Tricky Faults, Lax Construction and Wobbly Soil Fueled Destruction in Turkey

“We’re dealing with a fault system—a whole bunch of different faults and they are all talking to each other,” said Jonathan Delph, a seismologist at Purdue University who studies the geology of the region. 

Based on their close timing and proximity, the two earthquakes are considered part of the same seismic event that occurred at the intersection of three tectonic plates, according to Dara Goldberg, a USGS research geophysicist.

“We know that tectonic boundaries are the places that have significant seismic hazard and normally we think of those as being where two plates meet,” Dr. Goldberg said. “A triple junction is just that much more complicated because there are three plates meeting at the same spot.”

The first earthquake occurred on the East Anatolian fault, which runs in a north-south axis, while the second earthquake occurred on the intersecting Malatya fault which runs in an east-west direction. 

Dr. Goldberg said preliminary modeling by the USGS indicates that the initial shock was a so-called strike-slip earthquake that ruptured more than 60 miles of its fault, moving it by more than 10 feet. The magnitude 7.5 aftershock ruptured an 80-mile segment of its fault, shifting it by more than 35 feet.

The earthquakes were also very shallow, according to Stephen Hicks, a seismologist at the University College London. The first earthquake of 7.8 magnitude was 11 miles deep and the aftershock of 7.5 magnitude was 6 miles down. That meant the vast amount of energy unleashed didn’t have enough space to dissipate before hitting buildings on the surface, Dr. Hicks said.

That region of Turkey is rich in smaller faults—fracture points where two broken segments of rocky crust move past each other—because of the great plates colliding, said Dr. Delph.

“In rupturing the first fault we loaded stress onto another fault,” Dr. Delph said, that “caused that one to have its own main shock and aftershock sequence.” Larger magnitude quakes can transmit their stress to a larger area.

In August 1999, a magnitude 7.4 temblor struck about 53 miles southeast of Istanbul near Izmit, Turkey, killing more than 17,000 people. A second aftershock with a magnitude of 7.2 occurred three months later along the same fault.

“It isn’t unusual to have a major earthquake and then over some period of time, have another fault break,” said Jonathan Bray, distinguished professor of civil and environmental engineering at the University of California, Berkeley. “What’s a little unusual here is that we had a major magnitude 7.8 earthquake and then within about nine hours we had a 7.5. You don’t usually have them that close in time.”

The vast destruction that occurred on Monday was partly the result of something called liquefaction, a phenomenon in which intense shaking turns the soil underneath buildings’ foundations into a near-liquid state. Liquefaction is especially a problem in areas with a high water table near waterways, ports and rivers, according to Dr. Bray, who was part of a team that examined structures after the 1999 Izmit quake.

“Liquefaction causes the ground to fail and break apart,” said Dr. Bray. “Any kind of nonuniform ground deformation is very damaging to the structures above.” 

When Soil Liquefies

An earthquake’s violent shaking can turn solid ground to mush.

Liquefaction occurs in water-logged soil, in which the space between individual grains is filled with water, including low-lying areas near rivers, lakes, bays and oceans.

Earthquake vibrations can collapse those water-filled spaces, compressing soil volume and increasing internal water pressure. The mixture abruptly turns liquid, losing strength and stiffness.

Loose, granular

sediment layer

Soil saturated

with water

Soil particles

with high

water pressure

Soil particles

with normal

water pressure

Liquefaction can cause significant destruction. Riding on a layer of liquefied soil, ground at the surface, and even buildings, can be thrown back and forth or travel down slopes.

Buildings and houses can topple in the quake-created quicksand.

Roads can buckle and tilt. Piers, bridge supports and retaining walls can topple.

Water under pressure can be forced upward, pushing up mud or sand.

Liquefaction occurs in water-logged soil, in which the space between individual grains is filled with water, including low-lying areas near rivers, lakes, bays and oceans.

Earthquake vibrations can collapse those water-filled spaces, compressing soil volume and increasing internal water pressure. The mixture abruptly turns liquid, losing strength and stiffness.

