighborhoods just southeast of Downtown L.A.

Of course it needs to be noted that this fault crosses over major freeway and freeway interchanges, along with major railway lines and public transit lines. Chances are good that there will be some kind of damage to these lines in the event of a major earthquake.

Some may remember during the Northridge Earthquake railroad tracks actually bent in a few areas.

Of course, there are the fires that will likely break out, and the water-pipes that will burst.

One of the most dreadful worst case scenarios is a major earthquake occurring during the Santa Ana Winds.  

The study predicts total damage cost may come to $250 billion.

While a lot of the focus involving a major earthquake on the Puente Hills Fault has been focused on L.A. it is expected that Orange County will have severe damage with strong ground shaking expected in north Orange County. The M5.1 La Habra earthquake was hardly a dress rehearsal for what is expected in Orange County.


YouTube video created by USGS, SCEC and San Diego Super Computer Center showing the shaking expected from a major Puente Hills Fault earthquake. No copyright infringement intended; shown for educational purposes only. 

Not to be left out the Inland Empire is expected to receive strong shaking and some severe damage, particularly in southwestern San Bernardino County.

So, all around when you get right down to it just how bad will a major Puente Hills Fault earthquake be? Well, there is only really one way to find out, and that is when Mother Nature decides to show us. 

When Will Mother Nature Decide To Let Us Know?

Well, according to USGS researcher Ned Field, the lead author of the SCEC-USC study, a major rupture on the Puente Hills Fault occurs about once every 3,000 years. "In fact," says Mr. Field in the study, "as an individual your odds of dying of a heart attack or an auto accident are much greater than dying from this earthquake." 

If you are like most Southern Californians chances are when you hear that number you probably think there is not anything to worry about, because 3,000 years is a long way away. Well, here is the bad news, geologists and seismologists are not too sure where they are at in the cycle, such as if we are maybe 2,800 years away from the fault rupturing, or one month away from the fault rupturing.

Aside from knowing when the fault ruptures the SCEC-USC study found that the Puente Hills Fault has ruptured at least four times in the last 11,000 years, with earthquakes with magnitudes ranging from 7.2 to 7.5. 

It should be clear by now even if the next major rupture is a couple thousand years away the Puente Hills Fault is going to cause some problems for us in the years to come. 

It is worth noting that in the same March 2011 L.A. Downtown News story Mr. Jordan points out the San Andreas Fault is still a bigger threat to the area, because earthquakes there happen about every 100 to 200 years. 

The last major San Andreas' rupture in our area was the M7.9 1857 Fort Tejon Earthquake, which rupture along 225 miles of the San Andreas Fault beginning near Parkfield and rupturing south to the Cajon Pass.

The last major southern rupture of the San Andreas Fault between the Cajon Pass and the Salton Sea is believed to have occurred around 1690.

Why Does It Seem Like I Am Only Hearing About This Fault Now?

With such a dangerous fault running through Downtown L.A. you think you would of heard all about the Puente Hills Fault growing up or living in Southern California for many years. After all, at one point we have heard all about the San Andreas Fault, Newport-Inglewood Fault, Hollywood Fault, San Jacinto Fault, and Whittier Fault, among many others, but it seems like there has not been a lot said about this extraordinarily dangerous fault. 

You are not alone in thinking and believing you have only recently heard about the Puente Hills Fault, because it was just barely discovered in 1999, and it has taken a few years after that for officials to really understand the danger it poses. 

In the aftermath of the Northridge quake there was urgency among geologists and seismologists to attempt to find blind faults around Southern California.

So Do All These Recent Quakes Mean We're Going To Have a Big Quake?

The recent earthquakes in Orange County and L.A., along with the January 15 M4.4 shaker in Fontana, may simply mean, according to USGS, that Southern California is coming out of its "earthquake drought." 

As Doctor Lucy Jones from USGS has pointed out many times in various media interviews, following the aftermath of the 1994 Northridge Earthquake the L.A. area, aside from the little jolt here and there, has been very seismologically quiet. It seems possible the 1992 Landers/Big Bear earthquakes along with the Northridge quake may have relieved stress for a time in Southern California, but now we may be reentering a seismologically active period in Southern California.

It Could Happen Anytime!

Whether it is preceded by a series of noticeable earthquakes, or no quakes at all, a major earthquake can occur anytime in Southern California. 

We all want a direct answer to the unknowable, and that is WHEN is a major earthquake going to happen? The direct answer to that is this, at this time there is no accurate way to predict earthquakes, and thus there is no way of knowing when a major earthquake is going to occur.

Just Be Prepared!

For a lot of people the above answer is not the one they want to hear, but it is the only answer available. So, all we can do is prepare and have a plan in place when it does happen.

