摘要:

The 1987 MarchML6. 3 Edgecumbe earthquake occurred at shallow depth within the Whakatane Graben at the northeastern end of the onshore Taupo Volcanic Zone. The graben has formed where the north-south-trending North Island Shear Belt is intersected by the northeast-trending rift of the Taupo Volcanic Zone, and has been the site of continuing late Quaternary extension and subsidence. A history of displacement relative to sea level can be deduced from stratigraphy of drillholes on the downfaulted graben floor, and from outcrops of marine sediments on the uplifted graben margins. Subsidence had commenced before 0. 6 Ma (possibly at c. 1 Ma) and has continued to the present day at rates of c. 1–2 mm/year. Uplift of the western margin had commenced prior to 0. 4 Ma and the entire western rim was above sea level by c. 0. 3 Ma. Castlecliffian (mid Quaternary) marine sediments have been elevated to more than 300 m above present sea level, indicating long-term uplift rates of c. 1 mm/year. Uplift of the eastern margin has been at 0. 5 mm/year dunng the last 0. 12 Ma.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421383

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Seismic reflection profiles in the vicinity of the 1987 March 2 Edgecumbe earthquake have been used to determine the geological structure in the top 2 km under the southern Rangitaiki Plains. The greywacke basement is offset by about 1200 m on a fault plane that is estimated to have a 38° dip near the major fault associated with the earthquake at McCracken Road. The surface expression of the faulting, however, seems to be on one of a number of splay faults originating from the fault surface on the grey wacke basement.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421384

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

AnML5. 4 earthquake and associated sequence of smaller earthquakes were well recorded in 1977 in the Bay of Plenty region. Upper crustal aftershock origins clustered and migrated within an area 7 × 15 km elongated east-west The aftershocks were relatively swarm-like, producing a slope value for the frequency-magnitude relationship ofb= 1. 29 ± 0. 13, and were apparently of long-sequence duration, with decay coefficientp= 0. 67 ± 0. 03. A northeast-trending rupture fitted for the main shock, originating close to where foreshocks were centred, and passing between tight concentrations of later aftershock activity to either side.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421385

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The Edgecumbe earthquake sequence consisted of more than 600 shocks ofML≥ 3. 0 that occurred in an area of about 70 × 50 km in the Bay of Plenty, New Zealand, between 1987 February 21 and March 18. About 130 of these occurred prior to the main shock (ML6. 3) on March 2. The number, distribution, and variable sensitivity of the stations of the National Seismograph Network has meant that the accuracy of the epicentres computed from data from these stations alone is excellent: by comparison with epicentres determined from temporary station data, the absolute (systematic) uncertainty of epicentres in the vicinity of the i s is estimated to be about 2 km, and the relative uncertainty of epicentres is estimated to be ± 4 km.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421386

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Long-period body-wave modelling of the main shock of the 1987 March 2 Edgecumbe earthquake sequence reveals a multiple rupture event with a moment of 4. 3 × 1018N m. The first rupture originated at a depth of about 8 km and propagated to the surface and to the southwest. A second subevent occurred about 3 s later with an epicentre about 9 km to the southwest of the main-shock epicentre. The focal mechanism of the first subevent shows dominantly normal faulting. Comparison with the surface fault break indicates that the northeast-striking, northwest-dipping nodal plane was the fault plane and hence the horizontal slip vector trend was approximately 342°. The second subevent may correspond to the faulting observed on the Te Teko trace.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421387

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Moment tensor inversions of low-frequency teleseismic surface-wave and body-wave data for the 1987 March 2 Edgecumbe earthquake, New Zealand, give an average seismic moment of 7. 0 × 1018Nm. Measurements of seismic moment at low frequencies are converted to spectral levels normalised to 10 km distance and combined with the acceleration spectrum from a strong-motion seismogram recorded at about 15 km epicentral distance, to give acomposite source spectrum for the main shock covering more than four decades in frequency. The source spectrum shows a constant low-frequency level, a comer frequency of 0. 22 Hz, and a high-frequency slope of approximately ω-2. The comer frequency implies a fault radius of 6 km giving an average fault displacement of 2 m and a stress drop of 5 MPa. Integration of the velocity-squared spectrum indicates an energy release of 1. 1 × 1015J.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421388

