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
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
Examples: Bam, Iran. (Funning et al, 2005, etc) Aiquile,, Bolivia. (Funning et al, 2005a ) Nenana Mountain, Alaska. (Wright et al, 2003) Al Hoceima,, Morocco, 1994. (this study) Al Hoceima,, Morocco, 2004. (this study) Difficult to identify from surface observations. MAJOR UNIDENTIFIED SEISMIC HAZARD.
Seismology
Auxilliary Plane Tension Compression Compression Tension Fault Plane
Geodesy
30 km Fault NW-SE Fault NE-SW Fault Descending Ascending
10 km Fault ~ M6 NW-SE Fault NE-SW Fault Descending Ascending
1 km Fault ~ Point Source NW-SE Fault NE-SW Fault Descending Ascending
2004 Earthquake
Al Hoceima Region Algeria http://www.gly.fsu.edu/~salters
2004 Al Hoceima Earthquake M w 6.3. 24 th February 2004. ~600 dead. ~40,000 homeless.
2004 Al Hoceima : Surface Observations Institut Cartografic de Catalunya Rapport : GS-192/04 (fr)
2004 Al Hociema: Damage Map 20 km Area of soft sediments Ait Brahim et al, 2004
Preliminary Aftershock Locations
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
2004 Coseismic Interferogram 1.4 16 cm Ascending Track -1.4-14 cm First Image: 10 th December 2003 Second Image: 7 th July 2004
Descending Interferogram 18 1.4 cm -1.4-10 cm First Image: 13 th April 2003 Second Image: 6 th June 2004
Interferograms Ascending Descending NW-SE Model NW-SE Residual Fault Length: 8.8 km Slip: 1.4 m Root Mean Square Misfit: 21 mm
Interferograms Ascending Descending NE-SW Model Fault Length: 1.1 km Slip: 12.9 m NE-SW Residual Root Mean Square Misfit: 23 mm
Slip to Length Ratios NE-SW Model NW-SE Model From Funning (2005).
Synthetic Experiment
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
Ascending Descending Fault Length: 10 km Slip: 1 m Root Mean Square Misfit: 0.2 mm NW-SE Residual NW-SE Model Interferograms
Interferograms Ascending Descending NE-SW Model Fault Length: 2.0 km!! Slip: 9.1 m!! NE-SW Residual Root Mean Square Misfit: 6 mm
Slip to Length Ratios NE-SW Model Synthetic Parameters NW-SE Model
1994 Earthquake
1994 Al Hoceima Earthquake M w 6.0. 26 th May 1994. 2 dead. Significant Damage
Preliminary Observations: Surface Observations
Preliminary Observations: Intensity Map
Location from Seismology Harvard CMT NEIC El Alami 1998 Calvert 1997
1994 Coseismic Interferogram.
Earthquake Sequence
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+)
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
Aftershocks not consistent with NW-SE fault plane for 1994 event. => Pair of conjugate faults.
Conjugate Fault System
Al Hoceima Earthquake Sequence: 1994-2004
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.
Conclusions Al Hoceima Earthquakes occurred on a conjugate fault system 2004 Al Hoceima Earthquake occurred on a NW-SE right-lateral fault. 1994 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.
Variable Slip Model 0 Slip (m) Depth Peak Slip = 2.1 m At 8 km depth. 18 km 0 24 km Distance along strike
Errors on Slip Model. 0 Error (m) Depth 18 km 0 24 km Distance along strike
Monte Carlo Error Analysis 90 85 180 Tradeoffs Create 100 perturbed datasets and analyse. 10 9 8 Identify tradeoffs and errors 2 7 6 295 300 Strike 85 90 Dip 180 Rake Slip 8 9 10 Length 2 MinDepth
20 0 295 300 Strike 85 90 Dip 180 Rake Strike 295.4 ± 1.1 Dip 87.4 ± 1.5 Rake -179.2 ± 1.2 Slip 1.4 m ± 0.1 Slip 8 9 10 Length Length Min Depth 8.8 km ± 0.4 2.1 km ± 0.2 2 MinDepth
Resolution Inversion Depends on geometry between observation point and fault patch and smoothing factor.
Resolution
Recent Large Earthquakes Strike-slip Thrust