Short‐term probabilistic earthquake risk assessment considering time‐dependent b values

Abstract

Laboratory experiments highlight a systematic b value decrease during the stress increase period before failure, and some large natural events are known to show a precursory decrease in the b value. However, short‐term forecast models currently consider only the generic probability that an event can trigger subsequent seismicity in the near field. While the probability increase over a stationary Poissonian background is substantial, selected case studies have shown through cost‐benefit analysis that the absolute main shock probability remains too low to warrant significant mitigation actions. We analyze the probabilities considering both changes in the seismicity rates and temporal changes in the b value. The precursory b value decrease in the 2009 L'Aquila case results in an additional fiftyfold probability increase for a M6.3 event. Translated into time‐varying hazard and risk, these changes surpass the cost‐benefit threshold for short‐term evacuation.

The Effect of a Mainshock on the Size Distribution of the Aftershocks

“…Note also the 10% decrease in b value during the months to days prior to the mainshock. While this anomaly is consistent with selected case studies and laboratory studies that have reported dropping precursory b values (Gulia et al, ; Papadopoulos et al, ; Tormann et al, ), the number of premainshock stacked time series is only 8.…”

Foreshocks and short-term hazard assessment of large earthquakes using complex networks: the case of the 2009 L'Aquila earthquake

“…After the M L = 4.1 event, rate and magnitude of foreshocks increased and seismicity migrated toward the main shock nucleation zone. Gulia et al (2016) studied a circular area of 20 km radius, centered on the mainshock epicenter. They observed that a foreshock sequence started 3 months before the mainshock, activating a region of about 10 km length.…”

“…The mainshock was followed by abundant aftershocks, with the major events occurring on 7 April (M w = 5.6) and on 9 April (M w = 5.4). Immediately following the mainshock, Marzocchi and Lombardi (2009) began producing daily aftershock forecasts based on a stochastic model that combines the G-R (Gutenberg and Richter, 1944) distribution and space-time power-law decay of triggered shocks.…”

“…That is, a posteriori analysis of the INGV catalog data (http://bollettinosismico.rm.ingv.it) showed that from the beginning of 2006 up to the end of October 2008 the activity was relatively stable and remained in the state of background seismicity (seismicity rate r = 1.14, b = 1.09, where b is the slope of the magnitude-log frequency or G-R relation). In the earthquake magnitude domain, the magnitude-frequency (or G-R) relation (Gutenberg and Richter, 1944) reads as log N = a − bM. This relation, which describes the power-law decrease in the number of events with the increase in magnitude, has been found to apply in both clustered and non-clustered types of seismicity (see the review in Utsu, 2002); N is the cumulative number of events of magnitude equal to or larger than M, and a, b are parameters determined by the data.…”

Normalized rupture potential for small and large earthquakes along the Pacific Plate off Japan

“…We use the rich earthquake record along the Pacific Plate off Japan and investigate spatial NRP distributions and temporal changes for different target magnitudes. It is evident from seismicity records that earthquake activity (expressed by the a value in the Gutenberg-Richter law) changes greatly through space and time, and there is also plenty of evidence for significant spatiotemporal variability in the relative fraction of small to large magnitudes (expressed by the b value in the Gutenberg-Richter law) [e.g., Wiemer and Wyss, 1997;Enescu and Ito, 2001;Schorlemmer et al, 2004;Nanjo et al, 2012;Tormann et al, 2012Tormann et al, , 2013Gulia et al, 2016]. In particular, a recent study [Tormann et al, 2015] has documented a remarkable change in the size distribution of earthquakes along the Pacific Plate off Japan before and after the Tohoku event.…”