Imaging Unknown Faults in Christchurch, New Zealand, after a M6.2 Earthquake

Transcription

1 Imaging Unknown Faults in Christchurch, New Zealand, after a M6.2 Earthquake D.C. Lawton* (University of Calgary), M.B. Bertram (University of Calgary), K.W. Hall (University of Calgary), K.L. Bertram (University of Calgary) & J. Pettinga (University of Canterbury) Paper accepted for oral presentation at the 75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013 in London, England. Summary Approximately 41 line-km of high-fold reflection seismic data were recorded in and around the city of Christchurch, New Zealand, following a devastating Mw 6.3 earthquake on February 22, 2011 that caused the loss of 185 lives. The goal of the seismic program was to map previously unknown faults in and around the city for hazard assessment and to assist in the post-earthquake recovery effort. Seismic data were collected along six 2D lines, two of which were within the Christchurch metropolitan area and four were in rural areas west of the city. Recording conditions were challenging within the city, but good quality images were obtained along all of the seismic lines, with events interpretable to a depth of approximately 1.5 km. Numerous faults were imaged along the lines and these were interpreted in two groups older faults that showed clear offsets in deep (> 1 km) reflections and younger faults that showed displacement in shallow reflections. Some faults in the latter group were interpreted to be directly associated with hypocentres of shallow after-shocks in the region. These interpretations are now being incorporated into a risk assessment for further possible shallow earthquakes in the region.

2 Introduction On September 4, 2010, a Mw 7.1 earthquake (Darfield earthquake) struck the Christchurch region in New Zealand. The epicentre was centred about 40 km west of the city of Christchurch and caused significant damage but no loss of life. A key manifestation of the earthquake was a fault, called the Greendale Fault, which ruptured to the ground surface with a maximum right lateral displacement of 4.5 m and a vertical displacement of 1.5 m, upthrown to the south (Quigley et al., 2010). The fault trace at the surface was ~28 km long and was oriented approximately east-west, towards the city. On February 22, 2011, a Mw 6.3 aftershock struck with a shallow hypocentre very close to the city. This earthquake resulted in the loss of 185 lives and devastating damage ($23B) to the city infrastructure, buildings and homes. Since September, 2010, the region has experienced over 11,000 aftershocks, with over 3,300 of these being greater than 3M. Soon after the February 22, 2011 earthquake, the University of Calgary s Aries seismic recording system and Envirovibe source were air-freighted to New Zealand to undertake reflection seismic profiling in and around Christchurch to assist in identifying other potentially unknown fault systems for aftershock risk assessment and additional natural hazard identification. This paper describes the program and illustrates how exploration seismic technology was used for potential hazard identification in this area. Regional Tectonics New Zealand lies in a tectonically active area and straddles the boundary between the Australian and Pacific plates. Figure 1a shows a tectonic map of the south island of New Zealand, with the Christchurch region highlighted in the green circle. The major tectonic feature in the South Island is the Alpine Fault, with numerous splays to the north. The known fault network in the Canterbury Plains region around Christchurch is shown in Figure 1b, with most of the faults mapped to the west and north of Christchurch. The Greendale Fault (Figure 1b) was not previously known until it ruptured to surface during the Darfield earthquake. The right-lateral displacement across the Greendale Fault is illustrated in Figure 2a. It is postulated that stress transfer from the Darfield earthquake, may have triggered the Christchurch earthquake, with the epicentre 6 km south of downtown Christchurch city. Figure 1. (a) Tectonic and seismicity map of the South Island, NZ. Christchurch region is in the green circle (after Pettinga, 1986); (b) bedrock geology and faults in the region. The blue colours are Mesozoic greywackes that form the local basement; yellow are Holocene and Upper Pleistocene fluvial outwash sediments; purple are the Upper Miocene Lyttelton Volcanics.

3 Method The 2D seismic data were collected with a single 9,000 kg Envirovibe seismic source, with a Hz sweep over 20 s, with 4 sweeps stacked at each VP, and a VP interval of 10 m. The recording spread consisted of single, 10 Hz geophones spaced at 10 m and recorded with an Aram Aries recording system; the live spread had 400 geophones in a split-spread configuration with maximum source-receiver offset of 2 km. Seismic lines recorded in and around the city are shown in Figure 2b. Line 1 was recorded along the beach to the east of the city and Line 2 was recorded along streets near the city centre which were closed to public access due to severe infrastructure damage from the earthquake. Lines 3 and 5 through 7 were recorded west of the city to investigate step-overs and a seismicity gap between the aftershock patterns shown by the red dots in Figure 2b. Figure 2. (a) View across the Greendale Fault from the south side. The fence used to be straight; (b) Layout of 2D seismic lines recorded for this project. Red dots are epicentres of aftershocks recorded between September 2010 and March Map data courtesy 2011 Google. Results Figure 3a shows a shot gather from the seismic program recorded along Line 3 that crosses the Greendale Fault. A discontinuity in the first arrivals and offsets of reflections show the location of the fault clearly. The data were processed following a standard flow through to post-stack time migration and converted to depth using stacking velocities. Figure 3b shows the interpreted seismic image across the Greendale Fault. The fault zone is quite wide (~200 m) and deeper events show loss of reflectivity within the fault zone. It is interpreted to dip to the north, consistent with observations from moment tensor analysis of the fault displacement and fault plane orientation (Quigley et al., 2010). The increasing reflector offset with depth indicates that this fault is not a new fault but is a reactivated existing fault, even though no surface evidence for it has been documented throughout the time that this region has been settled. The deep reflector (1200 m) also has an older fault interpreted to offset it, creating a horst feature. Figure 3. (a) Shot gather across the Greendale Fault; (b) interpreted section showing a wide fault zone extending at depth.

