In this paper, we examine both the sequence and organisation of major shallow earthquakes occurred in various areas of the world from 1904 to 2017. We aim to describe their major features and how they are connected with foreshocks and aftershocks immediately close in time and space. Examining magnitude value’s fluctuations over time, we see that they form a basic pattern, consisting of three maxima, one of which is central, and two or more events preceding and following it, whose magnitude, in some cases, may be comparable. The retrospective analysis of earthquakes’ patterns of high comparable magnitude has allowed their classification along with the development of some statistically significant relationships between epicentral distance and magnitude difference and between time interval and delay among maxima as well as the identification of activation signals predicting their occurrence. The pattern we identified in seismic sequences analysis, in relation to minor shocks-generated activation signals’ positions may be used to obtain useful information for the evolutionary study of seismic sequences and for predicting double and multiple earthquakes. The graphic analysis procedure applied to the pattern enables us to know the period of seismic sequence’s greatest hazard after a strong earthquake.
Moderate and strong earthquakes may be preceded and followed by events of comparable magnitude forming a pattern only slightly different from the typical foreshocks-mainshock-aftershocks sequence (FMA).
The term Doublet Earthquakes (DEQ) is related to two seismic events showing similar magnitude and localisation values, and occurring after a short delay [
Previously performed studies classify as DEQ two earthquakes whose magnitude difference does not exceed 0.2 units, spatial separation is less than 100 km and time separation is a few years [
DEQ’s trigger mechanism is still not clear, but a possible explanation for their occurrence is that the first shock increases the static stress in the crusted adjacent volumes, which show different conditions, resistance and asperities [
In areas featuring large asperities having similar thickness, increases in stress are high and trigger the breaking of adjacent asperities, thus producing a sequence of two distinct earthquakes although with similar magnitude [
Asperities distribution is a simple pattern explaining the occurrence features of large earthquakes in many subduction areas [
Alternatively, the triggering of the second earthquake along the second fault may result from breaking propagation up to the end of the first fault [
DEQ occur across the world and unquestionably are a significant phenomenon related to the evaluation of the seismic hazard after a big event.
Their impact on buildings and infrastructures damaged during the first event may be greater during the second event and poses a danger to rescue teams after the first earthquake as well.
Using the FMA scheme, in this study we implemented a graphical approach to analyse strong individual earthquakes (EQ) and DEQ, obtain information on seismic sequence development over time, predict the magnitude of the second DEQ event and define activation signals before their occurrence.
The information obtained can be used to define seismic sequence’s highest hazards after a strong EQ and plan all actions required for persons and structures’ safety to limit the damage.
Observations of numerous seismic sequences have highlighted that inside them it is possible to detect patterns that may be described as warning signals for the most energetic shock. Theoretically, the seismic patterns that develop before an energetic event contain a series of information that allows, in some cases, to predict the shock’s energy, days to weeks early by their temporal development.
The analysis of historical earthquakes’ seismic sequences and, especially, of seismic patterns consisting of magnitude values’ fluctuations, identifies three types of shocks:
Foreshocks: premonitory shock;
Mainshock: primary shock―often single but sometimes multiple;
Aftershocks:
The preparation process that leads to the nucleation of large earthquakes usually is longer compared to smaller earthquakes’: this is also supported by the branched structure that in stronger earthquakes shows greater hierarchisation [
We also noted that some EQs are sometimes preceded by minor premonitory shocks, called foreshocks, which create specific short and medium-term patterns characterized by upward trend and higher development rate. Foreshocks may occur individually or in groups, while the time of occurrence between the last foreshocks and the main shock varies from a few hours to a few months depending on their order within the sequence [
Usually, first order foreshocks are followed by an aftershocks sequence that delays the main event occurrence, while in second order foreshocks, aftershocks are almost absent. The main shock (mainshock) has greater magnitude and is always followed by a series of lower magnitude shocks (aftershocks), lasting from a few weeks to some years, depending on mainshock magnitude. Aftershocks usually form a downward sequence, consisting of subsequent energy accumulation and release phases [
In principle, FMA sequence shows that an earthquake may generate other earthquakes that tend to cluster over time to form a premonitory pattern according to predetermined rules, such as TT-7S [
By observing magnitude values fluctuations within the seismic sequence, we note that during the energy accumulation phase, pattern TT-7S it is repeated over time with greater frequency, while in the energy release phase it appears as a strong fluctuation of magnitude values.
FMA pattern develops over a more or less short period and may feature some variations including multiple events of the same type that may be close in time and space (DEQ).
In
that the second minimum’s magnitude is lower than the first’s, we should draw the transition line from the first to third maximum (A), and update it if greater magnitude maxima are formed (
FMA pattern organisation in space, in time and magnitude probably is a single triggering process as well as a simple tool to distinguish mainshock from other shocks.
It is assumed that foreshocks’ magnitude values depend on past seismicity and are placed above the trend line in the aftershocks phase (green dashed line).
As a rule, in this analytical graphic approach, aftershocks are arranged according to decreasing minimum and maximum values (downward trend), while foreshocks are arranged based on increasing maximum and minimum values (upward trend) which are observed before major earthquakes.
During FMA formation, it is possible to observe the following development characteristics:
a) An increase in the number of shocks before the first maximum formation (F), followed by an aftershocks phase consisting of fewer shocks forming a first minimum;
b) The formation of the second maximum (M) characterised by a rapid energy release, and followed, in some cases, by a fast aftershocks phase characterised by shocks of decreasing magnitude forming a second minimum;
c) The formation of a third maximum (A) that often cannot exceed the second maximum’s magnitude value (M), but it is frequently above the first maximum’s magnitude value (F);
d) The most energetic phase ends when magnitude values fall below the transition line and, during the subsequent rise, fail to cross it, thus signifying the end of the increasing maximum and minimum formation as well as FMA pattern completion;
e) If it is inclined in the ongoing trend’s direction, the transition line has greater importance in FMA pattern formation. The higher the inclination, the larger the third maximum’s magnitude (A);
f) The activation signal preceding the second maximum (M) coincides with microsequence’s fourth shock DB-3SE [
g) The activation signal preceding the third maximum (A) coincides with the minimum formed after the second maximum (M).
