Moderate Magnitude Earthquakes Research Articles

SEISMICITY of the RUSSIAN PART of EAST EUROPEAN PLATFORM and ADJACENT TERRITORIES in 2015

It is reported that 41 stationary seismic stations, 2 arrays, and 7 temporary seismic stations, located in the area of Novovoronezh and Kursk nuclear stations, monitored seismicity of the Russian territory of the East European Platform (EEP) in 2015. The registration capabilities of the seismic network at the EEP as a whole were estimated based on the average station noise level and the equation for the energy decay of seismic phases. Zones with the best capabilities have been allocated. A feature of seismicity in 2015 is the manifestation of earthquakes of moderate magnitudes (ML=2.7–3.9) in the peripheral regions (in the southwest, west, and northwest) and in zones associated with paleorift structures: in the southwest – with the Dnieper Donetsk and in the northeast – with the Kirov-Kazhim and Soligalich (Central Russian) aulacogenes. The results of the macroseismic survey are given for the earthquake in Poltava on February 2, 2015, with M=3.7; focal mechanisms of two earthquakes (03.02.2015 and 12.06.2015) are constructed. According to the data of the Latvian Center, an earthquake was recorded in the region of Lithuania bordering the Kaliningrad region. Weaker natural seismicity with ML≤2.5 was recorded in Karelia and the regions bordering with Finland, near the Kandalaksha Bay, near the Khibiny, and Lovozersky massifs on the Kola Peninsula, and on the territory of the Voronezh crystalline massif.

The Interplay Between Seismic and Aseismic Slip Along the Chaman Fault Illuminated by InSAR

AbstractThe 700‐km‐long Chaman fault (CF) marks the western edge of the plate boundary between India and Eurasia. Although global plate models predict 2.3–3.6 cm/yr left‐lateral motion between both plates, the fault is known to have hosted few earthquakes in historical times. Recent geodetic measurements attested the presence of aseismic slip locally. To detail the interplay between fast and slow slip along the CF, we build three Interferometric Synthetic‐Aperture Radar time series of ground deformation covering the whole fault length over 5 years (2014–2019). We find that most of the active fault trace slips aseismically and continuously. From south to north, we identify three creeping fault portions: the Nushki, Central, and Qalat segments of lengths between 80 and 130 km. The loading rate is 1.2 ± 0.3 cm/yr for the two southernmost portions, while it is about 0.7 ± 0.2 cm/yr for the Qalat segment. The Central segment and the nearby locked segments have hosted the largest known historical earthquakes on the CF, and three moderate magnitude earthquakes in our observation period. We image these earthquakes for which modeled slip at depth (Mw5–5.6), time series of surface slip and deformation patterns argue toward large triggered aseismic slip. The June 2018 event displays postseismic moment 3–15 times greater than coseismic moment. Over the two decades covered by geodetic observations, continuous or triggered aseismic slip dominates along most of the fault and co‐locates with earthquakes. We observe that fault geometrical complexities delimit active segments and may be responsible for the kilometer‐scale intertwining between seismic and aseismic events.

The 2019–2020 Southwest Puerto Rico Earthquake Sequence: Seismicity and Faulting

Abstract The 2019–2020 Southwest Puerto Rico earthquake sequence ruptured multiple faults with several moderate magnitude earthquakes. Here, we investigate the seismotectonics of this fault system using high-precision hypocenter relocation and inversion of the near-field strong motions of the five largest events in the sequence (5.6≤Mw≤6.4) for kinematic rupture models. The Mw 6.4 mainshock occurred on a northeast-striking, southeast-dipping normal fault. The rupture nucleated offshore ∼15 km southeast of Indios at the depth of 8.6 km and extended southwest–northeast and up-dip with an average speed of 1.55 km/s, reaching the seafloor and shoreline after about 8 s. The 6 January 2020 (10:32:23) Mw 5.7 and the 7 January 2020 (11:18:46) Mw 5.8 events occurred on two east–southeast-striking, near-vertical, left-lateral strike-slip faults. However, the 7 January 2020 (08:34:05) Mw 5.6 normal-faulting aftershock, which occurred only 10 min after the Mw 6.4 normal-faulting mainshock, ruptured on a fault with almost the same strike as the mainshock but situated ∼8 km farther east, forming a set of parallel faults in the fault system. On 11 January 2020, an Mw 6.0 earthquake occurred on a north–northeast-striking, westing-dipping fault, orthogonal to the faults hosting the strike-slip earthquakes. We apply template matching for the detection of missed, small-magnitude earthquakes to study the spatial evolution of the main part of the sequence. Using the template-matching results along with Global Positioning System analysis, we image the temporal evolution of a foreshock sequence (Caja swarm). We propose that the swarm and the main sequence were a response to a tectonic transient that most affected the whole Puerto Rico Island.

