Seismica

The 1804 Alborán seismic series: Search for the source using seismic scenarios and static stress interactions

Yolanda de Pro Díaz, José Jesús Martínez Díaz, Carolina Canora Catalán — Mon, 14 Jul 2025 00:00:00 -0400

Linking historical earthquakes with the faults that caused them is crucial for seismic hazard assessment. Historical documentation describing the effects of an earthquake is a useful information source, from which we can compile the observed intensity field of the earthquake. In this work, we use intensity data from the catastrophic 1804 Alborán earthquake (south of Iberia) along with intensity simulations and coseismic stress transfer analysis to search for this earthquake's seismic source. We build intensity simulations for each fault proposed as a potential source, and compare these simulations with the intensity field. We also propose the possibility of the Alborán 1804 earthquake triggering the Dalías earthquake (European macroseismic intensity (IEMS-98) IX), which occurred seven months after, and analyze stress transfer between the possible sources of both earthquakes. Our results point to a conjunct rupture of the northern Al-Idrissi Fault segment and the North–South Faults as the most likely source for the Alborán earthquake.

Alaska Upper Crustal Velocities Revealed by Air-to-Ground Coupled Waves From the 2022 Hunga Tonga-Hunga Ha’apai Eruption

Tue, 08 Jul 2025 00:00:00 -0400

Pressure changes in the atmosphere couple to the solid earth, producing ground motions that contain information about local crustal elastic parameters. This type of air-to-ground coupled wave was observed globally following the largest explosion of the instrumental age, the climactic eruption of the Hunga Tonga-Hunga Ha’apai volcano on 15^th January, 2022. We utilize this unprecedented source, along with the presence of colocated seismometers, infrasound sensors, and barometers in Alaska, to examine coupling and reveal elastic parameters beneath the stations. We derive coupling spectra by forming seismic--to--pressure amplitude ratios as a function of frequency, and identify passbands of high coherence between the pressure and seismic records. By relating coupling spectra in high-coherence bands to elastic parameters, we estimate mean shear wave velocities under stations to a depth encompassing much of the upper crust. Our velocity estimates from low-frequency coupling exhibit good agreement with a previously existing tomographic velocity model from Berg et al. (2020), while estimates from high-frequency coupling show considerable scatter when compared to proxy V_s30, even though the overall values are reasonable. In addition to providing velocity estimates, our results also indicate that, for the broadband pressure signals from the Hunga Tonga-Hunga Ha’apai eruption, microseismic noise exerts a strong effect on the frequency bands where coupling is observed, and that the air-to-ground coupled waves exhibit significant complexity not necessarily described by theory. Our results show that coupling observations provide a simple forward observation of mean seismic velocities beneath seismoacoustic stations, without the need to resort to complex inversion schemes. It is remarkable that pressure waves generated thousands of kilometers away are able to reveal the seismic velocity structure of Alaska to several kilometers depth.

Investigating the D" Reflector Beneath the Indian Ocean with Source Arrays

Christine Thomas, Björn Holger Heyn, Lena Tölle, Rûna van Tent — Tue, 08 Jul 2025 00:00:00 -0400

We used seismic P-wave reflections to search for the discontinuity at the top of the D" region beneath the Indian Ocean. Due to a lack of seismic receiver arrays to target this region, we build source arrays using earthquakes in Indonesia and taking advantage of the long-running history of GEOSCOPE stations located in the western Indian Ocean and Antarctica, as well as three additional stations (Seychelles and Antarctica). Despite restricting the earthquake depth range, source-array stacks were difficult to interpret due to complications arising from differing earthquake depths, violating the plane wave assumption. Therefore, we use a source-array scatter imaging method, that does not rely on travel-times calculated for a plane wave. Using this technique in conjunction with source normalization, we found evidence for a D" P-wave reflector for several stations with reflector depths varying between 230-160 km above the CMB South of Australia and 190 to 270 km above the CMB beneath the Indian Ocean, where the depth of the reflector in the north of our study area is consistent with previously imaged D" depths using S-waves and agrees with receiver array data. We suggest that earlier imaged subducted lithosphere in this region is responsible for our D" reflections.