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 lookingat 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 ofseawater 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 at27 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 oscillatedwith 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.