Analysis of M 5.3 Sumbawa, Indonesia earthquake 2020 and its aftershocks based on hypocenter relocation from BMKG seismic stations

The 2020 Sumbawa earthquake of moderate magnitude (M 5.3) produced very significant aftershocks. Based on the computation of Utsu’s method, those aftershocks would be ended after the 20th day. Those earthquakes along 20 days were relocated using double-difference method. The relocation results show the southwest-northeast orientation and getting deeper into the northwest direction. Those two directions show the strike and the dip from the fault plane of the earthquake which was consistent with the focal mechanism released by the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG). Those results showed the majority of earthquakes occurred at a depth of shallower than 20 km. Those earthquake depths were fit with the previous study showing the crustal thickness beneath Sumbawa Island that was about 28 km. We also found that those earthquakes occurred at splay faults propagating to decollement structure. This study is beneficial for earthquake disaster mitigation especially in updating active faults on Sumbawa Island.

Earthquake vulnerability in the Himalaya by integrated multi-criteria decision models

Himalayan mountains are one of the most seismo-tectonically active zones on the surface of the earth. Recurring moderate and high magnitude earthquakes are not uncommon in this region. This paper maps the earthquake vulnerability in the region using integrated multi-criteria decision models. Factors which may influence vulnerability in a region can be categorized in social, geotechnical, structural, and physical parameters. We have used the analytical hierarchy process (AHP) approach to determine the weights of various parameters, which were further used to develop earthquake vulnerability maps for the study area using the VIseKriterijumska Optimizacija I Kompromisno Resenje (VIKOR), and Grey Relational Analysis (GRA) methods. There is good correlation between the vulnerability estimated through AHP-VIKOR and AHP-GRA methods. Our analysis indicates that more than 12% area may be under high to very-high vulnerability, whereas more than 44% population, and about 43% buildings are highly vulnerable to hazards due to earthquakes. The results would be useful for various hazard mitigation and infrastructure planning agencies working in the region.

The 2019 September 24, Mw = 6, Mirpur earthquake, NW Himalaya: Geodetic evidence for shallow, near-horizontal décollement rupture of the Main Himalayan Thrust

We present a source model for the 2019 Mw = 6 Mirpur earthquake, NW Himalaya using Interferometric Synthetic Aperture Radar (InSAR) measurements. Bayesian inversion of InSAR data from both ascending and descending orbits suggests that the earthquake ruptured a shallow (Depth ~ 5 km), near-horizontal (Dip ~2.5°) up-dip portion of the décollement of the Main Himalayan Thrust (MHT). The distributed slip model suggests a compact rupture terminating the up-dip end at the base of the Main Frontal Thrust (MFT) with higher slip (> 0.4 m) around the hypocentre, equivalent to a moment magnitude of Mw = 6. A shallow up-dip rupture of the MHT through a moderate magnitude earthquake is atypical as Himalayan earthquakes generally originate at the down-dip portion of the MHT and propagate towards south. We estimate a low effective coefficient of friction of 0.06 ± 0.02 from the slip model and suggest that high pore fluid pressures and/or a weak, lubricated portion of décollement could have caused a local, near-horizontal rupture at the base of the MFT. The 2019 Mirpur earthquake released a small fraction of the accumulated strain energy since the 1555 Kashmir earthquake. Coseismic Coulomb stress change analysis suggests a significant increase in stress on the locked, up-dip portion of the MHT and the frontal fold-thrust system. These findings compel a revisit of the seismic hazard assessment of the northwestern Himalaya.

