Repeating Earthquake Detection and Characterisation in New Zealand: A Catalogue for the Raukumara Peninsula, Northern Hikurangi Subduction Margin

<p>Repeating earthquakes provide a novel way of monitoring how stresses load faults between large earthquakes. In this thesis, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand and present New Zealand’s first long-duration repeating earthquake catalogue. This thesis addresses three primary objectives: (1) develop a method and composite criterion for identifying repeating earthquakes; (2) build a long-duration catalogue of repeating earthquakes for the Raukumara Peninsula; and (3) apply the method and composite criterion in different tectonic settings to investigate whether it can be applied more broadly elsewhere in New Zealand. The systematic identification of repeating earthquakes in New Zealand provides the first step in being able to monitor the state of stresses of New Zealand’s active faults in situ throughout the earthquake cycle. Studies elsewhere, particularly in Japan and California, have developed case-specific criteria for identifying repeating earthquakes. Building on these studies, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand, focusing on seismicity around the Raukumara Peninsula. Our composite criterion states that for events to be identified as repeating earthquakes, two or more events must have a normalised cross-correlation of at least 0.95 at two or more seismic stations, when calculated for 75% of the earthquake coda. Sensitivity to correlation window length, filtering frequency-band and correlation threshold were tested during the development of the composite criterion. These tests indicated that small perturbations to the parameter thresholds did not affect our ability to detect repeating earthquakes using the composite criterion. By applying our composite criterion to seismicity around the Raukumara Peninsula, we identified 62 repeating earthquake families occurring between 2003 and 2018, consisting of 160 individual earthquakes. These families have a magnitude range of MW 1.5–4.5, and have recurrence intervals and family durations of < 1–12 years. High-precision absolute and relative locations were calculated using manual phase picks and cross-correlation re-picking. Focal mechanisms for 56 of the families were also determined, using P-wave first motions, revealing predominantly strike-slip and normal faulting at shallow depths, low-angle reverse faulting along the subduction interface, and normal faulting in the subducting plate. We compared the timing of the repeating earthquakes to slow-slip events previously identified using geodetic measurements around the Raukumara Peninsula and observed that repeating earthquakes occurred during 26 of the 31 identified periods of slow-slip. We also compared the seismic moment– recurrence interval relationship of the Raukumara Peninsula repeating earthquakes to that of earthquakes near Parkfield, California, identified by Nadeau and Johnson (1998), and observed a similar functional relationship. Slip-rates of the Raukumara Peninsula repeating earthquake families were also calculated using a slip-rate–moment relationship and were found to vary from < 10mm/yr to 80mm/yr. We applied the method and composite criterion developed for the Raukumara Peninsula to two other locations to ensure it could be applied successfully in other New Zealand regions with different seismotectonic characteristics. Using our workflow, we successfully identified four families in Marlborough, and three families around Fiordland. These families differ from those identified around the Raukumara Peninsula in that they had relatively short recurrence intervals and family durations, of 2 minutes– 15 months. The ability of the composite criterion to identify these families confirms its suitability for further studies of repeating earthquakes throughout New Zealand.</p>

Repeating Earthquake Detection and Characterisation in New Zealand: A Catalogue for the Raukumara Peninsula, Northern Hikurangi Subduction Margin

