The time analysis of seismic events preceding several strong earthquakes occurred in recent decades throughout the world, has highlighted some foreshocks’ characteristics, which are helpful for their discrimination compared to other types of events. These features can be identified within the seismic sequence and used as strong events’ precursors. Through the energy release pattern analysis, which precedes any strong earthquakes, in this study we describe some graphical procedures suitable for distinguishing a foreshock from any other type of earthquake. We have broadly divided foreshocks into two classes, depending on their position within the energy release pattern, by describing some relationships between the foreshock’s magnitude and the following earthquake’s. The results obtained show how the energy release pattern of some major earthquakes has distinctive features and repeatability which it is possible to obtain information from in order to perform sufficiently reliable short-term forecasts.
From the observation of several seismic sequences, we infer that some earthquakes are preceded by events of increasing magnitude related to the main shock, which are called foreshocks [
Several studies conducted on seismic sequences show how the seismicity increased significantly before the mainshock. For example, 10 days before the 2009 L’Aquila mainshock occurred, the foreshock sequence was concentrated in the hanging-wall domain of the normal Paganica fault [
The foreshocks are one of the few well-documented precursors to large earthquakes: Therefore, understanding their very nature is crucial for earthquake prediction and hazard mitigation. Several studies use foreshock sequences with (combined with) a statistical and probabilistic approach [
We believe that foreshocks are the manifestation of an ongoing energy release process, with different spatial and temporal scales, which leads to the mainshock. During its manifestation time, the seismic sequence analysis may be an appropriate tool to highlight the various stages of preparing a strong event. Under this approach we have noticed that an area seismicity before major earthquakes shows a certain organization [
The analysis performed suggests that the space-time occurrence of foreshocks, ranges from a few hours up to years before the mainshock, and from a few kilometers to thousands of kilometers from mainshock. It is in fact believed that foreshocks may also occur in areas relatively far from the mainshocks and be part of an external energy release process, which influences the area by triggering the main event [
For a better understanding of foreshocks temporal organization during the energy release phase, we propose some simple graphical and numeric procedures that can be used for the preliminary forecast of a major event.
Foreshock sequences are the most obvious precursor to large earthquakes: therefore, understanding their origin and relation to mainshocks is crucial for earthquake prediction and hazard mitigation. Previous studies conducted on immediate foreshocks in California suggest that these events may be part of a mainshock rupture nucleation process, because estimated Coulomb stress changes from foreshocks are too small to produce stress triggering and observed foreshock areas scale with mainshock magnitude, are consistent with nucleation instead of earthquake-to-earthquake triggering [
Foreshocks occur close to the mainshock and most likely are part of the nucleation process [
Foreshocks’ magnitude values and position during the energy realease phase allow the implementation of useful models for predicting a strong earthquake. However, it is necessary to know the features that distinguish them from other earthquakes. Through the graphical analysis of the seismic sequence, it is possible to highlight the foreshocks’ characteristics and their grouping in various orders, based on magnitude and period.
Defined as the “foreshocks’ height”, magnitude represents the distance, in terms of energy, between a minimum and the subsequent maximum, while period is the time or the number of shocks between a foreshock and the next.
Depending on the period or the number of shocks between a foreshock and the next within the seismic sequence, it is possible to identify foreshocks of different magnitude orders. First order foreshocks characterize short-medium term time windows, while those of second order usually occur immediately before the mainshock and exactly in the same area where the mainshock occurs.
The time elapsing between the last second order foreshock and the mainshock may vary but it typically consists of few days, while those of first order may happen in different times and areas.
Based on foreshocks magnitude values and temporal position, which characterize the energy release phase, we obtain the following “Progressive Earthquakes”-type developmental patterns [
a) From one or more first order or short to medium-term foreshocks and mainshock (
b) From one second order or impending-term foreshock and mainshock (
c) From one or more first order or short to medium-term foreshocks, one second order or impending-term foreshock and mainshock (
d) From one or more various orders foreshocks and from multiple mainshocks (
By observing the graphs shown in
lease phase (green segment) triggered by small magnitude shocks. Major events, complementing the “Progressive Earthquakes” pattern, in turn trigger a long- term energy accumulation phase that consists of two or more energetic aftershocks.
By observing the graph of the Japanese seismic sequence we can notice two “Progressive Earthquakes”-type patterns consisting of two first-order foreshocks and one mainshock, in which the temporal distance between the second foreshock and the mainshock is greater compared to that between the first and the second foreshock, while the magnitude values decrease.
