Determination Of Local Magnitude Research Articles

Local magnitude determination for Turkey from two independent seismic networks and a consistent M4 + catalogue

SUMMARY Turkish seismicity is routinely recorded by two broad-band seismic networks: the Turkish National Seismic Network (AFAD ‘TU’ network) and the Bogazici University Kandilli Observatory And Earthquake Research Institute (KOERI ‘KO’ network). Each of them has over 200 seismic stations distributed across the country. We obtain a new uniform and consistent local magnitude (Ml) scale for Turkey using data from both networks, as well as the M4 + catalogue with Ml values that results from applying said scale to the events that occurred between 2007 and 2016 within the country. We measure the half peak-to-peak amplitude of the vertical component seismograms and estimate the attenuation parameters through an iterative procedure that fits the logarithm of the amplitude as a function of the hypocentral distance, the local magnitude, the station corrections and a constant accounting for possible systematic differences between amplitudes measured on horizontal seismographs and those measured on vertical seismographs. We additionally scale Ml to the moment magnitude measured by an independent source, obtaining a magnitude relationship accounting for attenuation adjustments, as well as differences between maximum horizontal and vertical amplitudes. The estimated Ml formula shows that station corrections from both networks regionally agree, and that non-zero corrections are centred on zero, within −0.48 and 0.46 magnitude units (corresponding to the 2.5 and the 97.5 quantiles). The new M4 + catalogue with consistent Ml values for AFAD and KOERI shows magnitude residuals that cannot be explained by a Gaussian distribution. We also show a very good agreement between our new Ml value and the original Ml values of AFAD and KOERI, with median differences below 0.1 magnitude units.

Determination of Local Magnitude for Earthquakes Recorded from the Texas Seismological Network (TexNet)

AbstractIn this study, we present a local magnitude (ML) relation for the earthquakes recorded from the Texas Seismological Network (TexNet) between the dates of 1 January 2017 and 31 July 2019. Using a comprehensive seismic dataset from earthquakes in Texas, we propose a distance correction term −logA0, which is consistent with the original definition of the Richter magnitude. The proposed distance correction calculation for the TexNet events accounts for the attenuation characteristics of the direct and refracted waves over different distance ranges. Regression analysis of Wood–Anderson amplitudes results in the following trilinear function, which represents the attenuation attributes of the events under investigation: −logA0={2.07×log(Rhyp)+0.0002×(Rhyp−100)−0.72Rhyp≤16 km1.54×log(Rhyp)+0.0002×(Rhyp−100)−0.0816 km<Rhyp≤105 km,0.29×log(Rhyp)+0.0002×(Rhyp−100)+2.45Rhyp>105 km in which Rhyp is the hypocentral distance (km). The derived distance correction relationship results in an accurate ML relationship for Texas that is unbiased over a 200 km distance range. Compared with other ML relations, the proposed relation in this study gives lower ML values over all distances than those calculated by Richter (1958), Hutton and Boore (1987), Babaie Mahani and Kao (2019), and Quinones et al. (2019) by an average of 0.01, 0.12, 0.16, and 0.15 units, respectively; this study’s proposed relation gives higher ML values over all distances than those calculated by Scales et al. (2017), Yenier (2017), and Greig et al. (2018) by an average of 0.28, 0.01, and 0.08 units, respectively.

Accurate Determination of Local Magnitude for Earthquakes in the Western Canada Sedimentary Basin

Accurate Determination of Local Magnitude for Earthquakes in the Western Canada Sedimentary Basin

Site amplifications and the effect on local magnitude determination at stations of the surface–downhole network in Taiwan

This study analyzed site effects including PGA and empirical transfer functions at 15 selected surface–downhole stations by using ground motion recordings of earthquakes with ML > 4 during 2012 and 2013 in Taiwan. In addition, we used all strong motion recordings of four large earthquakes (ML > 6) to generate intensity distribution maps and assess the differences in magnitude at the surface and downhole. The site amplification factors of the PGA were calculated using the ratio between the surface and downhole recordings. The mean PGA amplification factors ranged from 2 to an exaggerated value of 20 at different stations. In addition, the power law relationships between the PGAs at the surface and downhole were evaluated to understand how amplification varies as PGA increases. Strong ground motions with and without site effects throughout Taiwan could be observed by comparing intensity distribution maps generated using the surface and downhole accelerations from four large earthquakes with magnitudes > 6. Empirical transfer functions derived using the single-station and two-station methods at the same stations showed comparable dominant frequencies and amplification factors; however, the empirical transfer function derived using the two-station method showed clearer resonance peaks, not only at fundamental frequencies but also at higher mode resonance frequencies. The HHSR and the HVSR were highly similar, particularly at medium frequencies. This finding indicates that the HVSR can be used instead of the HHSR when only the surface recording is available. Moreover, the local magnitudes calculated using surface recordings were higher than those calculated using downhole recordings. The differences are attributed to the amplification caused by the sedimentary layers and resultant in 0.36, 0.46 and 0.49 on average for events with ML of > 6, 5–6, and 4–5. Furthermore, HHSRs at 5–10Hz and 1.1–1.7Hz were strongly correlated with PGA amplifications and ML differences, respectively.

