(upbeat music)
- [Narrator] On the night of September 7th,
a magnitude 7.0 earthquake rocked southwest Mexico
and ricocheted across social media.
(upbeat music)
Online, people posted about strange flashes in the sky
at the time of the earthquake.
The hashtag #apocalipsis gained traction.
What exactly was going on?
- I felt the earthquake very strongly here in Mexico City.
- As a seismologist, it's a cool thing to experience.
- During some part of the earthquake,
it's difficult to walk.
- I could feel how violent the movement was.
- [Narrator] The quake's epicenter was near Acapulco,
on the west coast, but it rattled Mexico City,
about 200 miles away.
At the time of the quake,
many reported seeing these flashes.
- Yes, I saw the lights, of course.
- [Narrator] There's some disagreement
as to the cause of this light, however.
- During the earthquake, we were having a thunderstorm.
- [Narrator] So maybe the flashes were lightning,
at least in part.
- I saw the flashes in the streets produced by electric arcs
because of the movement of the cables of the power lines.
- [Narrator] Others posted videos online
that showed transformers exploding,
which could have been another source of the light.
But there are other kinds of earthquake lights,
not caused by lightning or transformers.
They're white flashes that accompany
or precede large tremors,
especially in certain places like this one in Japan,
and scientists say they're real.
- But they're not understood.
- [Narrator] Here's what we think is going on.
It's basically static electricity.
Charge appears to build up within the Earth's crust,
which then gets discharged into the air,
creating a visible spark.
It's the same for any accumulation of electrons.
Like if you shuffle along a carpet in socks,
touch the doorknob, and feel a shock.
- You are generating a strong voltage between your finger
and whatever happens to draw the spark.
- [Narrator] Same thing when clouds in a thunderstorm
discharge lightning to the ground.
This rapid movement of electrons produces light.
When it comes to what's creating the charges
that may be responsible for earthquake lights,
there's one thing folks agree on.
- Nobody knows!
(laughs) So...
- [Narrator] There are a few theories.
Theory number one.
- Underneath the earth, there's huge pressures
and you grind things against one another.
- [Narrator] Put certain kinds of rocks in a vice,
clamp down, and you produce a voltage difference
across the stone.
Maybe that's what leads to the sparks.
Or maybe it's the friction from all that grinding.
Another theory is something called fracto-luminescence.
- So the act of making a fracture produces light.
- [Narrator] And naturally, earthquake activity
fractures rock as the waves ripple through the ground.
Troy Shinbrot has another theory
that comes from his efforts
to understand earthquake lights in the lab
using a tumbler filled with flour.
- We did the simplest, dumbest experiment,
and we seemed to see something.
- [Narrator] Tip the tumbler slowly,
and at a certain point, the flour slips.
- Before it slips,
you can see there's a little crack that appears,
and this crack generates measurable voltages.
And then because there is this crack,
the grains that are sitting on this unsupported region
start to slip.
Earthquakes are on a much larger scale
where the granular bed is the entire earth underneath us.
And the idea is that this crack generates a voltage
that is of such a large magnitude
that it produces earthquake light.
- [Narrator] Now, we don't know whether any of the flashes
during the recent Mexico quake
were indeed earthquake lights
caused by this discharge of static electricity
from the movement of the earth.
And if they were, we don't have the data
to know what force or stress within the Earth produced them.
Shinbrot wasn't there to document them,
and Miguel Santoyo is skeptical.
- It's not very probable that we have seen
that kind of light so far from the epicenter
here in Mexico City.
- [Narrator] So stand down the apocalypse alerts,
but maintain the early earthquake warning alerts,
because the ground will continue to shift beneath us.
- This is where we live, and we need to live with this.
And we need to be ready every day of the year.
(bright music)