The M W 6.2 Cass, New Zealand, earthquake of 24 November 1995: Reverse faulting in a strike slip region

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1 This article was downloaded by: [Boston University] On: 23 April 2013, At: 18:34 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK New Zealand Journal of Geology and Geophysics Publication details, including instructions for authors and subscription information: The M W 6.2 Cass, New Zealand, earthquake of 24 November 1995: Reverse faulting in a strike slip region Ken Gledhill a, Russell Robinson a, Terry Webb a, Rachel Abercrombie a, John Beavan a, Jim Cousins a & Donna Eberhart Phillips b a Gracefield Centre, Institute of Geological & Nuclear Sciences, P.O. Box , Lower Hutt, New Zealand b Institute of Geological & Nuclear Sciences, Private Bag 1930, Dunedin, New Zealand Version of record first published: 23 Mar To cite this article: Ken Gledhill, Russell Robinson, Terry Webb, Rachel Abercrombie, John Beavan, Jim Cousins & Donna Eberhart Phillips (2000): The M W 6.2 Cass, New Zealand, earthquake of 24 November 1995: Reverse faulting in a strike slip region, New Zealand Journal of Geology and Geophysics, 43:2, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 New Zealand Journal of Geology & Geophysics, 2000, Vol. 43: /00/ $7.00/0 The Royal Society of New Zealand The M W 6.2 Cass, New Zealand, earthquake of 24 November 1995: reverse faulting in a strike-slip region KEN GLEDHILL RUSSELL ROBINSON TERRY WEBB RACHEL ABERCROMBIE JOHN BEAVAN JIM COUSINS Institute of Geological & Nuclear Sciences Gracefield Centre P.O. Box Lower Hutt, New Zealand DONNA EBERHART-PHILLIPS Institute of Geological & Nuclear Sciences Private Bag 1930 Dunedin, New Zealand Abstract On 24 November 1995 an earthquake of moment magnitude M w 6.2 struck near the small settlement of Cass in the Southern Alps, South Island, New Zealand. Bodywave modelling using teleseismic arrivals gives an oblique reverse focal mechanism for the mainshock, with the fault plane striking approximately north-south, and a shallow centroid depth of 3-6 km. Aftershock recordings at the station SNZO near Wellington were used as empirical Green's functions to estimate a source time function duration of 7 s. A joint inversion for velocity and location of 169 selected events was used to derive a one-dimensional velocity model with station terms, and this velocity model was then used to relocate all recorded aftershocks. A subset of the best 803 events was then selected for further analysis. The apparent trend of the aftershock zone is NNW-SSE, with the mainshock near the centre. However, projections of the aftershocks on north-south and east-west crosssections show a band of activity shallowing to the south and dipping to the west. The north-striking, west-dipping nodal plane of the mainshock focal mechanism is therefore most '. ikely to be the fault plane. Early aftershocks occurred mainly to the south of the mainshock location, suggesting rupture to the south, a feature supported by the mainshock modelling. The aftershock focal mechanisms are mixed but reflect the regional stress field (NW-SE compression). Keywords Cass earthquake; focal mechanisms; aftershocks; strong ground motion INTRODUCTION On 1995 November 24 d 6 h 18 m UT, an earthquake of moment magnitude M w 6.2 struck near the small settlement of Cass on the Arthur's Pass road which crosses the Southern Alps in the South Island of New Zealand (Fig. 1). The earthquake was felt as far away as Blenheim in the north of the South Island and Timaru to the south. Because of a survey in progress (the Southern Alps Passive Seismic Experiment SAPSE; Eberhart-Phillips et al. 1996), several portable short-period seismographs, and one broad-band seismograph, were already deployed in the vicinity at the time. These supplement the stations of the New Zealand National Seismograph Network (NZNSN), which has an average station spacing of c. 100 km. The nearest temporary station was c. 20 km from the epicentre, and this was supplemented a day and a half later by the installation of the Poulter River station (POLA). This paper reports on the mainshock and the aftershocks 100 km G98048 Received 18 December 1998; accepted 2 December E 173 E Fig. 1 Location of the Cass mainshock (star) and the stations used in this study.

