By: Rafi Ahmad (MGI’s International Fellow)
This earthquake today, 12 May 2015, was felt all over north India. I have posted the USGS shaking intensity map below to clarify a couple of important points (really a response to questions asked to me over the last few days). A BLACK STAR on the map below marks the area on the surface of the Earth near Kodari, Nepal. This is the earthquake epicenter. The disturbance originated some 18 km below the surface in Kodari, Nepal. Please note that the energy generated by a fault break propogated outwards in the form of earthquake waves which were experienced all over the place. All those blue and yellow filled circles on the Shake map given below shows locations where people felt the shaking and communicated online to USGS via
Did you Feel it (DYFI) :http://earthquake.usgs.gov/earthq.../eventpage/us20002ejl...
LESSON: Please note that an earthquake need not originate below your feet to cause damage. Earthquakes occurring elsewhere may also cause serious damage. Why? I present two examples below which demonstrate how subsurface geology controls the damage. We cannot predict the next earthquake but there is good scientific data available to tell us what is likely to happen and where. Mexico City effect:
On September 19,1985, Mexico city suffered a catastrophic damage from a M8.1 earthquake which originated some 350km away on Mexico’s Pacific coast. Result: 9000 deaths; 30,000 injured; 50,000 homes destroyed including 500 buildings; 4billion dollars. Why? Most of the Mexico City is built on an old lake bed. The City centre where damage was most extensive is underlain by a thick deposit of very soft , high water-content sands and clays which amplified the earthquake waves. A cartoon to illustrate amplification of earthquake waves in sediments compared to rocky substrate- Source of the diagrams below.
The M7.8 Gorkha Earthquake of 25th April 2015 and 12th May 2015 earthquke caused severe damage, injuries and loss of lives in Kathmundu. Why ? The answer lies in the subsurface geology of Kathmundu. Like Mexico city, Kathmundu is also built on a depression underlain by lake sediments. This setting is particularly vulnerable to earthquake damage.
“Simplified geologic cross-section of the Kathmandu Valley showing basin-fill sediments. Lakebed deposits are labeled “lacustrine” whereas sediments deposited by rivers are labeled “fluvial”. After Sakai et al. Pleistocene rapid uplift of the Himalayan frontal ranges recorded in the Kathmandu and Siwalik basins, Palaeogeography, Palaeoclimatology, Palaeoecology, v. 241, p.16–27, 2006.” Why is the Indo-Gangetic Plain underlain by alluvial soils of all sorts is generally vulnerable to earthquake shaking as has been observed on 25th April 2015, 12th May 2015, 1934 Nepal-Bihar Earthquake and many other instances. We need to invest in research on detailed subsurface geotechnical mapping and/or compiling available data of cities located here. On the USGS Shake Map, one may note that in north India, most of the circles are blue and grey indicating Modified Mercalli Intensities II to V. The size of the circle refers to population in that city. For example, we felt shaking in Lucknow which is some 522km southeast from Kodari in Nepal. Generally, stronger the earthquake, violent is the shaking. Lesson: Invest in disaster risk reduction. Consider the case of Lucknow. I have simplified it to carry the point home. Earthquake Risk for Lucknow (R) = H (Earthquake Hazard) X V ( vulnerability). One may argue that H is constant. However, the value of V has increased several orders over the years and keeps ever increasing. The net result is an enhanced risk value. Strength of an earthquake:
I have been asked to explain how we represent strength of an earthquake. The STRENGTH of an earthquake is reported as its MAGNITUDE and/or INTENSITY. While every earthquake event has only one magnitude reflecting the energy released due to breakage and displacement on a geological fault; it has several intensities as explained below. The magnitude for today's earthquake was M7.3 as derived from the seismic waves recorded on a seismograph. This is commonly designated as Richter scale. Please disregard all that misinformation about the Richter Scale being an instrument. A black star on the map indicates the epicentre of today's earthquake. Epicentre marks an area on the surface of the Earth above the zone where the earthquake disturbance occurred below the Earth's surface. Please remember it is not a precisely defined point rather a zone. In today's earthquake, the area where disturbance occurred is estimated to be at a depth of 18km. All earthquakes originating within 30km depth are designated as a shallow events. Now coming to how we feel the shaking and related damage, the EARTHQUAKE INTENSITY. People in some areas feel a stronger shaking and more damage to buildings as compared to other areas. Why? Among various factors that control shaking, two are highlighted here. First, areas close to the epicentre experience stronger shaking. Second, the nature of substrate, soils/rock.
Soils like those found in the Indo-Gangetic Plain generally amplify the shaking and quite often such soils may lose strength by liquefying. In the 1934 earthquake there were widespread reports of liquefaction in Bihar and U.P. A convenient way to standardize the effects of how humans experience shaking and related damage, we use Modified Mercalli Scale which is shown on the Shake Map and designated as Roman numerals I to XII and colours from soft blue to warm red.
Today's earthquake had several intensities. See differently coloured circles. Kathmandu, yellow-orange circle, experienced shaking at level VIII compared to Lucknow , Blue circle, level IV, see western edge of the map. This is the level of shaking I had experienced today in Lucknow. This level of shaking corresponds to around average peak velocity 1-2 centimeters/second; average peak acceleration in the range of 0.015g-0.02g (g is gravity 9.8 meters/second/second). This sort of information is needed for building design. In north India, most of the circles are blue and grey indicating Modified Mercalli Intensities II to V. The size of the circle refers to population in that city. The shake map for the 12th May event is a mirror image of the event on the 25th April, 2015 and if I may say, that in 1934. Now you know it.
You may log on tohttp://earthquake.usgs.gov/earthquakes/ for more information. END.
Mona GeoInformatics Institute, University if the West Indies, Mona Campus, Kingston 7, Jamaica W.I. Web: www.monagis.com Tel: (876) 977-3160-2