scholarly journals An improved double-difference earthquake location algorithm using s P phases: application to the foreshock and aftershock sequences of the 2004 earthquake offshore of the Kii peninsula, Japan (Mw=7.5)

2006 ◽  
Vol 58 (7) ◽  
pp. 823-830 ◽  
Author(s):  
Ling Bai ◽  
Ichiro Kawasaki ◽  
Tianzhong Zhang ◽  
Yuzo Ishikawa
Keyword(s):  
Earthquake Location ◽  
Double Difference ◽  
Kii Peninsula ◽  
Aftershock Sequences ◽  
Location Algorithm

scholarly journals APPLICATION OF 3-D VELOCITY MODELS AND RAY TRACING IN DOUBLE DIFFERENCE EARTHQUAKE LOCATION ALGORITHMS: APPLICATION TO THE MYGDONIA BASIN (NORTHERN GREECE)

2004 ◽  
Vol 36 (3) ◽  
pp. 1396 ◽  
Author(s):  
O. C. Galanis ◽  
C. B. Papazachos ◽  
P. M. Hatzidimitriou ◽  
E. M. Scordilis
Keyword(s):  
Ray Tracing ◽  
Velocity Structure ◽  
Test Site ◽  
Earthquake Location ◽  
Double Difference ◽  
Northern Greece ◽  
Local Networks ◽  
Velocity Models ◽  
Earthquake Locations

In the past years there has been a growing demand for precise earthquake locations for seismotectonic and seismic hazard studies. Recently this has become possible because of the development of sophisticated location algorithms, as well as hardware resources. This is expected to lead to a better insight of seismicity in the near future. A well-known technique, which has been recently used for relocating earthquake data sets is the double difference algorithm. In its original implementation it makes use of a one-dimensional ray tracing routine to calculate seismic wave travel times. We have modified the implementation of the algorithm by incorporating a three-dimensional velocity model and ray tracing in order to relocate a set of earthquakes in the area of the Mygdonia Basin (Northern Greece). This area is covered by a permanent regional network and occasionally by temporary local networks. The velocity structure is very well known, as the Mygdonia Basin has been used as an international test site for seismological studies since 1993, which makes it an appropriate location for evaluating earthquake location algorithms, with the quality of the results depending only on the quality of the data and the algorithm itself. The new earthquake locations reveal details of the area's seismotectonic structure, which are blurred, if not misleading, when resolved by standard (routine) location algorithms.

Complex behaviour highlighted by earthquake aftershock and swarm sequences in Ubaye Region (French Western Alps). 

2021 ◽  
Author(s):  
Marion Baques ◽  
Louis De Barros ◽  
Maxime Godano ◽  
Hervé Jomard ◽  
Clara Duverger ◽  
...  
Keyword(s):  
Fault Plane ◽  
Joint Inversion ◽  
Fluid Pressure ◽  
Western Alps ◽  
Focal Mechanisms ◽  
High Rate ◽  
Double Difference ◽  
Complex Behaviour ◽  
Aftershock Sequences ◽  
Damaged Zone

<p>The Ubaye Region, where the city of Barcelonnette is settled, is the most seismically active region in the French Western Alps since at least two centuries. Seismicity in this area exhibits a dual behaviour, with mainshock-aftershock sequences alternating with abnormally high rate of seismicity associated with seismic swarms. Understanding processes triggering such a peculiar seismic behaviour is of primary importance in order to assess the seismic hazard in this region. The latest swarm activity started on February 26, 2012, with an earthquake of moment magnitude 4.2. It was followed two years later (on April 7, 2014) by a shock of magnitude Mw 4.8. From the first earthquake to the end of 2016, the seismic level has not returned to the background level and shares the same characteristics as a seismic swarm.</p><p>With the aim to discuss the seismogenic processes involved in the area, we focused on the two months following the 2014 mainshock (Mw=4.8). During this period, a dense temporary network (7 stations) was operating at a maximal distance of 10km from the epicentre area. We analysed this period starting with a double-difference relocation of ~ 6,000 earthquakes previously detected by template-matching. These hypocentres did not align on the fault plane of the 2014 mainshock, but on conjugated structures belonging to the 2-km wide damaged zone of the main fault plane and on remote structures with various orientations further away. We then computed 99 focal mechanisms from a joint inversion of P polarity and S/P ratio to clarify the geometry of the active structures. Many nodal planes are inconsistent with the structures deduced from the alignments of the earthquake locations. The stress-state orientation obtained from those focal mechanisms (σ<sub>1</sub> trending N27°± 5°, plunging 50°± 9°, a σ<sub>2</sub> trending N215°± 5°, plunging 40°± 9°, and a sub-horizontal σ<sub>3</sub> trending N122°± 3°) is consistent with those previously calculated in the area (Fojtíková and Vavryčuk, 2018). Nevertheless, some structures are unfavourably oriented to slip within this stress-field, suggesting that additional processes are required to explain them. As the presence of fluids was highlighted for the 2003-2004 and the 2012-2015 crisis, we calculated the fluid pressure needed to trigger slip on the planes from the focal mechanisms using Cauchy's equation. We found that a median fluid-overpressure of ~20 MPa (range between 0 to 50 MPa) is needed to cause slip.  Although the origin of fluids and how they are pressurized at depth remains open. The fluid processes seem to be the most favourable additional processes and were also proposed to explain the 2003-2004 crisis.</p>

scholarly journals Relative earthquake location procedure for clustered seismicity with a single station

2020 ◽  
Author(s):  
Francesco Grigoli ◽  
William Ellsworth ◽  
Miao Zhang ◽  
Mostafa Mousavi ◽  
Simone Cesca ◽  
...  
Keyword(s):  
Signal To Noise Ratio ◽  
Distance Geometry ◽  
Synthetic Data ◽  
Earthquake Location ◽  
Double Difference ◽  
Monitoring Networks ◽  
Seismic Stations ◽  
Single Station ◽  
Active Research ◽  
Distance Geometry Problem

Summary Earthquake location is one of the oldest problems in seismology, yet remains an active research topic. With dense seismic monitoring networks it is possible to obtain reliable locations for microearthquakes; however, in many cases dense networks are lacking, limiting the location accuracy, or preventing location when there are too few observations. For small events in all settings, recording may be sparse and location may be difficult due to low signal-to-noise ratio. In this work we introduce a new, distance-geometry-based method to locate seismicity clusters using only one or two seismic stations. A Distance Geometry Problem determines the location of sets of points based only on the distances between some member pairs. Applied to seismology, our approach allows earthquake location using the inter-event distance between earthquakes pairs, which can be estimated using only one or two seismic stations. We first validate the method with synthetic data that resemble common cluster shapes, and then test the method with two seismic sequences in California: the August 2014 Mw 6.0 Napa earthquake band the July 2019 Mw 6.4 Ridgecrest earthquake sequence. We demonstrate that our approach provides robust and reliable results even for a single station. When using two seismic stations, the results capture the same structures recovered with high resolution Double Difference locations based on multiple stations. The proposed method is particularly useful for poorly monitored areas, where only a limited number of stations are available.

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