The M6, November 22 2016, 00:19UTC earthquake, in the region of an ongoing “slow-slip” event off the North Island’s east coast, has characteristics of a low rupture velocity, “tsunami earthquake”, and may be a double event.
The Nov 22 event is flagged as unusual by the Early-est earthquake monitoring system because it has a relatively large, P-wave dominant period (Td) of 9.0sec, even larger than the Td=6.7sec of the tsunamigenic M7 earthquake in Japan a few hours earlier:
Such a high value for P dominant period suggests a deficiency of high-frequency energy, a characteristic of “tsunami earthquakes“, which have relatively low rupture velocity and may involve shallow rupture in relatively weak rocks.
This low dominant period is seen in a comparison of vertical ground displacement seismograms with a 2015, M5.7 earthquake in the same area, at station G_CAN 23° to the west of the earthquakes:
The 2016 event (lower trace) shows much higher amplitudes than the similar size, 2015 event (upper trace) and generally longer periods in the P wave, as detailed below:
Another striking feature of these seismograms is the longer duration and greater, long-period complexity of the 2016 P, S and Rayleigh surface waves relative to the 2015 event. The P wave complexity at station AU_MILA, 22° to the west of the event, suggests two similar ruptures about 20sec apart:
This separation would explain the complexity, duration and degradation of the 2016 surface waves (after about 00h30m on the full G_CAN seismogram). Rayleigh waves are strongest at around 20sec and thus subject to strong interference when there are two sources about 20sec apart. Such interference would also lead to an underestimate of the event size for the magnitude Ms, based on 20sec surface wave amplitudes, and perhaps also for centroid moment-tensor (CMT) Mw, based on long-period surface waves.
The facilities of IRIS Data Services, and specifically the IRIS Data Management Center, were used for access to waveforms, related metadata, and/or derived products used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR-1261681