A P-wave will give a jolt/alarm before the S-wave arrives


Have you ever heard someone say: “I woke up just before the earthquake hit, then it started shaking” ? I will not deny that some people are truly sensitive, yet I believe that many of the people with this story were awakened by the arrival of the p-wave some seconds before the s-wave (and other waves) that started the real shaking. Since the p-wave (“primary” or “pressure” wave) travels faster than a 60% slower s-wave (“secondary” or “shear” wave), it arrives first. It is just like being able to see lightening first, and then after some time period (seconds), the sound of the thunder finally arrives.

A common scenario goes something like this: (let’s do this like a short movie)


1 – We see a couple sleeping peacefully in bed. After this establishing shot, the whole house and the bedroom is shaken by a sharp, one-time jolt! [WHUMP! sound]


2 – Immediately after the jolt, we see the woman’s eyes pop wide-open and look anxiously from side to side.


3 – After 5 or 10 seconds pass, she appears safe and we see the woman sit-up in bed, look-around and begin to stretch. She stops to listen carefully to something that sounds like a train some distance away. The sound grows closer. After another 15 or 20 seconds, the sound is much closer as if the train just slammed into the house and the room, the house, and the whole neighborhood begin to shake, rattle and roll…

The End

It didn’t HAVE to be The End for this slumbering couple if they’d only used those precious 15 or 20 seconds to drop and cover, or my favorite: GET OUT!

There is a company that is marketing a device (or service?) that automates some initial emergency responses to a p-wave generated alarm. Macros are kicked into gear which open fire station doors, start emergency generators at hospitals and other applications before the major shaking even starts. Come to think of it, I bet it’s the p-wave that causes the sogs-stop-barking effect. This is one of many animal signs that we can pay attention to where it seems like ALL the neighborhood dogs are barking at once, then stop. This sign is said to offer about 30 to 60 second advance warning, of course this depends on distance to the epicenter.

With this understanding, some practical and interesting calculations can be made. Light travels at the, well, it’s called the “speed of light.” Guess what sound travels at? Yep, “the speed of sound.” I wonder what speed p-waves travel at? At approximately 186,000 miles per second, light could circle the Earth seven times in one second. If my figures are correct (I welcome any corrections), it would take sound something like 32 hours to circle the globe because sound travels at about 1/5th of a mile per second.

By knowing this we can estimate how far away lightning is by counting how many seconds pass before the sound arrives. If we count (“one thousand one, one thousand two…) for five seconds between seeing lightning flash and hearing thunder, we can know that the lightning is 5 miles away. Since lightning is no joke, here’s a link to some basic safety measures we might do well to be familiar with from the National Lightning Safety Institute.

What may be a practical take-home message from all this is to perhaps treat the arrival of a jolting p-wave as an alarm. Personally, I’ll be out the door with my grab-and-go kit (hopefully) before the arrival of the s-wave.

To estimate the distance (then magnitude), (on our way out) start counting seconds the moment we notice a p-wave (or sudden neighborhood-wide dog silence). Similar to the lightning/thunder illustration, we multiply the number of seconds separating the p- and s-wave arrival times by 5. If fifteen seconds passed, the epicenter is about 75 miles away. (5 x 15 = 75). As for magnitude, this is a very broad estimate…

Now that we timed timing the difference between the p-wave and s-wave, we can make an estimate of the distance to the epicenter. That is to say we can guess-up another quite rough estimate. Let’s say that we experience a kind of “medium” earthquake…a 6.0 which felt “strong,” yet the damage was “light.” If there were not very much time between the p-wave and s-wave, we can guess that that’s about the extent of it. On the other hand, if there was a great deal of time separating the two waves, then what we experienced locally was nowhere near as large as the epicenter. We could expect bigger and more numerous problems.

Alternately, we can turn on a radio and in a few minutes be well-informed. We can also wait what averages about 1 hour and 10 minutes to receive an e-mail from the USGS. Even with the delay, this service is actually pretty cool. The  USGS Earthquake Notification Service (ENS) is a free service that sends automated notification emails when earthquakes happen. You can dial-in different settings for different areas of the world. For world-wide I like to get email if an EQ is over 6.0. Locally, I have it set for 2.8 (and that might be too low). Tidal waves alerts arrive MUCH faster – in about 10 minutes. I have buttons to take you to the email sign-up section of those sites on my Prepare page.




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