Friday, February 25, 2011

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Tectonic considerations in New Zealand after the earthquakes of September 3, 2010 and February 21, 2011

Paul
Toys (1) - Giorgio De Luca (2)


Abstract: The earthquake of 21 February 2011 shows a different tectonic area of \u200b\u200bChristchurch, compared to the overall framework. The quake seems to be correlated with an event before the September 3, 2010, and then interpreted as aftershocks . The focal mechanism shows a motion to reverse fault that can not be related to the focal mechanism from slip faults right Previous seismic event. By analyzing the focal mechanisms, and related to the main Alpine Fault earthquake is possible to define the two as independent structures, which are caused by two kinematically different, but still subject to the Alpine Fault. These facilities are located in the global tectonic framework of New Zealand, offering a model tectonic appropriate. The framework of the tectonic stress does not change, showing a system of thrust tectonics constant and unchanging over time.


(1) Geologist of GeoResearch Center Italy - geoblog (website: www.georcit.blogspot.com ; mail: georcit@gmail.com ).
(2) Surveyor contributor GeoResearch Center Italy - geoblog (website: www.ricercasperimentale.blogspot.com ) .


Introduction
the day of the February 21, 2011 at 12:51 pm local (23:51 UTC) there was a strong earthquake of M 6.3 (Fig. 1), in the city of Christchurch (43.6 ° S, 172.7 ° E), New Zealand (USGS, 2010, 2011; TOYS, 2011; PIOMBINO, 2011). The quake, at first glance appears to correlate the event of September 3, 2010 (Fig. 2) (mainshock), and then interpreted as aftershocks of a seismic sequence migrant from west to east.


Figure 1: Location of the seismic
of February 21, 2011 (by: USGS, 2011).

Figure 2: Location of the seismic
of September 3, 2010 (by: USGS, 2010)

From Further analysis of focal mechanisms, this does not seem to be a tectonic structure which may relate the two events, and then the earthquake of February 21 they should be considered as a mainshock of a secondary structure (USGS, 2011) but related to Alpine Fault (TOYS, 2010).

The study conducted by authors want to focus sull'assetto and probable tectonic faults that caused the earthquake of February 21, 2011, as in Christchurch appears that there is an active tectonic structure (Institute of Geological and Nuclear SCIENCES - GNS, 2004; USGS, 2011) to justify a seismic event. It also wants to propose a tectonic model at the global scale, and linking the seismogenic faults and the tectonic stress field.

seismological and tectonic framework of the New Zealand

Figure 3: Tectonic Context of New Zealand
(by: MINISTRY FOR THE ENVIRONMENT)
From a tectonic point of view, at the global scale (Fig. 3 ), New Zealand is at a transform plate boundary type right with a NE-SW and that connects to the Australian plate and the Pacific NW to SE. The transform fault is described by various authors and called Alpine Fault (Wells, 1999; ROBINSON, 2003); ZACHARIASEN et al., 2006, UNIVERSITY OF OTAGO).

The event of 21 February può essere interpretato come parte di una sequenza inizia con il terremoto del 3 settembre 2010 con M 7,0 (fig. 1 e 2), solo sulla base di numerose scosse di assestamento avvenute secondo una direzione W–E, a partire dall’evento del 3 settembre ed estendendosi verso est (USGS, 2011; PIOMBINO, 2011).

Analizzando i meccanismi focali dei due eventi sismici precedentemente descritti (USGS, 2010, 2011), si nota come l'evento del 3 settembre (fig. 4) mostra un asse principale di massima compressione (asse P) a direzione NW–SE e un asse principale di massima estensione (asse T) a direzione NE–SW. Tale assetto dell’ellissoide degli sforzi è compatibile with a fault plane with the main direction E-W transcurrent kinematics to the right and a fault plane and auxiliary conjugate to the previous direction NS transcurrent motion to the left.

