ALBERT

Description: Earthquake source inversion is highly dependent on location determination and velocity models. Uncertainties in both the model parameters and the observations need to be rigorously incorporated into an inversion approach. Here, we show a probabilistic Bayesian method that allows formal inclusion of the uncertainties in the moment tensor inversion. This method allows the combination of different sets of far-field observations, such as P -wave and S -wave polarities and amplitude ratios, into one inversion. Additional observations can be included by deriving a suitable likelihood function from the uncertainties. This inversion produces samples from the source posterior probability distribution, including a best-fitting solution for the source mechanism and associated probability. The inversion can be constrained to the double-couple space or allowed to explore the gamut of moment tensor solutions, allowing volumetric and other non-double-couple components. The posterior probability of the double-couple and full moment tensor source models can be evaluated from the Bayesian evidence, using samples from the likelihood distributions for the two source models, producing an estimate of whether or not a source is double-couple. Such an approach is ideally suited to microseismic studies where there are many sources of uncertainty and it is often difficult to produce reliability estimates of the source mechanism, although this can be true of many other cases. Using full-waveform synthetic seismograms, we also show the effects of noise, location, network distribution and velocity model uncertainty on the source probability density function. The noise has the largest effect on the results, especially as it can affect other parts of the event processing. This uncertainty can lead to erroneous non-double-couple source probability distributions, even when no other uncertainties exist. Although including amplitude ratios can improve the constraint on the source probability distribution, the measurements are often systematically affected by noise, leading to deviation from their noise-free true values and consequently adversely affecting the source probability distribution, especially for the full moment tensor model. As an example of the application of this method, four events from the Krafla volcano in Iceland are inverted, which show clear differentiation between non-double-couple and double-couple sources, reflected in the posterior probability distributions for the source models.

Description: The polarity of the first motion of a seismic signal from an earthquake is an important constraint in earthquake source inversion. Microseismic events often have low signal-to-noise ratios, which may lead to difficulties estimating the correct first-motion polarities of the arrivals. This paper describes a probabilistic approach to polarity picking that can be both automated and combined with manual picking. This approach includes a quantitative estimate of the uncertainty of the polarity, improving calculation of the polarity probability density function for source inversion. It is sufficiently fast to be incorporated into an automatic processing workflow. When used in source inversion, the results are consistent with those from manual observations. In some cases, they produce a clearer constraint on the range of high-probability source mechanisms, and are better constrained than source mechanisms determined using a uniform probability of an incorrect polarity pick.

Description: Moment tensor analysis with a Bayesian approach was used to analyse a non-double-couple (non-DC) earthquake ( M w ~ 1) with a high isotropic (implosive) component within the Krafla caldera, Iceland. We deduce that the earthquake was generated by a closing crack at depth. The event is well located, with high signal-to-noise ratio and shows dilatational P -wave first arrivals at all stations where the first arrival can be picked with confidence. Coverage of the focal sphere is comprehensive and the source mechanism stable across the full range of uncertainties. The non-DC event lies within a cluster of microseismic activity including many DC events. Hence, we conclude that it is a true non-DC closing crack earthquake as a result of geothermal utilization and observed magma chamber deflation in the region at present.

Description: We present relatively relocated earthquake hypocentres for 〉1000 microearthquakes ( M L 〈 3) that occurred during the 2 weeks immediately prior to the 2010 March 20 fissure eruption at Fimmvörðuháls on the flank of Eyjafjallajökull volcano in Iceland. Our hypocentre locations lie predominantly in horizontally separated clusters spread over an area of 10 km 2 and approximately 4 km below sea level (5 km below the surface). Seismic activity in the final 4 d preceding the eruption extended to shallower levels 〈2 km below sea level and propagated to the surface at the Fimmvörðuháls eruption site on the day the eruption started. We demonstrate using synthetic data that the observed apparent ~1 km vertical elongation of seismic clusters is predominantly an artefact caused by only small errors (0.01–0.02 s) in arrival time data. Where the signal-to-noise ratio was sufficiently good to make subsample arrival time picks by cross-correlation of both P - and S -wave arrivals, the mean depth of 103 events in an individual cluster were constrained to 3.84 ± 0.06 km. Epicentral locations are significantly less vulnerable to arrival time errors than are depths for the seismic monitoring network we used. Within clusters of typically 100 recorded earthquakes, most of the arrivals exhibit similar waveforms and identical patterns of P -wave first-motion polarities across the entire monitoring network. The clusters of similar events comprise repetitive sources in the same location with the same orientations of failure, probably on the same rupture plane. The epicentral clustering and similarity of source mechanisms suggest that much of the seismicity was generated at approximately static constrictions to magma flow in an inflating sill complex. These constrictions may act as a form of valve in the country rock, which ruptures when the melt pressure exceeds a critical level, then reseals after a pulse of melt has passed through. This would generate recurring similar source mechanisms on the same weak fault plane as the connection between segments of the sill system is repeatedly refractured in the same location. We infer that the magmatic intrusion causing most of the seismicity was likely to be a laterally inflating complex of sills at about 4 km depth, with seismogenic pinch-points occurring between aseismic compartments of the sills, or between adjacent magma lobes as they inflated. During the final 4 d preceding the eruption onset between 22:30 and 23:30 UTC on 2010 March 20, the seismicity suggests that melt progressed upwards to a depth of ~2 km. This seismicity was probably caused by fracturing of the country rock at the margins of the propagating dyke. Subsequently, on the morning of the eruption a dyke propagated eastward from the region of precursory seismic activity to the Fimmvörðuháls eruption site.

