How the determination of earthquake parameters revolutionised seismology

A recent study from IIT Roorkee achieved “significant improvement” in hypocentral location of five earthquakes that occurred before the installation of the world-wide standardised seismograph network (WWSSN)

Seismology faces continuous challenges due to the unpredictable nature of earthquakes and the significant damage they inflict on societies. Understanding the parameters of an earthquake — epicentre, depth, magnitude, and fault mechanism — is crucial for several reasons, particularly in seismically active regions such as the Himalayas. Earthquakes help reveal active faults that may or may not be visible on the surface. By plotting earthquake epicentres over time, seismologists can also map fault lines that are not expressed on the surface and might go undetected. In the complex tectonic terrains like the Himalayas, where multiple fault systems interact, precise earthquake locations help distinguish between major thrust faults (e.g., Main Himalayan Thrust) and secondary faults, improving the tectonic models.

While hypocentre (the focus) is the point below the surface of the Earth where an earthquake originates, epicentre (geographic location) is its projection on the ground. The depth of focus, or the focal depth, can be shallow or deep — hundreds of kilometres; deep focal earthquakes rarely cause severe damage. The focal depth of the devastating 1993 Killari (Maharashtra) earthquake was only 7 km, while that of the 2001 Bhuj earthquake was 24 km.  

Earthquake parametres

Earthquakes occur on large faults, or weak zones, as in the Himalayas, resulting from continuous tectonic processes. There are also many faults on the Earth that remain dormant forever, and others that may generate occasional earthquakes, perhaps not so significant.  Then there are the seismically very active faults, which tend to generate frequent earthquakes, some very damaging.  Some of the largest earthquakes in global history have originated in the Himalayas, which features multiple fault lines along its 2,500-km-long, arcuate tectonic boundary between the Indian and Eurasian plates.

The fault lines formed in the Himalayas are well expressed by the spatial distribution of earthquakes that generally follow the arcuate shape of the Himalayas.  Earthquake epicentres are used to understand their spatial correlation with the faults. It is important to have precise knowledge of earthquake parameters because it helps identify potential faults and helps predict the size of an impending earthquake.

It is also important to identify faults that might not be visible on the surface but still have the potential to generate earthquakes. Such faults express themselves through earthquakes. Only by plotting the epicentres on a map of the region can the hidden faults be identified. Data generated over a long period is useful to determine the hazard associated with faults, giving a clear advantage in earthquake hazard assessment.  But, how well do we know about the locations of earthquakes, especially those which occurred before establishing a good network of earthquake recorders? Historical reports are known to skew the earthquake locations to the nearest population centres, and they may not reflect the actual association of the earthquakes with the fault that must have caused them.

The Himalayas, being one of the most seismically active regions, require continuous monitoring to mitigate risks for millions of people living in the region. A recent study by IIT Roorkee has helped refine seismic hazard models for better preparedness. The IIT Roorkee study achieved “significant improvement” in hypocentral location of five earthquakes that occurred before the installation of the world-wide standardised seismograph network (WWSSN). Obtaining accurate earthquake locations help in producing reliable catalogues and seismic hazard estimation. 

Evolution of earthquake parameter determination

Before the 1800s, when seismographs were first developed, earthquakes were documented based on human observations — how people felt the shaking or how structures were damaged. The work by the IIT Roorkee team aims to minimise the location errors, particularly for earthquakes recorded during periods of limited instrumentation, to enhance data reliability. Instrumental recording began in the 19th century, but it wasn’t until the 1960s, with sufficient seismic data available, that epicentres could be located more accurately. Advances in instrumentation and computational methods further improved the precision of determining epicentres and focal depths. In the recent study, hypocenter estimates of earthquakes after 1960s are consistent with those reported by others, and the location lies in the vicinity of a mapped active fault. As a result, earthquake catalogues have data of varying quality, ranging from historical accounts to sparse early recordings and modern high-accuracy measurements.

Errors in earthquake location

Despite instrumental data, errors in earthquake location arise due to multiple reasons. The primary method for locating an epicentre relies on the arrival times of P-waves (primary, faster-moving waves at about 6.2 km per second) and S-waves (secondary, slower waves at about 3.5 km per second in the Earth’s crust). Since P-waves arrive first, the time difference between the arrival of P and S waves at a seismograph indicates the distance from the earthquake’s origin. A delay of one second between the two waves indicates that they have originated from a source 8 km away, but anywhere within the periphery of a circle of the same radius. If there is a difference of 10 seconds between the arrival times of P and S waves at a particular recording station (A), the waves are from an earthquake about 80 km away, and the circle drawn with the focus as station A should have a radius of 80 km. If a second station (B) shows only two-second delay between the arrival of P and S waves, it is just about 16 km away and hence can be contained in a new circle of radius 16 km.  A third circle based on recorded data from another station (C) can be drawn, and if there is a point of intersection, the coordinates of that point are treated as the epicentre. If they do not intersect, but just define a small triangle, the epicentre can still be estimated as the midpoint of that triangle. Known as the triangulation method, this method is rather crude and insufficient, but it will provide a rough estimate of the geographical coordinates (latitude and longitude) of the epicentre. It is treated as crude and incomplete because of the limitations of manual determination, and there is no estimate of the hypocentral depth.

Challenge of velocity models

The primary limitation of the manual method is that it does not take into account the actual velocity of seismic waves as they pass through the rock layers. To enhance accuracy, the next stage involves constructing a velocity model of the subsurface rocks. However, this is the most challenging aspect of estimating earthquake locations because one must assume the speed at which the waves travel from an unknown point (the hypocentre) through a medium of unknown velocity marked by rock layers several kilometres thick. In geophysics, this kind of problem is known as an inverse problem, for which there is usually no unique solution. Furthermore, there is no way to verify the accuracy of the solution.

In the case of earthquake location, both the source of the signal (the hypocentre) and the characteristics of the rocks are unknown, and we are attempting to solve what is described as an ‘inverse’ problem.  This inherent uncertainty introduces errors that affect location accuracy. However, with advancements such as high-precision seismometers, modern computing, powerful algorithms, and AI-based models, future earthquake location estimates are anticipated to become significantly more accurate.

(This article has benefited from the comments and inputs from Kusala Rajendran)