diff --git a/common/main/game.h b/common/main/game.h
index b787cd585..450756808 100644
--- a/common/main/game.h
+++ b/common/main/game.h
@@ -179,6 +179,7 @@ struct d_game_unique_state : ::dcx::d_game_unique_state
 struct d_level_shared_seismic_state
 {
 	fix Level_shake_frequency;
+	fix Level_shake_duration;
 };
 
 struct d_level_unique_seismic_state
diff --git a/common/main/gamemine.h b/common/main/gamemine.h
index e5af0741b..fa2714c4e 100644
--- a/common/main/gamemine.h
+++ b/common/main/gamemine.h
@@ -148,7 +148,6 @@ int load_mine_data_compiled(PHYSFS_File *LoadFile, const char *Gamesave_current_
 #if defined(DXX_BUILD_DESCENT_II)
 namespace dsx {
 extern int d1_pig_present;
-extern fix Level_shake_duration;
 extern segnum_t Secret_return_segment;
 extern vms_matrix Secret_return_orient;
 
diff --git a/similar/main/gamemine.cpp b/similar/main/gamemine.cpp
index 775036bb9..29823eebe 100644
--- a/similar/main/gamemine.cpp
+++ b/similar/main/gamemine.cpp
@@ -85,7 +85,6 @@ static void segment2_read(shared_segment &s2, unique_segment &u2, PHYSFS_File *f
 
 #if defined(DXX_BUILD_DESCENT_I)
 #elif defined(DXX_BUILD_DESCENT_II)
-fix Level_shake_duration = 0;
 segnum_t Secret_return_segment;
 vms_matrix Secret_return_orient;
 
@@ -493,12 +492,12 @@ int load_mine_data(PHYSFS_File *LoadFile)
 #if defined(DXX_BUILD_DESCENT_II)
 	if (mine_top_fileinfo.fileinfo_version < 18) {
 		LevelSharedSeismicState.Level_shake_frequency = 0;
-		Level_shake_duration = 0;
+		LevelSharedSeismicState.Level_shake_duration = 0;
 		Secret_return_segment = segment_first;
 		Secret_return_orient = vmd_identity_matrix;
 	} else {
 		LevelSharedSeismicState.Level_shake_frequency = mine_fileinfo.level_shake_frequency << 12;
-		Level_shake_duration = mine_fileinfo.level_shake_duration << 12;
+		LevelSharedSeismicState.Level_shake_duration = mine_fileinfo.level_shake_duration << 12;
 		Secret_return_segment = mine_fileinfo.secret_return_segment;
 		Secret_return_orient = mine_fileinfo.secret_return_orient;
 	}
diff --git a/similar/main/weapon.cpp b/similar/main/weapon.cpp
index 8eb901af5..4dde7b71e 100644
--- a/similar/main/weapon.cpp
+++ b/similar/main/weapon.cpp
@@ -1131,7 +1131,7 @@ void init_seismic_disturbances()
 //	Return true if time to start a seismic disturbance.
 static bool seismic_disturbance_active()
 {
-	const auto level_shake_duration = Level_shake_duration;
+	const auto level_shake_duration = LevelSharedSeismicState.Level_shake_duration;
 	if (level_shake_duration < 1)
 		return false;
 
@@ -1158,7 +1158,7 @@ static void seismic_disturbance_frame(void)
 		if (seismic_disturbance_active()) {
 			int	fc, rx, rz;
 			fix delta_time = static_cast<fix>(GameTime64 - LevelUniqueSeismicState.Seismic_disturbance_end_time);
-			fc = abs(delta_time - Level_shake_duration / 2);
+			fc = abs(delta_time - LevelSharedSeismicState.Level_shake_duration / 2);
 			fc /= F1_0/16;
 			if (fc > 16)
 				fc = 16;