(Portland, OR)  --  Portland Police and Portland Fire & Rescue Arson Detectives are investigating two explosions that happened over the weekend.  Police say a book exchange box near Northeast 87th and Fremont Street was destroyed Saturday night.  Debris caused minor damage to two nearby houses.  Early Monday morning, a second explosion blew apart a brick mailbox near Northeast 148th and Burnside, sending bricks flying across the street.  No one was hurt in either explosion.  Detectives are asking anyone with surveillance video of the explosions to contact them.