Background and delayed-triggered swarms in the central Southern Alps, South Island, New Zealand

Abstract

Low-magnitude earthquake swarms (M L 2.8), consisting of up to 47 events of similar waveforms, have been observed repeatedly in the central Southern Alps, a rapidly uplifting orogen bounded by the transpressive Alpine Fault in the South Island of New Zealand. We compare nine background swarms recorded between November 2008 and April 2010 with five delayed-triggered swarms that occurred after the M W 7.8 Dusky Sound and the M W 7.1 Darfield (Canterbury) earthquakes. The two types of swarms are similar in terms of the magnitudes, depths, focal mechanisms, and interevent times of the constituent microearthquakes, and appear to both involve the rupture of steeply dipping faults in highly fractured crust in a 10 km 3 12 km area in the center of the SAMBA network. The delayed-triggered swarms occurred at similar epicentral distances (c. 4.53 the rupture length of the mainshocks) to the Dusky Sound and Darfield earthquakes, commenced shortly after the passage of the surface waves, continued for 5 and 2 days, respectively, and were followed in each case by a 2 day long quiescent period, which may suggest clockadvanced of faults in their failure-cycle. Triggering thresholds of 0.01 MPa proposed elsewhere are similar to the dynamic stress changes computed for the Southern Alps (0.09 MPa). However, as 98% of the locatable triggered events occurred several hours after the surface waves had passed, the dynamic stress changes associated with the surface waves themselves are unlikely to have triggered the earthquakes directly. Instead, we suggest that the locations and delays of the triggered swarms are more consistent with triggering by pore pressure diffusion.

Detecting Real Earthquakes Using Artificial Earthquakes: On the Use of Synthetic Waveforms in Matched‐Filter Earthquake Detection

“…Finally, we remove any detections with a SNR below 3.0 to allow construction of second-generation templates from the detections: low SNR detections make poor templates. All template sets detect events from the swarm and, in the case of the Green's function synthetics and the real template sets, outperform the previous analysis by Boese et al (2014) who made 17 detections ( Figure 1). We make 41, 29, and 18, detections, for the real, Green's function synthetics and naïve synthetics sets, respectively.…”

“…In contrast, due to the high waveform similarities often observed in swarms, matched‐filters work well in these situation (Shelly et al, ). We analyze a swarm in the central Southern Alps (New Zealand) previously studied by Boese et al (). We use data recorded on the SAMBA (Boese et al, ) and GeoNet (Petersen et al, ) networks on 24 May 2009.…”

Low‐frequency earthquakes reveal punctuated slow slip on the deep extent of the Alpine Fault, New Zealand

“…Detection of slow slip is also hindered by the constancy of LFE generation and hence slow slip, with brief swarms (tremor events) lasting 5–120 min suggesting similarly brief slip rate increases. Abrupt LFE detection rate increases coincident with large regional earthquakes may be indicative of triggering by transient or static stress shifts [ Boese et al ., ]. A full investigation of LFE triggering characteristics in the Southern Alps is the subject of ongoing research and beyond the scope of this paper.…”

“… Hypocentre locations of 14 LFE families identified in this study, with uncertainties plotted as blue cloud of probability density function. Local seismicity [ Boese et al ., ], deep seismicity [ Boese et al ., ], and previously identified tremor [ Wech et al ., ] are plotted as in the legend. Cross sections projected onto gray lines in map view.…”

Changes in hot spring temperature and hydrogeology of the Alpine Fault hanging wall, New Zealand, induced by distal South Island earthquakes

“…Some pertinent observations are as follows: (1 (Boese et al 2014). Although no specific triggered swarms were noted in the vicinity of Copland valley, swarms occurred several hours after the earthquakes, lasting for approximately the same period it takes for Copland spring to adjust to a new cooler background temperature.…”