On September 19, 1985, a magnitude 8.1
subduction-zone great earthquake occurred
beneath the coast of Michoacan Mexico.
Despite 350 kms from earthquake epicnter
to Mexico City, most of the 9,500 fatalities
and 30,000 serious injuries occurred there.
That quake prompted Mexico to both strengthen
its building codes and to develop an earthquake
alert system for all of Southern Mexico.
To understand how a subduction-zone earthquake
could cause such devastation so far away in the
Mexico City Basin, let’s start by looking at the tectonic setting.
The Cocos Plate dives beneath the
North American Plate at rates of 5–7 cm/year
offshore of Michoacan, Guerrero, and Oaxaca.
The 2,700 km-long Middle America Trench marks
subduction boundary from Mexico to Costa Rica.
In the past 40 years Mexico had 50 earthquakes
larger than M 6 with epicenters concentrated
near the plate boundary. During that interval,
a dozen M 7 or larger earthquakes
occurred in the subduction zone.
Red circles mark epicenters above site where
fault rupture began; yellow shows the rupture area.
Guerrero seismic gap is potential hazard zone.
How can we understand the relative energy
released by a Great earthquake of M 8 or M 9,
like the 1985 earthquake, compared to smaller
yet still damaging earthquakes?
The energy released by earthquakes increases by a factor of
approximately 32 for each unit increase in magnitude.
As an analogy, if breaking single strand of
spaghetti represents a M 5 earthquake,
then we need to break 32 to represent M 6,
1,024 strands for M 7, & nearly 33,000 noodles
to represent a M 8. To represent energy released
in a M 9 earthquake, like Japan 2011, we would
need to break more than a million noodles!
Chile 1960 was even bigger.
Most people think of an earthquake as a sudden
release of energy at the epicenter. In reality,
the earthquake is caused by sudden displacement of
rock across a fault at given depth within the earth at the
hypocenter, shown here at the top of subducting plate.
For great earthquakes, displacement at the
hypocenter is just the beginning.
In 1985, the rupture displaced an area 180 km long.
Zoom in to view the plate interaction.
The dense oceanic plate dives beneath the more-buoyant continental plate
at a rate of about 6 cm/yr.
Because tectonic plates are elastic like a spring,
converging plates build energy at the leading edge of the continent.
Locked by friction,the upper plate is forced back
and shortened, raising the land surface
GPS, not available in 1985,
is now able to measure that deformation.
During a megathrust event, the force of the
converging plates overcomes friction and causes
the overriding plate to move up the fault surface.
If it breaks all the way to the trench, as it did in
Japan in 2011, the leading edge will lurch seaward,
uplifting the seafloor, forming a mound of seawater
that spreads out as a tsunami.
The disaster of 1985 surprised seismologists
who thought a subduction-zone earthquake
would not produce such sudden ground motion,
or strong accelerations 350 km away in Mexico City.
Here’s why: if the seismic waves were traveling
through uniform material, shaking would be
1/2 as strong at 13 km, a 1/4 as strong at 27 km,
and an 1/8 as strong at 50 km. Mexico City
recorded up to 1G of acceleration, an amount
shown in this shake-table demonstration.
Acapulco, on bedrock closer to the epicenter,
had only moderate shaking. What explains the
stronger ground shaking in Mexico City?
Mexico City sits on remnants of a dried up lake
that once surrounded Aztec capital, Tenochtitlan,
now a sediment-filled basin surrounded by
hard volcanic-bedrock mountains.
Short-period seismic waves produced weak ground motion
as they passed through the bedrock but once inside the lake basin
the waves slowed and were amplified causing
the soft sediment to shake like Jello in a bowl.
In the broad Mexico City Basin
the seismic waves became trapped
and reverberated for many minutes in
a phenomenon known as basin amplification
as the waves bounced off the bedrock
becoming slower and bigger in the sediment.
The sediment “resonated” with 2-sec. period
for over a minute. The period of oscillation,
or shaking, is the time between two peaks.
In the bedrock, the seismic waves oscillated with
a 1-sec. period, but slowed to 2-sec. period
with higher amplitude as the waves
reverberated in the basin.
Before earthquake codes, buildings
were not always engineered to withstand
back-and-forth accelerations.
All buildings have a natural, period, or resonance
which is the number of seconds it takes for the
building to naturally vibrate back and forth.
If the period of ground motion matches the
natural resonance of a building, it will undergo
the largest oscillations and suffer the greatest damage.
Small buildings resonate naturally in 1-sec periods.
A high-rise building naturally resonates with a longer period.
For example, a 30-story building
resonates at a period of 3 sec.,
and a 50-story building at 5 seconds.
The result of two-second period was that
buildings 8–20 stories with similar natural
periods suffered the most damage while short buildings
and skyscrapers were relatively undamaged.
In 1991 seismic stations were deployed only in
the state of Guerrero, an area identified as the
most hazardous for proximity to Mexico City, and
the Seismic Alert System (SAS) was initially
designed to provide warning only to Mexico City.
It has since been expanded to issue warnings
to all large cities in the coverage area.
These 100 seismograph stations are the backbone
of Seismic Alert system as they detect and record
ground motion and transmit data to control centers
that rapidly analyze the data to determine
magnitude and location. For a moderate event,
a “preventive” warning goes out only to
government authorities and select users
like hospitals and the Mexico City subway.
For larger earthquakes, broader alert is broadcast
via TV, Radio, and loudspeakers to notify people of impending shaking.
On April 18th 2014, the Alert System was put to the test.
by coastal seismometers that immediately determined
the epicenter, depth, and magnitude.
The large size automatically prompted a public alert
to be issued to all cities within coverage area.
The 18 million people in the Mexico City Basin
received the earthquake early warning 71 seconds
before ground shaking arrived.
In 71 seconds people can move to safety, machinery is shut off,
rail and road traffic stopped,
emergency personnel & equipment readied for response.
Public education has been essential
to the success of this program.