3 256 New Zealand Journal of Geology and Geophysics, 2000, Vol S Fig. 2 Historical and recent earthquakes in the Cass region. The open circles are all earthquakes in the Seismological Observatory catalogue (Mi > 4) us until the end of Earthquakes of Mi 6 and greater are shown as solid circles that scale with magnitude. These include the North Canterbury earthquakes of 1881 and 1888, the Arthur's Pass earthquakes of 1929 and 1994 (which had two aftershocks of Mi 6.01, and the Lake Coleridge earthquake of km E E E of the Cass earthquake. The focal mechanism for the mainshock is derived using body-wave modelling, and the strong-motion and felt effects are discussed. The study includes the relocation of the mainshock and aftershocks using a one-dimensional velocity model derived by simultaneously inverting for the earthquake hypocentres and the velocity model. This provides better relative locations for the study of the aftershock distribution. A small number of aftershock focal mechanisms are also determined. TECTONIC SETTING New Zealand lies on the plate boundary between the Pacific and Australian plates. Under the North Island and the northern South Island, the Pacific plate is being subducted below the Australian plate. In contrast, the subduction is reversed in the Fiordland region in the southwest of the South Island, where the Australian plate is being subducted. In between lies a region of predominately strike-slip, accommodated by the Alpine Fault, a 600 km long fault system with a dextral offset of c. 460 km. In the last 6 m.y., the small but significant component of plate convergence has resulted in the uplift of the Southern Alps. The study region, where the Cass earthquake occurred, is in the southern part of the transition region between subduction and continental collision. This region has given rise to several large strike-slip earthquakes during the recorded history of New Zealand seismicity. Historical and recent earthquakes in the region Since 1881 there have been seven earthquakes of magnitude 6 and greater within c. 100 km of the location of the Cass 43.5 S earthquake (Fig. 2). The largest of these was the magnitude 7.3 North Canterbury earthquake of 1888 September 1 (Cowan 1991). This earthquake broke the Hope River segment of the Hope Fault causing a surface rupture of c. 30 km in length, with dextral offsets of between 1.5 and 2.6 m. It is thought to have had a strike-slip focal mechanism (Anderson et al. 1993). An earthquake of magnitude 6.8 in 1881 is given the same formal epicentre as the 1888 event, although it may have ruptured a different segment of the Hope Fault or another fault in the area. The 1929 Arthur's Pass earthquake (M w 7.1) is thought to have been a strike-slip event occurring on or near the Kakapo Fault. This earthquake caused extensive landsliding (Speight 1933), and much of our knowledge of this event comes from the mapping of the pattern of the landslides (Yang 1992). In the small settlements in the region, chimneys were toppled and furniture was thrown about. This earthquake was felt as far away as 700 km in the North Island. The 1946 June 26 Lake Coleridge earthquake (loci' magnitude MI 6.2) was felt over much of the South Island, and caused minor structural damage to homesteads in the region and at the Lake Coleridge hydro-electric power station (Downes 1995). It caused numerous landslides and had two foreshocks and a rich aftershock sequence which persisted until the end of The 1994 Arthur's Pass earthquake occurred at a different location to the 1929 event (see above and Fig. 2). After the mainshock, portable seismographs were deployed, and a GPS resurvey was carried out. The mainshock was at shallow depth (3-6 km) and was Myy 6.8. The reverse focal mechanism has the fault plane striking NE-SW (Abercrombie & Webb 1996; Abercrombie et al. in press).

4 Gledhill et al Cass earthquake 257 The aftershock sequence was complex, and featured two strike-slip aftershocks of ML 6.0 to the south. GPS-based dislocation modelling suggests that as well as the thrusting motion there was a considerable component of strike-slip motion. A geological reconnaissance survey of the area immediately after the mainshock found evidence of landsliding but no surface rupture (Robinson et al. 1995). A surface rupture would be expected for an earthquake of this size at shallow depth, but it is possible that the mountainous nature of the region conceals the rupture. THE MAINSHOCK Mainshock location Earthquakes in New Zealand are routinely located by the New Zealand Seismological Observatory (operated by the Institute of Geological & Nuclear Sciences) using data from the NZNSN and any available temporary stations. The mainshock location and depth, calculated during the routine processing at the Seismological Observatory, are given in Table 1. The formal location is 30 km ENE of the location of the 1994 Arthur's Pass earthquake, 20 km SSW of the 1929 Arthur's Pass epicentre, and 38 km southeast of the Alpine Fault. There are no recorded earthquakes that could be considered to be foreshocks. The only earthquake recorded after 19 November close to the position of the mainshock is very small {ML 1.7) and >10km from the mainshock location. This earthquake occurred about 10 h before the mainshock. However, analysis of the earthquake catalogue for this region suggests that this event is probably just part of the normal seismicity of the region. Body-wave modelling Method To determine the source parameters using teleseismic data, we use McCaffrey & Abers' (1988) version of Nabelek's (1984) inversion procedure, which minimises the misfit between the recorded and synthetic waveforms. We use P and SH broad-band and long-period waveforms recorded at Global Seismic Network (GSN) stations at distances between 30 and 80. To improve the signal to noise ratio at broadband GSN stations, the long-period traces are filtered to approximate a WWSSN long-period response. Synthetic seismograms are generated by combining direct (P or S) and reflected (pp and sp, or ss) phases from a point source (Langston & Helmberger 1975). The source structure is a layer over a halfspace, with velocities derived from a velocity inversion of aftershocks of