the darfield (canterbury, new Zealand) m 7.1 earthquake of september ...

The Darfield (Canterbury, New Zealand) Mw 7.1 Earthquake of September 2010: A Preliminary Seismological Report Ken Gledhill, John Ristau, Martin Reyners, Bill Fry, and Caroline Holden

Ken Gledhill, John Ristau, Martin Reyners, Bill Fry, and Caroline Holden GNS Science

INTRODUCTION At 04:35 on Saturday 4 September 2010 local time (16:35 on 3 September UT) a moment magnitude (Mw) 7.1 earthquake struck approximately 10 km southeast of the town of Darfield and within 40 km of New Zealand’s second largest city, Christchurch, causing extensive damage in the city and surrounding region. There was no loss of life due to a fortunate combination of strict building codes and the earthquake occurring at 04:35 local time when the streets were largely empty, but about 100 people were injured (two seriously) and many more were made homeless, at least temporarily. Current estimates put the total cost of repairs at NZ $4 billion (about US $3 billion). The key seismological features of the Darfield earthquake are summarized in Table 1. The epicenter and depth are very well constrained by the large number of nearby seismograph and strong-motion accelerometer sites (Figure 1). A surface rupture consistent with strike-slip faulting was quickly identified about 4 km south of the epicenter on a previously unknown fault, which has been named the Greendale fault, although various geophysical data sets have shown that the overall rupture process is more complex. Modern buildings in Christchurch performed very well in the earthquake, but many older brick and masonry buildings were badly damaged. Liquefaction and the associated lateral spreading and slumping caused extensive damage, even to new TABLE 1 The Key Seismological Features of the Darfield Earthquake Origin Time

September 3, 2010 at 16:35 UT (September 4, 2010 at 04:35 NZST) Epicenter 43.55 South, 172.17 East Depth 10.8 km Magnitude 7.1 MW Maximum Intensity MM 9 Location 9 km southeast of Darfield 37 km west of Christchurch

buildings, in areas near the coast with sandy soils (Cubrinovski and Green 2010). The earthquake was felt throughout the entire South Island and a large part of the North Island at distances greater than 900 km, with the maximum felt intensity estimated to be Modified Mercalli (MM) 9. Measured accelerations near source exceeded 1 g with several reading well over 0.5 g. A feature of the near-source strong-motion recordings is the high level of vertical acceleration. In 2001 the New Zealand Earthquake Commission (EQC) provided funding to GNS Science to launch the GeoNet project—an integrated geological hazard monitoring system and part of New Zealand’s comprehensive risk mitigation strategy. GeoNet and the data sets it produced are playing a key part in the response to the earthquake and the developing understanding of the event and its impacts. All GeoNet data and information are made freely available through the GeoNet Web site (http://www.geonet.org.nz). This paper provides a preliminary report on the seismological aspects of the earthquake and introduces the superb dataset collected by the GeoNet sensor networks in the region. The paper includes a brief discussion of the performance of GeoNet and outlines the impact of the “Internet age” on event response.

TECTONIC SETTING AND HISTORICAL SEISMICITY New Zealand straddles the boundary of the Pacific and the Australian plates and its active tectonics are dominated by three main features (Figure 1). Beneath the North Island and northern South Island the Pacific plate is subducting obliquely beneath the Australian plate at the Hikurangi trough. In contrast, in the Fiordland region in the southwest of the South Island subduction is reversed, with the Australian plate subducting obliquely beneath the Pacific plate at the Puysegur trench. Between these two subduction zones the plate boundary is characterized by continental convergence. The plate boundary at the surface is marked by the Alpine fault, a 650-km-long, right-lateral strike-slip fault that has had 480 km of displacement since the Late Oligocene-Early Miocene (e.g., Berryman et al. 1993). Paleoseismic evidence suggests that

