New constraints on seismicity in the Wellington region of New Zealand from relocated earthquake hypocentres

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1 Geophys. J. Int. (4) 158, doi: /j X x New constraints on seismicity in the Wellington region of New Zealand from relocated earthquake hypocentres Wen-xuan Du, 1 Clifford H. Thurber, 1 Martin Reyners, 2 Donna Eberhart-Phillips 3 and Haijiang Zhang 1 1 Department of Geology & Geophysics, University of Wisconsin-Madison, Madison, WI 5376, USA. clifft@ice.geology.wisc.edu 2 Institute of Geological and Nuclear Sciences, 69 Gracefield Road, Lower Hutt, New Zealand 3 Institute of Geological and Nuclear Sciences, Private Bag 19, Dunedin, New Zealand Accepted 4 May 11. Received 4 May 11; in original form 4 January 8 GJI Seismology SUMMARY The Wellington region of New Zealand overlies a strongly coupled portion of the shallow subduction interface between the Pacific and Australian plates. To better understand the active deformation in the area, we perform a double-difference (DD) relocation of 6825 local earthquakes that occurred between 199 and 1. Using both cross-correlation (CC) and bispectrum (BS) methods, we calculate high-quality waveform-based differential times (WBDTs) for event pairs. We manage to more than triple the S-wave differential time measurements after reasonably estimating many of the unpicked S arrivals. After relocation, the image of the double seismic zone beneath this region is greatly sharpened. We find several northeast-striking linear seismic features near Lake Wairarapa on the North Island, which may result from slip along normal faults within the subducting Pacific Plate. We further search for repeating events that might define creeping patches along the interplate thrust. Using a -s data window for CC, we find 287 event clusters with very high waveform similarities. Most of these clusters contain just a doublet that occurred within a short time period. The failure to find repeating earthquakes along the plate boundary in this 12-yr time period is consistent with the hypothesized strong coupling between the Pacific and Australian plates under the Wellington region. Key words: differential traveltimes, earthquake relocation, New Zealand, similar events. 1 INTRODUCTION The Wellington region of New Zealand lies on top of the southwestern end of the Hikurangi subduction zone, where the Pacific Plate is subducting obliquely under the Australian Plate at approximately 38 mm yr 1 (DeMets et al. 1994). Further to the south, the subduction changes into continental collision in the South Island and the plate motion is mainly accommodated through oblique slip on the Alpine fault. The downgoing subducting slab is well defined by the abundant deep-focus seismicity beneath this region (Anderson & Webb 1994) and the dipping seismic zone has a strike of approximately 45. Detailed studies have resolved two planes of seismicity in the slab, as has been observed in several other subduction zones (Robinson 1986). Reyners et al. (1997) determined the focal mechanisms of 145 events shallower than 1 in this region. They found that the stress regime in the subducted plate appears to be dominated by slab pull, with downdip extension in both planes of the double seismic zone. The Pacific and Australian plates appear to be strongly coupled over a downdip width of the plate interface of 7 and subduction thrust earthquakes of approximately Mw 8. could occur in this region (Reyners 1998). Several previous studies have examined the seismic activity in the Wellington region using the absolute arrival times of local earthquakes. Robinson (1986) derived a 3-D velocity model for this area using the arrival times of 129 events and a seismic network of 12 stations. He further used it to refine the locations of 7231 earthquakes that occurred in the period He defined the subducting plate interface using the upper envelope of the northwest dipping seismic zone and found that it is less than 4 deep beneath the Wellington region. Anderson & Webb (1994) studied the earthquakes in the period from 199 January to 1993 February using the upgraded New Zealand National Seismograph Network. They found that the seismic pattern for earthquakes shallower than 4 shows a general fabric that parallels the structural grain of northeasttrending faults in the overlying Australian Plate, but without any clear correlation with specific faults. Eberhart-Phillips & Reyners (1997) determined the 3-D velocity structure for the northern South Island and southernmost North Island using arrival times from 579 local earthquakes recorded during a temporary dense station deployment from 1993 October to 1994 March. They further used it to relocate 3146 earthquakes recorded in the same period, which have been interpreted by Reyners et al. (1997). They found across 188 C 4 RAS

