10:45 〜 12:15
[SCG45-P38] Comparison of slip distribution for four long-term slow slip events in southcentral Alaska
キーワード：スロースリップイベント、アラスカ、GNSS
Slow slip events (SSEs) are aseismic transient fault slip events, and can be categorized into long- and short-term SSEs. In southcentral Alaska, where the Pacific plate and the Yakutat microplate subduct beneath the North America plate, the 1964 M 9.2 Alaska earthquake ruptured the plate boundary fault over ~600 km (e.g., Christensen & Beck, 1994). Two long-term SSEs were identified in Upper Cook Inlet: the first in 1998-2001 and the second in 2009-2013 (Fu et al., 2015; Ohta et al., 2006), while three were observed in Lower Cook Inlet in 1995-2004, 2010-2011, and 2016-2017 (Li & Freymueller, 2018; Li et al., 2016; Wei et al., 2012). In addition, ~10 mm transient displacements from November 2008 to September 2009 can be recognized in GNSS stations on Kodiak Island landward of the western part of the 1964 rupture area. In this study, we focus on four long-term SSEs occurring after the late 2000s and estimate their slip distribution to investigate the interplate slip behavior in this region.
We used daily coordinates of 85 GNSS stations provided by the Nevada Geodetic Laboratory (Blewitt et al., 2018). Earthquake-related signals, seasonal oscillations, and common-mode errors were removed in the preprocessing (Cleveland et al., 1990; Wdowinski et al., 1997). We fit a linear function with an offset during a break to three components of GNSS timeseries for 6-month-long periods before and after SSE transients. The offset was regarded as SSE displacement. We then estimated the slip distribution of each event by a linear least-square method with Laplacian smoothing (Itoh & Nishimura, 2016; Nishimura, 2009). We discretized the slab surface (Hayes et al., 2018) into rectangular sub-faults and calculated Green’s function using the dislocation model in an elastic half-space (Okada, 1992). The slip direction on sub-faults was fixed to the relative plate motion between the Pacific and North America plates (DeMets et al., 2010).
Here, we describe the preliminary results for the 2008-2009 Kodiak and the 2016-2017 Lower Cook Inlet SSEs. The seismic moment of the Kodiak SSE was 5.28×1019 Nm (Mw 7.1) and its maximum slip reached ~26 mm. The slip area of the Kodiak SSE coincides with epicenters of low-frequency earthquakes observed by Brown et al. (2013). The Kodiak SSE is located at the down-dip extension of the 1964 Alaska earthquake (Christensen & Beck, 1994) and its slip area overlaps with that of the afterslip of the 1964 earthquake (e.g., Suito & Freymueller, 2009), similar to the Upper and Lower Cook Inlet SSEs (e.g., Ohta et al., 2006; Wei et al., 2012). The overlap between SSEs and afterslip was also reported in the Nankai (Sherrill & Johnson, 2021; Yarai & Ozawa, 2013) and Kuril (Okada & Nishimura, AGU Fall Meeting 2022) subduction zones and suggests the complex frictional properties on the plate interface.
On the other hand, the seismic moment of the Lower Cook Inlet SSE was 7.82×1019 Nm (Mw 7.2) with ~48 mm maximum slip. The slip area of the 2016-2017 SSE overlaps with that of the 1995-2004 event but is slightly northeast of that of the 2010-2011 event (Li et al., 2016). However, the magnitude and duration of the 2016-2017 event is smaller and shorter than those of the 1995-2004 event (Li et al., 2016). The variety of SSE characteristics in this region may be caused by the temporal change of the slip deficit rate (Li et al., 2016).
Acknowledgments
The GNSS data used in this study is from the stations operated by UNAVCO, University of Alaska Fairbanks, the United States Coast Guard, the Alaska Volcano Observatory, the Federal Aviation Administration, and the National Oceanic and Atmospheric Administration.
We used daily coordinates of 85 GNSS stations provided by the Nevada Geodetic Laboratory (Blewitt et al., 2018). Earthquake-related signals, seasonal oscillations, and common-mode errors were removed in the preprocessing (Cleveland et al., 1990; Wdowinski et al., 1997). We fit a linear function with an offset during a break to three components of GNSS timeseries for 6-month-long periods before and after SSE transients. The offset was regarded as SSE displacement. We then estimated the slip distribution of each event by a linear least-square method with Laplacian smoothing (Itoh & Nishimura, 2016; Nishimura, 2009). We discretized the slab surface (Hayes et al., 2018) into rectangular sub-faults and calculated Green’s function using the dislocation model in an elastic half-space (Okada, 1992). The slip direction on sub-faults was fixed to the relative plate motion between the Pacific and North America plates (DeMets et al., 2010).
Here, we describe the preliminary results for the 2008-2009 Kodiak and the 2016-2017 Lower Cook Inlet SSEs. The seismic moment of the Kodiak SSE was 5.28×1019 Nm (Mw 7.1) and its maximum slip reached ~26 mm. The slip area of the Kodiak SSE coincides with epicenters of low-frequency earthquakes observed by Brown et al. (2013). The Kodiak SSE is located at the down-dip extension of the 1964 Alaska earthquake (Christensen & Beck, 1994) and its slip area overlaps with that of the afterslip of the 1964 earthquake (e.g., Suito & Freymueller, 2009), similar to the Upper and Lower Cook Inlet SSEs (e.g., Ohta et al., 2006; Wei et al., 2012). The overlap between SSEs and afterslip was also reported in the Nankai (Sherrill & Johnson, 2021; Yarai & Ozawa, 2013) and Kuril (Okada & Nishimura, AGU Fall Meeting 2022) subduction zones and suggests the complex frictional properties on the plate interface.
On the other hand, the seismic moment of the Lower Cook Inlet SSE was 7.82×1019 Nm (Mw 7.2) with ~48 mm maximum slip. The slip area of the 2016-2017 SSE overlaps with that of the 1995-2004 event but is slightly northeast of that of the 2010-2011 event (Li et al., 2016). However, the magnitude and duration of the 2016-2017 event is smaller and shorter than those of the 1995-2004 event (Li et al., 2016). The variety of SSE characteristics in this region may be caused by the temporal change of the slip deficit rate (Li et al., 2016).
Acknowledgments
The GNSS data used in this study is from the stations operated by UNAVCO, University of Alaska Fairbanks, the United States Coast Guard, the Alaska Volcano Observatory, the Federal Aviation Administration, and the National Oceanic and Atmospheric Administration.