In the graph showing the seismic sequence of L’Aquila, we see that the energy release pattern consists of three first order foreshocks, two second order foreshocks and one mainshock. The three first-order foreshocks show an “additive/constant” model (green line) in which the foreshocks’ magnitude values show a proportionality relationships as the number of seismic events increases, while the second order foreshocks and the mainshock, show a multiplicative/amplified model (red line) where the magnitude values between the foreshocks and mainshock amplify as the number of seismic events raises. This amplified effect is due to the short-term occurrence between foreshocks and the mainshock, which usually lasts just a few days.
cerning 128 earthquakes occurred in various areas of the world and in Italy.
The graphs and
1) Many earthquakes occur less than thirty days after the last foreshock, but quite a number of foreshocks occurs twenty-four hours before the mainshock;
2) The frequency rate of foreshocks occurring within a radius of 50 km and over thirty days, decreases as distance and time increase;
3) Closer events in time and space, show higher magnitude;
4) Magnitude values decrease as distance and number of days increase.
The results obtained show that in some seismic sequences, the “Progressive Earthquakes”-type energy release phase tend to develop near the epicenter of the main shock.
No | Earthquake | Foreshock | Mainshock | Time (days) | Distance (km) | ||
---|---|---|---|---|---|---|---|
Date | M | Date | M | ||||
1 | Colfiorito - Italy | 26/09/1997 | 5.6 ML | 26/09/1997 | 5.8 ML | 1 | 1.24 |
2 | L’Aquila - Italy | 05/04/2009 | 3.9 Mw | 06/04/2009 | 6.1 Mw | 1 | 1.90 |
3 | Emilia - Italy | 19/05/2012 | 4.0 Mw | 20/05/2012 | 5.8 Mw | 1 | 2.14 |
4 | Central Italy | 26/10/2016 | 5.9 Mw | 30/10/2016 | 6.5 Mw | 4 | 8.70 |
5 | Vanuatu | 09/07/1980 | 6.9 Mw | 17/07/1980 | 8.0 Mw | 8 | 20.8 |
6 | Vanuatu | 28/11/1985 | 7.2 Ms | 28/11/1985 | 7.6 Ms | 1 | 8.53 |
7 | Vanuatu | 16/11/1985 | 6.8 Ms | 21/12/1985 | 7.6 Ms | 5 | 31.2 |
8 | Afghanistan | 05/03/1990 | 6.0 Ms | 25/03/1990 | 6.3 Ms | 10 | 15.1 |
9 | Japan | 09/03/2011 | 7.3 Mw | 11/03/2011 | 9.0 Mw | 2 | 44.3 |
10 | Japan | 18/07/1992 | 5.8 Mw | 18/07/1992 | 6.9 Mw | 1 | 6.96 |
11 | Nicaragua | 10/08/1992 | 5.7 Mw | 02/09/1992 | 7.7 Mw | 23 | 3.95 |
12 | Gulf of Alaska | 17/11/1987 | 7.0 ML | 30/11/1987 | 7.9 Mw | 13 | 29.5 |
13 | Filippine | 17/05/1992 | 7.1 Mw | 17/05/1992 | 7.3 Mw | 1 | 13.1 |
14 | Colombia | 17/10/1992 | 6.7 Ms | 18/10/1992 | 7.3 Ms | 1 | 26.3 |
15 | Denali-Alaska | 23/10/2002 | 6.7 Mw | 03/11/2002 | 7.9 Mw | 11 | 23.3 |
16 | Grecia | 01/02/2014 | 5.0 Mb | 03/02/2014 | 6.1 Mw | 2 | 14.0 |
Today it is believed that a foreshock is physically indistinguishable from any other earthquake, until a subsequent mainshock classifies it as such [
The methodology proposed to identify foreshocks is based on identifying the dynamic trendline that characterize the development of the energy accumulation and release phases [
As we can infer from
As their numerical value varies over time, these straight lines are called dynamic trendlines. Trendlines clearly show the direction the magnitude values are moving in both in the impending and long-term and until the trend remains unchanged.
A seismic events’s trend is considered unchanged as long as there are no clear signs of reversal, such as the trendline break. In fact, the crossing of the energy accumulation trendline (transition point) marks the beginning of an energy release phase that can be of “Flash earthquakes-” or “Progressive Earstquakes-” type [
Besides showing the ongoing trend, it seems that the trendlines momentarily prevent the magnitude values from raising and, in some cases, they allow knowing in advance certain levels of magnitude that will be achieved in the future. In fact, the
The trendline drawn from points 6 (first order foreshock) and 7 (aftershock) allows identifying the second order foreshocks 8 and 9.
An earthquake in the “Progressive Earthquakes”-type energy release pattern cannot be identified as foreshock or mainshock until a subsequent energetic aftershock is formed. In the absence of an energetic aftershock, the foreshock or mainshock is identified as provisional.
