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. 2012:2:846.
doi: 10.1038/srep00846. Epub 2012 Nov 14.

Spatial organization of foreshocks as a tool to forecast large earthquakes

E Lippiello  1 , W Marzocchi, L de Arcangelis, C Godano
Affiliations

Spatial organization of foreshocks as a tool to forecast large earthquakes

E Lippiello et al. Sci Rep. 2012.

Abstract

An increase in the number of smaller magnitude events, retrospectively named foreshocks, is often observed before large earthquakes. We show that the linear density probability of earthquakes occurring before and after small or intermediate mainshocks displays a symmetrical behavior, indicating that the size of the area fractured during the mainshock is encoded in the foreshock spatial organization. This observation can be used to discriminate spatial clustering due to foreshocks from the one induced by aftershocks and is implemented in an alarm-based model to forecast m > 6 earthquakes. A retrospective study of the last 19 years Southern California catalog shows that the daily occurrence probability presents isolated peaks closely located in time and space to the epicenters of five of the six m > 6 earthquakes. We find daily probabilities as high as 25% (in cells of size 0.04 × 0.04deg(2)), with significant probability gains with respect to standard models.

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Figures

Figure 1
Figure 1. Aftershocks and foreshocks spatio-temporal organization in Southern California.
Left upper panel. The linear density probability ρ(Δr) for foreshocks (filled circles) and aftershocks (empty diamonds) are obtained considering all events occurring within 12 hours from the mainshock. Different colors correspond to mainshocks in different magnitude classes, m ∈ [M, M + 1), and M = 2, 3, 4 for black, red and green symbols respectively. We restrict the distribution to Δr ≤ 3 km in order to reduce the contribution from background seismicity. Right upper panel The linear density probability ρ(Δr) averaged over 50 independent realizations of synthetic catalogs generated by the ETAS model. Details on the numerical procedure are given in the Methods. Data for aftershocks and foreshocks in numerical catalogs are indicated as continuous lines and pluses, respectively. Open diamonds refer to the aftershock ρ(Δr) in the experimental catalog. Black, red and green colors correspond to M = 2, 3, 4 respectively. Lower Panel. The inverse average distance formula image is plotted as function of time from the mainshock. Here ρ(Δr, t) is the linear density probability in the interval [–1.2t, –t] ([t, 1.2t]) before (after) mainshocks for foreshocks (filled circles) and aftershocks (empty diamonds), respectively. We average 1/Δr, instead of Δr, in order to reduce the influence of background seismicity and, for the same reason, we fix Rmax = 3 km. The same symbols as in upper panels are used.
Figure 2
Figure 2. Daily occurrence probability just before m > 6 earthquakes.
The probability formula image to have a m > 6 earthquake within 1 day in a cell of side 0.04°, is evaluated for the 6 largest events in Southern California just after the occurrence of the last event before mainshock. For each mainshock, we plot formula image over the entire Southern California region (upper panels), and a zoom over a box centered in the future mainshock epicenter (front panels). Black stars indicate the mainshock epicenter location and the values of formula image can be obtained from the color code bar.
Figure 3
Figure 3. Space-time Molchan diagrams.
The Molchan trajectories of the FS model relative to the RI reference model (black filled circles) and to the ETAS reference model (blue open diamonds). Points on the descending diagonal (red curve) indicate equivalent model performance. For points below the dot-dashed green line, the null hypothesis (equivalent probability for the two models) can be rejected with a confidence level larger than 99,99%.
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