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Supplementary Information
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The debate on the prognostic value of earthquake foreshocks: A meta-analysis
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Arnaud Mignan1
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1: Swiss Seismological Service, ETH Zurich, NO H66, Sonneggstrasse 5, CH-8092
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Zurich
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Contact: arnaud.mignan@sed.ethz.ch
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1. Data
I have reviewed almost one hundred references on accelerating seismic
12
release, relying mostly on the comprehensive review made by Ref. 18, and
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approximately one hundred references on short-term foreshocks. In order to build a
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comprehensive database, I have used the following approach: (i) Select all relevant
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references provided by the Google Scholar search engine based on keywords, such as
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“foreshock” or “accelerating moment release”; (ii) Select all relevant references listed
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in the reference list of each one of the articles already in the database; (iii) Repeat step
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(ii) until no more new reference is found. In that approach, only articles not cited by
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any of the references contained in the database may have been missed. Even if the
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completeness of the database cannot be averred, the database should nonetheless be
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representative of the foreshock literature. Of all the published studies, I have only
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found 37 in which foreshocks are observed and their possible origin explained,
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whether by earthquake triggering or loading due to aseismic slip (Table S1). I have
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not considered studies that infer one theory or the other based on changes of the slope
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of the GR law, on power-law time-to-failure fitting, or on event migration, since these
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patterns can be explained indiscriminately by both theories5,6,18. Such studies form the
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bulk of the two hundred references investigated. It should be noted that the results
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presented in Table S1 are based on the conclusions of the different peer-reviewed
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studies, whether they really do or do not represent the true foreshock process. No
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attempt has been made to rank the studies by the quality of their analyses. All 37
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studies are considered equally valid in the present work.
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Supplementary Table S1: Data used in the meta-analysis. Mainshock and minimum
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foreshock magnitudes are represented by M and mmin, respectively. T and L refer to
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earthquake triggering and to loading due to aseismic slip, respectively.
No.
1
Ref.
1
2
31
3
32
4
33
5
34
6
26
Data source
Data type
M
mmin
Foreshock Method
Result
1975 Haicheng,
Sequence
7.3
0.6
Anomalous Physics
No T
China
“The increase in shear stress on the mainshock fault caused by the foreshocks is very
small and […] direct triggering of the mainshock by the foreshocks is unlikely”, p. 4575
1979 Petatlan,
Sequence
7.6
1.6
Anomalous Heuristic L
Mexico
“[aseismic region 10 km wide between foreshocks and mainshock epicentre] suggests an
influence of the pre-main shock activity by an asperity existing along the Benioff zone.
The stress […] probably is transmitted from below”, p. 4261
San Andreas,
Sequences 5.0
2.0
Anomalous Heuristic No T
California
5.2
5.4
5.5
5.8
6.4
“If [the model of foreshocks triggering the mainshock] were true, the durations of the
sequences should decrease as the magnitudes of the foreshocks increase. No such
dependence is seen in these data”, p. 1377
California
Sequences 6.5
5.0
Anomalous Heuristic L
6.8
7.1
8.3
“This type of precursor [over extended area] seems to require the presence of several
major faults in close enough proximity to one another that moderate-size shocks are
selectively triggered on surrounding faults during the latter stages of the cycle of strain
buildup to large earthquakes”, p. 595
Central and
Stacking
2.4
1.5
Normal
Statistics T
Northern
California
“Foreshocks were explained as events which had an afterevent which happened to be
bigger than the triggering event”, p. 910
1992 Landers,
Sequence
7.3
1.5
Anomalous Physics
No T
California
“Modeling of the Coulomb stress changes due to all previous foreshocks indicates that
the foreshocks probably did not trigger each other”, p. 9865
7
35
8
36
9
37
10
38
11
39
12
40
13
41
14
42
15
6
16
27
17
7
California
Sequences
4.7
1.0
Anomalous Physics
No T
5.2
5.5
6.1
6.4
7.3
“In general, these foreshock sequences are not compatible with a cascading failure
nucleation model”, p. 22371
San Francisco Bay, Sequences 7.0
5.5
Anomalous Physics
L
California
7.1
7.9
“We find that the changes in moderate seismicity during this time period are broadly