The earthquake was preceded by three first order foreshocks (green stars) and by a second order foreshock (green square). As can be seen, the trendline drawn from points 1 and 2 shows a hypothetical medium―long term trend and allows estimating point 3’s magnitude (next minimum), while the intersection of the straight line passing through points 3 and 4 with the vertical line passing through the mainshock occurence point (point 5), provides the dynamic mainshock’s magnitude value. Alternatively, where the seismic sequence includes multiple first order foreshocks, the dynamic mainshock’s magnitude value is provided by the linear interpolation between them (dashed green line).
The use of a hierarchization process in the seismic sequence analysis, allows locating foreshocks and mainshock during the energy release phase. More accurate data on the foreshocks formation can be obtained from the seismic grid of the energy accumulation and release phases (
Obtained through the hierarchization process, the graph [
The extension of the first higher order seismic branch of the branched structure (branch 3a) up to the point of calculation allows identifying the transition point (full black circle) and the various orders foreshocks of the energy release phase. We can note how the various orders foreshocks and the mainshock are positioned above the dashed red line that represents the descending trendline drawn from the higher order seismic branch.
In some seismic sequences, it is apparent how a longer period after the last first order foreshock corresponds to a greater magnitude of the subsequent foreshock or mainshock.
ISF is an simple and very sensitive short-term oscillator, which can be used to monitor the strength and the developmental state of a seismic sequence and to detect the foreshocks as well as the energy accumulation phase’s trend.
ISF uses two types of seismic data: the magnitude values and the number of events recorded during the day. The greater is the variation of the magnitude
values and/or of the number of events, the greater the strength of the sequence. Therefore, whenever you have a daily increase of the magnitude value due to the occurrence of a foreshock or mainshock or a number of seismic events or of both, a peak is formed on the graph, followed by a gradual reduction in the amplitude of the ISF oscillation over time.
Due to the effect of the values fluctuations above the zero line, ISF has the appearance of a seismogram where, values above zero indicate the energy release phases, while negative values indicate the energy accumulation phases. ISF’s high values are associated with an increase in the number of earthquakes and/or high magnitude values. Positive and increasing index values show an energy release phase that is being strengthened, while positive and decreasing values show an energy accumulation phase. Values close to zero show that none of the two phases is ongoing and both anticipate a trend reversal.
a) If the magnitude value increases compared with the previous day, it means that an energy release phase is ongoing (green segment) ;
b) If the value decreases, it means that there is an energy accumulation phase ongoing (red segment);
c) If the value is equal, we have the same phase as the day before.
From the graph we can infer how, as the events progress, the magnitude values are not random but follow a trend according to increasing trends up to the process reversal. We can notice also growing trend phases, characterized by increasing maximum and minimum, and decreasing trend phases characterized by decreasing maximum and minimum.
The analysis of many seismic sequences revealed the existence of three main types of trends that can be identified in advance: (a) the main trend (lasting a few years); (b) the intermediate trend (lasting several months); (c) the minor trend (lasting a few weeks). It is therefore evident that there is not only one type of trend but different trends (one inside another) according to the observation time horizon.
To calculate the ISF, we simply identify the value of the daily maximum magnitude and multiply it by the number of daily shocks, attributing a positive or negative value depending on whether the value of the maximum magnitude of the day considered is greater (green segment) or smaller compared to that of the previous day (red segment). In the case where the maximum magnitude value of the day considered is equal to the previous day’s, the previous day ISF sign is attached to ISF.
The ISF graph related to central Italy (
Later, after a brief energy accumulation phase highlighted by a progressive decrease of ISF values, we observe a second peak in correspondence of the second order foreshock (green star) on 26 October whose magnitude was 5.9 Mw. The positive peak is followed by a negative minimum which corresponds to the triggering point of the energy release phase (red triangle) that ends on 30 October with the mainshock whose magnitude is 6.5 Mw, which is indicated on the graph by a positive ISF peak. The epicenter of the second order foreshock on
No | Earthquake Date | Magnitude | Identification | Order | Time (days) | Distance (km) |
---|---|---|---|---|---|---|
1 | 24/08/2016 | 6.0 MW | Foreshock | 1st Order | - | - |
2 | 26/10/2016 | 5.9 Mw | Foreshock | 2st Order | 64 | 24.98 |
3 | 30/10/2016 | 6.5 Mw | Mainshock | -- | 4 | 8.70 |
No | Earthquake Date | Magnitude | Identification | Order | Time (days) | Distance (km) |
---|---|---|---|---|---|---|
1 | 16/10/2016 | 4.0 Mw | Foreshock | 1st Order | - | - |
2 | 26/10/2016 | 5.4 Mw | Foreshock | 1st Order | 10 | 15.2 |
3 | 26/10/2016 | 5.9 Mw | Foreshock | 1st Order | <1 | 3.23 |
1 | 29/10/2016 | 4.1 Mw | Foreshock | 2st Order | 3 | 11.24 |
2 | 30/10/2016 | 6.5 Mw | Mainshock | -- | 1 | 8.70 |
26 October was located approximately 8.70 km from the main shock epicenter. The energy accumulation phase that is triggered after the mainshock is represented by a progressive and slow decrease in the oscillations amplitude of the ISF values.