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The Edgecumbe earthquake (1987 Mar 2d01h42m34sUT;ML6. 3,MS6. 6) was a normal faulting event accompanied by surface fauft breaks. The earthquake occurred in an onshore area of active back-arc extension characterised by recent volcanic activity. Over 100 aftershocks have been located using 11 portable, and 2 permanent, seismographs in the epicentral region. The events occurred during the period from 3 to 10 days after the main shock, and most have magnitudes of 3. 0 or greater. They have been located using a velocity model derived from the arrival-time data itself. Station terms, which form a part of that model, reflect the lateral variation in near-surface geology. The epicentres define a region 65 km long, in the northeast-southwest direction, and 10 km broad. This is much longer than the observed surface faulting or what would be expected from the main-shock magnitude. There is a gap in the aftershock distribution near the Kawerau Geothermal Field and the recently active andesite volcano, Mt Edgecumbe. Activity southwest of the gap may be part of secondary aftershock sequences because three large aftershocks soon after the main event occurred in that region. The gap in activity may then just be a coincidence. Computed depths of the aftershocks range from 0. 2 to 9. 6 km, most being from 4. 0 to 5. 9 km. The depth cutoff of 9. 6 km probably reflects a high temperature gradient as would be expected in a volcanic region. As is typical for normal faulting events, the main shock initiated near the bottom edge of the aftershock zone. The aftershocks near the main surface faulting appear to lie in the footwall and do not define the fault plane itself. Focal mechanisms have been obtained for 27 events; they are primarily of normal faulting type with varying degrees of strike-slip motion (right-lateral if a northeast-southwest fault plane is picked). The tension (T) axes are mostly horizontal and strongly clustered in the 150 ± 20° azimuth. The compression (P) axes are clustered in a vertical direction or, if not vertical, have a northwest-southeast azimuth. This stress regime reflects the tectonic situation of back-arc spreading and rift formation combined with a degree of shearing due to the oblique plate convergence zone to the east.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421389

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The Edgecumbe earthquake, 1987 March 2 (ML6. 3), was associated with renewed tectonic rupture on the Edgecumbe Fault, renewed movement on the Onepu and Rotoitipakau Faults, and several new surface breaks—the Awaiti, Otakiri, Te Teko, and Omeheu Faults. These northeast trending tectonic ruptures are widely distributed across the Rangitaiki Plains, range in length from 0. 5 to 7 km, and are mostly downthrown to the northwest. They are associated with warping and prominent fissuring. Maximum displacement, 2. 5 m vertical and 1. 8 m extensional, occurred near the middle of the Edgecumbe Fault trace. Trenching investigations revealed that perhaps two faulting events have occurred in the past 1850 years in addition to the 1987 event; the earlier one is unsubstantiated, but may have occurred at about 1850 years B. P., and another occurred at about 800 years B. P. (the time of deposition of the Kaharoa Ash). The 800 year B. P. event was associated with warping, but little fissuring.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421390

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The regional horizontal deformation associated with the 1987 March 2 Edgecumbe earthquake is estimated by comparing measurements made to 15 survey stations before and after the event The stations are located up to 25 km from the epicentre in the Rangitaiki Plains. The main feature of the deformation is an extension of 0. 8 (0. 1) m between the northwestern and southeastern edges of the plains. However, there is a complex pattern of deformation on the plains, with apparent ground strains of up to 300 ppm extension in regions which have been faulted, and up to 70 (30) ppm contraction in the centre of the plains between the main fault traces.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421391

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The Rangitaiki Plains in the Bay of Plenty region has been precise-levelled on several occasions since 1950. Following the 1987 Edgecumbe earthquake. relevelling was performed to determine the extent and magnitude of regional vertical deformation. A new route was installed and surveyed to extendheightcontrol. Results show that vertical deformation was confined to the Rangitaiki Plains, within which subsidence of at least 2 m occurred at the time of the earthquake. Repeated levelling since the earthquake shows that this subsidence is continuing and had reached, by October 1987, approximately 10% of that which occurred during the earthquake.

ISSN:0028-8306    

DOI:10.1080/00288306.1989.10421392

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor