Seven Questions and Answers about Earthquakes
- Tuesday, 5th May 2015
We are deeply saddened by the severe earthquake which struck Nepal on 25 April 2015, killing more than seven thousands of people hitherto (5 May 2015). Apart from levelling houses and destroying some renowned historical buildings, the earthquake even touched off a deadly avalanche in the Himalayas. It was the most powerful earthquake that hit Nepal since 1934 . Continued multiple aftershocks then occurred, with two of them reaching a magnitude of above 6.0.
Figure 1 Mr Mok Hing-yim, Observatory's Senior Scientific Officer (Head of the Observatory's Geophysics,
Time and Marine Meteorological Services) giving an account of the earthquake in Nepal.
1. What was the cause of this earthquake in Nepal ?
Nepal is located along the boundary of the Indian-Australian tectonic plate and the Eurasian tectonic plate (Figure 2), which belongs to an active seismic belt. The earthquake this time was caused by a collision between the two aforementioned tectonic plates (Figure 3).
Figure 2 Diagram showing the global distribution of tectonic plates (the location of
Nepal is indicated by the red arrow).
Figure 3 Schematic diagram illustrating the collision between the Indian-Australian tectonic plate
and the Eurasian tectonic plate.
2. The US Geological Survey said the Nepal's earthquake was 7.9 magnitude while there were some reported a magnitude of 8.1. It was also noticed that the magnitude was amended at a later time of reporting. What was the reason behind it ?
As different seismological monitoring centres receive data from different seismograph stations, their computed magnitudes may have slight differences. Besides, for stronger earthquakes, data from a larger number of seismograph stations and also seismograph stations located further away from the epicentre are normally required to obtain a more accurate analysis. Therefore the magnitude of stronger earthquake is often subject to amendments upon the reception of data from seismograph stations further away from the epicentre and recalculation.
Figure 4 Global Seismographic Network (till February 2015) . IRIS, IDA and USGS in the legend are the abbreviations of
Incorporated Institutions for Seismology Research, International Deployment of Accelerometers and US Geological
Survey respectively. The orange star over China denotes Hong Kong Po Shan Seismological Station.
3. What are the scales to quantify the intensity of an earthquake ?
Different countries or regions in the world adopt a certain earthquake intensity scale to quantify the tremor at different locations. In Hong Kong, the Modified Mercalli Scale (MMS)  is adopted. The MMS is scaled into 12 different levels, which are determined from the feel of tremor by human beings as well as the effects and severity of damages on building structures, etc. (Figure 5). The intensity of an earthquake at a certain location is related to both the magnitude of the earthquake and the distance of that location from the epicentre.
Figure 5 Modified Mercalli Scale (MMS).
4. How to monitor earthquakes and are they predictable ?
We set up seismograph station and utilise seismometers (Figure 6) to detect the seismic waves generated by earthquakes (Figure 7). Parameters such as origin time, epicenter, magnitude and depth of the earthquake can then be computed using the data collected from a network of seismograph stations. However, there is no reliable way yet to predict earthquakes even with the current state-of-the-art scientific knowledge and technology.
Figure 6 Broadband seismometer used for detecting global earthquakes.
Figure 7 Seismic waveforms recorded by seismograph stations for the Nepal M7.9 earthquake on 25 April 2015.
5. What are the threats associated with an earthquake and the precautionary measures ?
The main threats of earthquakes include the collapse of buildings, landslides, breakdown of electricity supplying systems, fire hazards caused by the leakage of gaseous fuels, etc. Strong submarine earthquakes may trigger tsunamis, posing severe threats to coastal residents. When earthquake occurs, the first thing is to keep calm and then seek refuge as appropriate. Safety rules can be found in the Hong Kong Observatory's webpage.
6. What was the world's deadliest earthquake recorded in recent years ?
If the casualties brought by tsunami triggered by an earthquake were also taken into account, the disastrous magnitude 9.1 earthquake that occurred west of northern Sumatra over the Indian Ocean on 26 December 2004 and its associated tsunami resulted in a death toll of over 220 thousands, which was the world's deadliest earthquake in recent years .
(ii)Figure 9 Aftermath of the tsunami in Thailand generated by the magnitude 9.1 earthquake in the Indian Ocean in 2004
(Photos: Courtesy of Dr Wong Wing-tak).
7. Is Hong Kong vulnerable to earthquake ?
Most earthquakes of the world occur along the boundaries of crustal tectonic plates. Hong Kong lies within the Eurasian Plate and is at a distance from the nearest plate boundary (Figure 10). It is not located at an active seismic belt. According to the assessment of the Geotechnical Engineering Office of the Civil Engineering and Development Department, the earthquake risk in Hong Kong is low to medium. Notwithstanding this, members of the public should know more about earthquakes and understand the safety rules during and after earthquakes, especially when they travel in areas where earthquakes frequently occur.
Figure 10 Position of Hong Kong within the global tectonic plates (indicated by the red arrow).
The Observatory always attaches great importance to the work on earthquake monitoring. Under the trend of globalisation, many people often travel to places outside Hong Kong and the messages of earthquakes occurring around the world are vital to the public. The Observatory endeavours to enhance the dissemination of information on earthquakes. The global Quick Earthquake Messages service was launched in 2011 (https://twitter.com/HKOEARTHQUAKE5C and https://twitter.com/HKOEARTHQUAKE6C). The information is also accessible on the mobile app "MyObservatory", the Observatory's webpage and Weibo, providing the public with timely information on global earthquakes.
 IRIS webpage
 Hong Kong Observatory's webpage
 USGS webpage