Variations in Apparent Stress and b Value Preceding the 2010 Mw8.8 Bio-Bío, Chile Earthquake

We calculated apparent stresses for 70 earthquakes (MW ≥ 5.0) occurring in the aftershock region of the 2010 MW8.8 Bio-Bío earthquake from January 1990 to September 2019. We identified that the average apparent stress was approximately 0.487 MPa between January 1990 and December 2005 and approximately 1.063 MPa within the period from January 2006 to January 2010. The latter one is 2.2-fold greater than the former, representing a significant difference as determined by a z test, with a 99% confidence level. Moreover, we analyzed the temporal evolution of the apparent stress and found that apparent stress rapidly increased from 0.43 to 1.2 MPa during the pre-event period from March 2006 to the occurrence of the Bio-Bío MW8.8 mainshock, and this increased apparent stress was found to be significant at the 98% confidence level. Furthermore, we calculated the spatial distribution of the apparent stress in the study region and observed two higher-apparent-stress regions, within one of which the epicenter of the MW8.8 event was located. On the basis of the inverse correlation between b value and stress, the temporal evolution and spatial distribution of b values were calculated and compared with those of the apparent stress. The comparison showed that the b values decreased approximately 4 years before the occurrence of the mainshock, while the apparent stress increased substantially; for the region of lower b, the apparent stress is higher, and vice versa. Therefore, the inverse correlation between b value and stress is supported by the results obtained in the present study and can be probably considered as one of the precursors to great earthquakes.

SOURCE PARAMETERS of STRONG EARTHQUAKES of the EARTH

For nine strongest earthquakes in Russia and the World, dynamic parameters were determined. They were calculated from the P-wave spectra recorded by IRIS-IDA digital equipmentat Obninsk (OBN), Talaya (TLY) and Arti (ARU) stations at the epicentral distances =30–80°.The following parameters are given: seismic moment, rupture length, stress drop and apparent stress, average displacement during rupture of earthquake. The moment magnitude Mw obtained from seismic moment M0 at Obninsk, Talaya, and Arti stations was calculate by the formula of H. Kanamori.A comparison of the obtained parameters M0 and Mw with the data of the GCMT international center showed their proximity.

Spatio-temporal variations of source parameters in the nucleation zone of the 6 April 2009, Mw 6.1 L'Aquila Earthquake

<p>We present the results of Brune stress drop (∆σ) and apparent stress (τa) variability of  earthquakes located in a small zone adjacent to the hypocenter of the damaging Mw 6.1 L'Aquila earthquake. Their magnitude ranges between  2.7 and 4.1. Interevent variability of stress drop and apparent stress results in a factor of 10, well beyond the individual‐event uncertainty. Radiation efficiency ηsw = τa/∆σ varies mostly between 0.1 and 0.2, but decreases in the days immediately before and after the main shock to values as low as 0.06. This may be related to the migration of the events occurring in those days into a focal volume with higher dynamic strength. The temporal change of ηsw might be interpreted as a spatial variation due to the earthquake migration into the locked portion of the fault originating the main shock. Furthermore, no variation in stress drop and apparent stress can be observed between foreshocks and aftershocks but the smallest and largest ∆σ result in a good correlation with the largest and smallest b‐values respectively, as already documented in literature in the rupture nucleation volume of large earthquakes.</p>

The Coda Calibration and Processing Tool: Java-Based Freeware for the Geophysical Community

