Irish earthquakes are as rare as hen’s teeth, which is really quite curious. The geology of Ireland is very similar to that of the rest of the British Isles [...]]]> An interesting event in the world of seismology yesterday, an Irish earthquake. The British Geological Survey report a magnitude 2.7 tremor felt in County Clare yesterday evening.

Irish earthquakes are as rare as hen’s teeth, which is really quite curious. The geology of Ireland is very similar to that of the rest of the British Isles and many of the geological structures in the UK can be traced into the Republic. However, whilst Great Britain has a modest level of seismicity, ‘Lesser’ Britain virtually has none.

Historical seismicity of the UK (yellow) from 1832 to 1970 for earthquakes of magnitude above 3.0 and instrumental seismicity (red) from 1970 to present for earthquakes with ML >2.0. Source BGS

Image source BGS.

One of the most sensible reasons for the lack of Irish earthquakes is that St Patrick banished them from Ireland at the same time he banished the snakes!

The region is underlain by Mercia Mudstone so it could be salt collapse related, but most of the salt extraction is the other side of Stafford. Nearby is [...]]]> An intriguing little earthquake from the south of the county this morning, a magnitude 1.3 tremor southwest of Gnosall (west of Stafford). This is the recording from Keele University.

The region is underlain by Mercia Mudstone so it could be salt collapse related, but most of the salt extraction is the other side of Stafford. Nearby is the enigmatically named “Coal Pit Bank Coverts” but there are no mines to my knowledge at the ‘pit’ is almost certainly a ‘marl-hole’ (marl being spread on the local fields to aid agriculture).

The ‘quake does lie between the epicentres of the Dudley 2002 M4.7 and the Stafford 1916 M4.6 earthquakes so could be related to a structure running north-south between the two, possibly an extension of the Malvern Line, but this event has been located very shallow by the BGS where as the larger events were at about 10km down.

Update: The BGS in their infinite wisdom have relocated the event to northeast of Gnosall and upped the magnitude to 1.4. However, the geology is broadly similar and most of the above comments still hold.

View Gnosall Quake in a larger map

Extracting the record shows a very strong body wave phase and surface wave train …

Matching this to the travel-time curves show that the strong phase is S, [...]]]> Another day, another large earthquake, this time in Northern Sumatra. This is 24 hour record that I saw when I broke my holiday for the second time this week.

Extracting the record shows a very strong body wave phase and surface wave train …

Matching this to the travel-time curves show that the strong phase is S, not P. Note that around 95° epicentral distance, Keele is not quite in the core shadow…

The P-wave is not visible because the path of the earthquake waves to Keele lies along the strike direction of the fault (northwestwards) and so P is nodal. S, however, is antinodal so we get a strong S-wave.

Full details of this earthquake are available from the USGS here.

Extracting the earthquake from the data stream gives this.

… and trying to fit it to the travel-time curves gives this …

Note that the P-wave is not recorded well. [...]]]> The Baja California Earthquake was recorded moderately well at Keele University, UK. This was the view on the display monitor in our foyer that greeted me this morning.

Extracting the earthquake from the data stream gives this.

… and trying to fit it to the travel-time curves gives this …

Note that the P-wave is not recorded well. This is probably due to the fact that with a north-west to south-east strike-slip fault mechanism, and the path to Keele is on a north-east azimuth. Consequently, the P-wave will be nodal towards Keele, that is almost no P-wave energy will be emitted in this direction.

This earthquake has spawned a whole series of aftershocks. Here is the current (as of 11:00 GMT 05/04/2010) map from the USGS data on Google Earth.

At 107° epicentral distance, Keele is just in the shadow zone so the direct P and S-waves are not recorded. However, even at a distance of almost 12,000 [...]]]> A very large earthquake, at 8.8, the sixth largest since modern seismic recording began around 1900, has struck off-shore southern Chile. More details from the USGS and Ole Nielsen.

At 107° epicentral distance, Keele is just in the shadow zone so the direct P and S-waves are not recorded. However, even at a distance of almost 12,000 km our seismometer went off-scale during the surface wave arrivals.

The earthquake magnitude has been determined by the Geological survey to be 1.3

The tremor is most likely related to subsidence or collapse due to dissolution of salt. The bedrock here is the Wilkesley Halite and in [...]]]> There was a small earthquake just south of Nantwich, Cheshire on January 31st, 2010 which we recorded here at Keele.

The earthquake magnitude has been determined by the Geological survey to be 1.3

The tremor is most likely related to subsidence or collapse due to dissolution of salt. The bedrock here is the Wilkesley Halite and in the Google Maps image of the earthquake location below salt dissolution features can be seen. A word of warning though as this location from the British Geological Survey is unlikely to be precise.

View Nantwich Quake in a larger map

This earthquake doesn’t appear to be that well recorded but the P and SS phases are identifiable.

The earthquake was a shallow left-lateral strike-slip earthquake along the Enriquillo-Plaintain Garden fault system, essentially a major component of the plate boundary [...]]]> Here is the seismogram of the M7 earthquake in Haiti on January 12, 2010 recorded at Keele University, UK.

This earthquake doesn’t appear to be that well recorded but the P and SS phases are identifiable.

The earthquake was a shallow left-lateral strike-slip earthquake along the Enriquillo-Plaintain Garden fault system, essentially a major component of the plate boundary between the North American and Caribbean plates. The Caribbean plate is moving eastwards with respect to North America by about 20 mm per year.

Much more on the tectonics of the area from Chris Rowan at Highly Allochthonous.

Recorded as part of the UK Schools Seismology project.