Moment magnitudes of two large Turkish earthquakes on February 6,2023 from long-period coda

Two large earthquakes(an earthquake doublet)occurred in south-central Turkey on February 6,2023,causing massive damages and casualties.The magnitudes and the relative sizes of the two mainshocks are essential information for scientific research and public awareness.There are obvious discrepancies among the results that have been reported so far,which may be revised and updated later.Here we applied a novel and reliable long-period coda moment magnitude method to the two large earthquakes.The moment magnitudes(with one standard error)are 7.95±0.013 and 7.86±0.012,respectively,which are larger than all the previous reports.The first mainshock,which matches the largest recorded earthquakes in the Turkish history,is slightly larger than the second one by 0.11±0.035 in magnitude or by 0.04 to 0.18 at 95%confidence level.

Relationships between moment magnitude and fault parameters:theoretical and semi-empirical relationships

Fault parameters are important in earthquake hazard analysis.In this paper,theoretical relationships between moment magnitude and fault parameters including subsurface rupture length,downdip rupture width,rupture area,and average slip over the fault surface are deduced based on seismological theory.These theoretical relationships are further simplified by applying similarity conditions and an unique form is established.Then,combining the simplified theoretical relationships between moment magnitude and fault parameters with seismic source data selected in this study,a practical semi-empirical relationship is established.The seismic source data selected is also to used to derive empirical relationships between moment magnitude and fault parameters by the ordinary least square regression method.Comparisons between semi-empirical relationships and empirical relationships show that the former depict distribution trends of data better than the latter.It is also observed that downdip rupture widths of strike slip faults are saturated when moment magnitude is more than 7.0,but downdip rupture widths of dip slip faults are not saturated in the molnent magnitude rangcs of this study.

Scaling relations of moment magnitude, local magnitude,and duration magnitude for earthquakes originated in northeast India

In this study, we aim to improve the scaling between the moment magnitude （Mw）, local magnitude （ME）, and the duration magnitude （MD） for 162 earthquakes in Shillong-Mikir plateau and its adjoining region of northeast India by extending the Mw estimates to lower magnitude earthquakes using spectral analysis of P-waves from vertical component seismograms. The Mw-ML and Mw-MD relationships are determined by linear regression analysis. It is found that, Mw values can be considered consistent with ME and My, within 0.1 and 0.2 magnitude units respectively, in 90 % of the cases. The scaling relationships investigated comply well with similar relationships in other regions in the world and in other seismogenic areas in the northeast India region.

The dependence of response spectrum on the tectonic ambient shear stress field

It has been analyzed the influence of the tectonic ambient shear stress value on response spectrum based on the previous theory. Based on the prediction equation BJF94 presented by the famous American researchers, CLB20, a new prediction formula is proposed by us, where it is introduced the influence of tectonic ambient shear stress value on response spectrum. BJF94 is the prediction equation, which mainly depends on strong ground motion data from western USA, while the prediction equation SEA99 is based on the strong ground motion data from exten-sional region all over the world. Comparing these two prediction equations in detail, it is found that after BJF94′s prediction value lg(Y) minus 0.16 logarithmic units, the value is very close to SEA99′s one. This case demonstrates that lg(Y) in extensional region is smaller; the differences of prediction equation are mainly owe to the differences of tectonic ambient shear stress value. If the factor of tectonic ambient shear stress value is included into the pre-diction equation, and the magnitude is used seismic moment magnitude to express, which is universal used around the world, and the distance is used the distance of fault project, which commonly used by many people, then re-gional differences of prediction equation will become much less, even vanish, and it can be constructed the uni-versal prediction equation proper to all over the world. The error in the earthquake-resistant design in China will be small if we directly use the results of response spectrum of USA (e.g. BJF94 or SEA99).

Towards fast focal mechanism inversion of shallow crustal earthquakes in the Chinese mainland

We have developed an automatic regional focal mechanism inversion system based on the Earthquake Rapid Report(ERR)system and the real-time three-component seismic waveform stream of 1000 broadband seismic stations provided by the China Earthquake Networks Center(CENC).The system can rapidly provide a double couple solution and centroid depth within 5–15 min after receiving earthquake information from the ERR system.The data processing is triggered by earthquake information obtained from the ERR system.The system is capable of determining the focal mechanism of all shallow-depth earthquakes in the Chinese mainland with a magnitude of 5.5–6.5.It utilizes waveform data recorded by seismic stations located within 500 km from the epicenter,enabling the reporting of a focal mechanism solution within 5–15 min of an earthquake occurrence.Additionally,the system can assign a corresponding grade(A B C)to the focal mechanism solution.We processed a total of 301 earthquakes that occurred from 2021 to June 2022,and after the quality control,166 of them were selected.These selected solutions were manually checked,and 160 of them were compiled in a focal mechanism catalog.This catalog can be conveniently downloaded online via the Internet.The automatic focal mechanism solution of earthquakes in eastern China exhibits a good agreement with that provided by the Global Centroid Moment Tensor(GCMT),when available.The average Kagan angle between this catalog and GCMT is 22
,and the average difference in MW is 0.17.Furthermore,compared with GCMT,the minimum magnitude of our catalog has been reduced from approximately 5.0 to 4.0.The correlation between the centroid depth and crustal thickness in the Chinese mainland confirms the distribution of the centroid depth.