Transcript What was the mechanism of this earthquake?

Magnitude 4.2 earthquake
Ramsgate, Kent, UK
Friday, 22 May, 2015 at 01:52:19 UTC
A magnitude 4.2 earthquake occurred near Ramsgate, Kent, in southeast England
22nd May 2015. The earthquakes occurred at a depth of 15 km (~9 mile).
Shaking was felt by people living in surrounding towns and villages. The strongest
ground shaking was felt up to around 25 km away from the epicentre.
The UK typically experiences around 3 earthquakes of this size every year.
Magnitude 4.2 earthquake
Ramsgate, Kent, UK
Friday, 22 May, 2015 at 01:52:19 UTC
Where was the earthquake felt?
Most shaking was felt within a 20 km radius of the earthquake’s
epicentre. Farther afiled, shaking was reported in Faversham,
Chatham, Basildon and Southend-on-Sea areas. The most distant
reports have been received from Norwich, North Walsham and
Cromer.
The strongest recorded felt intensity was Intensity IV (moderate
shaking).
Location of shaking reports (image courtesy BGS)
I.
Instrumental
Not felt by many people unless in favourable conditions.
II. Weak
Felt only by a few people at best, especially on the upper floors of buildings. Delicately
suspended objects may swing.
III. Slight
Felt quite noticeably by people indoors, especially on the upper floors of buildings.
Many to do not recognise it as an earthquake. Standing motor cars may rock slightly.
Vibration similar to the passing of a truck. Duration estimated.
IV. Moderate
Felt indoors by many people, outdoors by a few people during the day. At night, some
awakened.
V. Rather
Strong
Felt outside by most, may not be felt by some people in non-favourable conditions.
Dishes and windows may break and large bells will ring. Vibrations like train passing
close to house.
VI. Strong
Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes,
glassware broken; books fall off shelves; some heavy furniture moved or overturned; a
few instances of fallen plaster. Damage slight.
VII. Very
Strong
Difficult to stand; furniture broken; damage negligible in building of good design and
construction; slight to moderate in well-built ordinary structures; considerable damage
in poorly built or badly designed structures; some chimneys broken. Noticed by people
driving motor cars.
VIII.
Destructive
Damage slight in specially designed structures; considerable in ordinary substantial
buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys,
factory stacks, columns, monuments, walls. Heavy furniture moved.
IX. Violent
General panic; damage considerable in poorly designed structures, well designed
frame structures thrown out of plumb. Damage great in substantial buildings, with partial
collapse. Buildings shifted off foundations.
X. Intense
Some well build wooden structures destroyed; most masonry and frame structures
destroyed with foundation. Rails bent.
XI. Extreme
Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
XII.
Cataclysmic
Total destruction – everything is destroyed. Lines of sight and level distorted. Objects
thrown into the air. The ground moves in waves or ripples. Large amounts of rock move
position. Landscape altered, or leveled by several meters. In some cases, even the
routes of rivers are changed.
Modified Mercalli Intensity Scale
Magnitude 4.2 earthquake
Ramsgate, Kent, UK
Friday, 22 May, 2015 at 01:52:19 UTC
Past earthquakes in the region
Small-moderate sized earthquakes in
the UK are not unusual. South-east
England has experienced moderate
earthquakes in the past.
In April 2007, a magnitude 4.3
earthquake occurred just 25 km to
the southwest of the May 2015
event, close to the town of
Folkstone.
Historical evidence also suggests
magnitude 5 earthquakes in 1382
and 1580 struck the region.
Historic seismicity (since 1970) recorded in the region.
The earthquake epicentre is indicated by the red star. Earthquake locations from the
BGS catalogue.
Magnitude 4.2 earthquake
Ramsgate, Kent, UK
Friday, 22 May, 2015 at 01:52:19 UTC
What caused this earthquake to occur?
The UK does not lie on an active tectonic plate boundary (one of its nearest plate boundaries is the Mid-Atlantic
Ridge – located some 2000 km to the west). However, the stresses from these plate boundaries can be transferred
to the middle of the plates. This stress is sometimes released along pre-existing faults within the crust of the
Eurasian tectonic plate.
Earthquakes in the UK can also be caused by gradual uplift of the crust since the last ice age.
Many of the UK’s ancient faults are not seen on the surface – these are known as‘blind faults’. The lack of large
earthquakes in the UK also means we are unable to identify every fault in the UK that could cause an earthquake.
N. American
plate
Mid-Atlantic
Ridge
Eurasian plate
Alpine –
Pyrenees
mountain belt
Africa – Eurasia
collision zone
Map of major tectonic
boundaries in Western Europe
Magnitude 4.2 earthquake
Ramsgate, Kent, UK
Friday, 22 May, 2015 at 01:52:19 UTC
What was the mechanism of this earthquake?
Fortunately, due to the rather large size of this event compared to most UK earthquakes, faulting mechanisms are
available that are based on seismic data.
Although there are some discrepancies between the
‘beach ball’ patterns, they seem to indicate a mechanism
of reverse faulting with a slightly oblique (horizontal)
component.
Reverse faulting is where the hanging wall moves up
relative to the foot wall block.
Hanging wall
Foot wall
Magnitude 4.2 earthquake
Ramsgate, Kent, UK
Friday, 22 May, 2015 at 01:52:19 UTC
Recordings of the earthquake from the British Geological Survey
seismometer network
By finding the difference in arrival times between the P- and S-wave arrivals at different
seismic stations, we can calculate the distance of the earthquake from each receiver
(circles). If we do this for several stations (triangles), we can determine the approximate
epicentre of the earthquake (red star) by finding the common intersection point of these
circles.
Magnitude 4.2 earthquake
Ramsgate, Kent, UK
Friday, 22 May, 2015 at 01:52:19 UTC
Find out more …
•
BGS webpage for this earthquake
http://www.earthquakes.bgs.ac.uk/earthquakes/recent_events/20150522015216.html#page=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/uk-england-32840579
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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)
csem.org/
http://www.emsc-