2 Relocated seismicity for the Wellington region 189 Cook Strait that the upper plane of the double seismic zone was offset vertically and the character of seismicity in the overlying plate varied. Recently, accurate differential traveltimes obtained from seismic waveforms have been used to improve dramatically the relocation results for shallow crustal earthquakes (Got et al. 1994; Dodge et al. 1995; Shearer 1997; Rubin et al. 1999; Waldhauser & Ellsworth ; Rowe et al. 2; Schaff et al. 2). Such studies often use a cross-correlation (CC) technique to calculate the relative time delay between the waveforms of two events recorded at the same station to obtain sample-level (sometimes even subsample-level) relative pick accuracy. As a result, the relative location errors between earthquakes can be reduced from kilometres or hundreds of metres to tens of metres or less. Researchers often choose those time delay estimates for the relocation study when their associated CC coefficients exceed a specified threshold. The choice of an optimum threshold value is important but difficult. This is especially so for study regions where the number of stations is relatively small and coverage is not very dense. Du et al. (4) adopted the bispectrum (BS) method (Nikias & Raghuveer 1987; Nikias & Pan 1988) to calculate two additional estimates of the time delay and use them to verify (select or reject) the one computed with the CC technique. This BS verification process can provide quality control over the chosen CC time delays and potentially provide more differential times for close event pairs. Du et al. (4) found that the BS-verified CC time delays provide improved (smaller rms residual and more clustered) earthquake relocation results compared with those selected with the threshold criterion. Given that the Wellington region, with a population of over 4, overlies a strongly coupled portion of the shallow plate interface (Reyners 1998), it is important to understand active deformation in the area. Here we examine 12 yr of seismicity in the Wellington region from 199 to 1, recorded by a dense telemetered seismographnetwork.webuildontheworkofdu et al. (4), who studied 822 earthquakes in the Wellington region that occurred in We first apply both CC and BS methods to obtain reliable waveformbased differential times (WBDTs). Because the total number of S arrivals in the phase catalogue is smaller than one third of that for the P arrivals, we apply two techniques to estimate many of the unpicked S arrivals and then obtain accurate WBDTs for them. In the end, we more than triple the number of S-wave measurements. We then perform double-difference (DD) relocation and examine the patterns and structures revealed in the more clustered relocated seismicity. We further search for similar repeating earthquakes along the plate boundary that might define creeping patches along the plate interface, as reported by Igarashi et al. (3) for the northeastern Japan subduction zone. 2 DATA AND TECHNIQUE We analyse a total of 6825 New Zealand earthquakes that occurred in the Wellington region ( S, E) from 199 to 1 (Fig. 1). These events have magnitudes from 1.8 to 6.5. They were recorded at the permanent stations of the New Zealand National Seismograph Network and additional temporary stations that were set up periodically. These stations have a sampling rate of 5 samples per second. Forty-seven of them are located within of the centre of the study region and are used for the relocation. Twentytwo stations have waveform data for more than 1 per cent of the events and provide the main control for earthquake locations. Many stations do not have three components so we use only the vertical components in this study. We adopt the DD algorithm hypodd (Waldhauser & Ellsworth ) to perform the relocation study. This technique takes advantage of the fact that, if the hypocentral separation between two earthquakes is small compared to the event-station distance and the scale length of the velocity heterogeneity, the two ray paths between the events and a common station are very similar. Thus the difference in traveltimes for the two events observed at the common station can be directly related to the spatial offset between them. HypoDD uses both catalogue differential times (CTDTs) and WBDTs as input data for event relocation. The CTDT is calculated by subtracting the catalogue phase traveltimes of an event pair at a common station. There is a total of P arrivals and 58 S arrivals in the catalogue. Requiring a minimum of 8 common observations (or phase arrivals) for an event pair and a maximum of 1 pairs to be associated with an event within, we obtain P-wave and S-wave CTDTs. We find that for our data set, the total number of catalogue S arrivals is less than one third of that for P arrivals. Consequently for an event pair, the number of stations providing catalogue S picks for both events is smaller than that for catalogue P picks. Combined with the fact that the S-wave windows used for time delay calculations are usually contaminated by the P-wave coda, which decreases the waveform similarities and the CC coefficients, the total amount of selected S-wave WBDT is much smaller than that for P waves. The S-wave data, however, are important for controlling the trade-off between event origin time and focal depth (Gomberg et al. 199). Thus in this study, instead of manually picking those S arrivals unavailable from the phase catalogue, we adopt two techniques to reasonably estimate them. These additional approximate S picks are solely used to construct the appropriate data windows for S-wave time delay calculations. Because not all of the stations having two catalogue P picks for an event pair may have two catalogue S picks, we can further divide them into three subgroups with two (type I), one (type II) or zero (type III) catalogue S picks. For stations of type II, if the phase arrival times P 1 and S 1 are available for the first event and P 2 is available for the second one, we can estimate the missing S 2 as S 2 = S 1 P 1 + P 2. (1) We only perform the estimations for a subset of the missing S picks, however, when the event pairs they belong to are expected to be closely located. The underlying assumption is that for the closely spaced earthquakes, their individual (S P) phase arrival time differences are expected to be very similar. For stations of type III, we can estimate the two missing S picks with the theoretical S arrival times calculated from the catalogue event locations and a 1-D velocity model in the study area (Shearer 1997). We adopt the algorithm of Du et al. (4) to obtain WBDTs using both CC and BS methods and modify it slightly to accommodate the treatment of unpicked S arrivals at stations of type II and III. For a certain station, three time delay estimates in lag numbers are calculated for either P or S waves. k(cc) is the one determined with the CC technique using the bandpass filtered waveforms. If the associated CC coefficient is large enough, we will further extend the estimation into the subsample level by a coherence-weighted linear fitting of the cross-spectrum phase (Poupinet et al. 1984). k(bs1) and k(bs2) are two additional estimates obtained with the BS method, using the bandpass filtered and the raw waveforms, respectively. Basically, a CC time delay estimate k(cc) is trusted (or passes the verification) if its differences from both k(bs1) and k(bs2) do not exceed a tolerance limit lim (more details about the BS verification process can be found in Du et al. 4). The quality control provided