In some sequences, a precursor pattern forms before the second maximum (M) formation in FMA pattern, which consists of seven shocks at least with trend opposite to FMA pattern (
(RFMA), is specular to FMA pattern and usually is formed at the end of magnitude values’ upward trend.
Going into details, RFMA pattern development entails the formation of three minimum (points 2, 4, 6) and four maximum points (1, 3, 5, 7), where the second and sixth minimum and the third and the fifth maximum show increasing magnitude. In this pattern, if magnitude values do not form the third minimum (point 6), we have the early formation of a fourth maximum (M). In this case, the transition line is a horizontal line starting from the first maximum (point 3): the pattern is called early RFMA (PRFMA).
This pattern features some minor differences compared to standard FMA patterns:
1) Magnitude values of points 5 and 6 are usually higher than first and second minimum’s (points 2 and 4);
2) The transition line joining maximum points 3 and 5 (dashed red line) provides information on minimum mainshock’s magnitude value (MM) and on completion of the most energetic phase when magnitude values drop below it;
3) The first activation signal coincides with the second minimum (green triangle), while the second signal coincides with t pattern’s third minimum (red triangle).
The third maximum magnitude values’ range in FMA pattern can be calculated drawing from the first maximum (F) the line parallel to the transition line (dashed red line) by considering the distance between the transition and the parallel line (red arrow) as the expected magnitude values’ dynamic range (
In principle, after reaching a minimum in the aftershocks phase (point 2), magnitude values temporarily go back to about 50% of the second maximum amplitude (M), calculated based on the transition line, before starting descending again.
In some cases, they increase over 50% by forming two EQ having similar magnitude (DEQ) while sometimes the magnitude values exceed several times the transition line before definitely dropping and forming multiple earthquakes (MEQ).
After the magnitude values’ drop below the transition line, the minimum magnitude value can be calculated by measuring the distance between mainshock and transition line (green arrow) and drawing the latter from the breaking point downwards, perpendicularly to the transition line (
Often the minimum target is reached and exceeded during aftershocks phase. In this case, it is possible to estimate a second minimum target by projecting from magnitude value’s transit point below the transition line, in addition to the second maximum (green arrow) and the first maximum (red arrow) amplitude.
The preliminary range of fourth maximum magnitude values in RFMA (point 7) (
and parallel line as the expected magnitude values’ dynamic range. The fourth maximum magnitude value may also be computed by measuring the distance between the second minimum (point 4) and the transition line (green arrow) and projecting the latter from the second maximum (point 5) upwards (red arrow).
In order to classify DEQ occurred in various parts of the world, we analysed several seismic sequences using the catalogs of the National Institute of Geophysics and Volcanology (INGV) [
Based on seismicity fluctuation, we identified two earthquakes categories:
1) individual earthquakes;
2) double and multiple earthquakes.
In the first category, the largest shock is easily identifiable because of its greater magnitude compared to other earthquakes in the sequence.
Individual earthquakes include the following typologies:
(A) “Progressive Earthquakes” (
(B) “Flash Earthquakes” that are not preceded by foreshocks (
No | EARTHQUAKE | LATITUDE | LONGITUDE | DEPTH | MAGNITUDE |
---|---|---|---|---|---|
1 | Colfiorito | 44N - 42.7N | 14.2E - 11.3E | 0 - 50 | 2.0 - 7.0 |
2 | L’Aquila | 43.2N - 41.5N | 14.2E - 12.5E | 0 - 50 | 2.0 - 7.0 |
3 | Emilia | 45.2N - 44.7N | 11.7E - 10.5E | 0 - 50 | 2.0 - 7.0 |
4 | Central Italy | 43.4N - 42.4N | 13.7E - 12.6E | 0 - 50 | 2.0 - 7.0 |
No | Earthquakes | Date Event | Lat | Long | M | Date Mainshock (M) | M | Time Delays (ddhh:mm) | Distance (km) |
---|---|---|---|---|---|---|---|---|---|
1 | Colfiorito (F) | 23/09/1997 | 43.030 | 12.902 | 2.8 Md | 26/09/1997 | 5.6 ML | 02 05:07 | 1.60 |
2 | Colfiorito (S) | 25/09/1997 | 43.031 | 12.878 | 2.4 Md | 26/09/1997 | 5.6 ML | 00 05:04 | 3.19 |
3 | Colfiorito (F) | 26/09/1997 | 42.996 | 12.966 | 3.2 Md | 26/09/1997 | 5.8 ML | 00 01:10 | 4.50 |
4 | Colfiorito | 26/09/1997 | 43.028 | 12.867 | 2.5 Md | 26/09/1997 | 5.8 ML | 00 01:00 | 4.92 |
5 | Colfiorito (S) | 26/09/1997 | 43.030 | 12.886 | 2.1 Md | 26/09/1997 | 5.8 ML | 00 00:26 | 3.32 |
6 | Colfiorito | 26/09/1997 | 43.020 | 12.822 | 2.2 Md | 26/09/1997 | 5.8 ML | 00 00:21 | 8.47 |