Intraplate earthquakes in the Potiguar Basin, Brazil: Evidence for superposition of local and regional stresses and implications for moderate-size earthquake occurrence

The understanding of earthquake occurrence in intraplate areas has been one of the most challenging tasks in Seismology. The attenuation of seismic waves in these areas is lower compared to border plate regions. Therefore, even moderate magnitude earthquakes can represent a great hazard. In this case, assessing the stress field in the intraplate areas is important to better understanding the earthquake generating mechanisms. We analyzed 241 earthquakes of the 2017 seismic sequence in the NW part of the Potiguar Basin and estimated the stress regime. Seismically defined faults were determined for four high-precision relocated NW-SE- and NE-SW-trending clusters. Faulting intersection acting in one of these clusters was representative for suggesting an intersection model for intraplate earthquakes to explain why moderate size earthquakes occur in the study region. Another cluster presented a seismically defined fault vertically segmented with a clear spatiotemporal evolution and occurrence of almost entire activity in a very short period, which suggest energy build-up and rapid release between its fault segments. We calculated the composite focal mechanism for three clusters, with predominant strike-slip faulting with a sinistral movement. The inversion of focal mechanisms reinforces evidence for a superposition of local and regional stresses, with major compressional axis subparallel to the continental equatorial margin.

Identification of failure mechanisms in existing unreinforced masonry buildings in rural areas after April 4, 2019 earthquake in Turkey

Many unreinforced masonry buildings are completely destroyed after destructive earthquakes but, their failure mechanisms cannot be precisely determined. More specifically damage to such buildings is evinced during low or moderate magnitude earthquakes. The aim of this study is to identify the failure mechanisms of existing unreinforced masonry buildings and present a reconstruction strategy or offer strengthening procedures, in order to inhibit damage to masonry structures after destructive earthquakes. For this purpose, a moderate magnitude earthquake which occurred on April 4, 2019 near the township of Sivrice in Elazığ city, in the east of Turkey, was investigated. The magnitude of the studied seismic motion was expressed as M w = 5.2 by the Ministry of Interior Disaster and Emergency Management Presidency. Although this seismic effect was a moderate magnitude earthquake, significant failures occurred. Many masonry buildings and adobe dwellings in the villages of Sivrice were severely damaged by the earthquake. For field observations, ten villages located around the epicentre were visited 65 dwellings were observed and the damage patterns were assessed in detail. The observations showed that the main reasons for the damage to the dwellings were weak structural detailing of wall-to-wall and wall-to-roof connections, inhesion of earthen roofs, and a lack of bond beams in the structural walls. According to field observations, it is deduced that the performance of the undamaged masonry dwellings after earthquakes does not prove that they have adequate seismic safety. Therefore, reinforcing methods (shotcrete implementation, decreasing of space, corner reinforcing etc.) and innovative strengthening procedures (fibre reinforced polymer) are recommended for the undamaged existing unreinforced buildings at the end of the paper. • Damages to unreinforced existing masonry buildings were investigated during the April 4, 2019 Elazığ-Sivrice earthquake. • Reinforcing and repairing procedures for existing buildings were suggested. • Lessons learned from this earthquake were explained.

Crustal Configuration and Seismic Stability of the Eastern Indian Shield and Adjoining Regions: Insights for Incidents of Great Earthquakes in the Nepal-Bihar-Sikkim Himalaya

