Source Mechanism of the 2006 M_W5.1 Wen'an Earthquake Determined from a Joint Inversion of Local and Teleseismic Broadband Waveform Data

Abstract

On July 4th, 2006, a magnitude 5.1 earthquake occurred at Wen'an, ~100 km south of Beijing, which was felt at Beijing metropolitan area. To better understand the regional tectonics, we have inverted local and teleseismic broadband waveform data to determine the focal mechanism of this earthquake. We selected waveform data of 9 stations from the recently installed Beijing metropolitan digital Seismic Network (BSN). These stations are located within 600 km and cover a good azimuthal range to the earthquake. To better fit the lower amplitude P waveform, we employed two different weights for the P wave and surface wave arrivals, respectively. A grid search method was employed to find the strike, dip and slip of the earthquake that best fits the P and surface waveforms recorded at all the three components (the tangential component of the P‐wave arrivals was not used). Synthetic waveforms were computed with an F‐K method. Two crustal velocity models were used in the synthetic calculation to reflect a rapid east‐west transition in crustal structure observed by seismic and geological studies in the study area. The 3‐D grid search resulted in reasonable constraints on the fault geometry and the slip vector with a less well determined focal depth. As such we combined teleseismic waveform data from 8 stations of the Global Seismic Network in a joint inversion. Clearly identifiable depth phases (pP, sP) recorded by the teleseismic stations obviously provided a better constraint on the resulting source depth. Results from the joint inversion indicate that the Wen'an earthquake is mainly a right‐lateral strike slip event (–150°) which occurred at a near vertical (dip, ~80°) NNE trending (210°) fault. The estimated focal depth is 14~15 km, and the moment magnitude is 5.1. The estimated fault geometry here agrees well with aftershock distribution and is consistent with the major fault systems in the area which were developed under a NNE‐SSW oriented compressional stress field.

Finite‐Source Waveform Modeling of the 2006 Wen'an MW 4.9 Earthquake

“…Using first-motion polarity data from 45 local stations in the Beijing metropolitan digital Seismic Network (BSN), we determined the focal mechanism of the earthquake. The two conjugate nodal fault planes are (110 • , 65 • , 8 • ) and (16 • , 83 • , 155 • ) for (strike, dip, rake), which are consistent with the Harvard CMT solution and the mechanism determined by Huang et al [2] from a joint inversion of local and teleseismic waveform data. However, it has been difficult to further identify the actual rupture plane due to the lack of observational evidence in several aspects.…”

“…Although instruments are buried in boreholes, the thick sediments of Haihe Plain still brought on notable resonance at those sites. (2) The inclusion of the third patch does not change the fine features of first arrival produced by the simpler twopatch model, which we have used to determine the rupture plane, therefore does not contradict the knowledge of actual rupture plane and rupture direction inferred from the analysis of 2P models.…”

An attempt to detect temporal variations of crustal structure in the source area of the 2006 Wen-An earthquake in North China

“…No vertical exaggeration except for the topography here and in Figs. 5-16 and 19. to present have been performed (Zhang et al, 2007;Wang et al, 2009;Huang et al, 2009;Zheng et al, 2009), but the cause of the Wen-An earthquake is still not understood very well.…”

A study on the uncertainties of the centroid depth of the 2013 Lushan earthquake from teleseimic body wave data

“…In the local CAP method, centroid depth, fault plane solutions (strike/dip/rake) are inverted with grid-searching algorithm, thus capable of finding the global minimum. Therefore, the local CAP method has been broadly applied in source parameter inversions for earthquakes in US, China, Iran, and other regions (Huang et al 2009;Wei et al 2009;Tan et al 2010). The similar idea in local CAP method was applied for teleseismic body inversion to account for usually much stronger SH waves as compared to P waves, and the method is called as tCAP method (Wei 2009).…”