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Laura Ermert

Laura is a PhD student at ETH Zürich in Switzerland. She is working on ambient noise source inversion with cross-correlation techniques. Her goal on the blog is to showcase PhD students' and young researchers' results, as well as recent seismological highlights. You can reach Laura at lermert att student.ethz.ch.

To b, not to b, or to b with Voronoi?

To b, not to b, or to b with Voronoi?

The b-value is that parameter of the Gutenberg-Richter relation which controls the ratio of small to large earthquakes. Intriguing temporal and spatial variations of the b-value have been reported in recent years, for example sudden b-value changes at active fault zones.

Are such sharp spatial b-value variations merely a result of crude undersampling? To address this unsettling question, a recently published study presents a data-driven spatial subdivision to spare researchers arbitrary judgment on how to delineate different regions. Instead of a human researcher, the Bayesian information criterion decides on how many earthquakes are ‘enough’ to reliably estimate the b-value for one region. The study reports ‘irreconcilable differences’ with previously published b-value mapping techniques.

Applied to California, the new approach indeed shows significant b-value variations, but they occur on rather large spatial scales and can largely be attributed to biases induced by our limited capabilities to detect and record seismic events. A notable exception is the geysers geothermal region, where small events are actually more dominant than elsewhere, probably due to a different physical origin: many of these small events are geothermal rather than tectonic.

Does this prove the existence of small-scale b-value variations wrong? It doesn’t. The problem could rather be summarized as ‘what’s one seismologist’s noise is another one’s signal’: With larger subregions, the authors of the present study average out small fluctuations, which they — or rather the Bayesian information criterion — regard as random. Other researchers argue these are physically meaningful signals, even though observations might be scarce.

So…how should one be mapping b-value variations? Engaged debate on the procedure is certainly no harm to science. If you’d like to form you own opinion, read more here.