3 19 W.-X. Du et al. 4 S 41 S QRZ Australian Plate GFW DIW TCW Cook SNZO Wellington Kapiti Island KIW CAW MRW WEL OTW BHW WDW North Island MNG MTW MOW BLW MRZ AMW 42 S South Island THZ KHZ BBW CCW Strait Cape Campbell % - 99% 1% - % 1% - 1% Cape Palliser 38 mm yr 1 Hikurangi Trough Pacific Plate 43 S E 173 E 174 E 175 E 176 E 177 E Figure 1. Tectonic setting of the northern South Island and southern North Island. The arrow indicates the velocity of the Pacific Plate relative to the Australian Plate. The rectangular box outlines the Wellington region that is our study area. The circle with a radius of that centres on the study region encloses the seismic stations used for the relocation work. The station symbols indicate the number of events for which they provide the waveform data. They are denoted with triangles ( 99 per cent of events), squares (1 per cent of events) and circles (1 1 per cent of events). by the BS verification is especially important and useful for the S-wave WBDT estimation at stations of type II and III. In detail, we bandpass filter the waveforms between 1.5 and 8 Hz. The data window for the P-wave time delay calculation is 2.54 s, i.e. a total of 128 sample points with of them before the P arrival. It is divided into three 5 per cent overlapping segments each with 64 points for the BS calculation. A longer time window of 3.82 s is used for the S-wave time delay calculation to accommodate the uncertainty associated with the estimated S picks at stations of type II and III. This corresponds to 192 sample points with 5 of them before the S arrival. The S-wave window is divided into two 5 per cent overlapping segments each with 128 points for the BS calculation. After the BS verification with the tolerance limit lim set to one sample point, we obtain a total of P-wave and S-wave WBDTs with CC coefficient down to. (Fig. 2). This is 36 per cent more than what we can select using a CC coefficient cut-off of.7. The number of S-wave measurements at stations of type I is only Thus by reasonably estimating the unpicked S arrivals, we are able to more than triple the S-wave WBDTs. We use two 1-D velocity models for the DD relocation (Tables 1 and 2) and adopt a constant V p /V s ratio of One is a seven-layer model used by Robinson (1986). The other is an eight-layer model developed by Eberhart-Phillips & Reyners (1997). The two models generally provide very similar relocation results, but because they have different layer structures, they give slightly different depth estimates for those events with depths close to one of the velocity boundaries. The overall rms residual is smaller when we use the model of Eberhart-Phillips & Reyners (1997) and we present below the relocation results obtained with it. 3 RELOCATION RESULTS We are able to relocate a total of 6592 (or 96.5 per cent) out of the 6825 earthquakes. The overall rms residual for CTDTs is 95 ms and for WBDTs is 18 ms. The relative location uncertainties for events shallower than 4 are of the order of tens of metres because of the high-quality WBDTs. This result is based on the singular value decomposition (SVD) analysis using several data subsets and the analysis of event pairs with very high waveform similarities that is shown later. The deeper events have smaller numbers and a sparser distribution of observations than the shallow events. Thus

4 Relocated seismicity for the Wellington region x 14 P S 4 Total P estimates Total S estimates 3 Number CC Coefficient Figure 2. Distribution of the BS-verified CC differential times with respect to the CC coefficients for both P and S waves. The bin width for CC coefficients is.2. The parameters used for BS verification are CC lim(l) =., CC lim =.7, CC lim(u) =.8 and lim = 1 (for details see Du et al. 4). The differential times with associated CC coefficients between. and.7 belong to those event pairs with the maximum CC coefficient across all common stations larger than.8. Table 1. 1-D velocity model of Robinson (1986), V p /V s = Depth () to top of layer V p ( s 1 ) Table 2. 1-D velocity model of Eberhart-Phillips & Reyners (1997), V p /V s = Depth () to top of layer V p ( s 1 ) their relative locations are mostly controlled by the CTDTs and the relative location errors are of the order of hundreds of metres to kilometres. The absolute location errors are comparable to those for the initial catalogue locations, mainly because we are using 1-D velocity models for this subduction region, which has large lateral velocity variations (Eberhart-Phillips & Reyners 1997). Overall there are no systematic changes in hypocentres. The average change in epicentre is 1.6, but ranges up to The average change in focal depth is 1.8, but it can be as large as The relocated seismicity patterns are not very sensitive to the initial hypocentres of the events, although the absolute locations are. For example, we tried making the initial locations of the events deeper by 1. The relocated hypocentres are consistently deeper by an average of 7 m and the mean shift in epicentral positions is 26 m. 3.1 Large-scale features Fig. 3 shows in map view the comparison between the network and relocated epicentres. The relocated seismicity is quite clustered in many areas and we can identify prominent northeast-striking features. Fig. 4 shows in cross-section view the seismicity after projecting it along line H H in Fig. 3. After relocation, the image of the double seismic zone is greatly sharpened. Most noticeable is that the top of the upper plane is smoothly defined (Figs 4b and d). The thicknesses of both planes of the seismic zone are reduced from approximately 15 to less than 1. A separation of approximately exists between them. The upper plane dips at a very shallow angle to a depth of approximately and then steepens to 25 with increasing depth. We do not observe the expected convergence of the two planes of seismicity at depth because of our limited study region. We separate the events into two groups through line H H that crosses Cook Strait. For the cross-section on the southwestern side (Fig. 4d), the slab is mostly aseismic between the two planes of seismicity, while for that on the northeastern side (Fig. 4b), many earthquakes are found between the two planes. In the northeastern side, one prominent feature is that the offshore earthquakes beneath Cape Palliser have very confined focal depths after relocation (Fig. 4b). Their absolute depth values, however, should be interpreted with caution. Depths of offshore earthquakes are weakly constrained in standard network processing because of their poor azimuthal station coverage. Our analysis of the 199 Cape Palliser aftershock sequence, which will be shown later, indicates that after DD relocation these offshore earthquakes have very good relative locations because of the high-quality WBDTs. Their absolute locations especially in depth, however, depend strongly on their initial positions.