The Eastern Indian Shield (EIS) is comprised of the intracratonic (coal-bearing) Damodar Gondwana basin, rift-controlled extensional Lower Gangetic basin (LGB), and the downward flexed Indo-Gangetic basin (IGB). The present study involves the computations and mapping of the basement configuration, sediment thickness, Moho depth, and the residual isostatic gravity anomaly, based on 2-D gravity modeling. The sediment thickness in the area ranges between 0.0 and 6.5 km, and the Conrad discontinuity occurs at ∼17.0–20 km depth. The depth of the Moho varies between 36.0 and 41.5 km, with the maximum value beneath the Upper Gangetic basin (UGB), and the minimum of ∼36 km (uplifted Moho) in the southeastern part beneath the LGB. The maximum residual isostatic anomaly of +44 mGal in the southern part indicates the Singhbhum shear zone, LGB, and Rajmahal trap to be under-compensated, whereas the northern part recording the minimum residual isostatic anomaly of –87.0 mGal is over-compensated. Although the region experienced a few moderate-magnitude earthquakes in the past, small-magnitude earthquakes are sparsely distributed. The basement reactivation was possibly associated with a few events of magnitudes more than 4.0. Toward the south, in the Bay of Bengal (BOB), seismic activities of moderate size and shallow origin are confined between the aseismic 85 and 90°E ridges. The regions on the extreme north and south [along the Himalaya and the equatorial Indian Ocean (EIO)] are experienced moderate-to-great earthquakes over different times in the historical past, but the intervening EIS and the BOB have seismic stability. We propose that the two aseismic ridges are guiding the lithospheric stress fields, which are being further focused by the basement of the EIS, the BOB, and the N-S extended regional fault systems into the bending zone of the penetrating Indian lithosphere beneath the Himalaya. The minimum obliquity of the Indian plate and the transecting fault systems in the Foothills of the Himalaya channelize and enhance the stress field into the bending zone. The enhanced stress generates great earthquakes in the Nepal-Bihar-Sikkim Himalaya, and on being reflected back through the apparently stable EIS and BOB, the stress field creates deformation and great earthquakes in the EIO.

Characteristics of strong ground motion attenuation in the 2019 Mw 5.8 Changning earthquake, Sichuan, China

A moderate magnitude earthquake with Mw 5.8 occurred on June 17, 2019, in Changning County, Sichuan Province, China, causing 13 deaths, 226 injuries, and serious engineering damage. This earthquake induced heavier damage than earthquakes of similar magnitude. To explain this phenomenon in terms of ground motion characteristics, based on 58 sets of strong ground motions in this earthquake, the peak ground acceleration (PGA), peak ground velocity (PGV), acceleration response spectra (Sa), duration, and Arias intensity are analyzed. The results show that the PGA, PGV, and Sa are larger than the predicted values from some global ground motion models. The between-event residuals reveal that the source effects on the intermediate-period and long-period ground motions are stronger than those on short-period ground motions. Comparison of Arias intensity attenuation with the global models indicates that the energy of ground motions of the Changning earthquake is larger than those of earthquakes with the same magnitude.

The Traverse Ridge Paleoseismic Site and Ruptures Crossing the Boundary Between the Provo and Salt Lake City Segments of the Wasatch Fault Zone, Utah, United States

How structural segment boundaries modulate earthquake behavior is an important scientific and societal question, especially for the Wasatch fault zone (WFZ) where urban areas lie along multiple fault segments. The extent to which segment boundaries arrest ruptures, host moderate magnitude earthquakes, or transmit ruptures to adjacent fault segments is critical for understanding seismic hazard. To help address this outstanding issue, we conducted a paleoseismic investigation at the Traverse Ridge paleoseismic site (TR site) along the ∼7-km-long Fort Canyon segment boundary, which links the Provo (59 km) and Salt Lake City (40 km) segments of the WFZ. At the TR site, we logged two trenches which were cut across sub-parallel traces of the fault, separated by ∼175 m. Evidence from these exposures leads us to infer that at least 3 to 4 earthquakes have ruptured across the segment boundary in the Holocene. Radiocarbon dating of soil material developed below and above fault scarp colluvial packages and within a filled fissure constrains the age of the events. The most recent event ruptured the southern fault trace between 0.2 and 0.4 ka, the penultimate event ruptured the northern fault trace between 0.6 and 3.4 ka, and two prior events occurred between 1.4 and 6.2 ka (on the southern fault trace) and 7.2 and 8.1 ka (northern fault trace). Colluvial wedge heights of these events ranged from 0.7 to 1.2 m, indicating the segment boundary experiences surface ruptures with more than 1 m of vertical displacement. Given these estimates, we infer that these events were greater than Mw 6.7, with rupture extending across the entire segment boundary and portions of one or both adjacent fault segments. The Holocene recurrence of events at the TR site is lower than the closest paleoseismic sites at the adjacent fault segment endpoints. The contrasts in recurrence rates observed within 15 km of the Fort Canyon fault segment boundary may be explained conceptually by a leaky segment boundary model which permits spillover events, ruptures centered on the segment boundary, and segmented ruptures. The TR site demonstrates the utility of paleoseismology within segment boundaries which, through corroboration of displacement data, can demonstrate rupture connectivity between fault segments and test the validity of rupture models.

