Maluku Sea Plate Faulting Regime Analysis: A Preliminary Study

Present day Molucca or Maluku sea plate in the eastern of Indonesia possesses a complex tectonic setting. This complex tectonic setting has been formed due to the collision of an actively moving Eurasian plate and Philippine sea plate toward the Maluku sea plate. At the west, Maluku sea plate is subducting beneath Sangihe arc, which began in the early Miocene. While at the east, Maluku sea plate is subducting under Halmahera arc, since in the middle Miocene. These subduction processes take place up to the present. Therefore, it has formed Maluku sea plate into an inverted U-shape slab under a thickening accretionary complex. Seismicity distribution has clearly shown the U-shape slab. Earthquake events take place on the subducting slab, and interestingly on the above accretionary complex as well. Maluku sea plate might pose hazards to surrounding islands: northern Sulawesi, Halmahera island, Sangihe island and Talaud island. The possible hazard, for instance, a thrusting earthquake which may generate tsunami to the nearby islands. Hence, understanding its tectonic and seismicity signature, especially at the shallow part, are indeed important in the Maluku sea region. Faulting regime could be analyzed using focal mechanism ternary diagram analysis, by categorizing the focal mechanisms’ strike, dip and rake values. Thus, in this study we aim to analyze faulting regime and hazard potential in the complex. Maluku setting using ternary diagram analysis.

Nowcasting and multifractal features of the seismicity in the subduction zone of Tehuantepec Isthmus, southern México

<p>After the M8.2 earthquake occurred on September 07, 2017 at Isthmus of Tehuantepec, notable spatial and temporal changes where<br>registered, the temporal rate of occurrence increased and the spatial seismicity distribution showed a clear clusterization along<br>the region of collision of the Tehuantepec Transform/Ridge with the Middle America Trench off Chiapas. Also, the b-value in the<br>Gutenberg-Richer law showed changes in time. On the basis of that behavior we studied the sequence of magnitudes of the<br>earthquakes occurred within the Isthmus of Tehuantepec at southern Mexico from 2010 to 2020, by using the nowcasting method<br>and the multifractal detrended fluctuation analysis. Our findings suggest the b-value could depend on time and after the main-shock<br>M8.2, the underlying dynamics in the Tehuantepec ridge has been changed, which is clearly described by our analyses based on<br>nowcasting method and in the multifractality estimated changes.</p>

Recent faulting along Gorontalo fault based on seismicity data analysis and lineament mapping

We focused our study to characterize the geometry and activity of Gorontalo fault. We analysed reviewed the ISC seismic catalogue and the BMKG relocated earthquake events available for the time period of 1960 to 2021, located along the expected location of this fault. In addition, we analysed continuous record from local seismic observatory available for the same period. Further, we mapped the lineaments using 8.3-m resolution DEMNAS data. Tens on shallow earthquakes occurred in the vicinity of this fault with a range magnitude of M 2 to 3. Our lineament analysis however does not reveal distinctive pattern that may indicate the fault manifestation at the surface. The NW-SE trending lineaments are coincidence with the mapped trace of Gorontalo Fault. The weak surface manifestation of the fault scarp may be related to the tropical climatic condition of the area which may obliterate the faulting topography. However, we observed alignment of the seismicity distribution with the mapped NW-SE lineament, indicating that the lineament is likely representing active fault and these earthquakes are associated with faulting along this fault. Our study provide indication that the Gorontalo Fault is active and further study is necessary to investigate subsurface geometry and mitigate its seismic hazards.

An earthquake catalogue for seismic events in the Norman Wells region of the central Mackenzie Valley, Northwest Territories, using waveform data from local seismic stations

The development of unconventional hydrocarbon resources in the Norman Wells region of the Central Mackenzie Valley, Northwest Territories, has been explored by the energy industry. In early 2014, Conoco-Philips Canada conducted two multi-stage test operations of hydraulic fracturing (HF) in the region. In this study, we combine seismic data from the Canadian National Seismograph Network, four new stations established by the Northwest Territories Geoscience Office in collaboration with Natural Resources Canada in the Norman Wells region, and a local dense array installed by Conoco-Philips Canada to study the seismicity distribution during the pre-HF, HF and post-HF periods. We have identified and located 130 earthquakes within 100 km of the geographic centre of the local seismic network near Norman Wells for the pre-HF period (11 September 2013 - 7 February 2014). In comparison, 231 events are located during the HF period (8 February 2014 - 10 March 2014), and for the two post-HF periods, 11 March 2014 - 31 July 2014 and 27 February 2015 - 31 December 2015, we have catalogued 255 and 138 events, respectively. Source parameters and detailed phase pickings of each earthquake are given in the Appendices.

