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IntroductionEarthquakes have been felt in and around Johannesburgsince around 1908, about twenty years after gold miningstarted in this region (Gane et al., 1946). These tremorsare regularly felt in and around Johannesburg but withdifferent shaking strengths. However city residentsusually do not consider them a threat, since noimportant structural damage has yet been reported,although ground amplitudes of up to 300 microns havebeen observed (Gane et al., 1946).
In late 2013, residents of Gauteng and other nearbyprovinces were shaken by two moderately sizedearthquakes (Figure 1) resulting in widespread panic.On the morning (07:53 hours GMT) of 18 November2013, an earthquake of local magnitude, ML3.6 occurredin the Central Rand (Figure 1) and shook most of theJohannesburg area. The Council for Geoscience (CGS)gave the epicentre of the earthquake at coordinates,26.212°S and 27.920° E with errors of 3 km in longitudeand 2.5 km in latitude. Though the earthquake was feltwidely in the city, no injuries to the local population orserious damage to structures was reported. A secondearthquake of magnitude ML 3.9 occurred on 2 December 2013 at 19:18 hours GMT in the Bela-Belaarea, Limpopo Province, South Africa. The Council forGeoscience (CGS) located it at an epicentral location of 25.020°S and 28.456°E (Figure 1) about 20 km east ofBela-Bela. The error in the location for this earthquake
was higher as the location was determined using thesparsely distributed South African National SeismographNetwork, with an error in the longitude of 15 km and 6km in latitude. The shaking from this earthquake wasalso felt widely in the Limpopo, Mpumalanga, NorthWest and Gauteng Provinces. Though the earthquakewas felt widely, no injuries to the local population orstructural damage were reported.
The occurrence of such earthquakes is of concern tothe residents, government and other stakeholders (e.g. municipalities, insurance industry, constructionindustry, etc.) because repeated occurrence of suchearthquakes can result in damage to structures and eveninjury or loss of human life. Larger earthquakes in thesame areas could certainly result in more severe effects.Therefore, understanding the cause and effects of theseand similar earthquakes is essential to the managementof seismic risk in the country. To this end, the CGSembarked on an investigation of the effects of theseearthquakes as well as research to obtain information tohelp in understand their faulting mechanisms.
SeismicityThe monitoring of seismic activity linked to gold miningin the Gauteng region began with the installation of a200 kg Wiechert seismograph station at the UnionObservatory in 1910 (Gane, 1939, Gane et al., 1946).Since then, several institutions have over the years
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deployed seismograph stations throughout the countryfor different purposes. In 1970, a permanent network ofseven seismographs was deployed by the GeologicalSurvey (now the Council for Geoscience) following the29 September 1969 Ceres earthquake. These sevenstations collectively formed the South African National Seismograph Network (Saunders et al., 2008). The national network has since expanded and currentlyconsists of 30 stations. In addition, three clusters ofstations are being used to monitor seismicity in theWitwatersrand basin. Generally, the seismicity ofSouthern Africa is moderate, at a shallow depth and inmost cases difficult to correlate with geological features.This scattering of seismic foci is similar to the diffusepattern observed for intraplate regions around the world.
Seismicity in Gauteng is dominated by mining relatedearthquakes (Figure 1). Such earthquakes canoccasionally reach moderate magnitudes (ML 4 to 5). For instance, in 2005, a magnitude, ML5.3 earthquakeoccurred in Stilfontein resulting in the deaths of twominers, many injured underground, 58 people withminor injuries from damage to buildings on the surfaceand extensive property damage (Durrheim, et al., 2006).
Since 2009, the Council for Geoscience has beenmonitoring the seismicity within the abandoned miningregions of Johannesburg using a cluster network of 12 stations separated on average by a distance of 5 km.Currently ingress of water into the abandoned mines inJohannesburg has resulted in an increase in seismicitywithin the region, and hence increasing the risk to thepopulation. Thus, the CGS uses the cluster network toalso monitor the seismic effects of the water ingress(Cichowicz et al., 2010).
