Faulting Processes During Early-Stage Rifting: RAMP Response to the 2009-2010 Northern Malawi Earthquake Sequence

Faulting Processes During Early-Stage Rifting: RAMP Response to the 2009-2010 Northern Malawi Earthquake Sequence Figure 1.
Locations of main events from NEIC catalogue (red stars), seismic stations in RAMP deployment (white squares and text) and a subset of faults onshore (black and grey lines) and offshore (grey dotted lines). Purple dotted line indicates interpreted fault of Biggs et al. (2010) based on InSAR data.
On December 6, 2009, an unusual sequence of earthquakes began in the Karonga region in northern Malawi (Fig. 1). The sequence initiated with an Mw 5.8 event, which was followed 32 hours later by an Mw 5.9 event, and then nearly 12 days later by an Mw 6.0 event. Within this time span there were an additional six Mw > 5 events, with many more events that were at or below the threshold of the NEIC location system. Such events are very rare in the Malawi portion of the East African rift (EAR); prior to this sequence, the NEIC catalog [which dates to, 1973] contains only three events of M > 5 within the rift valley between 9-12oS latitude. This contrasts with both the Western and Eastern branches of the EAR through Kenya and Tanzania, which between them have experienced 115 M > 5 events in the same time period. Nearly all other moderate-size earthquakes within the Western Rift appear to be on major border faults, but the locations, mechanisms and event depths of the Karonga events imply that they occurred due to slip on normal faults within the hanging wall block. They offer a rare opportunity to evaluate how early-stage rifting is accommodated within the hanging wall of the system.
</p><p>The December 2009 earthquake sequence resulted in four casualties, 300 injuries and thousands of displaced people. Malawi currently has very limited capacity to monitor and locate such earthquakes; they have no permanent national network. With the support of IRIS PASSCAL, we undertook a Rapid Array Mobilization Program (RAMP) deployment at the request of the Director of the Malawi Geological Survey Department (MGSD). We deployed six broadband stations for a four-month period to monitoring aftershock activity. We recorded several thousand local events, and analysis is ongoing. This technical response complimented the post-event aid that was being provided by NGO’s and government agencies. Through our field activities, we explained the earthquakes to teachers, police officers and villagers. We provided television and radio interviews about earthquakes and the necessity of seismic monitoring, and blogged about our undertaking (http://blogs.ei.columbia.edu/ blog/tag/east-africa-rift/). We trained personnel from the MGSD in seismic equipment and field methods. This effort directly resulted in the purchase of equipment by the MGSD for 9 stations to begin a seismic network in Malawi; they purchased the same equipment employed in our RAMP deployment.
</p><p>Biggs, J., E. Nissen, T. Craig, J. Jackson, and D. P. Robinson (2010), Breaking up the hanging wall of a rift-border fault: the 2009 Karonga Earthquakes, Malawi, Geophys. Res. Lett., 37, doi:10.1029/2010GL043179.
</p><p>Acknowledgements: Funding for the RAMP deployment was provided by Columbia University through the Lamont-Doherty Earth Observatory and the Earth Institute. The IRIS PASSCAL program provided the instruments, as well as financial support for shipping. Data collection and instrument recovery supported by the NSF RAPID grant EAR-1019379.</p>


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