DESCRIPTION:

The M6.3 Spain EQ occurred on 25 January 2016 at 4:22:02 UTC in the Alboran Sea near the Strait of Gibraltar.

 

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

 

This earthquake has been preceded by a magnitude 5.0 foreshock on Thursday 21 January at 13:47 Morocco time. The mainshock has been followed by several aftershocks, many of them have been also felt in Spain and in Morocco. Two other recent destructive earthquakes in 1994 and 2004 near Al Hoceima highlight that the northern Moroccan margin is one of the most seismically active regions of the Western Mediterranean area (d'Acremont et al., 2014). Indeed, the mainshock occurred 50 km north of the 2004 M6.3 Al Hoceima earthquake (24 February 2004) that caused 630 victims.

 

ANALYSES:

 

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

The Catalogue data analysis started from January 2011 with a maximum distance of around 510 km from the epicentre. Figure 2 shows results from AMR and R-AMR analyses.R-AMR result shows a tendency in line with the sparse seismicity of the area (Figure 2, right): an almost flat behaviour until the last event (M5.0 on 21 January, red circle in figure) which, being close to the mainshock epicenter, contributes largely to the cumulative strain in such a way to sudden increase its value.

 

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 M6.3 Gibraltar 2016 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.

At threshold kt=2 or greater no anomalies were found.

Figure 3 shows an example of track analyzed by MASS method for Gibraltar EQ.

 

 

Figure 3:Example of track for Satellite A (January 10, 2016).

 

  1. Wavelet Analysis

As in the preceding Crete 2015 case, i.e. magnitude less than 7, the DbA is very small and the tracks close to it are few. The analysis does not evidence something relevant 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

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 M6.3 Gibraltar case study. A sample is classified as “anomaly” if the residual value exceeds a threshold kt=2.5 of 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 4 reports the cumulative number of anomaly tracks detected by NeLOG one month before and one month after the event.

 

 

Figure 4:Cumulative number of anomaly tracks detected by NeLOG one month before and one month after M6.3Gibraltar EQ. Threshold is kt = 2.5, the anomalies are selected only with geomagnetic quiet time (|Dst| ≤ 20 nT and ap≤ 10 nT) and in night time (22 ≤ LT ≤ 6).

The cumulate plot in Figure 4 is compatible with S-shape behavior, presenting an increment of the anomaly tracks just after the EQ. In addition, the lack of anomalies around 45 days is due to perturbed geomagnetic conditions, but this does not alter the shape of the cumulate around EQ.

 

  1. IONOSPHERE Ground-based

Ionosondes

By means of the analysis on the ionosonde data (El Arenosillo Ionospheric Observatory, Latitude: 37.1° N, Longitude:  6.7° E), one single anomaly has been identified on21 December 2015 at 14 UT, during which a geomagnetic storm occurred. In fact, bottom panel Figure 5 shows the ap of the recorded on 21 December 2015, that was larger than 100 in the first part of the UT day.

 

Figure 5: The ionospheric anomaly of the 21.12.15 using observed Δh’Es, δfbEs, and δfoF2 variations and 3-hour ap.

The considerations reported above suggest that the identified ionospheric anomaly is related to the effect in the ionosphere recorded at ground induced by geomagnetic storm and not to LAIC.

 

 

GNSS

For GNSS analysis, the period before the event was characterised by high geomagnetic activity and not useful for the SAFE scope.

 

 

CONCLUSION:

The M6.3 Gibraltar EQ was analyzed using both Swarm geomagnetic and ionospheric data searching for earthquake-related anomalies in the frame of LAIC theory.

Considering NeLOG method, in the first step of analysis a track has been here classified as anomalous if it has 5 or more anomaly samples (and not 11 or more as in other cases), because of the low magnitude (M6.3) of this event. TheR2 value obtain relaxing this constraintunderlines the discrepancy with respect to a linear fit, that is good indication for a possible correlation of the anomaly with the EQ. nEQ and C factors after the normalization are equal to 1, so they are not relevant. Then the analysis was repeated tagging a track as “anomaly” if it has at least 10 anomaly samples, but the comparison between the two approaches show they are equivalent.

MASS and NeSTAD method did not found any anomalies for this event, while ionosonde data analysis suggests that the detected anomaly is due to high geomagnetic activity and not to possible LAIC effect.