How to be a human seismometer
OK, so this is part of the blog where I divulge some of my deepest secrets. Have you ever wanted to impress people after an earthquake by making an eerily accurate estimate of the magnitude and epicenter? Here are my tips on how to do this.
Step 1: Know your surroundings
Before the ground starts shaking, look over some maps of your area, and be familiar with the seismic hazards you are likely to face. The USGS Quaternary Fault Database is a good starting point to familiarize yourself with the faults around you. Once you know the faults around you, and have a general idea of how they might behave, you are armed with the knowledge of some earthquake scenarios you can reasonably expect to experience at some point in the future.
Step 2: Know the Intensity
You won't be able to feel the magnitude of an earthquake, but you will feel the intensity. When the ground starts to move, you will need to be able to judge your Modified Mercalli Intensity. Here's my humorous version of it, if you can't make too much sense of the original:
I: Huh, look at that seismograph wiggle. (Only works if you're standing by a seismograph.)
II: Whoa, I feel dizzy. Why is that chandelier wobbling?
III: What the... Was that an earthquake?
IV: Wow! That was an earthquake!
V: Dude! Look at your bobblehead collection!
VI: Ah crap! There goes my bobblehead collection!
VII: Oh crap! Oh crap! This a big one!
VIII: Aaaaaaaaaaah!! Earthquake! Earthquake! AAAAAAAAH!!!
IX: Oh crap!!! We're all going to die!! We're all going to die!!
X: AAHHHHHHHH!!!!!! THE WHOLE DAMN STATE IS FALLING INTO THE OCEAN!!!!!!!
XI: AAAAAAAAAAAAAAAHHHHHHH!!!!!! WE'RE ALL DEAD!!!!!!!!!!!!!
XII: [nothing to say because everyone is actually dead]
Step 3: Direction
Now that the ground is actually shaking, you should be able to notice a distinctive direction to the motion. Once you have identified this direction, you have narrowed down the location of the quake to two small wedges, 180 degrees apart, centered on your location. The epicenter lies within one of these two wedges. The only exception to this is if this direction seems to change during the course of this earthquake. If it does, you know at that point you're dealing with a pretty major quake.
Step 4: Frequency
You can to a first approximation figure out the frequencies of the quake that you are experiencing. Do you hear rattling more than you feel a shaking? That's an indication of a high frequency seismic wave. You are either very close to a small quake, or you are experiencing the P-wave of a major quake. Do you feel a sharp shaking, or a gentle rocking like on a ship? Generally, the larger the quake, the lower frequencies you will notice. In very large quakes, you might notice everything moveable around you slowly rocking for several minutes after the quake.
Step 5: Duration
How long does the major shaking last? This will give you some idea of the magnitude. For a magnitude 3 quake nearby, you will feel a fairly intense shaking for only a few seconds. A magnitude 4 or 5 might last 10-20 seconds. If you feel shaking for over 30 seconds, you are dealing with a magnitude 6 or above.
Step 6: Synthesis
Once you have an idea of the intensity, direction, frequency and duration, you can begin to guess at magnitude and location. High frequency and short duration means a small quake, even if the intensity is high. A quake that lasts a minute or more, but is barely noticeable is likely a very large earthquake, but very far away. By knowing your surroundings, you can begin to guess at a location and magnitude. To get started I'll give a few examples of earthquakes that I've felt, and the results of my guesses according to these parameters:
Magnitude 3.1 about 4 miles away:
This minor earthquake occurred right below Portland back in January. The best description that I could think of for the motion was a "shiver". It was fairly high frequency, and felt like it was due south of my location. It lasted about 10 seconds. I guessed magnitude 3.5 near Scotts Mills. In reality, it was a magnitude 3.1 much closer to me than that. The USGS report showed that it was nearly due south from my location. I overestimated the magnitude because I thought the quake was more distant. I had neglected to take note of the recent increased activity on the East Bank Fault, where this earthquake occurred.
Distant magnitude 7.2:
Last June, we felt the very distant but very powerful Gorda Plate quake off the coast of California. This felt unlike any quake I had felt before, because of the great distance. The ground rocked like a ship's deck for about a minute or so, but there was no sharp shaking. I knew right away that this was a distant quake, and guessed a magnitude 7.1 near the Mendocino Triple Junction. This was probably my most accurate guess.
Deep magnitude 6.8:
The Nisqually Quake was one of the larger events to affect Portland recently. I was on a raised floor at work at the time of this quake, and didn't feel it firsthand. Based on reports from my coworkers that the quake had definitely come from the north, and lasted about 30 seconds, I guessed magnitude 7.0 near the Strait of Juan de Fuca.
Shallow magnitude 6.7:
I was about 16 miles from the epicenter of the Northridge Quake when it occured. I experienced very sharp shaking for about 40 seconds, with loose items flying everywhere off of shelves, but no real structural damage to my wood-frame apartment building. There was a definite WNW-ESE motion to the quake, and quite a bit of vertical motion as well. I knew this was a thrust quake immediately, but overestimated the magnitude at 7.0. I correctly located the epicenter as somewhere within the San Fernando Valley.
Somewhat distant magnitude 4.4:
This quake occurred during the aftershock sequence of Northridge, but I am mentioning it because the method of determining distance and direction allowed me to exclude it as a Northridge earthquake. The shaking was definitely not along the axis that I was used to feeling the Northridge aftershocks. It also lasted longer than I expected for the intensity: an indicator of greater distance. It turned out to be a Landers aftershock, and my perception of a difference from the Northridge shocks was vindicated.