The Considerations on Local Magnitude Determination to the Lwiro Seismographical Station

The Considerations on Local Magnitude Determination to the Lwiro Seismographical Station

California Integrated Seismic Network (CISN) Local Magnitude Determination in California and Vicinity

Determining local magnitude (M_L) in a manner that is uniform and internally consistent for earthquakes throughout California and the vicinity is an important component of the California Integrated Seismic Network (CISN). We present a new local magnitude attenuation function and corresponding station adjustments that are valid throughout California. The new attenuation function is an analytic function of the radial hypocentral distance between 1 and 500 km. Associated station adjustments are also available for 1185 horizontal seismometer and accelerometer channels from five seismic networks operating in California. The new attenuation function and adjustments provide several advantages to CISN. They allow a more robust M_L computation, the M_Ls are more consistent between northern and southern California than they have been in the past, and because adjustments are now available for more station-network-channel-location codes (SNCLs), M_Ls can be computed for small earthquakes in more locations than was previously possible. In addition to describing our method for calibrating the new CISN M_L, we also present a tool for adding adjustments for new or upgraded stations.

Research on local magnitude (M L ) scale in and near the Korean Peninsula

After synthesizing Wood-Anderson seismograms from broadband recordings at FDSN stations (BJT, SSE, INC and MDJ) in and near Korea, the empirical equations, for the vertical and horizontal components, respectively, for determination of local magnitude (M L ) in and near Korea, were estimated through a process of regression. Around 200 data, from events with epicentral distance ?D) ranging from 50 km to 1000 km, measured from synthetic Wood-Anderson seismograms were used. According to the regression with the constraint that the magnitude for an amplitude of 0.001 m measured at epicentral distance of 100 km is 3, the empirical formula (log 10 A 0 ) for the horizontal components is, with standard deviation (s) of 0.52, M L = log 10 A(D) + 1.71 log 10 A 0 (D) – 0.42 + C, and that for the vertical components is, with standard deviation (s) of 0.56, M L = log 10 A(D) +1.70 log 10 A 0 (D) – 0.4 + C, where, C is a station correction factor and A is the amplitude. This result shows that the attenuation in and near Korea is stronger than that in the East United States (Kim, 1998) and weaker than that in South California (Kanamori et al., 1993).

New observations of local seismicity by the SN-1 seafloor observatory in the Ionian Sea, off-shore Eastern Sicily (Italy)

SUMMARY Seismicity in Eastern Sicily as recorded by the Submarine Network-1 seafloor observatory (SN1) in the period from 2002 October to 2003 May is examined with the aim of identifying the as yet poorly known seismogenic zones placed in the Ionian basin, where some of the strongest earthquakes have occurred. A comparison between the seismic recordings of land networks and the seafloor station has allowed us to focus on low-magnitude seismicity only recorded by SN-1. We have analysed 239 high-quality events from among a total of 485 seismic signals not included in the land-based network bulletins. The waveform features and the possible source zones for those events are investigated by means of polarization and particle motion techniques. Most of the 239 events (213) are characterized by high values of rectilinearity typical of P- and S-arrival particle motions, while the remaining 26 events have different polarization features, with an emergent first phase and prevalently planar polarization. We have interpreted the latter signals as being associated to submarine landslides. From particle motion analysis, we have determined the azimuthal distribution of the events and the incidence angles of P waves in respect to the Observatory with the aim of determining their distribution in relation to the active but scarcely known structural setting of the off-shore area. Moreover, the integrated locations of some earthquakes occurring in the study area and recorded by SN-1 and land stations was performed to determine the apparent P-wave velocity necessary to calculate source-station distances. As an additional result of the integration, we have obtained more accurate locations of earthquakes occurring in the coastal and off-shore areas of Eastern Sicily, associated with reduced horizontal and vertical errors and significantly lower values of azimuthal gaps. Lastly, a location distribution of the 213 analysed events was obtained by setting two conditions: (1) a maximum epicentral distance to a fixed depth coinciding with the depth of the seafloor station and (2) a minimum epicentral distance associated to the maximum depth of events. Accordingly, two patterns of seismicity were determined for the maximum and the minimum expected spread of the hypocentres. The main features of both patterns are a diffuse seismicity in the Western Ionian basin with a major epicentre density SE of SN-1 and a depth of most of the events within 60 km. Local magnitude determination was also performed, taking into account an attenuation law proposed for Southeastern Sicily. Despite the uncertainties in the location distribution using single-station recordings, the results show diffuse seismicity all around SN-1 and, in particular, in the off-shore area.

