Box Counting Fractal Dimension and Frequency Size Distributon of Earthquakes in the Central Himalaya Region

To establish the relations between b-value and fractal dimension (D0) for the earthquake distribution, we study the regional variations of those parameters in the central Himalaya region. The earthquake catalog of 989 events (Mc = 4.0) from 1994.01.31 to 2020.10.28 was analyzed in the study. The study region is divided into two sub-regions (I) Region A: 27.3°N -30.3°N and 80°E -84.8°E (western Nepal and vicinity) and (II) Region B: 26.4°N -28.6°N and 84.8°E -88.4°E (eastern Nepal and vicinity). The b-value observed is within the range between 0.92 to 1.02 for region A and 0.64 to 0.74 for region B showing the homogeneous nature of the variation. The seismic a-value for those regions ranges respectively between 5.385 to 6.007 and 4.565 to 5.218. The low b-values and low seismicity noted for region B may be related with less heterogeneity and high strength in the crust. The high seismicity with average b-values obtained for region A may be related with high heterogeneity and low strength in the crust. The fractal dimension ≥1.74 for region A and ≥ 1.82 for region B indicate that the earthquakes were distributed over two-dimensional embedding space. The observed correlation between D0 and b is negative for western Nepal and positive for eastern Nepal while the correlation between D0 and a/b value is just opposite for the respective regions. The findings identify both regions as high-stress regions. The results coming from the study agree with the results of the preceding works and reveal information about the local disparity of stress and change in tectonic complexity in the central Himalaya region.

SEISMISITY of THE VOLCANIC AREAS of KAMCHATKA in 2015

The results of near real-time monitoring of the active Kamchatka volcanoes are described. Continuous monitoring was carried out using three remote methods: 1) seismic monitoring according to automatic telemetric seismic stations; 2) visual and video observation; 3) satellite observation of the thermal anomalies and the ash clouds. Annual results of seismic activity of the Northern (Shiveluch, Kluchevskoy, Bezymianny, Krestovsky, and Ushkovsky), the Avacha (Avachinsky, and Koryaksky), the Mutnovsky-Gorely volcano groups and the Kizimen volcano are presented. 5464 earthquakes with КS=1.8–8.1 were located for the Northern volcano group, 302 earthquakes with КS=1.7–5.7 – for the Avacha volcano group, 295 earthquakes with КS=2.1–6.8 for the Mutnovsky-Gorely volcano group, 462 earthquakes with КS=2.2–8.3 for Kizimen volcano, and 165 earthquakes with КS=2.5–8.4 for Zhupanovsky volcano in 2015. Maps of epicenters, quantities of seismic energy and earthquake distribution by energy classes are given. All periods of activity were fixed and investigated by remote methods in 2015: intensive volcanic activity of the Sheveluch volcano associated with a new cone; the summit explosive-effusive eruption of the Kluchevskoy volcano in January–April; and a continuation of seismic and volcanic activity of the Zhupanovsky volcano after 56-year quite period.

The TsuJal Amphibious Seismic Network: A Passive-Source Seismic Experiment in Western Mexico

The geodynamic complexity in the western Mexican margin is controlled by the multiple interactions between the Rivera, Pacific, Cocos, and North American plates, as evidenced by a high seismicity rate, most of whose hypocenters are poorly located. To mitigate this uncertainty with the aim of improving these hypocentral locations, we undertook the TsuJal Project, a passive seafloor seismic project conducted from April to November 2016. In addition to the Jalisco Seismic Network, 10 LCHEAPO 2000 Ocean Bottom Seismometers (OBSs) were deployed by the BO El Puma in a seafloor array from the Islas Marías Archipelago (Nayarit) to the offshore contact between the states of Colima and Michoacan. We located 445 earthquakes in four or more OBSs within the deployed array. Most of these earthquakes occurred in the contact region of the Rivera, Pacific, and Cocos plates, and a first analysis suggests the existence of three seismogenic zones (West, Center, and East) along the Rivera Transform fault that can be correlated with its morphological expression throughout the three seismogenic zones. The seismicity estimates that the Moho discontinuity is located at 10 km depth and supports earlier works regarding the West zone earthquake distribution. Subcrustal seismicity in the Central zone suggests that the Intra-Transform Spreading Basin domain is an ultra-low spreading ridge. A seismic swarm occurred during May and June 2016 between the eastern tip of the Paleo-Rivera Transform fault and the northern tip of the East Pacific Rise-Pacific Cocos Segment, illuminating some unidentified tectonic feature.