Description: Earthquakes are commonly located by linearized inversion of discrete arrival time picks made from signals recorded at a network of seismic stations. If mis-picks are made, these will contribute to the location, therefore causing potential bias. For data recorded by a dense seismic array, direct imaging methods can be applied instead. We describe the ‘coalescence microseismic mapping’ method, which is a bridge between the two approaches and will operate with seismic data recorded continuously on a sparse array. By continuously mapping scalar signals derived from the envelope of seismic arrivals we derive robust estimates of the spatio-temporal coordinates of the origins of seismic events. Noisy data are migrated away from the correct origin, so do not contribute to errors in location. The method is rooted in a Bayesian formulation of event location traveltime inversion, allows imaging of source locations and has the capacity to handle errors in modelled traveltimes. It has the advantage of working with any 3-D velocity model, which therefore may include anisotropy. It also automatically incorporates both P - and S -wave data. A multiresolution grid search leads to an efficient implementation, with a search over a larger domain including joint inversion for location and velocity structure possible where warranted by the data quality. We discuss the theory and implementation of this method and illustrate it with real data from microseismic events in Iceland caused by melt intrusion in the crust.

Description: We suggest that carbon dioxide exsolved from a mid-crustal basaltic dyke intrusion in Iceland migrated upwards and triggered shallow seismicity by allowing failure on pre-existing fractures under the relatively low elastic stresses (100–200 kPa; 1–2 bar) generated by the dyke inflation. Intense swarms of microseismicity accompanied magmatic intrusion into a dyke at depths of 13–19 km in the crust of Iceland's Northern Volcanic Rift Zone during 2007–2008. Contemporaneously, a series of small normal earthquakes, probably triggered by elastic stresses imposed by the dyke intrusion, occurred in the uppermost 4 km of crust: fault plane solutions from these are consistent with failure along the extensional fabric and surface fissure directions mapped in the area, suggesting that the faults failed along existing rift zone fabric even though the mid-crustal dyke is highly oblique to it. Several months after the melt froze in the mid-crust and seismicity associated with the intrusion had ceased, an upsurge in shallow microseismicity began in the updip projection of the dyke near the brittle–ductile transition at 6–7 km depth below sea level. This seismicity is caused by failure on right-lateral strike-slip faults, with fault planes orientated 23 ± 3°, which are identical with the 24 ± 2° orientation in this area of surface fractures and fissures caused by plate spreading and extension of the volcanic rift zone. However, these earthquakes have T -axes approximately aligned with the opening direction of the dyke, and the right-lateral sense of failure is opposite that of regional strike-slip faults. We suggest that the fractures occurred along pre-existing weaknesses generated by the pervasive fabric of the rift zone, but that the dyke opening in the mid-crust beneath it caused right-lateral failure. The seismicity commenced after a temporal delay of several months and has persisted for over 3 yr. We propose that fluids exsolved from the magma in the dyke, primarily carbon dioxide, percolated updip and to shallower depths predominantly along pre-existing fractures. Increased pore pressure from the volatiles reduced the effective normal compressive stress on faults, increasing the likelihood of failure and allowing the modest stress changes generated by the intrusion to cause failure. Propagation of volatiles through the crust would also account for the observed time delay between the intrusion at depth and the shallow earthquake clusters. A further short-lived cluster of earthquakes at 2–4 km depth beneath the surface exhibits left-lateral strike-slip faulting with epicentres well orientated along a lineation which is identical with other subparallel strike-slip faults in the area that transfer motion between two adjacent spreading segments. These shallow earthquakes lie beyond lobes of significant positive Coulomb stress change caused by the intrusion, implying minimal modifications to the stress field in their vicinity; hence, they continue to respond to the regional stress field rather than the local stress field generated by the dyke intrusion.