DETECTION OF REPEATING EARTHQUAKES OFF COSTA RICA AND NICARAGUA IN THE MIDDLE AMERICA SUBDUCTION ZONE
Repeating earthquakes are phenomena that commonly occur on creeping faults, such as those found at subducting plate boundaries. They result from repeated ruptures of seismic patches that are loaded by surrounding steady creep, and thus serve as useful indicators for resolving the spatiotemporal characteristics of fault creep. For instance, by monitoring periodic repeating earthquakes, it is possible to detect transient aseismic slip on the plate interface (e.g., Uchida et al., 2019).
Repeating earthquakes have been studied in subduction zones worldwide (Uchida & Bürgmann, 2019); however, their detailed spatial distribution remains poorly understood in many regions. For example, the offshore region of Costa Rica and Nicaragua, located in the Middle America subduction zone, is one such region. In this region, a spectrum of slow-to-fast seismic and geodetic events has been reported (e.g., Dixon et al., 2014). This region is also geomorphologically intriguing, exhibiting an uplift and subsidence pattern on the overriding plate side, consisting of an arc and a lowland zone. Since similar patterns are observed in the Nankai and Cascadia subduction zones, it is worthwhile to investigate whether this region exhibits similar or contrasting seismological characteristics. Such an investigation could also provide insights into the possible relationship between tectonic geomorphology and seismicity.
In light of the above, we conducted a detailed search for repeating earthquakes off the coasts of Costa Rica and Nicaragua. We used seismic waveform data from broadband seismometer networks in Costa Rica and Nicaragua, covering the period from 2015 to 2020. The event catalog was constructed using the following procedure: First, phase picking was performed using PhaseNet, a deep learning-based phase picker (Zhu & Beroza, 2019), with the globally trained model developed by Mousavi et al. (2019). Event association was then conducted based on the GaMMA method (Zhu et al., 2022), followed by event location using the hypomh algorithm (Hirata & Matsu’ura, 1987). After constructing the earthquake catalog, we applied an appropriate bandpass filter to the event waveforms and detected repeating earthquakes based on the waveform cross-correlation when the correlation coefficients exceeded 0.90 at three or more stations. For the detected repeating events, we calculated their recurrence intervals and estimated the fault slip amounts from the scaling laws for repeating earthquakes (e.g., Nadeau & Johnson, 1998).
As a preliminary result, we identified several repeating earthquake sequences (M ≥ 3.0, depth ~ 30 km) along the trench offshore of the Nicoya Peninsula. These events are distributed in the shallower part of the subducting plate interface. In contrast, offshore of Lake Nicaragua, the sequences were observed in the deeper part as well (M ≥ 4.0, depth ~ 80 km). We additionally observed numerous burst-type repeating earthquakes with recurrence intervals of less than one month. Based on these findings, we plan to further investigate the relationship between repeating earthquakes and previously reported slow slip events.