125-year-old Indian seismic puzzle solved

More than a century after a magnitude-8+ earthquake struck in India, researchers have identified the epicenter of the massive quake.
 

By Davitia James, Temblor Earthquake News Extern (@davitiaa)
 

Citation: James, D., 2021, 125-year-old Indian seismic puzzle solved, Temblor, http://doi.org/10.32858/temblor.223
 

On June 12, 1897, a magnitude-8+ earthquake rattled much of the Indian subcontinent. Since then, several locations have been proposed as the origin of the earthquake. Now, using original records of the event and 3D models of earthquake behavior, researchers have identified the epicenter, according to a new study published in The Seismic Record. Their work places the starting point at the intersection of the Chedrang and Oldham faults in the northwest region of the Shillong Plateau — a broad, flat area of the Himalayan foreland in northeastern India.
 

 

On top of the world

The Himalayan mountain range is the result of the Indian and Eurasian tectonic plates crashing into each other. Earth’s highest mountains above sea level are found in this region — Mount Everest is the most famous — and the area has been the setting of major earthquakes throughout recorded history. The 1897 earthquake, which came to be known as the Assam quake, caused the plateau to rise at least 33 feet (10 meters) in the north and resulted in widespread loss of life and structural damage. Reports suggest more than 1,500 people died and damage was extensive over an area almost the size of England (Oldham, 1899).
 

 

The great debate

Experts on the Assam quake generally accept that the moment magnitude — a description of the amount of energy let off by an earthquake — falls between magnitude-8.0 and -8.4. However, the location of the epicenter is less agreed on.

In 1899, British geologist Richard Dixon Oldham, a scientist with the Geological Society of India, wrote an extended report on this earthquake and first noted that different seismic waves arrive at different speeds. Using damage reports, felt reports, and reports of ground acceleration, along with field observations and very early seismographs from stations around the world, Oldham estimated that the epicenter was in the Shillong Plateau. He created a map with a roughly triangular area from which he hypothesized the rupture had begun.

His report and others over the last 124 years focused on north-dipping Himalayan faults, but a 2001 study (Bilham and England, 2001) instead placed the epicenter on a 68-mile-long (110-kilometer) fault that is now called the Oldham Fault. The Oldham is a buried fault that dips to the south, away from the Himalayas.

Buried faults do not reach the surface but still pose major threats. Around the world, scientists are investigating why these kinds of faults cause so much shaking and how to assess their risks.

The idea of the epicenter of this large quake occurring on a buried fault is controversial, says György Hetényi, co-author of the new study and professor at the University of Lausanne where both authors are based. Some experts tend to think that most large earthquakes — greater than magnitude-8 — must happen on visible faults, Hetényi says.
 

Finding the puzzle pieces

Hetényi and co-author Shiba Subedi decided to see if they could more precisely locate the epicenter of the 1897 quake, as well as the time of the event.

Subedi and Hetényi examined damage reports and first-hand observations of the quake — such as notes about when shaking hit train stations — as well as preserved seismic records from stations around the world. Though seismographs were invented just a few years prior to the 1897 Assam quake, a number had already been installed. Subedi and Hetényi combed historic records to calculate the arrival time of seismic waves. Then they combined these arrival times with modern velocity models to reveal how far away and when the quake happened.

Using arrival times and waveforms from international seismometers was difficult, says Hetényi. “The information on arrival times is not as precise as it is today.” Hetényi and Subedi could only use records in which arrival times included seconds. Additionally, Hetényi says, not all stations were operational when the quake struck. Some, for instance, were having their recording paper replaced at the exact moment of the quake, and thus had no record of the event. In the end, though, their search yielded P-wave and S-wave data from 32 different seismic stations.
 

 

Solving the puzzle

Hetényi and Subedi determined the quake’s start time was 11:06:46 UTC, plus or minus 15 seconds. With better constrained timing, and knowing how quickly different earthquake waves travel, they were able to calculate the location using a computer model that accounts for changes in the crust and mantle in different locations.

The resulting point was 26.0°N, 90.7°E. That puts the epicenter “at the western end of the Oldham Fault, at its junction with the Chedrang Fault,” the team wrote. This location lands inside Oldham’s 1899 outline and is farther north than most estimates, but within 25 miles (40 kilometers) of other suggestions, including one proposed by Oldham. The location removes other faults, like the Main Frontal Thrust that dips north below the Himalayas, from contention.
 

Many new mysteries to choose from

This is the first study to use original instrument recordings to refine epicenter estimates for the Assam earthquake. The study supports the idea that big earthquakes can happen on buried faults, says Roger Bilham, emeritus professor at the University of Colorado Boulder who was not involved with this study.

Finding this old, buried fault as the origin of a devastating earthquake is important for assessing risk in this part of the Himalayas. “Modern seismic hazard studies start with mapped faults at the surface,” says Hetényi. There are other buried faults in the area, so improving seismic coverage here is a key step to expanding the understanding of these subsurface faults, he says. In turn, he notes that this will hopefully improve preparation for future earthquakes in this region and other seismically active regions located within tectonic plates.

Bilham says he looks forward to future research in the area, particularly on the nearby Chedrang Fault. The northern edge of the Shillong Plateau shows nearly 36 feet (11 meters) of vertical displacement, and movement in the area has offset rocks crossing the Chedrang River, he notes. With faults that extend through most of the crust and possibly the most displacement ever measured for a single earthquake, Bilham says, this area of visible and hidden faults provides endless opportunities for more seismic sleuthing for decades to come.
 

References

Subedi, S. and Hetényi, G. (2021). Precise Locating of the Great 1897 Shillong Plateau Earthquake Using Teleseismic and Regional Seismic Phase Data, The Seismic Record. 1(3), 135–144, doi: 10.1785/0320210031.
 

Further Reading

Bilham, R. (2008). Tom La Touche and the great Assam earthquake of 12 June 1897: Letters from the epicenter, Seismol. Res. Lett. 79, no. 3, 426–437.

Chen, W. P., and P. Molnar (1990). Source parameters of earthquakes and intraplate deformation beneath the Shillong Plateau and the northern Indoburman ranges, J. Geophys. Res. 95, no. B8, 12,527–12,552.

England, P., and R. Bilham (2015). The Shillong Plateau and the great 1897 Assam earthquake, Tectonics 34, no. 9, 1792–1812.

Gutenberg, B. (1956). Great earthquakes 1896–1903, EOS Trans. Am. Geophys. Union 37, no. 5, 608–614.