local seismicity
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AbstractPeninsular Malaysia is tectonically situated on a stable craton (intraplate) and so far experiences relatively little earthquake activities, thus considered as a region with low seismicity. This study uses earthquake data from 59 events obtained from various sources in the period 1922 to 2020. The overall seismicity in the study area is low as expected due to the general intraplate setting. Earthquakes occurred onshore and offshore of Peninsular Malaysia between latitudes 1° and 7° N and longitudes 99° and 105° E. The seismicity pattern shows that the epicenters are distributed spatially in some parts of the peninsula and in the Malacca Strait with several epicenter zones. Most of earthquakes are associated with several preexisting faults and fault zones indicating that they are the major contributor to the local seismicity. Meanwhile, some further earthquakes were caused by activities related to reservoirs. Magnitudes are ranging from Mw 0.7 to 5.4 with the majority is Mw 1.0 + and 2.0 +. Hypocenters are located in between 1 and 167 km deep (shallow to intermediate earthquakes) with the majority being shallow earthquakes (1–70 km). The deepest earthquake located in the Straits of Malacca can be associated with a slab detachment broken off from the Sumatran Subduction Zone. Finally, this study contributes to the understanding of the intraplate seismicity of Peninsular Malaysia as a basis for seismic hazard and risk assessment.Article Highlights Earthquake assessment over the last 100 year reveals low but clear seismicity with an associated seismic hazard and risk for certain areas. Shallow, low-magnitude earthquakes associated with reservoir activities and preexisting faults reactivated by the nearby subduction zone. A deeper, low-magnitude earthquake can be related to slab detachment from the Sumatran subduction zone toward the east.

Abstract Stretching nearly the extent of the Canadian Cordillera, the Rocky Mountain trench (RMT) forms one of the longest valleys on Earth. Yet, the level of seismicity, and style of faulting, on the RMT remains poorly known. We assess earthquakes in the southern RMT using a temporary network of seismometers around Valemount, British Columbia, and identify active structures using a probabilistic earthquake catalog spanning from September 2017 to August 2018. Together with results from earlier geological and seismic studies, our new earthquake catalog provides a constraint on the geometry of subsurface faults and their level of activity during a year of recording. The tectonic analysis presented here benefits from the catalog of 47 earthquakes, including robust horizontal and vertical uncertainty quantification. The westward dip of the southern RMT fault is one of the prominent subsurface structures that we observe. The seismicity observed here occurs on smaller surrounding faults away from the RMT and shifts from the east to the west of the trench from north to south of Valemount. The change in distribution of earthquakes follows changes in the style of deformation along the length of the RMT. Focal mechanisms calculated for two earthquakes with particularly clear waveforms reveal northeast–southwest-oriented thrusting. The seismicity reveals a change in the pattern of deformation from narrowly focused transpression north of Valemount to more broadly distributed activity in an area characterized by normal faulting to the south. Six sets of repeating events detected here produce similar waveforms whose P waves exhibit correlation coefficients that exceed 0.7 and may result from the migration of fluids through the fractured crust.

AbstractThe 2011 magnitude (M) 9.0 Tohoku-oki earthquake was followed by seismicity activation in inland areas throughout Japan. An outstanding case is the M6.2 Northern Nagano earthquake, central Japan, occurred 13-h after the megathrust event, approximately 400 km away from its epicenter. The physical processes relating the occurrence of megathrust earthquakes and subsequent activation of relatively large inland earthquakes are not well understood. Here we use waveform data of a dense local seismic network to reveal with an unprecedented resolution the complex mechanisms leading to the occurrence of the M6.2 earthquake. We show that previously undetected small earthquakes initiated along the Nagano earthquake source fault at relatively short times after the Tohoku-oki megathrust earthquake, and the local seismicity continued intermittently until the occurrence of the M6.2 event, being likely ‘modulated’ by the arrival of surface waves from large, remote aftershocks off-shore Tohoku. About 1-h before the Nagano earthquake, there was an acceleration of micro-seismicity migrating towards its hypocenter. Migration speeds indicate potential localized slow-slip, culminating with the occurrence of the large inland earthquake, with fluids playing a seismicity-activation role at a regional scale.

Abstract We present a detailed analysis of the 10 June 2019 ML 4.0 Ohio earthquake sequence, which is the largest earthquake that struck Lake County, northeastern Ohio, since 1986. This sequence is well recorded by local seismic networks, which provides an unprecedented opportunity to understand the local seismotectonics. We utilize a waveform-based cross-correlation method to identify ∼12 times more events than reported by the Advanced National Seismic System (ANSS) Comprehensive Earthquake Catalog: the whole sequence started with several small earthquakes (ML 1–2) beginning 12 March 2019, and the last one occurred ∼1min immediately before the ML 4.0 mainshock; many previously unreported aftershocks (ML 0.3–2.2) are found, which were active for the first week after the mainshock; another major sequence with a 7 December 2019 ML 2.6 mainshock occurred and also started with a few smaller events beginning in mid-November and was followed by its own aftershocks. The relocated seismicity delineates a linear feature, orientation of which is consistent with the resolved focal plane that may correspond to the ruptured fault. Our results highlight that closer monitoring of local seismicity is crucial for understanding the seismotectonics and mitigating future seismic hazard around the southern Great Lakes.

