Sweet Spot Tremor Triggered by Intraslab Earthquakes in the Nankai Subduction Zone

Abstract:

Deep tectonic tremor has been observed at several major plate-bounding faults around the Pacific Rim. Tremor­ in these regions can be triggered by small stresses arising from solid earth tides as well as passing seismic waves of large, distant earthquakes. While large, distant earthquakes are capable of repeatedly triggering tremor in the same region (i.e., a sweet spot), it is less understood how intraslab earthquakes interact with sweet spot tremor areas. We conduct a systematic survey of tremor triggered in the Nankai subduction zone by intraslab earthquakes to better understand what governs fault slip along the Eurasian-Philippine Sea Plate boundary. We examine 3 tremor sweet spots in the Nankai subduction zone: Shikoku West, Kii North, and Tokai. In each region, we select earthquakes from the Japan Meteorological Agency (JMA) catalog that occur from mid-2009 to mid-2014 with magnitude (M) greater than 2, that occur within the down-going Philippine Sea Plate, and within a 300 km epicentral distance of the sweet spot region. Using these selection criteria, we obtain ~1,200 earthquakes in each region. We examine a tremor catalog immediately before and after these local events as well as visually inspect filtered waveforms from short-period Hi-net seismic stations surrounding the sweet spot areas to identify additional tremor signals. From our initial analysis, we have identified 18 clear cases of increased tremor activity immediately following intraslab earthquakes in Shikoku West, most of which occur down-dip of the Shikoku West sweet spot. In comparison, we have identified only 5 triggering earthquakes in Kii North, and our investigation at Tokai is still ongoing. Our results so far are in agreement with triggering susceptibility being dependent upon background activity rates, as has been suggested for remote triggering of microearthquakes in geothermal regions by large, distant earthquakes as well as for remotely triggered tremor in the Nankai subduction zone. Next, we plan to evaluate whether the observed triggering cases reflect a true causal relationship between intraslab earthquakes and interplate tremors or if the observations are due to coincidence. By doing so, we aim to better understand the physical mechanisms responsible for the triggering of the sweet spot tremor in the Nankai subduction zone.