4 Data acquisition along Lines 1 and 2 within the city was challenging, with high amplitude noise and difficult geophone and source coupling. Close monitoring had to be taken when operating the Envirovibe source near damaged buildings (Figure 4a) and amongst traffic (Figure 4b). Geophones were planted in holes drilled into the road pavement, or where grass verges were available. Figure 4. Seismic recording in Christchurch: (a) near damaged cathedral (b) in a high traffic area. Data quality was, however, better than expected due to the high source effort (long sweeps and diversity stacking). Figure 5 shows interpreted seismic sections from Lines 1 and 2. These show similar features. The image of Line 1 (Figure 5a) is dominated by an event that dips from south to north. This is interpreted to be the surface of volcanic rocks from the Lyttelton volcano, the centre of which outcrops southeast of Christchurch (Figure 1b). The seismic section has been converted to depth using a velocity function of 2000 m/s at sea-level to 2800 m/s at 1 km depth. Little seismic energy was able to penetrate the volcanic sequence and no reflections could be interpreted below the top volcanic reflector. Discontinuous, near-horizontal reflections from young, unconsolidated sediments lap out against the volcanic interface. These sediments are made up of alluvial to shallow marine sands and gravels deposited from large rivers draining the Southern Alps to the west. Two faults are interpreted on Line 1, identified because of significant offsets in the Lyttelton volcanic events. These faults are considered to post-date the volcanics but pre-date most of the younger sediments as no obvious offsets in reflection patterns can be observed in the sub-horizontal reflections. The fault interpreted near the centre of the line occurs on-trend with the Port Hills Fault, on which the Christchurch earthquake hypocentre was located, although it did not rupture to the ground surface. A second, minor fault is interpreted towards the north end of the line, and coincides with a gentle fold in the sedimentary section, indicating compression from the north. Figure 5. Interpreted, migrated sections from (a) Line 1 and (b) Line 2. Line 5 was recorded west of the city, with the goal to provide information about subsurface faults in an area where there is an apparent step-over in epicentres (Figure 2). This line is 16.5 km in length and was laid out along rural roads. Two families of faults are interpreted in his section. Shallow, recent faulting is prevalent in the south, and deeper older faults are interpreted in the northern part of the line, with significant structure in the acoustic basement.

5 Figure 6. Interpreted, migrated section from Line 5 Discussion The seismic program in the Christchurch region has successfully delineated a number of faults that were previously unknown. The general pattern of the fault systems in the area is shown in Figure 7, identified by the dashed white lines. Only the Greendale Fault ruptured to surface (solid white line). The faults identified through the seismic program are interpreted to be near-surface (< 1.5 km) expressions of deeper faults that are the loci of aftershock hypocentres shown in Figure 7. Linkages within the fault systems and step-overs between fault segments will be the topic of future studies. Figure 7. Interpretation of fault systems in the Christchurch region. Map data 2011 Europa Technologies and 2011 Google. Acknowledgements Funding for this project was provided by the New Zealand Crisis Management Centre and GNS Science. Sensor Geophysical kindly donated data processing for Lines 1 through 3. Logistic support from the CREWES Project and the Universities of Calgary and Canterbury. References Pettinga, J. R., Chamberlain, C.G., Yetton, M.D., Van Dissen, R.J. and Downes, G. (1998) "Earthquake Source Identification and Characterisation: Stage 1 (Part A)". Earthquake Hazard and Risk Assessment Study. Canterbury Reg. Council Publ. U98/ pages. Quigley, M., Villamor, P., Furlong, K., Beavan, J., Van Dissen, R., Litchfield, N., Stahl, T., Duffy, B., Bilderback, E., Noble, D., Barrell, D., Jongens, R., Cox, S., 2010, Previously Unknown Fault Shakes New Zealand's South Island, Eos, Transactions, AGU, Vol. 91, No. 49, p