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 .
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.
Still remember the Black rainstorm on 30 March last year? The torrential rain brought flashy lightning, loud thunders and roaring winds that night. Marble-sized hails that came with the heavy downpour fell over many places of Hong Kong. Locating the purple red echoes on the radar image is one of the possible methods to track hail (Figure 1). Actually, how does rainstorm form?
Figure 1 Radar image for 8 p.m. on 30 March 2014. Heavy rain was affecting areas covered by yellow to red echoes while hail might have manifested themselves as purple red echoes.
The development of rainstorm depends on a number of factors. In simple terms, they can be categorized into three areas:
1. Abundant supply of moisture In March or April every year, the southward advancement of cold air from inland China becomes less frequent while southerly airstream tends to affect the south China coast more often. This airstream normally takes a long sea track and therefore it is relatively warm and moist (Figure 2). As a result, the moisture over the south China coast increases. This ample supply of moisture is one of the critical factors favourable for precipitation.
Figure 2 A schematic diagram showing a warm and moist airstream generally affecting Hong Kong in March or April.
2. Uplifting of air There are a number of air lifting mechanisms. Trough of low pressure is amongst a relatively common weather system that triggers uplifting of air over southern China in spring and summer. The weather associated with a trough of low pressure is generally unstable. Since pressure along the trough axis is lower than that on both sides, the air in its vicinity tends to converge near the axis and is forced to rise. Water vapour in the air will condense into water droplets due to cooling in the uplifting process. The energy released during condensation will help lift the air further, forming cumulonimbus and precipitation (Figure 3). Apart from trough of low pressure, orography, high surface temperatures, frontal surface and upper-air weather systems or features such as jet streams, vortices, shear lines and upper-level divergence can also help uplift air. For example, when a trough of low pressure lingers over the south China coast in the presence of some of the lifting mechanisms mentioned above, the uplifting of air can become rather rapid and intense, triggering heavy rain.
Figure 3 Warm and moist air in the vicinity of a trough of low pressure lifting up to form cumulonimbus clouds and precipitation.
3. Atmospheric instability Warm air is less dense than cold air, and is thereby lighter. Hence warm air is easy to be lifted. When the lower atmosphere is filled with warm and moist air, the air will ascend easily and the atmosphere will become unstable. There are a number of indices commonly used to indicate the stability of the atmosphere. They include Level of Free Convection (LFC), Convection Available Potential Energy (CAPE) and K-index . Their values can be calculated from the daily ascent data. Their forecasts can also be derived from numerical weather prediction models for reference by forecasters.
When the factors in the above three areas occur at the same time, the chance of development of rainstorm will increase significantly. The threats posed by rainstorms are not only limited to flooding in low-lying areas, rivers and places with poor drainage, flash floods, or landslips in the vicinity of slopes due to heavy rain, but also severe thunderstorms and high gusts accompanied with rainstorms, or even hail, tornadoes, etc. Because of the random nature and uncertainty of rainstorm in its location of occurrence and evolution, forecasters need to utilize real-time observational data to effectively evaluate the short-term changes in its intensity, duration of influence and threats to Hong Kong so as to issue the appropriate rainstorm warning in a timely manner.
Below are some tips to get prepared for heavy rain:
1. Pay attention to the Observatory's 9-day forecast to see whether heavy rain or thunderstorm is mentioned in the forecast, and also the weather system that may bring unstable weather, like trough of low pressure or cold front, is included in the general situation;
2. Keep track of the latest local weather forecast and special weather tips;
3. When heavy rain is approaching, pay particular attention to the weather warnings and the corresponding precautionary measures.
If you wish to know more comprehensive weather information, check out the weather radar images, lightning location information, rainfall distribution map, rainfall nowcasting product, maximum gust information in regional weather etc. available on the Observatory's website or "MyObservatory" App. When inclement weather occurs, it is most important to make sure that you are staying in a safe place. Remember to stay alert to upcoming changes in weather, and take appropriate precautionary measures when necessary!
In winter, Hong Kong is frequented by the cold monsoon. You may notice that one feels colder on a windy and rainy day than a day at the same temperature but in fine weather or light wind. Why is there such a difference?
Our body produces heat continuously by metabolism in order to maintain a body temperature of around 37 degrees Celsius. When the ambient temperature is lower than the body temperature, body heat is lost to the environment, making one feel cold. Apart from air temperature, other meteorological conditions such as wind speed, humidity and solar radiation also affect the overall rate of heat loss. Since air movement helps carry away our body heat, we feel particularly cold under windy conditions when heat loss is faster. This phenomenon is called the 'wind chill effect'. Besides, we also feel particularly cold on rainy days. This is because water has higher heat conductivity than air and hence water droplets on our body can take away the body heat more efficiently. On the other hand, when we are exposed to sunshine on a fine day, the heat absorbed from solar radiation offsets part of the heat loss from the body, thus making us feel relatively warmer. On a gloomy day, however, we feel just the opposite. Therefore, if we are facing the winter wind and rain, we will have a stronger feeling of coldness. The temperature we perceive, commonly known as the apparent temperature, will be lower than the actual air temperature measured by instruments.
Depending on their climates, some countries devise wind chill index or apparent temperature, which specifically considers the heat transfer processes involved between the human body and the surrounding environment, as well as the responses of the human body tested under different meteorological conditions in a cold environment. However, there is so far no international standard on the calculation method and the applicable range of such wind chill indices. For example, the index currently used in the U.S. is only applicable when the actual air temperature is 10 degrees Celsius or below, while in Canada the index is to be used only at or below 5 degrees Celsius. It is noteworthy that the aforesaid indices normally do not include the effects of sunshine. As such, when there is sunshine in winter, such indices will be lower than the actual perceived temperature.
The climate in Hong Kong is different from the countries which widely adopt the wind chill indices. This is because there are not many days when air temperatures generally fall below 5 or 10 degrees Celsius. Hence the indices used in other countries may not be entirely applicable to Hong Kong. As compared to countries where snowstorms are common in winter, the wind chill effect in Hong Kong, especially in urban areas, is not particularly significant. For most of the time the perceived temperature does not differ much from the air temperature measured by instruments. If the apparent temperature is reported along with the actual air temperature, the public may get confused with the two similar numbers. To remind the public of the cold conditions in a simple and clear way, the Observatory now issues the Cold Weather Warning where the situation warrants, that has taken into account the combined effects of air temperature, wind speed and humidity, and reminds the public to take appropriate measures to keep themselves warm.