  Main-belt asteroids have been continuously colliding with one another since
they were formed. Its size distribution is primarily determined by the size
dependence of asteroid strength against catastrophic impacts. The strength
scaling law as a function of body size could depend on collision velocity, but
the relationship remains unknown especially under hypervelocity collisions
comparable to 10 km/sec. We present a wide-field imaging survey at ecliptic
latitude of around 25 deg for investigating the size distribution of small
main-belt asteroids which have highly inclined orbits. The analysis technique
allowing for efficient asteroid detections and high-accuracy photometric
measurements provide sufficient sample data to estimate the size distribution
of sub-km asteroids with inclinations larger than 14 deg. The best-fit
power-law slopes of the cumulative size distribution is 1.25 +/- 0.03 in the
diameter range of 0.6-1.0 km and 1.84 +/- 0.27 in 1.0-3.0 km. We provide a
simple size distribution model that takes into consideration the oscillations
of the power-law slope due to the transition from the gravity-scaled regime to
the strength-scaled regime. We find that the high-inclination population has a
shallow slope of the primary components of the size distribution compared to
the low-inclination populations. The asteroid population exposed to
hypervelocity impacts undergoes collisional processes that large bodies have a
higher disruptive strength and longer life-span relative to tiny bodies than
the ecliptic asteroids.
