Microearthquake analysis at local seismic networks in Iceland and Sweden and earthquake precursors

AbstractDuring continental rifting, strain and magmatism are believed to localize to narrow magmatic segments, while the rift margin is progressively abandoned. We integrate volcanological, geochemical, petrological and seismic data from the Ma’Alalta volcanic field (MVF) near the western margin of Afar, to show that the MVF is an active magmatic segment. Magmatism in MVF initiated with lava flows and large-volume, caldera-forming ignimbrites from a central edifice. However, the most recent magmatic activity shifted towards mafic lava fields, cinder cones and obsidian-rich silicic domes erupted from vents aligned NNW-SSE, defining a ~ 35-km-long magmatic segment. Along the same area, a NNW-SSE alignment of earthquakes was recorded by two local seismic networks (2005–2009 and 2011–2013). The geochemistry of the mafic rocks is similar to those of nearby axial volcanoes. Inferred magma storage depth from mineral geobarometry shows that a shallow, silicic chamber existed at ~ 5-km depth below the stratovolcano, while a stacked plumbing system with at least three magma storage levels between 9 and 24 km depth fed the recent basalts. We interpret the wide set of observations from the MVF as evidence that the area is an active magmatic segment, showing that localised axial extension can be heavily offset towards the rift margin.

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Indicators for site characterization at seismic station: recommendation from a dedicated survey

Bulletin of Earthquake Engineering ◽

10.1007/s10518-021-01136-7 ◽

2021 ◽

Author(s):

Giovanna Cultrera ◽

Cécile Cornou ◽

Giuseppe Di Giulio ◽

Pierre-Yves Bard

Keyword(s):

Seismic Station ◽

International Workshop ◽

Site Characterization ◽

Companion Paper ◽

Background Information ◽

Online Questionnaire ◽

Seismic Records ◽

Seismic Networks ◽

High Level ◽

Data Acquisition And Processing

AbstractIn recent years, the permanent seismic networks worldwide have largely increased, raising the amount of earthquake signals and the applications using seismic records. Although characterization of the soil properties at recording stations has a large impact on hazard estimates, it has not been implemented so far in a standardized way for reaching high-level metadata. To address this issue, we built an online questionnaire for the identification of the indicators useful for a reliable site characterization at a seismic station. We analysed the answers of a large number of experts in different fields, which allowed us to rank 24 different indicators and to identify the most relevant ones: fundamental frequency (f0), shear-wave velocity profile (VS), time-averaged Vs over 30 m (VS30), depth of seismological and engineering bedrock (Hseis_bed and Heng_bed), surface geology and soil class. Moreover, the questionnaire proposed two additional indices in terms of cost and difficulty to obtain a reliable value of each indicator, showing that the selection of the most relevant indicators results from a complex balance between physical relevancy, average cost and reliability. For each indicator we propose a summary report, provided as editable pdf, containing the background information of data acquisition and processing details, with the aim to homogenize site metadata information at European level and to define the quality of the site characterization (see companion paper Di Giulio et al. 2021). The selected indicators and the summary reports have been shared within European and worldwide scientific community and discussed in a dedicated international workshop. They represent a first attempt to reach a homogeneous set of high-level metadata for site characterization.

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Alarm systems based on a pair of short-term earthquake precursors

Bulletin of the Seismological Society of America ◽

10.1785/bssa0780041538 ◽

1988 ◽

Vol 78 (4) ◽

pp. 1538-1549

Author(s):

Giuseppe Grandori ◽

Elisa Guagenti ◽

Federico Perotti

Keyword(s):

Statistical Analysis ◽

Active Region ◽

Southern California ◽

Main Shock ◽

Weak Shock ◽

Earthquake Precursors ◽

Alarm System ◽

Short Term ◽

Alarm Systems

Abstract A statistical analysis of the foreshock-main shock correlation for a seismically active region in Italy is presented. It is found that the probability that a weak shock be followed within 2 days by a main shock is of the order of 2 per cent, while the probability that a main shock be preceded by a foreshock is of the order of 50 per cent. These results are quite similar to those found by L. Jones (1985) for southern California. The effectiveness of alarm systems based on a pair of short-term earthquake precursors is then analyzed. In particular, the analysis shows under what conditions the precursor, consisting of potential foreshocks, could be combined with another precursor to provide a reasonably effective alarm system.

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The Extended Integrated Particle Filter Method (IPFx) as a High-Performance Earthquake Early Warning System

Bulletin of the Seismological Society of America ◽

10.1785/0120210008 ◽

2021 ◽

Author(s):

Masumi Yamada ◽

Koji Tamaribuchi ◽

Stephen Wu

Keyword(s):

Particle Filter ◽

Early Warning ◽

Seismic Intensity ◽

Earthquake Early Warning ◽

Japan Meteorological Agency ◽

Estimation Accuracy ◽

Filter Method ◽

Source Estimation ◽

Single Station ◽

Seismic Networks

ABSTRACT An earthquake early warning (EEW) system rapidly analyzes seismic data to report the occurrence of an earthquake before strong shaking is felt at a site. In Japan, the integrated particle filter (IPF) method, a new source-estimation algorithm, was recently incorporated into the EEW system to improve the source-estimation accuracy during active seismicity. The problem of the current IPF method is that it uses the trigger information computed at each station in a specific format as the input and is therefore applicable to only limited seismic networks. This study proposes the extended IPF (IPFx) method to deal with continuous waveforms and merge all Japanese real-time seismic networks into a single framework. The new source determination algorithm processes seismic waveforms in two stages. The first stage (single-station processing) extracts trigger and amplitude information from continuous waveforms. The second stage (network processing) accumulates information from multiple stations and estimates the location and magnitude of ongoing earthquakes based on Bayesian inference. In 10 months of continuous online experiments, the IPFx method showed good performance in detecting earthquakes with maximum seismic intensity ≥3 in the Japan Meteorological Agency (JMA) catalog. By merging multiple seismic networks into a single EEW system, the warning time of the current EEW system can be improved further. The IPFx method provides accurate shaking estimation even at the beginning of event detection and achieves seismic intensity error <0.25 s after detecting an event. This method correctly avoided two major false alarms on 5 January 2018 and 30 July 2020. The IPFx method offers the potential of expanding the JMA IPF method to global seismic networks.