Moment magnitude determination of local seismic events recorded at selected Polish seismic stations

The paper presents the method of local magnitude determination used at Polish seismic stations to report events originating in one of the four regions of induced seismicity in Poland or its immediate vicinity. The method is based on recalculation of the seismic moment into magnitude, whereas the seismic moment is obtained from spectral analysis. The method has been introduced at Polish seismic stations in the late 1990s but as of yet had not been described in full because magnitude discrepancies have been found between the results of the individual stations. The authors have performed statistics of these differences, provide their explanation and calculate station corrections for each station and each event source region. The limitations of the method are also discussed. The method is found to be a good and reliable method of local magnitude determination provided the limitations are observed and station correction applied.

Local magnitude revaluation for recent Italian earthquakes (1981-1996)

In the framework of a joint project of the Italian Gruppo Nazionale per la Difesa dai Terremoti (GNDT) and the Istituto Nazionale di Geofisica (ING), devoted to the reorganization of the Italian earthquake instrumental database from 1981 to 1996, we approached the problem of the homogeneous determination of local magnitude in Italy. As a first step, we built a database of real Wood-Anderson amplitudes deduced from the available data of the two instruments that were in operation in Italy up to 1989 as well as of simulated Wood Anderson amplitudes computed by Very Broad Band recordings, available since 1990. The simultaneous computation of the attenuation function together with the magnitudes and the station residuals allows us to verify that, for the Italian area, this function does not significantly differ from the original one given by Richter. In the second step, the so obtained Ml magnitudes are used as a reference data set to estimate a new empirical relation for the duration magnitude Md using the unified phases database of the Italian National Seismic Network (RSNC) of ING and of the most important local seismic networks operating in Italy. The same data set was also employed to calibrate magnitude Ma based on amplitudes coming from short period vertical seismograms recorded by the RSNC automatic acquisition system. At last a set of reasonable criteria to choose the most reliable among Ml, Md and Ma has been formulated. The resulting set of magnitudes proved to be definitely better calibrated than the one obtained by procedures in use at RSNC.

The Taiwan Telemetered Seismographic Network

In 1972, the Taiwan Telemetered Seismographic Network (TTSN) was installed to monitor earthquakes occurring in an area from 21 to 26°N latitude and from 119 to 123°E longitude. The number of stations increased from six in 1972 to 24 in 1987. The stations are equipped with velocity-type seismometers with a natural frequency of 1 Hz. Since 1983, horizontal-component sensors have been added at several stations. Signals recorded by the field stations are transmitted to Taipei Recording Center through telephone lines or radio, and then recorded on FM magnetic tapes, paper, and nine-track magnetic tapes through A/D converters. Hypocenters of earthquakes are routinely determined by a HYPOC2 computer program. The size of an earthquake is estimated from the signal duration. Moreover, a simulated Wood-Anderson seismographic station is in operation at Taipei for the determination of local magnitude. A seismic work system (SWS) is being established for automatic data acquisition and data processing. To avoid clipping of signal amplitude, an Optimum Telemetry System is being linked to the TTSN.

震源近傍の加速度記録を用いた<i>M<sub>L</sub></i>の決定

震源近傍の加速度記録を用いた<i>M<sub>L</sub></i>の決定

Determination of local magnitude, ML, from strong-motion accelerograms

A technique is presented for determination of local magnitude, M_L, from strong-motion accelerograms. The accelerograph records are used as an acceleration input to the equation of motion of the Wood-Anderson torsion seismograph to produce a synthetic seismogram which is then read in the standard manner. When applied to 14 records from the San Fernando earthquake, the resulting M_L is 6.35, with a standard deviation of 0.26. This is in good agreement with the previously reported value of 6.3. The technique is also applied to other earthquakes in the western United States for which strong-motion records are available. An average value of M_L = 7.2 is obtained for the 1952 Kern County earthquake; this number is significantly smaller than the commonly used value of 7.7, which is more nearly a surface-wave magnitude. The method presented broadens the base from which M_L can be found and allows M_L to be determined in large earthquakes for which no standard assessment of local magnitude is possible. In addition, in instances where a large number of accelerograms are available, reliable values of M_L can be determined by averaging.