Resources To Help You Prepare

Prepare SoCal from The American Red Cross

Ready L.A. - City of L.A. Emergency Preparedness 

Los Angeles County - Emergency Preparedness

Ready O.C. - Orange County Emergency Preparedness 

San Bernardino County - Emergency Preparedness 

Riverside County Fire Department - Emergency Management

Ready Ventura County - Emergency Management  

San Diego County - Emergency Preparedness

Cal-OES - California Governor's Office of Emergency Services

Tips on Preparing an Emergency Kit from Ready.Gov

Did You Feel It? A 4.4 Earthquake Shakes L.A.

ENCINO - The largest earthquake to occur within the city of Los Angeles since the aftermath of the 1994 Northridge Earthquake shook the city and much of Southern California at 6:25 a.m. Monday when a magnitude 4.4 earthquake struck in the Santa Monica Mountains in the Sepulveda Pass.

The United States Geological Survey put the epicenter about two miles south-southeast of Encino. USGS officials at the California Institution of Technology, or as most of us know it, Caltech, say this is the strongest earthquake to occur in the Santa Monica Mountains on record, which only go back to 80 years when earthquakes were first recorded in that area. 

There have been some small aftershocks, mostly micro-quakes, which is to be expected. Caltech says Monday's earthquake is following the typical pattern of Southern California earthquakes with the mainshock and aftershocks.

As would be expected there was no serious damage, but there was some light property damage with items falling off shelves. There have been no reports of any injuries. Typically when you get into the magnitude 5 range that is when you start seeing serious property damage, like windows shattering and larger items being thrown about. Also older structures tend to sustain damage, sometimes serious damage, particularly with a quake in the upper M5 range. When you get into the magnitude 6 range with a quake that occurs under an urban area that is when you begin to see serious structural damage, even in a place like Southern California.

So far, as of this writing, no earthquake fault has been assigned to this earthquake. Earlier on Monday seismologists suggested this quake could be on an unknown fault in the Santa Monica Mountains. There are several fault lines near the epicenter of this quake, including the Santa Monica Fault.

In the first five-to-six hours after the M4.4 quake there is a five-percent chance of a larger earthquake occurring. That goes down to about one-percent of a larger quake occurring within the next three days. Historically a larger earthquake happening after a smaller jolt has been rare in Southern California.

Prior to Monday's earthquake the last large earthquake to occur within the the city of L.A. was on September 9, 2001 when a magnitude 4.2 hit near West Hollywood, which USGS believes was on the Newport-Inglewood Fault. 

The last significant earthquake in the L.A. basin was on May 17, 2009 when a M4.7 quake struck in Inglewood causing minor property damage. Seismologists believe this quake is consistent with slip on the Newport-Inglewood Fault.  

The last magnitude 5+ earthquake in the L.A. metro area was the 2008 M5.5 Chino Hills Earthquake.

Seismologists have described L.A. and Southern California as being in an "earthquake drought," where typically the L.A. area should have on average a M4+ once a year. Caltech says they will have to wait many months to see if this is the beginning of Southern California coming out of its "earthquake drought."

In the last year and a half there have been a series of minor jolts in the Marina Del Rey area. USGS officials at Caltech are not sure if Monday's earthquake and those quakes are related. Seismologists say quake clusters are not typical for Southern California. Again, officials stress, there is no telling what those Marina Del Rey quakes may mean, if anything.

It is worth noting that on March 9 there was a M6.8 earthquake off the Northern California coast about 50 miles west of Eureka. That quake, however, is much to far away to be associated with Monday's quake.

Around social media, and on television, some people have been suggesting, or outright claiming, this morning's earthquake was caused by the full moon or it is "earthquake weather." Many studies conducted by USGS and other agencies around the world have shown and proven many times over there is no such thing as earthquake weather. Furthermore those same agencies have shown there has been no correlation and causation between full moons and earthquakes.

Bottom line, there is no way to predict earthquakes at this time. The only earthquakes that can be predicted are aftershocks, and USGS officials believe there will be small aftershocks in the next few days from this Encino earthquake. 

All you can do is be prepared.

On This Date: The 1933 Long Beach Earthquake

LONG BEACH - On Sunday night a magnitude 6.8 earthquake struck about 50 miles west of Eureka in the Pacific Ocean, according to the United States Geological Survey, and it was felt throughout much of the Northern California and southern Oregon. With this earthquake being some distance from land there was not much damage.

Sunday night's earthquake struck at 10:18 p.m., and was under a couple hours shy of occurring on March 10, which is the anniversary of a much less powerful earthquake that also had an epicenter in the Pacific Ocean, but caused much severe damage.

On March 10, 1933, at 5:54 p.m. with a magnitude 6.4 the deadliest earthquake in Southern California history on record hit resulting in 120 reported deaths and caused, in 2014 dollars, $899,676,923 worth of damage.