Figure 4: Focal Mechanism
seismic event of September 3, 2010:
P = principal axis of maximum compression, T =
principal axis of maximum extension ;
(by: USGS, 2010)
The event of 21 February (Fig. 5), however, show a principal axis of maximum compression (P axis) in NW-SE direction and a principal axis of maximum extension (T-axis) direction to 19 ° N and 45 ° tilt. This structure of the efforts of the ellipsoid is compatible with a main fault plane in the direction of ENE-WSW oblique-reverse kinematics and fault plane and auxiliary conjugate to the previous, with NNW-SSE and also to oblique-reverse kinematics .
Both events show the same direction of the main axis of maximum compression (P axis), but different direction as far as the main axis of maximum extension axis (T). Moreover, the two focal mechanisms represent two different types of faults kinematically, the earthquake of September 3 shows a movement from slip faults, while the event of 21 February shows a movement to reverse fault.
Figure 5: Focal Mechanism
seismic event of 21 February 2011:
P = principal axis of maximum compression;
T = principal axis of maximum extension;
(by: USGS, 2011)
Conclusions


Figura 6 : Carta delle soluzioni dei meccanismi focali
(da: USGS, 2011).
 Dallo studio dei due meccanismi focali (fig. 4 e 5) si può notare come i due eventi sismici presentano una compatibilità nella direzione dell’asse principale di massima compressione (asse P, massimo stress tettonico di compressione) mentre non presentano compatibilità sia per quello che riguarda l’asse principale di massima estensione (asse T, massimo stress tettonico di estensione), sia dal punto di vista cinematico. Infatti, mentre l’evento del 3 settembre è caratterizzato da una cinematica dovuta ad una faglia trascorrente, l’evento del 21 febbraio è caratterizzato da una cinematica da faglia inversa. I due eventi devono essere perciò imputati a due mainshock distinti e legati a due strutture geologiche differenti.

Dal confronto tra i due meccanismi focali con quelli storici (fig. 6)(CMT, 2006) è possibile definire una corrispondenza con la Faglia Alpina per il primo evento sismico. Il meccanismo focale mostra il piano di faglia principale con una direzione W–E e inclinato ad alto angolo. Tale assetto strutturale può essere descritto come faglia secondaria trascorrente destra (sintetica) di tipo R (fig. 7a), oriented at low angle (around 15/20 °) (DAVIS, REYNOLDS, 1996) to the direction of the Alpine Fault (Fig. 8).

Figure 7: Model of tectonic structures associated with secondary
transform faults right: a) slip faults
secondary type R, R ' , P b) secondary reverse fault.
For the event of 21 February 2011, the structural framework changes. The focal mechanism shows a type of fault kinematically different from Previous to the earthquake (Fig. 4 and 5). Therefore we can say that the geological structure that caused the two earthquakes is not the same, but for the earthquake of February 21 should be treated with a compressive structure. This structure may be made in any way related to the Alpine Fault. In fact, in correspondence of shear zones (Fig. 7b) can generate thrust faults of or thrust faults, with an angle of about 45 ° to the direction of cutting main ( Davis, Reynolds, 1996 ). The level of the fault plane main direction ENE-WSW can precisely be described as secondary to inverse kinematics and fault-oriented according to the Alpine Fault (main cut) by an angle equal to approximately 45 ° to the fault plane dips towards SE with an angle of about 63 ° (Fig. 5 and 8). So the two minor tectonic structures are associated to the main structure of the Alpine Fault, a structure representing traspressiva right.

Figure 8: Model tectonics:
Rosso : Faglia Alpina;
Verde : faglia secondaria sintetica di tipo R;
Blue : faglia inversa ipotetica;
Frecce : Asse principale di massima compressione
(Asse P). 
 La struttura individuata sulla base dell’analisi del meccanismo focale inerente all’evento sismico del 21 febbraio, non mostra evidenze di tipo geomorfologico, se non quelle successive al terremoto e quindi deve considerarsi sepolta below the alluvial deposits.
area of \u200b\u200bChristchurch are not, however, buried and active tectonic structures mapped (Institute of Geological and Nuclear SCIENCES - GNS, 2004), and therefore this seismogenic structure (currently active) may have formed and have produced strong event earthquake of 21 February, and numerous cracks in the ground by cutting the surface deposits, or is this a fault ancient and buried under the alluvial deposits, which produced the earthquake reactivated (SCIENCE, 2011), justifying the high inclination fault plane.

Entrambe le strutture descritte sono compatibili con il medesimo asse principale di massima compressione (asse P) a direzione NW–SE. Tale assetto dello stress tettonico in entrambe i casi con la stessa direzione è comunque concorde con il paleostress tettonico alla scala globale (fig. 6). Tale situazione mostra come il regime di spinta tettonica tra le due placche non è cambiato nel tempo, mantenendo la medesima direzione di spostamento relativo delle due placche.



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