Identification of the Seismogenic Fault of the 1654 M 8.0 Tianshui Earthquake, Northeastern Tibetan Plateau

The 1654 M 8.0 Tianshui earthquake occurred in the triangle area bounded by the West Qinling fault (WQLF) and Lixian–Luojiabao fault (LLF) in the northeastern Tibetan plateau. Previous studies reported that the LLF is the source for this earthquake based on the historical records and the Holocene fault activities. However, topographic analyses, outcrop observations, trench excavations associated with the WQLF, together with the radiocarbon dating results reveal that (1) the most recent surface-rupturing earthquake (E1) occurred in the past 470 yr, which can only correspond to the 1654 Tianshui earthquake if the historic earthquakes record is complete. This result means that the seismogenic fault, which is responsible for the 1654 Tianshui earthquake is the WQLF, rather than the LLF as previously reported; (2) the penultimate morphogenic earthquake (E2) took place in the period of 2693–760 yr Cal B.P.; (3) the third recent large earthquake (E3) occurred in the period of 10,229–6032 yr Cal B.P. with a higher probability in this range of 9005–8596 yr Cal B.P.; and (4) in consideration of the double time span of event E3 when compared with event E2 and E1, there is a possibility that another morphogenic earthquake took place in the period of 8596–6032 yr Cal B.P., and then the fourth surface-rupturing event (E4) occurred in the period of 9005–8596 yr Cal B.P. Therefore, at least three or four Holocene slipping events have occurred upon the WQLF in the past ∼9000 yr, suggesting an average recurrence interval of large earthquakes of 2250–3000 yr. The new evidence associated with the source of the 1654 M 8.0 Tianshui earthquake and the recurrence interval of large earthquakes on the WQLF will throw light on the reassessment of seismic potential in this area.

A new approach to constrain the seismic origin for prehistoric turbidites as applied to the Dead Sea Basin

<p>Seismogenic turbidites are widely used for geohazard assessment. The use of turbidites as an earthquake indicator requires a clear demonstration that an earthquake, rather than non-seismic factors, is the most plausible trigger. The seismic origin is normally verified either by correlating the turbidites to historic earthquakes, or by demonstrating synchronous deposition over large areas of a basin. Correlating historic earthquakes could potentially constrain the seismic intensities necessary for triggering turbidites, however this method is not applicable to prehistoric events. In addition, the synchronous deposition of turbidites cannot be verified for a single core record.</p><p>Here, we propose a new approach to establish the seismic origin of prehistoric turbidites that involves analyzing in situ deformation that underlies each turbidite, as recorded in a 457 m-long core from the Dead Sea depocenter. These in situ deformations have been previously verified as seismites and could thus authenticate the trigger for each overlying turbidite. We also constrain the seismic intensities that triggered prehistoric turbidites by analyzing the degree of in situ deformation underlying each turbidite. Moreover, our high-resolution chemical and sedimentological data validate a long-lasting hypothesis that soft-sediment deformation in the Dead Sea formed at the sediment-water interface. In addition, we use our results to propose seven basic earthquake-related depositional scenarios preserved in depocenters located in tectonically active regions like the Dead Sea. These techniques and findings permit a more confident geohazard assessment in the region and act as a model for other similar tectonic settings, by improving the completeness of a paleoseismic archive.</p>

Muography as a new tool to study the historic earthquakes recorded in ancient burial mounds

Bidirectional muographic measurements were conducted at the Imashirozuka burial mound, Japan. The mound was built in the beginning of the 6th century as a megalithic tomb and later collapsed after a landslide caused by the 1596 Fushimi earthquake, one of the largest earthquakes that has occurred in Japan over the last few centuries. The measurements were conducted in order to find evidence of this past disaster recorded in this historical heritage site. As a result, the vertical low-density regions were found at the top of the mound. These regions were interpreted as large-scale vertical cracks that caused the translational collapse process behind the rotational landslide that was already found in prior trench-survey-based works. These results indicate that there was an intrinsic problem with the stability of the basic foundation of the Imashirozuka mound before the 1596 Fushimi earthquake.

