Seismic profiling across the ocean-land transition is a known challenge that requires a combination of data collection tech- niques. Seismic acquisition may require the use of swamp bug- gies, bay cables, ocean bottom hydrophones or seismometers, and/or marine air guns. The onshore-offshore seismic profil- ing technique, which uses moderate-to-widely spaced land instruments to record densely spaced marine air-gun sources, undershoots the coastline to provide detailed images of sub- surface regions that cannot be seen in either marine or land profiles. Intermediate to wide-angle raypath geometries are common in this technique. Useful analysis methods include arrival-time velocity tomography or wide-angle prestack migration, neither of which is based on vertical-incidence ray- paths. The creation of the IRIS PASSCAL pool of portable seis- mic recorders accelerated an era of investigations of continental crust using this onshore-offshore seismic profiling technique. During the past decade, the crustal seismology community based primarily in Europe and North America has used this method to improve our understanding of the 3D structure of passive and active continental margins, ocean-continent sub- duction zone and arc systems, and plate boundaries situated near coastlines. Recent targets include the Baltic shield, con- jugate margins on both sides of the mid-Atlantic spreading system, subducting slabs at Cascadia and the Andes, the Mendocino triple junction, the San Andreas strike-slip plate boundary, and as we report here, the Pacific/Indo-Australian transpressional plate boundary in southern New Zealand. This latter plate boundary, which has often been com- pared to the Big Bend of the San Andreas fault in southern California, is an example of extremely active strike-slip trans- lation and continent-continent convergence with one of the fastest present-day exhumation and erosion rates (8-10 mm/yr) in the world. Unlike the San Andreas fault, this southern New Zealand plate boundary has suppressed levels of modern seismicity which suggest either long recurrence rates for strong episodic earthquakes or a larger role for ductile creep or flu- ids-enhanced aseismic slip within the plate boundary fault zone. We used the onshore-offshore technique to conduct crustal imaging of this plate boundary in order to derive crustal structure and rheological behavior. Onshore-offshore profiling. Crustal onshore-offshore tran- sects cross tectonic targets that are often approximately par- allel to coastlines. This profiling takes advantage of an air-gun ship’s ability to shoot at 50-100 m intervals for hundreds of kilometers surrounding a tectonic target. This is coupled with onshore deployment of portable seismic recorders that are able to continuously record for at least 12 to 24+ hours. An indi- vidual instrument will collect the many air-gun signals to pro- vide a broad aperture of source-receiver offsets within a “common-receiver” gather (Figure 1). The offset range of the air-gun data is in part determined by the location of the instru- ment relative to the coastline (e.g., more inland instrument sites produce farther offsets). Because instruments in onshore-off- shore profiling are not rolled or redeployed during a set of air-gun ship tracks and are spaced a few kilometers apart, the onshore field effort is less intensive than standard CDP pro- filing. Reduction of onshore-offshore data requires the use of GPS-based air-gun shot times to extract seismograms from the continuously recorded data. The resulting common receiver gather is rich in resolution in that seismogram spacing is based on the air-gun shot interval; this spatial density greatly improves phase identification and traveltime picking. Seismic events that appear in many crustal-scale experiments are refractions through the crust and upper mantle (Pg and Pn, respectively), the Moho reflection (PmP) at the base of the crust, 256 THE LEADING EDGE MARCH 2003 Imaging a plate boundary using double-sided onshore-offshore seismic profiling DAVID OKAYA, University of Southern California, Los Angeles, U.S. TIM STERN, Victoria University of Wellington, New Zealand STEVE HOLBROOK and HARM VAN AVENDONK, University of Wyoming, Laramie, U.S. FRED DAVEY and STUART HENRYS, Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand Figure 1. The traveltime curves within an onshore-offshore common receiver gather are produced by marine air-gun sources recorded at a specified land instrument site (far offsets; blue raypaths) and by a collo- cated land refraction shot gather (near offsets; green raypaths) using the concept of reciprocity. Asterisk denotes site of collocated receiver gather's instrument and shot gather's explosion. Note opposite direction of ray- paths from air-gun and land explosion sources. Numerous onshore instru- ments provide many such common-receiver gathers, which can be used in tomographic inversions and other analyses. Other acquisition phases involve recording the airgun signals by OBS instruments (pink circle) and an MCS streamer. While these additional raypaths are illustrated (orange for OBS and red for MCS), their traveltime curves are not shown. Figure 2. Example of an onshore-offshore common receiver gather. For this gather the station location is shown in Figure 5 (red triangle) and the air-gun ship was in the Pacific Ocean. Seismic trace spacing is 50 m, an interval controlled by air-gun shot spacing. Vertical axis is reduced trav- eltime using 6.0 km/s. Crustal and upper mantle events are visible.