Last revised: 12/11/2002

Introduction

Since the morning of October 21, a relatively large number of earthquakes have been felt in the Greater Manchester area. At this time more than 100 earthquakes have been recorded on the British Geological Survey (BGS) seismograph network, of which most were reported felt. Minor damage has been reported for the largest event in the sequence, which can occur for earthquakes of this size.

The first earthquake that was felt occurred on 21 October at 07:45:15 (UTC) with a magnitude 3.2 ML. This was closely followed by a smaller event of 2.3 ML at 08:04:58. A few hours later at 11:42:34, there was a magnitude 3.9 ML earthquake, the largest recorded so far, which was strongly felt throughout the Greater Manchester area. About 22 seconds later was another earthquake with a magnitude 3.5 ML. Prior to those events, two earthquakes were recorded by the BGS seismograph stations on October 19 and one event on October 21 at 07:29:20. These three events were not felt. The activity has continued to the present time (complete list of events), however, the size and frequency of events has decreased.

While it appears somewhat unusual to get that many earthquakes within only a few days, this has been previously observed in the UK. Such earthquake sequences can occur in two ways. Firstly, moderate to large size earthquakes are usually followed by aftershocks, which occur due to readjustment to a new state of stress. The pattern of the aftershock sequence depends on the size of the event and the local tectonic setting. Normally, the largest aftershock is about one magnitude unit smaller than the main shock. For example, the magnitude 5.4 ML Lleyn Peninsula earthquake that occurred in North Wales in 1984 was followed by some 25 aftershocks in the subsequent two months, the largest of which was a magnitude 4.3 ML earthquake that occurred one month later. Secondly, earthquake swarms are sequences of earthquakes clustered in time and space without a clear distinction of main shock and aftershocks. Examples of such swarm activity in the UK are Comrie (1788-1801, 1839-46), Glenalmond (1970-72), Doune (1997) and Blackford (1997-98, 2000-01) in central Scotland, Constantine (1981, 1986, 1992-4) in Cornwall, and Johnstonbridge (mid1980s) and Dumfries (1991,1999) in the Borders.

The Manchester sequence appears to be an earthquake swarm, since the difference in magnitude between the largest event in the sequence and the other events is not as significant as expected in a main shock – aftershock sequence. However, most of the energy during the sequence was actually released in the double-event on October 21 at 11:42. The clustering of these events in time and space does suggest that there is a causal relationship between the events of the sequence.

Monitoring

Permanent Stations

The BGS operates a seismograph network of 146 stations distributed over the UK (station map). The field stations consist of a seismometer, a modulator and a radio for data transmission (Figure 1). These sites are typically in remote locations since the seismometers are sensitive to extremely small ground movements. The data from groups of sensors are transmitted to a number of central recording sites and continuously recorded on a computer. In the next stage, the data are collected and processed at the BGS in Edinburgh. The nearest networks to Manchester earthquakes are installed around Leeds and Keyworth as well as in North Wales and Cumbria (Figure 2). Sample seismograms are shown in Figure 3.

Figure 1. Example of a seismic station in the north-west Highlands of Scotland.

Figure 2. Permanent BGS seismograph stations around Manchester.

Figure 3. Some sample seismograms from stations of the permanent network for the event on October 21 at 07:45.

Deployment of Temporary Stations

The permanent network of seismograph stations operated by BGS allows determination of origin times, epicenters and depths of the Manchester earthquakes. However, the nearest seismograph station is approximately 24 km from the center of Manchester, so to better determine depth and epicenter and to better understand the relationship between seismicity and local geology, the BGS have installed three temporary seismograph stations around Manchester (Figure 4).

The closest of the temporary stations to the earthquake epicenters is situated at Manchester University. This will provide crucial information on how deep below the surface the focus of the earthquake is. The other two stations are positioned to complement the permanent stations to the east and southeast to maximize azimuthal coverage of the earthquakes and reduce uncertainty in the epicenters.

Figure 4. Locations of temporary seismograph stations (red squares) deployed around Manchester. Image produced from the Ordnance Survey Get-a-map service (http://www.ordnancesurvey.co.uk/getamap).

The N.E.R.C. (National Environment Research Council) Geophysical Equipment pool supplied the equipment for these stations, consisting of a seismometer (Figure 5) to measure the ground vibrations and a data logger to record the data (Figure 6). Data are time-stamped precisely using a GPS clock. Unlike the permanent stations, data from the temporary stations are not transferred to the BGS offices in Edinburgh in near real-time and has to be collected manually.

 Hanging objects swing considerably. China and glasses clatter together. Small, top-heavy and/or precariously supported objects may be shifted or fall down. Doors and windows swing open or shut. In a few cases window panes break. Liquids oscillate and may spill from well-filled containers. Animals indoors may become uneasy.

Figure 9. Map showing locations of felt reports (red symbols) and epicenter location (blue circles). The topography data is from the Ordnance Survey (License number: GD272191).

Geology

Geologically, the Manchester and Salford area straddles the southern part of the Carboniferous South Lancashire Coalfield and the northern part of the Permo-Triassic Cheshire Basin. The coalfield has been extensively worked from numerous collieries, including Patricroft, Bradford and Agecroft in the north Manchester city area. Coal mining ceased in this part of the coalfield in the late 1970s. The northeastern margin of the Cheshire Basin is heavily faulted. The throw of individual faults at the basin margin varies from tens of metres to over 1000 m. Coals that were worked from Bradford Colliery are in a fault block bounded to the north and east by the Bradford Fault. The Permo-Triassic rocks comprise the Sherwood Sandstone Group, which is the second most important aquifer in the UK.

Source Mechanism

Due to the movement of the tectonic plates, stresses build up within the Earth crust (the upper 35 km). The UK is on the Eurasian plate and receives compressional stresses due to the ridge-push force generated by the Mid-Atlantic ridge in the North Atlantic. These stresses are released through motion on pre-existing faults when the structure can no longer resist the stresses. This relative motion of the fault blocks is what happens during the earthquake. From the seismic data it is possible to identify the orientation of the fault and the direction of slip. This information is useful for identification of the fault on which an earthquake has occurred.

For the six largest events of the Manchester sequence, the source mechanism was determined based on the first motion polarities and the amplitude ratio of P and S waves. Figure 10 shows the fault plane solutions that were obtained. The six solutions are rather similar and basically strike-slip solutions. There are always two fault planes that explain the observed data equally well, and additional geological knowledge is required to identify which is the true fault plane. For Manchester, most of the faults in the epicentral area are oriented NW-SE, which therefore may appear to be the most likely candidate.

Figure 10. Source mechanism for 6 of the earthquakes in the Manchester sequence. The solution was obtained by using first motion polarities and amplitude ratios.

At the moment it is probably still too early to identify the fault of the earthquake sequence. Improvement of the earthquake location will be achieved through joint hypocenter location and use of data from the temporary stations.

Conclusion

The Manchester earthquake sequence that started on October 19 with more than 100 events due to its’ location in an urban environment was experienced by a large number of people. Not surprisingly, the events created an enormous media interest (BGS gave some 60 media interviews). The largest event on October 21 had a magnitude ML 3.9, and thus was only of moderate size at which no significant damage would be expected. The activity appears to be an earthquake swarm, since there is no clear distinction between a main shock and aftershocks. It seems likely that all events in the sequence originate from a relatively small source volume. This is supported by the similarities in source mechanism and waveform signals between the various events. The question on when the activity is going to stop cannot be answered at the moment, however, since October 25 the activity has decreased both with respect to numbers and size. The answer to the question on whether a larger event, following this sequence, is expected is clearly no.