Focal Mechanism Of Mainshock

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02 Nov 2017

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Introduction

The MS 7.0 Lushan earthquake occurred on 20 April 2013 at 08:02 Beijing time (00:02 UTC) in the morning of the day. Destruction was huge.Casualties numbered more than 200 people,more than ten thousand peoples were injured, and there were major economic losses. This event was one of the deadliest earthquakes in China during recent years, after the Wenchuan earthquake in 2008 and the Yushu earthquake in 2010.

The shock occurred in the Longmenshan fault systems(LFS), about 80km from 2008 Wenchuan earthquake epicenter(Fig.1). Longmenshan located near the plate eastern margin, was one of the most active fault. There are about ?? earthauke with magnitude larger than 6 occurred there since 19??. The LFS is consisted of four major faults, i.e the Maoxian fault(back fault), the Beichuan-Yingxiu fault(central), the Jiangyou-Guanxian fault(front fault) and the Dayi fault, which is considered to be a hidden fault. It was reported that the former three faults all ruptured during the 2008 Wenchuan earthquake. Also some evidence from geology indicate the Dayi fault was once active recently. Location results from China Earthquake Net Center(CENC) show that the mainshock and its aftershock just locate in the median region between Jiangyou-Guanxian fault and Dayi fault. So it is difficult to tell which fault is true fault plane.

In this paper, we report the preliminary results including relocation of the sequences and focal mechanisms of the mainshock and some bigger event. These results may be beneficial for understanding the process of the earthquake sequences.

Focal mechanism of Mainshock

The double-couple solution of the mainshock was determined using the "Cut-and-Paste(CAP)" technique (Zhao and Helmberger 1994; Zhu and Helmberger 1996). The main idea of this method is to divide the entire seismograms into two segments,i.e. Pnl wave and surface wave. Nearly all the stations within 250km from the epicenter are clipped because of the large surface wave amplitude. For surface wave records, only those stations with epicentral distance over 250km are used. We also analyzed the Pnl waveforms of regional seismograms from 32 stations with epicentral distances between 70 and 300 km and maximum azimuth gap of 32° (Figure.1).

Synthetic Green’s functions, derived from the three fundamental faults (Jost and Hermann, 1989), are used as the basis functions.These fundamental fault synthetics are combined with two 1D velocity structures typical to Longmenshan region(Zheng et al, 2009) and Sichuan basin (Wan et al, 2010) to form a library of Green’s functions that are used to match the observed waveforms. These velocity structures achieve good performance in the focal mechanism inversion of Wenchuan earthquake (Zheng et al., 2009)and Suining earthquake(Luo et al., 2011) which occur in the Sichuan basin.

The Green’s functions were generated from the frequency-wavenumber(F-K )technique (Zhu and Rivera 2002) basing on these two 1-D local crustal velocity models.

A focal depth of 15 km and Mw 6.7 correspond to minimum error between the recorded and synthetic waveforms (Figure 3).

The high cross-correlation coefficients (Figure 3B) confirm the similarity of the synthetics with the data, both for the Pnl waves and the surface waves. Because the "cut and paste" allows time shifts between portions of seismograms and synthetics, the results of this method are relatively insensitive to minor changes in the velocity model and to possible lateral crustal variations (Zhu and Helmberger 1996)

To avoid the waveform complexities that are not easily explained at some stations,

we simply discard them and concentrate on the high quality fits with cross-correlation values higher than xx.

The strike, dip, and rake of the focal mechanism are 28°,43°,92° respectively, corresponds to its auxiliary plain

It is generally believed that regional seismograms do not provide constrains on the dip angle of the focal mechanisms as good as the far field data. Here we also invert focal mechanism of the mainshock using the far field body wave records(Fig 4). The dip angle from far field body wave inversion was nearly the same to that from regional waveforms. This maybe imply the robustness of our results.

Both inversions obtained a focal depth of 15km

The errors in the strike, dip, and rake of the focal mechanism are about ±10°, ±10°, and ±10°, respectively.

Focal mechanism of aftershock

The same approach was adopted to determine focal mechanisms for the large events in this sequences using regional seismograms. There are 3 events with MS≥ 5.0 (Fig. 5). The focal mechanisms of these large earthquakes in the sequence are similar to the mainshock. The focal mechanisms of most of the events predominantly exhibit thrust motion on northeast- or northwest-striking nodal planes. Overall, the focal mechanisms uniformly suggest the presence of a coherent regional stress field.

Relocations

Event locations in the SSN catalog were routinely determined from picks of P- and S- wave arrival times. The relocation method used is hypo71 or hypo2000 with a 1-D velocity model comprising two flat uniform layers. Typical error in location is in the order of a few kilometers to a couple tens of kilometers. It is thus necessary to relocate the sequences for further analysis. Here we collected phases data of regional seismic network for the sequences during April20 to April23. The magnitude range from 2.5 to 7.0. In addition some portable stations were deployed after April22. The phase data of these portable stations were also collected and combined with phase data of permanent stations to relocate the earthquake sequences.