Seismicity in and around Bela-Bela is very differentfrom the Gauteng region and is characterised bygenerally diffuse minor earthquakes that do notnormally exceed local magnitude, ML 3.0 (Figure 1). The 2 December 2013 earthquake is one of the very fewmoderate earthquakes that have occurred in this area.Prior to this earthquake, the most recent earthquakerecorded in the region by the CGS was the 16 September2011 earthquake of ML 2.5. The biggest earthquake tooccur in Limpopo Province was the 5 October 1940Tzaneen earthquake of local magnitude, ML 5.0.However, no macroseismic information exists for thisearthquake. The source of the 2 December 2013earthquake is uncertain, though it is located close to the
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Figure 1. Seismicity map showing earthquakes (magnitude greater than 4.0 in Gauteng region) for the period between 1970 and 2013.
The location of the 18 November 2013 and 2 December 2013 earthquakes is also shown in the figure by the yellow stars and labels. The inset
is a map showing the location of the study area in South Africa.
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Warmbath fault, which has similar orientation to the faultplane solution obtained in this study.
The oblique nature of the collision between theZimbabwe craton and the Kaapvaal craton is believed tohave initiated or re-activated major transcurrent faultsystems in the region resulting in major structures suchas the Thabazimbi-Murchison lineament (Singh et al.,2011). Such faults are currently being reactivated by thepresent day stress field as can be seen by the scatteredseismicity located in and around the faults. Minor faultsthat also appear to be active include the Warmbath fault(Figure 2). Almost the entire region is associated with amoderate level of seismicity which cannot be associatedwith the sporadic mining activity in the area, thusproviding proof of seismogenic faults in the area.However, as observed for the 2 December 2013earthquake, it is difficult to conclusively associate theearthquakes with the known faults in the area.
Macroseismic survey and data analysisMacroseismic intensity data play an important role in the seismological, engineering and loss modelingcommunities (Midzi et al., 2013). They provide the muchneeded and more often unavailable information forconstraining the location and magnitude of historicalearthquakes as well as the reconstruction of shakingdistributions. The data can also be useful in the selection
of appropriate ground-motion prediction equations. This is done by either comparing intensity values toother regions of similar tectonics (e.g. Bakun andMcGarr, 2002; Allen and Wald, 2009) or by directcomparisons of intensity values to ground motionpredictions of peak ground acceleration and responsespectral ordinates (e.g. Scherbaum et al., 2009; Delavaud et al., 2009). In South Africa, Midzi et al.(2013) compiled an intensity database containing 57 earthquakes using the Modified Mercalli Intensityscale, MMI-56 (Richter, 1958).
Following the 18th November and 2 December 2013 earthquakes, the CGS conducted macroseismicsurveys to investigate the effects of the earthquakes. The macroseismic observations were compiled from avariety of sources, primarily questionnaires that aresupplemented by social media reports such as Twitter aswell as newspaper reports.
The observations used in this analysis were obtained asfollows:• Questionnaires submitted online on the CGS website
by individuals who felt the earthquake;• Distribution of questionnaires by CGS scientists at
homes, shopping malls, workplaces and schools, inand around Johannesburg;
• Twitter messages and newspaper reports.
Figure 2. Major faults in the vicinity of the 18 November and 2 December 2013 earthquakes.
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Figure 4. Distribution of questionnaires analysed for the 2 December 2013 earthquake. The black star represents the epicentral location
of the 02 December 2013 earthquake.
Figure 3. Spatial distribution of observations analysed for the 18th November 2013 earthquake according to observation types. The black
star represents the epicentral location of the earthquake. Numbers in brackets in the legend represent the total number of observations of that
type collected.
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18 November 2013 earthquake observationsA total of 262 observations were collected from theabove mentioned sources and are distributed as shownin Figure 3. As expected, the distribution of questionnairesproduced the most number of observations. Despite thegenerally good response received by the scientists in the field from many respondents, quite a few peoplewere suspicious and were not interested in participating.In future such investigations need much betterpreparation beforehand, through advertisements in the media.