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New Zealand seismicity 01/09/2005 - 01/09/2010 ML >= 2.5; depth  7.5) with recurrence intervals of ~200–300 years, with the most recent event in 1717 (e.g., Yetton et al. 1998; Cooper and Norris 1990; Rhoades and Van Dissen 2003; Sutherland et al. 2007). In the central South Island, the velocity of the Pacific plate relative to the Australian plate is 38 mm/yr at an azimuth of 248° (DeMets et al. 2010). The Alpine fault accommodates at least 70–75% of this relative plate motion (e.g., Norris and Cooper 2001; Sutherland et al. 2006). Paleoseismic studies

indicate 27 ± 5 mm/yr of strike-slip and 5–10 mm/yr of dipslip motion on the Alpine fault (Norris and Cooper 2001). The dip-slip component of motion is largely responsible for the uplift of the Southern Alps. A balancing of the plate motion budget across the central South Island using GPS, seismological, and geological data suggests that up to 5 mm/yr of active deformation is possible on faults distributed within the Southern Alps and up to 100 km to the east of the Alpine fault (Wallace et al. 2007).

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A number of M > 6–7 earthquakes have occurred in the Southern Alps and their eastern foothills in the past 150 years. These include 1888 North Canterbury Mw 7.1 (Cowan 1991), 1929 Arthur’s Pass Mw 7.0 (Doser et al. 1999), 1994 Arthur’s Pass Mw 6.7 (Abercrombie at al. 2000), and 1995 Cass Mw 6.2 (Gledhill et al. 2000). There are also a number of mapped active faults in the eastern foothills of the Southern Alps (see Stirling et al. 2008 for a summary). The fact that the Darfield earthquake was centered in the Canterbury plains, east of these foothills and where no active surface faults had previously been mapped, demonstrates that the zone of active deformation in the eastern South Island extends beyond the visible range front. Prior seismicity in the immediate region of the Darfield earthquake has been low, with no large earthquakes occurring since written records began 170 years ago. The strongest shaking experienced historically from local events was MM 7, both from a shallow M 4.7–4.9 earthquake near Christchurch (ca. 43.55°S 172.6°E) on 4 June 1869, and from a lower crustal M 5.6–5.8 earthquake near Lake Ellesmere (c. 43.8°S 172.5°E) on 31 August 1870 (G. Downes, personal communication, 2010). Crustal structure in the region of the Darfield earthquake is consistent with that of the Chatham Rise on the converging Pacific plate (Figure 1) (Reyners and Cowan 1993). The Chatham Rise is a marine plateau formed by fragmentation of the Gondwana continental margin in the mid- to late Cretaceous. It is cut by major east-west trending faults, which became established in the late Early Cretaceous (Wood and Herzer 1993).

aspects such as toppled chimneys and parapets, failure of gables and poorly secured face-loaded walls, and in-plane damage to masonry frames all being extensively documented (Dizhur et al. 2010). Overall the residential building stock, consisting predominantly of light timber frame construction, performed very well, with little structural damage due to ground shaking (Buchanan and Newcombe 2010). However, many older buildings (more than 20 years old) suffered damage due to falling chimneys. Close to the fault rupture, there was significant damage to building contents and a few broken windows due to strong shaking, although away from the fault zone very few windows were broken in any buildings (Buchanan and Newcombe 2010). The most significant structural damage to houses was from differential settlement of foundations induced by soil liquefaction and lateral spreading. More than 7,300 felt reports of the mainshock have been received to date with well over 100 MM 8 and some MM 9. Modified Mercalli intensities are derived automatically from the felt reports by an algorithm that only assigns intensities up to MM 8 (Coppola et al. 2010); MM 9 and above are assigned by engineers and do not appear on the map. Figure 3 is a contour map of the Canterbury region derived from the felt reports, and it shows the highest intensities in the epicentral region and extending east to the Christchurch region. Most of the reports of heavy damage came from Christchurch city as expected, and reports of damage have come from across the Canterbury region.

SURFACE FAULT RUPTURE

DATASET

The Darfield earthquake was the first earthquake known to produce ground-surface rupture in New Zealand since the 1987 Mw 6.5 Edgecumbe earthquake in the North Island (Beanland et al. 1989, 1990). The earthquake ruptured the previously unknown Greendale fault, which was buried beneath post-glacial, graveldeposited alluvial plains (Forsyth et al. 2008). The east-west striking surface rupture is located ~4 km south of the epicenter and extends for ~29.5 km mainly across low-relief pastoral farmland (Figure 2). Movement was predominantly right-lateral strike-slip with an average horizontal displacement of ~2.5 m and maximum displacements of ~5 m horizontally and ~1.5 m vertically (Quigley et al. 2010). The rupture is characterized by a series of en echelon left-stepping traces with a maximum stepover of 1 km and a large number of smaller step-overs.

