Real-time discrimination of earthquake foreshocks and aftershocks

Abstract

<p>Immediately after a large earthquake, the main question asked by the public and decision-makers is whether it was the mainshock or a foreshock to an even stronger event yet to come. So far, scientists can only offer empirical evidence from statistical compilations of past sequences, arguing that normally the aftershock sequence will decay gradually whereas the occurrence of a forthcoming larger event has a probability of a few per cent.</p><p>We analyse the average size distribution of aftershocks of the 2016 Amatrice–Norcia (Italy) and Kumamoto (Japan) earthquake sequences and we suggest that in many cases it may be possible to discriminate whether an ongoing sequence represents a decaying aftershock sequence or foreshocks to an upcoming large event.</p><p>We propose a simple traffic light classification (FTLS, Foreshock Traffic Light System) to assess in real time the level of concern about a subsequent larger event and test it against 58 sequences, achieving a classification accuracy of 95 per cent.</p><p>We finally test, in near-real-time, the performance of the FTLS to the 2019 Ridgecrest sequence, California: a Mw6.4 followed, about 2 days later, by a Mw7.1. We find that in the hours after the first Ridgecrest event (Mw 6.4, the b-value drops by 23% on average, when compared to the background value, resulting in a ‘red’ foreshock traffic light.</p><p>Mapping in space the changes in b, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. The second mainshock of magnitude 7.1 then indeed occurred in this location and after this event, the b-value increased by 26 percent over the background value, resulting in a green traffic light state.</p>

Detailed Investigation of the Foreshock Sequence of the 2010 Mw7.2 El Mayor‐Cucapah Earthquake

“…This interpretation is also consistent with recent observations of the foreshock sequences before the 1999 Izmit and 1999 Hector Mine mainshocks (Ellsworth & Bulut, 2018; Yoon et al, 2019). Finally, Figure S8 shows that the foreshock b values are lower than those of aftershocks, which is generally consistent with recent observations in other regions (e.g., Gulia & Wiemer, 2019; Tamaribuchi et al, 2018). However, we only applied template matching to the foreshock sequence, not the aftershock sequence.…”

“…The measured b value for foreshocks is lower than the value (0.96) of the aftershocks (Hauksson et al, 2010), which is also consistent with what we observed for catalog aftershocks with first 120 days (0.80, Figure S7). To verify whether our observation fits into the scenario of varying b values for foreshocks and aftershocks (Gulia & Wiemer, 2019), we applied a sliding window to measure b value using a constant number of 200 events by considering a time‐dependent Mc(t) (Hainzl, 2016). All 666 detected foreshocks were used for the foreshock window, while only ~2,730 catalog aftershocks within first 4 months from the waveform relocated catalog (the updated 1981–2018 catalog) were used for the aftershock period.…”

Seismic Analysis of the 2020 Magna, Utah, Earthquake Sequence: Evidence for a Listric Wasatch Fault

“…To monitor the potentially changing stress field following the mainshock, we calculated the b value time series of the ongoing sequence and compared it to the background value for the Salt Lake Valley area. Gulia and Wiemer (2019) proposed that time‐varying b value analysis can help identify whether ongoing earthquakes in a sequence are aftershocks of an earlier large event or foreshocks of a mainshock yet to occur. We experimented with different catalog subsets, magnitude cut‐offs, and time windows as described in Text S3.…”

“…We experimented with different catalog subsets, magnitude cut‐offs, and time windows as described in Text S3. Our methodology followed Gulia and Wiemer (2019) but with some variation, for instance in the use of a relatively large area to calculate the background b value because of the low historical rate of seismicity in the Magna region. We found that in the hours following the mainshock, the b value did decrease from the background level of ~0.8 to ~0.4, potentially signaling that these events were foreshocks.…”

“…Here we report on characteristics of the Magna earthquake sequence that are relevant to seismic hazard. We estimate the area of the mainshock rupture and interpret it in the context of an SLCS strain budget, and we perform a time‐dependent b value analysis to assess the likelihood that a larger event is yet to occur as part of the Magna sequence (Gulia & Wiemer, 2019). We also combine moment tensor inversion with aftershock detection and high‐precision relocation to constrain the SLCS fault architecture beneath the Salt Lake City metropolitan area.…”