Flank motion detected between 2010 and 2014 through InSAR time-series analysis at Pacaya Volcano, Guatemala

<p>Volcanic flank collapse has caused over 20,000 casualties in the past 400 years, and is one of the most dangerous hazards affecting communities and infrastructure near volcanoes. Flank instability has mostly been investigated at ocean volcanoes, due to their ability to trigger deadly tsunamis, however, these collapses are prevalent across volcanic settings, with all but one volcano in Guatemala with elevation over 2000m having experienced flank collapse, like Pacaya Volcano. At Pacaya, there is evidence for at least one past collapse, and transient SW flank motion has been identified accompanying vigorous eruptions in 2010 and 2014. We use InSAR time-series analysis to reveal, for the first time, long-term displacement of the SW flank of Pacaya during a period of volcanic quiescence from 2011-2013. This motion extended into 2014, with increased displacement rate attributed to dike intrusion during a major eruption. Subsequent static stress change analyses investigated the interactions between the modeled dike intrusion and detachment slip. Our research highlights that long-term flank motion might be more prevalent than currently recognized and that an awareness of existing structural weaknesses such as detachment faults and of possible magma-faulting interactions is vital when assessing the likelihood and style of volcanic flank collapse.</p>

A Stress-Similarity Triggering Model for Aftershocks of the Mw 6.4 and 7.1 Ridgecrest Earthquakes

ABSTRACT The July 2019 Mw 6.4 and 7.1 Ridgecrest earthquakes triggered numerous aftershocks, including clusters of off-fault aftershocks in an extensional stepover of the Garlock fault, near the town of Olancha, and near Panamint Valley. The locations of the off-fault aftershocks are consistent with the stress-similarity model of triggering, which hypothesizes that aftershocks preferentially occur in areas where the mainshock static stress change tensor is similar in orientation to the background stress tensor. The background stress field is determined from the inversion of earthquake focal mechanisms, with the spatial resolution adapted to the local density of earthquakes. The mainshock static stress change is computed using finite-source models for the Mw 6.4 foreshock and Mw 7.1 mainshock. I quantify the similarity between these two stress fields using the tensor dot product of the normalized deviatoric stress tensors. The off-fault aftershocks in the Garlock stepover and the Olancha area fall within lobes of positive stress similarity, whereas the aftershocks near Panamint Valley are partially within a lobe. The cluster in the Garlock fault stepover and the smaller of two clusters near Olancha occur in regions of locally anomalous background stress that results in higher stress similarity. I compute the spatial density of M≥2.0 aftershocks and find that the aftershock density increases as a function of stress similarity, with a factor of ∼15 difference between high stress-similarity and low stress-similarity areas. This result is robust with respect to the choice of mainshock model and the uncertainty of the background stress field. The aftershock density varies substantially inside the high stress-similarity lobes, however, indicating that other variable background conditions, such as material properties, temperature, and fluid pressure, may also be playing a role. Specifically, temperature and fluid pressure conditions might help explain the low rate of aftershocks in the Coso geothermal field.

Cascading Partial Rupture of the Flores Thrust during the 2018 Lombok Earthquake Sequence, Indonesia

A series of four Mw>6 earthquakes struck the northern region of Lombok, eastern Indonesia, in a span of three weeks from late July to mid-August 2018. The series was thought to be associated with the Flores thrust, but the exact mechanism causing the unusual earthquake series has remained elusive. Our Interferometric Synthetic Aperture Radar analysis, combined with insights from seismology, indicates that the events originated at different hypocenter depths with differing fault geometries, which may explain the cascading behavior of the events, and indicates that better imaging of active fault geometry might provide some insight into future rupture behavior on other similar thrust systems. Our static stress change calculations suggest that the earlier events in the sequence played a role in promoting the later events. In addition, the second event brought the most significant impact on a nearby volcano, by causing volumetric expansion at its shallow magma plumbing system and unclamping its magma ascent zone, which may potentially have an impact on its future eruptive activity. However, no volcanic activity has so far occurred after the earthquakes. Finally, our damage proxy maps suggest that the second event caused the greatest damage to buildings.

Earthquake static stress transfer in the 2013 Valencia Gulf (Spain) seismic sequence

On September 24th, 2013, a ML 3.6 earthquake struck in Valencia Gulf (Spain), near the Mediterranean coast of Castellon, roughly a week after the gas injections conducted in the area to develop an Underground Gas Storage had been halted. The event, felt by the nearby population, led to a sequence build-up of felt events which reached a maximum of ML 4.3 on October 2nd. Here, we study the role of static stress change as an earthquake triggering mechanism during the sequence, and provide quantitative assessment of the known faults final stress state. By means of the Coulomb Failure Function, the evolution of static stress is quantified both on fault planes derived from focal mechanism solutions (which act as source and receiver faults), and on the previously mapped structures in the area (receiver faults). Results show that static stress transfer could have acted as a partial trigger, and point towards an ESE-dipping structure as the most likely to have been activated during the sequence. Based on this approach, the influence of the studied events in the occurrence of future and potentially damaging earthquakes in the area would be, at most, of second order.

Quasi-Static Stress Change Around Mount Fuji Region Due to Tohoku Mega-Thrust Earthquake

The 2011 Tohoku mega-thrust earthquake caused huge crustal deformation over a wide are of Mainland Japan. Many mega-thrust earthquakes worldwide have triggered volcanic eruptions nearby, and it is assumed that stress changes due to the Tohoku earthquake resulted in a perturbation to the magma system. The objectives of our study is to evaluate this perturbation quantitatively and to analyze the mechanism of the interaction between mega-thrust earthquakes and volcanic eruptions. This paper focuses on quasi-static stress change due to viscous relaxation of a source region and the surrounding area.