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P-wave backazimuth anomalies observed by a small-aperture seismic array at Pinyon Flat, southern California: Implications for structure and source location

Bulletin of the Seismological Society of America ◽

10.1785/bssa0860020470 ◽

1996 ◽

Vol 86 (2) ◽

pp. 470-476 ◽

Cited By ~ 1

Author(s):

Cheng-Horng Lin ◽

S. W. Roecker

Keyword(s):

Southern California ◽

Seismic Array ◽

P Wave ◽

Small Aperture ◽

Data Set ◽

Single Interface ◽

Seismic Networks ◽

Dense Seismic Array ◽

Beamforming Technique

Abstract Seismograms of earthquakes and explosions recorded at local, regional, and teleseismic distances by a small-aperture, dense seismic array located on Pinyon Flat, in southern California, reveal large (±15°) backazimuth anomalies. We investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating backazimuth with an array using three different techniques: the broadband frequency-wavenumber (BBFK) technique, the polarization technique, and the beamforming technique. While each technique provided nearly the same direction as a most likely estimate, the beamforming estimate was associated with the smallest uncertainties. Backazimuth anomalies were then calculated for the entire data set by comparing the results from beamforming with backazimuths derived from earthquake locations reported by the Anza and Caltech seismic networks and the Preliminary Determination of Epicenters (PDE) Bulletin. These backazimuth anomalies have a simple sinelike dependence on azimuth, with the largest anomalies observed from the southeast and northwest directions. Such a trend may be explained as the effect of one or more interfaces dipping to the northeast beneath the array. A best-fit model of a single interface has a dip and strike of 20° and 315°, respectively, and a velocity contrast of 0.82 km/sec. Application of corrections computed from this simple model to ray directions significantly improves locations at all distances and directions, suggesting that this is an upper crustal feature. We confirm that knowledge of local structure can be very important for earthquake location by an array but also show that corrections computed from simple models may not only be adequate but superior to those determined by raytracing through smoothed laterally varying models.

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Rapid earthquake source characterization in the context of tsunami early warning

10.5194/egusphere-egu21-13218 ◽

2021 ◽

Author(s):

Alberto Armigliato ◽

Martina Zanetti ◽

Stefano Tinti ◽

Filippo Zaniboni ◽

Glauco Gallotti ◽

...

Keyword(s):

Focal Mechanism ◽

Early Warning ◽

Fault Plane ◽

Early Warning Systems ◽

Slip Distribution ◽

Generation Process ◽

Tsunami Early Warning ◽

Earthquake Focal Mechanism ◽

Run Up ◽

Seismic Networks

It is well known that for earthquake-generated tsunamis impacting near-field coastlines the focal mechanism, the position of the fault with respect to the coastline and the on fault slip distribution are key factors in determining the efficiency of the generation process and the distribution of the maximum run-up and inundation along the nearby coasts. The time needed to obtain the aforementioned information from the analysis of seismic records is usually too long compared to the time required to issue a timely tsunami warning/alert to the nearest coastlines. In the context of tsunami early warning systems, a big challenge is hence to be able to define 1) the relative position of the hypocenter and of the fault and 2) the earthquake focal mechanism, based only on the preliminary earthquake localization and magnitude estimation, which are made available by seismic networks soon after the earthquake occurs.In this study, the intrinsic unpredictability of the position of the hypocenter on the fault plane is studied through a probabilistic approach based on the analysis of two finite fault model datasets (SRCMOD and USGS) and by limiting the analysis to moderate-to-large shallow earthquakes (Mw &#160;6 and depth &#160;50 km). After a proper homogenization procedure needed to define a common geometry for all samples in the two datasets, the hypocentral positions are fitted with different probability density functions (PDFs) separately in the along-dip and along-strike directions.Regarding the focal mechanism determination, different approaches have been tested: the most successful is restricted to subduction-type earthquakes. It defines average values and uncertainties for strike, dip and rake angles based on a combination of a proper zonation of the main tsunamigenic subduction areas worldwide and of subduction zone geometries available from publicdatabases.The general workflow that we propose can be schematically outlined as follows. Once an earthquake occurs and the magnitude and hypocentral solutions are made available by seismic networks, it is possible to assign the focal mechanism by selecting the characteristic values for strike, dip and rake of the zone where the hypocenter falls into. Fault length and width, as well as the slip distribution on the fault plane, are computed through regression laws against magnitude proposed by previous studies. The resulting rectangular fault plane can be discretized into a matrix of subfaults: the position of the center of each subfault can be considered as a &#8220;realization&#8221; of the hypocenter position, which can then be assigned a probability. In this way, we can define a number of earthquake fault scenarios, each of which is assigned a probability, and we can run tsunami numerical simulations for each scenario to quantify the classical observables, such as water elevation time series in selected offshore/coastal tide-gauges, flow depth, run-up, inundation distance. The final results can be provided as probabilistic distributions of the different observables.The general approach, which is still in a proof-of-concept stage, is applied to the 16 September 2015 Illapel (Chile) tsunamigenic earthquake (Mw = 8.2). The comparison with the available tsunami observations is discussed with special attention devoted to the early-warning perspective.

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