a magnitude 7.3 earthquake occurred to the east of kathmandu, in an area close to mount everest....
TRANSCRIPT
A magnitude 7.3 earthquake occurred to the east of Kathmandu, in an area close to Mount Everest. This large earthquake is the largest aftershock so far of the devastating magnitude 7.8 earthquake that occurred 17 days earlier, further north in Nepal. While this earthquake is significantly smaller that the main shock on the 5 th of April 2015, it will be widely felt, and may cause damage, particularly to buildings already damaged by the mainshock. Given the mountainous terrain surrounding the epicenter, it is likely that more landslides and avalanches may be caused.
Magnitude 7.3 aftershock in NepalTuesday, 12th May, 2015 at 07:05:19 UTC
Location of the magnitude 7.8 main shock
Location of the magnitude 7.3 aftershock
Introduction
In this PowerPoint we will therefore have a slightly different structure. We will:
1.Give an overview of this significant aftershock earthquake
2.Give an update on some of the scientific observations that have been made in the two weeks since the mainshock
In this PowerPoint we will therefore have a slightly different structure. We will:
1.Give an overview of this significant aftershock earthquake
2.Give an update on some of the scientific observations that have been made in the two weeks since the mainshock
Location of the magnitude 7.8 main shock
Location of the magnitude 7.3 aftershock
Introduction
Magnitude 7.3 aftershock in NepalTuesday, 12th May, 2015 at 07:05:19 UTC
A magnitude 7.3 earthquake occurred to the east of Kathmandu, in an area close to Mount Everest. This large earthquake is the largest aftershock so far of the devastating magnitude 7.8 earthquake that occurred 17 days earlier, further north in Nepal. While this earthquake is significantly smaller that the main shock on the 5 th of April 2015, it will be widely felt, and may cause damage, particularly to buildings already damaged by the mainshock. Given the mountainous terrain surrounding the epicenter, it is likely that more landslides and avalanches may be caused.
Modified Mercalli Intensity Scale
Predicted ground shaking intensity (image courtesy USGS)
Where was the earthquake felt?
In this magnitude 7.3 earthquake, the greatest shaking is expected to have been very strong (VII) and possibly destructive (VIII) very close to the epicenter. Strong shaking is also expected in the Kathmandu Valley.
This is likely to cause damage especially to buildings that are already damaged from the mainshock in April. There is also a large potential for more landslides and avalanches in the mountainous areas close to the epicenter of this aftershock.
Magnitude 7.3 aftershock in NepalTuesday, 12th May, 2015 at 07:05:19 UTC
Past earthquakes in the region
The Himalayas are a very tectonically active part of the world. This means that earthquakes are not uncommon, with several earthquakes that are large enough to be felt occurring every year. Large earthquakes are however more rare.
In the aftermath of a large earthquake like that of the 25th of April, many smaller earthquakes (aftershocks) happen. The likelihood of large earthquakes like this large aftershock is also greatly increased.
This aftershock earthquake has occurred at the eastern end of the fault that ruptured in on the 25th of April. It is likely to have been triggered by the re-distribution of stresses on this fault by the magnitude 7.8 mainshock.
Map showing all events of magnitude greater than M 6.5 that have occurred in the area since 1900. The location of the April 2015
earthquake is indicated by the red star.(Earthquake data courtesy of the USGS and map data courtesy of
Google Earth)
M8.2, Jan 1934
M7.8, 25th April 2015 M7.3,
aftershock,12th May 2015
Magnitude 7.3 aftershock in NepalTuesday, 12th May, 2015 at 07:05:19 UTC
India plate
Eurasia plate
45 mm/yr
At the location of this earthquake, the India plate is under-thrusting beneath the Eurasia plate at a rate of about 45 mm per year. This make the region one of the most seismically hazardous on Earth. A series of thrust faults partially accommodate this convergence. The April 25, 2015 M 7.8 Nepal earthquake occurred as a result of thrust faulting on or near the main Himalayan frontal thrust between the India plate and the overriding Eurasia plate to the north. The aftershock on the 12th May 2015 appears to have ruptured with a very similar thrusting mechanism.
Map showing the location of the Himalayan thrust plate boundary fault
What caused this earthquake to occur?
Cartoon cross-section illustrating the collision between the Indian and Eurasian plates and the likely location of the 2015 earthquake.