the 1994 June 18 Arthur's Pass earthquake. This model had a 3 km thick layer with Vp = 5A4 km/s, Vs = 3.14 km/s, and p = 2600 kg/ m 3, over a halfspace with Vp = 6.03 km/s, Vs = 3.49 km/s, and p = 2800 kg/m 3, and is similar to the model later derived from Cass aftershocks. Receiver functions are calculated for a halfspace. Amplitudes are adjusted for geometrical spreading and for attenuation using Futterman's (1962) /* operator, with t* = 1 s for P and /* = 4 s for SH. Station coverage for the New Zealand region is poor to the north and east because there are only a few island stations in the Pacific Ocean, so stations tend to be clustered in the northwest. To compensate for this, seismograms are weighted according to azimuthal density (McCaffrey & Abers 1988). The inversion process (Nabelek 1984) is then used to adjust the strike, dip, rake, depth, and source time function, the latter being described by a series of overlapping triangles of prescribed number and duration. Iterations are continued until a minimum misfit between data and synthetics is found. We used the Harvard Centroid Moment Tensor (CMT) solution (Dziewonski et al. 1981) as a starting solution. Results Modelling shows that this earthquake was shallow and had a moment magnitude o Mw6.2. For a shallow event of this size, there is a strong trade-off between source depth and the duration of the source time function because the time interval between the arrival of direct and surface-reflected phases is similar to the length of the source time function. To constrain the depth better, we low-pass filter the clearest broad-band seismograms at 2 s, and compare these with synthetic seismograms for a best-fit source fixed at 2-12 km depths. For depths >8 km, the synthetics show a clear inflection due to the arrival of depth phases, which was not apparent in the data. Furthermore, the fit to the SH seismogram at station NWAO (Fig. 3A) degrades quite rapidly for depths >4 km. This seismogram appears to have a nodal direct arrival, but that cannot be achieved by adjusting the focal mechanism without degrading the fits at other stations. The only other way to fit that station is for the ss phase to arrive soon after direct S, requiring a shallow source. Inversions with the NWAO polarity reversed are unable to match this station, and so the problem is not instrumental. The waveforms and best-fit solution are shown in Fig. 3 A. The relatively high frequency arrival at station SPA may indicate a southward-propagating rupture. To fit this, we use a southward-directed line source with a rupture velocity of 3.0 km/s. The line source fits best when directed due south (to within 10 ), and with zero plunge (again to within 10 ). The improvement in fit over a point source is not significant under the Student's Mest, but the reduced root-mean-square (r.m.s.) is suggestive of a southwardpropagating rupture. For depths >4 km, the teleseismic source has duration of 4 s or less. The synthetic seismograms for the best-fit solution fit the data well at most stations (Fig. 3A). The poorest fits are at the close island stations (AFI and RAR) where noise levels are high. These stations are not included in the inversion. Their main contribution is in helping to resolve the depth, which we have to do separately. Table 1 The catalogue and relocated locations of the Cass earthquake. CUSP Relocation Year Month Day Origin time (h m s) Lat. ( S) Long. ( E) Depth (km) 7 9 M L

5 258 New Zealand Journal of Geology and Geophysics, 2000, Vol. 43 EM b Cass M =6.2 strike 1767 dip 45/ rake 447 depth 3km/M 2.6 x 10' Nm HN HN f Fig. 3A Body-wave solution for the Cass earthquake. The sub-title contains the strike, dip, and rake in degrees, depth in kilometres and scalar moment in Newtonmetres. Each waveform is labelled with the station code, identification letter, and a "d" indicating a digital record. Solid lines are the data, whereas dashed lines are the synthetic. Vertical bars mark the window used in the inversion, and a vertical scale is shown, al waveforms being normalised to that of an instrument with a gain of 3000 at a distance of 40. The source time function, its time scale, and the waveform time scale are also shown. The upper circle shows the P wave nodal planes on an equal area projection of the lower focal hemisphere, with the P-axis marked with a solid circle and the T-axis an open circle. The lower circle shows the SH wave nodal planes. Asterisks indicate stations not used in the inversion. We assess the degree of constraint on the derived source parameters by perturbing each of the strike, dip, rake, second strike, and depth in turn, and repeating the inversion with the perturbed parameter constrained. Our estimated uncertainty for each parameter is the value where the waveform fit is seen to deteriorate noticeably compared to the noise in the seismogram. The results are presented in Table 2. The body-wave mechanism agrees with the first motion solution within the estimated uncertainties, but differs significantly from both the Harvard and National Earthquake Information Centre (NEIC; Sipkin 1982) CMT solutions. In Fig. 4A, B, we compare the Harvard and NEIC CMT solutions, with the minimum misfit solution at stations covering a range of azimuths and for both P and SH waves. The minimum misfit solution is better at all stations. For shallow earthquakes, the CMT method has difficulty resolving the Mxz and Myz moment tensor elements that represent vertical dip-slip (Scott & Kanamori 1985). There is thus a strong trade-off between depth and the amount of dip-slip for very shallow earthquakes such as the Cass event, which may explain why both the Harvard (Fig. 4A) and NEIC CMT solutions, which have depths of 15 and 12 km, respectively, have too small a dip-slip component. Empirical Green's function analysis In order to resolve the inevitable trade-off between source duration, focal mechanism, and depth for moderate sized, shallow earthquakes such as Cass, we require an independent