Loose, granular

sediment layer

Soil saturated

with water

Soil particles

with high

water pressure

Soil particles

with normal

water pressure

Liquefaction can cause significant destruction. Riding on a layer of liquefied soil, ground at the surface, and even buildings, can be thrown back and forth or travel down slopes.

Buildings and houses can topple in the quake-created quicksand.

Roads can buckle and tilt. Piers, bridge supports and retaining walls can topple.

Water under pressure can be forced upward, pushing up mud or sand.

Liquefaction occurs in water-logged soil, in which the space between individual grains is filled with water, including low-lying areas near rivers, lakes, bays and oceans.

Earthquake vibrations can collapse those water-filled spaces, compressing soil volume and increasing internal water pressure. The mixture abruptly turns liquid, losing strength and stiffness.

Loose, granular

sediment layer

Soil saturated

with water

Soil particles

with normal

water pressure

Soil particles

with high

water pressure

Liquefaction can cause significant destruction. Riding on a layer of liquefied soil, ground at the surface, and even buildings, can be thrown back and forth or travel down slopes.

Buildings and houses can topple in the quake-created quicksand.

Roads can buckle and tilt. Piers, bridge supports and retaining walls can topple.

Water under pressure can be forced upward, pushing up mud or sand.

Liquefaction occurs in water-logged soil, in which the space between individual grains is filled with water, including low-lying areas near rivers, lakes, bays and oceans.

Loose, granular

sediment layer

Soil saturated

with water

Soil particles

with normal

water pressure

Earthquake vibrations can collapse those water-filled spaces, compressing soil volume and increasing internal water pressure. The mixture abruptly turns liquid, losing strength and stiffness.

Soil particles

with high

water pressure

Liquefaction can cause significant destruction. Riding on a layer of liquefied soil, ground at the surface, and even buildings, can be thrown back and forth or travel down slopes.

Buildings and houses can topple in the quake-created quicksand.

Roads can buckle and tilt. Piers, bridge supports and retaining walls can topple.

Water under pressure can be forced upward, pushing up mud or sand.

Liquefaction occurs in water-logged soil, in which the space between individual grains is filled with water, including low-lying areas near rivers, lakes, bays and oceans.

Loose, granular

sediment layer

Soil saturated

with water

Soil particles

with normal

water pressure

Earthquake vibrations can collapse those water-filled spaces, compressing soil volume and increasing internal water pressure. The mixture abruptly turns liquid, losing strength and stiffness.

Soil particles

with high

water pressure

Liquefaction can cause significant destruction. Riding on a layer of liquefied soil, ground at the surface, and even buildings, can be thrown back and forth or travel down slopes.

Buildings and houses can topple in the quake-created quicksand.

Roads can buckle and tilt. Piers, bridge supports and retaining walls can topple.

Water under pressure can be forced upward, pushing up mud or sand.

Building engineers can prevent liquefaction by pounding the ground with heavy weights to compact soil underneath the foundation before construction, Dr. Bray said. 

In addition to liquefaction, lax enforcement of building codes could have played an important role in fatalities and buildings collapsing. The second earthquake brought down buildings that were damaged but still standing after the first one, according to Polat Gülkan, professor of structural engineering at Başkent University in Ankara.

“Just the fact that the two earthquakes followed one after another doesn’t explain the whole problem,” Dr. Gülkan said. “There are other factors that caused the situation to be more critical than it needed to be.”

After the 1999 earthquakes, he said, Turkey introduced stricter building codes, which were last updated in 2018. 

“The trick here is not formulating this building supervision system, but properly implementing it,” Dr. Gülkan said. “I’m not sure that the same success has been achieved in that regard.”

—David Luhnow contributed to this article.

Write to Eric Niiler at eric.niiler@wsj.com, Aylin Woodward at aylin.woodward@wsj.com and Nidhi Subbaraman at nidhi.subbaraman@wsj.com