The Dudley earthquake of 22 September, 2002

The 4.7 ML Dudley earthquake on 22 September 2002 at 23:53 (UTC) was widely felt throughout England and Wales and was the largest earth-quake to occur onshore in the United Kingdom (UK) since the magnitude 5.1 ML Bishop's Castle earthquake in 1990. The earthquake hypocentre, determined from inversion of observed P- and S-wave travel-time data suggests a source depth of 14 km and this depth estimate is also supported by forward modelling of observed waveforms. Focal mechanisms obtained from both first motion polarites of local observations and moment tensor inversion of regional observations show left-lateral strike-slip faulting along a near vertical, near north-south striking fault plane whose orientation is in good agreement with the surface expression of the observed faults in the region. Two aftershocks were recorded within the location error ellipsoid of the mainshock. Comparison of the waveform signals revealed that the mainshock and aftershocks were nearly co-located and possibly had the same source mechanism. The observed peak ground acceleration is found to be less than that predicted using five empirical relations, which have been considered applicable in the UK. Seismic moment M0 and stress drop ẟo were measured from on-scale records where Lg arrivals were clear, and then used to give better estimates of the peak ground accelerations using a stochastic approach

Coordinated and Interoperable Seismological Data and Product Services in Europe: the EPOS Thematic Core Service for Seismology

In this article we describe EPOS Seismology, the Thematic Core Service consortium for the seismology domain within the European Plate Observing System infrastructure. EPOS Seismology was developed alongside the build-up of EPOS during the last decade, in close collaboration between the existing pan-European seismological initiatives ORFEUS (Observatories and Research Facilities for European Seismology), EMSC (Euro-Mediterranean Seismological Center) and EFEHR (European Facilities for Earthquake Hazard and Risk) and their respective communities. It provides on one hand a governance framework that allows a well-coordinated interaction of the seismological community services with EPOS and its bodies, and on the other hand it strengthens the coordination among the already existing seismological initiatives with regard to data, products and service provisioning and further development. Within the EPOS Delivery Framework, ORFEUS, EMSC and EFEHR provide a wide range of services that allow open access to a vast amount of seismological data and products, following and implementing the FAIR principles and supporting open science. Services include access to raw seismic waveforms of thousands of stations together with relevant station and data quality information, parametric earthquake information of recent and historical earthquakes together with advanced event-specific products like moment tensors or source models and further ancillary services, and comprehensive seismic hazard and risk information, covering latest European scale models and their underlying data. The services continue to be available on the well-established domain-specific platforms and websites, and are also consecutively integrated with the interoperable central EPOS data infrastructure. EPOS Seismology and its participating organizations provide a consistent framework for the future development of these services and their operation as EPOS services, closely coordinated also with other international seismological initiatives, and is well set to represent the European seismological research infrastructures and their stakeholders withi

Frontiers of Seismology

Frontiers of Seismology was a wide-ranging, cross-disciplinary meeting held in Edinburgh in April this year. Susanne Sargeant, Lars Ottemöller, Brian Baptie, Andy Bell, Andrew Curtis and Ian Main join forces to give a flavour of the meeting and the new strengths it revealed in seismology in the UK

Source parameters for the 28 April 2007 Mw 4.0 earthquake in Folkestone, United Kingdom

A moderate size earthquake (ML 4.3, Mw 4.0) occurred in southeastern England on 28 April 2007. The earthquake was of some significance as it caused damage in the town of Folkestone and produced the largest peak horizontal ground acceleration (PGA, 0.1g) measured in the United Kingdom to date. It was followed by 12 aftershocks between ML 0.8 and 1.7. The earthquake was the first of this size recorded by a significant number of newly installed broadband stations in the United Kingdom. The hypocenter of the event was at a depth of about 5 km beneath Folkestone, with an error ellipse indicating horizontal errors in a location of about 5 km. The depth was well constrained using a number of techniques, of which local travel-time inversion and teleseismic depth phase modelling are most reliable. A stress drop of 28.6 bars and a source radius of 0.5 km were determined from the analysis of displacement source spectra. We derived a near-surface attenuation factor =0.02 from the aftershock data that were used in the spectral analysis of the mainshock. Applying the horizontal to vertical (H/V) spectral ratio technique to microtremor data recorded at a station 2 km from the epicenter revealed site amplification at frequencies of 0.4 and 3.9 Hz. This amplification is likely to have contributed to the mainshock PGA of 0.1g measured at the same site. Similar site conditions may have been responsible for the damage in parts of Folkestone. The moment tensor computed from regional broadband data showed a strike-slip mechanism with a normal component and either right-lateral movement on a west-southwest–east-northeast-striking or left-lateral movement on a north-northwest–south-southeast-striking nodal plane. The north-northwest–south-southeast-striking nodal plane matches the trend of the main faults affecting the Kent coalfield and also possibly the Variscan front. It is thus possible that the causative fault was associated with the Variscan front, a major structural boundary at the northern limit of late Carboniferous folding and thrusting

Ground-motion difference between two moderate-size intraplate earthquakes in the United Kingdom

Two moderate-size earthquakes occurred in the United Kingdom, the first near Folkestone in 2007 with Mw 4.0 and the second near Market Rasen in 2008 with Mw 4.5. Both were strongly felt and caused some nonstructural damage. The earthquakes occurred at significantly different depths, the Folkestone earthquake at 5 km and the Market Rasen earthquake at 20 km. We determined the seismic moment and the stress drop of the two mainshocks, and two smaller earthquakes in the same locations, by modeling the source displacement spectra. We found stress drops of 30±34 bar and 344±136 bar for the Folkestone and Market Rasen mainshocks, respectively. This is a significant difference considering the earthquakes are only 275 km apart and both are of intraplate origin. We applied the stochastic ground-motion modeling technique and used the stress drop and seismic moment to compute vertical component peak ground acceleration. The modeled ground motions are consistent with the observations. We also computed vertical peak ground acceleration for a hypothetical Mw 6.0 high stress-drop (200 bar) earthquake and found that it would be 4.6 m/sec2 at 20 km hypocentral distance

A local magnitude scale ML for the United Kingdom

We have developed a new local magnitude scale ML for the United Kingdom (UK) to replace the Hutton and Boore (1987) scale developed for southern California, which has been used in the UK until now. The new UK scale is developed from 1482 observations of 85 earthquakes on 50 stations located across the British Isles and Ireland. Most of the observations are from epicentral distances of less than 600 km and only few from greater distances up to 900 km. The distance range of the scale is, therefore, 0–600 km. The amplitude observations were used to invert for the parameters defining distance dependence in the ML scale and station corrections. Synthetic tests showed that the inversion was robust. The new ML scale for the UK is given by ML=logA+0.95logR+0.00183R−1.76, in which A is horizontal‐component ground displacement amplitude in nanometers. The amplitudes are measured on traces that are filtered to simulate the Wood–Anderson seismograph. R is the hypocentral distance (in km). The UK scale is intermediate between scales determined for California and those of other intraplate areas such as Norway or the northeastern United States. The absolute station corrections found are all less than 0.5. The scale derived for the UK helps to reduce the overall variance of the mean magnitude estimates by 30%. Much of this improvement is due to the use of station corrections. Applying the UK scale to the database of recorded earthquakes results in a reduction of magnitude for earthquakes above ML 2 and a slight increase in magnitude for earthquakes below ML 2. The biggest change to the ML computation is likely to be for small earthquakes with few amplitude readings, where the use of station corrections makes a significant difference