广西 5.2 级地震：一个"地震少"地方的提醒

in Liunan District, Liuzhou City, Guangxi Zhuang Autonomous Region,

a magnitude 5.2 earthquake occurred.

Focal depth: 8 kilometers.

Epicenter at Taiyangcun Town, Liunan District,

about 16 kilometers from downtown Liuzhou.

This earthquake itself was not very high magnitude.

Magnitude 5.2,

among China's earthquakes in recent years,

doesn't count as severe.

But it drew national attention.

Why?

Because it happened in a place "that shouldn't have earthquakes."

China's earthquakes

are overwhelmingly concentrated in the west.

The edge of the Tibetan Plateau,

the western Sichuan Basin,

the Tianshan range in Xinjiang,

Yunnan.

These places

are at the front line where the Eurasian and Indian plates collide.

Plates compress, uplift, slip,

so earthquakes are frequent,

and some are extremely deadly.

The Wenchuan earthquake,

the Yushu earthquake,

the Ludian earthquake,

are all names everyone knows.

What about Guangxi?

Guangxi is in southern China,

within the interior of the South China block.

By traditional understanding,

the crust here is relatively stable,

earthquakes are few and small.

Over the past decades,

Guangxi has rarely had earthquakes above magnitude 5.

An inland city like Liuzhou

has almost never had one.

So this magnitude 5.2,

for Guangxi,

is a rare event.

Let's look closely at a few details of this earthquake.

First, it didn't happen in isolation.

On May 17,

that is, the day before the earthquake,

Liunan District had already had three small quakes:

7:46 a.m., magnitude 2.5;

5:07 p.m., magnitude 3.2;

10:54 p.m., magnitude 3.0.

These three small quakes,

in seismology, are called "foreshocks."

Not every major quake has foreshocks.

Sometimes

foreshocks get ignored.

People feel them,

but no one thinks a bigger one is coming.

This time the foreshock sequence in Liuzhou was clear.

In hindsight,

it was an obvious warning.

But beforehand,

no one knew whether a larger one would actually follow.

This is the hardest side of seismology —

prediction,

we still can't do it today.

Second, time and depth determined the damage.

0:21 a.m.,

the vast majority of people were in deep sleep.

A magnitude 5+ quake at this hour

means people had no reaction time.

Unlike during the day,

when you hear a sound and can run.

Being shaken awake in the middle of the night,

people need several seconds to register "this is an earthquake."

Those few seconds

are often the difference between life and death.

The focal depth was only 8 kilometers,

classifying this as a "shallow-focus earthquake."

The shallower the depth,

the more concentrated the energy reaching the surface.

The same magnitude 5.2,

if the focus had been 80 km down,

would have felt much weaker at the surface.

An 8-km shallow quake

pushes its destructive force straight up to the ground.

Combine the early hour with the shallow focus,

and that's why a 5.2

caused 13 buildings to collapse in Liuzhou.

Third, what kind of buildings collapsed?

Officials reported "roadside self-built houses."

This is the most common kind of dwelling in China's villages and small towns.

Built privately by an individual hiring workers,

not by a developer.

Some are decades old,

walls of bricks laid in mortar,

no steel reinforcement or concrete frame.

In Chinese they're called "brick-and-mortar" or, older still, "earth-and-brick" construction.

These houses

are comfortable in normal times,

with courtyards and sunlight.

But their earthquake resistance is poor.

The moment a strong quake hits,

the wall structure splits open in an instant,

then collapses.

In the Wenchuan earthquake,

this kind of self-built house was the first to fall.

The Yushu earthquake,

the Ludian earthquake,

the same.

China's newer high-rise apartments,

built to seismic standards,

generally don't fall in a magnitude 6 or 7.

So this time, all of Liuzhou's high-rises are fine.

Those affected

are the people living in old self-built houses.

There is a hidden unfairness in this.

People living in old self-built houses

tend to be lower-income families,

or elderly villagers.

They bear the bulk of an earthquake's cost.

Urban middle class in new high-rises

might only feel "a little shake."

This is the cruelty of earthquakes throughout history:

they aren't fair.

Fourth, what does Guangxi do now?

After this earthquake,

experts at the China Earthquake Administration said:

Guangxi's seismic belt

is more active than traditional understanding suggested.

The next few years will require continued monitoring.

What this means is:

the consensus that "Guangxi has few earthquakes"

may need revision.

Not that Guangxi will become Sichuan,

but rather,

"Guangxi doesn't need to worry about earthquakes at all" —

that idea

isn't necessarily right.

Going forward,

Guangxi's urban construction,

schools' emergency drills,

families' earthquake awareness,

all need to be reconsidered.

Fifth, what does this mean for the rest of China?

China has 1.4 billion people,

and many of them live in "low-earthquake" provinces.

Guangdong, Fujian, Jiangsu, Zhejiang, Hunan, Hubei.

People in these places

don't normally think about earthquakes at all.

But science tells us

the active vs. inactive phases of seismic belts

operate on decade and century timescales.

Places we think are "safe"

may just be ones where the last big quake was too long ago,

and we've lost the memory.

Historically,

Guangdong has had magnitude 7 quakes,

Fujian has had magnitude 6 quakes,

Hunan has had moderate quakes too.

We forgot.

This 5.2 in Guangxi

is not the apocalypse,

but it is a reminder.

A reminder to every person living in a "low-earthquake" place —

the city you live in

can shake too.

Finally, I want to say a few things.

Earthquakes are not moral events.

No one brings on an earthquake by doing something wrong.

An earthquake is the physical result of crustal movement.

What we can do

is not panic,

not fatalism,

but preparation.

Keep a simple emergency kit at home:

water,

a few days of food,

a flashlight,

some cash,

copies of important documents,

your regular medications.

Know that for the neighborhood you live in,

after an earthquake hits,

where the nearest open ground is.

Know where you and your family

will meet if you get separated.

These things

look excessive in normal times.

But the moment an earthquake comes,

you'll thank your past self for preparing.

The people of Liuzhou, Guangxi,

last night went through fear,

chaos,

and the long wait for sunrise.

Today,

what they need most is time,

money to rebuild,

the country's and society's care.

We watch the news,

and we think of them.

Okay.

That's all for today.

See you in the next story.