2 December 2013 earthquake observationsA total of 266 questionnaires were collected (Figure 4),with a total of 120 questionnaires completed online.People as far south as Sandton in Johannesburg, alsoreported feeling the shaking from the earthquake. The remaining 146 questionnaires were obtained fromthe distribution exercise. Despite the reluctance of manypeople in participating in the survey, enoughrespondents cooperated to make the analysis reliable.
Intensity Data Points (IDPs)The methodology followed to translate the collectedobservations into IDPs is essentially that recommendedby Musson and Cecic (2002) and implemented by Midziet al. (2013). The first step in the analysis of theobservations was to sort them according to places,where the places were defined as suburbs, towns ordistricts. The locations of the places were obtained usinga combination of different sources:
• Google Earth• The Geonames on-line database of the US National
All the individual intensity indicators per question in thequestionnaires for each place were summarised.Intensity values were then assigned for the sorted andgrouped observations by comparing the summary of theobservations for each place with the descriptions givenfor the intensity degrees in the MMI-56 scale. This wasdone by identifying which of the descriptions for thevarious intensity degrees best fits the sum of the datacollected for the particular place under consideration. As stated by Musson and Cecic (2002), it is important inthis process not to get lost in the pursuit of detail ofindividual diagnostics. The correct assignment is the onethat best expresses the generality of the observations.Following the process described above a total of 51 IDPswere created for the 18 November 2013 Earthquake(Figure 5) and 52 IDPs for the 2 December 2013earthquake (Figure 6). Although the 2 December 2014earthquake had a higher magnitude, the maximumintensity value observed was lower (Intensity level VI for
the 18 November earthquake compared to intensity level IV-V for the 2 December 2014 earthquake). In contrast the 2 December 2014 earthquake was feltover a wider area.
On comparing and analyzing the created IDPs, it wasobserved that in general, the IDPs for the 2 December2014 earthquake can be considered to be more reliableas they were created using many more observations thanthe IDPs for the 18 November 2013 earthquake (Figure 7). However, for both earthquakes, some of theIDPs cannot be considered to be reliable at all as theywere created using a single observation (eight singleobservation IDPs for the 18 November earthquake andseven for the 2 December 2014 earthquake). Those IDPscreated with many observations (e.g., 35 for Johannesburgand 18 for Roodepoort for the 18 November 2014earthquake; eight for Centurion, Randburg and Bela-Belafor the 2 December 2014 earthquake) are considered to bethe most reliable.
The distribution of intensity levels obtained for thefirst earthquake (18 November 2014) is shown in Figure 8, where most of the IDPs had intensity value ofI. Intensity level I refers to respondents who did not feelthe earthquake. However, quite a few moderate intensitylevels (IV to VI) were observed. The observeddistribution for the 2 December 2014 earthquake wasquite different, with shaking of maximum intensity valueIV-V, experienced at three places, Bela-Bela (20 km from the epicentre), Modimolle (about 30 km from the epicentre) and KwaMhlanga (about 50 km from theepicentre). Intensity level of IV was observed widely at26 places (Figure 9). The intensity values of IV were alsounexpectedly observed about 150km south in northernJohannesburg and southern Pretoria suburbs.
In trying to better understand the spatial distributionof the obtained IDPs, intensity-distance curves wereplotted (Figure 10). The value of these plots is that theyhelp to illustrate the importance of the collected data inunderstanding the attenuation of intensity levels, andhence attenuation of seismic waves in the region. The IDP distribution with distance for both earthquakes,thus intensity attenuation, is comparable to the shape ofthe Bakun and Scotti (2006) model, which was obtainedfor the French stable continental region. The decay ofintensity values with distance fits a consistent shape thatclearly shows the effect of attenuation on the groundmotion. Although the curve shows a smooth attenuationof intensity with distance, it was surprising that someobservers close to the 18 November 2013 epicentre (12 km) did not feel the shaking. The curve for the 2 December 2013 earthquake (Figure 10b) shows whatappears to be amplification of shaking at somedistances, 150km away in northern Johannesburg which might be due to site effects. This apparentamplification of ground motion is discussed further inthe next section. The investigation was carried out to tryand identify the possible cause of the observedamplification of the ground motion in northernJohannesburg.
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Figure 5. The IDP distribution for the 18th November 2013 earthquake. The location of the earthquake is also shown in the figure by a
black star.