6 Gledhill et al Cass earthquake 259 Relative Source Time Function Time (seconds) Fig. 3B The source time function for the Cass earthquake calculated using empirical Green's function techniques at station SNZO. estimate of either the depth or duration. The broad-band (GSN) station SNZO is 300 km distant from the Cass earthquake at an azimuth of 52. This station recorded both the mainshock and the largest aftershock (ML 5.2), and from these seismograms we are able to obtain an independent estimate of the source duration. We use the seismograms of the aftershock as an empirical Green's function to remove the propagation effects from the mainshock recordings. Deconvolving the aftershock seismogram from that of the mainshock results in an estimate of the source time function and hence the duration of the mainshock. The empirical Green's function technique has been used successfully to deconvolve the sources time function of earthquakes over a wide magnitude range using seismograms recorded at local, regional, and teleseismic distances. Mori & Hartzell (1990) used local P waves to deconvolve the source time function of an M L 4.6 earthquake. Dreger (1994) and Dreger et al. (1995) used local and regional broad-band seismograms to determine the slip distributions oimw 6-7 earthquakes, and Ammon et al. (1994) have extended the technique to use teleseismic recordings of large earthquakes. The method has also been used in New Zealand to obtain estimates of the source duration of the 1994 Arthur's Pass earthquake using SNZO and stations in Australia (Abercrombie et al. in press). The three basic assumptions of the method are that the smaller, empirical Green's function event is collocated with the main event (so that the propagation effects are identical for both earthquakes); that both events have the same focal mechanisms; and that the empirical Green's function event is small enough to be considered a deltafunction in comparison to the duration of the main earthquake. The largest aftershock to the Cass earthquake was one unit of magnitude smaller than the mainshock and so of sufficiently short duration to be used. It is c. 3 km from the mainshock, but this distance is negligible at the frequencies of concern, compared to the source duration and in comparison to the distance to SNZO. The aftershock also has a significantly different focal mechanism to that of the mainshock. This mismatch will severely affect the amplitude of the resulting source time function, but has a negligible effect on the duration which is the parameter of interest. These departures from the assumptions of the empirical Green's function method will decrease the signal-to-noise ratio of the resulting source time function but should not systematically bias the duration estimate. We select a range of time windows, including the S and surface waves and increasing amounts of coda (160^00 s), from the vertical and SH components of the mainshock and aftershock SNZO records. The corresponding mainshock and aftershock pairs are then deconvolved using the method of Mori & Hartzell (1990). The resulting source time functions are relatively noisy, as expected, and the apparent "double event" nature of the main pulse is likely to be an artefact, especially as the body-wave modelling shows no indication of a double source for the Cass earthquake. The duration of the source time function can be measured quite easily, however, to be c. 7 s. This is longer than that of the shallowest source (3 km) considered in the body-wave modelling. A source at 3^ km is preferred in the modelling, and the long duration obtained at SNZO is clearly inconsistent with a deeper, shorter source for the Cass earthquake. If the rupture propagated to the south, as suggested by the teleseismic modelling, then the 7 s source time function at station SNZO would imply a 5 s average duration. This is consistent with the teleseismic source time function for a rupture at a depth of c. 3 km. Both analyses of the mainshock therefore favour a source at 3-6 km depth, with a duration of c. 7 s, rupturing unilaterally to the south. Confirmation of the southward rupture, and further details of the slip distribution, will be possible in the future using the regional broad-band recordings of the SAPSE network. Source dimensions and stress drop The 1995 Cass earthquake had a oblique reverse focal mechanism with the fault plane striking approximately north-south. Initiation of the event started at c. 9 km, and the centroid depth was in the range 3-6 km. A source duration of c. 7 s is supported by both the body-wave modelling and the source time function calculated using empirical Green's functions. The early aftershocks (first 3 h) occurred mainly to the south of the mainshock location, suggesting rupture to the south. The body-wave modelling suggests a unilateral rupture and supports that rupture direction. If we assume rupture started at depth and propagated to the south, this has implications as far as the source Table 2 Source parameters of the Cass earthquake. Strike ( ) Dip O Rake ( ) Strike 2 n Body-waves 176 ±10 46 ±5 44 ± Depth (km) Moment (10 18 Nm)

7 260 New Zealand Journal of Geology and Geophysics, 2000, Vol. 43 THIS STUDY HARVARD Fig. 4A Comparison between the focal mechanism found in this study and the Harvard CMT. Both are lower hemisphere equal-area projections. dimensions and stress drop are concerned. The source dimensions would then not be the whole aftershock zone, but only the part of the zone to the south of the mainshock, giving a maximum length of c. 8 km (see Aftershock Distribution section). These source dimensions indicate a relatively high stress drop in the order of 100 bars (e.g., Kanamori & Anderson 1975). Another approach is to consider the source time function (Fig. 3B). This suggests a rupture duration of c. 3 s. Assuming that the rupture velocity is a little less than the shear-wave velocity, this gives a rupture length of c. 9 km, in reasonable agreement with the length of the aftershock zone to the south of the mainshock. GPS modelling The Cass earthquake occurred just outside a GPS network that had been surveyed in 1992 (Pearson et al. 1995), partially resurveyed in 1994 following the Arthur's Pass earthquake (Arnadottir et al. 1995), and resurveyed again in early Following the Cass earthquake, 11 stations of this network were resurveyed in early The displacement field, approximately corrected for interseismic