7 | Colfiorito | 26/09/1997 | 43.036 | 12.905 | 2.8 Md | 26/09/1997 | 5.8 ML | 00 00:16 | 2.17 |
8 | Colfiorito (F) | 26/09/1997 | 43.021 | 12.901 | 3.8 Md | 26/09/1997 | 5.8 ML | 00 00:07 | 2.06 |
9 | Colfiorito | 26/09/1997 | 43.018 | 12.913 | 5.6 ML | 26/09/1997 | 5.8 ML | 00 09:07 | 1.24 |
10 | L’Aquila (F) | 05/04/2009 | 42.325 | 13.382 | 3.9 Mw | 06/04/2009 | 6.1 Mw | 00 04:44 | 1.90 |
11 | L’Aquila | 05/04/2009 | 42.329 | 13.385 | 3.5 Mw | 06/04/2009 | 6.1 Mw | 00 02:53 | 1.50 |
12 | L’Aquila (S) | 05/04/2009 | 42.959 | 13.600 | 2.3 ML | 06/04/2009 | 6.1 Mw | 00 02:36 | 70.93 |
13 | Emilia (F) | 19/05/2012 | 44.911 | 11.247 | 4.0 Mw | 20/05/2012 | 5.8 Mw | 00 02:50 | 2.14 |
14 | Emilia (S) | 19/05/2012 | 44.903 | 11.293 | 2.2 ML | 20/05/2012 | 5.8 Mw | 00 02:21 | 2.41 |
15 | Emilia (S) | 29/05/2012 | 44.807 | 11.394 | 2.0 ML | 29/05/2012 | 5.6 Mw | 00 02:03 | 26.16 |
16 | Emilia | 29/05/2012 | 44.870 | 11.353 | 2.6 ML | 29/05/2012 | 5.6 Mw | 00 01:26 | 22.84 |
17 | Central Italy (F) | 16/08/2016 | 43.383 | 12.771 | 2.8 ML | 24/08/2016 | 6.0 Mw | 07 13:57 | 84.96 |
18 | Central Italy | 16/08/2016 | 42.832 | 13.025 | 2.2 ML | 24/08/2016 | 6.0 Mw | 07 05:54 | 22.65 |
19 | Central Italy (S) | 23/08/2016 | 43.057 | 12.988 | 2.0 ML | 24/08/2016 | 6.0 Mw | 00 06:48 | 44.67 |
20 | Central Italy (F) | 26/10/2016 | 42.907 | 13.116 | 3.2 ML | 26/10/2016 | 5.9 Mw | 00 00:18 | 1.82 |
21 | Central Italy | 26/10/2016 | 42.889 | 13.128 | 3.0 ML | 26/10/2016 | 5.9 Mw | 00 00:13 | 2.22 |
22 | Central Italy (S) | 26/10/2016 | 42.872 | 13.141 | 2.2 ML | 26/10/2016 | 5.9 Mw | 00 00:05 | 4.23 |
23 | Central Italy (F) | 26/10/2016 | 42.879 | 13.157 | 4.3 ML | 26/10/2016 | 5.9 Mw | 00 00:02 | 4.04 |
24 | Central Italy (F) | 30/10/2016 | 42.791 | 13.087 | 2.4 ML | 30/10/2016 | 6.5 Mw | 00 00:36 | 4.96 |
25 | Central Italy | 30/10/2016 | 43.062 | 13.068 | 2.1 ML | 30/10/2016 | 6.5 Mw | 00 00:24 | 25.81 |
26 | Central Italy (S) | 30/10/2016 | 42.927 | 13.029 | 2.0 ML | 30/10/2016 | 6.5 Mw | 00 00:09 | 12.05 |
As can be inferred, each second maximum (M) was preceded by shocks occurred with a maximum delay of about seven days and at a distance less than 85 km. Most activation signals (red triangle) were activated a few minutes before the second maximum (M), while foreshocks (F) occurred approximately within seven days of the mainshock (M). FMA pattern completion occurred the moment when magnitude values dropped under the transition line, while the third maximum was formed within the dynamic range established by the transition line and the relevant parallel line drawn from the first maximum.
Earthquake pairs with comparable magnitude are included in the second category: one of them is often identifiable due to its greater magnitude compared to the other.
To recognise and classify large DEQ, from the U.S. Geological Survey (USGS) catalog we selected 1237 DEQ, whose magnitude was equal to or greater than 6 M, which occurred in various areas of the world from 1904 to 2017.
The criteria we used to select DEQ were: difference in magnitude between the two events, which should not be greater than 1.0 units and hypocentral depth that must not exceed 50 km. For each pair of events we determined: time delay, distance in kilometers and difference in magnitude.
Our analyses highlighted that DEQ pattern has variances that allow us to group them into the types shown in
a) DEQ consisting of one foreshock and one mainshock (FM1) separated by several shocks (
b) DEQ consisting of one foreshock and one mainshock (FM2) separated by a minimum (
c) DEQ consisting of one consecutive foreshock and mainshock (FM3) (
d) DEQ consisting of two consecutive mainshocks (MM) (
e) DEQ consisting of one consecutive mainshock and aftershock (MA3) (
f) DEQ consisting of one mainshock and one aftershock (MA2) separated by a minimum (
g) DEQ consisting of one mainshock and one aftershock (MA1) separated by several shocks (
h) DEQ multiples (
In the
As can be seen, many DEQ feature a fair close difference in magnitude with relatively short delays between two pair’s events and occur close to each other.
Going into detail, 54.1% of DEQ shows a difference in magnitude equal to or less than 0.2 units, 83.6% less than 0.5 units and 93.4% less than 0.7 units. 50.9% is less than 30 km distant, 73.4% is less than 70 km distant and 81.6% is less than 100 km distant. 56.9% has a delay of less than one day, 81.1 less than three days and 95.7% less than 10 days. The greatest delay is 145 days.
The average distance is 61.3 km, the average delay 1.93 days and the average magnitude difference is 0.29 units. 59.7% of DEQ whose difference in magnitude is 0.2 units has a delay of less than one day, 83.2% less than two days and 96.7% less than ten days. 50.1% is less than 30 km distant, 72.2% is less than 70 km and 80.0% is less than 100 km.