<p>Repeating earthquakes provide a novel way of monitoring how stresses load faults between large earthquakes. In this thesis, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand and present New Zealand’s first long-duration repeating earthquake catalogue. This thesis addresses three primary objectives: (1) develop a method and composite criterion for identifying repeating earthquakes; (2) build a long-duration catalogue of repeating earthquakes for the Raukumara Peninsula; and (3) apply the method and composite criterion in different tectonic settings to investigate whether it can be applied more broadly elsewhere in New Zealand. The systematic identification of repeating earthquakes in New Zealand provides the first step in being able to monitor the state of stresses of New Zealand’s active faults in situ throughout the earthquake cycle. Studies elsewhere, particularly in Japan and California, have developed case-specific criteria for identifying repeating earthquakes. Building on these studies, we develop a method and composite criterion for identifying repeating earthquakes in New Zealand, focusing on seismicity around the Raukumara Peninsula. Our composite criterion states that for events to be identified as repeating earthquakes, two or more events must have a normalised cross-correlation of at least 0.95 at two or more seismic stations, when calculated for 75% of the earthquake coda. Sensitivity to correlation window length, filtering frequency-band and correlation threshold were tested during the development of the composite criterion. These tests indicated that small perturbations to the parameter thresholds did not affect our ability to detect repeating earthquakes using the composite criterion. By applying our composite criterion to seismicity around the Raukumara Peninsula, we identified 62 repeating earthquake families occurring between 2003 and 2018, consisting of 160 individual earthquakes. These families have a magnitude range of MW 1.5–4.5, and have recurrence intervals and family durations of < 1–12 years. High-precision absolute and relative locations were calculated using manual phase picks and cross-correlation re-picking. Focal mechanisms for 56 of the families were also determined, using P-wave first motions, revealing predominantly strike-slip and normal faulting at shallow depths, low-angle reverse faulting along the subduction interface, and normal faulting in the subducting plate. We compared the timing of the repeating earthquakes to slow-slip events previously identified using geodetic measurements around the Raukumara Peninsula and observed that repeating earthquakes occurred during 26 of the 31 identified periods of slow-slip. We also compared the seismic moment– recurrence interval relationship of the Raukumara Peninsula repeating earthquakes to that of earthquakes near Parkfield, California, identified by Nadeau and Johnson (1998), and observed a similar functional relationship. Slip-rates of the Raukumara Peninsula repeating earthquake families were also calculated using a slip-rate–moment relationship and were found to vary from < 10mm/yr to 80mm/yr. We applied the method and composite criterion developed for the Raukumara Peninsula to two other locations to ensure it could be applied successfully in other New Zealand regions with different seismotectonic characteristics. Using our workflow, we successfully identified four families in Marlborough, and three families around Fiordland. These families differ from those identified around the Raukumara Peninsula in that they had relatively short recurrence intervals and family durations, of 2 minutes– 15 months. The ability of the composite criterion to identify these families confirms its suitability for further studies of repeating earthquakes throughout New Zealand.</p>

An Evaluation of the Timing Accuracy of Global and Regional Seismic Stations and Networks

Clock accuracy is a basic parameter of any seismic station and has become increasingly important for seismology as the community seeks to refine structures and dynamic processes of the Earth. In this study, we measure the arrival time differences of moderate repeating earthquakes with magnitude 5.0–5.9 in the time range of 1991–2017 at the same seismic stations by cross-correlating their highly similar waveforms and thereby identify potential timing errors from the outliers of the measurements. The method has very high precision of about 10 ms and shows great potential to be used for routine inspection of the timing accuracy of historical and future digital seismic data. Here, we report 5131 probable cases of timing errors from 451 global and regional stations available from the Incorporated Research Institutions for Seismology Data Management Center, ranging from several tens of milliseconds to over 10 s. Clock accuracy seems to be a prevailing problem in permanent stations with long-running histories. Although most of the timing errors have already been tagged with low timing quality, there are quite a few exceptions, which call for greater attention from network operators and the seismological community. In addition, seismic studies, especially those on temporal changes of the Earth’s media from absolute arrival times, should be careful to avoid misinterpreting timing errors as temporal changes, which is indeed a problem in some previous studies of the Earth’s inner core boundary.

First Results for the Selection of Repeating Earthquakes in the Eastern Tien Shan (China)

This research is the first stage of seeking repeating earthquakes sequences (RES) in the modern orogeny active zones. The main idea is to find the possible influence of space weather parameters on the seismic process. This is the reason why I am interested in the satellite CSES-01 data. It is a tool that has monitored Earth’s seismo-electromagnetic activity since 2018. Presuming the “ionosphere-atmosphere-lithosphere” relation exists, it is necessary to involve both satellite and ground-based observational data. The seeking of the triggering mechanism still requires additional analysis of consistent geophysical ground-based networks (geomagnetic and seismic). The stations’ coordinates and instruments are presented. In this work, an earthquake catalog (NEIC) of 400 earthquakes with 2.5+ magnitude from 2015 to 2020 was used. The earthquakes epicenters are illustrated on Google Earth basemap (Landsat image) with geologic linear faults. It could help to find any correlation with relief surface or shear zones, which could be areas of nucleation. Some earthquake clusters were found in the Eastern Tien Shan (region of China), on the border with Kazakhstan and Kyrgyzstan, due to the K-means algorithm. Clustering helps group earthquakes into small families for further cross-correlation of seismic waveforms and the best match selection between the neighbors.