displacement between 1995 and 1996, is shown in Fig. 5. Because the Cass earthquake was outside the GPS network, we only have displacement data from within one quadrant so we are unable to formally invert the geodetic data for information about the faulting. We can, however, compare the observed displacement field with dislocation forwardmodel predictions using both the body-wave mechanism solution (Fig. 5) reported above and a fault plane that best fits the aftershock distribution reported below. Both models provide a fair fit to the observations, and it is not possible to discriminate between them based on the limited geodetic data. Strong ground motion and felt effects The Cass earthquake was felt widely in the central South Island. Felt reports were received from as far north as Blenheim and as far south as Timaru. A maximum felt intensity of Modified Mercalli 6 (Study Group 1992) was recorded at Arthur's Pass, Cass, and the Mt White Station nearby. However, insufficient felt reports were received to allow an isoseismal map to be drawn. The mainshock was recorded at 16 strong motion sites throughout the central South Island region. The strongest peak ground acceleration was a value of 0.155g at Flock Hill Station (17 km from the epicentre). The only other recording of above O.lg was that of 0.147g at Arthur's Pass Police Station (21 km from the epicentre). As well as the mainshock, 10 aftershocks were recorded at Arthur's Pass. Included among the mainshock records is a complete set of structural response records from a 12-storey reinforced concrete building in Christchurch. A similar set was recorded during the 1994 June 18 Arthur's Pass earthquake. Analysis of these two datasets is providing new information on the response of such buildings to strong ground motion (Zhao et al. 1996). Although the Arthur's Pass earthquake was much larger than the Cass earthquake, the peak ground accelerations in Christchurch were similar. A possible reason for this can be seen by comparing the Fourier amplitude spectrum for the two events (Fig. 6). The 1994 accelerogram had strong low-frequency content, as would be expected for this larger event. Peak acceleration is generally determined by the strength of the higher (1-10 Hz) frequency signal, which was broadly similar for the two accelerograms. THE AFTERSHOCK SEQUENCE Inversion and relocation Previous studies of large New Zealand earthquakes and their aftershocks (e.g., Robinson 1994) demonstrate the value of relocating the events using a seismic velocity mode! developed especially for that local region. We derive such a velocity model for the Cass region, and then use the routine phase-readings to relocate the earthquakes considered here, and recalculate the magnitudes. The earthquakes relocated are all those identified by the: routine processing procedure as occurring within the rectangular region: 42.80S to 43.05S, E to E, for the period 1995 November 19 to December 9 (1033 events in total). Stations used are those nine of the National Seismograph Network closest to the Cass earthquake, four temporary stations deployed in the region as part of the SAPSE experiment (Fig. 1; Table 3), and station POLA. which was especially installed to record aftershocks. All bat one of the stations (WVZ) recorded three components o f motion, and were all "short-period" types (1 Hz seismometer), except for ARPA which had a "broad-band''

8 Gledhill et al Cass earthquake Harvard 71/73/ km 71/73/159 free depth (5.2 km) 296/31/257 free depth (15 km) NEIC 161/81/4 12 km minimum misfit 176/45/ km Fig. 4B The wavefonn fits. The Harvard CMT solution is shown (top row) for comparison with the minimum misfit solution (bottom row) for the Cass earthquake. The Harvard solution is recalculated with a free depth (second row), and our solution is recalculated at the Harvard depth (third row), to compare the various source parameters. The NEIC solution is shown in the fourth row. Crosses mark waveforms judged not to fit acceptably. From this test, we conclude that our minimum misfit solution is significantly better than both *he Harvard and NEIC solutions H may29, ML 5.5 preliminary 50 mm Degrees East Cass, 95nov24 M.., Pig. 5 Observed horizontal surface displacement from the Cass earthquake compared with a dislocation model using the fault parameters derived by body-wave modelling in this paper. The error ellipses on the observed displacement vectors denote 95% confidence regions. The rectangle shows the surface projection of the west-dipping model fault plane. Mismatches between the model at three of the northwesternmost stations may be due to a nearby Mi 5.5 earthquake. sensor. The short-period instruments were EARSS digital seismographs (Gledhill et al. 1991) using Mark Products L4C-3D seismometers, and the broad-band instrument was a RefTek employing a Streckeisen STS2 seismometer. The EARSS use the time pips broadcast by the New Zealand National Radio stations for absolute timing, and the RefTeks use a GPS clock. The velocity model (Table 4) is 1-D (i.e., plane horizontal layers with P and S station terms), and is obtained by a joint inversion for velocity and location of 169 selected events. The selected events are all those with readings from at least five of the six closest stations. The procedure is described in more detail in Robinson (1994). Station terms for amplitude and duration magnitudes are also obtained. The inversion procedure cannot resolve the very shallow (<c. 2 km depth) structure, so that the four near-surface layers in Table 4 were added to the inversion model, based on results from the Arthur's Pass earthquake (Abercrombie et al. in press): they have little effect on the resulting locations. The station terms derived by the joint inversion are included in Table 3 and shown at the station locations in Fig. 7. The relocated events are classified as to quality (A-F), using the criteria listed in Robinson (1994). For this study, we mainly use the 803 events with quality A-D. Events of quality E have poor depth control and are restricted to a depth of 5.0 km, and events of quality F have inconsistent readings that prevent any reliable solution. The numbers of events in each class are shown in Table 5.