57.9% of DEQ with a difference in magnitude of 0.5 units has a delay of less than one day, 81.9% less than three days and 92.5% less than ten days. 50.9% is less than 30 km distant, 73.4% is less than 70 km and 81.5% is less than 100 km distant.
DEQ can be difficult to locate a priori, but in certain cases it is possible (patterns FM1, FM2, MA1 and MA2), to obtain the information that can be used to estimate their magnitude and occurrence time. Indeed, indications about when a DEQ may be formed, completed and the second event magnitude, are provided by reversal or extension points in relation to Fibonacci levels [
For example, patterns FM2 and MA2 contain an upward or downward ABCD pattern (
No | Earthquakes | Date | Lat | Long | Depth (km) | Magnitude | ∆M | Time Delays (ddhh:mm) | Distance (km) |
---|---|---|---|---|---|---|---|---|---|
1 | Russia | 25/06/1904 25/06/1904 | 52.864 51.565 | 160.445 161.417 | 15 30 | 7.5 Mw 7.7 Mw | 0.2 | 00 16:15 | 158.9 |
2 | Mongolia | 09/07/1905 23/07/1905 | 49.709 49.369 | 98.483 96.610 | 15 15 | 8.3 Mw 8.3 Mw | 0.0 | 13 17:06 | 140.3 |
3 | Vanuatu | 16/06/1910 09/11/1910 | −19.572 −16.289 | 169.438 166.904 | 100 20 | 7.8 Mw 7.3 Mw | 0.5 | 145 23:32 | 452.9 |
4 | Papua-Indonesia | 13/01/1916 13/01/1916 | −3.196 −3.987 | 135.731 138.011 | 25 35 | 7.1 Mw 7.7 Mw | 0.6 | 00 02:02 | 267.9 |
5 | Chile-Argentina | 02/03/1919 02/03/1919 | −43.800 −43.109 | −78.319 −71.695 | 15 15 | 7.2 Mw 7.2 Mw | 0.0 | 00 08:19 | 540.0 |
6 | Chile | 07/11/1922 11/11/1922 | −28.365 −28.293 | −71.96 −69.852 | 25 70 | 7.0 Mw 8.5 Mw | 0.5 | 03 05:32 | 206.5 |
7 | Japan | 01/09/1923 02/09/1923 | 35.413 35.007 | 139.298 139.926 | 15 15 | 8.1 Mw 7.8 Mw | 0.3 | 00 23:48 | 72.7 |
8 | China | 03/07/1924 11/07/1924 | 36.983 37.064 | 84.164 83.453 | 10 10 | 7.2 Mw 7.0 Mw | 0.2 | 08 15:04 | 63.8 |
9 | Molucca Sea | 03/05/1925 03/06/1925 | 1.190 1.292 | 126.01 126.01 | 15 15 | 7.1 Mw 7.0 Mw | 0.1 | 30 11:12 | 11.3 |
10 | Bulgaria | 14/04/1928 18/04/1928 | 42.329 42.356 | 25.717 25.109 | 10 15 | 7.1 Mw 7.1 Mw | 0.0 | 04 10:22 | 50.1 |
11 | Mexico | 17/06/1928 04/08/1928 09/10/1928 | 16.182 16.004 16.190 | −96.585 −98.209 97.502 | 20 20 25 | 7.9 Mw 7.2 Mw 7.5 Mw | 0.7 0.3 | 48 15:07 65 08:35 | 174.6 78.4 |
12 | Maule | 01/12/1928 02/12/1928 | −35.155 −35.685 | −72.105 −72.812 | 35 30 | 7.7 Mw 7.0 Mw | 0.7 | 00 24:14 | 87.1 |
13 | Aleutian Islands | 05/07/1929 07/07/1929 | 51.473 51.474 | −178.152 −177.771 | 35 35 | 7.0 Mw 7.3 Mw | 0.3 | 02 07:04 | 26.4 |
14 | China | 10/08/1931 18/08/1931 | 46.817 47.264 | 89.915 89.859 | 10 10 | 7.9 Mw 7.1 Mw | 0.8 | 07 17:03 | 49.9 |
15 | Solomon Islands | 03/10/1931 03/10/1931 03/10/1931 10/10/1931 | −11.117 −12.131 −10.931 −9.732 | 161.110 161.333 161.414 161.211 | 15 15 15 15 | 7.9 Mw 7.0 Mw 7.0 Mw 7.7 Mw | 0.9 0.0 0.7 | 00 02:05 00:01:29 06 01:32 | 115.3 133.7 134.0 |
16 | Mexico | 03/06/1932 18/06/1932 22/06/1932 | 19.786 19.419 19.373 | −103.784 −103.907 −104.224 | 15 15 25 | 8.1 Mw 7.8 Mw 7.7 Mw | 0.3 0.1 | 14 23:36 04 02:47 | 42.8 33.6 |
17 | Santa Cruz Islands | 18/07/1934 21/07/1934 | −11.936 −11.129 | 166.977 165.890 | 10 15 | 7.7 Mw 7.3 Mw | 0.4 | 02 10:38 | 148.6 |
18 | New Guinea | 20/09/1935 20/09/1935 | −3.824 −3.776 | 141.416 142.640 | 30 34 | 7.8 Mw 7.0 Mw | 0.8 | 00 03:37 | 135.9 |
---|---|---|---|---|---|---|---|---|---|
19 | Japan | 12/10/1935 18/10/1935 | 40.199 40.235 | 143.304 144.011 | 15 15 | 7.0 Mw 7.1 Mw | 0.1 | 05 07:27 | 60.12 |