The Darfield earthquake was very well recorded by both the broadband and strong-motion national-scale GeoNet networks (Petersen et al. 2010) and the Canterbury regional strong-motion network (CanNet) (Avery et al. 2004). All operational GeoNet real-time sites and a large number of the triggered strong-motion sites recorded the earthquake (a total of 295 sites) and provided waveform data to the GeoNet data center. There is also a set of building response records from the building array installed at the University of Canterbury. Some of the best near-field ground-shaking measurements of the Darfield earthquake were recorded by the sensors of the CanNet network, a set of low-cost accelerographs installed throughout the Canterbury plains and within Christchurch city (Avery et al. 2004). CanNet was installed as a part of GeoNet to capture the impacts of a rupture of the Alpine fault to the west, but it now is also playing a major role in unscrambling the complex nature of this earthquake. In total GeoNet obtained 38 strong ground motion recordings within 50 km of the epicenter. This exceptional dataset will be invaluable in defining the complex history of the rupture using inversion methods as well as recently developed source-tracking methods (e.g., Kao and Shan 2007). More information and analysis of the strong ground motion data are contained in Cousins and McVerry (2010). Despite the damage caused by the earthquake and the fact that power was lost in large parts of Christchurch city, data were successfully collected in near real time from most of the GeoNet

TYPES OF DAMAGE Modern buildings in Christchurch performed very well in the earthquake, but many older brick and masonry buildings were badly damaged. However, retrofitted buildings (even to only 33% of the current building code) sustained very little damage. The damage was consistent with projections for the scale of this earthquake, and indeed even greater damage might have been expected. In general, the nature of damage was consistent with observations previously made on the seismic performance of unreinforced masonry buildings in large earthquakes, with


CSHS 0.07 g

ASHS 0.16 g

CACS 0.31 g

DFHS 0.37 g HORC 0.82 g

GDLC 1.26 g

ROLC 0.74 g DSLC 0.32 g

LRSC 0.10 g

TPLC 0.88 g

REHS 0.22 g CBGS 0.12 g

LINC 0.92 g

RAKC 0.12 g SBRC 0.10 g

WSFC 0.07 g ADCS 0.06 g

DORC 0.08 g

▲▲ Figure 2. Map of central Canterbury showing the Greendale fault surface trace (black line), the epicenter of the Darfield earthquake (blue star), and the vertical PGAs at selected strong-motion stations.

MM

▲▲ Figure 3. Map showing the felt effects of the Darfield earthquake from reports received by the GeoNet Web site. Epicenter is denoted by the blue star. MM 9 and above values are assigned by engineers and are not derived automatically; therefore, no MM 9 values appear on this map. The highest intensities are from the epicentral region and extending east to the Christchurch region. Christchurch city region is outlined in black.


GeoNet first motion solution NP 1: 40/75/90 NP 2: 220/15/90

GeoNet regional moment tensor solution NP 1: 45/73/90 NP 2: 226/17/91 Mw 7.1 Mo: 6.10E19 Nm Centroid depth: 8 km USGS centroid moment tensor solution NP 1: 268/87/-166 NP 2: 178/77/-3 Mw 7.0 Mo: 3.50E19 Nm Centroid depth: 10 km

is dependent on the wavelength. High-frequency waves are sensitive to small-scale features whereas low-frequency waves are sensitive to larger features. When a distant earthquake is recorded, the source component of the signal is dominated by low-frequency content, as most of the source-generated highfrequency signal is attenuated during the wave’s propagation. Furthermore, as the wave trains from an earthquake propagate, they are dispersed, creating an elongation of the recorded coda. In a situation in which multiple ruptures contribute to the total energy release, dispersion can superimpose the individual signals, even if the individual mechanisms are spatially or temporally separated. This is likely to be the case with the Darfield earthquake, and the individual mechanisms can only be resolved using the near-source data recorded by GeoNet, which work is currently in progress.

