Relative relocation of earthquakes without a predefined velocity model: an example from a peculiar seismic cluster on Katla volcanos south-flank (

📝 Abstract

Relative relocation methods are commonly used to precisely relocate earthquake clusters consisting of similar waveforms. Repeating waveforms are often recorded at volcanoes, where, however, the crust structure is expected to contain strong heterogeneities and therefore the 1D velocity model assumption that is made in most location strategies is not likely to describe reality. A peculiar cluster of repeating low-frequency seismic events was recorded on the south flank of Katla volcano (Iceland) from 2011. As the hypocentres are located at the rim of the glacier, the seismicity may be due to volcanic or glacial processes. Information on the size and shape of the cluster may help constraining the source process. The extreme similarity of waveforms points to a very small spatial distribution of hypocentres. In order to extract meaningful information about size and shape of the cluster, we minimize uncertainty by optimizing the cross-correlation measurements and relative-relocation process. With a synthetic test we determine the best parameters for differential-time measurements and estimate their uncertainties, specifically for each waveform. We design a relocation strategy to work without a predefined velocity model, by formulating and inverting the problem to seek changes in both location and slowness, thus accounting for azimuth, take-off angles and velocity deviations from a 1D model. We solve the inversion explicitly in order to propagate data errors through the calculation. With this approach we are able to resolve a source volume few tens of meters wide on horizontal directions and around 100 meters in depth. There is no suggestion that the hypocentres lie on a single fault plane and the depth distribution indicates that their source is unlikely to be related to glacial processes as the ice thickness is not expected to exceed few tens of meters in the source area.

💡 Analysis

Relative relocation methods are commonly used to precisely relocate earthquake clusters consisting of similar waveforms. Repeating waveforms are often recorded at volcanoes, where, however, the crust structure is expected to contain strong heterogeneities and therefore the 1D velocity model assumption that is made in most location strategies is not likely to describe reality. A peculiar cluster of repeating low-frequency seismic events was recorded on the south flank of Katla volcano (Iceland) from 2011. As the hypocentres are located at the rim of the glacier, the seismicity may be due to volcanic or glacial processes. Information on the size and shape of the cluster may help constraining the source process. The extreme similarity of waveforms points to a very small spatial distribution of hypocentres. In order to extract meaningful information about size and shape of the cluster, we minimize uncertainty by optimizing the cross-correlation measurements and relative-relocation process. With a synthetic test we determine the best parameters for differential-time measurements and estimate their uncertainties, specifically for each waveform. We design a relocation strategy to work without a predefined velocity model, by formulating and inverting the problem to seek changes in both location and slowness, thus accounting for azimuth, take-off angles and velocity deviations from a 1D model. We solve the inversion explicitly in order to propagate data errors through the calculation. With this approach we are able to resolve a source volume few tens of meters wide on horizontal directions and around 100 meters in depth. There is no suggestion that the hypocentres lie on a single fault plane and the depth distribution indicates that their source is unlikely to be related to glacial processes as the ice thickness is not expected to exceed few tens of meters in the source area.

📄 Content

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Relative relocation of earthquakes without a predefined velocity model: an example from a peculiar seismic cluster on Katla volcano’s south-flank (Iceland)

Giulia Sgattoni1,2,3*, Ólafur Guðmundsson3, Páll Einarsson2, Federico Lucchi1

1 Department of Biological, Geological and Environmental Sciences, University of Bologna, Bologna, Italy 2 Institute of Earth Sciences, Science Institute, University of Iceland, Reykjavik, Iceland 3 Department of Earth Sciences, Uppsala University, Uppsala, Sweden *Corresponding author: giulia.sgattoni2@unibo.it

Summary Relative relocation methods are commonly used to precisely relocate earthquake clusters consisting of similar waveforms. Repeating waveforms are often recorded at volcanoes, where, however, the crust structure is expected to contain strong heterogeneities and therefore the 1D velocity model assumption that is made in most location strategies is not likely to describe reality. A peculiar cluster of repeating low-frequency seismic events was recorded on the south flank of Katla volcano (Iceland) from 2011. As the hypocentres are located at the rim of the glacier, the seismicity may be due to volcanic or glacial processes. Information on the size and shape of the cluster may help constraining the source process. The extreme similarity of waveforms points to a very small spatial distribution of hypocentres. In order to extract meaningful information about size and shape of the cluster, we minimize uncertainty by optimizing the cross-correlation measurements and relative-relocation process. With a synthetic test we determine the best parameters for differential-time measurements and estimate their uncertainties, specifically for each waveform. We design a relocation strategy to work without a predefined velocity model, by 2

formulating and inverting the problem to seek changes in both location and slowness, thus accounting for azimuth, take-off angles and velocity deviations from a 1D model. We solve the inversion explicitly in order to propagate data errors through the calculation. With this approach we are able to resolve a source volume few tens of meters wide on horizontal directions and around 100 meters in depth. There is no suggestion that the hypocentres lie on a single fault plane and the depth distribution indicates that their source is unlikely to be related to glacial processes as the ice thickness is not expected to exceed few tens of meters in the source area.

Keywords: Katla volcano; Cross-correlation; Relative relocation; slowness.

1. Introduction

Earthquake multiplets consist of very similar waveforms, often exceeding cross-correlation coefficients of 0.8 (Geller & Mueller 1980; Frémont & Malone 1987). They are common in tectonic and volcanic areas worldwide and they are likely to be caused by earthquakes occurring very close to each other and generated by similar, non-destructive, source processes (Geller & Mueller 1980). Because they consist of closely-spaced earthquakes, it is possible to determine relative relocation of the hypocentres with high accuracy (Poupinet et al. 1984; Fréchet 1985; Frémont & Malone 1987; Got et al. 1994; Slunga et al. 1995; Waldhauser & Ellsworth 2000; Thelen et al.
2008). The relative relocation method is based on the idea that closely-spaced events recorded at a common station will share similar path effects and site effects. If the hypocentral separation between two events is small compared to the station-hypocentre distance and scale length of velocity heterogeneities, and if the latter is big compared to the dominant wavelength of the waveforms, then the ray paths to a common station are similar and the relative time lag between the two events will depend on their spatial offset in the direction of the station (Waldhauser & Ellsworth 2000; Wolfe 2002).

Moreover, the location precision is improved by using high-precision waveform cross- correlation methods to determine the relative time measurements. This can be done either in the frequency domain (Poupinet et al. 1984) or in the time domain (Deichmann & Garcia-Fernandez 1992). The accuracy of the arrival-time differences between pairs of similar events is reported to be on the order of 0.001 s for micro-earthquakes recorded by local networks (e.g. Frémont & Malone 1987). This makes it possible to calculate the relative location between hypocentres with uncertainty on the order of a few meters to tens of meters (Waldhauser & Ellsworth 2000). 3

This is particularly useful at volcanoes, where earthquakes are often characterized by unclear phase onsets and their arrival-time determination can be highly imprecise with manual phase-picking. The relative location of earthquake multiplets is, therefore, a common practice at volcanoes worldwide. Got et al. (1994) relocated 250 earthquakes beneath Kïlauea that defined a nearly horizontal plane of seismicity at 8-km depth. Rowe et al. (2

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