Shallow slow earthquakes to decipher future catastrophic earthquakes in the Guerrero seismic gap

The Guerrero seismic gap is presumed to be a major source of seismic and tsunami hazard along the Mexican subduction zone. Until recently, there were limited observations at the shallow portion of the plate interface offshore Guerrero, so we deployed instruments there to better characterize the extent of the seismogenic zone. Here we report the discovery of episodic shallow tremors and potential slow slip events in Guerrero offshore. Their distribution, together with that of repeating earthquakes, seismicity, residual gravity and bathymetry, suggest that a portion of the shallow plate interface in the gap undergoes stable slip. This mechanical condition may not only explain the long return period of large earthquakes inside the gap, but also reveals why the rupture from past M < 8 earthquakes on adjacent megathrust segments did not propagate into the gap to result in much larger events. However, dynamic rupture effects could drive one of these nearby earthquakes to break through the entire Guerrero seismic gap.

Spatiotemporal analysis of repeating earthquakes near Porangahau, Hikurangi Margin, New Zealand

<p><b>The Hikurangi subduction zone beneath the eastern North Island, New Zealand exhibits a variety of fault-slip related phenomena including tsunami earthquakes, non-volcanic tremor, low-frequency earthquakes, episodic slow slip, and repeating earthquakes. The northern Hikurangi margin hosts shallow slow-slip and is weakly coupled to shallow depths. In contrast, the southern Hikurangi margin is strongly coupled, and only deep slow-slip has been observed. The transition in coupling occurs beneath the township of Porangahau, and is an exemplary focus region for studying how this change in locking is accommodated. </b></p><p>To examine slip processes beneath Porangahau, we have constructed and analysed a catalogue of repeating earthquakes that occurred between 2004 and 2018. Repeating earthquakes are thought to re-rupture the same fault patch at different times, and thus have nearly identical waveforms, locations and magnitudes. Because repeating earthquakes represent cyclic loading, they can be used to detect temporal and spatial changes of slip-rate at depth and hence monitor how stress is transferred to seismogenic zones. </p><p>To build a catalogue of repeating earthquakes we first clustered the GeoNet earthquake catalogue by distance and correlation to identify potential repeating events. We then used a stronger cross-correlation threshold of at least 0.95 normalised cross-correlation value at three or more stations to identify repeating earthquakes from the initial clusters. This threshold was determined by our group's previous work on the northern Hikurangi margin. We identified 225 families of repeating earthquakes, with each family having two or more earthquakes in the 14-year study period from 2004 to 2018. </p><p>We carried out manual phase picking and polarity identification for the most recent event in each family and computed absolute locations, local magnitudes calibrated with moment magnitude, and high-quality focal mechanisms. For the rest of the events in each family, we conducted cross-correlation re-picking to obtain precise relative locations and relative magnitudes. With precise locations and well-constrained focal mechanisms, we determined whether the repeating earthquake families originated within the Pacific Plate, Australian Plate or on the subduction interface. Most of the families are located within the Pacific Plate, and the majority of families that originate on the subduction interface are located near the township of Porangahau. At least 220 of the 532 identified repeating earthquakes locate at the transition from strong- to weak-coupling of the subduction interface near the township of Porangahau. </p><p>A variety of slow slip events have been detected near Porangahau in the last two decades. Even though some repeating earthquakes correlate spatially and temporally with slow slip events, temporal and spatial correlations between slow slip events and repeating earthquakes are scarce and sparse. The majority of repeating earthquakes are located up-dip or down-dip of modelled slow slip patches, with very few families having spatial correlation with slow slip events. We obtained a moment-recurrence interval relationship for the catalogue of repeating earthquakes near Porangahau and compared it to the relationship obtained by Nadeau and Johnson (1998) at Parkfield, California. Finally, we computed slip-rates using the families located on the subduction interface and obtained an average slip-rate of 13 mm/yr. The insights gained from this study lay the groundwork for future work constraining processes of strain accumulation at the creeping-to-locked transition zone near Porangahau.</p>