Figure 6. The spatial distribution of IDPs created for the 02 December 2013 earthquake. The location of the earthquake is also shown in the
figure by a black star.
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Figure 7. Number of observations used to create each IDP, (a) 18 November 2013 data and (b) 2 December 2013 data.
a
b
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Figure 8. Number of IDPs for each intensity level (18 November 2013 Earthquake).
Figure 9. Number of IDPs for each intensity level (2 December 2013 Earthquake).
Site effects on ground shaking (2 December 2013earthquake)Seismic site effects denote a set of different physicalphenomena arising from the propagation of seismicwaves in near-surface geological formations or ingeometrically irregular configurations at the earthsurface itself such as canyons, ridges and hilltops(Faccioli et al., 2002). These irregularities tend toproduce an increase in the amplitude of the groundvibrations generated at the site by the passage ofearthquake waves. A number of cases have beenobserved where the recorded ground motions andobserved earthquake damage point towards topographicamplification as the cause (e.g. the 1887 Western Liguriaearthquake (Faccioli et al., 2002), the 1971 San Fernandoearthquake (Boore, 1972), the 1983 Coalinga earthquake(Celebi, 1991), the 1985 Chile earthquake (Celebi, 1987),the 1987 Superstition Hills earthquake (Celebi, 1991), the1994 Northridge earthquake (Ashford and Sitar, 1994)and Egio earthquake (Athanasopoulos et al., 1999)).
One of the first studies to specifically consider theseismic response of soil slopes was conducted by Idrissand Seed (1967). Using the north to south component ofthe 18 May 1940 El Centro seismogram, they found thatthe magnitude of the peak surface acceleration was in allcases greater at the crest of the slope than at pointslower on the slope. However, when comparing the peaksurface acceleration at the crest to that at some distancebehind the crest, they found that in some cases theacceleration at the crest was much greater than the acceleration at a distance. Faccioli et al., (2002)proposed the following as the typical causes of theamplification of seismic waves on topographicirregularities:• a focusing (defocusing) phenomenon, due to
incidence of the waves on a locally convex (concave)surface profile, similar to those predicted bygeometrical optics.
• a dynamic phenomenon that can produce the resonantmotion of a whole hill, or mountain, if the wavesincident at the base meet certain requirements.
Boore (1972) studied topographic effects using thefinite-difference method and found amplification ofacceleration of about 2 at the crest and varying amountof amplification and attenuation along the surface of the slope from the crest to the base. Geli et al. (1988)arrived at a similar conclusion when they used a moredetailed model configuration using a layered profile and introduced nearby ridge effects. They also foundthat neighboring ridges may have a greater effect on site response. Athanasopoulos et al. (1999) studiedtopographic amplification by analysing a simplified 2-D profile of the Elgio town in Greece and found thatthe step-like topography greatly modifies the intensity ofmotion without affecting its frequency content. Pedersenet al. (1994) also studied local and regional earthquakedata from Greece by conducting analysis in thefrequency and time domain. In the frequency domain,
spectral ratios show amplifications of 1.5 to 3 at theridge top relative to the base of the ridge (Pedersen et al., 1994). Sitar and Clough (1983) used equivalenttwo dimensional finite-element models to analyse theseismic response of steep slopes in weakly cementedsands and found that acceleration tends to be amplifiedin the vicinity of the slope face. The macroseismicintensity studies for the 1887 Western Liguria earthquakeshowed the largest topographic anomaly at BussanaVecchia to be two intensity degrees (Faccioli et al.,2002). Available intensity-acceleration correlations andrecent numerical simulations consistently indicate thatthis increase can be approximately translated into anamplification factor of the order of 1.5 for maximumground acceleration.
According to the plot of intensity versus distance(Figure 10b), the intensities of five IDPs (Centurion,Midrand, Sandton, Randburg and Roodepoort) are muchhigher than would be expected given the magnitude ofthe earthquake as well as the epicentral distance of theIDPs. To try and explain these anomalous values,investigations were conducted to identify probablecauses of this apparent amplification of ground motion.Geological investigations show that all the observationsused in creating these IDPs are located on rocky ground.However, the observations were also shown to bemostly located on hilltops or on slopes of hills (Figures 11a to e). Given the examples given above onhow topography can cause the amplification of groundmotion, it was concluded that the topographic locationof the observations in the above mentioned IDPs,resulted in the amplification of shaking in that part of the region.