9 262 New Zealand Journal of Geology and Geophysics, 2000, Vol i i i i i M 0.40 Cass, Arthur's Pass, noise level (approx) 0.30 D. S Frequency (Hz) Fig. 6 Fourier amplitude spectra for the Cass and Arthur's Pass earthquakes recorded at the Christchurch Police Station. Aftershock distribution The 803 best located aftershocks (quality D or better) form a band of activity striking at an apparent azimuth of approximately 330, with the mainshock located just to the west of the centre of the band (Fig. 8). There is some indication of a different strike for the events to the north of the mainshock location compared to events to the south. The cross-sections in Fig. 9 give an indication of the extent of the aftershock zone. These cross-sections are plotted northsouth (cross-section 1) and west-east (cross-section 2) to check for agreement with the focal mechanism. The dense zone of aftershock activity has a length of c. 12 km (Fig. 8, 9 cross-section 1) with a width of c. 3 km. The aftershocks extend to a depth of c. 10 km and dip slightly to the west. The largest aftershock, of ML 5.2 occurred 26 h after the mainshock in the northwestern part of the aftershock zone. There is a group of aftershocks which do not follow the main 330 trend, but define a band striking NE-SW, Table 3 Station POLA ARPA LTZ CASA RK.IA MTTA DSZ AHAA EWZ KHZ MQZ QRZ THZ wvz ODZ Station positions and terms. Lat Long P-term (s) S-term (s) 'E 172"E 173"E 174*E Fig. 7 The station terms calculated by the joint inversion. P arrival station terms are shown next to each station, with S-arriva j terms in brackets. particularly on the eastern side of the main aftershock zone These aftershocks appear to follow the trend of the mainh strike-slip faults to the south of the Cass mainshock. Similar patterns of aftershocks well away from the mainshock fault plane have been observed elsewhere, mainly for strike-slip mainshocks (e.g., King et al. 1994). They can be explained by slip on small faults optimally oriented in the combined regional stress and induced stress fields, the latter due to slip in the mainshock. Calculations of these stress fields shov. that the off-fault aftershocks mostly occur where the Coulomb failure stress (CFS) has increased. The CFS takes into account the changes in both the shear and normal stresses. We have calculated the distribution of CFS around the Cass mainshock using the methods described in detai 1 Table 4 The velocity model. Layer Thickness (km) Depth (km) Vp (km/s) Vs (km; s

10 Gledhill et al Cass earthquake 263 Fig. 8 The distribution of aftershocks. Map view of the 803 aftershocks of quality D or better (see text). The known active faults are indicated, and the body-wave focal mechanism (upper hemisphere, equal-area projection) is shown. The line through the focal mechanism indicates the horizontal projection of the slip vector. The rectangles show the area covered by the cross-sections shown in Fig. 9. Magnitude (ML) in Robinson & McGinty (in press). The pattern of induced changes in CFS does indeed provide an explanation for the off-fault aftershocks (Fig. 10). Early aftershocks (first 3 h) occurred mainly to the south of the mainshock location, supporting the possibility of a south-rupturing mainshock. If we replot Fig. 8 and 9 restricting the dataset to only the better located earthquakes recorded after the Poulter River station (POLA) was installed, and showing only those events which occurred to the south of the mainshock location, a clearer picture of the possible rupture zone emerges (Fig. 11, 12). The crosssections shown in Fig. 12 indicate a diffuse band of earthquakes originating at a depth of c. 10 km, shallowing to the south (cross-section 1) and dipping at c. 45 to the west (cross-section 2). This suggests that the fault plane is the north-south-trending nodal plane of the focal mechanism which dips to the west at c. 45 (indicated on cross-section 2 by the dashed line). The fact that the diffuse band of aftershocks appears to strike at 330 is caused by the shallowing in depth of the band of aftershocks to the south. Table 5 Earthquakes in the various quality classes. Class A B C D E F Number % 'E 171'54'E 172 E The aftershocks of dip-slip earthquakes generally do not define an aftershock plane as clearly as those for strike-slip events. Examples of this in California include the 1971 San Fernando earthquake, the 1983 Coalinga and 1985 Kettleman Hills earthquakes (Ekstrom et al. 1992), and the 1994 Northridge earthquake (Hauksson et al. 1995). The picture in New Zealand is less clear. The 1994 Arthur's Pass earthquake has a very complex aftershock pattern, but the May 1990 Weber earthquake, which had a reverse faulting component, shows a clearly delineated aftershock zone (Robinson 1994). A factor in the current case may be the smearing caused by the lack of depth control. Aftershock statistics In most respects, the Cass aftershock sequence can be considered to be similar to many other aftershock sequences recorded in New Zealand (e.g., Adams & Lowry 1971; Robinspn et al. 1976; Robinson 1994) and elsewhere. The b-value of the aftershock sequence (Fig. 13) was close to one (1.08 ± 0.06), and the magnitude-frequency plot suggests that we have located all aftershocks of M L 2.3 and above. The fall-off in the number of aftershocks with time closely follows Omori's law with a/7-value of 0.90 ± 0.05 and an occurrencerate of 54 ± 8 events per day (M L 2.5 or greater) after one day (Fig. 14). Focal mechanisms We estimate the focal mechanisms of the mainshock, the three largest aftershocks, and 22 other aftershocks of M L > 3 which were recorded during the period when Poulter River (POLA) was operating, using the computer programs

