2:45 PM - 3:00 PM
[SVC28-11] Moment Tensors of Ring-Faulting at Active Volcanoes: Insights into Vertical-CLVD Earthquakes at the Sierra Negra Caldera, Galápagos Islands
Keywords:caldera, volcanic earthquake, ring fault, moment tensor
Large volcanic earthquakes (Mw > 5) with moment tensors (MTs) dominated by a vertical compensated-linear-vector-dipole (vertical-CLVD) component are sometimes generated by dip slip along a curved ring-fault system, which is often called ring-faulting. Ring-faulting causing vertical-CLVD earthquakes are known to be often associated with volcanic unrests at volcanic calderas (Shuler et al., 2013, JGR). However, relating their MTs determined from long-period seismic records to ring-fault parameters has proved difficult. The objective of this study is to find a robust way of estimating ring-fault parameters based on their MT solutions obtained from the records.
We first model the MTs of idealized ring-faulting and decompose into three deviatoric components representing the vertical dip-slip (DS), vertical strike-slip (SS), and vertical-CLVD (CLVD) mechanisms, respectively (Figure). We then show that the DS component is indeterminate from long-period seismic waves owing to its little seismic excitation at a very shallow source depth (< ~5 km) (Kanamori & Given, 1981, PEPI), whereas the SS and CLVD components are resolvable from long-period seismic waves. Considering the limited resolvability of the MT, we propose a new method for estimating ring-fault parameters using the two resolvable components. The arc angle of ring fault can be inferred from the dominancy of the CLVD component, whereas the ring-fault orientation can be constrained from the azimuth of the tension- or pressure-axis of the SS component.
For validation, we study a vertical-CLVD earthquake (Mw 5.5 on October 22, 2005 in UTC) that occurred during the 2005 volcanic activity at the Sierra Negra caldera, Galápagos Islands. We conduct the MT inversion using long-period (80–200 s) seismic data at far-field (epicentral distance: 12.6°–46.6°). Even when we change the source location and depth, the SS and CLVD components are stably estimated, whereas the estimation of the DS component is quite unstable. This confirms the resolvability of the SS and CLVD components from the far-field long-period seismic data. By considering the two resolvable components, we estimate the ring-fault parameters, i.e., the arc angle and the orientation of ring fault, which are consistent with those identified by previous geodetic studies and field surveys. We also consider ring-fault parameters of two recent vertical-CLVD earthquakes (Mw 5.3 on June 26, 2018; Mw 5.1 on July 5, 2018) that took place during the 2018 activity at the caldera, revealing significant differences between the two earthquakes in terms of slip direction and location.
These results show the usefulness of our method for estimating ring-fault parameters of vertical-CLVD earthquakes, which enables us to examine the kinematics and structures below active volcanoes with ring faults that are distributed globally. A better understanding of such vertical-CLVD earthquakes will provide insights into rock-fluid interactions between fault systems of volcanic edifices and magmatic processes, potentially leading to assessments of volcanic hazards. We also discuss the insufficient seismic excitation property of the ring-faulting at a shallow depth, and possible biases to the MT inversion caused by an isotropic source occurring nearby the ring-faulting.
We first model the MTs of idealized ring-faulting and decompose into three deviatoric components representing the vertical dip-slip (DS), vertical strike-slip (SS), and vertical-CLVD (CLVD) mechanisms, respectively (Figure). We then show that the DS component is indeterminate from long-period seismic waves owing to its little seismic excitation at a very shallow source depth (< ~5 km) (Kanamori & Given, 1981, PEPI), whereas the SS and CLVD components are resolvable from long-period seismic waves. Considering the limited resolvability of the MT, we propose a new method for estimating ring-fault parameters using the two resolvable components. The arc angle of ring fault can be inferred from the dominancy of the CLVD component, whereas the ring-fault orientation can be constrained from the azimuth of the tension- or pressure-axis of the SS component.
For validation, we study a vertical-CLVD earthquake (Mw 5.5 on October 22, 2005 in UTC) that occurred during the 2005 volcanic activity at the Sierra Negra caldera, Galápagos Islands. We conduct the MT inversion using long-period (80–200 s) seismic data at far-field (epicentral distance: 12.6°–46.6°). Even when we change the source location and depth, the SS and CLVD components are stably estimated, whereas the estimation of the DS component is quite unstable. This confirms the resolvability of the SS and CLVD components from the far-field long-period seismic data. By considering the two resolvable components, we estimate the ring-fault parameters, i.e., the arc angle and the orientation of ring fault, which are consistent with those identified by previous geodetic studies and field surveys. We also consider ring-fault parameters of two recent vertical-CLVD earthquakes (Mw 5.3 on June 26, 2018; Mw 5.1 on July 5, 2018) that took place during the 2018 activity at the caldera, revealing significant differences between the two earthquakes in terms of slip direction and location.
These results show the usefulness of our method for estimating ring-fault parameters of vertical-CLVD earthquakes, which enables us to examine the kinematics and structures below active volcanoes with ring faults that are distributed globally. A better understanding of such vertical-CLVD earthquakes will provide insights into rock-fluid interactions between fault systems of volcanic edifices and magmatic processes, potentially leading to assessments of volcanic hazards. We also discuss the insufficient seismic excitation property of the ring-faulting at a shallow depth, and possible biases to the MT inversion caused by an isotropic source occurring nearby the ring-faulting.