Determination of Earthquake Hypocenter Distribution using the Modified Joint Hypocenter Determination (MJHD) Method throughout the Palu-Koro Fault (Case Study: August-October, 2018)

The area of Sulawesi, especially along the Palu Koro Fault, is an area that is largely influenced by the confluence and movement of plates as well as regional fault activity pathways with high levels of seismicity. Determining the location of the hypocenter accurately through relocation is required in identifying the detailed tectonic structures in the area. Relocation of the hypocenter using the Modified Joint Hypocenter Determination (MJHD) method using the IASP91 velocity model in the period August to October 2018 with the arrival time data from BMKG catalog. The results of hypocenter relocation using the MJHD method show that from 132 earthquake distribution points to 63 earthquake hypocenter points after the relocation. The change in the location of the hypocenter was much denser along the Palu Koro Fault route than before the relocation as evidenced by the mean value of rms (root mean square) before relocation was 1.31 and after relocation it became smaller (0.61). Changes in parameter values after relocation using the MJHD method caused the distribution of the earthquake hypocenter to be tighter towards the Palu Koro fault than before the relocation, where the distribution had a random and scattered pattern.

Variations in Wedge Earthquake Distribution along the Strike Underlain by Thermally Controlled Hydrated Megathrusts

Accretionary wedge earthquakes usually occur in the overriding crust close to the trench or above the cold nose of the mantle wedge. However, the mechanism and temperature properties related to the slab dip angle remain poorly understood. Based on 3D thermal models to estimate the subduction wedge plate temperature and structure, we investigate the distribution of wedge earthquakes in Alaska, which has a varying slab dip angle along the trench. The horizontal distance of wedge-earthquake hypocenters significantly increases from the Aleutian Islands to south–central Alaska due to a transition from steep subduction to flat subduction. Slab dehydration inside the subducted Pacific plate indicates a simultaneous change in the distances between the intraslab metamorphic fronts and the Alaskan Trench at various depths, which is associated with the flattening of the Pacific plate eastward along the strike. The across-arc width of the wedge-earthquake source zone is consistent with the across-arc width of the surface high topography above the fully dehydrated megathrust, and the fluid upwelling spontaneously influences wedge seismotectonics and orogenesis.

Direct fault states assessment from wavefield properties: application to the 2009 L’Aquila earthquake

<p><span>Monitoring and investigating the physical states of active faults is essential to understand how earthquakes begin and the physical processes involved. Traditionally, fault-state investigation strategies use seismic catalogs whose completeness and accuracy may be limited. We propose to take benefit of the information encoded in the continuous seismograms in order to fully extract information about the fault physics. We calculate the covariance matrix spectrum of continuous seismograms at an array of stations and extract features (e.g. entropy, spectral width, variance and coherency) from the covariance matrix eigenvalue spectrum. Those features are related to seismic source characteristics (e.g. source localization, spectral content, duration...) in the time scale of analysis, and can be used to reveal different physical states of faults. The dominant frequency band of the seismic wavefield changes at different stages of fault activities. Therefore, we perform clustering to characterize the physical states of fault based on the extracted frequency-dependent features. We apply this approach to investigate the 2009 L’Aquila earthquake. At preparation phase of the L’Aquila earthquake, foreshocks are localized around the main active fault. In contrast, the aftershocks disperse in a more broaden area where the faults have been activated by the mainshock. The extracted features and corresponding clustering results are able to capture and distinguish those patterns of earthquake distribution. In addition, the locations of the seismic sources are encoded in the covariance matrix eigenvectors. Through clustering and migration of eigenvectors, we are able to reveal the spatial and temporal variation of the different seismic sources. The method is here applied to study recent earthquakes in Italy as the L’Aquila 2009, Emilia 2012 and Norcia 2016.</span></p>

Fault geometry and dynamics during the 1993-1998 uplift episode in Hengill, SW-Iceland

<p>The Hengill volcanic complex in SW-Iceland is located on a triple junction where two extensive and one conservative plate boundary meet. An uplift event, possibly caused by a magmatic intrusion, in the 1990ies caused a landrise of 8 cm over the period of 4 years and was accompanied by more than 90.000, mostly very small, earthquakes. We used cross-correlation to improve pick accuracy and applied a relative relocation algorithm to get high resolution earthquake locations of the earthquakes in the direct vicinity of the centre of the uplift. Relocated earthquake location reveal clustering and alignments of earthquakes that are mostly oriented in NNE and ENE direction. Then we recalculated focal mechanisms for the new locations and then use the Quakelook software to select the best fitting focal mechanism. Quakelook calculates a plane that best fits the locations of a cluster of earthquakes which then is compared to the database of possible focal mechanisms that all explain the polarity and amplitude data similar well. The projection of the slip vectors into the fault plane is then used to estimate the average movement along the fault. From the fault dynamics we learn about the stresses activating that fault.</p><p>The relocated earthquake distribution shows that the stresses induced by the uplift event must have been small in comparison to the regional stress since the activated faults do not respect the geometry of the uplift source but are rather in agreement to the regional stress field. The uplift did not cause any new breaks in the crust but rather reactivated existing faults which sub-optimally oriented in relation to the uplift.</p>