11 264 New Zealand Journal of Geology and Geophysics, 2000, Vol. 43 Cross Section km Cross Section 2 Fig. 9 Cross-section views of the 803 aftershocks shown in Fig. 8. Cross-section 1 is the north-south cross-section indicated in Fig. I (viewed from the west, and labelled 1). Cross-section 2 is the east-west cross-section indicated in Fig. 8 (viewed from the south anc labelled 2). The focal mechanism is shown on both cross-sections. Fig. 10 Map view of the distri bution of changes in Couloml Failure Stress (CFS) on optimally oriented faults due to slip durinj; the Cass mainshock. The calcu lations are for a depth of 5 km, anc the aftershocks shown (circles) an: those between 3 and 7 km deptl: and >3 km away from the mainshock fault plane, whose surface projection is shown. The regional stress is taken as the same found near the Arthur's Pass earthquake, 30 km to the west Darker regions represent places where the CFS has decreased and lighter areas where it hai increased. The aftershocks an: coloured in contrasting colours u the background, so the lack»1 events in the darker regions is noi just an illusion.

12 Gledhill et al Cass earthquake 265 Fig. 11 The distribution of aftershocks recorded on station POLA. Map view of the aftershocks recorded on the Poulter River station and located to the south of the mainshock location (see text). The known active faults are indicated. The rectangles show the area covered by the crosssections shown in Fig '48'S 17T54'S 172'S Cross Section 1 Cross Section km ±_ km -M5.0 km Fig. 12 Cross-section views of the aftershocks shown in Fig. 11. Cross-section 1 is the north-south cross-section indicated in Fig. 11 (viewed from the west, and labelled 1). Cross-section 2 is the east-west cross-section indicated in Fig. 11 (viewed from the south and labelled 2). The dashed line indicates the rupture as determined by the mainshock focal mechanism.

13 266 New Zealand Journal of Geology and Geophysics, 2000, Vol Frequency-Magnitude Plot for Cass CASS Aftershocks, Magnitude 2.5 and Greater b = 1.08 ±0.06 n(t) = A.C* p = 0.90 ± 0.05 log(a) = 1.73 ± Magnitude oooooo 10 Fig. 13 A frequency-magnitude plot for the earthquakes recorded Fig. 14 The fall-off in the number of aftershocks with time for during the Cass aftershock sequence. This plot indicates that the earthquakes of magnitude 2.5 and greater during the Cas:; catalogue is complete for earthquakes of magnitude 2.3 and greater. aftershock sequence. developed by Robinson & Webb (1996) based on the method of Schwartz (1995). This method uses both the first motions and amplitude P/S ratios to restrain the focal mechanisms. The envelopes of the complete theoretical seismograms are used to estimate the amplitude ratios rather than either the complete waveform, or just the maximum amplitudes of given phases. This reduces the dependence on the fine details and precise timing, but retains the complete seismograni approach. The resulting focal mechanisms show a variety ot orientations and mechanism types (Fig. 15). However, there is a tendency for the deeper events to have thrust mechanism:- Fig. 15 The focal mechanism- (upper hemisphere, equal-arsii projection) for the aftershock shown on an aftershock map. Tic focal mechanism for the main shock is displayed larger than tri-. rest. The numbers printed on the focal mechanisms give the depth of the earthquakes. 171'40'E 171'50'E 172 E

14 Gledhill et al Cass earthquake 267 IP-Axis Fig. 16 The P and T axes for the focal mechanisms. similar to the mainshock, and for the shallower events to have either normal or strike-slip mechanisms. This pattern was observed for the 1994 Northridge earthquake (Hauksson et al. 1995) and was interpreted as suggesting that the extensional strains in the hanging wall imposed by the mainshock were being released by both normal and strikeslip faulting aftershocks. Most of the focal mechanisms show some consistency in the position of the P and T axes (Fig. 16), suggesting NW-SE compression. Simple composite focal mechanism techniques (e.g., Robinson 1986) give a similar result. This result is in agreement with the regional stress field indicated by other means (e.g., Walcott 1984). The focal mechanism for the mainshock derived using the close stations agrees well with the mechanism derived above using body-wave modelling. Both are thrust mechanisms, with a nearly north-south nodal plane. SEISMIC HAZARD AND TECTONIC IMPLICATIONS Although the Cass and 1994 Arthur's Pass earthquakes occurred in regions of low population distant from a major city, the study of these earthquakes is important in assessing the hazard of the region and understanding the tectonics. New Zealand's third largest city, Christchurch, is situated within 100 km of these earthquakes, so an even larger earthquake in this region could cause considerable damage there. The recognition of this region as one of transition, from the subduction farther north to continentcontinent collision farther south, is important for hazard