Muography as a new tool to study the historic earthquakes recorded in ancient burial mounds

Bidirectional muographic measurements were conducted at the Imashirozuka burial mound, Japan. The mound was built in the beginning of the 6th century as a megalithic tomb and was later collapsed after a landslide caused by the 1596 Fushimi Earthquake, one of the largest earthquakes that have occurred in Japan over last few centuries. The measurements were conducted in order to find evidence of this past disaster recorded in this historical heritage sites. As a result, the vertical low-density regions were found at the top of the mound. These regions were interpreted as large-scale vertical cracks that caused the translational collapse process behind the rotational landslide that was already found in the prior trench-survey-based works. These results indicate that there was an intrinsic problem with the stability of the basic foundation of the Imashirozuka mound before the 1596 Fushimi Earthquake.

Active tectonics and seismicity in the Calabrian Arc subduction complex (Ionian Sea)

<p>The Calabria Arc (CA) is the narrowest subduction-rollback system on Earth, and it has been struck repeatedly by destructive historical earthquakes often associated with tsunamis. In spite of the detailed earthquake catalogue, the source parameters of most historic earthquakes are still debated, especially for earthquakes that may have been generated offshore.</p><p>The subduction system is characterized by an irregular plate boundary reflecting the presence of continental blocks, indenters, and different rates of continental collision. Convergence between Eurasia and Africa produces both compressive and transtensional deformation in the offshore accretionary complex. Shortening occurs along the outer deformation front and along splay faults accommodating differences in rheology and basal detachment depth. Two oppositely dipping strike-slip/transtensional fault systems, i.e., the Ionian (IF) and Alfeo-Etna (AEF) faults produce deep fragmentation of the subduction system and the collapse of the accretionary wedge, in agreement with geodetic models suggesting plate divergence in this region. Transtensional lithospheric faults segmenting the subduction system are punctuated by mantle-rooted diapirism driven by arc orthogonal rifting, collapse of the accretionary wedge, and deep fragmentation of the subduction system along pre-existing Mesozoic transform faults.</p><p>Seismological observations in the Western Ionian Sea highlight the presence of earthquake clusters along wide and deep-seated active tectonic structures, which were proposed as likely seismogenic sources for large magnitude historic earthquakes/tsunamis in the region. Low to moderate magnitude earthquakes occurring offshore were relocated using a new 1D velocity model for the Ionian Sea, constrained by geological and geophysical observations, which included data collected by NEMO-SN1 seafloor observatory. Seismological data from NEMO-SN1 were integrated with observations carried out by over 100 land stations of the INGV network, and led us to compile a map of 3D distribution for over 2600 events. 3D locations and focal mechanism analyses allowed us to highlight local lithospheric structure. Although seismicity appears scattered in a wide corridor of deformation within the subduction system, we observe alignments of events along main fault systems with strike-slip and extensional mechanisms. Moreover, results from seismological data analysis, i.e., misfits in the 3D distribution of hypocenters and tomographic maps, could be explained by the presence of an anomalous area between the two structures, characterized by thinned lithosphere probably caused by incipient rifting, as suggested by seismic reflection images and geodynamic interpretations.  </p>

The 2019Durrës, Albania earthquake sequence – preliminary results from a post-seismic campaign

<p>On 26<sup>th</sup> of November 2019 an M<sub>w</sub> 6.4 earthquake ruptured near the port town of Durrës, only 25 km from Tirana, the capital of Albania. It caused major damage and killed 51 people, making it the deadliest earthquake in 2019 worldwide.</p><p>The earthquake occurred on the eastern Adriatic margin, where the Adriatic micro-plate collides with Eurasia causing widespread distributed deformation and crustal shortening that built the peri-Adriatic orogenic belts. Convergence is accommodated in the external Dinarides/Albanides by thrust faulting, mostlyalong E-dipping low-angle detachments with subordinate W-dipping back-thrusts in the most external thrust belt segment. The deformation front, particularly along the southeastern Adriatic coast, is seismically highly active, manifested not only by this most recent event, but also, e.g., by one of the largest instrumentally recorded earthquakes in Europe, the 1979 M7.1 Montenegro event slightly further north and a number of disastrous historic earthquakes.</p><p>The 2019 Durrës mainshock was apparently relatively deep (~25 km) and of thrust type. It was preceded by significant foreshock activity starting in September 2019 with two M<sub>w</sub> 5.6 and 5.1 earthquakes a few kilometres south of the mainshock that also had a thrust mechanism, however with nodal planes differing from the mainshock, indicating that these occurred on a different fault.</p><p>Approximately two weeks after the mainshock, we installed a 30-station short-period seismic network to densely cover the epicentral area. We will present a preliminary analysis of the mainshock and its aftershock sequencehopefully elucidating the fault network responsible for the earthquake sequence.</p>