DESCRIPTION:

The 17th November ML6.0-Mw 6.4 (NOA) event occurred, as the result of strike slip faulting on or near the regional plate boundary between Nubia (Africa) and Eurasia plates, along a N20±5°E strike-slip fault with right-lateral sense of slip at 11 km of depth. Nubia converges with Eurasia at a rate of 9mm/yr towards the north-northwest. Nubian lithosphere subducts beneath the Aegean Sea along the Hellenic Arc, to the south of Crete and southeast of the November 17 earthquake. Relocation of seismicity and preliminary inversion of geodetic data suggest that the fault plane dips to east with an angle of about 70±5 degrees.

 

 

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

 

 

The mainshock has been followed by a magnitude 5.3 earthquake at 8:33 UTC.

The Lefkada earthquake occurred in an area that is considered as among the most active tectonic areas in Europe and one of the most active zones in the eastern Mediterranean region.

 

 

 

ANALYSES:

 

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

The Catalogue data analysis started from January 2010 with a maximum distance of around 620 km from the epicentre. Figure 2 shows results from AMR and R-AMR analyses.For this event R-AMR was not succeed in disclosing any acceleration in seismicity preceding the Lefkada mainshock.

 

Figure 2: AMR (left) and R-AMR (right) results.

 

 

  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 Lefkada 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 Lefkada EQ.

 

Figure 3:Example of anomalous track in Y magnetic component (Satellite A- October 28, 2015).

The cumulative number of anomaly tracks detected by MASS one month before and after the Lefkada EQ (threshold  kt= 2.0) is shown in Figure 4. In this case, cumulate plot is not compatible with an S-shape behavior, but some interesting aspects are present, like an activation 20 days before EQ. The general behavior is difficult to analyse as the disturbed days can obscure small effects in ionosphere produced by an eventual lithosphere-ionosphere coupling.

 

 

Figure 4: Cumulative number of anomaly detected by MASS one month before and one month after the M6.5 Lefkada EQ. Threshold is kt = 2.0  and the anomalies are selected only with geomagnetic quiet time (|Dst| ≤ 20 nT and ap≤ 10 nT). Only one  anomaly track is found in scalar intensity F of magnetic field.

 

 

  1. Wavelet Analysis

The Wavelet spectral analysis has evidenced the existence (in general) of anomalous families, each characterized by some features that altogether do not clarify whether they are linked to LAIC or not.

The DbA of this case lays approximately at the same latitude of the Japan 2016 event, so the analysis puts in evidence that they have in common a similar type of anomaly.

In Figure 5 a sequence of short-life and large amplitude signals appears just in correspondence of the epicenter, a fact that could suggest a possible lithospheric origin. However, by confronting the anomaly in figure (a) and (b) with that in (c), it should be noted that this latter is outside the DbA.

 

 

 

 

Figure 5: This sequence of figures shows a large and short live anomaly just at epicentral latitude: tracks in panel(a) and (b) pass through the DbA and so they could thought as being related to seismic activity. However, panel (c) shows the same feature outside the DbA, leaving the LAIC question unresolved.

 

Even in these cases, outside the DbA, nothing relevant emerges from wavelet analysis results.

 

 

  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

No tagged anomalies were found for this event by means of the tagging criteria applied on the NeSTAD anomaly parameters.

 

  1. Method II: NeLOG

The automatic search NeLOG of ionosphere electron density anomaly from Swarm data is applied to Lefkada M6.5 case study. A sample is classified as “anomaly” if the residual value exceeds a threshold ktof the RMS in the residuals from the fit. A track is selected as “anomaly” if it has more than 10 anomaly samples in DbA and if geomagnetic indices are |Dst|<20 nT and ap<10 nT. Tracks are selected within a mean local time between 22 and 6.

Figure 6 shows an example of anomaly track detected by NeLOG for the Lefkada EQ, while Figure 7 reports the cumulative number of anomaly tracks detected by NeLOG one month before and one month after the event.

 

Figure 6: An anomalous track identified by NeLOG (Sat Charlie 19 October 2015) before M6.5 Greece (Lefkada) 17 November 2015 EQ.

 

 

Figure 7:Cumulative number of anomaly tracks detected by NeLOG one month before and one month after M6.5 Lefkada (Greece) EQ. Threshold is kt = 2.0, the anomalies are selected only with geomagnetic quiet time (|Dst| ≤ 20 nT and ap≤ 10 nT) and in night time (22 ≤ LT ≤ 6).

 

  1. IONOSPHERE Ground-based

Ionosondes& GNSS

For this event, no suitable Ionosonde data were available. For GNSS analysis, the period before the event was characterized by high geomagnetic activity and not useful for the SAFE scope.

 

 

CONCLUSIONS:

The M6.5 Lefkada EQ was analyzed using both Swarm geomagnetic and ionospheric data searching for earthquake-related anomalies in the frame of LAIC theory.

Considering MASS method, the cumulate plot is not compatible with an S-shape behaviour, but some interesting aspects are present, like an activation 20 days before EQ. R2 factor is good, but also this could be an effect of quiet time selection and not of the physics of the system.

As regards ionospheric data analyses, NeSTAD method did not found any tagged anomalies for Lefkada M6.5 EQ. On the other hand, NeLOG results show that The R2factor is good, but, as for MASS, this could be an effect of the magnetic quiet time selection. After normalization process nEQ has a bad value, in contrast to C factor. The good normalized C factor is a positive condition to think that anomalies have internal source, so they could indicate a possible LAIC effect.