Extracting information from noise

Seismic waves created by sources such as wind, the ocean and human activity propagate inside the Earth all the time. Such waves are often regarded as ‘noise’ by seismologists, however, they travel through the Earth in exactly the same way as those waves from earthquakes. Recent advances in theory have shown that ambient noise recorded at two seismic stations can be combined to provide information about the properties of the Earth between the two stations. This is known as seismic interferometry and has revolutionised seismology over the past few years

Could it happen here?

Could a magnitude 9 earthquake and tsunami like the one that struck Japan affect the British Isles? The short answer is no. Huge mega-thrust earthquakes like this only happen at plate boundary subduction zones where one of the Earth’s tectonic plates is being pushed down, or subducted, beneath another. Places where this happens include Japan, Sumatra and South America all of which have had earthquakes of magnitude 8.5 or greater in the last few years that have resulted in tsunami. The British Isles sits in the middle of a tectonic plate, Eurasia

Seismogenesis and state of stress in the UK

In this paper I present a compilation of focal mechanisms for earthquakes with magnitudes greater than 3.0 ML in the British Isles that can be used to help constrain our understanding of seismicity and it's driving forces in the British Isles. The fault plane solutions consist of both previously published mechanisms for significant British earthquakes, and new solutions calculated from regional and local data for more recent and smaller earth- quakes that were previously unpublished. Focal mechanisms for earthquakes in the UK are dominantly strike-slip with northwest-southeast compression and northeast-southwest tension,or reverse, with northwest-southeast com- pression. In many cases there is also an oblique component to the slip. P and T axes from individual solutions are relatively well constrained in azimuth, though less so in dip, with P-axes orientation for most events clustering between north and north-west, indicating sub-horizontal compression. However, some spatial variation in P- and T-axes orientation is also apparent, with near north/northeast compression and east-west extension in north- west Scotland, changing to northwest-southeast compression in England and Wales. I estimate a best-fitting stress tensor, under the assumption of uniform stress using two different inversion methods for both the entire focal mechanism data set and two different subsets of the data. The results from the two different datasets suggest that there is a significant difference in the stress state between northwest Scotland and England and Wales. Calculated σ1 directions for England and Wales are northwest-southeast, consistent both with existing stress data and expected stresses from first order plate motions. By contrast, the inversion results for northwest Scotland show near east-west extension with possible σ1 and σ2 directions lying in a north south band, and that the magnitudes of σ1 and σ2 are similar. The relative magnitude of the principal stresses, R, determined for England and Wales suggests that the intermediate stress σ2 is close to the average value of σ1 and σ3 . The clear difference in the stress inversion results between northwest Scotland and England and Wales suggests that the principal stress directions expected from first order plate motions have been modified in Scotland by local stress conditions due to glacio-isostatic adjustment

Local earthquake tomography of Scotland

Scotland is a relatively aseismic region for the use of local earthquake tomography, but 40 yr of earthquakes recorded by a good and growing network make it possible. A careful selection is made from the earthquakes located by the British Geological Survey (BGS) over the last four decades to provide a data set maximising arrival time accuracy and ray path coverage of Scotland. A large number of 1-D velocity models with different layer geometries are considered and differentiated by employing quarry blasts as ground-truth events. Then, SIMULPS14 is used to produce a robust 3-D tomographic P-wave velocity model for Scotland. In areas of high resolution the model shows good agreement with previously published interpretations of seismic refraction and reflection experiments. However, the model shows relatively little lateral variation in seismic velocity except at shallow depths, where sedimentary basins such as the Midland Valley are apparent. At greater depths, higher velocities in the northwest parts of the model suggest that the thickness of crust increases towards the south and east. This observation is also in agreement with previous studies. Quarry blasts used as ground truth events and relocated with the preferred 3-D model are shown to be markedly more accurate than when located with the existing BGS 1-D velocity model

