The 9th International Workshop on Statistical Seismology, 14 – 18 June, Potsdam, Germany

SPATIOTEMPORAL VARIATION OF B-VALUE IN MOLUCCA COLLISION ZONE USING HIGH PRECISION RELOCATED HYPOCENTERS

Hasbi Ash Shiddiqi(1), Sri Widiyantoro(2), Andri Dian Nugraha(2), Mohamad Ramdhan(3), Wandono(3), Sutiyono(3), TitiHandayani(3)

(1) Earth Sciences Graduate Program, Faculty of Earth Sciences and Technology, Institut Teknologi Bandung, Indonesia(2)Global Geophysical Research Group, Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Indonesia

(2) Earth Sciences Graduate Program, Faculty of Earth Sciences and Technology, Institut Teknologi Bandung, Indonesia(3)Agency for Meteorology, Climatology and Geophysics (BMKG), Jakarta, Indonesia

(1) h.a.shiddiqi@gmail.com

INTRODUCTION

DATA AND METHODS

RESULTS

CONCLUSIONS

ACKNOWLEDGMENTS

High seismicity in the Molucca Collision Zone (MCZ) is a result of complex tectonic setting in and around thisregion. The Molucca Sea Plate (MSP) collides with the Halmahera arc in the east and with the Sangihe arc inthe west and slab of MSP forms a reversed V-shape dipping beneath the Halmahera and Sangihe arcs. Theshallow part of MCZ is highly deformed crust and most of large earthquakes occurred in this area.

Teleseismic Double-Difference RelocationWe relocated nearly 10,000 earthquakes in the Molucca Sea and surrounding regions by applying teleseismicdouble-difference relocation method (DD relocation) by Pesicek et al. (2010). We performed relocationinversion utilizing local, regional and teleseismic arrival time data from BMKG data catalog for the period 2009-2015. The 3D regional seismic velocity model has been taken from Widiyantoro and van der Hilst (1997) tocalculate seismic travel-time.

Frequency Magnitude Distribution (FMD)Statistical analysis between the distribution of earthquakes’ magnitudes with their total number isrepresented by Gutenberg-Richter law (1994):

𝑙𝑜𝑔10𝑁 = 𝑎 − 𝑏𝑀

Where N is number of events with a magnitude ≥ M while a and b are constants. Constant b or b-value is animportant tool for determining the stress state of a region. In this study, we utilized ZMAP code (Wiemer,2001) to analyze the spatiotemporal distribution of b-value in the Molucca sea region. For this analysis, weused DD relocated hypocenters and ISC catalog (1990 to 2015).

Our results show that travel-time RMS residuals were reduced 42% compared to BMKG catalog. Cross-sectionsof DD relocated seismicity on this region clearly show two opposing Wadati-Benioff zone that representsgeometry of subduction slabs beneath this region.

For the first step, we analyze FMD for each zone that determined based on seismicity distribution, largeearthquakes history and geological setting (see Figure 2 for the zone locations). Based on our result, zone 2 isthe most active part where largest earthquake occurred in 2007 (Mw 7.5). The b-value for zone 2 is relativelylower compared to the two other zones. Depth section of b-value for zone 2 is suitable with depth of severallarge earthquakes (between 15 to 50 km).

Our result reveals a low b-value anomaly around the central part of Molucca Sea where Mw 7.1 earthquakessequence occurred several months later. Another areas with significant low b-value are in the southwest andsoutheast of MCZ. We interpreted the low b-value in the southeast area is related to the northern SorongFault. The northern part of MCZ has a relatively high b-value that is possibly related to stress release after theMw 7.1 2009 earthquake.

Figure 1. Tectonics Setting of Molucca Collision Zone and surrounding regions along with relocated earthquakes with Mw ≥ 4.5. Figure 3. Representation of b-value variation for each zone: FMD (left), temporal b-value with blue arrows indicates the

occurrence of earthquakes with Mw between 6 and 7; red arrows indicate earthquakes with Mw 7 or larger (middle); and b-value as function of depth (right). Figure 3. A. b-value variation for zone 1, Figure 3. B . b-value variation for zone

2, and Figure 4. C. b-value variation for zone 2.

We have successfully relocated BMKG earthquakes which refined tectonic structures and Wadati-Benioff zonein the MCZ area clearly. FMD analysis using relocated events and ISC catalog shows a distinctive low b-valueanomaly in the central part of MCZ where an earthquake with Mw 7.2 occurred in November 2014. Depth-profile of b-value reveals a low anomaly at shallow depths between 15 and 50 km, which agrees well with thedepth of several large earthquakes in this region. There are also significant low b-value anomalies in thesouthern part of MCZ related to the northern Sorong Fault and the Sangihe Thrust.

Figure 4. Map view of b-value variation in the Molucca Collision Zone using ISC and DD relocated catalog from 2000 to mid of 2014 preceding Mw 7.1 November 2014 earthquakes sequence. b-value spatial variation is shown on left figure

and standard deviation of b-value is shown on right figure.

We gratefully acknowledge BMKG for access to their earthquake data catalog . We thank the Graduate Research onEarthquake and Active Tectonics (GREAT), Faculty of Earth Science and Technology - ITB, and the Near SurfaceGeophysics Laboratory, Faculty of Petroleum and Mining Engineering - ITB for supporting this research. HAS would liketo thank Pepen Supendi from BMKG for fruitful discussion related to FMD and b-value analysis. Some figures in thisposter were created using Generic Mapping Tools (Wessel and Smith, 1991). Topographic contour was taken from SRTM15 Plus. Tectonic boundaries are adopted from Bird (2003), and Hall and Wilson (2000).

Figure 5. b-value variation for each cross-section. Cross-section positions are based on Figure 2.

In this study we aim to provide a catalog of

relocated hypocenters and interpret their relation to

the tectonic process. Furthermore, we

investigate the spatiotemporal variation of

b-value from relocated BMKG earthquakes and

also the International Seismological Centre (ISC)

catalog to obtain information regarding

tectonic process and its relation to large

earthquakes in this area.

Figure 2. Teleseismic double-difference relocation result from April 2009 to May 2015. Cross-section areas are depicted by black boxes and focus zones are depicted by blue boxes (left panel). Cross-Sections of BMKG catalog and DD

relocation results beneath the Molucca Collision Zone are displayed in the right panel. Volcanoes are represented by black triangles.