consistent with our model of elastic strain accumulation and release on the major faults”,
p. 786
California
Sequences 6.8
5.1
Anomalous Heuristic No T
6.9
7.0
7.1
7.3
7.5
“There is no evidence [precursors distributed over large distances] to suggest that the
precursor earthquakes directly influence the time and location of the subsequent strong
earthquake”, p. 5794
Japan
Stacking
5.0
3.0
Normal
Statistics T
“[Foreshocks as mainshock triggers] is compatible with the relationship between a
mainshock and aftershocks […]. However the values of exponent of the power law […]
are significantly smaller than those for similarly stacked aftershocks”, p. 381
Global
Stacking
6.0
5.0
Normal
Statistics T
7.0
“The world generic [aftershock cluster] model would better represent worldwide
foreshock-related conditional probabilities”, p. 4766
New Zealand
Stacking
4.0
4.0
Normal
Statistics T
“Our results are consistent with models in which foreshocks are a manifestation of the
same process as aftershocks, but the huge range allowed makes such consistency less
meaningful”, p. 467
California, Nevada Stacking
5.0
2.0
Normal
Statistics T
“The data are consistent with our hypothesis that main shock and aftershock magnitude
are independent, and that foreshocks are simply small main shocks with large
aftershocks”, p. 5
1999 Hector Mine, Sequence
7.1
2.0
Normal
Statistics T
California
“It is probable [described as “circumstantial evidence”, p. 1] that the 1999 M7.1 Hector
Mine earthquake was not triggered directly by the 1992 M7.3 Landers earthquake but
rather by its own foreshocks, which were themselves triggered directly or indirectly by
the Landers earthquake”, p. 12
1998 Sendai, Japan Sequence
5.0
1.4
Anomalous Physics
No T
“Cumulative Coulomb stress changes at the hypocenters of those foreshocks and the
mainshock do not support the cascade model for generation of the foreshocks and the
mainshock”, p. 2476
Southern
Stacking
3.0
3.0
Normal
Statistics T
California
“The cascade model described here [ETAS] is sufficient to explain the properties of
foreshocks in time, space and magnitude”, p. 9
1999 Hector Mine, Sequence
7.1
1.3
Anomalous Heuristic No T
California
“Qualitatively, the lack of triggering in the foreshock sequence, the foreshock
mechanisms and the mainshock location in the stress shadow of the foreshocks do not
suggest static stress triggering”, p. 4
Global
Stacking
6.0
5.6
Normal
Statistics T
18
43
19
44
20
45
21
46
22
19
23
47
24
24
25
48
26
49
27
50
28
51
California
2.2;4.8 2.2
“A single physical triggering mechanism is responsible for the occurrence of aftershocks,
foreshocks, and multiplets”, p. 88
Southern
Stacking
5.4
2.5
Normal
Statistics T
California
“The SCEDC statistics satisfy an ETAS model with  = 0.8-0.9, consistent with previous
catalogue studies”, p. 459
East Pacific Rise
Stacking
5.4
2.5
Anomalous Statistics No T
“We can reject the ETAS hypothesis that the clustering of foreshocks, mainshocks and
aftershocks […] can be described by the same triggering mechanism”, p. 459
2003 Tokachi-oki,
Sequence
8.0
0.0
Anomalous Physics
L
Japan
1.5
“Each anomaly [deviation from ETAS] is explained by change in Coulomb failure stress,
caused by aseismic slip”, p. 1
Sumatra
Sequences 9.0
4.5
Anomalous Physics
L
8.7
“The outlined region corresponds to the 0.02 bar Coulomb-pre-stress contour […]. The
earthquakes within this contour demonstrate AMR”, p. 642
California
Sequences 6.5
3.5
Anomalous Physics
L
6.6
6.7
7.0
7.1
7.3
7.5
“AMR is shown to occur preferentially in the lobes of the backslip stress field predicted
by the stress accumulation model”, p. 1
2004 Niigata-Ken- Sequence
6.6
2.0
Anomalous Physics
L
Chuetsu, Japan
“The quiescence and activation, which took place in the zone of negative and positive
increments of the Coulomb failure stress, respectively, were possibly caused by aseismic
slip on or near the focal fault plane”, p. 1
Southern
Stacking
2.0
2.0
Normal
Statistics T
California, global
5.7
5.7
“The ability of foreshocks to trigger larger earthquakes is consistent with the triggering
capability of mainshocks. Thus, […] foreshocks have no more predictive power than
other earthquakes”, p. 2147
California, Nevada Stacking
6.5
4.0
Normal
Statistics T
“These tests [comparison to ETAS simulations] demonstrate that apparent AMR may
arise from […] normal foreshock and aftershock activity”, p. 1
1997 UmbriaSequence
6.0
2.3
Anomalous Statistics L
Marche, Italy
“We then show that background events located in that spatiotemporal window
[quiescence due to aseismic slip] form a clear acceleration, as expected by the NonCritical PAST”, p. 1
New Zealand
Stacking
4.0
4.0
Normal
Statistics T
“New Zealand’s foreshock probabilities follow this generic law [Gutenberg-Richter law
and modified Omori law] with slightly smaller decay exponents compared to Californian
foreshocks and New Zealand aftershocks”, p. 2158
Japan
Stacking
4.3
4.2
Normal
Statistics T
New Zealand
4.1
4.0
Southern
3.5
3.4
California
“Even if the physical interpretation is unclear, a clustering model where spontaneous
events and triggered events have different triggering behaviors can be used to assess the
risk of foreshocks”, p. 1
Greece
Sequences 6.4
3.5
Anomalous Physics
L
6.5
6.6
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20
30
52
“Studying the annual rate of occurrence it is found that in areas of positive pre-stress
changes a sharp increase of the number of earthquakes is observed several years before
the occurrence of the main shock”, p. 367
1999 Izmit, Turkey Sequence
7.6
0.3
Anomalous Heuristic L
"The signal consisted of a succession of repetitive seismic bursts, accelerating with time,
[...] These observations show that the earthquake was preceded for 44 minutes by a phase
of slow slip", p. 877
Southern
Stacking
3.0
2.0
Normal
Statistics T
California
“These foreshocks are an expected consequence of earthquake interaction and clustering.