Isoseismal mapsUsing the obtained IDPs, isoseismal maps were createdfor both earthquakes by gridding the obtained intensityvalues (Figure 12). The created maps have lines joiningareas of equal intensities. The warmer colours indicatehigher intensities whilst the lower intensities areindicated by the colder colours. Thus, areas with thesame colour indicate that they experienced the sameintensities and are thus linked together. As expected theepicentres are located within areas that experienced the strongest shaking. This provides confidence in theinstrumental location. It is clear here that the shakingduring the 2 December 2013 earthquake was felt over amuch larger area even considering the earthquake isslightly larger in magnitude.
Fault plane solutionTo help understand the source of the earthquake, faultplane solutions were determined using data recorded atmany seismograph stations (56 for the 18 November2013 earthquake and 20 for the 2 December 2013earthquake) scattered all over southern Africa. These stations are part of the South Africa NationalNetwork as well as cluster networks in and around theJohannesburg, Klerksdorp and Carletonville mining areas.
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Figure 10. (a) The Intensity-distance curve for the 50 IDPs obtained for the 18 November 2013 earthquake. (b) The Intensity-distance curve
for the 52 IDPs obtained for the 2 December 2013 earthquake. The black lines in both figures were inserted to imitate the shape of the Bakun
and Scotti (2006) model. Highlighted are outlying IDPs with higher than expected intensity values (Centurion, Midrand, Sandton, Randburg
and Roodepoort).
b
a
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Figure 11. Location of observations used in creating IDPs at (a) Centurion, (b) Midrand, (c) Randburg, (d) Roodepoort and (e) Sandton
respectively, plotted on maps showing topographic levels in the region. The insert in (a) represents the location of the study area.
a
b
CENTURION
MIDRAND
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Figure 11. Location of observations used in creating IDPs at (a) Centurion, (b) Midrand, (c) Randburg, (d) Roodepoort and (e) Sandton
respectively, plotted on maps showing topographic levels in the region. The insert in (a) represents the location of the study area, (continued).
c
d
RANDBURG
ROODEPOORT
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The solutions were determined using P-wave firstmotion polarities that were read off all the collectedwaveform data. A full description of the processfollowed is given in the manual for the SEISAN software(Ottemöller et al., 2013), where the primary programused is FOCMEC (Snoke et al., 1984). FOCMEC has goodcontrol because it performs an efficient systematicsearch of the focal sphere and reports acceptablesolutions based on a selection criteria for the number ofpolarity uncertainties. The selection criteria for bothpolarities and angles allow for correction or weightingsfor near-nodal solutions.
The velocity model by Midzi et al. (2010) was usedin the determination of take-off angles. The data wereweighted according to the nature of the polarity onset.That is, a clear P-wave polarity was given a full weight of a unity value but where the onset was not very clear, the weight was reduced by half. To confirmthe stability of the results, two other programs in theSEISAN software package were utilised to determine the fault plane solutions using the same data set, HASH (Hardebeck and Shearer, 2002; 2003) and PINV (Suetsugu, 1998). Both programs produced results similar to those produced using FOCMEC (Figure 13).
18 November 2013 earthquakeThe focal mechanism solution of the 18 November 2013earthquake shows normal faulting with an oblique leftlateral component (Figure 13). The main nodal plane’sparameters are: Strike = 72°, Dip = 51° and Rake = -77°.The strike of the nodal planes is in agreement with thefaults identified in the area (Figure 14). However,identifying the actual fault where faulting occurred isimpossible given the error in the location of theepicentre is estimated as about 3 km.
2 December 2013 earthquakeThe fault plane solution obtained for the 2 December2013 earthquake was similar to that of the 18 November2013 earthquake, as it also shows normal faulting with avery slight oblique left lateral movement (Figure 13).However, the nodal plane parameters are slightlydifferent: Strike = 135°, Dip = 45° and Rake = -82°. Thesolution is observed to have orientation similar to that ofthe nearby Warmbath fault which strikes in a northwestto southeast direction (Figure 2).