5 192 W.-X. Du et al. 4.6 S 4.8 S H 41 S 41.2 S 41.4 S 41.8 S 42 S 5 (A) H 174 E E 175 E E 176 E 4.6 S 4.8 S H 41 S 41.2 S Lake Wairarapa 41.4 S Cape Palliser 41.8 S 42 S Cape Campbell 5 H (B) 174 E E 175 E E 176 E Figure 3. Map view of the earthquake locations (a) before and (b) after DD relocation. The three rectangles enclose the subregions to be shown in more detail later. The H H is a projection line with a strike of 135. Its centre is denoted with an open circle.

6 Relocated seismicity for the Wellington region 193 Depth () Northeastern Side Wellington Lake Wairarapa Cape Palliser (A) Southwestern Side Cape Campbell (C) Depth () Wellington Lake Wairarapa Distance () Cape Palliser (B) Cape Campbell Distance () (D) Figure 4. Cross-section view of the event locations by projecting onto the line H H in Fig. 3. Earthquakes to the northeast of the projection line are shown in (a) before and (b) after relocation. Those to the southwest of the projection line are shown in (c) before and (d) after relocation. Open circles represent the 233 earthquakes that are not relocated by DD algorithm. Overall we find that for our studied events the relative locations over a scale of 5 to 1 are excellent. However, because there is no real improvement in absolute locations, the relative event positions over a scale of to should be interpreted cautiously. Anderson & Webb (1994) did not observe a clear correlation of earthquakes shallower than 4 under the North Island with any specific northeast-trending faults in the overlying Australian Plate. From Fig. 4(b) we notice that the majority of the events shallower than 4 fall into the upper plane of the seismic zone. They do not occur in the overlying plate and thus do not correlate well with the crustal faults. Below, we pick out three subareas to illustrate in detail the patterns or structures in the shallow part of the relocated seismicity. 3.2 Lake Wairarapa Fig. 5 shows the relocation results for the subregion near Lake Wairarapa on the North Island (Fig. 3). In map view, the relocated earthquakes form several parallel northeast-striking linear structures that are perpendicular to the strike of the subducting slab (Fig. 5b). Although the seismic features show similar orientations to the crustal faults in the area, for the most part there is no clear association between them. Fig. 5(d) shows a cross-section view of the relocated seismicity by projecting the earthquakes onto the line H H having a strike of 135. The majority of the events fall in a depth range between 24 and. Two highly clustered parallel linear structures can be observed close to the centre of the H H line (Fig. 5b). The one to the southeast has a length of approximately.8 and is located between 25 and 26 depth. The other one to the northwest is longer with a length around 1.1 and deeper with a depth range between 27 and 28. Fig. 5(d) shows that these two seismic structures can be connected by a plane with a dip of 77 and thus may be related to each other. Reyners et al. (1997) determined the focal mechanisms for 145 earthquakes that occurred between 1993 October and 1994 March in our study region. Three of them belong to the shallower and shorter seismic structure. They occurred within a 7-min period on 1993 December 8 and had magnitudes of 3.5, 2.8 and 2.7. Examination of the event catalogue reveals that the latter two smaller events are aftershocks of the first one. Fig. 5(b) displays the fault plane solutions for the three earthquakes. They all show normal-faulting mechanisms. We further notice that all of the three nodal planes with the steeper dips (83,68 and 63 )have strikes in a northeast direction. Although these nodal planes are oriented more easterly than the trend of the seismic structures (Fig. 5b), they are very likely to be the fault planes. We further notice that the earthquakes from these two linear structures have very similar waveforms at several recording stations. This suggests that they have similar normal-faulting mechanisms. Reading et al. (1) investigated the depth of the plate interface across the southern North Island using converted seismic phases SP and PS. For the region near Lake Wairarapa they determined a depth of approximately. Combined with the fact that these two

