A weak subducting slab at intermediate depths below northeast Japan
Data files
Sep 25, 2023 version files 401.35 KB
-
README.md
2.83 KB
-
Transect_Focal_Mechanisms.csv
398.52 KB
Abstract
Knowledge of the state of stress in subducting slabs is essential for understanding their mechanical behavior and the physical processes that generate earthquakes. Here we develop a novel framework that uses a high-resolution focal mechanism catalog to determine, for the first time, the full deviatoric stress tensor within the subducting slab at intermediate depths. We show that by combining the static stress calculated from coseismic slip distributions with the stress orientations before and after the mainshock, that deviatoric stress within the slab at intermediate depths must be very low ( ∼ 1 MPa). These results preclude earthquake source mechanisms that require large background driving stresses, favoring a mechanically weak subducting slab, thus providing quantitative constraints on the physical processes that generate intermediate-depth earthquakes.
README: A weak subducting slab at intermediate depths below northeast Japan
https://doi.org/10.5061/dryad.bg79cnph1
We use waveform data from F-net (Fukuyama, 1998) and Hi-Net (Okada, 2004) to analyze moment tensor solutions, hypocenters and first-motion focal mechanisms of intraplate earthquakes occurring in NE Japan (36.5 < lat <42.5) and (138.5 < lon < 142.5) from 2008 to 2021 (M > 2.8 ). The NIED focal mechanisms were constrained using the methods, P-wave polarity observations, and hypocentral information of the NIED catalog and the the MOWLAS system (Kimura, 2012). This catalog of 3852 focal mechanisms is combined with 632 mechanisms (2002-2008, M > 2.8) from Kita et al. 2010 to constrain the state of stress within the deep subducting Pacific plate from 2002 to 2021 (depths > 60 km). We then select ~2200 focal mechanisms for analysis which occur within the subducting slab downdip of and near the mainshock rupture area, (i.e. within a 150 km wide transect centered on the epicenter and parallel to the slab dip at depths > 65 km < 200 km.
Description of the data and file structure
The focal mechanism data, and their spatial relationship with respect to the plate interface of the subducting slab (Kita et al., 2010) are compiled in a CSV text file. Each row reports the event information for a single focal mechanism with columns:
| year | |
|---|---|
| month | Event Origin Time |
| day | Event Origin Time |
| hour | Event Origin Time |
| minute | Event Origin Time |
| second | Event Origin Time |
| lat | Event Origin Latitude in decimal degrees |
| lon | Event Origin Longitude in decimal degrees |
| depth | Event Origin Depth in kilometers |
| Mw | Moment magnitude |
| distance_along_transect | Horizontal Distance in the Analysis Transect Plane in kilometers |
| perp_dist | Horizontal Distance in the Direction Perpendicular to the Analysis Transect Plane in kilometers |
| perp_dist_to_plate | Minimum Distance Perpendicular to the Plate Interface in kilometers |
| strike | The fault-trace direction in decimal degrees (0 to 360, relative to North) |
| dip | The angle of the fault in decimal degrees (0 to 90, relative to horizontal) |
| rake | The direction the hanging wall moves during rupture, measured relative to the fault strike (between -180 and 180 decimal degrees) |
Sharing/Access information
Links to other publicly accessible locations of the data:
- The p-wave polarity focal mechanisms were obtained from NIED using the online MOWLAS system (http://www.hinet.bosai.go.jp/).
- The hypocenters were obtained from the JMA (https://www.data.jma.go.jp/svd/eqev/data/bulletin/hypo).
- The F-Net moment tensors were obtained from the NIED (https://www.fnet.bosai.go.jp/event/search.php?LANG=en).
Methods
The focal mechanism data, and their spatial relationship with respect to the plate interface of the subducting slab (Kita et. al., 2010). The p-wave polarity focal mechanisms were obtained from NIED using the online MOWLAS system. The hypocenters were obtained from the JMA. The F-Net moment tensors were obtained from the NIED.