DISTRIBUTION OF AFTERSHOCKS ▲▲ Figure 4. The GeoNet first motion solution (top), GeoNet regional moment tensor (middle), and U.S. Geological Survey centroid moment tensor (bottom) focal mechanisms of the Darfield earthquake with some of the key parameters noted: the strike/dip/rake of both nodal planes (NP), and for the moment tensor solutions Mw , seismic moment (Mo ), and centroid depth. All focal mechanisms are shown as lower hemisphere projections.

sensor networks in the region. At one stage up to 50% of the cell phone sites within the region were unavailable, and while this may have slowed some of the strong-motion records that use cell phone technology for data transfer, there was no overall data loss. Communications with one site, which relays data through the University of Canterbury, and the strong-motion building array at the university, were lost, but the data from these sites were retrieved successfully once power was restored.

SOURCE MECHANISM Several estimates of the focal mechanism of the Darfield earthquake are available. The first motion and regional moment tensor focal mechanisms are in very good agreement, but quite different from the teleseismic moment tensor solutions (Figure 4). The teleseismic moment tensor solutions indicate a strike-slip source mechanism in agreement with the Greendale fault trace orientation, whereas the regional moment tensor and first motion source mechanisms show reverse faulting (Global CMT Project, see http://www.globalcmt.org; for NEIC/USGS solutions see http://earthquake.usgs.gov/earthquakes/eqinthenews/2010/ us2010atbj/#scitech). Complex rupture processes involving both reverse and strike-slip faulting have been observed for a number of earthquakes both in the local Canterbury region, e.g., 1994 Arthur’s Pass (Abercrombie et al. 2000) and 1995 Cass (Gledhill et al. 2000); and overseas, e.g., 2010 Haiti, Mw 7.0 (Hayes et al. 2010), 2008 Wenchuan Mw 7.9 (Zhang and Ge 2010), 2002 Denali Mw 7.9 (Eberhart-Phillips et al. 2003). Source properties of large earthquakes have historically been analyzed by looking at waveforms recorded at regional or greater distances. The smallest feature that a wave can resolve

The Darfield earthquake has been followed by a reasonably energetic aftershock sequence with more than 4,000 located events through the end of January 2011 (Figure 5). As of the end of January 2011 there have been 14 aftershocks of local magnitude (ML) 5.0 or greater and 155 of ML 4.0–5.0, but because of the shallow depth and proximity to the city many more have been felt by residents in the region, some of which are as small as ML 2.5. Several of the events in the M L 5.0 range have caused additional damage and concern, particularly those within or very near Christchurch city. The largest aftershock was an ML 5.6 event that occurred about 20 minutes after the mainshock. To better record the aftershock sequence, GNS Science installed an additional 10 portable seismographs and three accelerographs within the aftershock zone, and 10 accelerographs within Christchurch city. Several other institutions have installed portable seismographs to assist regional studies. Most of the aftershocks are clustered near the surface trace of the Greendale fault but there are some significant features to point out. The aftershock zone is fairly diffuse with a large number of events continuing many kilometers east of the fault trace, and the vast majority of the events are quite shallow at 7.0

▲▲ Figure 5. A map of the preliminary aftershock locations. More than 3,300 aftershocks have been located through 31 December 2010. Note the “finger” of aftershocks trending north from the epicenter and the cluster of aftershocks at the western end of the fault trace. Yellow stars are ML ≥ 5.0 aftershocks and the purple star points to the epicenter. Also noted is an ML 4.9 aftershock on 26 December 2010, which caused a significant amount of damage in the Christchurch city center.