7 194 W.-X. Du et al. Catalog Locations Relocated Locations 41.2 S S (A) H 41.2 S S (B) H S S S S 41.3 S S E E E E H 41.3 S S E E E E H 22 (C) 22 (D) Depth () Depth () Distance () Distance () Figure 5. Seismicity in the subregion near Lake Wairarapa. (a) Catalogue locations in map view. (b) Relocated locations in map view. The projection line H H has a strike of 135 and its centre ( S, E) is denoted by an open circle. The focal mechanisms (lower hemisphere projection), determined by Reyners et al. (1997), of three earthquakes in the shallower linear structure are shown. Shaded areas represent compressional motions. (c) Catalogue locations along H H in cross-section view. (d) Relocated locations along H H in cross-section view. Focal mechanisms of the three earthquakes are shown (back projected onto the section). The dashed line that connects the two seismic structures together has a dip of 77. seismic structures fall into the upper plane of the double seismic zone in Fig. 4(b), we think that they may represent activity on a normal fault within the subducting Pacific Plate. We performed several different relocation tests to make sure that the observed linear seismic structures are not artefacts, such as changing the 1-D velocity structure, changing the number and configuration of the stations used and changing the initial starting locations of the events. We find that the observed linear seismic structures are robust features in the relocated seismicity. The quality of the relocation results can also be checked directly with the waveform data. Fig. 6 displays the vertical-component waveforms recorded at station OTW for events in each of the two seismic structures. The P arrivals of the earthquakes are aligned by CC to that of a reference event (denoted with a star). The earthquakes are aligned vertically upward with increasing hypocentral distances to station OTW, computed from their relocated locations. The S waves of the shallower structure (Fig. 6a) arrive approximately 4 s after the P waves, while those of the deeper one (Fig. 6b) come a little later, as expected. The two groups of events have similar P waves butdifferent-lookings waves. For both groups, we observe the time move-out of S waves with increasing hypocentral distances. The differences in the hypocentral distances for the farthest and closest events in the two groups are.8 and 1., respectively. The related differences (S P)intime between the farthest and closest events are both approximately.8 s, or four sample points. Because the (S P) time is roughly 4 s for the event with a hypocentral distance of 32.6 from station OTW, then for two events with a difference in hypocentral distance of.8 we expect a difference in (S P) time of.9 s, in good agreement with the observed differences. The above waveform examination corroborates our relocation results, demonstrating that the observed linear seismic structures are not relocation artefacts. McGinty et al. () inverted for the stress tensor orientation for the northern South Island and southern North Island using the Coulomb failure criterion and P-wave first motions. They found that for the upper plane of the dipping seismic zone beneath the southern North Island, the stress regime favours normal faulting relative to the subducted plate, with motion on a near-vertical fault plane oriented along the strike of the subducted plate being preferred. Thus, we think that the several other parallel linear seismic structures in this subregion with steep dip angles may result from similar normal faulting on other faults within the subducting Pacific Plate. This also explains why these linear features do not align with the crustal faults in the overlying Australian Plate.

8 Station OTW (azm = 268 ) Relocated seismicity for the Wellington region * (A) * (B) Time (sec) 33.5 Figure 6. Vertical-component waveforms at station OTW for earthquakes in the (a) shallower and (b) deeper linear seismic structures shown in Fig. 5. Waveforms are bandpass filtered between 1.5 and 8 Hz. The time window is 1 s before and 7 s after the P arrival. The P arrivals of the earthquakes are aligned by CC to that of a reference event ( ). The events are aligned vertically by the size of their hypocentral distances to station OTW. The two vertical dashed lines are used to demonstrate the move-out of S waves. The differences (S P)intime between the events with the largest and smallest distances are.8 s for both structures. 3.3 Cape Campbell Earthquakes with depths shallower than in our study area are most abundant in the Cape Campbell subregion (Fig. 3). Depths of shallow offshore earthquakes are weakly constrained in standard network processing (Fig. 7c). We can observe several prominent clusters with steep vertical extension in the shallow part of the relocated seismicity (Fig. 7d). Some of them probably can be associated with the offshore extension of the crustal faults on the South Island. In the upper plane of the double seismic zone, we find two prominent clusters of events with depths at approximately 25 and 29, respectively. The shallow cluster has a very small dip angle. Two events in the shallow cluster have previously determined focal mechanisms (Figs 7b and d, Reyners et al. 1997). They are the low-angle thrust-type events that indicate the position of the subducting plate interface. Fig. 8 shows the vertical-component waveforms for 37 events in the shallow cluster including the two with determined focal mechanisms. The high degree of similarity among them suggests that this shallow cluster can be interpreted as a group of thrust events

9 196 W.-X. Du et al S H 41.5 S H S 41.7 S 41.8 S (A) E E E E H 41.8 S (B) E E E E H 1 1 Depth () 4 Depth () (C) Distance () (D) Distance () Figure 7. Earthquake locations (a and c) before and (b and d) after relocation for the Cape Campbell region. (a) and (b) are map views; (c) and (d) are cross-section views after projecting onto line H H. Focal mechanisms, determined by Reyners et al. (1997), of two events in the shallower cluster at a depth of approximately 25 are shown [lower hemisphere projection in (b) and back projected onto the section in (d)]. The projection line H H has a strike of 135 and its centre (41.65 S, E) is denoted by an open circle. at or possibly just above the plate interface. However, given that we do not see other clusters with a very small dip angle near the plate interface, it is likely that low-angle thrust type events are still quite rare in the Wellington region, as suggested by Reyners et al. (1997). The deeper cluster has a northeast-striking trend and a very large dip angle. It may be similar to what we observe in the Lake Wairarapa region on the North Island, i.e. seismic activity on a normal fault within the subducting Pacific Plate. We, however, do not have focal mechanism information to check this conjecture. 3.4 Cape Palliser The Cape Palliser region is located at the southeastern tip of the North Island (Fig. 3). On 199 October 4, a M L 5.3 earthquake occurred in this area and was followed by another M L 5.3 event 26 hr later. The two main shocks were accompanied by a large number of aftershocks. Fig. 9 shows the relocation results for 269 earthquakes that occurred during the 6 months after the first main shock. We separate this event sequence into three consecutive time periods to show its temporal evolution. The first period (Figs 9a and b) covers the 26 hr between the two main shocks. We observe that the majority of the events fall into a tight depth range between 23 and 24, while their horizontal positions are more scattered. A few earthquakes in the western side have shallower depths between and 21. In the second period that spans 1 days after the second main shock (Figs 9c and d), many aftershocks occurred in the western side and form a tight cluster with depths between and 22. The third period (Figs 9e and f) extends to 6 months after the second main shock. The majority of the events are located in the western branch and propagated toward the North Island. We want to point out that the tight depth distribution for the relocated events is mainly attributed to the high-quality WBDTs. Fig. 1 shows the vertical-component waveforms of 25 earthquakes in both the deeper eastern (depths between 23 and 24 ) and shallower western (depths between