No | Earthquake Date | Magnitude | Identification | Order | Time (days) | Distance (km) |
---|---|---|---|---|---|---|
1 | 09/03/2011 | 7.3 MW | Foreshock | 1st Order | - | - |
2 | 11/03/2011 | 9.0 Mw | Mainshock | --- | 2 | 44.3 |
ber 2016, the occurrence time and the distance between them.
A quick method for estimating the mainshock magnitude is shown in
that reports the seismic sequence in the earthquake occured in the Gulf of Alaska on 30 November 1987.
As can be seen, the mainshock was preceded by a foreshock with a magnitude of 7.0 Mw recorded on l7 November 1987. To calculate the magnitude of the mainshock, the first step is to draw the branched structure [
The empirical relations developed can be used to assess the magnitude of various order foreshock or “Progressive Earthquakes”-type pattern mainshock by know- ing the magnitude of the previous foreshock.
The relationships were obtained from the study of M > 3.8 foreshocks recorded by the Italian seismological network NIED network and by the USGS network between 1970 and 2016.
The average Mainshock Magnitude (MM) value is estimated through the following empirical relationship, obtained from the graphs displayed in
Relationship valid for all areas of the world with a foreshock magnitude (MF) in Mw:
Relationship valid for the Italian territory with a foreshock magnitude (MF) in Mw.
In the second procedure, the expected earthquake magnitude value is obtained with a formula empirically derived from a statistical analysis of the magnitude values of first and second order foreshocks that preceded the mainshocks in different areas of the world.
The equation to calculate the magnitude is as follows:
Relationship valid for all areas of the world with a foreshock magnitude (MF) in Mw:
The study of various order foreshocks the “Progressive Earthquake”-type energy release phase consists of, has provided a different methods of comparison between foreshocks and big earthquakes, leading to a better understanding of the phases regulating the preparation process of strong earthquakes.
The results obtained show that in some seismic sequences, the “Progressive Earthquake”-type energy release phase tends to develop near the main shock epicenter.
In particular, the analysis of the distances in time and space between foreshocks and mainshock has highlighted how some foreshocks occur shortly before the mainshock, within a radius of 50 kilometers from its epicenter. We also pointed out how the foreshocks frequency rate decreases as the distance from the mainshock epicenter and time increase, while the magnitude values decrease as the distance and the number of days increase.
The graphical analysis of the seismic sequence has shown how the foreshocks lie above the trendline that joins decreasing magnitude maximum values that form during the energy accumulation or above the seismic branch extension whose order is greater compared to the branched structure.
An in-depth examination of the branched structure showed how during the energy release phase only the branches seismic energy release of different order convey to foreshocks or mainshock, while in the energy accumulation phase the seismic branches of both energy accumulation and release phases convey to aftershock.
ISF (Index Simplified Force) can be used to obtain information about the strength of the seismic sequence in a given moment, to locate the foreshocks and to monitor the energy accumulation and release phase progress.
Based on the data resulting from the observations of several seismic sequences, we obtained some graphical and numeric procedures to determine the mainshock’s magnitude value using the trendline, the branched structure’s higher order seismic branch or the foreshock magnitude value.
We believe that the described method shows the foreshock identification limit during seismic sequences. Actually, as is not always possible to identify the event that precedes a strong earthquake, one prefers to classify it as provisional foreshock. The subsequent evolution of the seismic sequence will provide more information so that an appropriate foreshocks classification can be implemented.
The aforementioned procedures show how, starting from the analysis of the “Progressive Earthquakes”-type energy release phase, it is possible to obtain information to locate the foreshocks and make sufficiently reliable short-term forecasts. Seismic sequence’s evolution monitoring is essential to identify correctly the foreshock that precedes strong events.
Riga, G. and Balocchi, P. (2017) How to Identify Foreshocks in Seismic Sequences to Predict Strong Earthquakes. Open Journal of Earth- quake Research, 6, 55-71. https://doi.org/10.4236/ojer.2017.61003