UK earthquake monitoring 2002/2003

The aims of the Seismic Monitoring and Information Service are to develop and maintain a national database of seismic activity in the UK for use in seismic hazard assessment, and to provide near-immediate responses to the occurrence, or reported occurrence, of significant events. The British Geological Survey (BGS) has been charged with the task of operating and further developing a uniform network of seismograph stations throughout the UK in order to acquire standardised data on a long-term basis. The project is supported by a group of organisations under the chairmanship of the Office of the Deputy Prime Minister (ODPM) with major financial input from the Natural Environment Research Council (NERC). This Customer Group is listed in Annex A. In the 14th year of the project (April 2002 to March 2003), three subnetworks were upgraded with the installation of data loggers running under the QNX operating system, and a strong motion instrument was installed at Hartland in North Devon. The increasing number of acceleration records being captured by strong motion instruments, is feeding into a better understanding of attenuation and seismic hazard in the UK. Some 235 earthquakes were located by the monitoring network in 2002, with 87 of them having magnitudes of 2.0 ML or greater (Annex B). A total of 42 events in this magnitude category were reported as felt along with 6 smaller ones. Nine strong-motion records were captured from six of the nineteen sites now equipped with strong motion instruments. The largest earthquake in the reporting year, with a magnitude of 4.7 ML, occurred near Dudley on 22 September. It was felt up to 337 km away and over an area of 126,000 km2 (Isoseismal 3 EMS) and reached a maximum intensity of 5 on the European Macroseismic Scale (EMS, Annex H). A peak ground acceleration of 153 mms -2 was recorded on the three-component accelerometer at Keyworth, a distance of 83 km from the epicentre. The focal mechanism indicates strike slip movement along near vertical fault planes striking either NNE-SSW or WNW-ESE. The following month, an earthquake sequence commenced near Manchester with 117 events located, 37 of which were felt by the local population. The sequence caused widespread alarm in the greater Manchester area. The largest offshore earthquake occurred in the central North Sea on 12 October 2002 with a magnitude of 3.5 ML, approximately 70 km east of the Shetland Islands. In addition to earthquakes, BGS frequently receives reports of seismic events felt and heard, which on investigation prove to be sonic booms, spurious or in coalfield areas, where much of the activity is probably induced by mining. During the reporting period, data from six sonic events were processed and reported upon following public concern or media attention. All significant felt events and some others were reported rapidly to the Customer Group through seismic alerts sent by e-mail. The alerts were also published on the Internet (http://www.earthquakes.bgs.ac.uk). Monthly seismic bulletins were issued six weeks in arrears and, following revision, were compiled into an annual bulletin (Simpson, 2003). In all these reporting areas, scheduled targets have been met or surpassed. Maintenance and protection of historical archives, another primary goal of the project, has continued and has been enhanced by donations of the Soil Mechanics UK data, from a study in the early 1980’s and the British Association for the Advancement of Science Seismological Committee archives. The environmental monitoring stations at Eskdalemuir and Hartland observatories recorded a variety of parameters throughout the year and the data are now accessible on-line through an Internet connection

Probabilistic Forecasting of Hydraulic Fracturing Induced Seismicity Using an Injection-Rate Driven ETAS Model

The development of robust forecasts of human‐induced seismicity is highly desirable to mitigate the effects of disturbing or damaging earthquakes. We assess the performance of a well‐established statistical model, the epidemic‐type aftershock sequence (ETAS) model, with a catalog of ∼93,000 microearthquakes observed at the Preston New Road (PNR, United Kingdom) unconventional shale gas site during, and after hydraulic fracturing of the PNR‐1z and PNR‐2 wells. Because ETAS was developed for slower loading rate tectonic seismicity, to account for seismicity caused by pressurized fluid, we also generate three modified ETAS with background rates proportional to injection rates. We find that (1) the standard ETAS captures low seismicity between and after injections but is outperformed by the modified model during high‐seismicity periods, and (2) the injection‐rate driven ETAS substantially improves when the forecast is calibrated on sleeve‐specific pumping data. We finally forecast out‐of‐sample the PNR‐2 seismicity using the average response to injection observed at PNR‐1z, achieving better predictive skills than the in‐sample standard ETAS. The insights from this study contribute toward producing informative seismicity forecasts for real‐time decision making and risk mitigation techniques during unconventional shale gas development

Constructing new seismograms from old earthquakes: retrospective seismology at multiple length scales