Step 1: Know your surroundings
Before the ground starts shaking, look over some maps of your area, and be familiar with the seismic hazards you are likely to face. The USGS Quaternary Fault Database is a good starting point to familiarize yourself with the faults around you. Once you know the faults around you, and have a general idea of how they might behave, you are armed with the knowledge of some earthquake scenarios you can reasonably expect to experience at some point in the future.
Step 2: Know the Intensity
You won't be able to feel the magnitude of an earthquake, but you will feel the intensity. When the ground starts to move, you will need to be able to judge your Modified Mercalli Intensity. Here's my humorous version of it, if you can't make too much sense of the original:
I: Huh, look at that seismograph wiggle. (Only works if you're standing by a seismograph.)
II: Whoa, I feel dizzy. Why is that chandelier wobbling?
III: What the... Was that an earthquake?
IV: Wow! That was an earthquake!
V: Dude! Look at your bobblehead collection!
VI: Ah crap! There goes my bobblehead collection!
VII: Oh crap! Oh crap! This a big one!
VIII: Aaaaaaaaaaah!! Earthquake! Earthquake! AAAAAAAAH!!!
IX: Oh crap!!! We're all going to die!! We're all going to die!!
X: AAHHHHHHHH!!!!!! THE WHOLE DAMN STATE IS FALLING INTO THE OCEAN!!!!!!!
XI: AAAAAAAAAAAAAAAHHHHHHH!!!!!! WE'RE ALL DEAD!!!!!!!!!!!!!
XII: [nothing to say because everyone is actually dead]
Step 3: Direction
Now that the ground is actually shaking, you should be able to notice a distinctive direction to the motion. Once you have identified this direction, you have narrowed down the location of the quake to two small wedges, 180 degrees apart, centered on your location. The epicenter lies within one of these two wedges. The only exception to this is if this direction seems to change during the course of this earthquake. If it does, you know at that point you're dealing with a pretty major quake.
Step 4: Frequency
You can to a first approximation figure out the frequencies of the quake that you are experiencing. Do you hear rattling more than you feel a shaking? That's an indication of a high frequency seismic wave. You are either very close to a small quake, or you are experiencing the P-wave of a major quake. Do you feel a sharp shaking, or a gentle rocking like on a ship? Generally, the larger the quake, the lower frequencies you will notice. In very large quakes, you might notice everything moveable around you slowly rocking for several minutes after the quake.
Step 5: Duration
How long does the major shaking last? This will give you some idea of the magnitude. For a magnitude 3 quake nearby, you will feel a fairly intense shaking for only a few seconds. A magnitude 4 or 5 might last 10-20 seconds. If you feel shaking for over 30 seconds, you are dealing with a magnitude 6 or above.
Step 6: Synthesis
Once you have an idea of the intensity, direction, frequency and duration, you can begin to guess at magnitude and location. High frequency and short duration means a small quake, even if the intensity is high. A quake that lasts a minute or more, but is barely noticeable is likely a very large earthquake, but very far away. By knowing your surroundings, you can begin to guess at a location and magnitude. To get started I'll give a few examples of earthquakes that I've felt, and the results of my guesses according to these parameters:
Magnitude 3.1 about 4 miles away:
This minor earthquake occurred right below Portland back in January. The best description that I could think of for the motion was a "shiver". It was fairly high frequency, and felt like it was due south of my location. It lasted about 10 seconds. I guessed magnitude 3.5 near Scotts Mills. In reality, it was a magnitude 3.1 much closer to me than that. The USGS report showed that it was nearly due south from my location. I overestimated the magnitude because I thought the quake was more distant. I had neglected to take note of the recent increased activity on the East Bank Fault, where this earthquake occurred.
Distant magnitude 7.2:
Last June, we felt the very distant but very powerful Gorda Plate quake off the coast of California. This felt unlike any quake I had felt before, because of the great distance. The ground rocked like a ship's deck for about a minute or so, but there was no sharp shaking. I knew right away that this was a distant quake, and guessed a magnitude 7.1 near the Mendocino Triple Junction. This was probably my most accurate guess.
Deep magnitude 6.8:
The Nisqually Quake was one of the larger events to affect Portland recently. I was on a raised floor at work at the time of this quake, and didn't feel it firsthand. Based on reports from my coworkers that the quake had definitely come from the north, and lasted about 30 seconds, I guessed magnitude 7.0 near the Strait of Juan de Fuca.
Shallow magnitude 6.7:
I was about 16 miles from the epicenter of the Northridge Quake when it occured. I experienced very sharp shaking for about 40 seconds, with loose items flying everywhere off of shelves, but no real structural damage to my wood-frame apartment building. There was a definite WNW-ESE motion to the quake, and quite a bit of vertical motion as well. I knew this was a thrust quake immediately, but overestimated the magnitude at 7.0. I correctly located the epicenter as somewhere within the San Fernando Valley.
Somewhat distant magnitude 4.4:
This quake occurred during the aftershock sequence of Northridge, but I am mentioning it because the method of determining distance and direction allowed me to exclude it as a Northridge earthquake. The shaking was definitely not along the axis that I was used to feeling the Northridge aftershocks. It also lasted longer than I expected for the intensity: an indicator of greater distance. It turned out to be a Landers aftershock, and my perception of a difference from the Northridge shocks was vindicated.
posted by grishnash at 10:51 PM
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