DESCRIPTION:

 

 

Figure 1: Location of the EQ epicenter and Dobrovolsky area.

 

The highest rates of seismicity in the Mediterranean region are found along the Hellenic subduction zone of southern Italy.

The main event was followed by a rich aftershock activity, with 94 aftershocks with magnitude range 1.7- 5.2 occurred in the first 4 days.

The majority of the aftershocks are forming an almost N-S trending cloud between Kasos and western Crete extend in depth from 20 to ~40 km.

 

 

ANALYSES:

 

  1. SEISMOLOGY
  2. Accelerated Moment Release (AMR) & Revised-AMR

The Catalogue data analysis started from January 2011 with a maximum distance of around 420 km from the epicentre. Figure 2 shows results from AMR and R-AMR analyses.The effect of the south-west seismic sequence of 2013 is visible in the pronounced jump (highlighted by the red oval). However, it is also evident that R-AMR analysis confirms the findings that no acceleration preceded the main rupture.

 

 

Figure 2: The Reduced Cumulative Strain for 5 years before the mainshock. With the exception of a sequence in 2013 (red ellipse), no significant acceleration appears before the mainshock.

 

 

  1. SWARM – GEOMAGNETISM

Two different approaches were developed to search EQ precursor using geomagnetic Swarm data: MASS method and Wavelet method. Both analyses are based on Level 1B MAGxLR Swarm product.

 

  1. MASS algorithm (Magnetic Swarm anomaly detection by Spline analysis)

The algorithm MASS (Magnetic Swarm anomaly detection by Spline analysis)was applied to Crete 2015 earthquake with different thresholds, while the moving window was fixed at 3.0°. The algorithm analyses all tracks in DbA (Dobrovolsky Area) one month before and one after the EQ. The tracks are marked as “anomaly” only if the centre of the moving window is in DbA and if geomagnetic conditions are quiet. Figure 3 shows an example of anomalous tracks detected by MASS method for Crete EQ.

 

Figure 3:Example of anomalous track in Y magnetic component (Satellite A- April 24, 2015).

The cumulative number of anomaly tracks detected by MASS one month before and after the Crete EQ (threshold  kt= 2.0) is shown in Figure 4. MASS algorithm has not detected any anomaly before EQ and it has detected only a few anomaly tracks after EQ. So the method fails in the searching of possible EQ-related precursor for this event.

 

 

Figure 4: Cumulative number of anomaly detected by MASS one month before and one month after the M6.1 Crete EQ. Threshold is kt = 2.0  and the anomalies are selected only with geomagnetic quiet time (|Dst| ≤ 20 nT and ap≤ 10 nT).No anomaly tracks are found in scalar intensity F of magnetic field.

 

 

  1. Wavelet Analysis

 

For this specific event the DbA is very small and there are very few tracks close to this area. The analysis does not show any evidence related to LAIC.

 

  1. SWARM – IONOSPHERE from Satellite

Satellite-based data for the ionospheric characterization of the EQ-related events are mainly those referred to the LP (Langmuir Probe) instrument aboard the SWARM satellites. The electron density Ne is the relevant parameter used for the ionospheric characterization in the frame of SAFE project. Two different methods were developed to analyze Swarm ionospheric data: NeSTAD and NeLOG.

 

  1. Method I: NeSTAD

The NeSTAD analysis has been applied to the Swarm constellation data (LP and IBI data )available in the period from 17 March to 16 May have been used to derive the track anomaly parameters.The NeSTAD has been initialized with the mild outliers mode (k=1.5) and with an “excess area” parameter equal to 0.1.

Then, to tag the interesting track anomalies for this particular event, the following criteria have been applied:

  • R>Rthr=0.7 and standard deviation of the filtered track below σthr=0.1 or standard deviation of the filtered track above σthr=0.1 independently of R value.
  • Only night time tracks have been selected (18 to 06 LT), because during night time and at mid latitudes the ionosphere is expected to be less turbulent.
  • Quiet ionospheric conditions (absolute value of Dst in the considered day not exceeding 20 nT).

An example of tagged anomaly is provided in figure 5

 

Figure 5: Identified anomaly with the NeSTAD algorithm and tagged as interesting for the Crete EQ event. Tagged anomaly refers to Swarm Bravo satellite on 31 March 2015.

 

 

Figure 6: - Cumulative number of anomalies identified through the NeSTAD algorithm for Swarm satellite Bravo.

 

Figure 6 shows the cumulative number of the tagged anomalies through the NeSTAD algorithm for Bravo satellite. The black dashed line indicates the day in which the Crete M6.1 EQ event occurred. The red boxes indicate the days in which disturbed geomagnetic conditions have been recorded and that are not included in the tagging process. The two blue dashed lines indicate the time interval during which the given Swarm satellite covered the DbA of the event at nighttime.

 

  1. Method II: NeLOG

The automatic search NeLOG of ionosphere electron density anomaly from Swarm data is applied to Crete M6.1 case study, but no anomaly track have been detected in the DbA. Then the search for anomaly tracks was extended in a circular area of 10° radius centred on epicentre, and 18 anomaly tracks are extracted and represented as cumulative number in Figure 7.

 

 

Figure 7: Cumulative number of the electron density anomaly tracks detected one month before and one month after M6.1 Crete(Greece) 16/04/2015 EQ in a circular area of 10 degree radius centred on epicentre.

 

  1. IONOSPHERE Ground-based

Ionosondes& GNSS

For this event, no suitable Ionosonde and GNSS data were available.

 

 

CONCLUSION:

The M6.1 Crete EQ was analyzed using both Swarm geomagnetic and ionospheric data searching for earthquake-related anomalies in the frame of LAIC theory.

This event has the lowest magnitude among the 12 EQ considered, so this could be the reason for which different analyses fail studying this event.

In particular, MASS algorithm did not detect any anomaly before EQ but only few anomaly tracks after the event. The failure could be due to two reasons: the Crete 2015 earthquake is the one with the lowest magnitude among the 12 events studied; in addition, the possible coupling effect between lithosphere and ionosphere could be masked by the disturbed geomagnetic conditions in the period around the earthquake.

Moreover NeLOG algorithm found 21 tracks with geomagnetic quiet conditions and in Dobrovolsky area, none of these tracks has more than 10 anomalous samples. Extending the search to a wider geographic area (radius = 10 degrees) some anomalies are detected, but the shape seems to be compatible with random effect (linear shape) so this method fails analyzing this event.

Finally, the cumulative number of anomalies derived by tagging procedure on the NeSTAD output allowed identifying anomalies before the event for Bravo and anomalies after the event for Alpha and Charlie. Also in this case, according to the Dst criterion used by the NeSTAD algorithm, many days around the event have been found to be disturbed, limiting the possibility to identifying any anomaly possibly related to LAIC in the proximity of the EQ.