Buried Strike Slip Faults: The 1994 and 2004 Al Hoceima, Morocco Earthquakes.

Transcription

1 Buried Strike Slip Faults: The 1994 and 2004 Al Hoceima, Morocco Earthquakes. Juliet Biggs 1, Eric Bergman 2, Brian Emmerson 3, Gareth Funning 4, James Jackson 3, Barry Parsons 1,Tim Wright 1. 1 COMET, Dept. of Earth Sciences. University of Oxford. 2 CIEI, Dept, of Physics, University of Colorado, Boulder 3 COMET, Dept. of Earth Sciences.University of Cambridge 4 Berkeley Seismological Laboratory, University of California, Berkeley

2 Strike-Slip Slip Earthquakes: SURFACE RUPTURE BURIED (blind) Surface Rupture Denali Earthquake, Alaska Bam Earthquake, Iran 7m offset at surface 20 cm offset at surface 2 m at depth

3 Examples: Bam, Iran. (Funning et al, 2005, etc) Aiquile,, Bolivia. (Funning et al, 2005a ) Nenana Mountain, Alaska. (Wright et al, 2003) Al Hoceima,, Morocco, (this study) Al Hoceima,, Morocco, (this study) Difficult to identify from surface observations. MAJOR UNIDENTIFIED SEISMIC HAZARD.

4 Seismology

5 Auxilliary Plane Tension Compression Compression Tension Fault Plane

6 Geodesy

7 30 km Fault NW-SE Fault NE-SW Fault Descending Ascending

8 10 km Fault ~ M6 NW-SE Fault NE-SW Fault Descending Ascending

9 1 km Fault ~ Point Source NW-SE Fault NE-SW Fault Descending Ascending

10 2004 Earthquake

11 Al Hoceima Region Algeria

12 2004 Al Hoceima Earthquake M w th February ~600 dead. ~40,000 homeless.

13 2004 Al Hoceima : Surface Observations Institut Cartografic de Catalunya Rapport : GS-192/04 (fr)

14 2004 Al Hociema: Damage Map 20 km Area of soft sediments Ait Brahim et al, 2004

15 Preliminary Aftershock Locations

16 B I L L d M A 2 d Y AK d H I A d A R U d W M Q d A BK T d H Y B * d A T D d F U R I d K M B O d L S Z d T S U M d L V C d N N A d S D V d S JG d T E I G d H KT d T U C d R S S D d F F C d F R B d R E S d A LE d Nodal Planes: ~25 o ~295 o

17 2004 Coseismic Interferogram cm Ascending Track cm First Image: 10 th December 2003 Second Image: 7 th July 2004

18 Descending Interferogram cm cm First Image: 13 th April 2003 Second Image: 6 th June 2004

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20 Interferograms Ascending Descending NW-SE Model NW-SE Residual Fault Length: 8.8 km Slip: 1.4 m Root Mean Square Misfit: 21 mm

21 Interferograms Ascending Descending NE-SW Model Fault Length: 1.1 km Slip: 12.9 m NE-SW Residual Root Mean Square Misfit: 23 mm

22 Slip to Length Ratios NE-SW Model NW-SE Model From Funning (2005).

23 Synthetic Experiment

24 Is this a general result or specific to Al Hoceima? Inversion assuming fault plane is fault plane Invent input earthquake parameters Produce Interferograms Compare output results to input Test effect of: fault orientation, atmospheric noise, coherence pattern. Inversion assuming auxiliary plane is fault plane

25 Ascending Descending Fault Length: 10 km Slip: 1 m Root Mean Square Misfit: 0.2 mm NW-SE Residual NW-SE Model Interferograms

26 Interferograms Ascending Descending NE-SW Model Fault Length: 2.0 km!! Slip: 9.1 m!! NE-SW Residual Root Mean Square Misfit: 6 mm

27 Slip to Length Ratios NE-SW Model Synthetic Parameters NW-SE Model

28 1994 Earthquake

29 1994 Al Hoceima Earthquake M w th May dead. Significant Damage

30 Preliminary Observations: Surface Observations

31 Preliminary Observations: Intensity Map

32 Location from Seismology Harvard CMT NEIC El Alami 1998 Calvert 1997

33 1994 Coseismic Interferogram.

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36 Earthquake Sequence

37 Combining Seismology + InSAR to study earthquake sequences Seismology Detect Small Events Poor absolute location (but good relative location) InSAR Accurate absolute locations Events with significant surface deformation (i.e. shallow, M5-6+)

38 Hypocentroidal Decomposition Method (based on Jordan + Sverdrup, 1981) Step 1: Relative locations of events within cluster using seismology. Step 2: Absolute location of entire cluster use InSAR as ground truth. Relocate 58 Events: 10 associated with 1994 event 19 associated with 2004 event 2004 Earthquake 1994 Earthquake

39 Aftershocks not consistent with NW-SE fault plane for 1994 event. => Pair of conjugate faults.

40 Conjugate Fault System

41 Al Hoceima Earthquake Sequence:

42 Regional Context Strike Slip Thrust Faulting Tectonics of transition zone between Al Hoceima and El Asnam remains unclear. May only become clear once further larger earthquakes have occurred.

43 Conclusions Al Hoceima Earthquakes occurred on a conjugate fault system 2004 Al Hoceima Earthquake occurred on a NW-SE right-lateral fault Al Hoceima Earthquake occurred on a NE-SW left-lateral lateral fault. Significant seismic hazard associated with the region between Al Hoceima and El Asnam.

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47 Variable Slip Model 0 Slip (m) Depth Peak Slip = 2.1 m At 8 km depth. 18 km 0 24 km Distance along strike

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49 Errors on Slip Model. 0 Error (m) Depth 18 km 0 24 km Distance along strike

50 Monte Carlo Error Analysis Tradeoffs Create 100 perturbed datasets and analyse Identify tradeoffs and errors Strike Dip 180 Rake Slip Length 2 MinDepth

51 Strike Dip 180 Rake Strike ± 1.1 Dip 87.4 ± 1.5 Rake ± 1.2 Slip 1.4 m ± 0.1 Slip Length Length Min Depth 8.8 km ± km ± MinDepth

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53 Resolution Inversion Depends on geometry between observation point and fault patch and smoothing factor.

54 Resolution

55 Recent Large Earthquakes Strike-slip Thrust