Local Travel-time Tomography Research Articles

Automated Earthquake Detection and Local Travel Time Tomography in the South‐Central Andes (32–35°S): Implications for Regional Tectonics

AbstractIn the South‐Central Andes, the crustal structures driving the tectonic evolution of the Andean Cordillera remain unresolved. So far, most seismological studies focused on the subduction interface, leaving crustal seismicity and its relationship with crustal deformation and Andean volcanism mostly unconstrained. However, because of their large number compared to higher magnitude events, the characterization of small‐magnitude crustal earthquakes is key to identify active structures and better constrain the tectonic models. In this work, we exploit 53 months of continuously recorded, three‐component waveforms from the permanent seismic network in central Chile using a deep‐learning approach to improve the detection of small‐magnitude earthquakes. To increase station coverage, we also use the seismic phases obtained from a previous temporary seismic deployment. We use the obtained seismicity catalog to refine tomographic models of that region, revealing a more detailed architecture of the Chilean forearc. Travel times calculated in the new 3‐D velocity model allowed us to locate ∼14,000 earthquakes. Refined double‐difference relocations of ∼4,900 events located beneath the West Andean Thrust suggest a large‐scale, west‐dipping structure which, together with the west‐verging tectonic front, likely contributed to the uplift and crustal deformation during the past ∼20 Myr.

The deep subsurface structure beneath Lusi and the adjacent volcanic chain inferred from local travel-time tomography

Since 2006, the Lusi mud eruption is ongoing in the northeast of Java Island, Indonesia. We perform a 3D seismic tomography of the area surrounding Lusi and the adjacent Arjuno-Welirang volcanic complex using P-wave arrivals of 797 local earthquakes recorded on 31 seismic stations for two years between 2015 and 2016. Resolution tests show that we can reasonably well constrain the subsurface velocity structure beneath Lusi down to a depth of 20 km. The results of our tomography reveal that Lusi has a shallow plumbing system that is interconnected with the neighboring Penanggungan volcano. Their joint system reaches down to a depth of 20 km. The Arjuno-Welirang volcanic complex has its own deep-rooted plumbing system that might be connected to Penanggungan volcano and Lusi along the Watukosek Fault zone. The connection of Lusi with this large reservoir volume elucidates the longevity of the Lusi eruption.

Crustal seismic structure beneath the Deccan Traps area (Gujarat, India), from local travel-time tomography

The Gujarat region in western India is known for its intra-plate seismic activity, including the Mw 7.7 Bhuj earthquake, a reverse-faulting event that reactivated normal faults of the Mesozoic Kachchh rift zone. The Late Cretaceous Deccan Traps, one of the largest igneous provinces on the Earth, cover the southern part of Gujarat. This study is aimed at bringing light to the crustal rift zone structure and likely origin of the Traps based on the velocity structure of the crust beneath Gujarat. Tomographic inversion of the Gujarat region was done using the non-linear, passive-source tomographic algorithm, LOTOS. We use high-quality arrival times of 22,280 P and 22,040S waves from 3555 events recorded from August 2006 to May 2011 at 83 permanent and temporary stations installed in Gujarat state by the Institute of Seismological Research (ISR). We conclude that the resulting high-velocity anomalies, which reach down to the Moho, are most likely related to intrusives associated with the Deccan Traps. Low velocity anomalies are found in sediment-filled Mesozoic rift basins and are related to weakened zones of faults and fracturing. A low-velocity anomaly in the north of the region coincides with the seismogenic zone of the reactivated Kachchh rift system, which is apparently associated with the channel of the outpouring of Deccan basalt.

Crustal Seismic Velocities of the Rwenzori Region, East African Rift, from Local Travel-Time Tomography: Evidence for Low-Velocity Anomalies beneath the Mountain Range

We investigate the upper crustal velocity structure beneath the Rwenzori Mountains in western Uganda. This mountain range of nonvolcanic origin is situated within the western branch of the East African rift and reaches altitudes of more than 5000 m. The cause for the extreme uplift within a rifting environment is currently being debated. The local tomographic inversion described here is based on 2053 earth­ quakes recorded by a network of up to 35 stations covering an area of 140 x 90 km2• The deployment was limited by the international border between Uganda and the Democratic Republic of the Congo, such that a number of recorded events lie outside the station perimeter. We perform synthetic tests to assess the effect of location uncertainty on the results. The tests show that the resolution is good between 3 and 15 km depth within a restricted area covered by the array. However, smearing can be significant in some parts. The inversion for P-and S-wave velocity anomalies is performed independently and agrees well. The interpretation of the results is based on a synthetic model that reproduces the same pattern of anomalies as that obtained after inversion of the real data. Our models exhibit a significant negative velocity anomaly (up to -8%) beneath the central Rwenzori Mountains. This could be an indication for active magmatic intrusions beneath the mountains in relation to the rifting. The pre­ sence of low velocities in the northwest of the range, within the rift, may be related to magmatic processes beneath the Buranga hot springs. Higher velocities are found elsewhere beneath the eastern rift shoulder and are thought to be related to old cratonic crust.

Tomographic imaging of seismic velocity structure in and around the Onikobe volcanic area, northeastern Japan: implications for fluid distribution

We estimated three-dimensional P- ( V p) and S-wave velocity ( V s) and V p/ V s structures in and around the Onikobe volcanic area, northeastern Japan, by local travel time tomography. We used travel time data from source and receiver pairs located within and outside the study area, which plays an important role in obtaining the optimum ray coverage and in elucidating the deeper structure more accurately. Detailed information on deeper structures is essential for imaging the complete volcanic system from the magmatic source zone through areas of shallow hydrothermal circulation. More than 50 000 travel time data for the P-waves and 35 000 for the S-waves were used to image the velocity structure. Our results show the following dominant features: (1) two conduits in the upper crust with low V p and low V s indicative of H 2O-rich fluid pathways: one lying beneath Naruko volcano, the other beneath the focal area of the 1962 Northern Miyagi earthquake ( M6.5); (2) an underlying broad region in the lower crust with low V p, low V s and high V p/ V s, suggestive of a zone of partial melt, from which the fluids in (1) are derived; and (3) low V p/ V s areas near the surface of the Sanzugawa and Onikobe calderas, suggesting a diffuse vapor-saturated cap.