Their existence and location near the eventual rupture does not in itself indicate a
preparatory strain accumulation process beyond ordinary earthquake triggering”, p. 5
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21
32
53
33
54
34
25
35
5
36
55
37
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2011 Tohoku,
Sequence
9.0
2.0
Anomalous Heuristic L
Japan
"The time history of quasi-static slip along the plate interface, based on small repeating
earthquakes that were part of the migrating seismicity, suggests that two sequences
involved slow-slip transients", p. 705
Southern
Stacking
2.0
2.0
Anomalous Statistics No T
California
“We show that the linear density probability of earthquakes occurring before and after
[…] mainshocks displays a symmetrical behavior […]. This observation can be used to
discriminate spatial clustering due to foreshocks from the one induced by aftershocks”, p.
1
2011 Tohoku,
Sequence
9.0
1.2
Normal
Statistics T
Japan
“The whole sequence [2 day-long foreshock sequence initiated by a Mw7.3 earthquake]
can be readily explained without the need to invoke aseismic transients”, p. 1
Italy, Southern
Stacking
5.5
4.0
Normal
Statistics T
California
6.0
“The results indicate that the foreshock activity observed in the real catalogs is
compatible with what is expected by the ETAS model”, p. 1
2009 L’Aquila,
Sequence
6.3
1.8;3.3 Anomalous Statistics L
Italy
“Based on the computation of the temporal joint log-likelihood, we found that the
theoretical time series [predicted by the NC PAST] provides a good match to the
observations”, p. 4
2009 L’Aquila,
Sequence
6.3
3.4;6.3 Normal
Statistics No L
Italy
“Only for a minimum magnitude cutoff Mco ≥ 3.4 is the time series better fitted by a
Poisson process [95% of events shown not to produce any aftershock]”, p. 4
California
Stacking
2.0
1.5
Normal
Statistics T
“We can distinguish among three different kinds of spatial clustering […] (i) Direct
triggering. […] (ii) Correlated triggering. […]”, p. 12
California
Stacking
2.0
1.5
Anomalous Statistics L
[…] (iii) Correlations originating from some underlying physical process”, p. 12
North Pacific
Stacking
6.5
2.5
Anomalous Statistics No T
“The observed pattern seems to preclude triggering of the mainshock by foreshocks for
most of the subduction earthquakes of the data set”, p. 302
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2. Additional references
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March 14, 1979, Petatlan, Mexico Earthquake (Ms = 7.6). J. Geophys. Res. 88,
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4247-4262 (1983).
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32. Jones, L. M. Foreshocks (1966-1980) in the San Andreas System, California. Bull.
Seismol. Soc. Am. 74, 1361-1380 (1984).
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33. Sykes, L. R. & Jaumé, S. C. Seismic activity on neighbouring faults as a long-
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term precursor to large earthquakes in the San Francisco Bay area. Nature 348,
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595-599 (1990).
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47
48
34. Shaw, B. E. Generalized Omori law for aftershocks and foreshocks from a simple
dynamics. Geophys. Res. Lett. 20, 907-910 (1993).
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California foreshock sequences: Implications for the earthquake initiation process.
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J. Geophys. Res. 101, 22371-22392 (1996).
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36. Jaumé, S. C. & Sykes, L. R. Evolution of moderate seismicity in the San
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Francisco Bay region, 1850 to 1993: Seismicity changes related to the occurrence
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