Synthetic seismicity distribution in Guerrero–Oaxaca subduction zone, Mexico, and its implications on the role of asperities in Gutenberg–Richter law

Seismicity and magnitude distributions are fundamental for seismic hazard analysis. The Mexican subduction margin along the Pacific Coast is one of the most active seismic zones in the world, which makes it an optimal region for observation and experimentation analyses. Some remarkable seismicity features have been observed on a subvolume of this subduction region, suggesting that the observed simplicity of earthquake sources arises from the rupturing of single asperities. This subregion has been named SUB3 in a recent seismotectonic regionalization of Mexico. In this work, we numerically test this hypothesis using the TREMOL (sThochastic Rupture Earthquake MOdeL) v0.1.0 code. As test cases, we choose four of the most significant recent events (6.5 < Mw < 7.8) that occurred in the Guerrero–Oaxaca region (SUB3) during the period 1988–2018, and whose associated seismic histories are well recorded in the regional catalogs. Synthetic seismicity results show a reasonable fit to the real data, which improves when the available data from the real events increase. These results give support to the hypothesis that single-asperity ruptures are a distinctive feature that controls seismicity in SUB3. Moreover, a fault aspect ratio sensitivity analysis is carried out to study how the synthetic seismicity varies. Our results indicate that asperity shape is an important modeling parameter controlling the frequency–magnitude distribution of synthetic data. Therefore, TREMOL provides appropriate means to model complex seismicity curves, such as those observed in the SUB3 region, and highlights its usefulness as a tool to shed additional light on the earthquake process.

How Alpine seismicity relates to lithospheric strength

Despite the amount of research focused on the Alpine orogen, different hypotheses still exist regarding varying seismicity distribution patterns throughout the region. Previous measurement-constrained regional 3D models of lithospheric density distribution and thermal field facilitate the generation of an observation-based rheological model of the region. Long term lithospheric strength was then calculated and compared to observed seismicity patterns, showing that the highest strengths within the crust (~ 1 GPa) and upper mantle (> 2 GPa), occur at temperatures characteristic for specific phase transitions (crust: 200–400 °C; mantle: ~ 600 °C) with almost all seismicity occurring in in these regions. Correlation in the northern and southern forelands between crustal and lithospheric strengths and seismicity show different patterns of event distribution, reflecting their different tectonic settings. Seismicity in the plate boundary setting of the southern foreland corresponds to the integrated lithospheric strength, occurring mainly in the weaker domains surrounding the strong Adriatic indenter. However, in the intraplate setting of the northern foreland seismicity instead corresponds to the crustal strength, mainly occurring in the weaker and warmer crust beneath the URG. Results generated in this study are available for open access use to further discussions on the region.

Anatomy of active volcanic edifice at the Kusatsu–Shirane volcano, Japan, by magnetotellurics: hydrothermal implications for volcanic unrests

We aimed to perform three-dimensional imaging of the underlying geothermal system to a depth of 2 km using magnetotellurics (MT) at around the Yugama crater, the Kusatsu–Shirane Volcano, Japan, which is known to have frequent phreatic eruptions. We deployed 91 MT sites focusing around the peak area of 2 km × 2 km with typical spacings of 200 m. The full tensor impedances and the magnetic transfer functions were inverted, using an unstructured tetrahedral finite element code to include the topographic effect. The final model showed (1) low-permeability bell-shaped clay cap (C1) as the near-surface conductor, (2) brine reservoir as a deep conductor (C3) at a depth of 1.5 km from the surface, and (3) a vertical conductor (C2) connecting the deep conductor to the clay cap which implies an established fluid path. The columnar high-seismicity distribution to the east of the C2 conductor implies that the flushed vapor and magmatic gas was released from the brine reservoir by breaking the silica cap at the brittle–ductile transition. The past magnetization/demagnetization sources and the inflation source of the 2014 unrest are located just below the clay cap, consistent with the clay capped geothermal model underlain by brine reservoir. The resistivity model showed the architecture of the magmatic–hydrothermal system, which can explain the episodic volcanic unrest.