10 Station BLW (dist = 98 ; azm = 76 ) Relocated seismicity for the Wellington region 197 * * Time (sec) Figure 8. Vertical-component waveforms at station BLW of 37 events in the shallow cluster with depths of approximately 25 (Fig. 7d). Waveforms are bandpass filtered between 1.5 and 8 Hz. The 1-s time window starts 1 s before the P arrival. The two events with low-angle-thrust focal mechanisms determined by Reyners et al. (1997) are denoted by. and 22 ) branches. The P waves are very similar within each group and differ significantly from those in the other group. The S waves look more similar for events in the western branch, consistent with the fact that they more clustered in space than those in the deeper, eastern group. Although the relocated aftershocks appear to be very clustered, especially in their depth distributions, their absolute locations are not very well resolved. This mainly results from the fact that this sequence is located offshore and no seismic stations exist to the south and east. After including the most distant station KHZ on the South Island (Fig. 1) with an epicentral distance of approximately 18, the largest azimuthal gap in station coverage for the sequence is still more than 18. Luo (1992) analysed the seismic phases of nine aftershocks in this sequence to study the subduction interface and crustal structure in the Cape Palliser region. He used the four closest stations with one of them having three components

11 198 W.-X. Du et al S Period I 41.5 Period I Cape Palliser Latitude ( S) S (A) E E E 41.7 (B) Depth () 41.5 S Period II 41.5 Period II Latitude ( S) S (C) E E E 41.7 (D) Depth () 41.5 S Period III 41.5 Period III Latitude ( S) S (E) E E E 41.7 (F) Depth () Figure 9. Aftershock sequence of two M L Cape Palliser earthquakes. Event locations before and after the relocation are represented with open squares and filled circles, respectively. (a and b) The activity for the 26 hr between the two main shocks. (c and d) The activity for the 1 days after the second main shock. (e and f) The activity to 6 months after the second main shock. and determined focal depths ranging from 12 to 15, which are on average 6 shallower than their network locations. He argued that these depths are close to that of the plate interface in the region (Davey & Smith 1983; Davey et al. 1986). Waldhauser & Ellsworth () demonstrated that for the DD algorithm, consistent errors in initial absolute locations can cause systematic location errors in the relocated seismicity. To deal with the possible scenario that the catalogue depths determined for events in this region are systematically deeper than their true values as suggested by Luo (1992), we repeat the DD relocation five times by decreasing the initial depths of all events by the same amount, from 1 to 5. We find that for each individual relocation, the relocated events have shallower depths as expected but their geometry remains very similar to that shown in Fig. 9. This indicates that the high-quality WBDTs control the relative positions of this aftershock sequence very well, but we do not have good control on the absolute depths. 4 SEARCH FOR REPEATING EARTHQUAKES Using waveform similarity analysis, Igarashi et al. (3) detected 321 earthquake clusters with very similar waveforms (CC coefficient above.95) that are bandpass filtered between 1 and 4 Hz, on the plate boundary in the northeastern Japan subduction zone.

12 Relocated seismicity for the Wellington region 199 BHW (dist=55; azm=295 ) WDW (dist=56; azm=315 ) CAW (dist=66; azm=3 ) Time (sec) Time (sec) Time (sec) Figure 1. Vertical-component waveforms at three stations BHW, WDW and CAW. Waveforms are bandpass filtered between 1.5 and 8 Hz. The 14-s time window starts 1 s before the P arrival. The waveforms above and below the thick horizontal lines correspond to 25 earthquakes in the deeper, eastern and shallower, western branches in Fig. 9, respectively.

