Seismic Warning Systems

I have been meaning to write the second installment of the Three D’s for quite a while now, but the recent magnitude 7 earthquake in Haiti motivated me to finally do it. Here is the second of our Three D’s: Direction.

Recall from the first installment of the Three D’s that earthquakes are not points on a map as represented by epicenter, but in fact occur on faults, geometric planes slicing through the Earth’s crust. The epicenter represents the point on the surface where the earthquake began. The implication here is that an earthquake does not rupture an entire fault all at the same moment – it has a beginning and an end. Actually, earthquakes behave a lot like zippers, starting at one end of a fault (or sometimes in the middle) and “unzipping” the fault at a rate of about 3 km per second. Make no mistake; this is quite fast, about 10 times the speed of sound and much faster than even the fastest jets ever flown. But jets can provide us with a useful analogy nevertheless, because earthquakes can generate an effect similar to a sonic boom!

In slow flight, the sound waves from the passage of the jet through the air radiate away from the jet in all directions. If the jet is sufficiently slow, you can hear it before it arrives because the sound waves get ahead of the jet and reach your ears before the jet does. As a jet travels at near-sonic or super-sonic speeds, these sound waves cannot get ahead of the jet, because they are moving at about the same speed. Instead, the sound waves “pile up” at the nose of the jet and form a shock wave, which we hear as a sonic boom when the jet passes over our heads. Behind the jet we hear a lot of small sound waves, comparatively quiet by themselves. In front of the jet they add up together and create a loud bang.

ShakeMap for the 2003 Magnitude 6.5 San Simeon Earthquake

In the same way earthquake ruptures, which unzip at almost the same speed as S-waves, cause these seismic waves to pile up ahead of the rupture and cause much more severe shaking in the direction of rupture than behind the rupture. This effect is called directivity by seismologists. One earthquake that exemplifies this effect is the 2003 San Simeon, CA Earthquake.

The figure shows the ShakeMap for this magnitude 6.5 event. This earthquake ruptured from northwest to southeast, along the black line which represents the fault, starting at the star which represents the epicenter. The colors on the map represent shaking intensity with hotter colors meaning worse shaking. The map shows how the fault “projected” its energy toward the town of Paso Robles 25 miles to the southeast, where the shaking was more intense than it was in San Simeon, 5 miles to the southwest of the epicenter. In this earthquake, the town of Paso Robles suffered much worse because the earthquake rupture was directed toward the city than if the same fault segment had ruptured from southeast to northwest, away from Paso Robles.

So we now know two of the three factors that affect the degree of shaking you might feel in an earthquake: Distance from the fault (rather than from the epicenter), and the Direction in which the fault ruptured. Stay tuned for the exciting conclusion of our three part series, when we take a look at the final D: Dirt.

Gilead Wurman
Chief Seismologist
Seismic Warning Systems, Inc.

This past week has been very exciting for residents of the Owens Valley, to the East of the Sierra Nevada in California. There have been over 500 earthquakes larger than Magnitude 1 in an area of about 26 miles by 39 miles, an extraordinary number. This includes six earthquakes Magnitude 4.5 or greater, the largest being a Magnitude 5.2 earthquake that occurred around 6:15pm local time on Friday Oct. 2. This is an earthquake swarm!

What makes an earthquake swarm different from simply a 5.2 earthquake with lots of aftershocks? For starters, the largest earthquake wasn’t the first in the sequence. In fact, there were two other large events just before the 5.2: a 4.8 at 6:09 and a 4.9 at 6:10. The entire sequence actually began with a Magnitude 5 nearly two days earlier, just after 3am on the morning of Thursday Oct. 1. So that’s one big difference: there is no discernable “mainshock” for the aftershocks to follow. Another difference is there are a lot of large earthquakes for the number of small earthquakes in the sequence. Take a look at the chart, which shows the number of earthquakes larger than some minimum magnitude. The number of earthquakes at each larger magnitude decays exponentially according to a law called the Gutenberg-Richter relationship. According to this law, each time we go up by one magnitude unit we should see a factor of 10 fewer earthquakes. That is, if we have 500 earthquakes with magnitude > 1, we should have only 50 earthquakes with magnitude > 2, only five with magnitude > 3, most likely no magnitude > 4 earthquakes, and almost certainly no magnitude > 5 earthquakes. As you can see from this chart, we have over 100 earthquakes with magnitude > 2, about 20 with magnitude > 3, and 8 with magnitude > 4. So there are a lot more large earthquakes than we would expect!