To relocate earthquakes with an improved precision, we first employed the hybrid velocity models used to invert focal mechanisms and adopted hypo2000 to re-determine their absolute location. The sequences after relocation with hypo2000 show nearly north-east striking of 30°.Depths of the sequences are very diverse, ranging from 2km to 30km(Fig. 5), nearly involves the whole crust. As aresult, the reliability of the results was decreased. It is hard to find obvious dip trending from the depth section.

We further used the double-difference (DD) method [Waldhauser and Ellsworth, 2000] to better obtain the locations of the entire sequences. The method used here for relocating earthquake hypocenters is a double-difference algorithm known as hypoDD (Waldhauser and Ellsworth, 2000), which solves for event hypocenters and origin times using differential travel-time data from catalog phase picks and/or waveform cross correlations. It does so by minimizing the residuals between the observed and theoretical travel times (double differences) for pairs of earthquakes by adjusting the vector difference between the hypocenters. The relocated hypocenters are determined by solving the double-difference equation for all hypocentral pairs at all stations. A benefit of this method is that it considers the difference between closely space events and therefore reduces the error from unmodeled velocity variations between the station and the event pair.

Relocating events with hypoDD results in precise relative locations and reduces error between events. This may illuminate faults if most of the events are located on the same structure.

Results from both synthetic-data and real-data studies show that relative errors are reduced by a factor of ~2 using catalog arrival times alone and are reduced by a further factor fof ~2 using catalog arrival times alone and are reduced by a further factor of ~5-10 if cross-correlation data are also used(Waldhauser and Ellsworth, 2000; Hauksson and Shearer, 2005).Cross-correlation information is incorporated into hypoDD by using the precise differential arrival times weighted according to the cross-correlation coefficient (squared coherency to include negative correlations). Cross-correlation coefficient (ccc) between all waveform pairs on the same station were then determined using a 2.5s window beginning 0.5 s before the P arrival with the maximum time shift 0.5s. The time shift corresponding to the maximum ccc is considered to be the differential arrival times between two events.Those events having seismogram pairs with ccc's greater than 0.70 were selected.

We relocated the mainshock and aftershocks, using the 1-D velocity model and HypoDD (Waldhauser and Ellsworth 2000). The relocation focal depth of the mainshock is about 14km, which is consistent with that from waveform modeling. The relocations show similar string to either mainshock nodal plane (Figure 4). The two depth cross sections reveal the details of the depth distribution of the earthquakes in the swarm.The tight distribution of earthquakes in the A–A’ cross section shows that probably the bulk of the swarm events occurred on only one vertical fault. The seismicity distribution in the B–B0 cross section suggests the presence of two clusters along this southwest-striking plane, separated by less than 2 km.

In addition, the aftershocks occurred in depth less than 22 km. Overall, the events located in the southern part are shallow than other parts about ~5-10km. No events with depth larger than 13km were found here. It seems that there is a "transition zone" in the midway between A and A’ , where the earthquakes own more deep source. The seismicity distribution in the B–B0 cross section suggests the presence of two clusters along this southwest-striking plane, separated by less than 2 km.

It is thus not easy to determine from the spatial aftershock patterns which of the two steeply dipping nodal planes ruptured in the mainshock. Further, the mainshock is located at a depth of 15 km, midway between the Jiangyou-Guanxian fault and Dayi fault, and because of its large focal depth, projecting the nodal planes up to dipping surface faults is difficult at best

The two depth cross sections show the details of the depth distribution of the earthquakes in the swarm.The tight distribution of earthquakes in the A–A’ cross section shows that probably the bulk of the swarm events occurred on only one vertical fault. The seismicity distribution in the B–B0 cross section suggests the

presence of two clusters along this southwest-striking plane, separated by less than 2 km.

Most of the slip is distributed in the depth range from 3 to 6 km and extends approximately 6 km along strike, with a maximum slip amplitude of approximately 0.4 m. The slip model reveals clear unilateral rupture directivity and two asperities,

with the first one above the hypocenter and the second approximately 3 km to the southwest along the strike

Station map.

Velocity model.(two models)

Tele Cap

Local cap.

This well known fault plane - auxiliary

plane ambiguity cannot be resolved from the seismic

radiation of a point source. Hence studies of locations of

aftershocks, surface faulting, rupture directivity, or static

final displacements (Backus, 1977a) need to be done in

order to resolve this ambiguity.

Discussion and Conclusion

讨论和结论:

主震机制解

大的余震震源机制解,震源深度

余震深度分布,平面分布

Relocation

Relocation section

Some big events wavefom fit

table.

153

14.570

2013-04-21

17:05:22.74

30.33

103.01

009

5.4

038

39

79

5.1

012

13.159

232

8.190

2013-04-20

11:34:15.52

30.19

102.89

010

5.3

049

41

108

5.1

013

6.117

288

15.020

2013-04-20

08:02:47.18

30.30

102.97

012

7.0

028

43

92

6.7

015

13.674



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