13 11 W.-X. Du et al. They found that most of the clusters were not located within the subducting Pacific Plate. They further argued that these events represent repeating slips of small asperities on the plate boundary with a dimension of.1 to 1 that are loaded by stable sliding (creep) in the surrounding area. In this work, we also make an effort to look for similar event clusters in our study region. For this calculation, we bandpass filter the waveforms between 1.5 and 5 Hz. The -s time window we choose for waveform CC starts 1 s before the catalogue P arrival. This window length is sufficiently long to include both P and S waves for most events because we only examine stations within. We use the maximum CC coefficient of an event pair across all common stations to represent their waveform similarity. We then use a dendrogram-based clustering algorithm (Lance & Williams 1967) to group the earthquakes into clusters. We choose acccoefficient cut-off of.98, which is higher than the.95 used by Igarashi et al. (3) because they impose that similarity constraint at two or more stations for an event pair. We tried relaxing the clustering criteria, i.e. decreasing the CC coefficient cut-off value to.97,.96 and.95. In these cases, we obtain a larger number of clusters with more events in some clusters as expected, but the patterns of time separation for events in the clusters remain very similar to what we describe below. We find a total of 287 clusters composed of a total of 621 events, which represents 9 per cent of the earthquakes we examine. They have magnitudes ranging from 2. to 3.8. We first examine the spatial separations between the events in each cluster. After relocation, the average horizontal and vertical separations decrease from 1.5 to 66 m and from 1.6 to 57 m, respectively. Considering the fact that for WBDTs with not very high CC coefficients we can only achieve a resolution on the level of a sample point, which is.2 s, the reduction in the spatial separation is reasonable for these event pairs that very likely ruptured in the same location. The majority of the clusters contain just a pair of earthquakes (a doublet) and the largest one has four events. When we examine the time separation between consecutive events in each cluster, we find that 44 per cent of the event pairs occurred within 1 day and 74 per cent of them occurred within 1 month. The largest observed time separation is 7.6 yr. Fig. 11 shows the locations of 46 such event pairs with time separations larger than 6 months. Comparing Fig. 11(b) with Figs 4(b) and (d) indicates that most of the event pairs occurred in the upper plane of the double seismic zone. Their depth distribution suggests that they are mainly located within the subducting Pacific Plate, rather than along the plate boundary as reported by Igarashi et al. (3). Most of the event clusters we find 4.6 S 4.8 S H 41 S 41.2 S 41.4 S 41.8 S 42 S (A) E E 175 E E 176 E H Depth () 1 4 (B) Distance () Figure 11. Locations of 46 event pairs with very high waveform similarity (the maximum CC coefficient across all common stations larger than.98). Time separations for events in each pair are larger than 6 months. (a) Map view. (b) Cross-section view.

14 Relocated seismicity for the Wellington region 111 correspond to swarms or aftershock activity. We do not find a single cluster within which several events occurred in a relatively regular manner in the 12-yr period we examine. Igarashi et al. (3) reported that the repeating earthquakes that occurred continuously are not associated with either the large moment release areas of recent large interplate earthquakes or the strongly coupled areas inferred from GPS data analysis. Thus, the failure to find such repeating events in our study area is consistent with the hypothesized strong plate coupling beneath the Wellington region. Under the northern South Island, which is a little to the south of our study region, the plates are thought to be permanently locked with no active subduction occurring (Reyners et al. 1997; Reyners 1998). The dextral strike-slip Marlborough fault system there accommodates 8 1 per cent of the total motion between the Australian and Pacific plates (Holt & Haines 1995). In our study area, the plate interface is thought to be strongly coupled. Reyners et al. (1997) found that subhorizontal compressional strain predominates in the overlying Australian Plate. Their finding is consistent with recent GPS results, which indicate that the relative velocity of the Wellington region is approximately one-half of the predicted value between the Australian and Pacific plates (Larson & Freymueller 1995; Darby & Beavan 1). The finding of low-angle thrust events in this region, however, indicates that the plate interface is not completely locked. Further to the northeast, the degree of plate coupling is thought to decrease from strong to weak (Reyners 1998). We think that the chances are better that repeating events could be found along the plate interface under the northeastern part of the North Island. 5 CONCLUSIONS Using the waveform data of 6825 local earthquakes that occurred between 199 and 1, we perform a DD relocation study for the Wellington region of New Zealand. After relocation, the image of the double seismic zone beneath this region is greatly sharpened. Both planes of the double seismic zone have a thickness of less than 1 and there is a separation of approximately between them. We find several northeast-striking linear seismic features near Lake Wairarapa in the North Island. Their depth distribution, geometry and focal mechanisms suggest that these events resulted from slip along normal faults within the subducting Pacific Plate. In the region near Cape Campbell, we find a group of low-angle thrust events at a depth of 25 based on its geometry, focal mechanism information and waveform similarity. We also examine in detail the M L Palliser earthquake sequence. As a result of the poor azimuthal station coverage, we are not able to resolve well their absolute locations. Their relative positions, however, are controlled well by the high-quality WBDTs. The majority of the aftershocks can be separated into a deeper eastern branch with a confined depth range between 23 and 24 and a western branch that is shallower (between and 22 ) and more spatially clustered. The results show that DD relocation with waveform CC is valuable for studies of subduction zones, revealing crustal features and details of the plate interface. We attempt to find similar repeating earthquakes along the subducting plate boundary such as reported by Igarashi et al. (3) for northeastern Japan. Using a high CC coefficient cut-off at.98, we find 287 event clusters with very high waveform similarity. Most of these clusters contain just a doublet that occurred within a short time period. We do not find a single cluster within which several events occurred in a continuous manner in the 12-yr period we examine. This result is consistent with the hypothesized strong coupling between the Pacific and Australian plates under the Wellington region. ACKNOWLEDGMENTS The authors acknowledge the helpful comments and reviews by Anya Reading, Rob van der Hilst and an anonymous reviewer. The authors thank those involved in running the Wellington seismograph network and analysing the data from it, particularly Russell Robinson, Brian Ferris, Jan Harris and Diane Maunder. The GMT system (Wessel & Smith 1998) was used to make several figures. The authors are grateful to Michael Brudzinski for helping with some GMT plots. This material is based upon work supported by the National Science Foundation under Grant No. EAR REFERENCES Anderson, H. & Webb, T., New Zealand seismicity: patterns revealed by the upgraded National Seismograph Network, N. Z. J. Geol. Geophys., 37, Darby, D. & Beavan, J., 1. Evidence from GPS measurements for contemporary interplate coupling on the southern Hikurangi subduction thrust and for partitioning of strain in the upper plate, J. geophys. Res., 16, Davey, F.J. & Smith, E.G.C., A crustal seismic reflection-refraction experiment across the subducted Pacific Plate under Wellington, New Zealand, Phys. Earth planet. Int., 31, Davey, F.J., Hampton, M., Lewis, K., Childs, J., Pettinga, J. & Fisher, M., The structure of a growing accretionary prism, the Hikurangi margin, New Zealand, Geology, 14, DeMets, C., Gordon, R.G., Argus, D.F. & Stein, S., Effect of recent revisions to the geomagnetic reversal timescale on estimates of current plate motions, Geophys. Res. Lett., 21, Dodge, D.A., Beroza, G.C. & Ellsworth, W.L., Foreshock sequence of the 1992 Landers, California, earthquake and its implications for earthquake nucleation, J. geophys. Res., 1, Du, W.X., Thurber, C.H. & Eberhart-Phillips, D., 4. Earthquake relocation using cross-correlation time delay estimates verified with the bispectrum method, Bull. seism. Soc. Am., 94, Eberhart-Phillips, D. & Reyners, M., Continental subduction and three-dimensional crustal structure: The northern Southern Island, New Zealand, J. geophys. Res., 12, Gomberg, J.S., Shedlock, K.M. & Roecker, S.W., 199. The effect of S-wave arrival times on the accuracy of hypocenter estimation, Bull. seism. Soc. Am., 8, Got, J.-L., Fréchet, J. & Klein, F.W., Deep fault plane geometry inferred from multiple relative relocation beneath the south flank of Kilauea, J. geophys. Res., 99, Holt, W.E. & Haines, A.J., The kinematics of northern South Island, New Zealand, determined from geologic strain rates, J. geophys. Res., 1, Igarashi, T., Matsuzawa, T. & Hasegawa, A., 3. Repeating earthquakes and interplate aseismic slip in the northeastern Japan subduction zone, J. geophys. Res., 18(B5), 2249, doi:1.129/2jb19. Lance, G.N. & Williams, W.T., A general theory for classificatory sorting strategies, 1. hierarchical systems, Comput. J., 1, Larson, K.M. & Freymueller, J., Relative motions of the Australian, Pacific and Antarctic plates estimated by the Global Positioning System, Geophys. Res. Lett., 22, Luo, X., Subduction interface and crustal structure in the Cape Palliser region, North Island, New Zealand, from observations of Cape Palliser earthquakes, N. Z. J. Geol. Geophys., 35, McGinty, P., Reyners, M. & Robinson, R.,. Stress directions in the shallow part of the Hikurangi subduction zone, New Zealand, from the inversion of earthquake first motions, Geophys. J. Int., 142,