<p>The coda magnitude method of <em>Mayeda and Walter</em> (1996) provides stable source spectra and moment magnitudes (<em>M</em><em><sub>w</sub></em>) for local to regional events from as few as one station that are virtually insensitive to source and path heterogeneity. The method allows for a consistent measure of <em>M</em><em><sub>w</sub></em> over a broad range of event sizes rather than relying on empirical magnitude relationships that attempt to tie various narrowband relative magnitudes (<em>e.g.,</em> <em>M</em><em><sub>L</sub>, M<sub>D</sub>, m<sub>b</sub></em>, etc.) to absolute <em>M</em><em><sub>w </sub></em>derived from long-period waveform modeling. The use of <em>S</em>-coda and <em>P</em>-coda envelopes has been well documented over the past several decades for stable source spectra, apparent stress scaling, and hazard studies. However, up until recently, the method requires extensive calibration effort and routine operational use was limited only to proprietary US NDC software. The Coda Calibration Tool (CCT) stems from a multi-year collaboration between the US NDC and LLNL scientists with the goal of developing a fast and easy Java-based, platform independent coda envelope calibration and processing tool. We present an overview of the tool and advantages of the method along with several calibration examples, all of which are freely available to the public via GitHub (https://github.com/LLNL/coda-calibration-tool). Once a region is calibrated, the tool can then be used in routine processing to obtain stable source spectra and associated source information (<em>e.g.</em>, <em>M</em><em><sub>w</sub></em>, radiated seismic energy, apparent stress, corner frequency, source discrimination on event type and/or depth). As more events are recorded or new stations added, simple updates to the calibration can be performed. All calibration and measurement information (<em>e.g.,</em> site and path correction terms, raw & measured amplitudes, errors, etc.) is stored within an internal database that can be queried for future use. We welcome future collaboration, testing and suggestions by the geophysical community.  </p>

DOUBLET of DOMBAI EARTHQUAKES 2013 in FOCAL ZONE of CHKHALTA EARTHQUAKE 1963

Two Dombai earthquakes were recorded on March 26, 2013, at 23h35m with КР=11.9 and on May 28, 2013, at 00h09m with КР=11.9 (Mw GCMT=4.9 and 5.2) in the focal area of two strong (I0=IX and I0=VII at MSK 64) Chkhalta of 1963 and Teberda of 1905 earthquakes. They were accompanied by aftershock pro-cesses. The estimates of dynamic parameters of foci (for March 26, 2013, then May 28, 2013): the rupture length is L=3.4, 3.8 km, the stress drop =55, 79 Pa, the apparent stress drop=1, 3 Pa, and the average displacement along the rupture u=0.25, 0.33 m, were obtained from the spectra of S-waves at “Anapa” and “Kislovodsk” stations. Earthquakes occurred under the action of near-horizontal (PL=12°, PL=18°) com-pressive stresses directed north-northeast (AZM=21°, AZM=30°). The nodal planes of both mechanisms have a similar strike. The type of movement in both foci is a reverse fault with some strike-slip components. The stretches of nodal planes and the types of movements in the foci are in good agreement with the geodynamic setting of the axial structures of the Greater Caucasus, and also are similar to the mechanism of the destructive Chkhalta earthquake of 1963.

Detecting long-lasting transients of earthquake activity on a fault system by monitoring apparent stress, ground motion and clustering

Damaging earthquakes result from the evolution of stress in the brittle upper-crust, but the understanding of the mechanics of faulting cannot be achieved by only studying the large ones, which are rare. Considering a fault as a complex system, microearthquakes allow to set a benchmark in the system evolution. Here, we investigate the possibility to detect when a fault system starts deviating from a predefined benchmark behavior by monitoring the temporal and spatial variability of different micro-and-small magnitude earthquakes properties. We follow the temporal evolution of the apparent stress and of the event-specific residuals of ground shaking. Temporal and spatial clustering properties of microearthquakes are monitored as well. We focus on a fault system located in Southern Italy, where the Mw 6.9 Irpinia earthquake occurred in 1980. Following the temporal evolution of earthquakes parameters and their time-space distribution, we can identify two long-lasting phases in the seismicity patterns that are likely related to high pressure fluids in the shallow crust, which were otherwise impossible to decipher. Monitoring temporal and spatial variability of micro-to-small earthquakes source parameters at near fault observatories can have high potential as tool for providing us with new understanding of how the machine generating large earthquakes works.