Tag Archives: seismology

Harnessing Your Personal Seismometer to Measure the Size of An Earthquake

It’s not difficult to turn yourself into a personal seismometer to calculate the approximate magnitude of an earthquake that you experience. I have employed this technique myself when feeling the all too common earthquakes in Tokyo for example.

In fact, by this means scientists have been able to deduce the size of some earthquakes long before the earliest earthquake recordings. One key measure of the size of the November 1, 1755 Great Lisbon earthquake, for example, is based on what was reported by the “personal seismometers” of Lisbon.

Lisbon seen from the east during the earthquake. Exaggerated fires and damage effects. People fleeing in the foreground. (Copper engraving, Netherlands, 1756) – Image and caption from the National Information Service for Earthquake Engineering image library via UC Berkeley Seismology Laboratory

So How Do You Become a Seismometer?

As soon as you feel that unsettling earthquake vibration, your most important action to become a seismometer is immediately to note the time. When the vibrations have finally calmed down, check how much time has elapsed. Did the vibrations last for ten seconds, or maybe two minutes?

Now to calculate the size of the earthquake

The duration of the vibrations helps to estimate the fault length. Fault ruptures that generate earthquake vibrations typically break at a speed of about two kilometers per second. So, a 100km long fault that starts to break at one end will take 50 seconds to rupture. If the rupture spreads symmetrically from the middle of the fault, it could all be over in half that time.

The fastest body wave (push-pull) vibrations radiate away from the fault at about 5km/sec, while the slowest up and down and side to side surface waves travel at around 2km/second. We call the procession of vibrations radiating away from the fault the “wave-train.” The wave train comprises vibrations traveling at different speeds, like a crowd of people some of whom start off running while others are dawdling. As a result the wave-train of vibrations takes longer to pass the further you are from the fault—by around 30 seconds per 100km.

If you are very close to the fault, the direction of fault rupture can also be important for how long the vibrations last. Yet these subtleties are not so significant because there are such big differences in how the length of fault rupture varies with magnitude.

Magnitude

Fault Length Shaking duration

Mw 5

5km

2-3 seconds

Mw 6

15km

6-10 seconds

Mw 7

60km

20-40 seconds

Mw 8

200km

1-2 minutes

Mw 9 500km

3-5 minutes

Shaking intensity tells you the distance from the fault rupture

As you note the duration of the vibrations, also pay attention to the strength of the shaking.  For earthquakes above magnitude 6, this will tell you approximately how far you are away from the fault. If the most poorly constructed buildings are starting to disintegrate, then you are probably within 20-50km of the fault rupture; if the shaking feels like a long slow motion, you are at least 200km away.

Tsunami height confirms the magnitude of the earthquake

Tsunami height is also a good measure of the size of the earthquake. The tsunami is generated by the sudden change in the elevation of the sea floor that accompanies the fault rupture. And the overall volume of the displaced water will typically be a function of the area of the fault that ruptures and the displacement. There is even a “tsunami magnitude” based on the amplitude of the tsunami relative to distance from the fault source.

Estimating The Magnitude Of Lisbon 

We know from the level of damage in Lisbon caused by the 1755 earthquake that the city was probably less than 100km from the fault rupture. We also have consistent reports that the shaking in the city lasted six minutes, which means the actual duration of fault rupture was probably about four minutes long. This puts the earthquake into the “close to Mw9” range—the largest earthquake in Europe for the last 500 years.

The earthquake’s accompanying tsunami reached heights of 20 meters in the western Algarve, confirming the earthquake was in the Mw9 range.

Safety Comes First

Next time you feel an earthquake remember self-preservation should always come first. “Drop” (beneath a table or bed), “cover and hold” is good advice if you are in a well-constructed building.  If you are at the coast and feel an earthquake lasting more than a minute, you should immediately move to higher ground. Also, tsunamis can travel beyond where the earthquake is felt. If you ever see the sea slowly recede, then a tsunami is coming.

Let us know your experiences of earthquakes.

The Curious Story of the “Epicenter”

The word epicenter was coined in the mid-19th century to mean the point at the surface above the source of an earthquake. After discarding explanations, such as “thunderstorms in caverns” or “electrical discharges,” earthquakes were thought to be underground chemical explosions.

Source: USGS

Two historical earthquakes—1891 in Japan and 1906 in California—made it clear that a sudden movement along a fault caused earthquakes. The fault that broke in 1906 was almost 300 miles long. It made no sense to consider the source of the earthquake as a single location. The word epicenter should have gone the way of other words attached to redundant scientific theories like “phlogiston” or the “aether.”

But instead the term epicenter underwent a strange resurrection.

With the development of seismic recorders at the start of the 20th century, seismologists focused on identifying the time of arrival of the first seismic waves from an earthquake. By running time backwards from the array of recorders they could pinpoint where the earthquake initiated. The point at the surface above where the fault started to break was termed the “epicenter.” For small earthquakes, the fault will not have broken far from the epicenter, but for big earthquakes, the rupture can extend hundreds of kilometres. The vibrations radiate from all along the fault rupture.

In the early 20th century, seismologists developed direct contacts with the press and radio to provide information on earthquakes. Savvy journalists asked for the location of the “epicenter”—because that was the only location seismologists could give. The term “epicenter” entered everyday language: outbreaks of disease or civil disorder could all have “epicenters.” Graphics departments in newspapers and TV news now map the location of the earthquake epicenter and run rings around it—like ripples from a stone thrown into a pond—as if the earthquake originates from a point, exactly as in the chemical explosion theory 150 years ago.

The bigger the earthquake, the more misleading this becomes. The epicenter of the 2008 Wenchuan earthquake in China was at the southwest end of a fault rupture almost 250km long. In the 1995 Kobe, Japan earthquake, the epicenter was far to the southwest even though the fault rupture ran right through the city. In the great Mw9 2011 Japan earthquake, the fault rupture extended for around 400km. In each case TV news showed a point with rings around it.

In the Kathmandu earthquake in April 2015, television news showed the epicenter as situated 100km to the west of the city, but in fact the rupture had passed right underneath Kathmandu. The practice is not only misleading, but potentially dangerous. In Nepal the biggest aftershocks were occurring 200km away from the epicenter, at the eastern end of the rupture close to Mt Everest.

How can we get news media to stop asking for the epicenter and start demanding a map of the fault rupture? The term “epicenter” has an important technical meaning in seismology; it defines where the fault starts to break. For the last century it was a convenient way for seismologists to pacify journalists by giving them the easily calculated location of the epicenter. Today, within a few hours, seismologists can deliver a reasonable map of the fault rupture. More than a century after the discovery that a fault rupture causes earthquakes, it is time this is recognized and communicated by the news.