GeoNet

USGS

M 4 5 6 7 reverse strike-slip other

▲▲ Figure 6. Aftershock focal mechanisms derived from regional moment tensor solutions. The epicenter is denoted by the yellow star, and the GeoNet and USGS mainshock moment tensor solutions are shown by the yellow focal mechanisms on the right side of the figure. Reverse faulting (red mechanisms) is evident in the western part of the aftershock zone, to the north of the epicenter, and to the east in the Christchurch vicinity. Strike-slip faulting (green mechanisms) dominates along the Greendale fault surface trace. Normal or oblique faulting (blue mechanisms) are also scattered throughout the aftershock region.


the largest aftershock, as the waveforms were obscured by the coda of the mainshock. The focal mechanisms (Figure 6) show a variety of styles; however, most are either strike-slip or reverse faulting mechanisms. Mechanisms along the eastern part of the Greendale fault are mainly strike-slip and vary between mechanisms that closely match the U.S. Geological Survey and Global CMT Project mainshock solutions, and those with a P-axis rotated counter-clockwise similar to the overall P-axis trend for earthquakes in the South Island. Many of the mechanisms in the western part of the aftershock zone, in close proximity to the epicenter and to the east near Christchurch, are reverse faulting or have a large reverse-faulting component.

DISCUSSION There are several lines of evidence that suggest that this earthquake is not a simple strike-slip event. The hypocenter of the Darfield earthquake is very well constrained (within at most ±0.5 km) but is about 4 km north of the surface trace of the Greendale fault. This cannot be explained by location uncertainty or by a shallow dipping fault as a strike-slip mechanism will be near vertical. The second important piece of evidence comes from the various estimates of the focal mechanism of the earthquake. There is a clear difference between the teleseismic moment tensor methods, which indicate a strike-slip source mechanism in agreement with the Greendale fault trace orientation, and the regional moment tensor and first-motion source mechanisms (Figure 4), which are in close agreement and show reverse faulting. The teleseismic moment tensor methods provide an average over the whole event, whereas the other two methods are modeling the nature of the first part of the rupture. The USGS broadband energy solution also indicates a complex event with at least two subevents (the NEIC/USGS energy solution can be found at http://earthquake.usgs.gov/ earthquakes/eqinthenews/2010/us2010atbj/neic_atbj_e.php). The aftershock distribution (Figure 5) shows a NNWSSE oriented “finger” of aftershocks off the main alignment of aftershocks, particularly if only the early aftershocks are considered. The aftershock focal mechanisms show a variety of faulting styles, providing additional evidence for the complex nature of the rupture process. There is a cluster of aftershocks at the western end of the fault trace where the focal mechanisms for the larger events are predominantly reverse faulting. Additionally, near-source strong-motion stations show unusually high vertical accelerations that require an initial reverse component to the event. Preliminary geodetic modeling (Beavan, Samsonov, Motagh et al. 2010) using GPS and differential InSAR require several fault segments to be active during the earthquake. The geodetic results are consistent with an initial Mw 6.5 rupture on a blind reverse fault, dipping steeply to the southeast, which triggered right-lateral rupture on the Greendale fault where the majority of the moment release occurred (Mw 7.0). A number of other reverse faults, including a blind thrust associated with the aftershocks to the west of the Greendale fault (Figure 5),

were also active, giving Mw 7.1 for all modeled fault segments (Beavan, Samsonov, Motagh et al. 2010). Preliminary strong motion data modeling also favours a composite fault rupture. The proposed sequence from this preliminary modeling suggests initiation on a blind thrust located at the hypocenter, with an estimated Mw 6.3, which continues with right-lateral strike-slip rupture of the 30-km-long Greendale fault plane, Mw 6.9, and ends with the rupture of a blind thrust near the western end of the surface fault trace with an estimated Mw 6.5. Waveform fit residuals for stations east of the fault trace suggest that the subsurface strikeslip rupture may have extended further east. Further studies may clarify whether the waveform fit may have been influenced by, for example, effects from guided waves or reflected waves off Banks Peninsula to the east. The above observations are consistent with the earthquake beginning as a steeply dipping reverse-faulting event at the determined hypocenter, then continuing by triggering the Greendale fault in a right-lateral strike-slip sense to accommodate the displacement and the regional stress. A full multidisciplinary study is required to quantify the styles and the sequence of the ruptures, but no other hypothesis explains the seismological observations. In a New Zealand and international context this is not a particularly unusual scenario, with the 1994 Arthur’s Pass (Abercrombie et al. 2000) and 1995 Cass (Gledhill et al. 2000) earthquakes in the Canterbury region both likely to have been complex events. International examples include the 1992 Landers earthquake in California (Abercrombie and Mori 1994) and the 2010 Haiti earthquake, which exhibited complex rupture processes similar to Darfield (Hayes et al. 2010). The difference in the case of the Darfield earthquake is that the mainshock was very well recorded by many nearby instruments. Considerable research is still required to fully characterize the Darfield earthquake and its impacts. From a seismological point of view this will involve the integration of various source-modeling techniques to form a preferred source model and analysis of the aftershocks to put the earthquake in a tectonic context. A full multidisciplinary study involving geodesy (GPS and satellite radar imagery), seismology (strongmotion, aftershock, and numerical modeling studies), and geology is underway to constrain the rupture process. This is important both in a regional tectonic context and to help in the understanding of the patterns of damage and liquefaction.