20 | Japan | 05/11/1938 05/11/1938 06/11/1938 06/11/1938 | 36.966 37.166 37.393 37.019 | 142.090 142.221 142.303 142.430 | 35 35 30 25 | 7.8 Mw 7.7 Mw 7.7 Mw 7.6 Mw | 0.1 0.0 0.1 | 00 02:07 00 22:03 00 12:45 | 25.1 51.1 43.1 |
21 | Costa Rica | 05/12/1941 06/12/1941 | 8.396 8.523 | −83.457 −84.528 | 20 15 | 7.3 Mw 7.0 Mw | 0.3 | 01 00:37 | 118.6 |
22 | Dominican Republic | 04/08/1946 08/08/1946 | 19.083 19.538 | −69.248 −69.657 | 15 15 | 7.5 Mw 7.0 Mw | 0.5 | 03 19:37 | 66.3 |
23 | Mariana Islands | 13/06/1947 19/06/1947 | 21.722 21.600 | 145.567 145.464 | 35 35 | 7.0 Mw 7.2 Mw | 0.2 | 05 11:10 | 17.2 |
24 | Tierra del Fuego | 17/12/1949 17/12/1949 | −53.923 −53.911 | −69.596 −69.753 | 10 10 | 7.7 Mw 7.3 Mw | 0.4 | 00 08:14 | 10.4 |
25 | Taiwan | 22/10/1951 22/10/1951 | 23.917 23.775 | 121.343 121.393 | 25 20 | 7.2 Mw 7.0 Mw | 0.2 | 00 02:14 | 16.6 |
26 | Taiwan | 24/11/1951 24/11/1951 | 23.046 23.092 | 121.249 121.214 | 25 30 | 7.3 Mw 7.8 Mw | 0.5 | 00 00:03 | 6.2 |
27 | Aleutian Islands | 11/03/1957 11/03/1957 12/03/1957 14/03/1957 16/03/1957 | 52.691 51.339 51.481 51.196 51.419 | −169.191 −178.602 −177.243 −176.733 −178.870 | 35 25 20 25 25 | 7.1 Mw 7.0 Mw 7.1 Mw 7.1 Mw 7.2 Mw | 0.1 0.1 0.0 0.1 | 00 04:57 00 20:50 02 03:02 01 11:47 | 660.8 95.6 47.5 115.8 |
28 | Greece | 24/04/1957 25/04/1957 | 36.493 36.405 | 28.829 28.699 | 35 35 | 7.1 Mw 7.3 Mw | 0.2 | 00 07:15 | 15.2 |
29 | Japan | 20/03/1960 23/03/1960 | 39.869 39.635 | 143.228 143.316 | 15 15 | 8.0 Mw 7.0 Mw | 1.0 | 02 07:16 | 27.1 |
30 | Chile | 21/05/1960 22/05/1960 | −37.824 −37.775 | −73.353 −73.017 | 25 25 | 8.1 Mw 7.1 Mw | 1.0 | 01 00:28 | 30.0 |
31 | Japan | 16/01/1961 16/01/1961 | 36.121 36.226 | 141.758 141.815 | 30 30 | 7.2 Mw 7.0 Mw | 0.2 | 00 04:52 | 12.7 |
32 | Japan | 12/04/1962 23/04/1962 | 38.022 42.506 | 142.789 143.734 | 28 60 | 7.3 Mw 7.1 Mw | 0.2 | 11 05:06 | 505 |
33 | Indonesia | 15/04/1963 16/04/1963 | −0.975 −1.050 | 128.07 128.043 | 30 30 | 7.1 Mw 7.1 Mw | 0.0 | 00 00:26 | 8.8 |
34 | Santa Cruz Islands | 15/09/1963 17/09/1963 | −10.522 −10.466 | 165.642 165.360 | 35 45 | 7.4 Mw 7.2 Mw | 0.2 | 02 18:34 | 31.4 |
35 | Kuril Islands | 13/10/1963 20/10/1963 | 44.872 44.726 | 149.483 150.547 | 35 28.2 | 8.5 Mw 7.8 Mw | 0.7 | 06 19:36 | 85.5 |
36 | Kuril Islands | 11/06/1965 11/06/1965 | 44.608 44.578 | 149.022 148.699 | 40.7 58 | 7.0 Mw 7.2 Mw | 0.2 | 00 00:01 | 25.8 |
---|---|---|---|---|---|---|---|---|---|
37 | Vanuatu | 11/08/1965 11/08/1965 13/08/1965 | −15.449 −15.861 −15.871 | 166.980 167.092 166.960 | 25 30 25 | 7.2 Mw 7.6 Mw 7.4 Mw | 0.4 0.2 | 00 18:51 01 14:09 | 47.4 14.2 |
38 | Santa Cruz Islands | 31/12/1966 31/12/1966 | −12.091 −12.326 | 166.552 166.491 | 55 35 | 7.8 Mw 7.1 Mw | 0.7 | 00 03:52 | 26.9 |
39 | Japan | 16/05/1968 16/05/1968 | 40.860 41.430 | 143.435 142.864 | 29.9 25 | 8.2 Mw 7.9 Mw | 0.3 | 00 09:50 | 79.4 |
40 | Kuril Islands | 11/08/1969 14/08/1969 | 43.599 43.313 | 147.385 147.647 | 25 27.5 | 7.5 Mw 7.1 Mw | 0.4 | 02 16:52 | 38.19 |
41 | Papua New Guinea | 14/07/1971 26/07/1971 | −5.524 −4.817 | 153.850 153.172 | 40 40 | 8.0 Ms 8.1 Ms | 0.1 | 11 19:12 | 108.7 |
42 | Russia | 02/08/1971 05/09/1971 | 41.415 46.505 | 143.416 141.199 | 57.8 18.1 | 7.1 Mw 7.3 Mw | 0.2 | 34 11:11 | 593.1 |
43 | Japan | 17/06/1973 24/06/1973 | 43.233 43.318 | 145.785 146.442 | 48 50 | 7.7 Mw 7.1 Mw | 0.6 | 06 22:48 | 54.0 |
44 | Solomon Islands | 31/01/1974 01/02/1974 | −7.461 −7.383 | 155.894 155.575 | 34 40 | 7.0 Ms 7.1 Ms | 0.1 | 00 03:42 | 36.2 |
45 | Papua New Guinea | 20/07/1975 20/07/1975 | −6.590 −7.104 | 155.054 155.152 | 49 44 | 7.9 Ms 7.7 Ms | 0.2 | 00 05:17 | 58.2 |