Commonly referred to as the Long Beach Earthquake, as damage was great in Long Beach and Compton with many collapsed buildings, the epicenter of this earthquake was actually about a mile west of Newport Beach in the Pacific Ocean.


Aftermath of the 1933 earthquake along Main Street in Compton.  No copyright infringement intended; shown for historic and educational purposes only.   

As Carey McWilliams would point out, many boosters and chamber of commerce types often sold Southern California, aside from being the land of sunshine and oranges, as a place where big earthquakes do not ever happen, despite the fact that within the last 34 years leading up to 1933 there were at least three notable earthquakes in Southern California. Of course much of the same was said and done in San Francisco before and after the 1906 earthquake with their boosters claiming it was just a great fire that destroyed most of San Francisco in 1906 and ignoring large quakes in the Bay Area prior to 1906.

Nonetheless this earthquake was really the first event that brought to light many hazards and potential hazards earthquakes pose to modern Southern California.

With so many collapsed buildings the most important thing to come from the 1933 earthquake was a piece of legislation called, The Field Act. The Field Act was the first major piece of legislation that mandated earthquake resistant construction, which was specifically aimed for schools. When this earthquake struck many schools collapsed due mainly to unreinforced masonry construction, and, as has been pointed out many times over and still worth repeating, had this earthquake struck just a few hours earlier many, perhaps hundreds, of school children likely would have perished. Records show at least 250 schools were destroyed in this earthquake.


Severe damage at Thomas Jefferson Junior High School in Long Beach as a result of the 1933 earthquake. No copyright infringement intended; shown for historic and educational purposes only.

The 1933 earthquake then and now vividly shows the dangers the Newport-Inglewood Fault poses to the Los Angeles basin. Many geologists and seismologists believe this fault can produce a magnitude 7.4 earthquake, and that such a major event on this fault could be much worse in terms of damage and casualties in L.A. and Orange County than perhaps an event on the San Andreas Fault.

With its epicenter just off the coast in Newport Beach this quake still holds the record as being the most deadly and damaging earthquake in Orange County history with major damage in Santa Ana, Garden Grove and Anaheim.

Damage to a Santa Ana building as a result of the 1933 earthquake. No copyright infringement intended; shown for historic and educational purposes only.   

Historic film footage of the aftermath of the Long Beach Earthquake. No copyright infringement intended; shown for historic and educational purposes only.

A very important lesson we should all take from this earthquake is that it should dispel the myth that big earthquakes in Southern California only occur in the middle of the night or early morning. For anybody who grew up or lived in the Southern California for the past 25-plus years probably know that our major earthquakes have occurred during the early morning hours, and this has lead some people to believe big quakes only hit around that time. As USGS and other scientists have stressed many times over, it is simply luck that Southern California's recent large earthquakes have hit in the early morning. Seismologists and emergency planners stress that a major earthquake could hit at anytime, as this one did in 1933 at 5:54 p.m.

As old photographs show the Southern California coast, particularly in the Huntington Beach area, used to be dotted with oil wells. It was reported about two hours before the earthquake the pressure in Huntington Beach oil wells dropped. Geologists and seismologists have studied pressure drops in oil wells before and after earthquakes, but however at this time the whole correlation and causation of pressure drops and earthquakes has not added up, so to speak.

It would not be until the 1971 Sylmar Earthquake that the L.A. basin would experience its next damaging and deadly earthquake, and not until the 1952 Kern County Earthquakes that California would have its next damaging and deadly earthquake.

The Newport-Inglewood Fault is still highly active, and was likely responsible for the May 17, 2009 M4.7 earthquake near Inglewood. This fault may possibly be responsible for the series of minor earthquakes in and around the Marina Del Rey area within the last year and a half.

In very modern times the Newport-Inglewood Fault made it presence known in Orange County when on April 7, 1989 a M4.7 occurred right under Newport Beach. That quake caused items to fall off shelves at the Newport Beach Fashion Center and other such property damage around Orange County. It was the last notable sized earthquake in Orange County.

There is that whole thing about history and learning from the past, and not repeating it. Sometimes earthquakes have a tendency to strike the same area twice or more and that cannot be helped, but to avoid mistakes of the past we can all be prepared.

The 1933 earthquake shows big quakes can hit in the middle of rush hour, and an earthquake of this size with its Newport Beach epicenter can and likely will cause damage in the L.A. and O.C. metro area. Any time an earthquake at or above magnitude 6 hits in an urban area that is when you can expect damage, even in an area that is suppose to be "built" to resist quake damage.

As we always do when we talk about earthquakes on this site we stress the importance of being prepared, and remember, that at this time there is no way to predict earthquakes.