Crustal Deformation and Fault Strength of the Sulawesi Subduction Zone

AbstractThis paper investigates the seismicity and rheology of the North‐Sulawesi subduction zone. Body‐wave modeling is used to estimate focal mechanisms and centroid depths of moderate magnitude (M5–M6.5) earthquakes on the North Sulawesi megathrust and surrounding region. The slip vectors of megathrust earthquakes radiate outward from Sulawesi, indicating motion that is incompatible with the relative motion of two rigid plates. Instead, the observed deformation implies lateral spreading of high topography, controlled by gravitational potential energy contrasts. This finding suggests that the observed deformation of Sulawesi results from stresses transmitted through the lithosphere, rather than basal tractions due to circulation in the mantle. Our modeling of the force balance on the megathrust shows that the subduction megathrust is weak, with an average shear stress of ∼13 MPa and an effective coefficient of friction of 0.03. Elsewhere in Sulawesi, slip vectors of other earthquakes suggest similar potential‐energy‐driven deformation is present, but at significantly slower rates. Our results show the importance of lateral rheology contrasts in determining deformation rate, and hence seismic hazard, in response to a given driving force.

Strong-Motions From Damaging Moderate Magnitude (5.9 ≥ Mw) Earthquakes in Japan Recorded by K-NET and KiK-net

Strong-motions from 79 moderate magnitude (5.9 ≥ Mw) earthquakes that caused various degrees of impact on humans and built-environment in Japan between 1996 and 2019, after the start of K-NET and KiK-net, are presented. As such, most of the earthquakes occurred beneath the land, and agencies had reported damages from events as small as Mw 4.1. Together, large peak ground accelerations exceeding 500 cm/s2were recorded during 15 earthquakes, reaching approximately 1128 cm/s2during one event. Similarly, large peak ground velocities exceeding 30 cm/s were recorded during ten earthquakes, reaching about 76 cm/s to the maximum. Most of the large values aforementioned were recorded within a hypocentral distance of approximately 30 km and on soil site conditions. Intermediate to long-period ground motions are of growing concerns in urban areas located on sedimentary basins with mid-rise and high-rise buildings. The threshold magnitude for the large ground motions at the periods of about 2–5 s is not well understood. In March 2013, the Japan Meteorological Agency introduced four levels of long-period ground motion intensity (LPGMI) based on absolute velocity response spectra (AVRS) between 1.6 and 7.8 s. In the present data set, LPGMI of level 3 (AVRS 50–100 cm/s) and level 4 (AVRS >100 cm/s) were observed each at a single site from shallow-focus earthquakes of Mw 5.8 and 5.9 at distances of approximately 12 and 15 km, respectively. However, the peak response periods were relatively short (1.6–1.8 s). The data showed that LPGM from moderate earthquakes is of lower concern regarding earthquake early warning at distances beyond 200 km. The 2018 Mw 5.5 North Osaka earthquake, despite the moderate magnitude, caused the loss of six human lives and brought significant damage to buildings and lifelines. Comparing the data with the ground motion prediction equations (GMPEs) and other events suggested that the North Osaka earthquake was probably a higher stress drop event. These moderate earthquakes’ observations hinted that the commonly used GMPEs in Japan may not sufficiently grasp these earthquakes' hazards.