46 | Uzbekistan | 08/04/1976 17/05/1976 | 40.311 40.381 | 63.773 63.472 | 33 10 | 7.0 Ms 7.0 Ms | 0.0 | 39 00:18 | 26.67 |
47 | China | 27/07/1976 28/07/1976 | 39.664 39.57 | 117.978 117.978 | 23 26 | 7.4 Mw 7.4 Ms | 0.0 | 00 15:03 | 10.4 |
48 | Solomon Islands | 20/04/1977 20/04/1977 20/04/1977 | −9.890 −9.844 −9.965 | 160.348 160.822 160.731 | 19 33 33 | 7.5 Ms 7.5 Ms 7.5 Ms | 0.0 0.0 | 00 00:07 00 04:35 | 52.2 16.7 |
49 | Kuril Islands | 23/03/1978 24/03/1978 | 44.932 44.244 | 148.439 148.862 | 33 33 | 7.5 Ms 7.6 Ms | 0.1 | 01 16:32 | 83.5 |
50 | Mexico | 07/06/1982 07/06/1982 | 16.610 16.560 | −98.150 −98.360 | 40 33 | 7.2 Ms 7.0 Ms | 0.2 | 00 03:20 | 23.0 |
51 | Chile | 03/03/1985 04/03/1985 09/04/1985 | −33.135 −33.207 −34.131 | −71.871 −71.663 −71.618 | 33 33 37.8 | 8.0 Mw 7.4 Mw 7.2 Mw | 0.6 0.2 | 00 01:45 05 01:24 | 20.9 102.8 |
52 | Papua New Guinea | 10/05/1985 03/07/1985 | −5.599 −4.439 | 151.045 152.828 | 26.7 33 | 7.2 Mw 7.3 Mw | 0.1 | 53 13:00 | 235.9 |
53 | Afghanistan | 29/07/1985 23/08/1985 | 36.190 39.431 | 70.896 75.224 | 98.7 6.8 | 7.4 Mw 7.0 Mw | 0.4 | 25 04:47 | 523.7 |
54 | Mexico | 19/09/1985 21/09/1985 | 18.190 17.802 | −102.533 −101.647 | 27.9 30.8 | 8.0 Mw 7.6 Mw | 0.4 | 01 12:20 | 103.2 |
55 | Vanuatu | 28/11/1985 21/12/1985 | −13.987 −13.966 | 166.185 166.516 | 33 43 | 7.0 Mw 7.1 Mw | 0.1 | 22 21:24 | 35.8 |
---|---|---|---|---|---|---|---|---|---|
56 | Chile | 05/03/1987 05/03/1987 | −24.388 −24.495 | −70.161 −70.701 | 62.3 34.8 | 7.6 Mw 7.0 Mw | 0.6 | 00 01:38 | 55.9 |
57 | Papua New Guinea | 12/10/1987 16/10/1987 | −7.288 −6.266 | 154.371 149.06 | 24.7 47.8 | 7.0 Mw 7.4 Mw | 0.4 | 04 06:51 | 597.3 |
58 | Fiji Islands | 03/03/1990 05/03/1990 | −22.122 −18.318 | 175.163 168.063 | 33.2 20.7 | 7.6 Mw 7.1 Mw | 0.5 | 02 04:22 | 852.9 |
59 | Sudan | 20/05/1990 24/05/1990 | 5.121 5.358 | 32.145 31.848 | 14.9 16 | 7.2 Mw 7.1 Mw | 0.1 | 04 17:38 | 42.1 |
60 | Philippines | 17/05/1992 17/05/1992 | 7.239 7.191 | 126.645 126.762 | 32.8 33 | 7.1 Mw 7.3 Mw | 0.2 | 00 00:26 | 13.9 |
61 | Kuril Islands | 04/10/1994 09/10/1994 | 43.773 43.905 | 147.321 147.916 | 14 33 | 8.3 Mw 7.3 Mw | 1.0 | 04 18:33 | 49.9 |
62 | Japan | 28/12/1994 06/01/1995 | 40.525 40.246 | 143.419 142.175 | 26.5 26.9 | 7.8 Mw 7.7 Mw | 0.1 | 09 10:18 | 109.8 |
63 | Philippines | 21/04/1995 05/05/1995 | 12.059 12.626 | 125.580 125.297 | 20.7 16 | 7.2 Mw 7.1 Mw | 0.1 | 14 03:19 | 70.1 |
64 | Papua New Guinea | 16/08/1995 16/08/1995 | −5.799 −5.771 | 154.178 154.347 | 30.1 33 | 7.7 Mw 7.2 Mw | 0.5 | 00 12:43 | 18.9 |
65 | Aleutian Islands | 10/06/1996 10/06/1996 | 51.564 51.478 | −177.632 −176.847 | 33 26.3 | 7.9 Mw 7.3Mw | 0.6 | 00 11:21 | 55.1 |
66 | Banda Sea | 09/11/1998 29/11/1998 | −6.92 −2.071 | 128.946 124.891 | 33 33 | 7.0 Mw 7.7Mw | 0.7 | 20 08:32 | 701.9 |
67 | Papua New Guinea | 29/10/2000 16/11/2000 16/11/2000 17/11/2000 | −4.766 −3.980 −5.233 −5.496 | 153.945 152.169 153.102 151.781 | 50 33 30 33 | 7.0 Mw 8.0Mw 7.8 Mw 7.8Mw | 1.0 0.2 0.0 | 17 20:17 00 02:48 01 13:19 | 215.4 173.5 149.1 |
68 | Peru | 23/06/2001 07/07/2001 | −16.265 −17.543 | −73.641 −72.077 | 33 33 | 8.4Mw 7.6 Mw | 0.8 | 13 03:05 | 218.8 |
69 | Japan | 25/09/2003 25/09/2003 | 41.815 41.774 | 143.593 143.593 | 27 33 | 8.3Mw 7.4 Mw | 0.9 | 00 01:18 | 4.5 |
70 | Loyalty Islands | 27/12/2003 03/01/2004 | −22.015 −22.015 | 169.766 169.683 | 10 22 | 7.3 Mw 7.1 Mw | 0.2 | 07 00:23 | 8.5 |
71 | Papua, Indonesia | 05/02/2004 07/02/2004 | −3.615 −4.003 | 135.538 135.023 | 16.6 10 | 7.0 Mw 7.3 Mw | 0.3 | 01 05:37 | 71.6 |