Seismotectonic analysis of the 2014 seismic swarm at the Western Corinth Gulf (Greece)

&lt;p&gt;The Corinth Rift (Greece) is one of the most seismically active regions in Europe and has been studied extensively during the past decades. It is characterized by normal faulting and extension rates between 6 and 15 mm&amp;#160;yr&amp;#8722;1 in an approximately N10E&amp;#176; direction. The seismicity of the area is continuously monitored by the stations of the Corinth Rift Laboratory Network (CRL Net). The availability of a dense permanent seismological network allows the extensive analysis of the seismic swarms which occur frequently. In this study, the September 2014 swarm located at the western part of the Corinth Gulf is analyzed. Initially, more than 4000 automatically located events, of a two month period, were relocated using the HYPODD algorithm, incorporating both catalogue and cross-correlation differential traveltimes. Consequently, the initial seismic cloud was separated into several smaller, densely concentrated clusters. Double difference relocation was also applied to 707 manually located events in order to investigate the Vp/Vs ratio variation, due to its sensitivity in pore fluids. The swarm&amp;#8217;s parameters such as seismicity distribution and moment tensors were combined with the seismotectonic data of the area. The results indicate an initial activation of the Psathopyrgos normal fault; afterwards the seismicity extended both towards East and West, while most events occurred at the western part of the study area. The seismicity distribution revealed a main activation of the North &amp;#8211; dipping faults. The seismicity migration with respect to pore pressure changes due to fluid movements was investigated through diffusivity calculations. The diffusivity value was found to be 4.5m&lt;sup&gt;2&lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;, which is consistent with results of previous studies in the area. The results of the investigation of the fault- zone hydraulic behavior provide evidence for the fluid &amp;#8211; triggered earthquake swarms and the related rock physical properties.&lt;/p&gt;

Lithological and structural control on the seismicity distribution in Central Italy

&lt;p&gt;In the seismically active region of Central Italy, national (permanent) and local (not-permanent) seismic networks provided very accurate location of the seismicity recorded during the major seismic sequences occurred in the last 25 years (e.g. 1997-1998; 2009; 2016-2017), as well as of the background seismicity registered in the intervening periods.&amp;#160; In the same region, a network of seismic reflection profiles, originally acquired for oil exploration purposes, is also available, effectively imaging the geological structure at depth, to be compared with the seismicity distribution.&amp;#160;&lt;/p&gt;&lt;p&gt;This comparison reveals that, if the position of the brittle/ductile transition exerts a role at regional scale for the occurrence of crustal seismicity, at a more local scale the depth and thickness of the seismogenic layer is mostly controlled by the contrasting rheological properties of the different lithological groups involved in the upper crust.&amp;#160;&lt;/p&gt;&lt;p&gt;The upper crust stratigraphy, including the sedimentary cover and the uppermost part of the basement, consists of alternated strong (rigid, e.g. carbonates and dolostones) end weak (not-rigid, e.g. shales, sandstones, and phyllites) layers. This mechanically complex multilayer is involved in a belt of imbricated thrusts (Late Miocene-Early Pliocene), displaced by subsequent extensional (normal) faults (Late Pliocene-present), responsible for the observed regional seismicity. The top of the basement s.l. (composed of clastic sedimentary and slightly metamorphosed rocks) is involved in major thrusts. &amp;#160;For these different lithological units, combined field and lab studies of fault rock properties have documented localized and potentially unstable deformation occurring in granular mineral phases (carbonates) and distributed and stable slip within phyllosilicate-rich shear zones (shales and phyllites).&lt;/p&gt;&lt;p&gt;By comparing the geological structure with the seismicity distribution, we observed that:&lt;/p&gt;&lt;p&gt;-&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;The seismicity cut-off (i.e. the bottom of the seismogenic layer) is structurally (not thermally) controlled, and grossly corresponds to the top basement; the upper boundary of the seismogenic layer corresponds to the top of carbonates.&lt;/p&gt;&lt;p&gt;-&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;Most seismicity occurs within the rigid layers (carbonates and evaporites), and do not penetrate the turbidites and basements rocks.&lt;/p&gt;&lt;p&gt;-&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Close to the axial region of the mountain range, where the larger amount of shortening is observed, the presence thrust sheets from the previous compressional phase, significantly affect the seismicity distribution and propagation.&lt;/p&gt;&lt;p&gt;-&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;Major east-dipping extensional detachments, recognized in the seismic profiles, are also marked by distinctive seismicity alignments.&lt;/p&gt;