Two submarine earthquakes (Mw 5.8) occurred near volcanic islands, Curtis and Cheeseman, in the Kermadec Arc in 2009 and 2017. Following both earthquakes, similar tsunamis with wave heights of about a meter, larger than expected from their moderate seismic magnitudes, were observed by coastal tide gauges. We investigate the source mechanism for both earthquakes by analyzing tsunami and seismic data of the 2017 event. Tsunami waveform analysis indicates that the earthquake uplifted a submerged caldera around the islands. Combined analysis of tsunami and seismic data suggests that trapdoor faulting, or sudden intra-caldera fault slip interacted with magma reservoir deformation, occurred due to magma overpressure, possibly in association with caldera resurgence. The earthquake scaling relationship for trapdoor faulting events at three calderas deviates from that for regular earthquakes, possibly due to the fault-reservoir interaction in calderas.

Time-domain analyses of seismic waveforms have revealed diverse source complexity in large earthquakes (Mw>7). However, source characteristics of small earthquakes have been studied by assuming a simple rupture pattern in the frequency domain. This study utilized high-quality seismic network data from Japan to systematically address the source complexities and radiated energies of Mw 3–7 earthquakes in the time domain. We first determined the apparent moment-rate functions (AMRFs) of the earthquakes using the empirical Green’s functions. Some of the AMRFs showed multiple peaks, suggesting complex ruptures at multiple patches. We then estimated the radiated energies (𝐸𝑅) of 1736 events having more than ten reliable AMRFs. The scaled energy (𝑒𝑅=𝐸𝑅/𝑀0) did not strongly depend on the seismic moment (𝑀0), focal mechanisms, or depth. The median value of 𝑒𝑅 was 3.7×10-5, which is comparable to those of previous studies; however, 𝑒𝑅 varied by approximately one order of magnitude among earthquakes. Additionally, we measured the source complexity based on the radiated energy enhancement factor (𝑅𝐸𝐸𝐹). The values of 𝑅𝐸𝐸𝐹 differed among earthquakes, implying diverse source complexity. The values of 𝑅𝐸𝐸𝐹 did not show strong scale dependence for Mw 3–7 earthquakes, suggesting that the source diversity of smaller earthquakes is similar to that of larger earthquakes at their representative spatial scales. Applying a simple spectral model (e.g., the ω2-source model) to complex ruptures may produce substantial estimation errors of source parameters.

We determined the rupture sequence of the Aug 12, 2021 Mw 8.2 South Sandwich Island earthquake which, according to the reports from the National Earthquake Information Center, appears to be a complex sequence in both time and space. Notable tsunamis were recorded by tide gauges at global distances. Given the complexity of this event, we used a multiple subevent inversion method that can accommodate complex variations of fault geometry, location, depth, and temporal characteristics of the event. We found that the rupture initiated as a regular deep thrust earthquake; it then triggered a shallow dominantly slow subevent extending ~200 km to the south, and ended with 2 other regular subevents. The total duration is ~260s, unusually long for an Mw 8.2 event. Our result is qualitatively consistent with other moment tensor solutions and the observed mB-Mw and MS-Mw relations, and provides a more quantitative space-temporal pattern of this unusual sequence.