- Keep in mind that to
a geoscientist in 1906,
the theory of plate tectonics,
the fundamental mechanism
by which we understand
how the planet works
wouldn't be formally accepted
for another 60 years.
The earthquake struck
at 5:12 in the morning
on April 18 1906.
The Fault slipped 21 feet
in a matter of seconds.
And as it slipped, it
sent out waves of energy
earthquake waves, that rippled out
into the surrounding areas.
Police Sergeant Jessie Cook
was standing on Davis
Street in San Francisco
and he watched the ground
undulate beneath him,
and he later wrote,
"It was as
if the waves of the ocean
were coming toward me,
billowing as they came.
Davis street split open
right in front of me.
A gaping trench about six feet deep
and half full of water
suddenly yawned and
sprang up on the sidewalk
at the southeast corner,
while the walls of the building
I had marked for my asylum
began tottering.
Before I could get into
the shelter of the doorway,
the walls had actually fallen inward,
but the stacked up cases of produce
that filled the place
prevented them from collapsing."
Part of what makes the 1906 earthquake
so important to a
geoscientists like myself
is that so many people made
observations like this.
They noted the time and the
intensity of the shaking,
and they documented the
destruction using photography,
which is a new concept in
documenting earthquakes.
In the years that followed the earthquake,
a team of researchers gathered together
to study the earthquake in greater detail
than had ever been done before.
And the team of researchers was led
by this stud of a man,
the head of the geology department
at UC Berkeley, Go Bears.
His name was Andrew Lawson.
And Lawson and his
colleagues set out again
to document the earthquake
in greater detail
than had ever been attempted before,
because they had access
to much more information,
a lot more data.
So they began to conduct
interviews with the survivors.
They documented the destruction themselves
taking photographs.
And they made countless
measurements around the Bay Area
and collected the only
92 seismographic readings
of the earthquake
none of which came from the United States.
So that was hard work in 1906.
In the end, Lawson and his
team produced a report,
cleverly called the Lawson Report,
and it totally revolutionized
the burgeoning field of seismology.
It was also a 400 page document
that very few people read.
(audience laughs)
But hidden in those pages,
embedded in those changes
were the seeds of ideas,
that would ripple out
and become embedded in the rock record.
They began by just making observations.
Lawson and his colleagues noted
that the earthquake
appeared to have started
on the San Andreas Fault.
And while that might sound obvious today,
that wasn't known back then,
In fact, in 1906, most
people, geologists included
thought that earthquakes
and all of the shaking
created the fault,
rather than the fault
generating the earthquake.
Lawson and his colleagues also noted
that the fault had slipped horizontally,
rather than vertically.
And again, while that seems like,
sort of a trivial observation,
it slipped horizontally,
at the time it was thought
that earthquakes could
only be generated by vertical motions.
So with these two seemingly simple,
deceptively simple observations,
the Lawson Report is already
radically transforming
our understanding of the natural world
and our relationship with it.
Lawson also teamed up with a
man named Grove Karl Gilbert,
who is a legend to nerdy
geologists like myself.
- (audience laughs)
And Lawson and Gilbert,
set out to map, all 600 plus miles
of the San Andreas Fault
through California.
Not an easy job in 1906,
on foot and horseback
through a lot of poison oak.
But they ended up making that trace
and they ended up documenting
again, all of these
motions along the fault,
these deceptively simple observations.
And while those are very important,
perhaps the most revolutionary concept
to be found in the Lawson report
belongs to this man,
a geologist from Johns Hopkins University
named Henry Fielding Reid.
And Reid and his colleagues
came out to the Bay Area
very soon after the earthquake.
And they began to make
their own observations,
and specifically, they were interested
in how the landscape had changed,
you know, before and after the earthquake.
And what Reid and his colleagues noted,
is that the fault didn't
appear to have slipped
the same amount everywhere.
In fact, they saw that it
slipped a much greater distance
where the earthquake epicenter was,
and then as you moved away
from that epicenter along
the San Andreas Fault,
it seemed to slip less and
less and less and less,
as though the crust itself were absorbing
some of that motion, a
little bit like a rubber band
just sort of elastically
absorbing something.
This led Reid and his colleagues
to propose a new theory
on the working mechanics of earthquakes.
They postulated that stress and strain
would build up on a fault over time,
a little bit like stretching
out that rubber band,
until eventually the friction on the fault
could no longer hold and
it would slip and generate an earthquake,
the rubber band snapping back into place.
This theory became known
as elastic rebound theory.
And to this day,
it is still the best
working hypothesis we have
on the fundamentals of earthquakes.