72 | Sumatra, Indonesia | 12/09/2007 12/09/2007 13/09/2007 | −4.438 −2.625 −2.130 | 101.367 100.841 99.627 | 34 35 22 | 8.4Mw 7.9 Mw 7.0 Mw | 0.5 0.9 | 00 12:39 00 03:46 | 209.9 145.7 |
73 | Papua, Indonesia | 03/01/2009 03/01/2009 | −0.414 −0.691 | 132.885 133.305 | 17 23 | 7.7 Mw 7.4 Mw | 0.3 | 00 02:50 | 55.9 |
74 | Kermadec Islands | 18/02/2009 19/03/2009 | −27.424 −23.043 | −176.330 −174.660 | 25 31 | 7.0 Mw 7.6 Mw | 0.6 | 28 20:24 | 515.3 |
---|---|---|---|---|---|---|---|---|---|
75 | Vanuatu | 07/10/2009 07/10/2009 | −12.517 −13.093 | 166.382 166.497 | 35 31.1 | 7.8 Mw 7.4 Mw | 0.4 | 00 00:55 | 62.2 |
76 | Chile | 27/02/2010 11/03/2010 | −37.773 −34.326 | −75.048 −71.799 | 35 18 | 7.4 Mw 7.0 Mw | 0.4 | 12 06:54 | 481.8 |
77 | Sumatra, Indonesia | 06/04/2010 09/05/2010 | 2.383 3.748 | 97.048 96.018 | 31 38 | 7.8 Mw 7.2 Mw | 0.6 | 32 07:44 | 190.0 |
78 | Papua New Guinea | 18/07/2010 04/08/2010 | −5.931 −5.746 | 150.590 150.765 | 35 44 | 7.3 Mw 7.0 Mw | 0.3 | I7 08:27 | 28.2 |
79 | Japan | 11/03/2011 11/03/2011 07/04/2011 | 36.281 38.058 38.276 | 141.111 144.590 141.588 | 42.6 18.6 42 | 7.9 Mw 7.7 Mw 7.1 Mw | 0.2 0.6 | 00 00:10 27 08:07 | 366.1 305.1 |
80 | Vanuatu | 20/08/2011 20/08/2011 | −18.365 −18.311 | 168.143 168.218 | 32 28 | 7.2 Mw 7.1 Mw | 0.1 | 00 01:24 | 9.9 |
81 | Sumatra | 11/04/2012 11/04/2012 | 2.327 0.802 | 93.063 92.463 | 20 25.1 | 8.6 Mw 8.2 Mw | 0.4 | 00 02:05 | 182.2 |
81 | Solomon Islands | 06/02/2013 06/02/2013 06/02/2013 08/02/2013 | −10.799 −11.183 −10.499 −10.928 | 165.114 164.882 165.588 166.018 | 24 10 8.8 21 | 8.0 Mw 7.1 Mw 7.0 Mw 7.1 Mw | 0.9 0.1 0.1 | 00 00:11 00 00:31 02 13:32 | 49.6 108.3 66.9 |
82 | Chile | 01/04/2014 03/04/2014 | −19.609 −20.570 | −70.7691 −70.4931 | 25 22.4 | 8.2 Mw 7.7 Mw | 0.5 | 01 02:57 | 110.7 |
83 | Solomon Islands | 12/04/2014 13/04/2014 | −11.270 −11.463 | 162.1481 162.0511 | 22.56 39 | 7.6 Mw 8.4 Mw | 0.8 | 00 16:22 | 23.9 |
84 | Mexico | 08/09/2017 19/09/2017 | 15.0678 18.5462 | −93.715 −98.487 | 69.65 51 | 8.1 Mw 7.1 Mw | 1.0 | 11 13:25 | 638.4 |
a) Segment BC is approximately 61.8% - 76.4% of segment AB;
b) Segment CD has approximately a ± 0.5 units magnitude compared to point B.
The closer the C point to point B (segment BC less than 50%) the greater is DEQ formation probability. Point E indicates the final magnitude values’ drop below the transition line up to a magnitude value greater than the one indicated by point C. Usually, this type of pattern takes a few minutes to complete, and we may assume that only one asperity breaks.
FM1 and MA1 patterns consisting of two EQ separated by multiple, minor shocks, have a high predictability ratio if we observe the “Butterfly” configuration (butterfly pattern) drawn by the shocks sequence between the two maxima.
“Butterfly” configuration (
・ Points 1 and 5, the first and the second earthquake;
・ Point 3, the second relative maximum that forms after point 1;
・ Points 2 and 4 the first and the second minima that form before and after point 3.
To define the configuration, we must connect the first maximum (point 1) with the first minimum (point 2) and then the latter with the second maximum (point 3) that confirms the point 2 previously identified. From point 3 the magnitude values begin to drop until they form the second minimum (point 4) and, finally, the point 5 will complete the “butterfly” pattern. It is possible to draw multiple, temporary “Butterfly” patterns between two separate EQ with the consequent formation of multiple points 3 of increasing magnitude. In this case, for the pattern construction we should consider the last point 3 formed.
