DESCRIPTION:

The Mw 7.8 Gorkha (Nepal) earthquake struck Nepal on April 25, 2015 at 06:11:26 UTC with an epicentre at about 80 km northwest of Kathmandu, the capital of Nepal, in the Himalaya Arc. The hypocenter was at a focal depth of 10-15 km.

 

Figure 1: Epicentre of the M7.8 Nepal (Gorkha) 2015 EQ. The projected red circle represents the limit of the DbA.

 

This event occurred on a thrust fault between the subducting India plate and the overriding Eurasia plate to the north. At the epicentral area, the India plate is converging with Eurasia at a rate of 45mm/yr towards the north-northeast.

A large number of aftershocks followed the mainshock but according to the NEIC, ISC and NSC catalog was not preceded by any foreshocks within ~3 weeks. Aftershocks are propagated from west to east in an area of about 130 km to the east of the epicenter at a strike angle of 295° .Most of the aftershocks occurred at a shallow depth within 15 km of depth.

Among the largest aftershocks, the May12, 2015 M7.3 earthquake at about 150 km to the east.

 

ANALYSES:

 

  1. SEISMOLOGY

 

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

The Accelerating Moment Release (AMR) idea was developed by Bowman et al. (1998) and is based on the hypothesis that prior to a large earthquake the stress field in the vicinity of the next rupture increases in such a way that one can observe an acceleration of the background seismicity following a power-law function. The proposed revision of the AMR method is based on the introduction of a “reduced” Benioff strain for the earthquakes of the seismic sequence where, for the same magnitude and after a certain distance from the main-shock epicentre, the closer the events the more they are weighted.

The USGS Catalogue analysis for Nepal event starts from January 2010 and data are extracted around the epicentre of the main-shock with a radius of 2200km (the corresponding DbA). AMR applied of this Catalogue shows a large event in 2013.

 

Figure 2: Sequence of trials for seeking the R-AMR optimum set of parameters for Nepal region. The upper figures result from a decrease of the maximum radius (green border).

 

The application of the R-AMR methodology provides the best results in detecting the precursory seismic acceleration, when compared with those found by ordinary AMR technique, especially when using USGS catalog.

 

 

  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 M7.8 Nepal 2015 earthquake with different thresholds, while the moving window was fixed at 3.0°. The algorithm analyses all tracks in DbA one month before and one after the EQ. The tracks are marked as “anomaly” only if the center 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 Nepal EQ.

 

Figure 3:Example of anomalous track in Y magnetic component (Satellite A- May 25, 2015).

The cumulative number of anomaly detected by MASS one month before and after theM7.8 25 April 2015 Nepal EQ (threshold kt= 3.25) is shown in Figure 4. In addition, we observe a change in the slope of the cumulative number of anomalies shortly after the EQ, which can be a good indication for a possible post-seismic LAIC effect.

 

 

Figure 4: Cumulative number of anomaly detected by MASS one month before and one month after M7.8 25 April 2015 Nepal EQ. Threshold is kt = 3. 25, the anomalies are selected without geomagnetic index selection.

 

 

  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.

 

 

Figure 5: The panels  (a) and (b) show adjacent tracks on the day of the mainshock. The frequency content shows a short period wave appearing for little time. The signal detected just over the epicenter seems to be a remnant of the preceding wave and nothing reliably attributable to LAIC.

 

The short period wave emerging in Figure 5(a) seems appearing in Figure 5(b) and no other anomaly is recognizable. On the contrary, the anomaly in 6(a) and 6(b) lasts for at least one hour and a half increasing its frequency content. However, approximately the same wave appears two days later outside DbA. Thus, we cannot say nor exclude its lithospheric origin.

 

 

Figure 6: The upper (a) and middle (b) panels show two adjacent tracks passing through the DbA where a persistent anomaly appears showing a slight southward direction together with a frequency content increase.

 

 

  1. SWARM – IONOSPHERE from Satellite

. Satellite-based data for the ionospheric characterization of the EQ 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 of the EQ events. 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. In particular, LP and IBI data available in the period from 26 March to 26 April 2015have 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=0.85 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 morning tracks have been selected (02-06 LT), to remove the impact of the equatorial fountain during the day and to minimize the impact of the plasma bubble formation during night-times
  • Quiet ionospheric conditions (absolute value of Dst in the considered day not exceeding 20 nT).

An example of tagged anomaly is provided in figure 7.

 

Figure 7: Identified anomaly with the NeSTAD algorithm and tagged as interesting for the M7.8  Nepal EQ event. Tagged anomaly refers to Swarm Alpha satellite on 23 April 2015.

 

 

Figure 8: - Cumulative number of anomalies identified through the NeSTAD algorithm for Swarm satellite Alpha.

 

Figure 8 shows the cumulative number of the tagged anomalies through the NeSTAD algorithm for Alpha satellite. The black dashed line indicates the day in which the Nepal M7.8 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 morning time.

 

  1. Method II: NeLOG

The automatic search NeLOG of ionosphere electron density anomaly from Swarm data is applied to Nepal M7.8 case study.A sample is classified as anomaly if the residual value exceeds a threshold kt times the RMS of the residual after the spline fit (in this case kt=3). A track is selected as an “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.

 

 

Figure9: Anomaly detected by NeLOG (satellite Alpha, 24 April 2015)immediately before the Nepal M7.8 EQ.

It is very interesting that around the EQ the shape of the cumulative number of anomaly tracks is highly compatible with a sigmoid and all the evaluation factors (R, nEQ, C) give good indication for possible LAIC effect.

 

 

Figure10: Cumulative number of anomaly samples detected by NeLOG one month before and one month after M7.8 25 April 2015 Nepal EQ. Threshold is kt = 3.0, the anomalies are selected only with geomagnetic quiet condition.

 

  1. IONOSPHERE Ground-based

Ionosondes& GNSS

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

 

CONCLUSIONS:

Different approaches have been used to study the M7.8 Nepal EQ event searching for earthquake-related anomalies in the frame of LAIC theory.

Considering MASS algorithm, three disturbed time-windows in terms of geomagnetic condition have been identified in the analyzed period, so the analysis were performed using weak constraints on magnetic indexes or without. For this event, the change in the slope of the cumulative number of anomalies shortly after the EQ, which can be a good indication for a possible post-seismic LAIC effect.

For NeSTAD algorithm the cumulative number of anomalies obtained by tagging procedure on the three Swarm satellites allowed identifying a meaningful increase of the cumulative frequency in the proximity of the EQ for Alpha and Charlie. It is interesting also to note how the slope of the cumulative frequency curve presented a steep change right after the EQ. Such behavior seems to be in good agreement with the sigmoid law as in the critical point theory.

On the contrary, wavelet analysis for Nepal EQ does not help in clarifying the origin of the found anomalies; the discrimination of the source of emerging signals from the background by applying this method alone may need the adoption of some other more complex scheme.