<p>There are many reports on electromagnetic pre-earthquake phenomena such as geomagnetic, ionospheric, and atmospheric anomalous changes. Ionospheric anomaly preceding large earthquakes is one of the most promising phenomena. Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) model has been proposed to explain these phenomena. In this study, to evaluate the possibility of chemical channel of LAIC by observation, we have installed sensors for atmospheric electric field, atmospheric ion concentration, atmospheric Rn concentration, underground Rn concentration (GRC), and weather elements at Asahi station, Boso, Japan. Since the atmospheric electricity parameters are very much influenced by weather factors, it is necessary to remove these effects as much as possible. In this aim, we apply the MSSA (Multi-channel Singular Spectral Analysis) to remove these influences from the variation of GRC and estimate the underground Rn flux (GRF). We investigated the correlations (1) between GRF and precipitation and (2) between GRF and the local seismic activity around Asahi station. The preliminary results show that there is a tendency of correlation (1) between GRF and heavy rain and (2) between GRF and local seismicity within an epicenter distance of 50 km from the station.</p>

Abstract Thanks to the new permanent seismic stations installed in the Franz Joseph Land and Severnaya Zemlya arctic archipelagoes, it has become possible at present to record earthquakes occurring in the eastern Gakkel ridge with a much lower detection threshold than that provided by the global network. At present, the lowest recorded magnitude is ML 2.4 and the magnitude of completeness is 3.4. We examined the results of seismic monitoring conducted from December 2016 through January 2020 to show that the earthquake epicenters are not uniformly distributed both in space and over time within the eastern part of the ridge. There were periods of quiescence and seismic activity. Most of the epicenters are confined to the area between 86° and 95.0° E. Relative location techniques were used to locate the single major swarm of earthquakes recorded so far. Most earthquakes were recorded by two or three stations only, so that relative location techniques have been able to yield reliable data for an analysis of the swarm. We showed that there have been actually two swarms that contained different numbers of events. The earthquakes in the larger swarm were occurring nonuniformly over time and clustered at certain depths. The ML scale was calibrated for the Eurasian Arctic based on records of the seismic stations installed in the Svalbard Archipelago, Franz Joseph Land, and on Severnaya Zemlya: −logA0(R)=1.5×logR100+1.0×10−4(R−100)+3.0. The results will help expand our knowledge of the tectonic and magmatic processes occurring within the ultraslow Gakkel ridge, which are reflected in the local seismicity.

Abstract. The city of Auckland, New Zealand (Tāmaki Makaurau, Aotearoa), sits on top of an active volcanic field. Seismic stations in and around the city monitor activity of the Auckland Volcanic Field (AVF) and provide data to image its subsurface. The seismic sensors – some positioned at the surface and others in boreholes – are generally noisier during the day than during nighttime. For most stations, weekdays are noisier than weekends, proving human activity contributes to recordings of seismic noise, even on seismographs as deep as 384 m below the surface and as far as 15 km from Auckland's Central Business District. Lockdown measures in New Zealand to battle the spread of COVID-19 allow us to separate sources of seismic energy and evaluate both the quality of the monitoring network and the level of local seismicity. A matched-filtering scheme based on template matching with known earthquakes improved the existing catalogue of five known local earthquakes to 35 for the period between 1 November 2019 and 15 June 2020. However, the Level-4 lockdown from 25 March to 27 April – with its drop in anthropogenic seismic noise above 1 Hz – did not mark an enhanced detection level. Nevertheless, it may be that wind and ocean swell mask the presence of weak local seismicity, particularly near surface-mounted seismographs in the Hauraki Gulf that show much higher levels of noise than the rest of the local network.

In this study, we improved and adapted existing signal processing methods on vast geomagnetic field data to investigate the correlations between various earthquake properties and characteristics of possible geomagnetic precursors. The data from 10 magnetometer stations were utilized to detect precursory ultra-low frequency emission and estimate the source direction for 34 earthquakes occurring between the year 2007–2016 in Southeast Asia, East Asia, and South America regions. As a result, possible precursors of 20 earthquakes were identified (58.82% detection rate). Weak correlations were obtained when all precursors were considered. However, statistically significant and strong linear correlations (r ≥ 0.60, p < 0.05) were found when the precursors from two closely located stations in Japan (Onagawa (ONW) and Tohno (TNO)) were exclusively investigated. For these stations, it was found that the lead time of the precursor is strongly (or very strongly) correlated with the earthquake magnitude, the local seismicity index, and the hypocentral depth. In addition, the error percentage of the estimated direction showed a strong correlation with the hypocentral depth. It is concluded that, when the study area is restricted to a specific location, the earthquake properties are more likely to have correlations with several characteristics of the possible precursors.

Assessment of local seismicity caused by human engineering activity is a necessity after several earthquakes have been recorded in world practice, which destroyed a hydraulic structure, a dam and accompanied by human casualties. The Tupalang, Gissarak and Pachkamar reservoirs, operated in southern Uzbekistan, are located in the mountain system of the southern part of the Tien Shan seismogenic fault. According to the general seismic zoning, the zones where these reservoirs are located are classified as 8-9 magnitude shakes during strong earthquakes. Localisation of local earthquakes associated with the operation of these reservoirs is necessary for taking operational measures to prevent possible emergencies associated with an increase in the level of technogenic seismicity. Methods for assessing the local seismic regime were used to track the level of local seismicity. The catalogs of local earthquakes occurring in the zone of active influence of reservoirs have been compiled. The seismic activity levels and changes in the angle of inclination of the recurrence of earthquakes were determined for each object separately. The dependence of the variation in the level of local seismicity on the mode of operation of reservoirs has been revealed. Several evidence of the negative impact of large hydraulic reservoirs on the deformation and seismic state of the zones of influence of reservoirs in southern Uzbekistan have been identified.