Low-Frequency Earthquakes Downdip of Deep Slow Slip Beneath the North Island of New Zealand

We report the first catalog of low-frequency earthquakes in the Hikurangi subduction zone, located beneath the Kaimanawa Range of the North Island at 50 km depth, downdip of regularly recurring (every 4-5 years) deep M7 slow slip events. To systematically detect low-frequency earthquakes within the regional continuous seismic data, we utilized a matched-filter approach with template waveforms derived from previous observations of tectonic tremor. We built our catalog of 36 low-frequency earthquake sources, that produced almost 21,000 events over more than a decade, with two matched-filter search iterations. In each iteration, the detections were gathered into families and their coherent waveforms processed and stacked to extract high-quality waveforms, allowing us to pick seismic phase arrivals to locate the low-frequency earthquakes. We highlight three characteristic features to validate that our detected events are indeed low-frequency earthquakes: the eponymous deficit of high frequencies in their seismic waveforms, the episodic swarms of activity that define their activity through time, and their location at the plate boundary with a double-couple source mechanism and geometry consistent with the subduction interface. Considering the observed low-frequency earthquakes' relationship to neighboring slow slip, we observe the event swarms to occur much more frequently than the M7 slow slip events located just updip. Similar to other deep low-frequency earthquakes in other subduction zones, we suggest that this characteristic clustering in time is driven by more frequent, smaller slow slip events that are not clearly observable at the surface. Slow slip is episodic fault slip that lasts days, weeks or months, rather than the rapid ruptures of regular earthquakes. Geodetic observations of the surface displacement produced by slow slip suggest that their timing and location influence the seismic cycle of nearby faults and may even trigger large earthquakes. Although slow slip does not produce seismic radiation itself, slow slip is often accompanied by tiny repetitive seismic signals. These tiny seismic events, called low-frequency earthquakes, can act as a powerful indicator of when and where slow slip is happening. In this study, we develop a new approach to detect low-frequency earthquakes within continuous seismic waveforms, revealing the first observations of low-frequency earthquakes in the Hikurangi subduction zone beneath the North Island of New Zealand. Our catalog of low-frequency earthquakes suggests a complex pattern of slow fault slip at depth, with more frequent activity than geodetic data alone would suggest. The observed low-frequency earthquake activity in the Hikurangi subduction zone thus represents a unique opportunity to study the slip history at depth beneath the North Island of New Zealand. 36 low-frequency earthquake sources are extracted from continuous waveforms through template matching, deblurring, and unsupervised learning Low-frequency earthquake sources locate close to the plate boundary with source mechanisms consistent with the subduction interface Detected low-frequency earthquakes are likely triggered by small, frequent, and deep slow slip not geodetically observable at the surface