On the statistical significance of foreshock sequences in Southern California

Abstract

Earthquake foreshocks may provide information that is critical to short-term earthquake forecasting. However, foreshocks are far from ubiquitously observed, which makes the interpretation of ongoing seismic sequences problematic. Based on a statistical analysis, Trugman and Ross (2019, https:// doi.org/10.1029/2019GL083725) suggested that as much as 72% of all mainshocks in Southern California is preceded by foreshock sequences. In this study, we reassess the analysis of Trugman and Ross (2019, https://doi.org/10.1029/2019GL083725), and we evaluate the impact of the assumptions made by these authors. Using an alternative statistical approach, we find that only 15 out of 46 mainshocks (33%) are preceded by significantly elevated seismicity rates. When accounting for temporal fluctuations in the background seismicity, only 18% of the analyzed foreshock sequences remain unexplained by the background seismicity. These results imply that even in a highly complete earthquake catalog, the majority of earthquakes do not exhibit detectable foreshock sequences.substantially less than that reported by T&R (20-30% versus 70%, respectively). Only about half of these foreshock sequences can be explained by random fluctuations in the background seismicity rate, which suggests that in some cases elevated seismicity rates are uniquely associated with periods preceding mainshocks. Geophysical Research Letters 10.1029/2019GL086224seismicity rates, while about half of that fraction is a priori anticipated based on the ubiquitous fluctuations in the background seismicity rate. In other words, we expect that only about 15% of all mainshocks exhibit a foreshock sequence uniquely associated with the earthquake preparation process.

Seismic and Aseismic Fault Slip During the Initiation Phase of the 2017 MW = 6.9 Valparaíso Earthquake

“…The analysis of an highly complete earthquake catalog in Southern California showed that 72% of M W ≥ 4 earthquakes in the region are preceded by an elevated seismic activity compared with the background seismicity rate (Trugman & Ross, 2019), suggesting that foreshock activity is more ubiquitous than previously thought. However, a recent reanalysis of the same catalog suggested that a much smaller portion of these foreshock sequences were really anomalous and could not be attributed to temporal fluctuations in background seismicity rate (van den Ende & Ampuero, 2020). Although anomalous foreshock sequences currently appear to be the exceptional, the improvement of near‐fault geodetic and seismological observational capabilities are essential to bridge the gap between natural fault observations and laboratory experiments, where foreshocks are commonly observed.…”

Frequency‐Magnitude Statistics of Laboratory Foreshocks Vary With Shear Velocity, Fault Slip Rate, and Shear Stress

“…Foreshock patterns in nature can be challenging to identify due to sparseness in seismicity and/or a lack of network coverage (Bakun et al., 2005 ). However, there are well documented examples of increased foreshock activity prior to the mainshock (Bouchon et al., 2013 ; Brodsky & Lay, 2014 ; Ellsworth & Bulut, 2018 ; Gulia & Wiemer, 2019 ; Gulia et al., 2016 ; Kato et al., 2016 ; Nanjo et al., 2012 ; Papadopoulos et al., 2010 ; Trugman & Ross, 2019 ; van den Ende & Ampuero, 2020 ; Wyss & Lee, 1973 ; Yoon et al., 2019 ). In some cases, the Gutenberg‐Richter b ‐value decreases prior to the mainshock (Gulia et al., 2016 ; Nanjo et al., 2012 ); implying that foreshock magnitude increases systematically as the fault approaches failure.…”

“…However, this can lead to misguided conclusions about how foreshock patterns evolve in space and time. For instance, Trugman and Ross ( 2019 ) implemented a template matching approach (Quake‐Template‐Matching or QTM) and demonstrated that foreshock sequences may be more common than previously thought (see also van den Ende & Ampuero, 2020 ). This observation was driven by the fact that the QTM catalog was able to lower the magnitude of completeness well below that of standard catalogs.…”

Rare Occurrences of Non‐cascading Foreshock Activity in Southern California

“…These results suggest that detailed earthquake detections could bear important information about an impending earthquake. The seismic activity observed in the 20‐day window before M ≥ 4 earthquakes was later re‐evaluated by van den Ende and Ampuero (2020, V&A from here on) to investigate in which cases these increases in seismicity were significant compared to the natural fluctuations of the seismicity rate. In their approach, V&A choose to test seismic activities smoothed at 20 days against a model that accounts for increases in seismicity.…”