Discussion and conclusionSince pumping in abandoned mines was stopped,seismicity in and around Johannesburg has generallyincreased due to increase in pore pressure and decreasein cohesion on faults as the water table rose. The CGS
Figure 11. Location of observations used in creating IDPs at (a) Centurion, (b) Midrand, (c) Randburg, (d) Roodepoort and (e) Sandton
respectively, plotted on maps showing topographic levels in the region. The insert in (a) represents the location of the study area, (continued).
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Figure 12. (a) An isoseismal map of the 18 November 2013 earthquake, which shows bands of colours of equal intensity, according to the
MMI scale. High intensity values are indicated by the red colour while the green colour indicates low intensity values. The epicentre of
the 18th November earthquake is also shown in the figure as a black star. (b) An isoseismal map of the 2 December 2013 earthquake, which
shows colours of equal intensity, according to MMI scale. High intensity values are indicated by the red colour while the green colour indicates
the low intensity values. The black star in this figure represents the epicentre of the 2 December 2013 earthquake.
a
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has improved monitoring of the seismicity through theinstallation of a cluster of stations in the Central Randregion. It is hoped this improved monitoring will resultin better understanding of the source and effects of theearthquakes in the region to assist in the mitigation ofpossible disastrous consequences for the city and itsinhabitants. The 18 November 2013 earthquake is one ofthe many earthquakes that have occurred in this area.Although this earthquake did not cause serious damage,it is worth noting that it was felt by many peoplethroughout Johannesburg and its surroundings. It causeda lot of panic within the community, especially thoseclose to the epicentre. Shaking of MMI intensity degreeIV was observed at a number of places with the highestintensity of VI observed in the Fleurhof suburb, which islocated about 13km west of the Johannesburg citycentre. The information obtained from the isoseismalmap (Figure 12a), provides a good indication of the extent of the ground motion experienced by thecommunity and is thus helpful in other studies such asseismic hazard assessments. The shaking experiencedduring the 2 December 2013 earthquake was felt over amuch larger area, in the southern part of Limpopo, inparts of Mpumalanga, Gauteng and North WestProvinces. Surprisingly, strong shaking of intensity IVwas also experienced in northern Johannesburg about150 km from the epicentre. Investigations show that thisapparent amplification of the ground motion inJohannesburg can be linked to topographic effects.
Fault plane solutions that were calculated for bothearthquakes showed normal faulting, which is consistentwith the general style of faulting in the region. However,the process of associating earthquakes with faults toidentify active faults in the region is a tricky process. It can be improved by reducing epicentral locationerrors, usually by improving earthquake monitoring, aswell as improving velocity models used in the analysisof the data. Geological and geophysical investigationscan also help to identify active faults in the region. Such investigations would include paleoseismicinvestigations to identify actual slip or rupture on thefaults. The apparent shape and extent of shakingobserved during the 2 December 2013 earthquake(Figure 12b) is also of concern and shows earthquakesources far to the north of Johannesburg and Pretoriashould be taken into account when determining thehazard and risk of the two cities.
AcknowledgementsThe analysis which was done in order to derive theepicentre was performed using data from the Council ofGeoscience seismic station networks including thecluster networks in and around Johannesburg,Carletonville and Klerksdorp. We extend our sincereappreciation to the Council for Geoscience for fundingthe macroseismic intensity survey. The authors are alsograteful to the reviewers (Ms M. Singh and ananonymous reviewer) as well as Professor R. Durrheim,for their constructive comments which certainly helpedto improve the manuscript.
Figure 13. The focal mechanism solutions obtained using the
SEISAN algorithms FOCMEC, PINV and HASH (Havskov
and Ottemöller, 2010) for the 18 November 2013 earthquake, and
FOCMEC for the 2 December 2013 earthquake.
Figure 14. Correlation of the focal mechanism solution with local
faults at the epicentre of the 18 November 2013 earthquake.
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