Spatiotemporal analysis of repeating earthquakes near Porangahau, Hikurangi Margin, New Zealand

<p><b>The Hikurangi subduction zone beneath the eastern North Island, New Zealand exhibits a variety of fault-slip related phenomena including tsunami earthquakes, non-volcanic tremor, low-frequency earthquakes, episodic slow slip, and repeating earthquakes. The northern Hikurangi margin hosts shallow slow-slip and is weakly coupled to shallow depths. In contrast, the southern Hikurangi margin is strongly coupled, and only deep slow-slip has been observed. The transition in coupling occurs beneath the township of Porangahau, and is an exemplary focus region for studying how this change in locking is accommodated. </b></p><p>To examine slip processes beneath Porangahau, we have constructed and analysed a catalogue of repeating earthquakes that occurred between 2004 and 2018. Repeating earthquakes are thought to re-rupture the same fault patch at different times, and thus have nearly identical waveforms, locations and magnitudes. Because repeating earthquakes represent cyclic loading, they can be used to detect temporal and spatial changes of slip-rate at depth and hence monitor how stress is transferred to seismogenic zones. </p><p>To build a catalogue of repeating earthquakes we first clustered the GeoNet earthquake catalogue by distance and correlation to identify potential repeating events. We then used a stronger cross-correlation threshold of at least 0.95 normalised cross-correlation value at three or more stations to identify repeating earthquakes from the initial clusters. This threshold was determined by our group's previous work on the northern Hikurangi margin. We identified 225 families of repeating earthquakes, with each family having two or more earthquakes in the 14-year study period from 2004 to 2018. </p><p>We carried out manual phase picking and polarity identification for the most recent event in each family and computed absolute locations, local magnitudes calibrated with moment magnitude, and high-quality focal mechanisms. For the rest of the events in each family, we conducted cross-correlation re-picking to obtain precise relative locations and relative magnitudes. With precise locations and well-constrained focal mechanisms, we determined whether the repeating earthquake families originated within the Pacific Plate, Australian Plate or on the subduction interface. Most of the families are located within the Pacific Plate, and the majority of families that originate on the subduction interface are located near the township of Porangahau. At least 220 of the 532 identified repeating earthquakes locate at the transition from strong- to weak-coupling of the subduction interface near the township of Porangahau. </p><p>A variety of slow slip events have been detected near Porangahau in the last two decades. Even though some repeating earthquakes correlate spatially and temporally with slow slip events, temporal and spatial correlations between slow slip events and repeating earthquakes are scarce and sparse. The majority of repeating earthquakes are located up-dip or down-dip of modelled slow slip patches, with very few families having spatial correlation with slow slip events. We obtained a moment-recurrence interval relationship for the catalogue of repeating earthquakes near Porangahau and compared it to the relationship obtained by Nadeau and Johnson (1998) at Parkfield, California. Finally, we computed slip-rates using the families located on the subduction interface and obtained an average slip-rate of 13 mm/yr. The insights gained from this study lay the groundwork for future work constraining processes of strain accumulation at the creeping-to-locked transition zone near Porangahau.</p>

Repeating earthquakes and ground deformation reveal the structure and triggering mechanisms of the Pernicana fault, Mt. Etna

Structure and dynamics of fault systems can be investigated using repeating earthquakes as repeatable seismic sources, alongside ground deformation measurements. Here we utilise a dataset of repeating earthquakes which occurred between 2000 and 2019 along the transtensive Pernicana fault system on the northeast flank of Mount Etna, Italy, to investigate the fault structure, as well as the triggering mechanisms of the seismicity. By grouping the repeating earthquakes into families and integrating the seismic data with GPS measurements of ground deformation, we identify four distinct portions of the fault. Each portion shows a different behaviour in terms of seismicity, repeating earthquakes and ground deformation, which we attribute to structural differences including a segmentation of the fault plane at depth. The recurrence intervals of repeating earthquake families display a low degree of regularity which suggests an episodic triggering mechanism, such as magma intrusion, rather than displacement under a constant stress.