Once defined, the “Butterfly” pattern allows obtaining the following information:
a) The higher the point 3 (above 50% level), the greater the probability that points 2 and 4 are increasing and the point 5 is greater in magnitude compared to point 1 (point 1 is a foreshock, while point 5 is a mainshock);
b) The lower the point 3 (below 50% level), the greater the probability that points 2 and 4 are decreasing and the point 5 is smaller in magnitude compared to point 1 (point 1 is a mainshock, while the point 5 is an aftershock);
c) Usually, between points 1 and 2 and 3 and 4, the minimum magnitude values have a decreasing trend, while trend is increasing between points 2 and 3 and 4 and 5;
d) The minimum magnitude value of the second maximum is given by point 1 and 3 average magnitude values (point 6);
e) The second maximum highest magnitude value is empirically calculated by adding to (if points 2 and 4 are upwards) or subtracting from (if points 2 and 4 are downwards) the first maximum magnitude value, the magnitude difference between points 2 and 4;
f) The activation signal coincides with point 4 (red triangle);
g) Usually, the area formed by points 1, 2 and 3 (pattern’s left sector) is larger if minima 2 and 4 are upward, while it is smaller if the contrary happens;
h) Second DEQ earthquake is more frequently of “Flash Earthquake” type (i.e., foreshocks do not precede it);
i) Often some minor shocks that occur before DEQ are close to their epicenters (
This pattern takes longer to complete (from a few hours to several days) and, given its asymmetry highlighted by point 3 position, we may assume the breaking of many asperities, faults or different rocks.
The second maximum’s magnitude values range within the “Butterfly” pattern can be estimated by the following procedure as well:
M 1 = P M + 0.618 ⋅ ( M P 1 − M P 2 ) (1)
M 2 = P M + 0.382 ⋅ ( M P 1 − M P 2 ) (2)
where,
M1 = magnitude value of the range upper limit;
M2 = magnitude value of the range lower limit;
P M = ( M P 1 + M P 2 + M P 3 ) / 3 (3)
MP1, MP2, MP3 = magnitude values of points 1, 2 and 3 in the pattern;
0.382 and 0.618 are Fibonacci levels statistically achieved by the magnitude values of the second event in the pattern.
The procedure we present here can also be used to assess the magnitude value of the second earthquake (point D) in patterns FM2 and MA2 by considering the following points:
MP1 = point B magnitude value;
MP2 = point A magnitude value;
MP3 = point C magnitude value.
173.5 km distance from the first. The third EQ occurred with 1 days 13 hours 19 minutes delay and at a 149.1 distance km from the second.
The longer delay observed in third EQ compared to the first is also highlighted by the higher number of shocks occurred. After the first seismic event, we observe a decrease in magnitude values up to almost 100% (
After the initial 8.0 Mw magnitude earthquake, we observe a SE-oriented migration of subsequent shocks epicenters (red circles), where we see a higher concentration of seismicity in a region surrounding the second 7.8 Mw magnitude EQ (yellow circle). During the aftershock phase, we note that some epicenters (magenta-colored) are arranged around the epicenter of the third 7.8 Mw event.
This information suggests that the most complex DEQ e MEQ patterns can be related to multiple asperities having different sizes and thickness, located along the same or adjacent faults.
In fact, along the faults with large asperities having similar thickness, the breaking of one of them leads to an increasing stress in the adjacent ones by triggering their breaking and hence the formation of distinct, but similar earthquakes pairs [
In this paper, we used a graphic and statistical approach to classify double and multiple earthquakes and identify the activation signals that allow collecting information about the time of the second earthquake, whose magnitude is comparable to the first, may happen.
The observations made on magnitude values fluctuations over time, underline that FMA pattern allows identifying the energy release phase closure as well as the mainshock compared to that of other major earthquakes in the pattern.
In some seismic sequences, FMA pattern consists of two or more earthquakes of magnitude comparable: we may classify them depending on the number of lower magnitude shocks that separate them.
The detailed analysis of 1237 DEQ occurred across the world show that, as a rule, an earthquake within the FMA pattern can trigger a second large event close in time and space.
The results we obtained show that a 0.5 units magnitude difference, a spatial separation not exceeding 100 km and 10 days time separation are DEQ’s most common characteristics.
In different types of DEQ ,we noticed that as the number of shocks between the first and the second event increases, even the distance between the events basically increases (the earthquake pairs are most likely associated with the breaking of several asperities along the same or adjacent faults), while in the absence of a shock or a few shocks between DEQ, the distance between events essentially decreases (earthquake pairs are most likely associated with repeated breaking of the same asperity or fault).
Besides, in DEQ separated by several lower magnitude shocks, a “Butterfly” pattern is formed, which allows obtaining early information on the second earthquake pairs’ magnitude and when this will happen.
Patterns consisting of more than two earthquakes with comparable magnitude (MEQ) differ from basic FMA pattern, and probably they are formed in certain complex tectonic areas where crustal asperieties having different sizes and thickness, may be several.
Usually, these patterns result from a mainshock and two or more foreshocks/aftershocks having similar magnitude although differently spaced in time and space.
Large earthquakes with comparable magnitude are not rare and in some areas could represent an underestimated risk.
In our approach, we believe that the position of the trigger point immediately generated after the first earthquake in some types of DEQ, is the most hazardous point and, therefore, its identification is crucial to reduce the risks that rescue teams are exposed to if the following earthquake features a comparable seismic energy.
Riga, G. and Balocchi, P. (2018) Double Earthquakes Classification and Seismic Precursors. Open Journal of Earthquake Research, 7, 1-27. https://doi.org/10.4236/ojer.2018.71001