Crustal Strength Variations Inferred From Earthquake Stress Drop at Axial Seamount Surrounding the 2015 Eruption

Abstract

Variations in stress drops of earthquakes associated with the April and May 2015 eruption of Axial Seamount, on the Juan de Fuca Ridge, suggest a reduction in crustal strength as a result of the eruption. Seismicity during the inflation and deflation periods was well recorded by ocean bottom seismometers located within and along the caldera. We use these nearby recordings and an empirical Green's function spectral ratio method to obtain corner frequencies for stress drops of earthquakes on caldera ring faults. We find stress drops from 0.6 to 43 MPa for 423 ring fault earthquakes (1.6 ≤ MW ≤ 3.6) and an average stress drop two times higher during the inflation period (6.4 MPa) prior to the eruption, than during the subsequent deflation (3.2 MPa). Stress drops also correlate with spatially varying shear wave speed, possibly reflecting a region of pervasive cracking in the northern caldera.

Resolution and uncertainties in estimates of earthquake stress drop and energy release

“…Ruhl et al [72] suggested that temporal variations in stress drop during a swarm sequence may reflect long-term spatial variations, combined with temporal variation in the spatial distribution of the earthquakes. Yoshida et al [178] reported temporal variation in stress drop associated with fluid migration and seismicity rate in a productive swarm in NE Japan, Goertz-Allmann et al [179] and Kwiatek et al [132] reported a decrease in stress drop associated with higher pore fluid pressure during geothermal injection in Switzerland and El Salvador, respecrively and Moyer et al [180] reported a systematic change in stress drop for earthquakes following an eruption at the Axial seamount. To varying degree, all these observations are subject to possible artefacts from temporal variation in the surrounding medium (e.g.…”

“…[179] and Kwiatek et al [132] reported a decrease in stress drop associated with higher pore fluid pressure during geothermal injection in Switzerland and El Salvador, respecrively and Moyer et al . [180] reported a systematic change in stress drop for earthquakes following an eruption at the Axial seamount. To varying degree, all these observations are subject to possible artefacts from temporal variation in the surrounding medium (e.g.…”

Quantifying Magma Overpressure Beneath a Submarine Caldera: A Mechanical Modeling Approach to Tsunamigenic Trapdoor Faulting Near Kita‐Ioto Island, Japan

“…Some studies reported complete or near‐complete stress drop during tectonic earthquakes (Hasegawa et al., 2011; Ross et al., 2017), while the stress drop ratio can be partial and vary from earthquake to earthquake (Hardebeck & Okada, 2018). For intra‐caldera earthquakes, several recent studies estimated stress drop during caldera collapses (Moyer et al., 2020; T. A. Wang et al., 2022), but our knowledge on the stress drop ratio in calderas is poor and the ratio may vary from caldera to caldera.…”

Megathrust Stress Drop as Trigger of Aftershock Seismicity: Insights From the 2011 Tohoku Earthquake, Japan

“…We then calculate average stress drop values ($$\overline{{\increment}\sigma }$$ Forearc ) at 10‐km depth intervals with 50% overlap (Text S3 and S4 and Figures S8 and S9 in Supporting Information ). The averaged values better reflect bulk stress conditions in the forearc and are better suited for comparison with modeled stresses (Moyer et al., 2020).…”

“…We then calculate average stress drop values ( 𝐴𝐴 ∆𝜎𝜎 Forearc ) at 10-km depth intervals with 50% overlap (Text S3 and S4 and Figures S8 and S9 in Supporting Information S1). The averaged values better reflect bulk stress conditions in the forearc and are better suited for comparison with modeled stresses (Moyer et al, 2020).…”