A Stochastic View of the 2020 Elazığ Mw 6.8 Earthquake (Turkey)

AbstractUntil the Mw 6.8 Elazığ earthquake ruptured the central portion of the East Anatolian Fault (EAF, Turkey) on January 24, 2020, the region had only experienced moderate magnitude (Mw < 6.2) earthquakes over the last century. We use geodetic data to constrain a model of subsurface fault slip. We adopt an unregularized Bayesian sampling approach relying solely on physically justifiable prior information and account for uncertainties in both the assumed elastic structure and fault geometry. The rupture of the Elazığ earthquake was mostly unilateral, with two primary disconnected regions of slip. This rupture pattern may be controlled by structural complexity. Both the Elazığ and 2010 Mw 6.1 Kovancılar events ruptured portions of the central EAF that are believed to be coupled during interseismic periods, and the Palu segment is the last portion of the EAF showing a large fault slip deficit which has not yet ruptured in the last 145 years.

Source and Attenuation Properties of the 2012 Moe, Southeastern Australia, Earthquake Sequence

AbstractThe 19 June 2012 local magnitude ML 5.4 (Mw 5.1) Moe earthquake, which occurred within the Australian stable continental region, was the largest seismic event for the state of Victoria, for more than 30 yr. Seismic networks in the southeast Australian region yielded many high-quality recordings of the moderate-magnitude earthquake mainshock and its largest aftershock (ML 4.4 and Mw 4.3) at a range of distances from the epicenter. The source and attenuation characteristics of the earthquake sequence are analyzed. Almost 15,000 felt reports were received following the mainshock, and its ground motions tripped a number of coal-fired power generators in the region amounting to the loss of, approximately, 1955 MW of generation capacity. The attenuation of macroseismic intensities is shown to mimic the attenuation shape of eastern North America (ENA) models but requires an interevent bias to reduce predicted intensities. Furthermore, instrumental ground-motion recordings are compared to ground-motion models (GMMs) considered applicable for the southeastern Australian (SEA) region. Some GMMs developed for ENA and SEA provide reasonable estimates of the recorded ground motions of spectral acceleration within epicentral distances of, approximately, 100 km. The mean Next Generation Attenuation-East GMM, recently developed for stable ENA, performs relatively poorly for the 2012 Moe earthquake sequence, particularly, for short-period accelerations. These observations will help inform future seismic hazard assessments for eastern Australia.

Long-period Strong Ground Motions in the Tokyo Bay Area Observed during Moderate-magnitude Earthquakes that Occurred in the Nankai Trough

Long-period Strong Ground Motions in the Tokyo Bay Area Observed during Moderate-magnitude Earthquakes that Occurred in the Nankai Trough

GWFM: A Global Catalog of Moderate-Magnitude Earthquakes Studied Using Teleseismic Body Waves

AbstractWe present a compilation of 2131 high-fidelity mechanisms and centroid depths of moderate-magnitude earthquakes derived using synthetic body-waveform modeling (the Global Waveform-Modelled Earthquake Catalog v1.0—gWFM), which can be visualized and downloaded online (see Data and Resources). In this article, we describe the methods used to construct the gWFM and present a comparison between the earthquake depths and focal mechanisms in the gWFM with those derived by the International Seismological Centre, Global Centroid Moment Tensor (Global CMT) project, and the U.S. Geological Survey (USGS) W-phase, as well as 60 events studied using geodesy. We find that 20%–30% of the earthquakes in routine global catalogs have depths that differ by more than 10 km from those in the gWFM. Shallow-crustal earthquakes of Mw 5–6 are typically the worst located in depth by routine catalogs. Over 90% of the earthquakes in the gWFM are within ±15° in strike, ±5° in dip, and ±15° in rake of the Global CMT and USGS W-phase best double-couple moment tensor solutions. However, the mechanisms of shallow Mw 5–6 earthquakes in the routine catalogs can be inaccurate, due to the well-known insensitivity of long-period surface waves to the vertical dip-slip components of the moment tensor. The gWFM is an archive of well-constrained earthquake source parameters, though it will continue to update as new earthquake mechanisms and depths are published, thereby remaining an up-to-date research tool.