THE DARFIELD EARTHQUAKE AND GEONET The Darfield earthquake was the first earthquake to impact New Zealand in the “Internet age.” This term includes social media sites such as Twitter and Facebook where GeoNet earthquake reports are posted and have a large New Zealand following. This was important for how information about the earthquake was delivered to responding agencies, the media, and the wider community. The GeoNet Web site played a very important role in this process, and the open data access policy


allowed many third party Web sites to play a part by providing the earthquake information in new and exciting ways. This event was the first full-scale test of GeoNet and the related response functions since the inception of the facility in 2001, although we have responded to four other large earthquakes, the largest being the Mw 7.8 Dusky Sound earthquake of July 2009 (Fry et al. 2010; Beavan, Samsonov, Denys et al. 2010), and three tsunami events in recent years. During the earthquake and ongoing aftershock sequence the overall GeoNet facility performed as designed. This included the sensor networks, data transmission, reception, and analysis and dissemination systems. GNS Science is the scientific adviser to the New Zealand Ministry of Civil Defence and Emergency Management on geological perils. During the Darfield earthquake the established procedures and relationships were fully utilized, with GeoNet data and information providing the basis for sound scientific advice for the national response effort. The Darfield earthquake and its aftershocks caused a great deal of traffic to the GeoNet Web site. In the first five days after the quake, the Web site served more traffic than for the entire 2009 year, and during the month of September the site received 564 million hits. More than 56,000 felt reports were received during September, adding to the load on the Web server infrastructure. This huge increase in Web traffic over a very short period of time presented some challenges and occasionally pushed the servers to their limits. Due to the flexible design of the Web site hosting, we were able to expand the capacity by installing additional Web servers as interest in the aftershocks grew.

POSTSCRIPT A destructive aftershock of local magnitude (ML) 6.3 and shallow depth struck approximately 10 km southeast of downtown Christchurch at 12:51 on Tuesday 22 February 2011 local daylight time (23:51 on 21 February UT), causing extensive damage and loss of life (currently estimated at 180 deaths) in the central city and eastern suburbs. This earthquake was very energetic (energy magnitude [Me] 6.7) with recorded maximum vertical accelerations of 2.2 g near the epicenter. This resulted in more extensive liquefaction and higher levels of building damage than the Darfield mainshock. Once again the event was very well-recorded by the GeoNet network, but no surface rupture has been found. The authors will provide a more detailed report on this earthquake at a later date.

ACKNOWLEDGMENTS The analysts, technicians, and IT staff at GeoNet played a crucial role in the response to the Darfield earthquake and the authors would especially like to recognize and thank them for their vital contributions. We acknowledge the New Zealand GeoNet project sponsors EQC, GNS Science, and LINZ for providing data and images used in this paper. Stephen Bannister and John Beavan provided many helpful comments that greatly improved this manuscript. Some of the figures were created using Generic Mapping Tools (GMT, Wessel and

Smith 1991). Regional moment tensor solutions were computed using the mtpackagev1.1 package developed by Doug Dreger of the Berkeley Seismological Laboratory, and Green’s functions were computed using the FKPROG software developed by Chandan Saidia of URS.

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