If energy emitted by a seismic source such as an earthquake is recorded on a suitable backbone array of seismometers, source-receiver interferometry (SRI) is a method that allows those recordings to be projected to the location of another target seismometer, providing an estimate of the seismogram that would have been recorded at that location. Since the other seismometer may not have been deployed at the time at which the source occurred, this renders possible the concept of “retrospective seismology” whereby the installation of a sensor at one period of time allows the construction of virtual seismograms as though that sensor had been active before or after its period of installation. Here we construct such virtual seismograms on target sensors in both industrial seismic and earthquake seismology settings, using both active seismic sources and ambient seismic noise to construct SRI propagators, and on length scales ranging over 5 orders of magnitude from ∼40 m to ∼2500 km. In each case we compare seismograms constructed at target sensors by SRI to those actually recorded on the same sensors. We show that spatial integrations required by interferometric theory can be calculated over irregular receiver arrays by embedding these arrays within 2-D spatial Voronoi cells, thus improving spatial interpolation and interferometric results. The results of SRI are significantly improved by restricting the backbone receiver array to include approximately those receivers that provide a stationary-phase contribution to the interferometric integrals. Finally, we apply both correlation-correlation and correlation-convolution SRI and show that the latter constructs fewer nonphysical arrivals

Locating micro-seismic sources with a single seismometer channel using coda wave interferometry

A novel source location method based on coda wave interferometry (CWI) was applied to a microseismic data set of mining-induced events recorded in Nottinghamshire, England. CWI uses scattered waves in the coda of seismograms to estimate the differences between two seismic states. We used CWI to estimate the distances between pairs of earthquake locations, which are then used jointly to determine the relative location of a cluster of events using a probabilistic framework. We evaluated two improvements to this location technique: These account for the impact of a large difference in the dominant wavelength of a recording made on different instruments, and they standardize the selection of parameters to be used when implementing the method. Although the method has been shown to produce reasonable estimates on larger earthquakes, we tested the method for microseismic events with shorter distinguishable codas in recorded waveforms, and hence, fewer recorded scattered waves. The earthquake location results are highly consistent when using different individual seismometer channels, showing that it is possible to locate event clusters with a single-channel seismometer. We thus extend the potential applications of this cost-effective method to seismic events over a wider range of magnitudes

SISMIKO: emergency network deployment and data sharing for the 2016 central Italy seismic sequence

At 01:36 UTC (03:36 local time) on August 24th 2016, an earthquake Mw 6.0 struck an extensive sector of the central Apennines (coordinates: latitude 42.70° N, longitude 13.23° E, 8.0 km depth). The earthquake caused about 300 casualties and severe damage to the historical buildings and economic activity in an area located near the borders of the Umbria, Lazio, Abruzzo and Marche regions. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) located in few minutes the hypocenter near Accumoli, a small town in the province of Rieti. In the hours after the quake, dozens of events were recorded by the National Seismic Network (Rete Sismica Nazionale, RSN) of the INGV, many of which had a ML > 3.0. The density and coverage of the RSN in the epicentral area meant the epicenter and magnitude of the main event and subsequent shocks that followed it in the early hours of the seismic sequence were well constrained. However, in order to better constrain the localizations of the aftershock hypocenters, especially the depths, a denser seismic monitoring network was needed. Just after the mainshock, SISMIKO, the coordinating body of the emergency seismic network at INGV, was activated in order to install a temporary seismic network integrated with the existing permanent network in the epicentral area. From August the 24th to the 30th, SISMIKO deployed eighteen seismic stations, generally six components (equipped with both velocimeter and accelerometer), with thirteen of the seismic station transmitting in real-time to the INGV seismic monitoring room in Rome. The design and geometry of the temporary network was decided in consolation with other groups who were deploying seismic stations in the region, namely EMERSITO (a group studying site-effects), and the emergency Italian strong motion network (RAN) managed by the National Civil Protection Department (DPC). Further 25 BB temporary seismic stations were deployed by colleagues of the British Geological Survey (BGS) and the School of Geosciences, University of Edinburgh in collaboration with INGV. All data acquired from SISMIKO stations, are quickly available at the European Integrated Data Archive (EIDA). The data acquired by the SISMIKO stations were included in the preliminary analysis that was performed by the Bollettino Sismico Italiano (BSI), the Centro Nazionale Terremoti (CNT) staff working in Ancona, and the INGV-MI, described below