Evolution of the multifractal parameters along different steps of a seismic activity. The example of Canterbury 2000–2018 (New Zealand)

The multifractal detrended fluctuation algorithm is applied to a series of distances and elapsed times between consecutive earthquakes recorded along the years 2000–2018 in the Canterbury region (New Zealand). The time evolution of several multifractal parameters (Hurst exponent, Hölder central and maximum exponents, spectral amplitude, asymmetry, and complexity index) is analyzed. Peaks of multifractal parameters, with statistical significance exceeding 95%, are associated with three earthquakes of notable magnitude (equaling or exceeding Mw = 5.7). Additionally, some other peaks are also associated with the swarm of earthquakes of moderate magnitude. Possible shortcomings created by this assignment to mainshocks or swarms can be removed by comparing the results corresponding to elapsed times and interevent distances between consecutive events. Additionally, the Buishand test, which is used to verify the statistical significance of the detected peaks, also discriminates between mainshocks of notable seismic magnitude and swarms of earthquakes with moderate magnitude. The obtained results, based on the multifractal structure of the seismic activity, could also represent some advances in predicting, with sufficient time, forthcoming mainshocks of high magnitude and mitigate their destructive effects.

Testing Ground-Motion Prediction Equations against Moderate Magnitude Earthquake Data Recorded in Korea

ABSTRACT The selection of ground-motion prediction equations (GMPEs) to perform seismic hazard assessments is challenging for stable continental regions that lack a sufficient number of recordings. In this study, we implement various ranking methods to test the efficiencies of a wide range of GMPEs against the recordings from three of the largest magnitude inland earthquakes that occurred in the Korean Peninsula, which belongs to an intraplate region with low seismicity. In this context, we select a total of 14 GMPEs developed for active shallow crustal zones (Next Generation Attenuation-West2 [NGA-West2] project), stable continental regions, and Korea. Three statistical approaches, including the classical residual, log-likelihood (LLH), and Euclidean distance-based ranking (EDR) methods, are used to evaluate the performance of the GMPEs. The residual analyses show that for the very short spectral period (T≤0.1 s), regionally developed GMPEs perform the best, whereas the NGA-West2 GMPEs outperform other equations for short (0.2≤T≤0.5 s) and medium to long periods (T≥0.75 s). The LLH approach is shown to favor a stable continental region GMPE that has the highest standard deviation. The EDR method, which can account for both aleatory uncertainties and model bias, is found to favor the NGA-West2 and Korean GMPEs. NGA-West2 GMPEs show the lowest model bias, whereas the Korean GMPEs exhibit the lowest residual bias. Five GMPEs selected based on the EDR method are recommended for a probabilistic seismic hazard analysis in Korea. For deterministic hazard assessment, using the Korean GMPEs for the very short spectral period and NGA-West2 GMPEs for short and medium to long periods is recommended. Overall, the stable continental region GMPEs are demonstrated to perform poorly when tested against the earthquakes recorded in Korea.

Slip Partitioning in the 2016 Alboran Sea Earthquake Sequence (Western Mediterranean)

A MW=5.1 earthquake on January 21st 2016 marked the beginning of a significant seismic sequence in the southern Alboran Sea, culminating in a MW=6.3 earthquake on January 25th, and continuing with further moderate magnitude earthquakes until March. We use data from 35 seismic broadband stations in Spain, Morocco and Portugal to relocate the seismicity, estimate seismic moment tensors, and isolate regional apparent source time functions for the main earthquake. Relocation and regional moment tensor inversion consistently yield very shallow depths for the majority of events. We obtain 50 moment tensors for the sequence, showing a mixture of strike-slip faulting for the foreshock and the main event and reverse faulting for the major aftershocks. The leading role of reverse focal mechanisms among the aftershocks may be explained by the geometry of the fault network. The mainshock nucleates at a bend along the left-lateral Al-Idrisi fault, introducing local transpression within the transtensional Alboran Basin. The shallow depths of the 2016 Alboran Sea earthquakes may favour slip-partitioning on the involved faults. Apparent source durations for the main event suggest a ~21 km long, asymmetric rupture that propagates primarily towards NE into the restraining fault segment, with fast rupture speed of ~3.0 km/s. Consistently, the inversion for laterally variable fault displacement situates the main slip in the restraining segment. The partitioning into strike-slip rupture and dip-slip aftershocks confirms a non-optimal orientation of this segment, and suggests that the 2016 event settled a slip deficit from previous ruptures that could not propagate into the stronger restraining segment.