estimates. Frequent moderate earthquakes have occurred since the beginning of European settlement, and it is likely that these earthquakes will continue. Earlier earthquakes were mainly strike-slip events, but both the Arthur's Pass and Cass earthquakes had thrusting mechanisms consistent with processes causing the uplift of the Southern Alps. Recent earthquakes in the transition region between the subduction in the northern South Island and the strike-slip Alpine Fault have not occurred on mapped faults. The 1990 Tennyson earthquake (Anderson et al. 1993; McGinty et al. 1997), the 1994 Arthur's Pass earthquake (Robinson & McGinty in press; Abercrombie et al. in press), and the Cass earthquake under discussion here have no identified surface expression other than some landsliding and hillside slumping. This has important implications for hazard estimation, and suggests that the overall tectonics of a region are very important for hazard estimation. A similar situation has been highlighted in California following the 1994 Northridge earthquake (Hauksson et al. 1995). The Northridge earthquake is the most recent in a series of Californian earthquakes which have occurred on "blind thrusts", reverse faults which have no direct surface fault traces (e.g., Ekstrom et al. 1992). Accurate hazard estimates must obviously consider this type of event. Another similarity between the Californian Northridge earthquake and recent New Zealand earthquakes within the transition region in the South Island relates to the possibility of earthquake triggering. Stein et al. (1994) used dislocation modelling to argue that the 1994 Northridge earthquake was triggered by the stress loading from the 1971 San Fernanado earthquake. In the New Zealand context, Robinson & McGinty (in press) suggest that the Cass earthquake may have been triggered by the 1994 Arthur's Pass earthquake, which occurred 30 km west of the Cass earthquake. This is concluded after calculating the change caused by the Arthur's Pass earthquake in the Coulomb failure stress for the fault orientation of the Cass earthquake (Robinson & McGinty in press, fig. 9). This suggests the possibility of a timedependent hazard. After a large earthquake, the potential hazard in the surrounding region may increase for a period of time. What does the Cass earthquake add to our knowledge of the tectonics of this region? Both the 1994 Arthur's Pass and the Cass earthquake mechanisms have significant thrust components in a region where previously only strike-slip earthquakes have been observed. However, whereas the 1994 Arthur's Pass earthquake has a possible fault plane in agreement widi the strike of the active faults in the region (NE-SW), the Cass earthquake has a fault plane well away from the active strike-slip fault directions. This suggests a block rotation model with both strike-slip and reverse faults within the same region resulting from the oblique convergence at the plate boundary. Previous models for this region of transition, between subduction and strike-slip motion (e.g., Anderson et al. 1993), have invoked partitioning between the reverse faulting region in Buller, and the strike-slip region farther east in Marlborough, to accommodate the oblique convergence in this region. We now have two earthquakes with significant amounts of reverse faulting in this region of past strike-slip activity, although geodetic GPS modelling suggests that there was a strike-slip component associated with the Arthur's Pass earthquake, possibly as post-seismic slip, or associated with the largest aftershocks. It may also be significant that the locations of these earthquakes are in the southernmost part of the transition region.

15 268 New Zealand Journal of Geology and Geophysics, 2000, Vol. 4.3 CONCLUSIONS The Cass M w 6.2 earthquake of 1995 November 24 had an oblique reverse focal mechanism with an approximately north-south fault plane dipping at 45 to the west. The aftershocks form a band with an apparent strike of 330. The mainshock is located near the centre of the aftershocks, with the early aftershocks occurring mainly to the south suggesting rupture to the south. North-south and east-west cross-sections through the aftershock distribution show a diffuse band of aftershocks which shallow to the south and dip at c. 45 to the west. Dislocation models based on GPS measurements are compatible with the derived focal mechanisms but provide little extra constraint. The aftershock focal mechanisms are mixed but reflect the regional stress field (NW-SE compression). The deeper aftershocks tend to have reverse faulting mechanisms similar to the mainshock, while shallower aftershocks tend to have normal or strike-slip mechanisms. An important feature of the Cass earthquake is that both the mainshock and aftershock sequence occurred within a dense (by New Zealand standards) seismograph network, so that the detailed study of the mainshock has not had to rely on the sparse New Zealand National Seismograph Network. ACKNOWLEDGMENTS Valuable help with the fieldwork was provided by Tim O'Neill. Some of the data used in this study was collected by IRIS/ PASSCAL instruments, and the IRIS DMC provided teleseismic waveform data. This study was supported by the New Zealand Foundation for Research, Science and Technology. 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