15 112 W.-X. Du et al. Nikias, C.L. & Pan, R., Time delay estimation in unknown Gaussian spatially correlated noise, IEEE Trans. Acoust. Speech Signal Processing, 36, Nikias, C.L. & Raghuveer, M.R., Bispectrum estimation: A digital signal processing framework, Proc. IEEE, 75, Poupinet, G., Ellsworth, W.L. & Fréchet, J., Monitoring velocity variations in the crust using earthquake doublets: an application to the Calaveras fault, California, J. geophys. Res., 89, Reading, A.M., Gubbins, D. & Mao, W., 1. A multiphase seismic investigation of the shallow subduction zone, southern North Island, New Zealand, Geophys. J. Int., 147, Reyners, M., Plate coupling and the hazard of large subduction thrust earthquakes at the Hikurangi subduction zone, New Zealand, N. Z. J. Geol. Geophys., 41, Reyners, M., Robinson, R. & McGinty, P., Plate coupling in the northern South Island and southernmost North Island, New Zealand, as illuminated by earthquake focal mechanisms, J. geophys. Res., 12, Robinson, R., Seismicity, structure and tectonics of the Wellington region, New Zealand, Geophys. J. R. astr. Soc., 87, Rowe, C.A., Aster, R.C., Phillips, W.S., Jones, R.H., Borchers, B. & Fehler M.C., 2. Using automated, high-precision repicking to improve delineation of microseismic structures at the Soultz geothermal reservoir, Pure appl. Geophys., 159, Rubin, A.M., Gillard, D. & Got, J.-L., Streaks of microearthquakes along creeping faults., Nature, 4, Schaff, D.P., Bokelmann, G.H.R., Beroza, G.C., Waldhauser, F. & Ellsworth, W.L., 2. High-resolution image of Calaveras Fault seismicity., J. geophys. Res., 17(B9), 2186, doi:1.129/1jb633. Shearer, P.M., Improving local earthquake locations using the L1 norm and waveform cross correlation: Application to the Whittier Narrows, California. aftershock sequence, J. geophys. Res., 12, Waldhauser, F. & Ellsworth, W.L.,. A double-difference earthquake location algorithm: Method and application to the northern Hayward Fault, California, Bull. seism. Soc. Am., 9, Wessel, P. & Smith, W.H.F., New version of the generic mapping tools released, EOS, Trans. Am. geophys. Un., 79, 579.