Why is this important? Who cares if it’s a swarm or a regular earthquake? Why do I ask so many rhetorical questions? That’s the professor in me, sorry! The answer is that the Owens Valley was the site of one of the three largest earthquakes in California’s recorded history. You’ve probably heard of the 1906 Great San Francisco earthquake, which was about a Magnitude 8. If you live in Southern California you may have heard of the 1857 Fort Tejon earthquake, a Magnitude 8 earthquake that ruptured the San Andreas Fault south of Parkfield. There was yet another Magnitude 8 earthquake in 1872, the Lone Pine earthquake which ruptured the Owens Valley Fault, killed 27 people out of the 300 or so who lived in Lone Pine, and destroyed 52 of the 59 houses in the town. This earthquake was felt throughout California and Nevada, including in Yosemite where it started rockslides and woke the famed naturalist John Muir. So this is an interesting sequence of events, occurring as it does very near to the fault that ruptured in 1872. It is unclear whether the recent earthquakes are on the same fault, as they are several miles to the East of the fault zone, but this swarm suggests the possibility that something may be happening underground.

Gilead Wurman
Chief Seismologist
Seismic Warning Systems, Inc.

In the first of a three part series on the science of seismology, Gilead Wurman, Chief Seismologist at Seismic Warning Systems explains why “further away” does not necessarily mean “safer” when it comes to earthquakes.

“The three most important factors in real estate are location, location, location.”

Yes, it’s true what they say: when buying property the three things to consider are location, location, and location. This isn’t true, however, when it comes to predicting the damage from an earthquake. When the media show a map of a major earthquake, they typically show the “location” of the quake, or the epicenter, as a point on the map surrounded by ever-widening circles in a big bullseye. Looking at these maps, it’s no surprise that most people believe the closer you are to the epicenter, the worse the shaking. In reality though, the epicenter may not tell the whole story. To really understand where the shaking is bad we need to consider The Three D’s: Distance, Direction, and Dirt. Today we will look at the concept of “Distance.” Over the coming weeks, check back here for further discussions on the concepts of “Direction” and “Dirt.”

Earthquakes, and large earthquakes especially, are not located at a single point in the Earth. Rather, they break along a fault in the ground, a line on the surface of the Earth. A small earthquake might break a patch of fault a few hundred feet long, but large earthquakes may break large sections of faults dozens, or even hundreds of miles long. The Great San Francisco Earthquake of 1906 broke a section of the San Andreas Fault from Cape Mendocino in Northern California all the way to San Juan Bautista near Monterey, a distance of nearly 300 miles! In a major earthquake, your distance from the epicenter, where the earthquake starts, is not as important as your distance from the breaking fault.

1906 Earthquake Intensity (Boatright and Bundock, 2005)

In the 1906 Earthquake, San Francisco was only about 10 miles from the epicenter. Santa Rosa, north of the San Francisco Bay, was more than 50 miles from the epicenter, but only about 20 miles from the breaking fault. By some reports, it experienced more severe shaking than San Francisco itself! So we must stop thinking of earthquakes as just epicenters, or points on a map. Instead we should think of them as faults, or lines on a map. And next time we ponder the possibility of a major earthquake, we should worry less about its location, and more about our distance from the fault.

Gilead Wurman
Chief Seismologist
Seismic Warning Systems, Inc.

Sunday’s 4.7 earthquake in LA might not have been big by California standards, but if you were sitting in the Starbucks in Torrance at Hawthorne and Artesia Blvds you may have got more than a sprinkling of cinnamon in your latte. As these pictures from LA Times photographer Jay L. Clendenin show, a huge shard of glass fell, injuring one person.

Here’s the link for a bigger version plus other pictures: LA Times Quake Pix.

There is a lot of talk about the seismic safety of our school buildings here in California, and laws like the Field Act and the Private Schools Building Safety Act have been passed to try and improve it. But most injuries in earthquakes actually come from non-structural hazards such as ceiling fittings, bookcases, things on shelves, etc. That’s why it’s essential that students and staff get away from windows and under desks AS SOON AS POSSIBLE!

During the Northridge earthquake of 1994, none of the fire stations of Battalion 15 could open their doors, either due to loss of power or the doors jamming. They had to manually open either the front or rear doors - by flashlight - as it was pitch black at 4.30am! This of course severely delayed their response to the emergency. It was remarkable that the firefighters themselves sustained no injuries. You can find a harrowing account from the the Battalion Commander himself here: Battalion 15 Northridge Earthquake.

Yosemite’s famous Ahwahnee hotel, built in 1927, needs $60m in seismic retrofit repairs reports the San Jose Mercury News. This would be part of $137m in overall repairs needed said the acting superintendent. But on the plus side, Mary Lou Zoback of RMS in Newark says the shaking wouldn’t be as bad as the 2002 report says, and that they need a new report… A 7.6 quake shook the Lone Pine area in 1872.

We are pleased to announce that Seismic Warning Systems has started blogging! You’ll find lots of interesting posts over time, with information on the latest happenings in earthquake early warning, seismic retrofitting, earthquake preparedness etc.