Indian plate
Eurasian plate
Mainshock:
Aftershock:
Focal mechanism of the Nepal earthquake showing thrusting (from the USGS)
Magnitude 7.3 aftershock in NepalTuesday, 12th May, 2015 at 07:05:19 UTC
Recordings of the earthquake from the British Geological Survey seismometer network
Plot similar seismograms yourself at: http://ds.iris.edu/wilber3/find_stations/5111620
These seismometers are located some 7000 km (4500 miles) from the earthquake’s epicentre. The P-waves are the first to arrive, but the later-arriving surface waves have the strongest amplitude.
Magnitude 7.3 aftershock in NepalTuesday, 12th May, 2015 at 07:05:19 UTC
NEPAL
INDIA
CHINA
Kathmandu
Himalaya
Aftershocks from Mw 7.8 earthquake, 25th April 2015
This earthquake is the largest in a series of aftershocks of the magnitude 7.8 main shock earthquake that occurred in Nepal on the 25th of April.
The mainshock is shown in red on the map, and magnitude 7.3 aftershock is shown in yellow. Other aftershocks are shown by blue circles.
The aftershocks roughly show the area of the Himalayan thrust faults that ruptured on the 25th of April. You can see that today’s large aftershock occurs at the very end of this rupture area.
Map showing all aftershocks with magnitude greater than 4 recorded by the USGS. The mainshock location is given by the red
star.(Earthquake data courtesy of the USGS)
Magnitude 7.3 aftershock in NepalTuesday, 12th May, 2015 at 07:05:19 UTC
Location of the magnitude 7.8 main shock (25th April 2015)
Location of the magnitude 7.3 aftershock (12th May 2015)
Magnitude comparison
The magnitude of earthquakes are reported on a logarithmic scale. Meaning that this magnitude 7.3 aftershock is approximately 6 times less powerful than the main shock.
On average about 15 earthquakes with 7 – 8 occur every year. Many of these though occur in remote areas of the ocean, or deep in the Earth, and so have a far smaller human impact.
25th April 2015 Mw 7.8 Nepal main shock
12th May 2015 Mw 7.3 Nepal after shock
2010 Mw 6.3 L’Aquila, Italy
Magnitude 7.3 aftershock in NepalTuesday, 12th May, 2015 at 07:05:19 UTC
The 7.8 Mw Nepal Earthquake – an update
• The fault that ruptured during the Nepal main shock was more than 80 km long, and the amount of slip on this fault varies over its area.
• The figure to the left shows that in some areas of the fault the slip was as much as 400 cm, and that the largest slip was to the south east of the epicentre.
• The grey graph in the bottom left shows the amount of energy released from the onset time of the earthquake (0 seconds).
The 7.8 Mw Nepal Earthquake – an update
The 7.8 Mw Nepal Earthquake – an update
Kathmandu valley is
built on ancient lake (amplifies
ground shaking)
Kathmandu
Steep snow-covered mountains: avalanches
Lack of earthquake-
resistant buildings and
poor education
Villages on steep Himalayan foothills:
landslides
Shallow depth and large magnitude of
earthquake
Why was damage so large?
• USGS webpage for this earthquakehttp://earthquake.usgs.gov/earthquakes/eventpage/us20002926#general_summary
• BGS (British Geological Survey) – seismology and earthquakes – frequently asked questions http://www.earthquakes.bgs.ac.uk/education/faqs/faq_index.html
• BBC News article on this earthquake http://www.bbc.co.uk/news/science-environment-32462763
• IRIS special event page http://ds.iris.edu/ds/nodes/dmc/specialevents/2015/04/25/nepal/
• IRIS (Incorporated Research Institutions for Seismology) – learning about earthquakes http://www.iris.edu/hq/programs/education_and_outreach/students
• UK School Seismology Project – classroom activities, videos and support documents http://www.bgs.ac.uk/schoolseismology/home.html
• USGS (United States Geological Survey) – FAQs, glossary, posters, animations http://earthquake.usgs.gov/learn/
• EMSC (European Mediterranean Seismological Centre) http://www.emsc-csem.org/
Find out more …
Saturday, 25 April, 2015 at 06:11:26 UTC
Magnitude 7.8 earthquakeNepal