DOI: 10.5281/zenodo.15858365
title: Supplementary files to "Focal mechanisms in the southeastern South Island of Aotearoa New Zealand indicate scale-dependent partitioning of transpressional strain"

description: This repository includes supplementary material to the in review artice

Williams, J., Eberhart-Phillips, D., Bourguignon S., Stirling, M., Reyners, M., Upton, P., Focal mechanisms in the southeastern South Island of Aotearoa New Zealand indicate scale-dependent partitioning of transpressional strain. In review with Seismica

Contents of this repository are:



bb_plots_otagonet.zip & bb_plots_sosa.zip: plots of the 96 and 30 focal mechanisms resolved from the OtagoNet array and Southland Otago Seismic Array (SOSA) data using HASH (Hardebeck and Shearer 2002). Focal mechanisms are plotted in sets of 4. The focal mechanisms are plotted as in Figure 2 in the main text, so that for each event, the first column depicts the recording stations's projection and polarity in a lower hemisphere equal area stereonet, the P- and T-axes of the preferred and accepted nodal planes, and the preferred nodal planes. In the centre column, we show the S-wave/P-wave amplitude ratio at each event's recording station, and the preferred and accepted nodal planes. The right column depicts the preferred focal mechanism as a beach ball plot. The letters A-D after 'Q' in the event label indicate the focal mechanism quality (Table 2, main text).

combined_nodal_planes.csv: catalog of 126 focal mechanisms obtained from the OtagoNet array and Southland Otago Seismic Array (SOSA) in the southeastern South Island of New Zealand. For A-C quality events, indicated nodal plane is the one selected by STRESSINVERSE as the preferred 'fault plane' (Vavryčuk 2014). For D quality events, the default nodal plane returned by HASH is indicated. Event quality A-D criteria are described by Table 2 in the main text.

cosa_fmfm_sesi.csv: catalog of 11 focal mechanisms constrained from the Central Otago Seismic Array (COSA) that are located within our southeastern South Island study area (Warren-Smith et al 2017).

ref_instability_info.csv: list of the instability for each SOSA, OtagoNet, and COSA focal mechanism within the stress state resolved by randomly sampling nodal planes and in STRESSINVERSE's final iteration. Focal mechanism instability is defined following Eq. 13 in Vavryčuk (2014). This file also indicates which nodal plane is selected for the final stress inversion.

snz_rmt_nodal_planes.csv: catalog of events in the southeastern South Island from the GeoNet regional moment tensor catalog (Ristau et al 2013; https://github.com/GeoNet/data/tree/main/moment-tensor) up to 28/02/2025. The listed nodal planes do not necessarily indicate the event's fault plane.

SuppFigsS1-S6.pdf: This file contains the following figures:



Fig S1: map indicating how focal mechanisms were grouped using a k-means clustering algorithm.

Fig S2: histograms for the focal mechanism and region moment tensor event magnitudes documented in this study.

Fig S3: lower hemisphere equal area stereonet indicating the orientation of the focal mechanism's dextral and sinistral fault planes as inferred by STRESSINVERSE.

Fig S4: lower hemisphere equal area stereonet indicating the orientation of principal stress axes and focal mechanism fault planes for event clusters 1 and 2, where the clusters are derived using a k-means algorithm.

Fig S5: Histogram of shape rations R resolved from 100 stress inversions of the southeastern South Island focal mechanisms, where in each inversion, the focal mechanism fault planes are randomly sampled.

Fig S6: As for Fig. S5, but for the stress inversions using the regional moment tensor solutions.




References



Hardebeck, J. L., & Shearer, P. M. (2002). A new method for determining first-motion focal mechanisms. Bulletin of the Seismological Society of America, 92(6), 2264-2276.

Ristau, J. (2013). Update of regional moment tensor analysis for earthquakes in New Zealand and adjacent offshore regions. Bulletin of the Seismological Society of America, 103(4), 2520-2533.

Vavryčuk, V. (2014). Iterative joint inversion for stress and fault orientations from focal mechanisms. Geophysical Journal International, 199(1), 69-77.

Warren‐Smith, E., Lamb, S., Stern, T. A., & Smith, E. (2017). Microseismicity in southern South Island, New Zealand: Implications for the mechanism of crustal deformation adjacent to a major continental transform. Journal of Geophysical Research: Solid Earth, 122(11), 9208-9227.